National Workshop on Boiler Corrosion , 11-12th April, 1995, NML Jamshedpur, INDIA BOILER CORROSION - METALLURGICAL ASPECT R.N. GHOSH National Metallurgical Laboratory Jamshedpur 831 007 Introduction Boilers are used to generate steam either to drive a turbine for electric power generation or to run plant and machineries in a process or manufacturing industry. They use fossil fuels or a nuclear reactor as the primary heat source. Our discussion however is mainly con- cerned with the metallurgical as- pect of the fossil fuel fired boilers only. The design of a boiler is based on the principle of heat balance. The heat of combustion of fossil fuel determines the amount and qual- ity of steam generated. Fig.1 gives a schematic view of the various com- ponents of a boiler. Within the fur- nace, flame temperature may ap- proach 1650°C. The furnace wall which is made up of tubes through which water keeps flowing absorbs the heat and thus reduces the flue gas temperature to 900-1100°C. As a result steam at saturation tempera- ture which is a function of boiler pressure is generated. Heat transfer in this stage is primarily through radiation and convection. The flue gas temperature is further reduced by super-heating or reheating of steam in superheaters and reheaters. To extract more heat and to improve overall efficiency, an economizer preheats the boiler feed water to its boiling point. The flue gas travels through an air preheater which heats the combustion air and then makes its way up the stack. Under steady state condition the typical ranges of heat flux in the various parts of a boiler are given in Table-1. Table-1 Typical values of heat flux and temperatures of steam/water in various parts of a boiler Boiler component Heat Flux W/m2 Steam/Water Temperature Economizer 15,000-32,000 3000C Water wall 125,000-400,000 4000C Reheater 19,000-38,000 6000C Superheater 31,000-63,000 600°C B-1
Transcript
National Workshop on Boiler Corrosion , 11-12th April, 1995, NML Jamshedpur, INDIA
BOILER CORROSION - METALLURGICAL ASPECT
R.N. GHOSHNational Metallurgical Laboratory
Jamshedpur 831 007
Introduction
Boilers are used to generate
steam either to drive a turbine for
electric power generation or to run
plant and machineries in a process
or manufacturing industry. They
use fossil fuels or a nuclear reactor
as the primary heat source. Our
discussion however is mainly con-
cerned with the metallurgical as-
pect of the fossil fuel fired boilers
only. The design of a boiler is based
on the principle of heat balance.
The heat of combustion of fossil
fuel determines the amount and qual-
ity of steam generated. Fig.1 gives
a schematic view of the various com-
ponents of a boiler. Within the fur-
nace, flame temperature may ap-
proach 1650°C. The furnace wall
which is made up of tubes through
which water keeps flowing absorbsthe heat and thus reduces the fluegas temperature to 900-1100°C. Asa result steam at saturation tempera-ture which is a function of boilerpressure is generated. Heat transferin this stage is primarily throughradiation and convection. The fluegas temperature is further reducedby super-heating or reheating ofsteam in superheaters and reheaters.To extract more heat and to improveoverall efficiency, an economizerpreheats the boiler feed water to itsboiling point. The flue gas travelsthrough an air preheater which heatsthe combustion air and then makesits way up the stack. Under steadystate condition the typical ranges ofheat flux in the various parts of aboiler are given in Table-1.
Fig. 1 : Schematic view of various components of a boiler
Temperature Profile from Flue Gas to Steam
To
Clean Tube Tube + Int. Scale
T4T3T1
Tube + ext. Scale
Note That only Internal scale is likely to raise metal wall temp.
