c 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 10.1002/14356007.b01 07 Construction Materials in Chemical Industry 1 Construction Materials in Chemical Industry Hubert Gr ¨ afen, Bayer AG, Leverkusen, Federal Republic of Germany 1. Introduction .............. 2 2. Material Requirements ....... 2 2.1. Processability and Joining ..... 3 2.2. Mechanical Stability and its De- pendence on Temperature ..... 3 2.3. Corrosion Resistance ........ 5 2.4. Resistance to Wear .......... 6 3. Choice of Materials ......... 6 4. Quality Assurance through Mate- rial Tests and Checking of Fabri- cation and Functioning ....... 8 5. Properties and Applications of Materials ................ 9 5.1. Steels ................... 9 5.1.1. Unalloyed and Low-Alloy Steels for Vessels and Pipelines ......... 9 5.1.2. Steels with High-Temperature Strength ................. 11 5.1.3. Heat-Resistant Steels ......... 12 5.1.4. Steels for Low Temperatures .... 13 5.1.5. Steels Resistant to Pressurized Hy- drogen .................. 13 5.1.6. Stainless Steels ............. 14 5.1.6.1. Technical Properties .......... 18 5.1.6.2. Chemical Properties .......... 20 5.1.6.3. Development State of Stainless Cr – Ni Steels ................. 24 5.2. Cast Iron ................ 25 5.3. Nickel and Nickel Alloys ...... 26 5.3.1. Nickel – Copper Alloys ........ 26 5.3.2. Nickel – Chromium Alloys ...... 27 5.3.3. Nickel – Molybdenum and Nickel – Molybdenum – Chromium Alloys . 27 5.4. Aluminum and Aluminum Alloys 28 5.5. Copper and Copper Alloys .... 31 5.6. Lead and Lead Alloys ........ 33 5.7. Zinc and Zinc Alloys ........ 34 5.8. Tin and Tin Alloys .......... 35 5.9. Titanium, Zirconium, Niobium, and Tantalum ............. 35 5.10. Organic Materials .......... 38 5.10.1. Selection Criteria ........... 39 5.10.2. Properties and Application Criteria 39 5.10.3. Thermosetting Plastics ........ 41 5.11. Inorganic Nonmetallic Materials 42 5.11.1. Glass ................... 42 5.11.2. Graphite ................. 43 5.11.3. Refractory and Acid-Resistant Bricks .................. 43 5.11.4. Engineering Ceramics ........ 44 6. References ............... 45 1. Introduction Components of chemical plant are generally subjected to thermal, chemical, and mechani- cal stresses. The combination of these stresses places very heavy demands on plant materials, especially with regard to corrosion. Thus even unalloyed and low-alloy steels have to meet very strict quality requirements. They must have high purity, for example, and their nonmetallic inclu- sions must be finely dispersed, since they af- fect the ability of plant to withstand corrosion cracking, such as stress corrosion cracking and damage by hydrogen. By far the most important materials are the highly alloyed stainless steels and nickel-based alloys, though aluminum, cop- per and their alloys, and refractory metals, and organic and inorganic materials are also impor- tant. In this article the applications of materials in process plant manufacture are described and development trends are discussed. 2. Material Requirements In the choice of materials for production plant in the chemical industry, there are three basic considerations: 1) The processibility of the material in its com- mercially available form (sheet, piping, pro- files, etc.) 2) The ability of materials to withstand produc- tion processes. This is a complex property that includes mechanical stability and its de- pendence on temperature, resistance to cor- rosion, and possibly resistance to wear also. 3) The costs of materials, of their processing,
Components of chemical plant are generallysubjected to thermal, chemical, and mechani-cal stresses. The combination of these stressesplaces very heavy demands on plant materials,especially with regard to corrosion. Thus evenunalloyed and low-alloy steels have tomeet verystrict quality requirements. Theymust have highpurity, for example, and their nonmetallic inclu-sions must be finely dispersed, since they af-fect the ability of plant to withstand corrosioncracking, such as stress corrosion cracking anddamage by hydrogen. By far the most importantmaterials are the highly alloyed stainless steelsand nickel-based alloys, though aluminum, cop-per and their alloys, and refractory metals, andorganic and inorganic materials are also impor-tant. In this article the applications of materials
in process plant manufacture are described anddevelopment trends are discussed.
2. Material Requirements
In the choice of materials for production plantin the chemical industry, there are three basicconsiderations:
1) The processibility of the material in its com-mercially available form (sheet, piping, pro-files, etc.)
2) The ability of materials to withstand produc-tion processes. This is a complex propertythat includes mechanical stability and its de-pendence on temperature, resistance to cor-rosion, and possibly resistance to wear also.
3) The costs of materials, of their processing,
2 Construction Materials in Chemical Industry
and of the inspections of the chemical appa-ratus during its useful lifetime.
As so many factors have to be taken into con-sideration, choosing materials is not easy, espe-cially if the plant is to be used for a recentlydeveloped chemical process. The prospect of asuitable choice is best where the material scien-tist, chemical engineer, plant designer, and plantengineer have worked closely together [1], [2].
2.1. Processability and Joining
Shaping (e.g., bending, rounding, and flang-ing), separating (e.g., cutting and machining)and joining (e.g., welding, bonding, and piperolling) are particularly important processes inthe fabrication of chemical plant. Coating andprocesses that modify the properties of mate-rials (e.g., quenching, tempering, nitriding, agehardening) are also widely used. The choice offabrication processes depends on the propertiesof the material concerned, e.g., on its suitabilityfor cold shaping or welding. Fabrication mustnot affect materials so drastically that their re-sistance to the conditions of subsequent use issignificantly impaired.
Joint welding is by far the most importantjoining process in chemical plant fabrication,while build-up welding is used widely both forcoating in original plant fabrication, and for re-pairs [3–5]. The weldability of a component de-pends on the weldability of the material andon the design and fabrication of the part [6].Although each factor may be decisive on itsown, the interplay of factors must never be over-looked. It is thus pointless to choose a steel withthe highest possible yield strength unless onehas checked that dangerous peak stress will notoccur (through deficiencies of design) and thatwelding defects will not be caused by the use ofwelding techniques unsuited to the material.
Bonding is used for materials that cannot bewelded or whose properties are changed exces-sively by welding [7], [8]. As adhesive bondscannot be exposed to elevated temperatures, thismethodof joining parts has acquired little impor-tance for chemical apparatus.
2.2. Mechanical Stability and itsDependence on Temperature [9]
Chemical apparatus in use is subjected to widelydiffering mechanical stresses. The possible vari-ations of mechanical stress with time are shownschematically in Figure 1. These stresses maybe monoaxial, biaxial, or triaxial.
Figure 1. Forms of the time function of mechanical stress
The behavior of materials under polyaxialstress – the states of stress in plant componentsare almost always of this kind – is seldomknownas such, because strength data are usually avail-able only as yield strength and tensile strengthfor loads exerted monoaxially in a tensile test.Therefore, where a particular exposure is con-cerned, the material’s mechanical stability mustbe assessed by comparing its knownmechanicalstrength values with a stress calculated accord-ing to the appropriate theory of strength [10].
In the fabrication of chemical apparatus pref-erence is given to materials that are easilyshaped, since they react to the application of ex-cessive force by undergoing energy-consumingchanges of shape instead of simply breaking.Temperature has an important influence on themechanical stability ofmaterials. As the temper-ature rises, strength decreases, ease of shapingincreases, and creep behavior becomes the mainfactor determining mechanical stability.
Thus, above a limit temperature, which de-pends on the material, the yield strength or ten-sile strength at the envisaged operating tempera-ture can no longer serve as a characteristic valuefor calculating the stress of a chemical apparatus(Fig. 2, see next page) [9].
The strength of metals increases with de-creasing temperature. Therefore, componentsintended for use at room temperature might beexpected to be more resistant to loads exertedat lower temperatures. That this is only partlytrue is explained by the deformation behaviorof materials. The characteristic strain values ofmetals tend to drop suddenly as the tempera-
Construction Materials in Chemical Industry 3
ture decreases, and this is accompanied by anincrease in the tendency to undergo brittle frac-ture. The inherent risk of brittle fracture is notaccounted for directly by strength calculations.
Figure 2. Dependence of characteristic strength data ontemperature (schematic)
Susceptibility to brittle fracture is representedby values for elongation at rupture and reductionof area in the tensile test, and also by the tran-sition temperature obtained from a plot of thenotched-bar impact energy versus temperature(Fig. 3) (see also →Mechanical Properties andTesting ofMetallicMaterials). In choosing ama-terial to suit a particular set of requirements, ade-quate safety must be ensured by taking this tran-sition temperature into consideration, as well asthe influences exerted on the material during itsproduction and processing. In the case of high-strengthmaterials, it is also advisable to performfracture toughness testing [11].
Figure 3. Transition temperature Tt of the notched-bar im-pact energya) High level, fracture on working; b) Transition range,mixed fracture; c) Low level, brittle fracture
Where alternating stresses occur, determin-ing suitable characteristic material values that
will enable a part to bedesigned so that it satisfiessafety requirements is often difficult, becausethe combinations of loads exerted in practicaluse cannot be accounted for fully by measur-ing the alternating stress endurance limit (e.g.,by the Wohler method). The shape and surfacecondition of the part must also be considered(Fig. 4). More realistic criteria can be obtainedby determining the resistance to service condi-tions of components themselves. The AD (Ar-beitsgemeinschaft Druckbehalter) leaflets [12]on calculations for pressure vessels or their com-ponents are based on the assumption that thesevessels are normally subjected to static loads. Ifthe pressure fluctuates, the resultant additionalstresses can be taken into account according toAD leaflet S 1, in which allowance for themis made by reducing the permissible stresses(Fig. 5). Prediction of useful lifetime is particu-larly difficultwhen alternating stresses and creepstresses are superimposed.
Figure 4. Influence of the surface quality on alternatingstress tests on high-presure tubes (nominal bore 6mm; nom-inal pressure 250MPa) made of alloy 30CrMoV9 (mate-rial no. 1.7707; HB30≈ 3000MPa; tensile strength = 900 –1100MPa)a) Cold-worked; b)Nitrided; c) Electropolished; d)Nitrided;e) Polished; f) and g) As-delivered
In such cases there is still no acceptedmethodfor prediction of useful life. It would also appearunrealistic to seek “universally valid” theoriesand better to simulate typical stress processes intest specimens similar to plant components, thusproviding the design engineer with useful lifedata for particular kinds of materials and prob-lems. According to the available experience it isbest to design plant and components so that they
4 Construction Materials in Chemical Industry
can withstand the stress that gives the shortestuseful life.
Figure 5. Admissible number of cycles for pressure vesselsas a function of stressa) Range of alternating compressive stress = 90 %;b) Range of alternating compressive stress = 100 %
2.3. Corrosion Resistance (→Corrosion)
The longevity of most apparatus depends notonly on mechanical loads but also on the natureand spectrum of the corrosive ambient medium.As a rule the characteristic symptom of corro-sion damage is not the loss of material but animpairment of function and load-bearing capac-ity. Uniform corrosion, for example, may causea considerable loss of mass before the service-ability of a part begins to suffer. Localized orselective corrosion proceeds rapidly towards theinside of a part, such that a notching effect is ex-erted on parts that bear mechanical loads. Cor-rosion of this kind may rapidly cause vessels toleak, or parts to fail through low-ductility frac-ture.
If the surface becomes creviced or pitted,the stress becomes nonuniform, and the opera-tional stability of the component under alternat-ing stresses is impaired. This also reduces thestatic fatigue resistance under constant load, asshownby the example inFigure 6 [13]. The staticfatigue resistance of copper samples exposed si-multaneously to corrosion and mechanical load-ing is impaired to a much greater extent that ofprecorroded samples that have already under-gone a considerably greater loss of mass andare then stressed mechanically. This exemplifiesthe interaction between corrosion and creep. For
iron-based and nickel-based alloys, this interre-lation affects the static load fatigue resistanceof parts exposed to corrosion processes at hightemperatures.
Figure 6. Creep rupture behavior of copper at 95 ◦Ca) 0.5MH2SO4 (air saturated); b) 0.5MH2SO4 (air); c) Air(precorroded in 0.5M H2SO4); d) Air∆m =Mass loss, wt %
Basically it may be assumed that the behaviorof a part exposed to corrosive influences dependslargely on the stability of the material, which, inturn, depends mainly on the film which forms atthe surface. Efforts are therefore being made tofind alloys that can improve the conditions forsurface film formation. It is also necessary toensure that the microstructures of alloys remainas stable as possible, so that demixing (which,for example, could impair the passivation ca-pability of alloys) cannot occur in welding, hotforming, etc. This is particularly important be-cause precipitation may strengthen the tendencytowards local activation (local removal of thepassive layer), whereby local corrosion may beinitiated.
Highly localized corrosion that results fromsimultaneous chemical attack and mechani-cal stress is particularly serious. The crite-rion of failure here is the occurrence of low-ductility fracture. Stress corrosion cracking, fa-tigue cracking, and water-induced cracking arephenomena of this kind. They are particularlyimportant with regard to serviceability, usefullife, and operational reliability.
The special danger of anodic stress corrosioncracking and hydrogen-induced cracking is thatthey can rarely be detected while the cracks arespreading and before the apparatus starts to leak
Construction Materials in Chemical Industry 5
or a part breaks. Although anodic stress corro-sion cracks outwardly resemble brittle fractures,the metal itself retains its ductility.
The initiation of cracking by the variousstress corrosion crackingmechanisms that resultfrom the action of a given corrodent on a givenmaterial depends very much on the mechanicalstresses involved.
In the case of fatigue cracking the loss of fa-tigue strength for finite life and loss of fatiguestrength have caused considerable difficulties inthe dimensioning of parts.
2.4. Resistance to Wear (see also→Abrasion and Erosion)
Since all production processes, except chemi-cal reactions, involve physical operations suchas comminution, conveyence, and separation ofphases, the wear resistance of materials is im-portant for many types of chemical apparatus.Models of the various kinds of wear have beendeveloped [14]; in practice, however, the con-ditions are complex, for not only may severalskinds of wear occur simultaneously, but overlapand interaction of wear and corrosion processescan also arise.
Thus, as a rule, only practical trials reveal theactual stress conditions involved. Compared tocorrosion, however, wear is less problematic be-cause it does not cause cracklike damage, and itseffects aremore easily repaired, e.g., by build-upwelding.
3. Choice of Materials
Designers of chemical apparatus must pay care-ful attention to materials as well as the purposefor which the apparatus is intended. This is im-portant at all stages, from initial planning to thedetailed drawing of the finalized design. For aselection process, however, not just the functionand properties needed by the components, butalso the operations bywhich the components arefabricated from the material, must be taken intoconsideration. Not only do these operations callfor definite properties on the part of materials,theymay also affect the behavior of componentsunder service conditions.
The demands made on chemical plant in useare becoming increasingly strict, and the vari-ety of fabrication processes and range of ma-terials available are growing. These factors, to-gether with the need to use economical fabrica-tion techniques, have given the choice of mate-rials a complexity that calls for systematic con-sideration. A basic procedure for the selectionof materials for chemical plant is shown in Fig-ure 7 (see next page). Clearly the demands ofthe application and those of the fabrication pro-cess must be taken into account as fully as possi-ble so that the overall requirements are correctlyformulated and properly interpreted in the fix-ing of verifiable property data. At the same timethe behavior of materials under operating con-ditions and those of fabrication must be knownso that one can judge reliably whether or not agiven material is suitable for a given purpose.
Often, these preconditions cannot be met en-tirely. In many cases, therefore, model experi-ments and serial testing are necessary as em-pirical aids to selection. Establishing and theninterpreting the combination of mechanical andcorrosion-chemical exposures, all of which areoften supplemented by wear, is particularly dif-ficult for chemical plant. Frequently, corrosionbehavior is treated in the selection procedure asthe decisive criterion. In fact, experience in theoperation of chemical plant has shown thatmuchof the damage that occurs to chemical appara-tus arises from the use of materials that are in-sufficiently resistant to corrosion under practicalconditions, and that damage of purely mechan-ical origin is generally much less frequent [15].
Therefore, technical rules exist for designingand dimensioning apparatus to meet combinedmechanical and thermal stresses. The standard-ized calculation procedures based on strengthand ductility data are intended to provide thedesigner with a proven basis on which to work.However, where wear processes (corrosion andabrasion) are concerned, that is not possible;here, instead, special design criteria must beworked out in each individual case.
