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PREVENTION FROM INTER-GRANUALCORROSION

Sometimes Intergranular corrosion is also called

intercrystalline corrosion or interdendritic corrosion. In thepresence of tensile stress, cracking may occur along grain

boundaries. This type of corrosion is frequently calledinterranular stress corrosion cracking (IGSCC) or intergranular

corrosion cracking.

In most cases of corrosion, the grain boundaries behave inessentially the same way as the grains themselves. The grain

boundaries can undergo marked localized attack while the restof the material remains unaffected. The alloy disintegrates and

loses its mechanical properties.

This type of corrosion is due either to the presence ofimpurities in the boundaries, or to local enrichment ordepletion of one or more alloying elements. For example, small

quantities of iron in aluminum or titanium (iron has a lowsolubility), segregate to the grain boundaries where they can

induce intergranular corrosion. Certain precipitate

phases (Mg5Al8, Mg2Si, MgZn2, MnAl6, etc.) are also known tocause or enhance intergranular attack of high strength

aluminum alloys, particularly in chloride-rich media.

The exfoliation corrosion phenomenon observed in rolledaluminum alloys is usually, but not always, intergranular innature. In this case, the corrosion products occupy a larger

volume than the metal "consumed", generating a high pressureon the slivers of uncorroded metal, leading to the formation of

blisters. Numerous alloy types can undergo intergranular

attack, but the most important practical example is theintergranular corrosion of austenitic stainless steels, related to

chromium depletion in the vicinity of the boundaries, due to theintergranular precipitation of chromium carbides (Cr23C6),

during a "sensitizing" heat treatment or thermal cycle.

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Exfoliation corrosion morphology inside a water pipelineIntergranular or intercrystalline means between grains or

crystals. As the name suggests, this is a form of corrosiveattack that progresses preferentially along interdendritic paths

(the grain bourdaries). Positive identification of this type ofcorrosion usually requires microstructure examination under a

microscopy although sometimes it is visually recognizable as in

the case of weld decay.

The photos show the microstructure of a type 304 stainless

steel. The figure on the left is the normalized microstructureand the one on the right is the "sensitized" structure and is

susceptible to intergranular corrosion or intergranular stress

corrosion cracking.This type of attack results from local differences in

composition, such as coring commonly encountered in alloycastings. Grain boundary precipitation, notably chromium

carbides in stainless steels, is a well recognized and acceptedmechanism of intergranular corrosion. The precipitation of

chromium carbides consumed the alloying element - chromium

from a narrow band along the grain boundary and this makesthe zone anodic to the unaffected grains. The chromium

depleted zone becomes the preferential path for corrosionattack or crack propagation if under tensile stress.

Intergranular Corrosion occurs when a grain boundary area is

preferentially attacked because of the presence of precipitatesin these regions.

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Grain boundaries are preferred sites for SegregationPrecipitation.

Two types of segregates and precipitatesIntermetallics (intermediate constitutes): Formed from metal

atoms having identifiable chemical formulae. Can either beanodic or cathodic to the metal?

Compounds: Formed between metals and the non-metallicelements, H, C, Si, N and O.Fe23C6 and MnS in steel are

cathodic to ferrite.

Any metal in which intermetallics or compounds are present atgrain boundaries will be susceptible to intergranular stress

corrosion cracking. Austenitic stainless steels are mostsusceptible to intergranular corrosion. 18-8 or type 304

stainless steel: Fe, 18%Cr, 8%Ni; When C% < 0.03%, only the

austenite phase is stable. When C% > 0.03% austenite andferrite mixed carbide (FeCr)23C6 are stable.

The proportions of carbide obtained are dependent upon the

rate of cooling

Fast cooling by water/oil quenching from > 1000oC suppressescarbide formation.

If the material is reheated within the range 600-850oC, carbideprecipitation will occur at the grain boundaries.

The material is thus said to be sensitized and is in a dangerous

condition - susceptible to Intergranular corrosion cracking

If the material is reheated below 600oC, the rate of diffusion ofCr is too slow for carbide precipitation to occur.12%Cr + Fe === "stainless" steels

precipitation of carbide(FeCr) 23C6 causes Cr depletion (Cr

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Prevention

Use low carbon (e.g. 304L, 316L) grade of stainless steels.

Lower the C content to below 0.03%, so that the carbides arenot stable.

Use stabilized grades alloyed with titanium (for example type

321) or niobium (for example type 347). Titanium and niobiumare strong carbide- formers. They react with the carbon to form

the corresponding carbides thereby preventing chromiumdepletion.

Use high temperature solution heat treatment to dissolve the

precipitates. (Post welds heat treatment of sensitized steel).Most likely cause of failure was intergranular cracking initiated

by a network of grain boundary precipitates.

CONTROL FOR AUSTENITIC STAINLISS STEELS:

Three methods are used to control or minimize intergranualarcorrosion of the austenitic stainless steels:

Employing high temperature solution heat treatment,

commonly termed quench-annealing or solution quenching.adding elements that are strong carbide formers( calledstabilizers), and

Lowering the carbon content to below 0.03%.

