Inter Granular Corrosion

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GAZIANTEP UNIVERSITY

ENGINEERING FACULTY

INTERGRANULAR CORROSION

COURSE: ME470 CORROSION AND SURFACETREATMENT

PREPARED BY:ERSN AYDINSTUDENT NUMBER:200744142

GIVEN TO:ASST. PROF. DR. NECP FAZIL YILMAZ

2012


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CHAPTER 1

INTRODUCTION

The purpose of this report is to present the facts and the theory concerning the nature,

detection, and prevention of intergranular corrosion in stainless steels. Stainless steels are

common material of construction as these offer a wide range of corrosion resistance along

with good fabrication and mechanical properties to many industrial environments e.g.chemical, nuclear etc. At the same time, these alloys are prone to microstructural changes

along with changes in chemistry when exposed to sensitization temperatures possibly due to

faulty heat treatment or welding operation. In certain environments, sensitized steels corrode

preferentially along the grain boundary leaving the matrix unattacked, termed as intergranular

corrosion (IGC). This causes frequent catastrophic failure of engineering components. These

sensitized stainless steels when subjected under load in certain corrosive atmospheres,

experience crack propagation through the boundary leading to even faster failures.


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CHAPTER 2

CORROSION

Corrosion can be defined as the destructive attack of a metal by a chemical or electrochemical

reaction with its environment. The air oxidation of hot steel forming an iron oxide coating is

an example of chemical attack. Corrosion of this type seldom occurs in the oil field. The

second type, electrochemical corrosion, occurs at a solid-liquid interface. This occurs innearly every instance where oil field waters contact steel equipment. The corrosion commonly

occurring in water handling systems consists mainly of acid gas attack. Acid gas corrosion is

caused, for the most part, by H2S. No doubt CO2 causes some corrosion, especially when

aided by dissolved oxygen. However, the comparative effect of CO2 has been grossly over-

emphasized.

In order to have electrochemical corrosion, the following four factors are necessary:

-anode

-cathode

-electrolyte

-external connection.

Remove any of these and corrosion will cease. This is the principle of corrosion mitigation; to

remove one of the four components necessary for the corrosion reaction. The anode and

cathode are called electrodes. The anode is the electrode at which oxidation occurs or where

electricity leaves the metal and enters the electrolyte. For iron, the anode reaction is shown in

equation (1), where iron is oxidized to the ferrous ion.

Fe = Fe++ + 2e-

H+ = H2 e-


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Reduction occurs at the cathode. An example is the reduction of hydrogen ion, shown in

equation (2).

The electrolyte of interest here is water. Its function is not only to carry corrosive materials to

the surface but also to provide a medium of electron transfer utilizing ions. Any connection,

excluding the electrolyte, between the anode and the cathode is an external connection.

Anodes and cathodes both exist on steel surfaces. The body of the steel serves as an external

connection. The severity of corrosion depends upon the potential generated between the anode

and cathode.

A good example of corrosion is a dry cell. The center carbon electrode is the cathode. A zinc

case serves as anode. These electrodes are separated by an electrolyte that is essentially

ammonium chloride solution. When an external connection in the form of a flashlight bulb is

attached, corrosion occurs as zinc goes into solution, and the bulb glows. The greater the flow

of electricity through the cell, the greater is the amount of zinc that corrodes. The relationship

between the amount of zinc corroding and the flow of current is quantitative.


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CHAPTER 3

INTERGRANULAR CORROSION

3.1 Definition of Intergranular Corrosion

Intergranular corrosion is a form of localized surface attack in which a narrow path is

corroded along the grain boundaries of a metal. No corrosion is found in the areas away from

the grain boundaries. It may initiate on the surface and then proceed by local cell action in the

immediate vicinity of a grain boundary. Depletion of chromium in the grain boundary regions

results in intergranular corrosion of stainless steels. This form of corrosion is mainly

associated with high temperatures resulting from heat treatments. Heat treatments experienced

during welding can cause such high temperatures. The corrosion can occur besides the weld

area, especially for materials with high carbon content.