A shortlist of materials can be drawn upfrom corrosion tables, which give the corrosionbehavior of specific materials [16], [17]. Nev-ertheless, the most extensive tables can nevertake into consideration all the conditions of agiven practical case. Almost always there ismore than one corrosive agent, because chemi-
6 Construction Materials in Chemical Industry
Figure 7. Basic procedure for the choice of materials for chemical apparatus
cal processes normally involve startingmaterialsand end products, intermediates and byproducts,often with unknown properties, and solids thatpromote wear. Furthermore, reactions are influ-encedbypressure and temperature. Thus the cor-rosion behavior of materials cannot be given intables with any degree of accuracy, and termssuch as resistant or nonresistant may be inap-propriate in an actual application. The results oflaboratory tests are consideraly more useful.
The most reliable predictions are those basedonplant tests performed either in a plant engagedin practical production or in a pilot plant. Often,however, such tests are very laborious. Even incases where tests have been carried out, corro-sion may still occur, possibly because the stabil-ity of a material has been impaired by excessivecold forming or by excessive heat input in weld-ing, or because the states of the material in thetest plant and in the subsequently constructedchemical apparatus were not identical.
In view of this complexity, close coopera-tion between material scientists, chemical engi-neers, chemists, designers, apparatus manufac-turers, and –where chemical apparatus subjectto compulsory testing, e.g., pressure vessels, isconcerned – the representatives of the officiallyrecognized supervisory authorities, is particu-larly desirable.
Because corrosion reactions are so complexand depend on so many factors, an assessmentof the corrosion behavior of a given material –corrodent system will differ greatly accordingto the method of investigation. False interpreta-tions can be avoided and comparison improvedif one has a good knowledge of the kinds of in-formation provided by the chosen test methodas well as knowing its limitations. It is thereforeunderstandable that standardized corrosion testsare few in number or contain more general in-structions. Frequently, they cover only the statesof materials that are most favorable from the as-pect of corrosion chemistry (that is to say the ho-
Construction Materials in Chemical Industry 7
mogeneous structural states existing at the timeof delivery), describing, for example, the testingof stainless steels for resistance to intergranularcorrosion (DIN 50 914) or the testing of stain-less steels in boiling nitric acid (DIN 50 921 andASTM A 262-70). A comparison of the sensi-tivities of unalloyed and low-alloyed steels tointergranular stress corrosion cracking is givenin DIN 50 915.
A common feature of all investigation meth-ods is that they refer to the material, and not di-rectly to the stresses that occur in practice. Sup-plementary chemical or electrochemical testssimulating the conditions that occur in practiceare therefore necessary in the selection of ma-terials. Chemical tests are performed on sam-ples exposed to gaseous and liquid corrodents atthe operating pressure and temperature, whereasin electrochemical corrosion tests, the depen-dence of corrosion on potential is investigatedand provides information on the effects of vari-ables which alter the potential.
The results of chemical corrosion tests andelectrochemical corrosion tests may differ fun-damentally. The differences arise because inchemical tests the corrosion potential may varywith time. In electrochemical tests the potentialis fixed, so that, though they are more informa-tive in some respects, their practical value maybe limited in others.
The characteristic stability data needed in thechoice of materials can be obtained in twoways:
1) By inserting suitable material specimens inpilot plants or in existing, but not yet entirelysatisfactory, production plants,
2) By performing laboratory corrosion tests.
With regard to the corrosive medium in cor-rosion tests, careful attention must be paid toa number of important factors, such concentra-tion, temperature, dissolved gases, impurities,solid matter, and rate of flow. As there are somany parameters, tests should use suitable spec-imens in a plant and under conditions as closeas possible to those of practical use.
The specimens should be placed at severalrepresentative locations. In a distillation column,for example, they should be placed in the pit, inthe vicinity of the feed point, and in the head.
The shape of a specimen used depends onthe types of corrosion expected. Where generalcorrosion predominates, it is sufficient to use
welded sheets that have the surface quality of theparts to be used in practice. If stress corrosioncracking is involved, plastically and elasticallystressed specimens or tuning fork specimens areused.
Material specimens are generally attached tointerior parts, such as an agitator or a thermome-ter protection tube. To prevent polarization ofspecimens through contact with plant compo-nents, and consequent falsification of measure-ments, the fixing screws are placed in insulatingsheaths.
In addition to the exposure of test specimens,the parts of a pilot plant itself should be exam-ined as a source of further information on cor-rosion behavior. For critical plant units such asheat exchangers, it has been found advisable toremove a tube from time to time, to cut it open,and to examine the inside and outside surfacesfor corrosion symptoms.
If stability tests cannot be performed in pilotinstallations, laboratory tests under conditionsas close as possible to those of practice must becarried out. They should also be carried out asa supplementary measure in cases where planttests have not clearly revealed the time depen-dency of the corrosion processes, and severerconditions of attack may therefore help to com-plete the picture.
General guidelines on the conduct of corro-sion tests are given in DIN 50 905. These guide-lines should be followed as a route to recipro-cally comparable results that can be transferredto the plant component in question.
For further details on chemical and electro-chemical corrosion tests, see →Corrosion.
4. Quality Assurance throughMaterial Tests and Checking ofFabrication and Functioning
To ensure that a chemical apparatuswill be prop-erly fabricated and that its functioning free fromdisturbances, many tests must be carried out be-fore and during fabrication, before the apparatusis put into operation, andwhile it is in actual use.At the stage of testing of materials when theyleave the factory and arrive at the plant manufac-turer’s establishment it is normally fairly easy toascertain whether or not they are of the required
8 Construction Materials in Chemical Industry
quality, as the required property data will havebeen agreed on the basis of quality standards orsimilar specifications. The same cannot be saidof fabrication checks, which include, in partic-ular, checking the construction for compliancewith the blueprints and examining the weldingwork. Here the test engineer has more latitude,especially in interpreting the results of nonde-structive tests. It must be emphasized that thefirst step in fabrication testing should be a thor-ough visual inspection; this gives the tester anoverall impression of the care takenby themanu-facturer. Checking compliance with legislationandofficial regulations ismainly the responsibil-ity of the Technical Control Associations and theplant safety inspection departments of the largechemical firms. Functioning tests check the be-havior of the apparatus under conditions close tothose of practice. Of exceptional importance arethe regular inspections of plant in use that are in-tended to reveal incipient damage early enoughto ensure that plant shut-down can be avoidedand any necessary repairs need not be under-taken in haste. Once again, visual inspectiontakes precedence. Much emphasis is also placedon nondestructivemethods – thicknessmeasure-ment, for example, and tests that reveal initialcracking [18]. Testing of plant in service alsoprovides knowledge thatmay be useful in select-ing materials for new chemical apparatus by re-vealing weaknesses of design, materials, or fab-rication.
5. Properties and Applications ofMaterials
5.1. Steels (→Steel)
Steels are still the materials most commonlyused for chemical plant. Their variety of alloycompositions and the range of variation of theirproperties permit an exceptional degree of adap-tation to practical requirements.
5.1.1. Unalloyed and Low-Alloy Steels forVessels and Pipelines
Sheet and piping made of unalloyed and low-alloyed steels with carbon contents up to ca.0.25wt% are used extensively in chemical plantfor vessels (pressure vessels, storage vessels,etc.) and pipelines that are not exposed to partic-ularly severe corrosion. The main standards anddesignations of several typical steels are com-piled in Table 1. Comparable steels with com-parable properties are described in the standardspecifications of other countries – see, for ex-ample, SAE, AISI, ASTM (United States), BS(Great Britain), NF (France), SS (Sweden), UNI(Italy) and NBN (Belgium). The steels are nor-mally processed in the normalized state.
Owing to the introduction of weldable fine-grained structuralsteels with yield strengths of
Table 1. Quality standards for unalloyed and low-alloy steels widely used in chemical plant
Standard Short title Typical steels
Short name Material number
DIN 17 100 Steels for general structural purposes RSt 37 – 2 1.0038St 44 – 2 1.0044
DIN 1629 Seamless unalloyed circular steel tubes for St 37.0 1.0254special requirements St 44.0 1.0256
St 52.0 1.0421
DIN 17 155 Steel plates and strips for pressure purposes H I 1.0345H II 1.042515Mo 3 1.5415
DIN 17 102 Weldable, finegrained construction steels StE 285 1.0486(normalized) StE 355 1.0562
WStE 460 1.8935
DIN 17 175 Seamless tubes of heat-resistant steels St 35.8 1.0305St 45.8 1.040515Mo 3 1.5415
≥ 360MPa, boiler plate consisting of unal-loyed steels, whose yield strengths are consid-erably lower, is used on a smaller scale thanit was formerly. The use of high-strength heat-treated structural steels with yield strengths of≥ 700MPa [19–23] will presumably increase.Pipes consisting of welded steel strip will beused increasingly instead of seamless pipes [24].
Steels are suitable for fusion welding by allprocesses, but the welding filler metals mustbe suited to the base material. Specific weld-ing guidelines must be adhered to increasinglyas the alloy content and yield strength rise; theseguidelines can be found by consulting either thestandards ormanufacturers of steels andweldingfiller materials. Processing guidelines for weld-able fine-grained structural steels are compiledin DIN standards.
Normalized fine-grained structural steels arenow very popular. They combine the excellentstrength and toughness imparted by fine-grainhardening [25] with the advantages offered bymodern ladle metallurgy in respect of purity andmicroalloying.
The blast or shot injection of calcium com-pounds to reduce the sulfide content of steel(TN process) and the addition of cerium and zir-conium to produce globular, finely distributedsulfide inclusions should also be mentioned.These processes improve purity, block segrega-tion, weldability, and toughness.
In special cases, these desulfurization meth-ods enable the sulfur content to be reduced to be-low 0.005 %. Consequently, and through the in-fluence exerted on the nature of the precipitation,the anisotropy of the steel’s properties is reducedconsiderably, as may be seen from the percent-age reductions of area at fracture of fine-grainedstructural steels in the thickness direction of thesheet (Fig. 8) [26]. Figure 9 [26] shows that thefatigue resistance is also improved.
Where damage occurs, the leak-before-failure behavior imparted by high toughness isexceptionally favorable because the risk of sud-den failure as a result of unstable crack propaga-tion is practically eliminated. The high purity offine-grained steels, together with the fine disper-sion of their nonmetallic inclusions, notably thesulfides, is very favorable with regard to corro-sion resistance. Coarse sulfides promote the for-mation and growth of cracks both in hydrogen-
Figure 8. Influence of sulfur content on the reduction of areaat rupture in the thickness direction of StE 355 steel platesa) 40 – 55mm plate, 0.015 – 0.054 % S; b) 40mm plate,≤ 0.010%S; c)≥ 50mmplate,≤ 0.006%S; d) 20 – 50mmplate, TN treated, 0.002 % S
Figure 9. Influence of the TN process on the dynamicstrengthof steel StE 355 (stress ratioR = 0, thickness of plate:50mm)A) Untreated; B) Calcium treated (TN process); a) Trans-verse; b) Longitudinal; c) Normal
10 Construction Materials in Chemical Industry
induced crack formation and in stress corrosioncracking [27]. Upon chemical exposure to aque-ous solutions, the sulfides, especially under theinfluence of the hydrolytic acidification that oc-curs in cracks, may be converted to H2S, whichis a vigorous promoter of hydrogen cracking.
It is now believed that crack growth in anodicstress corrosion cracking is also promoted by hy-drogen. That would explain why steel StE 355,in particular, behaves favorably towards suchcrack-initiating media as alkalis, nitrates, andliquid ammonia. This steel is now a standardmaterial for these media. For greater safety, thefinished apparatus should be subjected to stressrelief annealing.
Considerable progress has been madethrough efforts to improve the economy of struc-tural steels by raising their strength. Startingfrom steel St 52-3, it has been possible to raisethe yield strength from 355 to ca. 900MPa. Thefirst step in this direction consisted in raising thealloy contents of normalized structural steels.But the scope for improvement in this way waslimited by the fact that increasing the alloy con-tent impairs the cold cracking resistance inweld-ing. Agreement has therefore been reached thatthe yield strength of normalized steels shouldnot exceed 500MPa.
Considerably higher yield strengths, even atlow alloy contents, are obtainable by quenchingand tempering. In the case of heat-resistant struc-tural steels, air tempering is preferred. Waterquenching followed by tempering, on the otherhand, is more favorable for steel needing hightoughness. By far the most important represen-tative of this group of steels in terms of quan-tity is StE 690, whose minimum yield strengthis 690MPa (Euronorm 137). Steels with com-parable yield strengths are also used on a largescale for pressure vessels and other chemical ap-paratus, such as the pressure-bearing parts ofmultistory vessels. StE 890 has been used to anincreasing extent since the mid 1980s [28].
5.1.2. Steels with High-TemperatureStrength
High-temperature structural steels are usedin the chemical industry mainly for heavilystressed parts of steam boilers (e.g., collectors,drums, piping) and for pressure vessels and tubes
operating at up to ca 800 ◦C [29]. These materi-als are also very important in reactor technology.According to their maximum service tempera-tures that can be maintained for long periods,the following groups are distinguished:
1) For temperatures up to ca. 400 ◦C: un-alloyed steels, in some cases melted asfine-grained structural steels, whose mainstrength-related property is the high-tempe-rature yield strength. These steels are stan-dardized in DIN 17 155 (boiler plate types),DIN 17 175 (pipes) and DIN 17 102 (fine-grained structural steels) and are suitable forall fusion welding methods. Typical steelsbelonging to this group are the grades HI(1.0345), St 35.8 (1.0305), and WStE 355(1.0565).
2) For temperatures of 400 – 550 ◦C: unalloyedand low-alloy steels having good long-term,high-temperature strength-related propertyvalues. The main alloying elements are man-ganese (up to 1.3wt%), chromium (up to2.5wt%), andmolybdenum (up to 1.2wt%),vanadiumalso being used occasionally (up to0.5wt%). These materials are standardizedinDIN17 155,DIN17 175,DIN17 240 (nutsand bolts), and DIN 17 245 (steel castings).These steels are suitable for fusion welding,though preheating and post-welding anneal-ing may be necessary, details of which canbe found in the standards.
3) For temperatures of 550 – 600 ◦C: marten-sitic steels containing up to ca. 12wt% chro-mium and additions of molybdenum, vana-dium, nickel, and tungsten [30]. These arestandardized in DIN 17 459 (sheet, pipes,forgings) and in DIN 17 245 (steel castings).These steels are supplied in the quenchedand tempered state; a typical representa-tive is X 20CrMoV12 1 (1.4922). Being air-hardenable, these steels must be kept at 250to 450 ◦C during welding, after which theymust be cooled to ca. 120 ◦C and then imme-diately annealed.
Table 2. Approximate temperature limits for the use of steels in a weakly oxidizing flue-gas atmosphere
Steel Material number Standard Approximate temperature, ◦C
St 35.8 1.0305 DIN 17 175 500St 45.8 1.0405 DIN 17 175 50015Mo 3 1.5415 DIN 17 155 53013CrMo 44 1.7335 DIN 17 155 56010CrMo 9 10 1.7380 DIN 17 175 590X 20CrMoV12 1 1.4922 DIN 17 175 600X 8CrNiNb 16 13 1.4961 DIN 17 459 750X 8CrNiMoNb 16 16 1.4981 DIN 17 459 750
and X 8NiCrAlTi 32 21 (material no.1.4959), which are standardized in DIN17 459.
These steels are welded with filler metals ofthe same composition, and at the lowest possi-ble heat input to avoid hot cracking of the weldmetal.
As austenitic steels are very expensive, theyare used only in the zones which reach thehighest temperatures, outside of which low-alloy steels are used. Special techniques haveto be used to form welds between ferritic andaustenitic steels because of the diffusion pro-cesses that occur in welding and because of thedifferences in thermal expansion [31].
For temperatures above 800 ◦C only iron –chromium– nickel alloys and nonferrous alloysbased on cobalt or nickel are suitable [32], [33];see also Section 5.3.
Table 2 gives approximate figures for thehighest temperatures at which a number of steelswith high strength at elevated temperature can beused in a weakly oxidizing flue-gas atmosphere.Long-term heat stability values are compiled in[34]; the importance of these values in calculat-ing the useful lives of components subjected tothis exposure is discussed in [35].