Fig. 2 : Temperature profile across the tithe - wall showingthe effect of scale formation of metal temperature
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Cold reheoter pipe
R.N. GHOSH
Clearly it is at the water wallwhere the heat flux is maximum, thetemperature difference betweensteam and flue gas is expected to bethe highest. The effective resis-tance to heat transfer is determinedby the sum total of the resistance ofthe ash or soot or scale deposits onthe external tube surface, tube wall,internal scale and the thin film ofsteam on the inner wall. Fig.2 givesa schematic representation of thetemperature profile from flue gastemperature (To) to bulk steam tem-perature (Ts) for clean, as well asinternally / externally scaled condi-tions. Note that formation of inter-nal scale raises tube metal tempera-ture. However formation of ash orshoot deposit or scale formation onthe external surface although wouldreduce efficiency of heat transfer itdoes not alter the tube wall tem-perature. To maintain high effi-ciency removal of ash/soot depositand periodic cleaning of internalsurface is necessary. The steamside heat transfer is a function is offluid velocity, density and tube borediameter. Depending on the magni-tude of heat flux there is scope ofoverheating as well (Fig. 3). It isthe water wall which is more sus-ceptible to such overheating as thereARE both steam and water & highheat flux.
The boiler components are thusrequired to withstand high tempera-ture and pressure in an environmentof steam/water and flue gases. Steelis the obvious choice for construc-tion of boiler. This is primarilybecause of its availability in variousforms and sizes, low cost, ease of
fabrication corrosion resistance andhigh strength . Steel is primarily analloy of iron and carbon althoughmost commercial grades contain ahost of other elements e.g., Si, Mn,S, P, Cr, Ni etc. Whilst some ofthem are harmful , many are benefi-cial as well. By controlling alloyaddition , heat treatment andthermomechanical processing awide range of properties could bedeveloped in steel. Boilers aremostly made up of plain carbon andlow alloy steel containing Cr & Mo.The choice of steel depends on theoperating condition of the boilercomponents . For examplesuperheater , reheater tubes or steampipes where very good hightemperature properties and oxidationresistance is required steels having2.25Cr IMo or 1Cr 0.5Mo 0.25Vsteels are preferred . In boilergenerating steam at 650 ° C or highereven high alloy ferritic or austeniticsteels are also being used . Howeverin cases where very high mechanicalproperties are not essential or wherethe difficulties of welding make highalloy additions undesirable oruneconomic leaner steels are used.Typical examples are 1Cr 0.5Mo &0.5 Mo steel.
Commercial steels invariablycontain residual or tramp elements.Those universally present are S &P, coming from the ores, fuel orslags used to refine steel. Someores contain significant amounts ofAs, Sb, Bi and Sn. Besides Sn andCu may come from scrap. Theeffect of these elements withoutexception is damaging. Whenpresent in steel they migrate to grain
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Fig. 3 : Variation of steam/water and metaltemperature at different levels of- heatflux
(a) and (h) : The effect of scale thickness on the temperature gradient acrossthe scale; (c) and (d) : Thee ffectof Scale thickness on the temperature of themetal at the outer surface of the tube. Th graphs are based on a continuous,uniform scale, with a thermal conductivity of 14.4 W/m . K (lOBtu . in./h .ft2 . °F.) on tubes in the film-boiling regrion of 690-kPa (100-psi) system
boundaries and markedly reducetheir resistance to fracture. Thesehave been identified as the maincause of embrittlement. Embrittledsteels exhibit intergranular fracture.This tendency is promoted by cer-tain alloy additions viz., C, Mn, Siand Ni but retarded by Mo.Embrittlement can be minimised byreducing the content of content ofresidual elements from levels ofabout 0 .01% to 0.001% by carefulchoice of scrap and slagging.
boiler design is inherently conser-
vative.
Overheating Failure
Overheating results from
restriction of flow within a heated
tube or from localized heating of the
tube wall. Mild restriction of flow
favours a small degree of
overheating. This leads to failure by
stress rupture. Such failures are
characterized by thick tip rupture
with little ductility (Fig. 4).