Corrosion technology, therefore suspects that the attack which is observed along the weld in

the failed pipe material is as a result of poor heat treatment during welding. This is justified

considering the fact that the corroding section of the pipe is the material with higher carbon

content. Chromium is added to steels to improve their corrosion resistance. Stainless steels

contain at least 10 % chromium. The chromium rich oxide film ( Cr23C6 ) is thin, adherent and

very protective. But if heated into range 510 790 0C, the steels sensitize and become

prone to inter-granular corrosion.

During welding, material temperature reaches to around 9500

to 14500

F leading to reaction

between chromium and carbon to form chromium carbides, which deplete the chromium at

the weld interface. This process, which renders the material susceptible to corrosion as a

result of the depletion of the chromium in the pipe material, is known as sensitization.

Sensitization involves the precipitation of chromium carbide ( Cr23C6 ) at the grain boundaries

due to high temperatures. Steels with more than 0.02 % of carbon content are more prone to

this effect as seen in the 316 grade. The carbon diffuses readily and the disorder in the


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boundaries provides nucleation sites. This depletes the grain boundaries of chromium

resulting in metals with low corrosion resistance.

The grain boundaries and the weld heat affected zone when exposed to a corrosive

environment results to a preferential attack, often generally referred to as known as weld

decay.

Also, the rough nature of the weld between the two metals suggests spoor heat treatment

during the offshore welding when compared to the smooth welding appearance at the far end

of the corroding section. Corrosion technology, therefore, believes that welding was not

followed by proper post heat treatment. Normally, after welding operations, the equipment is

quench-annealed to eliminate susceptibility to weld decay. This involves heating to

temperatures between 1066 and 11210C, followed by rapid water cooling to ensure that the

precipitated chromium carbide dissolves to maintain a more homogeneous alloy. This is a

very important process but corrosion technology suspects that the welder must have avoided

this due to unknown reasons.

It is a type of deterioration which occurs after the corrosion occurred near the grain

boundaries of material. Intergranular corrosion is especially observed on austenitic chromium

nickel steels and aluminum copper alloys. Ferritic stainless steels have a sensation against to

this type of corrosion even under very limited circumstances. The most distinct property of

intergranular corrosion is that corrosion speed reachs to the highest values near grain

boundaries inspite of loss of very small weight. This situation causes decomposition of the

materials throughout the cross-sectional area by exposed to corrosion. While particles keep

their shape and compactness, they expose to decomposition between grains. As a result of this

we need to expect some important changes on the behaviors of metals. The most important

one is that the mechanic strength is reduced to zero where corrosion occurs. For example, it is

possible to get a material from chromium nickel steel to powder form by crushing with hands

which is exposed to intergranular corrosion. There is no change on the outside appearance and

dimensions of the materials. These conditions make hard to observe and control of

intergranular corrosion.


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3.2 Mechanism of Intergranular Corrosion

There are three mechanisms that have been identified as causing intergranular corrosion in

various situations.

1) The first mechanism is the selective attack of grain boundary material due to its high

energy content. Metal crystals form in an ordered arrangement of atoms because this ordered

arrangement has a lower energy content than a disordered arrangement. Grain boundaries are

highly disordered as they are at the boundaries of crystals which, although they are internally

ordered, have random orientation with respect to each other. The disordered grain boundary is

often 10 to 100 atoms wide and these atoms have a higher energy than the surrounding atoms.

Higher energy material can be more chemically active than lower energy material and thus,the grain boundary material can be anodic with respect to the surrounding grains. When this

occurs, the anodic area is small and the cathodic area is large, thus, rapid attack can occur.

The result is that the individual grains are no longer joined with the strong grain boundary

glue and disintegrate leaving a powdery residue and rough grainy surface.