Commercial solution-quenching treatments consist of heating

to 1950 to 2050 F followed by water-quenching. Chromiumcarbide is dissolved at these temperatures, and a more

homogeneous alloy is obtained. Most of the austenitic stainless

steels are supplied in this condition. If welding is used duringfabrication, the equipment must be quench-annealed to

eliminate susceptibility to weld decay. This poses an expensiveproblem for large equipment and, in fact, furnaces are not

available for heat-treating very large vessels. In addition,

welding is sometimes necessary in the customers plant tomake repairs or, for example, to attach a nozzle to a vessel.

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Quenching or rapid cooling from the solution temperature isvery important. If cooling is slow, the entire structure would be

susceptible to intergranual corrosion.

The strong carbide formers or stabilizing elements, columbium

(or columbium plus tantalum) and titanium, are used toproduce types 347 & 321 stainless steels, respectively. These

elements have a much grater affinity for carbon than doeschromium and are added in sufficient quantity to combine with

all of the carbon in steel. The stabilized steels eliminate the

economic and other objections of solution- quenching theunstabilized steels after fabrication or weld repair.

Lowering the carbon to below 0.03% (type 304L) does not

permit sufficient carbide to form to cause intrgranualar attackin most applications. One producer calls these the extra- low-

carbon (ELC) steels. The situation is same as above except that

here weld decay is absent in the low carbon plate. The verticaltrenches are due to a weld bead deposited on the back surface

of the specimen.

The original 18-8 steels contained around 0.20% carbon, but

this was quickly reduced to 0.08% because of rapid and seriousweld-decay failures. Lowering the carbon content much below

0.08% was not possible until it was discovered that it was

possible to below oxygen through the melt to burn out carbonand until low-carbon ferrochrome was developed.

These stainless steels have a high solubility for carbon when in

the molten state and therefore have a tremendous propensityfor picking up carbon. For example, the intent of the low-

carbon grades is obviated when the welder carefully cleans the

beveled plate with an oily or greasy rag before welding!

A few isolated carbides that may appear in type 304L are not

destructive for many applications in which a continuous

network of carbides would be catastrophic. In fact, thesusceptibility to intergranualar corrosion of the austeniticstainless steels can be reduced by severely cold working the

alloy. Cold-working produces smaller grains and many slip

lines, which provide a much larger surface for carbideprecipitation. This is not, however a recommended or practical

procedure.

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SENSITIZATION

Sensitization of metals involves the creation ofgalvaniccorrosion cells within the microstructure of an alloy.

Certain alloys when exposed to a temperature characterized as

a sensitizing temperature become particularly susceptibleto intergranular corrosion. In a corrosive atmosphere, the grain

interfaces of these sensitized alloys become very reactive andintergranular corrosion results. This is characterized by a

localized attack at an adjacent to grain boundaries withrelatively little corrosion of the grains themselves. The alloy

disintegrates (grains fall out) and/or loses its strength.

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 thegrain boundary areas. Thus in certain aluminium alloys, small

amounts ofiron have been shown to segregate in the grainboundaries and cause intergranular corrosion. Also, it has been

shown that the zinc content of a brass is higher at the grainboundaries and subject to such corrosion.

High-strength aluminium alloys such as the Duralumin-typealloys (Al-Cu) which depend upon precipitated phases for

strengthening are susceptible to intergranular corrosionfollowing sensitization at temperatures of about 120C. Nickel-

rich alloys such as Inconel 600 and Incoloy 800 show similarsusceptibility. Die-castzinc alloys containing aluminum exhibitintergranular corrosion by steam in a marine atmosphere. Cr-

Mn and Cr-Mn-Ni steels are also susceptible to intergranularcorrosion following sensitization in the temperature range of

400-850C. In the case of the austeniticstainless steels, when

these steels are sensitized by being heated in the temperaturerange of about 500 to 800C, depletion of chromium in the

grain boundary region occurs, resulting in susceptibility tointergranular corrosion. Such sensitization of austenitic

stainless steels can readily occur because of temperatureservice requirements, as in steam generators, or as a result ofsubsequent welding of the formed structure.

Several methods have been used to control or minimize the

intergranular corrosion of susceptible alloys, particularly ofthe austenitic stainless steels. Thus a high-temperature

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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 forattaching other structures.

Another control technique for preventing intergranular

corrosion involves incorporating strong carbide formers or

stabilizing elements such asniobium or titanium in the stainlesssteels. Such elements have a much greater affinity

for carbon than does chromium; carbide formation with theseelements reduces the carbon available in the alloy for

formation ofchromium carbides. Such a stabilized titanium-

bearing austenitic chromium-nickel-copper stainless steel isshown in U.S. Pat. No. 3,562,781. Or the stainless steel may

initially be reduced in carbon content below 0.03 percent sothat insufficient carbon is provided for carbide formation.

These techniques are expensive and only partially effective

since sensitization may occur with time. The low-carbonsteels also frequently exhibit lower strengths at high

temperatures.

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