The types of steel used invarious parts of a boiler are given inFig.1. Possibly the best way todescribe the metallurgical aspect ofboiler design is to discuss why inspite of judicious selection ofmaterial some of the components failin service. A survey conducted byone laboratory over a period of 12years based on 413 investigationsindicate overheating as the maincause of failure. Table-2 gives asummary of this survey. Howeverdefective material does not alwayscause a component to fail soon afterbeing put into service. Even massivedefects were found to be present insome areas without causing fractureto occur until after considerableperiod of service. This is because
Rapid overheating is most
likely to occur at start up or during
periods of rapid fluctuation in steam
demand or during full power
operation it may be caused by sudden
loss of circulation for example feed
pump failure or local loss of circu-
lation. This leads to failure by ten-
sile fracture which is transgranular
in nature. Such failures are accom-
panied by extensive tube swelling
and thinning down of tube wall
(Fig.5).
During operation a widevariety of deposits can form oneither sides of the tubes. This canalso cause overheating failureby changing the heat transfer
Table-2Sample failure statistics of boiler components
Fig. 4 . Characteristic thin-lip rapture andassociated rnicrostructural features
Type 321 stainless steel (ASME SA-213, Grade TP321 H)superheater tube that failed by thick-ip stress rupture
(a) Overall view showing a typical fishmouth rupture. Approximately'/2 x. (b) Unetched sectionfro location between arrows in (a) showing extensive transverse cracking adjacent to the mainfracture (at right). Approximately 4'/z x. (c) Specimen etched electrolytically in 60% HNO3 (nitric
acid) showing intergranular nature of cracking. 100x
Fig. 5 : Characteristic thin-lip raptureThin-lip rupture in a 64-mm (2'/2 in.) outside-diameter x 2.7-mm (0.105-in.)
wall thickness carbon steel furnace-wall tube that was caused by rapid overheatingKnve-edge wall thinning at longitudinal main rupture is shown in corss section in the
inset. Note secondary circumferential fracture at left end of the longitudinal main rupture.
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characteristics of a tube bank or ofan individual tube. Fire side depos-its either insulate portions of theheat transfer surface or in cases theseare thick enough the flow pattern offlue gases through a tube bank isaltered . In both cases the metal walltemperature of regions devoid ofdeposits will be higher than in re-gions where deposits are present.
When waterside deposits are pre-
sents tube wall temperature in-
creases in the region of deposits.Figure 3 shows the effect of scale
thickness on metal wall tempera-
ture. The increase is more pro-
nounced in case tubes of smaller
diameter. It can even go beyond the
recommended limit for the steel.
Therefore it is necessary to keep the
water side surface free of adherent
scale particularly in the regions of
high flux.
Such overheating significantlyalters the structure and properties ofthe steel tube. Prolonged overheat-ing at temperatures below AC I (thetemperature at which austenite be-gins to form) in carbon and lowalloy steel causes decomposition ofpearlite (or Bainite ) into ferrite andspheroidal carbides. This weakensthe tube material. If continued over-heating persists it could cause for-mation of voids at grain boundariesand eventual grain separation, re-sulting in stress rupture failure.
Rapid overheating of boiler tubesmade of carbon and low alloy steelusually results in failure because ofdecrease in yield strength. If failuretakes place below recrystallization
temperature microstructure nearfracture would exhibit elongatedgrains. Rupture that occurs at tem-peratures greater than AC1 may ex-hibit pearlitic/bainitic areas formedas a result of quenching effect ofescaping water or steam on partlyaustenitic structure existing at thetime of rupture.
Microstructural evidence as de-scribed above may not always betaken as conclusive proof of over-heating. Such features may havebeen present at the time of installa-tion. To confirm it is therefore nec-essary to examine microstructuresof either adjacent tube or unusedtube of the same lot. Careful exami-nation of microstructures along withhardness and oxide scale thicknessmeasurement nevertheless can givea very good estimate of the extent ofoverheating. Such estimates couldprove extremely helpful in calculat-ing the remaining life of variouscomponents of a boiler. To facili-tate such computation a computersoftware called RELIEF has beendeveloped at NML. A demonstra-tion could be arranged for those whomay be interested.