2) A second mechanism is selective attack of grain boundary material that has a different

composition from the surrounding grains. When metals crystallize from the molten state, the

crystals tend to be more pure than the molten material. This is because the pure metal crystals

are more ordered and have a lower energy content than if they contained large amounts of

impurities. In some cases, most of the impurities are concentrated at the grain boundaries.

When the composition of this impure material causes it to be more anodic than the

surrounding grains, rapid attack can occur with results similar to those described above. When

the composition of the impure grain boundary material causes it to be more cathodic than the

surrounding grains, the favorable anode/cathode area ratio makes this situation relatively

innocuous. Contamination of grain boundaries can sometimes also occur after manufacture.

Mercury on aluminium can penetrate and contaminate the grain boundaries and cause

subsequent intergranular attack. This is why mercury and mercury compounds are prohibited

aboard aluminium ships or on aircraft.

3) A third mechanism is selective attack adjacent to the grain boundaries due to the local

depletion of an alloying element. This form of attack can occur in many stainless steels. It is

called sensitization. Many stainless steels rely on a combination of nickel and chromium for


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their corrosion resistance. As both nickel and chromium are expensive, they are added only in

amounts necessary to obtain the necessary corrosion resistance. Another element, which is

commonly present in ail steels, is carbon. In stainless steels, carbon atoms tend to concentrate

at the grain boundaries as an impurity during solidification. Chromium carbides can form

adjacent to the grain boundaries during welding and heat treatment. When these compounds

form, the chromium is removed from the alloy adjacent to the grain boundaries and the

resulting alloy does not have enough chromium content to remain passive. Again, there is a

very unfavourable anode/cathode area ratio and rapid attack can occur. Three different

methods are used to avoid this type of attack in stainless steels during welding or other

heating.

a. The first method to avoid sensitization is through heat treatment. At high temperatures

(above 1,800F), chromium carbides are unstable and will redissolve if they have formed. At

low temperatures, (below 1,000F) the chromium and carbon atoms cannot move and

formation of chromium carbides is prevented. Formation of the chromium carbides is a

problem primarily in the ranges of 1,100 to 1,600F. When welding stainless steel, some area

adjacent to the weld is likely to reach this temperature range long enough to form amounts of

chromium carbides. When this occurs, or when the alloy is otherwise sensitized, it should be

heated to temperatures above 1,800F to redissolve the carbides, then rapidly cooled to below

1,000F to avoid carbide formation.

b. The second method used to avoid sensitization in stainless steels is to reduce the carbon

content of the alloy to very low levels. These low carbon grades (such as 304 L and 316 L; L

stands for low carbon) do not have enough carbon to form carbides and is thus resistant to

sensitization during welding. Care must be taken, however, to not introduce additional carbon

during welding from contamination, such as can be caused by oil or grease.

c. The third method used to avoid sensitization in the stainless steels is to intentionally add an

element that will combine with the carbon but is not required for passivity of the alloy.

Titanium and niobium have a greater affinity for carbon than chromium. They are added to

the alloy during manufacture in amounts to combine with all of the carbon present in the alloy

and thus inhibit sensitization. Type 321 stainless steel contains titanium and Type 347

stainless steel contains niobium. These alloys, or the low carbon grades, should be used whenwelding without heat treatment is required.


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The older weight loss, electrical conductivity, and microscopic tests are primarily useful in the

laboratory. The electropolishing test gives every promise of being a positive, rapid, and

sensitive test that will find wide application in field testing equipment, particularly in the

vicinity of welds.

3.4 Sensitization

Sensitization refers to the precipitation of carbides at grain boundaries in a stainless steel or

alloy, causing the steel or alloy to be susceptible to intergranular corrosion or intergranular

stress corrosion cracking.