5.1.3. Heat-Resistant Steels
Heat-resistant steels are those that have goodstrength-related property data and are distin-guished by exceptional resistance to exposurefor short or long periods to hot gases or combus-tion products at temperatures exceeding 550 ◦C,and which are thus resistant to scaling [36]. Ro-tary furnaces, cracking units, and muffle fur-naces are examples of chemical plant in whichthese conditions arise.
Resistance to scaling is obtained mainlyby alloying with chromium, but further im-provements are possible if silicon and alu-minum are added. The most commonlyused heat-resistant steels are standardized inStahl-Eisen-Werkstoffblatt 470-76. Distinc-tions are made between the ferritic steels,such as X 10CrAl 7 (1.4713), X 10CrAl 13(1.4724) andX 10CrAl 24 (1.4762); the ferritic-austenitic steel X 20CrNiSi25 4 (1.4828); andaustenitic steels such as X 15CrNiSi 20 12(1.4828), X 15CrNiSi 25 20 (1.4841) andX 10CrNiAlTi 32 20 (1.4876).
The above-mentioned publication also giveslong-term heat resistance values for periods ofup to 105 h. The highest service temperatureslisted, which extend to 1200 ◦C, apply to air.They may be reduced greatly by admixtures tothe air, e.g., of water vapor or sulfur-containingor carburizingmatter, because the reaction prod-ucts do not form sufficiently thick surface layers.
Within certain temperature ranges, heatresis-tant steels tend to become brittle. Ferritic steelswith ≥ 12wt% chromium do so between 400and 530 ◦C. This so-called 475 ◦C embrittle-ment can be eliminated by annealing brieflyabove 600 ◦C. In the case of ferritic steels with≥ 17wt% chromium and austenitic steels, anintermetallic iron – chromium σ-phase, whichcauses severe embrittlement, is formed between600 and 900 ◦C. It endangers particularly theweld interfaces.
Within the same temperature range,austenitic steels precipitate chromium carbides,which further reduce the toughness. The steelmost severely affected is X 15CrNiSi 25 20(1.4841), which should be used only above900 ◦C. The σ-phase and the carbides can beredissolved by annealing above 1000 ◦C, fol-lowed by quenching.
12 Construction Materials in Chemical Industry
The heat-resistant steels can be welded bythe usual methods, provided the guidelines foralloyed steels are followed. Suitable filler met-als are listed in Stahl-Eisen-Werkstoffblatt 470-76. It should be noted that in ferritic steels with≥ 12wt% chromium, coarse grains, which canno longer be removed by heat treatment, areformed at temperatures above 900 ◦C.
Heat-resistant cast steel, as used in themanufacture of pipes by centrifugal casting,for example, is standardized in Stahl-Eisen-Werkstoffblatt 471-76.
5.1.4. Steels for Low Temperatures
Refrigeration is very important in the chem-ical industry, e.g., in the fractional distilla-tion of hydrocarbons or storage and trans-portation of liquid gases. Steels for the pres-sure vessels must still have sufficient tough-ness at the lowest operating temperature. Thedegree of low-temperature toughness dependsparticularly on the steel’s composition and heattreatment. Notched-bar impact energies, mea-sured onDVM(DeutscherVerband furMaterial-forschung und -prufung e. V., Berlin) longitudi-nal specimens, of ≥ 40 and ≥ 60 J/cm2, for caststeel and other steels, respectively, are regardedas evidence of sufficient low-temperature tough-ness. Hence the lowest service temperature of asteel with good low-temperature tenacity is thatat which the notch impact energy is still abovethis limit. Steels for low temperatures can be di-vided into four groups:
1) Unalloyed aluminum-killed steels for servicetemperatures down to − 50 ◦C in the nor-malized state and− 80 ◦C in the heat-treatedstate; e.g., TTSt 35 (1.1101).
2) Unalloyed and low-alloy weldable fine-grained structural steels in the normal-ized state for service temperatures down to− 60 ◦C, e.g., TStE 380 (1.8910).
3) Nickel-alloyed heat-treatable steels with1.5 – 9wt% Ni for service temperatures of− 100 to − 190 ◦C, e.g., 12Ni 19 (1.5680).
4) Austenitic chromium– nickel steels for ser-vice temperatures extending close to absolutezero; e.g., the steels specified in DIN 17 440and DIN 17 441.
The relevant quality standards are DIN17 173 and DIN 17 174 for groups 1 and 3
and DIN 17 102 for group 2, and Stahl-Eisen-Werkstoffblatt 685 – 82 for cast steel with goodlow-temperature toughness.
In the welding of steels for low temperaturesit is necessary to use filler materials that givea weld metal whose strength and toughness areequal to those of the base material [37–39].
5.1.5. Steels Resistant to PressurizedHydrogen [40], [41]
Steels for high-pressure plant in the chemicalindustry can divided into the following twogroups:
1) Steels for components subjected to purelymechanical loads or to pressurized hydrogen,in both cases at temperatures of ≤ 200 ◦C,
2) Steels for components exposed to pressur-ized hydrogen at temperatures > 200 ◦C,
For the conditions of group 1, as presentin the production of low-density polyeth-ylene (pressures of up to about 400MPa),mainly unalloyed and low-alloy heat-treatablesteels are used; these are standardized in DIN17 200 and in the case of large forgings, inStahl-Eisen-Werkstoffblatt 550-76. If relativelygood corrosion resistance is required, stainlessheat-treatable chromium and chromium– nickelsteels according to DIN 17 440, and occasion-ally hardenable stainless steels, are used instead[42]. The periodic alternation of the pressuresubjects the components to a severe fatigue-inducing stress. Their shape stability thereforedepends decisively not just on the choice of ma-terials, but also on their being designed to suit theconditions of exposure as well as on satisfactoryfabrication and installation [43].
Hot pressurized hydrogen, which is neededin many high-pressure syntheses (e.g., ammo-nia synthesis and pressure hydrogenation) dis-sociates on the surface of many steels, diffusesinto the metal, and reacts with carbon to formmethane, thus causing decarburization, embrit-tlement, and cracking:
Fe3C+2H2 → 3 Fe +CH4
This reaction between hydrogen and cemen-tite occurs mainly at the grain boundaries, the
Construction Materials in Chemical Industry 13
Table 3. Composition of pressurized-hydrogen-resistant steels
grains thus losing their cohesion. Hydrogen andmethane accumulate, giving rise to high pressurewhose cleaving action leads to internal microc-racks. Together with the stresses exerted on thecomponent and the loss of mechanical strength,this results finally in pronounced brittle fracture.
The resistance of carbon steels to attack bypressurized hydrogen depends partly on the am-bient conditions, but also on the microstruc-ture, cold working, effects of welding, impu-rities, and heat treatment. Elements that formstable carbides, such as chromium, molybde-num, tungsten, vanadium, titanium, and nio-bium, can be added to the steel to prevent thereaction between hydrogen and cementite. Thepressurized-hydrogen-resistant steels now in usecontain up to 16wt% Cr, and also in manycases 0.2 – 1.5wt% Mo. Some types are addi-tionally alloyedwith up to 0.85wt%V(Table 3).The low-alloy ferritic steels are suitable for fu-sion welding, provided that they are preheated.The chromium steels have to be kept at 250 –450 ◦C throughout welding, and partially an-nealed immediately afterwards. The austeniticsteel (1.4988) tends to suffer hot cracking onwelding. Short-term tests give very little indica-tion of the limits to the use of steels of this kind,because the time factor is much too importantand the attack of pressurized hydrogen on steelshas a significant incubation period. As an illus-tration, Figure 10 gives stability limits for vari-ous steels, as compiled by Nelson [44]. It canbe seen that even at low hydrogen partial pres-sures (e.g., 2MPa) an unalloyed steel should beused only below 300 ◦C. Austenitic steels with18 % Cr and ca. 9 % Ni have good resistanceto pressurized hydrogen; they can be exposed to
hydrogen throughout the range of temperaturesused in normal high-pressure processes.
Figure 10. Resistance diagram for the attack of pressurizedhydrogen on steels (Nelson diagram)a) 1.25Cr 0.5Mo steel; b) 5.0 Cr 0.5Mo steel; c) 8.0 Cr0.5Mo steel; d) 2.25Cr 1.0Mo steel; e) 2.0 Cr 0.5Mo steel;f ) 1.25Cr 0.5Mo steel; g) 1.0 Cr 0.5Mo steel; h) 0.5Mosteel; i) Mild steel
5.1.6. Stainless Steels [45]
Among the highly alloyed steels, the most im-portant group comprises those that are chem-ically resistant. They have chromium contents≥ 14wt %. As the steels of this group re-sist heat as well as chemicals, this group alsoincludes a considerable number of the heat-resistant steels discussed in Section 5.1.3. Of-ten they contain nickel in addition to chromi-um. Themore highly alloyed corrosion-resistant(acid-resistant) steels must additionally with-stand general corrosion and localized corrosionin relatively aggressive corrodents (salt solu-tions; acids, even at fairly high concentrations).Characteristic alloying components, apart fromchromium and nickel, are molybdenum, copper,and in some cases silicon.
14 Construction Materials in Chemical Industry
There are many kinds of steels and alloys,each being represented by a material number(DIN 17 007) or by chemical composition (DIN17 006).
Themechanical properties of steels are deter-mined by the microstructure, which depends oncomposition and heat treatment. Stainless steelsare divided according tomicrostructure into fourgroups:
1) Martensitic (hardenable) steels with> 0.12wt % C and ≤ 15wt % Cr. They arehardened at temperatures > 1000 ◦C (e.g.cutting steels). After being hardened theycan be improved by tempering at 500 to600 ◦C– their strength thus being reduced toa desired lower level – and henceforth com-bine high strength with good ductility.In the heat treatment, a ferritic structure, withprecipitated carbides of high chromium con-tent of type M23C6 or M7C3, is formed. Fix-ation of chromium and the formation of chro-mium-depleted zones reduces the resistanceto corrosion. To equalize the chromium con-tent of the ferritic matrix again it is necessaryto anneal the steel for a fairly long time atelevated temperature. Although this restoresthe corrosion resistance to some extent, it im-pairs the strength. As some of the chromiumis still bound as carbide, the corrosion resis-tance after annealling procedure is still con-siderably poorer than that of the martensite.
2) Ferritic steels with body-centered cubic lat-tice (α-phase). Those of greatest importanceare the ferritic chromium steels with ca.17wt % chromium. Their mechanical andtechnical properties, however, are unsatis-factory; when welded, they tend to becomecoarse-grained and brittle. Nevertheless theyhave the advantage of resisting stress corro-sion cracking in chloride-containing media.
3) Austenitic steels with face-centered cubiclattice (γ-phase). Particularly important arethe austenitic chromium– nickel steels withca. 18wt % Cr and 10wt % Ni, but withoutaddition of molybdenum. Their strength isrelatively low and their ductility very good.In chloride-containing corrodents, however,they undergo transgranular stress corrosioncracking.
sistance to stress corrosion cracking. Theyare therefore used mainly under conditionswhichmay cause fatigue corrosion and stresscorrosion cracking in austenitic steels.
Pure iron has a ferritic structure. The alloy-ing elements that may be added to iron are ei-ther ferrite-forming (chromium, molybdenum,silicon, titanium) or austenite-forming (carbon,nitrogen, nickel, manganese).
The austenitic chromium– nickel steels, withor without molybdenum, are used in thesolution-heat-treated state (solution heat treat-ment temperature 1000 – 1100 ◦C). At solu-tion heat treatment temperatures these steelsare close to the boundary of the α + γ fieldin the iron – chromium– nickel phase diagram(Fig. 11). As the content of chromium increases,more nickel or nitrogen must be added to main-tain the austenitic structure. If the content ofmo-lybdenum is raised, then it is also necessary toincrease the content of austenitizers and/or toreduce the chromium content.
Figure 11. Ternary phase diagram of Fe –Cr –Ni (sectionat 1100 ◦C)
With increasing chromium content, and es-pecially increasing molybdenum content, thetendency to segregate intermetallic compounds(σ-phase, χ-phase, Laves phase, Fe2Mo), in-creases greatly (see Fig. 12). If the precipitationtendency is strong enough, intermetallic com-pounds may already be precipitated during the
Construction Materials in Chemical Industry 15
relatively short-lived heat exposure of welding.Hence the addition ofmolybdenum,which is de-sirable for reasons of corrosion chemistry, has anupper limit. In the case of austenitic chromium–nickel steels it is about 6wt % Mo.
Figure 12. Precipitation fields of chromium-rich carbidesM23C6, σ- and χ-phase, molybdenum-rich Laves phase(Fe2Mo), and of a complex nitride (Z-phase) for the steelX 2CrNiMo 18 14 3
Addition of nitrogen retards the precipita-tion of the intermetallic phases and compoundsconsiderably, as well as improving the mechan-ical properties by introducing nitrogen atomsinto interstices in the metal lattice. Nitrogen-alloyed austenitic and ferritic – austenitic steelsare therefore of industrial importance.
In ferritic – austenitic steels, the ferritizerschromium and molybdenum are enriched in theferrite phase, whose nickel content is corre-spondingly depleted. The austenitic phase con-tains, conversely, more nickel and less chromi-um and molybdenum. These differences of con-centration are relatively slight, however, and areonly important for the use of ferritic – austeniticsteels in borderline cases. The alloy compositionof these steels is also chosen such that the ferriticand austenitic phases are present in the structurein approximately equal proportions and the chro-mium content of the austenite is not less than17wt %.
Annealing within certain temperature rangesmay cause carbides, nitrides, and intermetalliccompounds to be precipitated in the microstruc-ture of stainless steels. Owing to the special im-portance of chromium and molybdenum in im-parting corrosion resistance to stainless steels,the precipitation of phases and compounds thatcontain these elements exert strong effects. Thesegregation of chromium- and molybdenum-richphases depletes thematrix in these elements.
The matrix therefore loses its resistance to cor-rosion.
In ferritic chromium steels and austeniticchromium– nickel steels, the chromium-rich σ-phase may precipitate on annealing at 600 –900 ◦C. For ferritic chromium steels with upto ca. 18wt % chromium this precipitation hasno importance, while in the case of molybde-num-free austenitic chromium– nickel steels itis, at the most, important only in connectionwith the welded material. For reasons of weld-ing technique (prevention of hot cracking) thewelded material of normal austenitic steels al-most always contains some δ-ferrite, which de-composes when the steel is annealed. In a corro-sion exposure this decomposed δ-ferrite may beselectively dissolved. Damagemust be expectedwhere a coherent δ-ferrite network is present(proportion of δ-ferrite in the structure> 10 %).
The designations of themost important stain-less steels for chemical plant are compiled inDIN 17 440 (Table 4, see next page).
Chemical engineering also uses a number ofother special materials notable for their high re-sistance to pitting corrosion and stress corrosioncracking, as well as to mineral acids (Table 5,see next page).
Particularly high molybdenum contents arepresent in the steels listed in Table 6 (see nextpage). The stability of the austenite is due tonickel and nitrogen.
Unlike ferritic and austenitic steels, fer-ritic – austenitic steels have a two-phase struc-ture, which, in contrast to the compositionof austenitic steels, is obtained by raising thecontents of ferrite-stabilizing elements, suchas chromium and silicon, and reducing theaustenite-stabilizing nickel content (Table 7, seepage after next). By virtue of their high yieldstrength of at least 450MPa, steels of this kindare used for components which, while exposedto corrosive media, are additionally subjected towear (cavitation, erosion) and vibration. Beingmore resistant than commercial austenitic steelsto stress corrosion cracking in neutral chloridesolutions, they are also being used increasinglyto handle aggressive coolingwater.A commonlyusedmaterial of this group is X 2CrNiMoN22 5(1.4462), which, in addition to about 22wt%Cr, has a nickel content of about 5.5wt%, anMo content of about 3wt%, and an austeniticstructure proportion of about 60 % [46], [47].
A specially developed austenitic steel thatcontains about 4wt% Si (X 2CrNiSi 18 15,1.4361) and has already undergone trials un-der practical conditions has remarkably good re-sistance to highly concentrated nitric acid andchromic acid. As Figure 13 shows, its advan-tages are particularly pronounced at HNO3 con-centrations above 75 %, where it is superior tostainless steels of all other types [48]. In themeantime a variant has been developed for ex-tremely severe HNO3 exposure (5.3wt% Si,17.5wt% Ni, 0.015wt% C) [49].