Austenitic steel however exhib-its different kinds of microstruc-tural instability. These are carbideor ferrite or 6 phase precipitation.All these may shorten the stressrupture life of stainless steels andcan enhance the probability of tubefailure with prolonged overheating.
Embrittlement
Metallurgical changes within the
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tube metal are responsible for rup- Methane is a large molecule and
tures caused by embrittlement. The cannot diffuse out of steel. It col-
main mechanisms are hydrogen dam- lects at grain boundaries and when
age and graphitization. the pressure is high enough, leaves
cracks behind.
Hydrogen damage takes place
primarily in steel components. Dis-
continuous intergranular cracking
and decarburization are its charac-
teristic features. Such cracking re-
suits from precipitation of hydrogen
or methane. Tubes that have under-
gone such damage often exhibit win-
dow type fracture.
Hydrogen is one of the normal
product of corrosion reaction be-
tween iron and water in both acidic
or basic conditions.
2 H+ + Fe = Fc2+ + 2 11 (acidic conditions)
20H-+Fe=FcO22-+2 H (basic cond.)
Hydrogen damage is usually
associated with thick internal de-
posits. The metal temperature un-der this deposit is usually higher.
plain carbon steelThe structure is ferrite and graphite with only a
trace of spheroidized carbon remaining. Etchedwith nital. (500 x)
Temperature-time plot of pearlite decomposition
by the competing mechanisms of spheroidization& graphitization in carbon and low-alloy steels.The curve for spheroidization is for conversion ofone-half of the carbon in 0.15% C steel tospheroidal carbides. The curve for graphitization isfor conversion ofone-half of the carbon in aluminumdeoxidized 0.5% Mo cast steel to nodular graphite.
Fig. 7 : Kinetics of graphitization and spheroidization
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tubings. Economizers , steam pipesand other components subjected to425-550°C for several thousandhours are more likely to be embrittledby graphitization . The heat affectedzones ( HAZ) adjacent to welds areamong the more likely locations forgraphitization to occur.
Water Side Corrosion
This is one of the main cause offorced outages of steam generators.Corrosion in boiler takes places by avariety of mechanisms . It is mostcommon in water walls and econo-mizers. These are usually made ofplain carbon steel although in a fewcases low chromium ferritic steelshave been used. The metal walltemperature is less than 400°C.
A thin film of Fe304 that formson the water side of the tube acts asa protective layer. This forms as aresult of the following reaction.
3 Fe + 4 H2O = Fe304 + 4 H2.
The oxide consists of two layers.In practice the outer layer is seldomformed , because as the iron diffusesoutward the Fe304 formed at theouter interface usually becomes en-trained in the boiler water flow andthen deposits in another region ofthe boiler . The protective nature ofthis layer depends on pH of thewater and on the amount of contami-nants.
Fig. 6, gives the effect of pH onrate of corrosion of steel by water.Magnetite is unstable and soluble atpH <5 and at pH >12.
The significant categories ofwater side corrosion failure mecha-nisms are caustic corrosion , hydro-
gen damage ( already discussed) and
pitting ( localized corrosion ). In all
these cases the amount of corrosionproduct deposited on the tube wall
plays an important role. Caustic
corrosion & hydrogen damage re-
sult from the break down of the pro-
tective Fe304 layer by the concen-
tration of corrosive chemicals within
a wall deposit. Caustic corrosion
develops as the concentration of
NaOH in the deposit builds up toraise the pH. At high pH, Fe304
layer becomes soluble and rapid cor-
rosion takes place.
Caustic corrosion is caused by
the selective deposition of corro-
sion products and NaOH at loca-
tions of high flux. Porous deposits
accumulating in high heat input ar-
eas promote concentration of NaOH
by a process known as wick boiling.