Certain alloys when exposed to a temperature characterized as a sensitizing temperature

become particularly susceptible to intergranular corrosion. In a corrosive atmosphere, the

grain interfaces of these sensitized alloys become very reactive and intergranular corrosion

results. This is characterized by a localized attack at an adjacent to grain boundaries with

relatively little corrosion of the grains themselves. The alloy disintegrates (grains fall out)

and/or loses its strength.

The photos show the typical microstructure of a normalized (unsensitized) type 304 stainlesssteel and a heavily sensitized steel. The dark lines in the sensitized microstructure are

networks of chromium carbides precipitated along the grain boundaries.

Intergranular corrosion is generally considered to be caused by the segregation of impurities

at the grain boundaries or by enrichment or depletion of one of the alloying elements in the

grain boundary areas. Thus in certain aluminum alloys, small amounts ofiron have been

shown to segregate in the grain boundaries and cause intergranular corrosion. Also, it has

been shown that the zinc content of a brass is higher at the grain boundaries and subject to

such corrosion. High-strength aluminum alloys such as the Duralumin-type alloys (Al-Cu)

which depend upon precipitated phases for strengthening are susceptible to intergranular

corrosion following sensitization at temperatures of about 120C. Nickel-rich alloys such

as Inconel 600 and Incoloy 800 show similar susceptibility. Die-cast zinc alloys containing

aluminum exhibit intergranular corrosion by steam in a marine atmosphere. Cr-Mn and Cr-

Mn-Ni steels are also susceptible to intergranular corrosion following sensitization in the
http://www.corrosionclinic.com/types_of_corrosion/intergranular_corrosion_cracking.htmhttp://www.corrosionclinic.com/types_of_corrosion/intergranular_corrosion_cracking.htmhttp://en.wikipedia.org/wiki/Grain_boundaryhttp://en.wikipedia.org/wiki/Corrosionhttp://en.wikipedia.org/wiki/Aluminium_alloyhttp://en.wikipedia.org/wiki/Ironhttp://en.wikipedia.org/wiki/Zinchttp://en.wikipedia.org/wiki/Brasshttp://en.wikipedia.org/wiki/Aluminiumhttp://en.wikipedia.org/wiki/Duraluminhttp://en.wikipedia.org/wiki/Nickelhttp://en.wikipedia.org/wiki/Inconelhttp://en.wikipedia.org/wiki/Incoloyhttp://en.wikipedia.org/wiki/Die_castinghttp://en.wikipedia.org/wiki/Zinchttp://en.wikipedia.org/wiki/Steamhttp://en.wikipedia.org/wiki/Steelshttp://en.wikipedia.org/wiki/Steelshttp://en.wikipedia.org/wiki/Steamhttp://en.wikipedia.org/wiki/Zinchttp://en.wikipedia.org/wiki/Die_castinghttp://en.wikipedia.org/wiki/Incoloyhttp://en.wikipedia.org/wiki/Inconelhttp://en.wikipedia.org/wiki/Nickelhttp://en.wikipedia.org/wiki/Duraluminhttp://en.wikipedia.org/wiki/Aluminiumhttp://en.wikipedia.org/wiki/Brasshttp://en.wikipedia.org/wiki/Zinchttp://en.wikipedia.org/wiki/Ironhttp://en.wikipedia.org/wiki/Aluminium_alloyhttp://en.wikipedia.org/wiki/Corrosionhttp://en.wikipedia.org/wiki/Grain_boundaryhttp://www.corrosionclinic.com/types_of_corrosion/intergranular_corrosion_cracking.htmhttp://www.corrosionclinic.com/types_of_corrosion/intergranular_corrosion_cracking.htm


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temperature range of 400-850C. In the case of the austenitic stainless steels, when these

steels are sensitized by being heated in the temperature range of about 500 to 800C,

depletion of chromium in the grain boundary region occurs, resulting in susceptibility to

intergranular corrosion. Such sensitization of austenitic stainless steels can readily occur

because of temperature service requirements, as in steam generators, or as a result of

subsequent welding of the formed structure.