Figure 13. Area-related mass loss rate of stainless steels inboiling nitric acid (exposure time: 50 – 90 d)a) X 2CrNiSi 18 15; b) X 2CrNi 18 9; c) X 1CrMo 26 1;d) X 1CrNi 25 21
Ferritic steels have also undergone special de-velopment. Because the mechanical propertiesof the welded joints were unsatisfactory, thecommercial types used until now were rarelychosen for chemical plant. A substantial im-provement may be expected from the titanium-stabilized chromium and chromium–molybde-num steels of low carbon and nitrogen con-tent and from the nickel-alloyed chromium–molybdenum steels. Progress in the field of
ferritic steels has been marked by the use ofspecial methods for obtaining low C+N con-tents in their manufacture (ELI = extra low in-terstitial steels) [50]. Three groups can now bedistinguished: 1) steels with 25 – 28wt % Cr,without nickel, in some cases with an addi-tion of Mo, and in all cases with a C+N con-tent limited to 0.015wt% (superferrites); 2) mo-lybdenum-containing steels with 18wt % Cr,which, although the proportions of the afore-mentioned interstitial elements have been low-ered, are additionally stabilized with titaniumor niobium; and 3) stabilized molybdenum-and nickel-containing ferritic steels with highCr contents, such as X 1CrNiMoNb 28 4 2 (Re-manit 4575) and X 2CrNiMoTi 25 4 4 (Monit).
The superior toughness of the last-mentionedsteel types, as compared with that of a conven-tional Cr steel, is illustrated by the transitiontemperature of the notch impact strength.
Stabilized ferritic steels should be of partic-ular interest in connection with the handling ofcorrosive media, especially chloride-containingcooling waters (e.g., river water), owing to theirgood corrosion properties, suitability for weld-ing, and availability as semifinished products, aswell as for economic reasons [51].
For reasons of cost, and because of certaindesign problems, stainless cast steel is often pre-ferred to stainless rolled and wrought steels as amaterial for chemical plant. The chemical com-position and mechanical properties of stainlesscast steels are fairly similar to those of the rolledand wrought steels. Table 8 lists several impor-tant types, especially those used for chemicalpumps.
5.1.6.1. Technical Properties
The mechanical properties of the four groups ofstainless steels are shown in Table 9.
Table 9.Mechanical properties of the four main steel types
Steel type Ferritic a Martensitic b Austenitic c Austenitic – ferritic d
Yield strength, MPa ca. 300 450 – 600 ca. 230 e ca. 500 f
Tensile strength, MPa 450 – 650 600 – 950 ca. 600 ca. 750Fracture strain, % ca. 25 14 – 18 45 ca. 30Notch impact energy, J g 30 – 55 ca. 150
a Normal steels with 16 – 18wt % Cr, annealed.b Quenched and annealed.c Solution annealed.d Annealed at 1100 ◦C and quenched.e 0.2 % yield strength.f 1 % yield strength;g DVM specimen, longitudinal.
Between their melting points and room tem-perature, ferritic steels are pure or mainly fer-ritic. The production of a fine-grained structurerequires forming at temperatures below 800 ◦C,heating beyond 800 ◦C, and cooling. Heatingabove 1000 ◦C (welding) enlarges the grain, thusembrittling the steel and causing precipitationof carbides. Temperatures above 1100 ◦C affectthe structure to such an extent that the steelbreaks when subsequently loaded. These steelscannot be hardened by heat treatment. Beingless expensive than austenitic steels, and moreresistant to chloride-containing solutions, theyare used for pipelines intended for chloride-containing waters, as well as for heat exchang-ers and condensers. Martensitic steels exist only
in the pearlitic and martensitic forms, withoutintermediate stages. They are hardened by heat-ing to 980 – 1100 ◦C, depending on carbon con-tent, cooling in oil or air, and tempering at atemperature above 600 ◦C. The steels contain-ing less than 0.4wt% carbon serve as heat-treatable materials with good mechanical prop-erties, while those with higher carbon contentsare used as hardened steels. Certain types areresistant to very high temperatures and to pres-surized water. Having good resistance to erosionand cavitation as well as high fatigue strength,they are used also for propellers and impellers.Austenitic steels with low chromium and nickelcontents are so unstable that martensite maybe formed at low temperatures or through cold
Construction Materials in Chemical Industry 19
forming. High chromium contents favor the for-mation of ferritic structures. Fine granulation re-sults from recrystallization during hot forming.The grain-growth rate is much lower than that offerritic steel. The final heat treatment consists ofheating to 1000 – 1100 ◦C and quenching in wa-ter or air. Hardening by heat treatment is not pos-sible; the strength is improved by cold or hot –cold forming. The most conspicuous propertiesof these steels are a lowyield strength, high resis-tance to fracture, high strength at elevated tem-peratures, resistance to pressurized water, heatresistance, permanent strain at low loads, and atendency to creep. An important application isthe fabrication of pressure vessels.
In austenitic – ferritic steels the ratio of fer-rite to austenite is determined by the compo-sition and heat treatment. It increases with theannealing temperature and rate of cooling. At-tention is drawn to the risk of 475 ◦C embrit-tlement (hardness increase, loss of toughness,and loss of chemical resistance through sepa-ration into phases of high iron and chromiumcontent [52] and σ-phase formation), as wellas to the fact that, in comparison with purelyaustenitic steels, these steels are less sensitiveto stress corrosion cracking. Their applicationsinclude pump manufacture. Special grades withhigh wear resistance are used for bushings.
Ferritic, austenitic, and ferritic – austeniticstainless steels can be welded by practically allthe well-known methods. The argon arc (TIG,MIG) process, resistance welding (spot or seamwelding), and the plasma arc welding and elec-tron beam welding are particularly suitable forferritic chromium steels. Austenitic chromium–nickel steel is recommended as the electrodema-terial in addition to material of the same com-position as the base. If, for reasons of corrosion(particularly the risk of stress corrosion crack-ing), either of these materials cannot be used,an austenitic filler can serve for the lower layersand a ferritic filler for the cover pass. In view ofthe marked grain growth tendency, as little heatas possible should be introduced. It is thereforeadvisable to use thin electrodes and lowcurrents.
As scale formation may be accompaniedby the formation of chromium-depleted surfacezones, protective gas should be used to pre-vent scale formation on welding. Cleanliness ofthe seam, which must be without splashes andundercutting, is also important. Where the de-
mands are particularly severe, as in the case ofcomponents subjected to vibration, the surfacequality can be improved by subsequent grinding.
Unstabilized 17wt% chromium steels thathave beenweldedmust always be annealed sincethey are otherwise sensitive to intergranular cor-rosion, even inmild corrodents.Where this treat-ment is impracticable, stabilized grades shouldbe used. Although the latter benefit from heattreatment, it is not absolutely necessary. Stabi-lized steels and molybdenum-containing steelsalso exhibit lower growth tendency. Apart fromimparting resistance to intergranular corrosion,additions of stabilizing elements also improvethe resistance to pitting corrosion.
The principal welding methods for austeniticsteels are TIG, MIG, and submerged-arc weld-ing. Apart from using steels that have no inter-granular susceptibility after welding (stabilizedsteels and ELC types) it is particularly impor-tant to avoid hot cracking and loss of corrosionresistance through precipitation of intermetal-lic phases.Appropriate recommendations can befound in the processing guidelines.
5.1.6.2. Chemical Properties
The resistance of stainless steels to corrosivemedia is determined by their passive behavior,which depends in turn on the alloying elementsand their concentrations.
In this connection, chromium is particularlyimportant, but the effects of nickel, molybde-num, and copper are also significant. In partic-ular, chromium reduces the passivation currentdensity and the residual current density of thesteel in the passive state. The main effect ofmolybdenum is a reduction of the passivationcurrent density and facilitation of repassivation,which promotes the transition from the active tothe passive state.
In general it is considered that a corrosionrate of 0.1mm/a (0.1 gm−2 h−1) represents thehighest degree of corrosion resistance attainableunder practical conditions. Corresponding datacan be found in stability tables and diagrams andinmanufacturers’ literature.Often, however, sta-bility diagrams, of which Figure 14 is an exam-ple, apply only to chemically pure corrodents. Inthe case of sulfuric acid, for example, even rel-atively small amounts of oxidizing agents(NO−
2 ,
20 Construction Materials in Chemical Industry
Figure 14. Isocorrosion curves of chromium– nickel –mo-lybdenum stainless steels in sulfuric acidA) 0.05 % C, 17 % Cr, 13 %Ni, 5 %Mo; B) 0.05 % C, 20 %Cr, 25 % Ni, 3 % Mo, 2 % Cu
Fe3+) permit the useof stainless steels in sulfuricacid at concentrations up to ca. 50 % and tem-peratures considerably higher than those shownin Figure 14. Thus figures for resistance to purecorrodents should in many cases be regarded asinitial approximations. The influences of venti-lation, i.e., the corrodent’s oxygen content, andof the state of motion (e.g., laminar or turbulentflow) must be taken into account also.
Uniform surface corrosion is of relativelymi-nor importance for stainless steels. Localizedcorrosion, however, may lead quickly to com-ponent failure even where the corrosion rate islow, and must therefore be avoided as far as pos-sible. Normally, the susceptibility of metallicmaterials to localized corrosion increases withthe resistance to general corrosion. In localizedcorrosion of stainless steels, the locally activeareas are very small compared to the area ofthe passive surface. Therefore, the local anodicdissolution current density is very high. Dam-age caused to passive materials by stress corro-sion cracking and fatigue corrosion, which oc-cur with virtually no loss of mass, is particu-larly hazardous. The corrosion may penetrateeven relatively thick components within a shorttime. The types of localized corrosion that aremost important in stainless steels are intergran-ular corrosion (grain-boundary attack), pittingcorrosion (pit formation), stress corrosion (cor-rosion cracking), and fatigue corrosion (corro-sion fatigue cracking).
Intergranular corrosion is a selective processin which the areas close to the grain boundariesare corroded. In all cases it has one of the fol-lowing causes:
1) Depletion of alloying elements important tochemical resistance in areas close to grainboundaries, caused by formation of precipi-tates, in which these elements have accumu-lated, at the grain boundaries. In joint weld-ing, these precipitates may be formed in theheat-affected zones bordering on the weldseams.
2) Chemical attack on precipitates at the grainboundaries, e.g., by concentrated nitric acidon precipitates of titanium carbonitrides intitanium-stabilized steels.
3) Preferential attack by accumulations of ac-companying and trace elements in the steel,e.g., phosphorus, silicon, and boron (which
Construction Materials in Chemical Industry 21
stimulate the anodic metal dissolution at thegrain boundaries).
4) Cell formation between the matrix and pre-cipitates at the grain boundaries.
In stainless steels, the depletion of the al-loying elements chromium andmolybdenum re-sulting from precipitation of carbides with highchromiumandmolybdenumcontents is themostimportant cause of intergranular corrosion in in-dustry.
At low temperatures the solubility of car-bon in austenitic chromium– nickel steels andnickel – chromium alloys is very low, but it in-creases markedly as the temperatures rises. Thecarbon dissolved in the austenitic solid solutionat high annealing temperatures may precipitateat the grain boundaries at lower annealing tem-peratures as a carbide of type M23C6 with highchromium andmolybdenum content. In this wayzones of depleted chromium and molybdenumcontent, which grow together to form coherentchromium-depleted areas as the carbide precip-itation progresses, are formed round the pre-cipitated carbides. Through the precipitation ofchromium-rich carbides (75 – 90wt% Cr) thechromium content in the vicinity of the grainboundaries falls below the so-called resistancelimit of ca. 12wt % chromium.
Figure 15. Diagram of grain-boundary disintegrationa) Solubility limit for carbon; b) Precipitation of chromium-rich carbides; c) Intergranular corrosion
The susceptibility to intergranular corrosionis indicated by the grain-boundary corrosionfields in the plot of annealing time versus anneal-ing temperature (Fig. 15, see above). The shapeand sizes of these fields are best described by the
highest temperature that still causes susceptibil-ity to intergranular corrosion and by the shortestannealing time leading to sensitization of grainboundaries. This shortest annealing time (repre-sented by the “nose” of the grain-boundary cor-rosion field) is situated in the temperature rangearound 650 ◦C for molybdenum-free austeniticsteels and in the range around 750 ◦C for mo-lybdenum-containing steels.
When the temperature falls after welding, itpasses through the range in which austeniticchromium– nickel steels become susceptible tointergranular corrosion. The following steps aretaken to make the steel resistant to intergranularcorrosion after welding:
1) The carbon content is reduced to levels atwhich neither temperature nor the exposuretime causes sensitization (ELC steels).
2) Stabilization with additions of thecarbide-forming element titanium or nio-bium/tantalum. The affinity of these alloyingelements for carbon is greater than that ofchromium. They form special carbides orcarbonitrides of the type M (C,N). Due tothe affinity of both elements for nitrogen,the steel’s nitrogen content, as well as itscarbon content, must be taken into accountin calculating the stabilization ratio.
In ferritic chromium steels, the rate at whichchromium-rich carbides are precipitated is veryhigh. After annealing at temperatures above850 ◦C, even very rapid cooling cannot pre-vent their precipitation, and the formation ofa structure susceptible to intergranular corro-sion. In the temperature range around 800 ◦C,however, the very high diffusion rates permit-ted by the body-centered cubic lattice of theα mixed crystal causes rapid replenishment ofchromium by diffusion from the chromium-rich matrix into the chromium-depleted grainboundary areas. As a reduction of the car-bon content would not eliminate intergranu-lar susceptibility until an extremely low levelhad been reached (< 0.005wt %), stabilization,even of ferritic chromium steels, by addition ofniobium/tantalum, and especially titanium, hasgreat industrial importance. Pitting corrosion ofstainless steels occurs only when they are in thepassive state, and is then caused almost exclu-sively by chloride ions in aqueous solution. Itis therefore also known as chloride corrosion.
22 Construction Materials in Chemical Industry
In the initiating step of pitting corrosion chlo-ride ions are adsorbed locally by the passivelayer,whereupon the passivity breaks down. Themechanism of this corrosion has not yet beenfully explained.
In the pitting corrosion of stainless steels, theinfluence of potential is important. Pitting cor-rosion only occurs when the potential exceedsthe pitting potential. In the presence of a corro-sive medium, pitting corrosion therefore occursonly if the corrosion potential of the steel ismorepositive than the pitting potential.
With regard to the attacking medium, themain factors by which the pitting potential islowered (and therefore the risk of pitting corro-sion heightened) are increase of chloride con-centration and temperature, and reduction of pH[53].
With regard to the material, the pitting poten-tial can be shifted towards more positive poten-tials by raising the chromium content and espe-cially themolybdenum content; the alloying ele-ments nickel and manganese, on the other hand,have practically no influence.
The effects of chromium and molybdenumare described, and the resistances of the variousalloys estimated, with the help of the pitting re-sistance equivalent PRE [54]:
PRE = wt %Cr + 3.3 × wt %Mo
Over a wide range of percentages of theseelements, the pitting potential depends linearlyon the pitting resistance equivalent, in as far asthe structure is that given by solution annealing.Deviations are attributable to homogenizationof the structure by relatively high nitrogen con-tents. Stress corrosion cracking, which is like-wise initiated mainly by chlorides, accounts formost of the damage suffered by stainless steels.The basic prerequisites of stress corrosion crack-ing are as follows:
1) The action of a specific corrodent on a ma-terial susceptible to stress corrosion crack-ing, i.e., the presence of a critical mate-rial/corrodent system.
2) Static tensile stresses, possibly superim-posed by infrequent dynamic loads. Onlytensile loads are effective. Pressure does notcause stress corrosion cracking; in fact, inmany cases it is an effective means of pre-venting corrosion of this type.
Stress corrosion cracking of stainless steelswith mainly transgranular progression of thecorrosion cracksmay occur in the following cor-rodents:
2) Alkaline solutions (threshold temperature at40 – 50wt%NaOH, 100 ◦C). In addition, in-tergranular cracking occurs at high tempera-tures in dilute alkaline solutions.
If the steel is in the sensitized state in whichit is susceptible to intergranular cracking, andthe tensile stress and potential exceed criticalvalues, intergranular stress corrosion crackingoccurs also in, e.g., water at high temperatures(Fig. 16) [55].