As a result caustic level could go up
from 200 ppm in bulk water to over
200,000 ppm adjacent to the tube
surface.
Susceptibility of caustic corro-
sion can be minimized by restricting
the entry of deposit forming materi-
als and periodic removal of water-
side deposit by chemical cleaning.Careful monitoring and control of
water chemistry is necessary to pre-
vent accumulation of NaOH.
Pitting or localized corrosionresults from oxygen attack or acidicconditions on the internal surface ofthe boiler tubes. This can lead toperforation of the tube wall particu-
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any when a small area becomes an-odic with respect to the rest of thetube surface. This condition candevelop from an exposure of thetubes to water with high acid oroxygen concentrations or at crev-ices.
The oxygen in water reacts withand rapidly removes the hydrogenproduced at the cathode , thus pro-motes cathodic reaction. The oxy-gen will also oxidize the Fe2+ ion toproduce Hematite (Fe203) as thecorrosion product which covers cra-ter like perforations in the tube wall.
Chromium containing steelswhen exposed to 500°C-650°C de-velops spinet type oxides consist-ing of two layers. Their relativethicknesses depend on the Cr con-tent of the steel. In superheater &reheaters the outer layer is invari-ably Fe304 and the inner layer is anFe-Cr spinel type oxide containingthe alloying elements of steel. Suchlayers normally protects further oxi-dation. However there are two waysin which this protection can break-down (i) excessive growth & exfo-liation, ( ii) stress corrosion crack-ing (SCC).
Pitting failure can occur any-
where in the boiler, particularly in
economizer, superheater, reheater
and the non-heated portion of the
water wall tubes. For full protec-
tion against pitting during shut down
it is necessary to keep the boiler
filled with hydrazine treated water
and blanketed or capped with nitro-gen. Oxygen pitting of economizer
can be minimized by proper opera-tion of deaerators and their heaters;
by elimination of air leakage and by
injection of oxygen scavenger.
Superheaters and reheaters un-like economizers and water walltubes are designed for a finite life.This is based on a conservative creepfailure criterion. To produce steamat 540°C or beyond, the tube walltemperature could exceed 600°Cespecially in the last stages of su-perheat and reheat sections. De-pending on the temperature/designtube material could vary from car-bon to low alloy ferritic to austen-itic steels.
For ferritic steel corrosion ini-tially follows a parabolic law. How-
ever depending on temperature at alatter stage it might change over tolinear kinetics . This is associatedwith formation of multiple layers of
oxide. As a result of the difference
in thermal expansion between thescale and the tube exfoliation oc-curs. when oxide exfoliates and
travels into the turbine componentsit may cause serious solid particle
erosion problem.
Austenitic stainless steels be-cause of higher Cr content corrodemore slowly than ferritic steels -under the same steam conditions.However here as well exfoliationcould take place due to either break-down of the inner layer or void for-mation at oxide/oxide interface.Nevertheless in comparison to fer-ritic steel the problem of exfoliationin this case is much less.
Replacement of superheater/reheater rubes with chromate treated
Differential scanning calorim-etry of such deposits indicates melt-ing point in the range 335-410°C.
Role of chlorine of fire side cor-
rosion is rather difficult to define.
However under reducing conditions
HCl or NaCl can render oxide scale
less effective for corrosion protec-
tion either through blister or crack
formation or through reactions with
oxides to form volatile products.
Molten salt related corrosion of
water wall tubes seldom occurs in
oil fired boilers. This is because of
low ash content and absence of chlo-
rine in the oil. Therefore the depos-
its formed in oil fired boiler tubes
are very thin and do not provide
conditions for formation of
pyrosulfates. Moreover the chance
of local overheating due to forma-
tion of deposits is much less. The
metal wall temperature in the water
wall is usually below 400°C which
is lower than the melting points of
any possible compounds between
Na and V oxides.