Several methods have been used to control or minimize the intergranular corrosion of

susceptible alloys, particularly of the austenitic stainless steels. For example, a high-

temperature solution heat treatment, commonly termed solution-annealing, quench-annealing

or solution-quenching, has been used. The alloy is heated to a temperature of about 1,060 to1,120C and then water quenched. This method is generally unsuitable for treating large

assemblies, and also ineffective where welding is subsequently used for making repairs or for

attaching other structures.

Another control technique for preventing intergranular corrosion involves incorporating

strong carbide formers or stabilizing elements such as niobium or titanium in the stainless

steels. Such elements have a much greater affinity for carbon than does chromium; carbide

formation with these elements reduces the carbon available in the alloy for formation

ofchromium carbides. Such a stabilized titanium-bearing austenitic chromium-nickel-copper

stainless steel is shown in U.S. Pat. No. 3,562,781. Or the stainless steel may initially be

reduced in carbon content below 0.03 percent so that insufficient carbon is provided for

carbide formation. These techniques are expensive and only partially effective since

sensitization may occur with time. The low-carbon steels also frequently exhibit lower

strengths at high temperatures.
http://en.wikipedia.org/wiki/Austenitichttp://en.wikipedia.org/wiki/Stainless_steelhttp://en.wikipedia.org/wiki/Steam_generatorhttp://en.wikipedia.org/wiki/Weldinghttp://en.wikipedia.org/wiki/Stainless_steelhttp://en.wikipedia.org/wiki/Heat_treatmenthttp://en.wikipedia.org/wiki/Annealing_(metallurgy)http://en.wikipedia.org/wiki/Quenchhttp://en.wikipedia.org/wiki/Carbidehttp://en.wikipedia.org/wiki/Niobiumhttp://en.wikipedia.org/wiki/Titaniumhttp://en.wikipedia.org/wiki/Carbonhttp://en.wikipedia.org/wiki/Chromiumhttp://en.wikipedia.org/wiki/Chromium_carbidehttp://en.wikipedia.org/wiki/Low-carbon_steelhttp://en.wikipedia.org/wiki/Low-carbon_steelhttp://en.wikipedia.org/wiki/Chromium_carbidehttp://en.wikipedia.org/wiki/Chromiumhttp://en.wikipedia.org/wiki/Carbonhttp://en.wikipedia.org/wiki/Titaniumhttp://en.wikipedia.org/wiki/Niobiumhttp://en.wikipedia.org/wiki/Carbidehttp://en.wikipedia.org/wiki/Quenchhttp://en.wikipedia.org/wiki/Annealing_(metallurgy)http://en.wikipedia.org/wiki/Heat_treatmenthttp://en.wikipedia.org/wiki/Stainless_steelhttp://en.wikipedia.org/wiki/Weldinghttp://en.wikipedia.org/wiki/Steam_generatorhttp://en.wikipedia.org/wiki/Stainless_steelhttp://en.wikipedia.org/wiki/Austenitic


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3.5 Weld Decay

Any stainless steel contains 13% or higher chromium. Because of the large amount of

chromium, stainless steels are kept free from corrosion due to the chromium oxide forming a

rigid membrane on their surfaces when subjected to such corrosive media as air or oxidizing

acids (e.g. nitric acid). Austenitic stainless steel contains (in addition to chromium) nickel,

molybdenum, and copper to provide the corrosion resistance against nonoxidizing acids (such

as hydrochloric and sulfuric acid) and reducing acids (such as saline solution and sulfurous

acid).

The typical austenitic stainless steel, Type 304 (18%Cr-8%Ni), is used for a wide range of

applications due to excellent mechanical properties, workability, weldability, in addition tosuperior corrosion resistance. However, the weld heat-affected zone of Type 304 may be

attacked by selective corrosion, when it is exposed to a severe corrosive environment. The

attack is called "weld decay," which is caused by intergranular corrosion. Fig. 1 shows weld

decay that occurred on both sides of the seam weld of a 304 pipe of a hot dilute nitric process.