Figure 16. Concentration ranges of dissolved oxygen andchloride that may lead to stress corrosion cracking (SSC) oftype 304 stainless steels in water at temperatures in the range260 – 300 ◦C. Applied stresses in excess of yield strengthand test times in excess of 1000 h, or strain rates greaterthan 10−5 s−1
•=Sensitized, SCC; ◦=Sensitized, no SCC;�=Annealed,SCC; �=Annealed, no SCC
Under conditions that cause stress corrosioncracking the following materials can be used in-stead of normal stainless austenitic 18Cr 10Nisteels: stainless ferritic chromium steels with17 – 28wt % chromium, with or without mo-lybdenum; austenitic – ferritic steels (duplexsteels), whose use is restricted in some casesto temperatures below 300 ◦C; austenitic steelswith increased nickel and/or molybdenum con-tent; and nickel-based alloys.Apart frommolyb-denum, an alloying element that decisively im-proves the stress corrosion cracking resistance
Construction Materials in Chemical Industry 23
of stainless austenitic steels is nickel (at nickelcontents exceeding about 40wt %, stress corro-sion cracking no longer occurs). Corrosion fa-tigue is the term used for crack damage whichoccurs in a component exposed simultaneouslyto alternating mechanical stress and a corrodent.It may be caused in all metallic materials by un-specific corrodents, differing in this respect verymuch from stress corrosion cracking, for whoseoccurrence specific corrodents are responsible.The main symptom of corrosion fatigue is theloss of endurance strength.
Fatigue cracking of stainless steels differs ac-cording to whether they are in the active or pas-sive states. In the active state numerous cracksare formed, whereas in the passive state fewcracks, or perhaps just a single precrack, areformed. Damage to stainless steel plant throughfatigue cracking is, however, relatively rare [56].
5.1.6.3. Development State of StainlessCr –Ni Steels
The corrosion resistance of stainless steels isbenefitting from the attainment of particularlyhigh degrees of purity, as given by electroslagremelting, for example. At the same time thestrength-related properties are being improvedwithout loss of ductility.
To make possible the increases in Cr and Mocontent on which these improvements dependit is necessary to increase the nitrogen content.This is achieved by renitrogenizing the steel un-der pressure and simultaneously adding alloyingelements that increase the nitrogen’s solubility[57], [58], e.g., manganese.
Of two steels now undergoing tri-als, namely X 3CrNiMoNoN23 27 6 4 andX 3CrNiMnMoNbN23 17 8 4, the latter is su-perior in respect of strength-related propertiesin consequence of precipitation hardening bya finely dispersed intermetallic Z-phase (0.2 %yield strength at room temperature ca. 500MPa;at 500 ◦C,> 300MPa). This steel,which iswith-out Nb and therefore free from precipitation, issuperior in ductility (notch impact energy atroom temperature > 300 J), though at 500 ◦C itstill has a 0.2 % yield strength of ca. 250MPa[59].
The stress corrosion cracking behavior ofsteels of this type is shown inFigure 17.The steel
with the highest N content is the one most resis-tant to boiling 35 % MgCl2 solution. In boilingseawater all the steels achieved times to failureof more than 1000 h at loads above the 0.2 %yield strength [60].
Figure 17. Stress corrosion behavior of nitrogen-containingstainless steels in boilingMgCl2 solution (35wt %, 123 ◦C)
The steel X 3CrNiMnMoNbN23 17 5 3(1.4565) is commercially available. With apitting resistance equivalent of about 48 (inwhich the high nitrogen content is taken intoaccount) it is more resistant to pitting corrosionthan any other stainless steel (cf. Fig. 18). OnlyNiCr 22Mo 9Nb, better known as Incoloy 625,performs comparably well.
Figure 18. Pitting resistance equivalent (PRE) and criticalpitting corrosion temperature for various steels and Incoloy625 in a 10wt % solution of FeCl3· 6H2O
Some mechanical properties of alloy 1.4565are as follows:
24 Construction Materials in Chemical Industry
0.2 % Yield strength ≥ 500N/mm2
0.2% Yield strengthat 300 ◦C ≥ 300N/mm2
Notch impact energyat − 269 ◦C ≥200 J
Elongation > 50 % at room temperatureReduction of area > 70 % at room temperature
As the nitrogen contents of the aforemen-tioned steels in the molten state are below thesolubility limit, these materials have good weld-ing behavior. The filler metals used for themat present are relatively highly alloyed nickel-based materials of the Ni –Mo –Cr type.
5.2. Cast Iron [60], [61] (→Steel)
The term cast iron is used for unalloyed, low-alloy, and highly alloyed iron – carbon cast ma-terials containing ca. 2 – 5wt % carbon and0.8 – 3wt % silicon, the carbon being mainlyin the form of graphite. The fundamental struc-ture of unalloyed and low-alloy cast iron isferritic, ferritic – pearlitic, or pearlitic; that ofhighly-alloyed cast ironmay be any of these, butalso austenitic. The main alloying elements arenickel, chromium, and copper. The main qualitystandards are DIN 1691, for unalloyed and low-alloy lamellar graphite cast iron; DIN 1693, forunalloyed and low-alloy spheroidal graphite castiron; and DIN 1694, for austenitic cast iron.
Mechanical properties of the various types ofcast iron are listed in Table 10. The strength ofgray cast iron depends on the thickness of thecasting’s cross section (wall thickness). Empir-ical values for the strength of castings are givenin Figure 19.
The exceptionally low plastic deformabilityand impact strength of lamellar graphite cast ironin comparison with nodular graphite cast ironare explained by the graphite lamellae, whichact as internal notches. Unalloyed and low-alloylamellar graphite cast iron and nodular graphitecast iron are attacked only slightly by pure sul-
furic acid and pure phosphoric acid. Their re-sistance to soil corrosion and seawater attackis also good. Depending on their compositionand the form of the graphite, austenitic cast ma-terials [62], [63] withstand corrosion by alka-lis and sulfuric acid. The austenitic grades thatwithstand moderately high and high tempera-tures are used for pumps, valves, cylinder lin-ings, waste-gas pipes, and furnace parts, for ex-ample. The corrosion-resistant grades are usedin the food, artificial silk, and plastics indus-tries as well as for pipes and vessels. Men-tion should also be made of the thermal shockstability of GGL(laminar)NiCr 30 3 (materialno. 0.6676) and the low-temperature ductility ofGGG(nodular)NiMn 23 4 (material no. 0.7673).
Figure 19. Mean tensile strength of gray cast iron versusthe section thickness of the casting (according to DIN 1691)
A special grade that withstands hot nitric acidand hot sulfuric acid is the iron – silicon mate-rial containing 14 – 18wt % silicon. Its hardnessand brittleness constitute a great disadvantagebecause they restrict the choice of shaping tech-niques to casting and grinding.
Lamellar and nodular graphite cast iron canbe welded hot with filler materials of the same
Table 10.Mechanical properties of the various types of cast iron
Cast iron type Tensile strength σu, MPa Fracture strain, % Notch impact energy, J
composition, and cold with filler materials ofdifferent composition [64], [65].
5.3. Nickel and Nickel Alloys [66–68](→Nickel, →Nickel Alloys)
Owing to their excellent corrosion resistance tovarious aggressive media, nickel and its alloysare of growing importance in chemical engi-neering. Furthermore, Ni –Cr alloys, especiallythose with additions of silicon and aluminum,have good high-temperature strength by virtueof their high creep strengths, and good heat-re-sistance to hot gases and combustion products.
The most important unalloyed nickelgrades are Ni 99.6 (2.4060), nickel content≥ 99.6wt %; LC-Ni 99.6 (2.4061), nickel con-tent ≥ 99.6wt %; Ni 99.2 (2.4066), nickel con-tent ≥ 99.2wt %; LC-Ni 99 (2.4060), nickelcontent ≥ 99.0wt %.
The nickel content (according toDIN 17 740)may include up to 1wt % cobalt. Ni 99.6 maycontain up to 0.08wt % carbon, and Ni 99.2up to 0.1wt % carbon; after prolonged expo-sure to temperatures above about 300 ◦C, thecarbon may be precipitated as grain boundarygraphite, thus making the material brittle [69].Frequent use is therefore made of LC (low-carbon) Ni, which has a maximum carbon con-tent of 0.02wt % [70], [71].
Plant consisting of nickel or nickel-platedsteels is used mainly to produce and processalkalis [71], [72]. The good resistance of purenickel to sodium hydroxide solution can be seenin Figure 20. Nickel is also used to handle acidhalides [73], seawater, brackish water, dilute air-free and nonoxidizing mineral acids and salt so-lutions, and fatty acids. It can also be exposedto dry chlorine or hydrogen chloride at temper-atures up to 535 ◦C.
Nickel has good high-temperature strength.Up to about 300 ◦C its yield strength and ten-sile strength undergo practically no change [69].Nickel is very sensitive to sulfur and sulfur com-pounds [69], [70], [74]. Above 400 ◦C sulfurpenetrates nickel along the grain boundaries andforms a eutectic mixture of nickel and nickelsulfide, embrittling the material. This process israpid at 550 – 650 ◦C. The surface must there-fore be cleaned thoroughly before welding and
each annealing treatment. Sulfur-free lubricantsmust be used in hot and cold forming. The fur-nace atmosphere for heat treatment should besulfur-free and reducing or neutral; if freedomfrom sulfur cannot be ensured, the furnace at-mosphere should be slightly oxidizing.
Figure 20. Corrosion resistance of pure nickel in sodiumhydroxide, related to concentration and temperature [72]
All the welding methods normally used forsteel, except submerged-arc welding [74], aresuitable for nickel. Suitable weld filler mate-rials are available for combinations with un-alloyed or low-alloy steels, austenitic stainlesssteels, and nickel – copper, copper – nickel, andnickel –molybdenum alloys. Nickel parts canalso be joined by brazing and soldering.
Cast nickel [70] has the characteristic prop-erties of forged nickel, but is always slightly al-loyed with carbon, silicon, and manganese toimprove the flow of the molten metal and den-sity of the castings.
5.3.1. Nickel –Copper Alloys
In chemical engineering only two alloys withmore than 50wt % nickel have gained impor-tance, namely NiCu30Fe (2.4360, Inconel alloy400) and NiCu30Al (2.4375), which are stan-dardized in DIN 17 743. Both are sold as sheets,strip, pipe, rods, wires, and forgings.
The alloy NiCu30Fe can be exposed to sea-water and brackish water, solutions of nonoxi-
26 Construction Materials in Chemical Industry
dizing salts, alkalis [75], cold or warm nonox-idizing acids [73], and dry bromine. It is alsoresistant to hydrofluoric acid [76], though stresscorrosion cracking may occur in hydrofluoricacid vapors containing air, as well as to a lesserextent in the liquid phase. Stress corrosion crack-ing has been caused in these alloys by corrosivemedia containing mercury or its compounds.Where there is a risk of stress corrosion cracking,stress relief annealing should be carried out at550 ◦C, and in exceptional cases at up to 650 ◦C,and followed by slow cooling in the furnace. AsNiCu30Fe is expensive, solid sheet is often re-placed in the manufacture of process plant bysteel sheet plated with this alloy.
NiCu30Al is an age-hardenable alloy withsimilar corrosion behavior to NiCu30Fe. Be-cause of the risk of stress corrosion cracking itshould be used in the age-hardened state; it can-not be exposed to hydrofluoric acid, however.
Both materials are weldable by the usualmethods; they can also be brazed. Soft solderingis less frequently used. Like pure nickel, both al-loys are sensitive to sulfur and sulfur compoundsabove ca. 400 ◦C.
5.3.2. Nickel –Chromium Alloys [68]
Nickel – chromium alloys, with various ironcontents and up to ca. 9wt % molybdenum and2wt % copper, are normally used where thecorrosion resistance of highly alloyed stainlesssteels is inadequate. They exhibit good resis-tance to acids and to pitting corrosion, crevicecorrosion, and stress corrosion cracking. Thetypes most important in chemical engineering,together with their main applications, are listedin Table 11.
The molybdenum-free alloy NiCr15Fe, likenickel, has good resistance to caustic alkalis,even surpassing it in this respect under oxidizingconditions in consequence of its high chromiumcontent [77].
NiCr15Fe, NiCr23Fe and NiCr29Fe havegreater high-temperature strength than nickeland can be exposed to oxidizing sulfur-free at-mospheres at temperatures up to 1100 ◦C, andto reducing sulfur-free atmospheres at up to1150 ◦C. These alloys are also used in nitrid-ing gases (480 – 600 ◦C) and under carburizingconditions (800 – 950 ◦C). Like pure nickel andthe other nickel alloys, they are sensitive to sul-fur and its compounds at temperatures aboveca. 400 ◦C; this must be taken into accountwhere processing, e.g., bywelding, is envisaged.Nickel – chromium– iron –molybdenum alloyscombine the good resistance of nickel –molyb-denum alloys under reducing conditions withthe good resistance of nickel – chromium alloysunder oxidizing conditions. This explains theirgood resistance to acids and salt solutions. Withthe low-carbon grades and those stabilized withtitanium or niobium there is virtually no risk ofintergranular corrosion.
Chromium– nickel casting alloys (60/40,50/50 and 35/65) are used in chemical engineer-ing and have good resistance to corrosion by fuelash [78].
Nickel –molybdenum alloys are not passi-fiable, since they contain no chromium. Theirgood resistance to reducing acids is explainedby the low rate of corrosion in the active state,for which molybdenum is responsible.
The alloy NiMo30 (2.4810), consisting ofnickel, 26 – 30wt%molybdenumand5 – 7wt%iron withstands hydrochloric acid at all con-centrations. Nickel –molybdenum alloys of thiskind cannot be used under oxidizing conditions,however, because even atmospheric oxygen in-creases the rate of corrosion considerably. Al-loying with chromium makes them passive inoxidizing corrodents, and therefore resistant tocorrosion under these conditions also. In addi-tion they retain their low rates of corrosion inthe active state; therefore, like NiMo16Cr16Ti(2.4610) and NiCr21Mo14W (2.4608), for ex-ample, they can be used under reducing and ox-idizing conditions. NiCr21Mo14W has proveditself under severely oxidizing conditions.
Since the corrosion resistance of nickel –molybdenum and nickel –molybdenum– chro-mium alloys depends on molybdenum and/orchromium, precipitation of intermetallic com-pounds containing these elements maymake thematerials susceptible to intergranular corrosion.Precipitation of molybdenum-rich compoundsleads to intergranular susceptibility in the activestate (in hydrochloric acid, for example), whileprecipitation of compounds rich in chromiumcauses susceptibility to intergranular corrosionand intergranular crack formation in the passivestate [79].
Exact proportioning of the alloying elements,but, above all, minimization of the silicon andcarbon contents (< 0.03wt %) and additionalstabilization [80], [81], give alloys that toleratehot forming, annealing, and welding without ac-quiring intergranular sensitivity; this is demon-
strated by the time – temperature precipitationdiagram of Hastelloy alloy C-4 (Fig. 21).
Figure 21. Time – temperature sensitization diagrams forHastelloy alloy C-276 (a) and Hastelloy alloy C- 4 (b); testfor intercrystalline corrosion in accordance with ASTM G-28
Hot and cold forming of these high-nickelmaterials is difficult owing to their high waterresistance and work-hardening tendency. Theyareweldable by all the commonly usedmethods.In the production of weld joints it is advisable,in the interest of good corrosion resistance, tostart by making a thin root pass and then to weldthe filler and cover passes while cooling the rootside of the weld. If the design and size of a com-ponent permit heat treatment, solution annealingat 1150 ◦C, followed by water quenching, is rec-ommended [79]. Steel can be explosion-claddedwith any of these alloys, and they are often usedfor this purpose.Above 400 ◦C they are sensitiveto sulfur and sulfur compounds, like all materi-als with high nickel contents; this must be takeninto account in connection with annealing andwelding.
5.4. Aluminum and Aluminum Alloys(→Aluminum, →Aluminum Alloys)
The high strength and low density of aluminummake it highly suitable for lightweight struc-tures. Its economic value in the chemical in-dustry, however, depends more on its corrosionbehavior [82], [83]. Pure and very pure alu-minum according to DIN 1712 comprise thegrades Al 99, Al 99.5, Al 99.8, and Al 99.99.
The good corrosion resistance of aluminum,in spite of its very negative standard electrode
28 Construction Materials in Chemical Industry
potential of − 1.66V, is due to the formationof nonconducting Al2O3 passive films. The cor-rosion resistance depends on the purity of thematerial, which for chemical plant should notbe below 99.5wt % Al. As the protective Al2O3surface layer is largely insoluble in the pH rangeof 4.5 – 8.8 (Fig. 22), aluminum materials havevery good resistance to corrosion in approxi-mately neutral aqueous media. The corrosionbehavior and the main applications are listed inTable 13 [82].