Problems of fire side corrosion
can be overcome either by changing
the operating conditions or the tube
material. Where corrosion results
from the reducing conditions nearthe tube wall adjustment of burnersand proper fuel & air distributioncan be helpful. An alternate methodcould be to introduce a flow of airalong the walls through openings inthe membrane between water walltubes. This is often referred to as aircurtain. This can be implementedin conjunction with local monitor-ing of the gas composition.
Removal of chemical species
responsible for corrosion from the
fuel is an another alternative to
combat fire side corrosion. Stan-
dard coal washing technique can
remove half of the sulfur and al-
kali metal content from the coal.
However washing generally does
not remove chlorine. Blending of
coal to reduce average content of
corrosive species is an alternative
strategy. However all these would
certainly involve additional fuel
cost.
Material solutions to fire side
corrosion problems involve either
replacement of the tubes with more
corrosion resistant material or by
application of protective coatings.
In cases where tube wall thinning
rate is only slightly greater than
allowable, replacement of tubes
with same material and regular
monitoring of wall thickness is rec-
ommended. For severe wall thin-
ning conditions steel containing
upto 9% Cr exhibit similar perfor-
mance. In this case possibly
thicker plains carbon steel tubes or
composite coextruded tubes with
an outer layer of austenitic stain-
less steel could be used.
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I Typical microstructures of 0.18% C steel boiler tubesthat ruptured as a result of rapid overheating
(a) Elongated grains near tensile rupture resultingfrom rapid overheating below the recrystallizationtemperature. (b) Mixed structure near ruptureresulting from rapid overheating between Acs, andAc3 amd sinsegiemt gienching by escaping water or
steam . Both etched with 2% vital, (250x).
(a) Voids (black ) in grain boundaries andspeheroidization (light, globular), both of which arecharacteristic of tertiary creep, (250x) ( b) Intergranularseparation adjacent to facture surface (top). 50xMottled areas in both specimens are regions wherepearlite has decomposed into ferrite and spheroidalcarbides. Both etched with 2% nital
Fig. 8 : Typical microstructuralchange as a result of rapid over
heating
Micrograph of an etched specimen from a carbonsteel boiler tube. Decarburization and discontinuousintergranular cracking resulted from hydrogendamage ((250x)
Fig. 9 : Typical microstructuralchange as a result of prolonged
overheating below critical temperature
A Window Fracture :Typical results from hydrogendamage in carbon or low-alloy steel boiler tubes
Fig. 10 : Typical microstructural changes andnature of tube rupture as a result of hydrogen damage
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Enrichment of the surface by Alor Cr can also improve corrosionresistance . This can be attained by anumber of methods . Diffusion coat-ing, flame or plasma spraying hasbeen used to apply high Cr, high Al,or Fe-Al-Cr alloy coatings on waterwall tubes. Although such alloysexhibit excellent corrosion resis-tance in laboratory testing , there aremany technical problems concern-ing reproducibility , ease of fabrica-tion and joining are required to betackled before their commercial use.
Concluding Remarks
A short overview of the materialrelated problems of a boiler has beenpresented. Steel being the most com-monly use material our discussionhas been mostly centered aroundsteel. The fact that many of the steelcomponents in a boiler are known tohave survived more than 50 years ofservice life without any appreciable
loss of strength, toughness and sec-tion size goes to show that majorityof the problems could be solved byhaving closer control on the operat-ing condition, the quality of waterand the quality of fuel. Neverthe-less there is ample scope for im-proving the efficiency of boiler byproper choice of steel, better designand possibly by introducing specialmaterial at critical locations. In spiteof the best effort on the part of de-signers, fabricators, and operators,component failures often take place.A proper analysis of these not onlyhelps identify the cause of failurebut also suggests remedial measuresto overcome them. A number ofcase studies concerning failure ofboiler components will be discussedin subsequent lectures. In majorityof the cases corrosion either directlyor indirectly responsible for failure.The points discussed here will pos-sibly help you appreciate that. q