Figure 1: Weld decay occurring on both sides of a 304-pipe weld for a hot dilute nitric process

line


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Weld areas are heated at high temperatures in arc welding. Figure 2 shows the temperature

distribution and the heat-affected zone in a weld.

Figure 2: Temperature distribution and the heat-affected zone in a 304 stainless steel weld

In the carbide precipitation zone (as shown in figure 2) chromium combines with carbon and

precipitates chromium carbides at the grain boundaries, depleting the corrosion-resistible,

uncombined chromium at or adjacent to the grain boundaries. This phenomenon is called

sensitization because the areas along the grain boundaries became sensitive to corrosion. In

order to control the sensitization of the heat-affected zone, use


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What causes knife-line attack ?

For stabilized stainless steels and alloys, carbon is bonded with stabilizers (Ti or Nb) and no

weld decay occurs in the heat affected zone during welding. In the event of a subsequent heat

treatment or welding, however, precipitation of chromium carbide is possible and this leaves

the narrow band adjacent to the fusion line susceptible to intergranular corrosion.

3.7 Methods of Prevention of Intergranular Corrosion

The chromium carbide theory assumes that the precipitation of chromium rich carbides at the

grain boundaries depletes the adjacent areas of chromium and thus opens up a vulnerablo

pathway for corrosion in the vicinity of the grain boundaries. The obvious methods of

preventing intergranular corrosion, then, are three:(1) maintain the carbon in solution by

proper heat treatment; (2) reduce the amount of carbon available for combination with

chromium to the point where no chromium carbide will precipitate; (3) control the chromium

carbide precipitation in such a manner as not to leave a continuous , chromium-depleted

network.

Welding, burning, riveting, high temperature service, and occasional unpredictable heatings in

construction and repair involve holding the 18-8 in the sensitization range for varying lengths

of time. Under such conditions, the carbides will precipitate, sensitizing the metal to

intergranular attack. It should be emphasized that there are a great number of applications in

which the possibility of reheating 18-8 to the sensitization range during service or repairs is

negligible. When reheating is not a factor, it is but necessary to insure that 18-8 has been

properly quenched, retaining the carbides in solution.

18-8 is quenched not only to keep the carbides in solution but also, and primarily, to produce

a homogeneous austenitic structure. The temperatures employed may be inferred. Kinzel and

Franks state that homogenization in the 1000-12000C range followed by rapid cooling in

water, oil, or even air retains the carbides in solution and producer fully softened,

homogeneous austenite. It is general practice to keep the finishing temperature for hot work

above 9000C, preferably above lOOO

0C, and it is mandatory to quench to the fully softened

structure with carbides in solution before pickling.
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An increase in the temperature from which quenched improves the ductility, lowers the tensile

strength, and increases the grain size. Bain, Abom and Rutherford have shown that an

increase in grain size markedly increases the severity of intergranular attack particularly for

short periods of time in the sensitization range. Quenching temperatures higher than 12000C

are to be avoided and, in general, the lowest temperature in the 1000-12000C range which

gives complete homogenization in a reasonable time is to be preferred.

If it is feasible to follow any reheating operation such as welding with an anneal of the full

section, the carbides precipitated during the welding operation may be redissolved and once

more retained in homogeneous austenite upon quenching to room temperature.

A note of caution should be added at this point. Quenching does not produce uniform cooling

throughout a section. The interior cools much slower than the surface. The thicker the section,

the greater is the difference in surface and interior cooling rates. Thus with sections of some

thickness it is quite probable that no possible quenching operation will cool the center portion

fast enough to prevent precipitation of the carbides. Ordinarily it is the outside surface which

is expected to resist corrosion, but the possibility of drilling, tapping, punching, and

machining operations exposing interior, sensitized metal to the corroding medium cannot be

overlooked.