Figure 22. Influence of pH on the solubility of the Al2O3film on aluminum
Because the Al2O3 passive film is non-conducting, relatively thick corrosion-inhibitingAl2O3 layers can be produced by anodic oxida-tion. They consist of a very thin and almost non-porous dielectric underlayer (barrier film) and
a fine-pored top layer (Fig. 23), which can bedyed and compacted [84]. Hard anodizing is ananodic oxidation process that gives particularlyhard and wear-resistant oxide films for techni-cal purposes [85]. Apart from aluminum itself,numerous wrought and cast alloys based on Al –Mg, Al –Mn, Al –Mg – Si, andAl – Zn –Mg aresuitable for hard anodizing.
Figure 23. Structure of an oxide film on aluminum, pro-duced by anodizing
High-purity aluminum is used in chemi-cal engineering mainly as a cladding material.
Table 13. Examples for the corrosion behavior of aluminum alloys in various media
∗ 1 = good resistance; 2 = resistant; 3 = low resistance; 4 = still usable; 5 = barely resistant; 6 = not resistant
Construction Materials in Chemical Industry 29
Super-pure aluminum and AlSi 12 are outstand-ingly resistant to highly concentrated nitric acid(Fig. 24), even at high temperatures. They alsowithstand concentrated acetic acid well [86].
Figure 24. Corrosion behavior of Al 99.5 in nitric acid
The main alloying elements that are used toincrease the strength of aluminum are Mg, Si,Zn, Cu, and Mn. Wrought alloys are standard-ized in DIN 1725, Part 1, and cast alloys in DIN1725, Part 2. Both groups include self-hardening(alloy hardening) and hardenable (precipitationhardening) materials.
Aluminum–manganese alloys are of the self-hardening type.Alloys containing 0.8 – 1.5wt%Mn are easy to process and exhibit corrosion re-sistance similar to that of pure aluminum.
The main representatives of the self-hardening materials are Al –Mg alloys, whosestrength is further improved by cold form-ing. Their corrosion resistance is approximatelyequal to that of pure aluminum (Al99.5), whichthey even surpass where seawater is concerned(cf. Table 13). AlMg3, AlMg5, AlMg2Mn0.8,and AlMg4.5Mn therefore belong to the groupof aluminummaterials known as seawater resis-tant.
However, with increasing magnesium con-tent (especially for Mg >5wt%), the suscep-tibility of Al –Mg alloys to intergranular corro-sion increases through the formation of Al8Mg5grain-boundary precipitates (Fig. 25). There isalso a risk of susceptibility to stress corrosioncracking [87].
The stability can be raised by reducing themagnesium content and adding manganese. A
typical representative of the materials obtainedin this way is AlMg4.5Mn, which has goodstrength-related properties and corrosion resis-tance and is used both for purposes involvingseawater contact and in chemical engineering.Additions of Mn and Cr improve the resistanceto chloride-induced pitting corrosion [88].
Figure 25. Breakthrough potentials for intergranularcorrosiona) Al8Mg5; b) AlMg7 phase with grain boundary precipi-tates; c) AlMg7 phase, homogenized
Age-hardenable Al –Cu –Mg alloys (2.8 –4.8wt % Cu, 0.4 – 1.8wt % Mg) have highstrength (e.g., AlCu4Mg2: σu= 440MPa) butlow corrosion resistance. Apart from havinglittle resistance to atmospheric corrosion, theyare particularly susceptible to stress corrosioncracking [89].
Age-hardenable Al –Mg – Si alloys withgood heat forming behavior (0.4 – 3.5wt %Mg,0.3 – 1.5wt % Si) are more resistant to corro-sion than Al –Cu –Mg alloys. Although rela-tively high silicon contents raise the strength,they reduce the resistance to intergranular cor-rosion by causing the formation of heteroge-neous grain boundary precipitates. Therefore,where corrosion is expected, it is preferable tochoose materials with low silicon contents (e.g.,AlMgSi0.5). These materials, however, are notfully resistant to pitting and intergranular corro-sion in media with high chloride contents.
After artificial ageing, the alloys based onAl – Zn –Mg, and especially Al – Zn –Mg –Cu,
30 Construction Materials in Chemical Industry
have higher strength (up to 530MPa) than allother aluminum materials, but they are suscep-tible to pitting and intergranular corrosion inchloride-containing media.
Important cast aluminum alloys for mechani-cal and chemical plant are those based on Al – Si(5 to 20 % Si), Al –Mg (3 to 10 % Mg), or Al –Mg – Si or Al – Si –Mg; apart from having highstrength and adequate ductility, they are distin-guished by good corrosion resistance, includingresistance to seawater.
Various treatments and methods are used toprotect aluminum fromcorrosion.High-strengthaluminum alloys, for example, are frequentlyplated with pure aluminum (Al99.5). Nonmetal-lic inorganic coatings such as chromate andphosphate increase the corrosion resistance andalso serve as primers for paints [83].
The thickness of the natural oxide filmon alu-minum can be increased by boiling the metal indeionized water or by steaming it, as a result ofwhich an AlOOH (boehmite) layer with a thick-ness of 1 – 2µm is formed above the Al2O3 bar-rier layer [84]. Another possibility is the use ofimpressed current or sacrificial anodes (zinc) toreduce the potential below the values at whichthe risk of pitting or intergranular corrosion iscritical [84].
5.5. Copper and Copper Alloys [90], [91](→Copper and →Copper Alloys)
The good corrosion behavior of copper andcopper-based alloys has led to their extensiveuse under moist atmospheric conditions and inthe handling of drinking water, industrial wa-ter, and water at high temperatures. Their manyoutdoor applications as fresh water pipes, fit-tings, condensers, heat exchangers in seawa-ter desalination plants, in chemical apparatus,and for many other purposes are explained notonly by good resistance to corrosion but also bytheir good workability, strength-related proper-ties, and high thermal and electrical conductiv-ity.
In accordance with its position in theelectrochemical series (standard potentialCu/Cu+ = 0.34V) copper (DIN 1787) has goodresistance to corrosion, as do high-copper alloys.The good resistance to approximately neutral toalkaline aqueous media (not including water
containing NH3) results from the formation ofoxide films which consist of Cu2O or CuO (de-pending on the nature of the medium and thecorrosion potential) and which afford good pro-tection. In outdoor applications, including thosein marine climates, copper is substantially re-sistant; this explains its use in the constructionindustry. Its main field of application, however,is plant for drinking, cooling, and industrial wa-ter [82]. Here, too, it has good resistance, thoughunder unfavorable conditions (as when depositsare formed or the nature of the water is unsuit-able) pitting corrosion cannot be entirely ruledout [92].
In inorganic and organic acids the corrosionrate depends largely on the presence of oxidiz-ing agents. In nonoxidizing acids in the absenceof oxygen and at room temperature this rate re-mains low; in oxidizing acids such as sulfuricacid, it increases with the oxygen content.
In aqueous ammonia solutions under condi-tions in which surface films are not formed (highalkalinity), copper is attacked severely. In ap-proximately neutral solutions capable of form-ing surface films, especially aqueousmedia con-taining NO−
2 , some susceptibility to stress cor-rosion cracking, even on the part of pure copper,cannot be entirely excluded [93].
Alloying, particularly with Zn, Al, Ni, orSn, raises the strength of copper. In the caseof Cu –Zn alloys (DIN 17 660) single phaseα-phase alloys (Zn ≤ 37wt %) and (α +β)-alloys (37 – 46wt % Zn) are distinguished, theformer being more resistant to corrosion. The(α +β)-alloys tend to suffer preferential attackon the β-phase, which has the higher Zn con-tent. The alloys of both types, but above all the(α +β)-alloys, suffer dezincification corrosionin chloride-containing waters (Fig. 26, see nextpage). Dezincification of the α-phase alloys iscounteractedby addingAs, Sn, Sb, andPas addi-tional alloying elements [94].Among the ternaryalloys, CuZn28Sn and CuZn20Al (2wt % Al)have been used successfully for many years asmaterials for cooler, condenser, and heat ex-changer tubes in cooling waters with relativelylow contamination levels. CuZn20Al is remark-able also for its good resistance to seawater andalso to erosion – corrosion.
In coolingwater containingNH3 orH2S thereis a risk of stress corrosion cracking. The criti-cal stress for ammonia-inducedstress corrosion
Construction Materials in Chemical Industry 31
Figure 26. Dezincification of α-brass in tap water
Figure 27. Influence of the content of alloying elements onammonia-induced SCC of copper alloysa) CuAl; b) CuNi; c) CuZnUpper curves: tensile strength; Lower curves: SCC tresholdstress
cracking in Cu –Zn materials decreases with in-creasing Zn content, and is only about 10MPafor Cu70Zn30 (Fig. 27) [95]. Residual stressesin components that have not been sufficientlyannealed may therefore suffice to initiate crackformation. To prevent stress corrosion cracking,stress relief annealing, followed by testing forresidual stresses according to DIN 50 916, isnecessary.
Of the Cu –Al alloys standardized inDIN 17 665, the homogeneous α-alloys(Al≤ 7.8wt %) are among those that withstandcorrosion, examples being CuAl5, CuAl5As
and CuAl8. In their resistance to seawater theyeven surpass pure copper [82]. In addition to theincreased Al content, their improved resistanceresults from the formation of adherent, highlyprotective oxide films that contain Cu2O andAl2O3 [96].
The two-phase (α +β)-alloys (Al>7.8wt%)are less resistant. The heterogeneous multiphasealloys (Al >10wt %), which, apart from the α-phase, also contain the γ2-phase and (dependingon the heat treatment) martensitic phases, suffernot only corrosion at relatively high rates, butalso preferential dealuminization. Additions ofNi, Fe, and Mn improve the corrosion behav-ior of the heterogeneous alloys (e.g., CuAl11Ni,CuAl10Fe, CuAl9Mn). Aluminum bronzes areused in process plant manufacture, where theirapplications include pump parts (gears) and fit-tings, including those of seawater pumps, andcondenser tubing.
Although the homogeneous α-alloys arethose most resistant to uniform corrosion anddealuminization, they show heightened suscep-tibility to ammonia-induced stress corrosioncracking; this applies particularly to Cu –Alma-terials with about 4wt % Al (Fig. 27). The het-erogeneous materials with Al >8wt %, on theother hand, have good resistance to stress corro-sion cracking, but are sensitive to selective cor-rosion [95].
Technically interesting Cu – Snmaterials (tinbronzes) standardized in DIN 17 662 containup to 9wt % and up to 14wt % Sn, respec-tively, depending on whether they are wroughtalloys or cast alloys [82]. Their strength dependson the tin content and degree of cold forming.With favorable working properties and resis-tance to alternating stresses, together with re-sistance to corrosion in neutral salt solutionsand alkaline solutions (except those containingNH3), they are used for screws, springs (CuSn2),pipes (CuSn6) and chemical plant components(CuSn4, CuSn6).
Cast materials consisting of binary Cu – Snbronzes and multicomponent alloys with addi-tions of Pb (Sn – Pb bronzes) or Zn (gun metal)have proved themselves in marine technology,machinery, and process plant. Owing to their lu-brication properties, which are outstanding insome cases, they are used for bearings, screws,worm gears, toothedwheels, pumps, and turbineblades, etc.
32 Construction Materials in Chemical Industry
Copper – nickel alloys (DIN 17 664 and DIN17 658) form a continous mixed-crystal series,which improves their corrosion behavior. TheCu –Ni materials of technical interest (CuNi10,CuNi20, CuNi30), which generally contain ironandmanganese additions, are among the copper-based materials most resistant to corrosion. Atlevels of up to about 1.5wt %, iron further im-proves the corrosion behavior. The good corro-sion resistance of the frequently used materi-als CuNi10Fe and CuNi30Fe results from theformation of oxidic surface films consisting ofCu2O on the metal side and complex corrosionproducts with high percentages of Fe and Ni onthe solution side [97].
The high corrosion resistance of Cu –Ni –Fe materials, especially in seawater, but alsoin ammonia-containing media (brackish water),explains their growing importance in marinetechnology (seawater desalination plants) andas tube materials for condensers, coolers, andheat exchangers [82]. These alloys also with-stand erosion – corrosion and cavitation well.Chloride-induced pitting corrosion occurs occa-sionally under unfavorable conditions. In com-parison with other copper-based materials, Cu –Ni – Fe alloys with Ni contents of ≥ 10 % ex-hibit substantially better resistance to ammonia-induced intergranular stress corrosion cracking(Fig. 27). They are immune to stress corrosioncracking in chloride-containing media [97].
As the Cu –Ni – Fe alloys of technical in-terest tend to form Fe –Ni-rich grain bound-ary precipitates at 350 – 650 ◦C [98], it is as-sumed that the homogenized, precipitation-freematerials are those most resistant to intergran-ular stress corrosion cracking. Hence the ho-mogenized state has always been preferred forcomponent fabrication. Investigations into theinfluence of the state of precipitation on thestress corrosion cracking behavior of the alloyCuNi10Fe1.5 in ammonia-containing solutionshave shown, however, that specific heat treat-ments are capable of producing material statesconsiderably superior to the homogeneous states[97] (Fig. 28). It has also been demonstrated thatthe stress corrosion cracking behavior is deter-mined exclusively by the hardness of the mate-rial [97].
The variety of copper-based materials is suchthat reliable usedepends on very careful choice:
Figure 28. Mean crack velocity v of CuNi10Fe1.5 in 1M[(NH4)2SO4+NH3] (pH 9, 50 ◦C) as a function of the elec-trode potentiala) Homogenized; b) Aged (50 h, 500 ◦C)
the material must be exactly suited to the condi-tions of use.
5.6. Lead and Lead Alloys [82],[99](→Lead and →Lead Alloys)
Lead owes its good corrosion resistance to itsability to form dense, firmly adherent surfacefilms consisting of lead sulfates, carbonates,or oxides, depending on the corrosive medium[99].
In outdoor applications and approximatelyneutral waters the protection from corrosionarises from the formation of basic lead carbon-ates of low solubility (Pb(OH) 2 · 2 PbCO3),which may also contain lead sulfates. In acidsand alkalis, lead normally suffers severe surfacecorrosion (Fig. 29). However, it is distinguishedby outstanding resistance to sulfuric acid andgood resistance to phosphoric acid and chromicacid; this results from the low solubility of thesulfate, phosphate, or chromate surface films[99]. Lead is therefore a useful material for fit-tings, pipes, pump parts, and other mechanicalcomponents and also for leadings (reaction ves-sels, electrolysis tanks) [82], [100].
For greater strength and finer grain, leadis alloyed with antimony (0.5 – 13wt % Sb).Lead – antimony alloys can be age hardened(precipitation hardening), which gives them fur-ther strength (hard lead). These alloys are usedparticularly where mechanical stresses must bewithstood in addition to corrosion, as in the caseof accumulator plates, pumps, valves, and im-pellers [82]. The corrosion resistance can be fur-ther improved by small additions of As and Se.
Construction Materials in Chemical Industry 33
Figure 29. Dependence of the corrosion behavior of lead inaqueous solutions on pHa) HNO3; b) Acetic acid; c) Ba(OH)2; d) NaOH
Compared to pure lead, alloys based on Pb –Cu or Pb –Cu – Sn, and also alloys of the Pb –Cu – Sn – Pd type with extremely low contentsof Cu (0.01 – 0.1wt %), Sn (0.05 – 1.12wt %),and Pd (0.10wt %), have improved resistance tocorrosion by hot and boiling sulfuric acid [101];see Table 14.
Table 14. Linear corrosion rate of multicomponent lead alloys inboiling sulfuric acid
Alloy Corrosion rate, mm/a
50 %H2SO4
70 %H2SO4
80 %H2SO4
Pb 99.9 Cu 0.48 8.23 a
Pb Cu Pd (0.06, 0.1) 0.12 0.21 0.22Pb Cu Au (0.06, 0.1) 1.86 0.10 0.30Pb Sb Pd (1.1, 0.1) 0.17 0.19 a
Pb Cu Sn Pd (0.05, 0.12, 0.10) 0.01 0.10 0.26Pb Cu Sn Pd (0.10, 0.13, 0.2) 0.01 0.05 0.19Pb Cu Sn Au (0.04, 0.05, 0.10) 0.15 0.23 1.95Pb Ni Sn Pd (0.10, 0.10, 0.10) 0.09 0.28 2.80Pb Te Sn Pd (0.10, 0.10, 0.10) 0.09 0.29 3.50
a Dissolves.