If there is a distinct probability of heating 18-8 to temperatures in the sensitizing range during

service or repair, and such heating, cannot be followed by an annealing treatment,

intergranular corrosion can still be prevented by reducing the effective carbon content to a

point where no chromium carbides will precipitate on cooling.

It is possible to produce an 18-8 steel with less than 0.02% carbon (the solubility limit 1.14) ,

and such a steel is not susceptible to intergranular corrosion, even when heated for long

periods in the sensitizing range. Becket and Franks found very low carbon 18-8 to be

noticeably magnetic, however, indicating that it was no longer homogeneous austenite. The

difficulties encountered in producing an 18-8 steel with less than 0.02% carbon increase the

cost sharply. Rather than reduce the carbon content all the way down to 0.02%, it becomes

more economical to add an alloying element which is a stronger carbide former than

chromium, thus effectively accomplishing the same result.


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Either titanium or columbium as alloy additions are eminently successful in preventing,

intergranular corrosion in 18-8. The carbon content is held to as low a value as practical,

0.06% to 0.1% in order to reduce the amount of alloy addition. The amount of titanium

required is 6 times the carbon content in excess of 0.02%, whereas the amount of columbium

required is 10 times the total carbon content. Titanium and columbium both are stronger

carbide formers than chromium, and both, therefore, form a more stable carbide.

The function of the carbide forming alloy additions in preventing intergranular corrosion is to

prevent chromium depletion by replacing chromium in the chromium rich carbides. Of

necessity, the alloy additions must be given an opportunity to replace the chromium in order

to be effective. This is accomplished by a "stabilizing" heat treatment at 850-9000C for 2 to 4

hours. At these temperatures, even though the chromium carbide forms first, diffusion of

titanium or columbium is sufficient to rapidly displace the chromium from the carbide.

It is well to remember that if a titanium stabilized alloy is subsequently heated above 900C,

as in welding, part of the stable titanium carbide goes into solution and on cooling some

chromium carbide precipitates, producing; slight sensitization. Even the slight sensitization

thus produced is often noticeable in titanium stabilized 18-8 welded plate. Columbium carbide

does not appear to dissolve in the area of the weld to the extent necessary to produce

subsequent sensitization on cooling. Columbium stabilized 18-8 has a slight advantage over

the titanium stabilized alloy in welded construction.

In the process of welding, molten metal from the rod is deposited on the cooler plates to be

joined. There is a high loss of titanium through oxidation if titanium stabilized rod is used.

Columbium stabilized rod gives very satisfactory results, and is universally preferred.

Titanium stabilized plate can be successfully welded with columbium stabilized rod, but the

slight sensitization resulting from partial solution of the titanium carbide in the heated area

near the weld may be objectionable. Columbium, too, has its disadvantages; for over 1%

columbium decreases the ductility and toughness, but this is not a serious limitation provided

carbon is held below 0.1%.

Other alloy additions have been suggested, such as tantalum, vanadium, zirconium, and

uranium to perform the same function as titanium and columbium, but large amounts arerequired and no serious attempts have been made to develop such additions for general use.


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It is interesting to note that molybdenum, while markedly increasing the resistance of 18-8 to

corrosion and particularly to pitting, reduces but does not eliminate intergranular corrosion.

According to Franks, Binder, and Bishop, columbium in the amount of 10 times the carbon

content is required, as in ordinary 18-8, to provide immunity in the molybdenum alloy. Such

an 18-8 alloy with both Mo and Cb has a wide range of applications under the severest

corrosion conditions, but would be quite expensive.

It is possible to make the excess carbide in 18-8 precipitate in such a manner as not to leave a

continuous chromium depleted pathway for corrosion. Many investigators have shown the

ordinary 18-8 can be immunized against intergranular corrosion by cold working fully

annealed stock 50 to 75 % and reheating in the 760- 8500C range for a few hours. Cold

working 18-8 introduces a great number of slip planes where the carbide can and will

precipitate, thus producing a general precipitation and reducing the amount of carbide

precipitation at the grain boundaries. When heated to the higher temperatures where

appreciable chromium diffusion occurs, the time for recovery is drastically reduced since

much less grain boundary, chromium-depletion has occurred.

It is also possible to introduce a small amount of ferrite and have the carbides precipitate in

the ferrite primarily, instead of at the grain boundaries. Payson took excellent

photomicrographs showing the concentration of carbide precipitates in the ferrite. The

presence of stable ferrite along with austenite may be induced by alloy additions, increasing

the chromium content, and, according to Uhlig, by control of the nitrogen content. Payson

lists the following elements in varying amounts as promoting the tendency for stable ferrite,

tungsten, molybdenum, vanadium, silicon, chromium (in amounts greater than 18%), and

titanium. He attributed the prevention of intergranular corrosion in titanium stabilized 18-8 in

part, at least, to its stable ferrite forming tendencies.

Molybdenum was first added to 18-8 to reduce intergranular corrosion through the formation

of small amounts of ferrite. It was found so effective in reducing pitting and general corrosion

that the present tendency is to keep molybdenum around 2%, increase the nickel content, and

prevent the formation of ferrite, thus restricting the effect of molybdenum to decreasing


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pitting and improving general corrosion resistance. Then if resistance to intergranular

corrosion is required, a columbium addition can be made.

Payson and Krivobok both attempted to induce small amounts of ferrite with the hope that the

carbides precipitating primarily in the ferrite would not precipitate at the grain boundaries in

sufficient quantities to deplete the chromium content on short time heating and cooling: cycles

as in welding. Neither Payson's alloy additions nor Krivobok's variations of the Cr-Ni ratio to

produce small amounts of ferrite led to a sound solution, but they did lay the groundwork for

the development of an interesting alloy developed by Scherer.

Scherer investigated the effect of increasing the chromium content of 18-8. He found, as

Payson had, that with 22-23% chromium, instead of homogeneous austenite there was 10-

20% ferrite present. He also found, as Payson, Krivobok, and other investigators had not, that

on heating at 6000C. for a few hours longer than necessary to produce sensitization, the high

chromium alloy recovered its immunity to intergranular corrosion.

Furthermore, many reheats above 900C, and subsequent cooling did not reestablish a

sensitized condition as would be expected. Scherers results were confirmed and amplified by

Hougardy, and Tofaute and Schottky. It is interesting to note that Tofaute and Sohottky

replaced a portion of the nickel content with nitrogen as sevaral other investigators have done.

The primary objection to the high chromium and other 18-8 alloy modifications with 10-20%

a table ferrite is a decided tendency to hot shortness.

The high chromium, partially ferritic, stainless alloy developed by Schererhas possible

application in welded constriction where the presence of 15% ferrite does not produce

undesirable mechanical properties, and where the ferrite does not lower the corrosion resistant

to the environment to be met in service. In general, however, titanium and columbium

stabilized alloys are to be preferred unless, as seems unlikely, there develops a considerable

price differential in favor of the high chromium steel.


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CHAPTER 4

CONCLUSION

The purpose of this report is investigation of intergranular corrosion. Intergranular attack

caused by high grain boundary energies or impurities at the grain boundaries results in attackwith a grainy residue and rough surface. Under high magnification, the individual grains are

often visible. Intergranular attack of aluminum alloys is associated with pitting or other

localized attack. Sensitization in stainless steels has a similar grainy appearance. When caused

by welding it is often localized in narrow bands adjacent to the weld and is sometimes called

knife line attack..

As a result, investigation of how it is occurring, how we can prevent of intergranular

corrosion is presented in this report.


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APPENDIX

Examples for intergranular corrosion


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Inter Granular Corrosion

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