As the solubilities of Cu and Pd in lead are ex-tremely low, these alloying elements are presentin the precipitated state, and fine dispersion ofthe particles is beneficial. The precipitates are
capable of reducing the hydrogen overvoltageand accelerating oxygen reduction, thus shiftingthe potential of the metal into the passive rangeand consequently improving its corrosion resis-tance (Fig. 30) [101].
Figure 30. Potential – time curves of lead alloys in boiling70 % H2SO4a) Pb Cu Sn Pd (0.05 0.12 0.1); b) Pb 99.9 Cu – 0.1 Pd; c) Pb99.985 – 0.1 Pd; d) Pb Cu Sn Au (0.04 0.05 0.1); e) Pb CuSn (0.12 0.12); f) Pb 99.985 – 0.1Au; g) Pb 99.9 Cu; h) Pb99.985
Accordingly, alloys consisting of highpuritylead and copper, or where particularly good cor-rosion resistance is needed, of lead and pal-ladium, are used for chemical apparatus. ThePb –Cu – Sn – Pd alloys also have relatively highcreep strength [101].
5.7. Zinc and Zinc Alloys (→Zinc Alloys)
In DIN 1706 a distinction is made between high-purity zinc (Zn 99.995, Zn 99.99, Zn 99.95) andtechnical zinc (Zn 99.5, Zn 98.5 and Zn 97.5).
Through outdoor exposure and even mildchemical exposure (contact with drinking wa-ter, for example) zinc forms protective surfacefilms that consist mainly of basic zinc carbon-ates and which adhere to the metallic zinc fairlyfirmly. Zinc is severely attacked by hotwater andsteam. A fairly high corrosion rate can also beexpected in industrial air containing SO2 [82].Zinc is not resistant to acids and strong alkalis.
The main use of zinc is the protection of steelfrom corrosion, where it is used mainly in theform of metallic coatings, but also as sacrificialanodes.
The most important zinc alloys are the diecasting alloys GD-ZnAl 4 and GD-ZnAl 4Cu 1;
34 Construction Materials in Chemical Industry
in plant manufacture they are of only minor im-portance.
5.8. Tin and Tin Alloys (→Tin Alloys)
The purity requirements for tin as specified inDIN 1704 range from 98 to 99.90wt % Sn.
Due to the formation of oxidic surface filmsof low solubility, tin has good corrosion behav-ior in outdoor applications and in water [82].Tinis attacked severely by halogens, halogen com-pounds, and alkalis. It is substantially resistantto numerous foods and drinks. It also has goodresistance to corrosion by organic acids (acetic,citric, maleic, tartaric, and lactic acid), espe-cially in the absence of oxygen. Tin is usedmainly in the food industry as a protectivemetal-lic coating on appliances and containers.
Tin – lead alloys with 30 – 60wt % Sn (e.g.,SnPb40, PbSn50, PbSn40), possibly with ad-ditions of Sb and Cu, are used as soft sol-ders (DIN 1707). Tin alloys with Sn contentsof 80 – 91wt % are important bearing materials(sliding bearings); examples are Sn80Sb12 (5 –7wt % Cu, 11 – 13wt % Sb, 1 – 3wt % Pb) andSn80Sb18 (16 – 20wt % Sb, 1 – 3wt % Pb).
The refractory metals of groups 4 and 5 of theperiodic table have now gained considerable im-portance in the fabrication of chemical plant.Having excellent passivity, which results fromthe formation of oxidic surface films, such ma-terials as titanium, tantalum, niobium, and zirco-nium are outstandingly resistant to many media.
InVdTUV-Werkstoffblatt 230/1 unalloyed ti-tanium is allocated to four groups having differ-ent degrees of purity and strength (Table 15). Asthe formingbehavior of types belonging togroupIV is limited, only those belonging to groups Ito III are normally used in chemical engineer-ing. The corrosion resistance of these types canbe improved by alloying with 0.15 – 0.25wt %palladium [104].
In the United States, unalloyed titanium (C.P.Ti = commercially pure titanium) is classified infour groups (grades 1 to 4; see Table 16) but thepercentages of permissible additives are greaterthan those laid down in VdTUV-Werkstoffblatt230/1. Twopalladium-alloyed titanium types aretermed grade 7 and grade 11.
The high price of Pd explains the introduc-tion of the molybdenum- and nickel-containing
Table 15. Grades of unalloyed titanium
Material Material number Max. impurity content, wt % Yield strength
Fe O N C H (1 %), MPa
Ti I 3.7025 0.15 0.12 0.05 0.06 0.013 200Ti II 3.7035 0.20 0.18 0.05 0.06 0.013 270Ti III 3.7055 0.25 0.25 0.05 0.06 0.013 350Ti IV 3.7065 0.30 0.35 0.05 0.06 0.013 410
alloy grade 12, which, like the ruthenium- andnickel-containing alloy Ticorex A, is superiorto unalloyed titanium in crevice corrosion resis-tance.
Finally,mention should bemade of a titaniumalloy containing 5 % tantalum which has beendeveloped to the production stage in Japan. Ithas improved resistance to nitric acid.
A Werkstoffblatt for the alloy tantalum – 2.5tungsten, in which growing interest is beingshown on account of its strength, is in prepa-ration.
Themechanical properties of titanium, zirco-nium and tantalum, and also a selection of theirphysical data, are compiled in Tables 17 and 18.
The fundamental corrosion-chemical proper-ties of these materials for chemical plant are
1) The stability of titaniumunder oxidizing con-ditions
2) The stability of zirconium under reducingand alkaline conditions
3) The stability of tantalum under oxidizing andreducing conditions.
In the case of titanium, which is the most fa-vorable material in this group with respect todensity, particular attention must be drawn toits resistance to chlorides and oxidizing agents.For example, titanium has been used success-fully for heat exchangers exposed to seawater,
The use of palladium as an alloying elementextends the range of applications of titanium inacids with reducing effects, such as sulfuric acidand hydrochloric acid, and improves the resis-tance to crevice corrosion [104].
Corrosion resistance data for titanium, zir-conium, and tantalum are compiled in Table 19(see next page). A common feature of the specialmetals is a pronounced sensitivity to hydroflu-oric acid and fluorides, with a risk of hydrogenembrittlement in the case of extreme exposures.Titanium cannot be used in hydrochloric acidor sulfuric acid. However, its resistance to thesemedia can be improved considerably by addingoxidizing agents. Titanium and zirconium can-not be used in fuming nitric acid, because ofthe possibility of pyrophoric reactions and stresscorrosion cracking.
Of the refractory metals, tantalum has by farthe best corrosion behavior in hot, concentratedmineral acids, apart from hydrofluoric acid. Inhot, concentrated sulfuric acid its corrosion re-sistance is comparable with that of glass or castferrosilicon and is surpassed only by that of no-ble metals such as gold and platinum. It is alsohighly resistant to hydrochloric acid and phos-phoric acid and has no pyrophoric tendency in
Table 17.Mechanical properties of titanium, zirconium, and tantalum
Table 19. Corrosion resistance of titanium, zirconium, and tantalum
Medium Concentration, wt % T , ◦C Corrosion rate, mm/a
Titanium Zirconium Tantalum
Hydrochloric 5 20 <0.05 <0.05 <0.001acid 15 35 2.4 <0.08 <0.001(aerated) 37 35 15.0 <0.08 <0.001Sulfuric acid 10 35 1.2 <0.05 <0.001(aerated) 40 35 8.5 <0.05 <0.001Nitric acid (fuming) inflamm. in air inflamm. in air resistant
0.001 all not resistant not resistant not resistantSodium 10 100 <0.05 <0.05 1.0hydroxide 40 80 <0.1 <0.05 not resistant
fuming nitric acid. Tantalum is, however, at-tacked by oleum, even at room temperature, andalso by hot alkaline solutions. Although it hashigh ductility, its strength can be increased con-siderably by alloying with 2 – 10wt % W, withlittle effect on the corrosion resistance. Tanta-lum – tungsten alloys exhibit good resistance tosulfuric acid, which may be seen in Figure 31[105]. The alloy most favorable in this respect isTa2.5W.
At normal pressure tantalum and its alloywith 2.5wt % tungsten are completely resistantto hydrochloric acid at all concentrations andtemperatures [106]. If, however, hydrochloricacid is handled under pressure in tantalum appa-ratus, damage through hydrogen absorption, andfinally through hydrogen embrittlement, is pos-sible. An autoclave insert failed in consequenceof hydrogen embrittlement after being in use foronly 8 h. The range of conditions under whichtantalum is at risk through hydrogen embrittle-ment is shown in Figure 32.
Figure 32. Comparison of the corrosion resistance of tan-talum and Hastelloy alloy B in hydrochloric acid
Alloying tantalum with niobium gives favor-ably priced materials with similar resistance tocorrosion that have a good chance of becomingestablished in the chemical industry and of play-ing a part similar in importance to that of tanta-lum itself. This applies particularly to use in ni-tric acid. Figure 33 shows the corrosion rates oftantalum – niobiumalloys in sulfuric acid (70%)at 165 ◦C. As, however, the rate of corrosion in-creases with the niobium content, only alloyscontaining up to 40wt % Nb are of technicalinterest. The mechanical properties of tantalum,tantalum alloys, and niobium are shown in Ta-ble 20.
Table 20.Mechanical properties of tantalum, tantalum alloys, andniobium
Figure 33. Corrosion rates of tantalum – niobium in boiling70 vol% sulfuric acid at 165 ◦Ca) Ta–75Nb; b) Ta–60Nb; c) Ta–50Nb; d) Ta–40Nb; e) Ta–25 Nb; f) Ta
Under oxidizing conditions zirconium is of-ten less resistant than titanium. Its resistanceto nonoxidizing media nevertheless greatly ex-ceeds that of titanium. Zirconium is thus practi-cally passive in hot, fluoride-free sulfuric acid.Zirconium, like titanium, is resistant to many or-ganic acids, but is severely attacked by hydroflu-oric acid. Unlike the other refractory metals, zir-conium is resistant to hot alkaline solutions andis therefore the most suitable material for thesemedia in many cases.
Zirconium also has good resistance to cor-rosion by hydrochloric acid (Fig. 34), though itis vital to ensure that oxidizing agents, such asFe(III) or Cu(II) compounds, are not present. At30 ◦C and an acid concentration of 32 %, as lit-tle as 5 ppm of such a compound increases thecorrosion rate ten-fold (from 7.6×10−4mm/a to7.6×10−3mm/a) [107], [108]. For critical cor-rosion conditions, heat treatment for 3 h at 750 –790 ◦C, especially after welding, is advisable inorder to maximize the resistance [109].
Refractorymetals have a wide range of appli-cations in the chemical industry. They are usedfor reactors, columns, agitators, pipelines, fit-tings, bellows, and pumps, for example. Sensingheads, such as thermo-feelers, magnetic floatsand membranes, that must also function reli-ably under adverse conditions are protected bycladding with special metals. Heat exchangersmade fromspecialmetals and consistingof coils,rods, candle-shaped elements, or tube bundlesare used to cool or heat corrosive media.
Figure 34. Corrosion of zirconium in hydrochloric acida) 5mm/a; b) 0.5mm/a; c) 0.13mm/a
The components may consist entirely of thespecial metal or of suitable composites. In manycases nonadherent lining or sheathing of the sub-strate material is sufficient; cladding is appro-priate where heavy loads are exerted, good heattransmission is needed, or negative pressure ex-ists. As the special metals tend to form inter-metallic phases with steel at high temperatures(1000 ◦C), they are normally applied by explo-sion cladding, possibly with intermediate layers(e.g., copper) [110], [111].
Although special metals have good weld-ing behavior, their reactions with atmosphericgases must be taken into account. Hence weld-ing is only possible under an inert gas or in highvacuum. It is essential to avoid alloying withiron-based materials in welding. Since the melt-ing point of tantalum, for example, is roughlytwice as high as that of steel, specially designedbutt-welds are necessary in the processing ofexplosion-cladded metal sheet (cf. Fig. 35, nextpage) [112].
5.10. Organic Materials [113–115]
To an increasing extent the growing require-ments on the availability and reliability of chem-ically exposed plant components are necessitat-ing the use of particularly corrosion-resistantmaterials. In this respect the chemical natureof organic materials gives them certain advan-tages over metallic and inorganic nonmetallicmaterials, provided the operating temperaturesdo not reach critical levels. In many cases they
38 Construction Materials in Chemical Industry
are well suited for plant that handles the mediamost commonly encountered at chemical works,namely hydrochloric acid, caustic soda solu-tion, hypochlorite solution, sulfuric acid, wa-ters, and salt solutions of all kinds. The maintypes of organic materials are thermoplastics,thermosetting plastics, elastomers, glass-fiber-reinforced plastics, and foamed plastics. Com-posite and surface coating materials are beingused increasingly too (see →Corrosion).
Figure 35. Design of a butt-weld for explosion-claddedsheet (mild steel/tantalum with copper intermediate layer)– · – Hardness curve in tantalum cover strip; —- Hardnesscurve in tantalum cladding; – – – Hardness curve in copperintermediate layer
One of the most important processing meth-ods for plastics is welding. A survey of the weld-ing methods is given in [116]. A comprehen-sive account of the design of plastic components,with detailed consideration of strength calcula-tions and dimensioning, is contained in VDI-Taschenbuch T 21 [117].
5.10.1. Selection Criteria
The stability required of plastics varies accord-ing to whether a plastic component must sim-ply withstand corrosion, or fulfil structural func-tions also. In general no risk arises if the phys-ical properties of a coating or facing consistingof a synthetic material are impaired through theaction of a medium; but the same process in astatically or dynamically stressed part may leadto its premature failure. Similarly, swelling andstress corrosion cracking may be nonhazardous,and the quality of joints relatively unimportant,in nonstructural plastics, but hazardous, or crit-ical, in those with structural functions. The per-meability of polymers is less serious in compo-nents than in coatings, especially those onmetal-lic substrates. Thus the envisaged conditions ofuse must be carefully considered before mate-rials are chosen. As important parameters, thesolutions to which the components will be ex-posed, including all their consituents (such assolvents, amines, phenols, compoundswith highvapor pressures, oxidizing agents), the maxi-mum and long-term service temperatures, thepressures and vacuum conditions, and the staticand dynamic loads, must receive special atten-tion. Only when the various factors have beenestablished can a list of foreseeably suitable ma-terials be drawn up for more detailed consider-ation.
5.10.2. Properties and Application Criteria
Unlike the surface removal corrosion of metallicmaterials, the action of liquid media on organicmaterials leads mainly to swelling, caused byabsorption of the medium. In many cases thedegree of swelling of an organic material in in-dividual media and the dependence of swellingon time and temperature as well as its possiblereversibility constitute adequate criteria for thedetermination of suitability. The stability datapublished in manufacturers’ and other literatureare usually based on swelling tests. These dataare adequate where coatings or linings, e.g., ofrubber or plastics, are concerned, and in a num-ber of other cases.
Where static or dynamically loaded parts areconcerned, swelling tests alone must be consid-ered tentative, enabling one merely to draw up
Construction Materials in Chemical Industry 39
a shortlist of materials for further consideration.Here it is farmore important to ascertain how thestrength ofmaterials is affected by prolonged ex-posure to chemicals, the degree to which creepoccurs, and whether or not the conditions arefavorable to stress corrosion cracking. Lossesof long period creep resistance caused by liquidmedia are preferably expressed as resistance fac-tors. A resistance factor is the ratio of the time tofailure in the test medium to the time to failure inwater [118]. In somemedia this ratio depends onthemechanical stresses exerted on the sample; inother cases it is largely independent of them. Re-sistance factors for glass-reinforced plastics canbe derived from the differences in creep behaviorin tensile tests under various ambient conditions(as ratios between creep moduli, for example)[119].
The stress corrosion crackingof organic poly-mers is generally an entirely physical process,with diffusion and swelling processes and in-ternal and/or external stresses playing a majorpart [120]. The term is used only where crack-ing caused by exposure to chemicals, like thatof metals, occurs exclusively in the presence oftensile stresses. Tests are performed in variousmedia and at various temperatures on panels orpipes subjected to constant deformation or con-stant stress in the long period tensile creep test[118]. Susceptibility to stress corrosion crack-ing is increased by exposure to wetting agents,organic acids, solvents, and oxidizing media, byshaping and jointing operations, by internal andexternal stresses, by prolonged exposure to hightemperatures in processing, by temperature al-teration, and by notching. Inmany cases the sen-sitivity offinishedparts to stress corrosion crack-ing can be reduced by thermal after-treatment.
The thermoplastics most widely used inchemical engineering are PVC [poly(vinyl chlo-ride)], PE (polyethylene), PP (polypropylene)and PB (polybutene).
Poly(vinyl chloride) has been used fordecades due to its outstanding resistance tochemicals. It is harder and tougher than otherthermoplastics, but at temperatures below 20 ◦Cit has less impact strength than polyethylene(PE) and polybutene (PB). Above its soften-ing temperature of 80 ◦C, poly(vinyl chloride)can be shaped by plastic deformation. The mostfavorable deformation range is 110 – 130 ◦C.
Original shaping (e.g., by injection molding,extrusion, or welding) requires higher temper-atures, however (170 – 200 ◦C). The upper ser-vice temperature limit is 60 ◦C, or possibly alower temperature, depending on the chemicalexposure.
Polyethylene is a partly crystalline mate-rial with high impact strength at temperaturesdown to ca. − 100 ◦C. High-density polyethyl-ene is preferred for chemical plant because ithas greater strength and stiffness than the low-density types. It can be shaped thermoplasti-cally at temperatures above 130 ◦C and weldedat ca. 200 ◦C. The upper service temperaturelimit is 60 ◦C, or 80 ◦C if glass fibers are in-corporated. The resistance to nonoxidizing me-dia is excellent. The swelling caused by hydro-carbons leads to losses of strength that can betaken into account with the aid of resistance fac-tors. The susceptibility to stress corrosion crack-ing increases with the degree of crystallization,but falls with increasing molecular mass. Highmolecularmassmaterial is less easily processed,however, and its properties may suffer if it is ex-posed to elevated processing temperatures forlong periods.
Polypropylene (PP) has properties similarto those of polyethylene, but greater impact,notched impact, and shock resistance, especiallyat temperatures below 0 ◦C. The upper servicetemperature limit is 80 ◦C, or 100 ◦C if glassfibers are present. Polypropylene is almost asresistant to chemicals as hard polyethylene, butmore sensitive to oxidizing media and morestrongly swelling in organic solvents. In com-mon with hard polyethylene it has a certain per-meability to organic media.
The applications of polypropylene includeexhaust ducts, internal chimney tubing, sepa-rators, rotary filters, filter fabrics and pressureplates, hydrocyclones, and gas scrubbers.
Polybutene (PB) is one of the more recentpolymericmaterials that have gained acceptancein chemical engineering. Its strength propertieslie between those of soft polyethylene (LD-PE)and polypropylene. Its long period creep resis-tance exceeds even that of polypropylene. Al-though polybutene is soluble in several organic
40 Construction Materials in Chemical Industry
solvents at elevated temperatures, its suscepti-bility to stress corrosion cracking is low. Theupper service temperature limit is 90 ◦C. Themain applications of polybutene are high-tempe-rature pressure pipes, filter pressure plates, strip-wound vessels, and pumps.
Polytetrafluorethylene (PTFE) is the ther-mally and chemically most resistant plastic usedin chemical engineering. Its service tempera-ture range extends from − 200 to 250 ◦C. It ischemically resistant to virtuallly all media ex-ceptmolten alkalis and elemental fluorine.How-ever, fillers added to the plastic to improve itsextrudability ormechanical strengthmay reduceits resistance to chemicals. The outstanding abh-esive behavior and exceptionally favorable slid-ing properties deserve specialmention. Applica-tions include pipe linings, bellows, hoses, pumpparts, tube bundle heat exchangers, steam re-lease nozzles, conveyor belts, seals, stuffing boxpackings, and filter fabrics.
Poly(vinylidene fluoride) (PVDF) is a part-ly crystalline plastic of high thermal stability. Itcan be used at temperatures up to 100 ◦C, or,if glass-filled, up to about 120 ◦C. PVDF is re-sistant to most organic and inorganic acids. Inalkalis, especially caustic soda solution, it tendsto suffer stress corrosion cracking. Its main ap-plication is in pipelines used to convey acids.
Elastomers and ebonite based on natural orsynthetic rubber [121], [82] are used mainlyfor surface protection (see →Corrosion). Elas-tomers also have many applications in chemi-cal engineering as seals, membranes, bellows,vibration dampers, hoses, and Moineau pumpstators, as well as being used for space-savingfolding containers for the transportation of cor-rosive materials by land and sea.
5.10.3. Thermosetting Plastics
The main materials in this group are phe-nolic, furane, epoxy, vinyl ester, and unsatu-rated polyester (UP) resins. They are filled withglass fibers, graphite, or quartz and processed totamping compositions, hotmolding compounds,sheet, and putties. Being brittle, they are rarely
used directly for mechanically stressed parts un-less reinforcing materials and fillers have beenincorporated. Having high cross-link density,thermosetting plastics are insoluble and can-not be melted, and their strength and long pe-riod creep resistance are without the marked de-pendence on temperature exhibited by thermo-plastics and elastomers. Individual parts can bejoined to one another with catalytically harden-ing cements.
Solid parts made from glass-reinforced reac-tion resins are distinguished not only by chem-ical stability but also by relatively low weight,together with high strength, and by good emer-gency running behavior. They are being used in-creasingly for composite structures exploitingthe high stability and heat resistance of glass-fiber-reinforced reactive resins with the supe-rior chemical resistance of thermoplastic coat-ings and linings.
For exposure to aggressive media, parts con-sisting entirely of glass-reinforced plastics usu-ally receive an at least 2.5mm thick coating con-sisting mainly of resin. This protective coating,in which the proportion of glass is low, actsas a barrier to corrosion and protects the load-bearing, glass-reinforced structure from the ac-tion of destructive media. The resins used areunsaturated polyester, vinyl ester, epoxy, or fu-rane resins.
Table 21 gives a general idea of the cor-rosion resistance of reaction resins used inthe fabrication of apparatus and pipes. Glass-fiber-reinforced resins are used for corrosion-resistant articles of all kinds, especially pipesfor waste water and waste air disposal, stacks,siphons, vats, transportation and storage con-tainers, down-pipes, filter pressure plates, andframes. The upper service temperature limits ex-tend to about 120 ◦C, depending on the resin inquestion.
Table 21. Chemical resistance of reaction resins∗
Resistance towards
Type of resin Acids Alkalis Solvents Oxidizingagents
∗+= resistant; 0 = conditionally resistant; −= not resistant
Construction Materials in Chemical Industry 41
More frequently, however, parts consistingof glass-reinforced plastics are provided withlinings. The lining materials most widely usedat present are the thermoplastics polypropylene(PP), unplasticized polyvinyl chloride (PVC-U)and polyvinylidene fluoride (PVDF).
In the fabrication of composite structures,thermoplastic liners are normally produced as afirst step and than reinforced with a glass-filledreaction resin. Thermoplastic linings consistingof sheetmaterial are placed end to end and joinedby welding. The normal welding techniques arehot gas, heated tool, and hot gas extrusion weld-ing.
Figure 36. PVC-lined UP-GF apparatus
Figure 36 depicts a PVC-lined UP-GF ap-paratus, seen here as a demonstration exhibitwith parts removed to show structural details.Another application is shown in Figure 37. Thisis a storage facility for solutions containing HClandNaOCl, consistingofUP-GF tanks andpipeswith PVC liners. The design of parts consist-ing of glass-reinforced plastics is described in[122]. Properties, processing, and applications
are described in [123]. The use of organic mate-rials in the form of corrosion-resistant coatingsand linings represents an important applicationin chemical engineering (see →Corrosion).
Figure 37. UP-GF tanks and pipes with PVC liners, con-taining HCl and NaOCl solutions
5.11. Inorganic Nonmetallic Materials
5.11.1. Glass [124], [125]
Pipelines, apparatus, and even entire plants arenow being produced increasingly from borosili-cate glass in order to solve exceptionally criticalcorrosion problems. A complete range of glassparts for assembly by the user, ranging from sim-ple pipe sections andmoldings tofittings, vesselsand column components, and even to complexheat exchangers and their accessories, is nowcommercially available.
Borosilicate glass has a very wide range ofuses and can be exposed permanently to temper-atures up to 200 ◦C. It is permanently stable tovirtually all media except hydrofluoric acid andstrong alkalis. Above 200 ◦C it is attacked fairlyseverely by all acids, especially phosphoric acid[126]. Other advantages include the smoothnessof its surfaces, catalytic inertness, and nontoxi-city.
Glass can be combined with other materials –plastics and metals, for example – thus open-ing up a wide range of technical possibilities.Glass apparatuses for heating purposes can beprovided with elements consisting of stainlesssteel, highly corrosion-resistant nickel alloys, ti-tanium, or tantalum. Pumps and valves of allkinds can be sealed with polytetrafluoroethyl-ene components. PTFE is also used in the form
42 Construction Materials in Chemical Industry
of bellows to compensate for thermal expansionand to damp vibration. Important applications ofborosilicate glass include distillation and rectifi-cation units, absorbers and gas scrubbers, nitricacid concentrators, acid recovery plant, cleaningunits, and piping.
Although quartz glass is very resistant to ther-mal shock due to its extremely low coefficientof thermal expansion, its high cost limits techni-cal use to special articles (e.g., observation win-dows).
The use of enamel coatings, made from vit-reous enamels and partly crystalline enamels,to protect tanks, apparatus, columns, pipelines,pumps and other items from corrosion is de-scribed in →Corrosion).
5.11.2. Graphite [82], [127], [128]
Graphite has been used as a special material inchemical engineering since the mid-1930s. Heatexchangers, simply cemented together frompanels and strip, were the first application. Now,all structural elements, such as pipes, solid andhollow cylinders, slabs, panels, and profiles, canbe produced from impregnated graphite (hard-fired carbon or electrographite).
As the various graphite production meth-ods give porous materials, impregnation withphenol – formaldehyde or furane condensationresins by the vacuum-pressure process is nec-essary if impermeability to gases and liquidsis required, the effect of this treatment beingthat all the pores are closed. The quality ofthe finished material depends on the proper-ties of the chosen resin. This applies particu-larly to the permeability, corrosion resistance,and upper service temperature limits (which arein the region of 165 ◦C). For greater stabilityand higher service temperature limits, alterna-tive impregnating agents, such as polytetrafluor-ethylene, are used. Carbon filling of the pores ingraphite can be carried out by impregnatingwithcokable materials, which are then decomposedthermally, this treatment being repeated severaltimes. The resultingmaterial has good corrosionresistance and a high service temperature limit,but is less strong and more brittle than the resin-impregnated grades.
Resin-impregnated electrographite is thesynthetic carbon material most widely used in
chemical engineering. It is highly resistant tocorrosion by organic and inorganic acids, alka-lis, alcohols and many solvents, and other or-ganic compounds. It is easily worked, has highthermal conductivity, and withstands even se-vere temperature shocks. As graphite is a brittlematerial with practically no plastic deformation,the parts must be so designed that tensile andshear stresses are avoided as far as possible.
Themain applications of graphite are heat ex-changers of all kinds, columns, sprinkling cool-ers, centrifugal pumps, valves, pipelines, andsmall parts of many kinds. The use of graphitein plant for production of dry hydrogen chlorideand in the treatment of waste water and wasteair deserves special mention.
5.11.3. Refractory and Acid-ResistantBricks [129–132]
This term covers ceramic materials that are stillfree from deformation at temperatures exceed-ing 1600 ◦C and still do not soften at those ex-ceeding 1800 ◦C. Bricks are normally shapedand fired, but are sometimes used in the form ofcastings. Apart from refractoriness, such prop-erties as softening under pressure, thermal con-ductivity, chemical resistance, resistance to tem-perature alternation, abrasion resistance, shapestability, and slagging tendency are of great im-portance.
Refractory bricks serve to enclose spaces inwhich reactions take place and must last as longas possible under the given conditions. In viewofthe variety of these conditions they must be cho-sen for then intended application in accordancewith their chemical and technological proper-ties. The following classification of refractorymaterials is based on their main consituents:products of high silica content, alumina-basedmaterials, basic refractory bricks, neutral refrac-tory products, carbon bricks, and silicon carbidebricks.
For corrosion protection in difficult cases,chemical apparatus can be lined with corrosion-resistant bricks, tiles, or moldings. These are at-tached to the wall or to one another with mortaror putty. Sealing and insulating layers may beused additionally between the wall of the appa-ratus and the lining.
Construction Materials in Chemical Industry 43
5.11.4. Engineering Ceramics [133], [134]
Oxide Ceramics. The most important oxideceramic material is aluminum oxide, Al2O3.Beryllium oxide has applications as a moderatorin nuclear reactors, and zirconium dioxide as arocket material. In Table 22 the main mechan-ical and thermal properties of aluminum oxideare compared with those of hard metal. A con-spicuous property of Al2O3 is its great hardness.The strength values are practically constant overa wide temperature range. This is because the in-crease in plasticity (and hence in the ability todissipate local peak stresses) which occurs asthe temperature rises compensates for the ther-mally induced loss of strength.On the other handthe impact strength (unnotched) of 0.2 J/cm2 at20 ◦C, is low compared to the notched impactstrength of 100 J/cm2 for structural steel. ThusAl2O3 is a brittle material.
Table 22. Properties of Al2O3 ceramic and a hard metal
Alumina has very good corrosion resis-tance, withstanding acids, molten metals, manyglasses, and slag. It has advantages overmetals particularly where corrosive and ther-mal stresses are superimposed on mechanicalstresses.
One of the commonest applications of alu-mina ceramics is therefore the sealing of pas-sages leading into vessels and spaces in whichcorrosive media are present. In the form of sliderings andmechanical seals alumina is superior toall other materials in resistance to corrosion andwear. Its use is therefore leading to the replace-ment of conventional stuffing boxes on rotat-ing shafts by mechanical seals, especially wherehigh rotation speeds occur. Aluminum oxide canbe used in oxidizing and reducing atmospheres
and under vacuum at working temperatures ofup to 1950 ◦C.
Silicon Carbide. Silicon carbide has notonly high strength (bending strength up to650MPa), but also a low coefficient of thermalexpansion of only 4.8×10−6K−1 and a highthermal conductivity of 42WK−1m−1. Theseproperties explain the very good resistance ofsilicon carbide products to changes of tempera-ture. Since silicon carbide also has excellent re-sistance to corrosion, it is used as a structuralma-terial for components subjected to severe ther-mal and chemical loads.
Silicon carbide is used mainly as a grindingagent, e.g., for grey cast iron, nonferrous met-als, glass, and ceramics. With great resistance totemperature alternation and corrosion, SiC ther-mocouple protection tubes are used in contactwith liquid aluminum in aluminum holding fur-naces. Silicon carbide products are used for themuffles of indirectly heated smelting furnacesfor zinc, aluminum, copper and their alloys, andfor tank linings. With good electrical conduc-tivity at high temperatures, coupled with resis-tance to oxidation, SiCheating elements are usedin oxidizing atmospheres at temperatures up to1500 ◦C.
Silicon Nitride. The most remarkable prop-erties of siliconnitride include good resistance tochemical attack by acids and molten nonferrousmetals, resistance to temperaturefluctuation (co-efficient of thermal expansion 3×10−6K−1),and heat resistance. The bending strength ofhot-pressed silicon nitride is about 700MPa at20 ◦C and still as high as 290MPa at 1400 ◦C.At 1000 ◦C dense silicon nitride still has a 100-h long-term creep resistance of 200MPa. In thecase of the aluminum industry, materials areneeded that are not wetted by the molten metal,have good resistance to temperature fluctua-tion, and high strength at temperatures up to ca.800 ◦C. As silicon nitride meets these require-ments, it is used for, among other things, ther-mocouple protection tubes and ascending pipesin low-pressure casting. The hardness of siliconnitride gives it high wear resistance, which, to-gether with the low friction coefficient (µ= 0.1 –0.2), favors its use as a material for bearings andsliding contacts. Slide rings, roller bearings, and
44 Construction Materials in Chemical Industry
tube drawing plugs are typical applications. Sil-icon nitride is also used as a cutting material.
Ceramicmaterials are often used in the chem-ical industry in the form of thermally sprayedcoatings that afford protection against wear andcorrosion. This field of application is describedin →Corrosion).
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