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SEMINAR

ON

DETECTING

SUSCEPTIBILITY TO

INTERGRANULAR CORROSION

PREPARED BY:

NAME:TRUSHIT K. NASHIKKAR

ROLL NO. :913

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INTERGRANULAR CORROSION

The microstructure ofmetals and alloys is made up ofgrains, separated by grain boundaries.

Intergranular corrosion is localized attack along thegrain boundaries, or immediately adjacent to grainboundaries, while the bulk of the grains remain largelyunaffected.

This form of corrosion is usually associated withchemical segregation effects (impurities have atendency to be enriched at grain boundaries) or specificphases precipitated on the grain boundaries.

Such precipitation can produce zones of reducedcorrosion resistance in the immediate vicinity.

MECHANISM

http://corrosion-doctors.org/MatSelect/corrmetals.htmhttp://corrosion-doctors.org/MatSelect/corralloys.htmhttp://corrosion-doctors.org/MatSelect/corrmetals.htmhttp://corrosion-doctors.org/MatSelect/corralloys.htm

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Microscope view of a polished cross section of a material

attacked by intergranular corrosion

What causes intergranular corrosion?

This type of attack results from local differences incomposition, such as coring commonly encountered inalloy castings.

Grain boundary precipitation, notably chromiumcarbides in stainless steels, is a well recognized andaccepted mechanism of intergranular corrosion.

The precipitation of chromium carbides consumed thealloying element - chromium from a narrow band alongthe grain boundary and this makes the zone anodic tothe unaffected grains.

The chromium depleted zone becomes the preferentialpath for corrosion attack or crack propagation if undertensile stress.

CHROMIUM PROFILE ACROSS GRAIN:

http://en.wikipedia.org/wiki/File:Intergranular_corrosion.JPG

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In nickel alloys and austenitic stainless steels, wherechromium is added for corrosion resistance, themechanism involved is formation ofchromiumcarbide at the grain boundaries, forming chromium-depleted zones (this process is called sensitization).

Around 12% chromium is minimally required to ensurepassivation, mechanism by which a thin invisible layerforms at the surface of stainless steels.

This layer protects the metal from corrosiveenvironments and it is, thus, stainless.

Selective leaching often involve grain boundarydepletion mechanisms.

These zones also act as local galvanic couples,causing local galvanic corrosion.

http://en.wikipedia.org/wiki/Chromium_carbidehttp://en.wikipedia.org/wiki/Chromium_carbidehttp://en.wikipedia.org/wiki/Sensitization_effecthttp://en.wikipedia.org/wiki/Selective_leachinghttp://en.wikipedia.org/wiki/Galvanic_couplehttp://en.wikipedia.org/wiki/Galvanic_corrosionhttp://en.wikipedia.org/wiki/Chromium_carbidehttp://en.wikipedia.org/wiki/Chromium_carbidehttp://en.wikipedia.org/wiki/Sensitization_effecthttp://en.wikipedia.org/wiki/Selective_leachinghttp://en.wikipedia.org/wiki/Galvanic_couplehttp://en.wikipedia.org/wiki/Galvanic_corrosion

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This condition happens when the material is heated totemperature around 700 C for too long time, and oftenhappens during welding or an improperheat treatment.

When zones of such material form due to welding, theresulting corrosion is termed weld decay.

Stainless steels can be stabilized against this behaviorby addition oftitanium, niobium, ortantalum, whichform titanium carbide, niobium carbide and tantalumcarbide preferentially to chromium carbide, by loweringthe content ofcarbon in the steel and in case of welding

also in the filler metal under 0.02%, or by heating theentire part above 1000 C and quenching it in water,leading to dissolution of the chromium carbide in thegrains and then preventing its precipitation.

Another possibility is to keep the welded parts thinenough so that, upon cooling, the metal dissipates heattoo quickly for chromium carbide to precipitate.

Other related kind of intergranular corrosion istermed knifeline attack (KLA).

Knifeline attack impacts steels stabilized by niobium,such as 347 stainless steel.

Titanium, niobium, and theircarbides dissolve in steelat very high temperatures.

At some cooling regimes, niobium carbide does notprecipitate, and the steel then behaves like unstabilizedsteel, forming chromium carbide instead.

http://en.wikipedia.org/wiki/Weldinghttp://en.wikipedia.org/wiki/Heat_treatmenthttp://en.wikipedia.org/wiki/Titaniumhttp://en.wikipedia.org/wiki/Niobiumhttp://en.wikipedia.org/wiki/Tantalumhttp://en.wikipedia.org/wiki/Titanium_carbidehttp://en.wikipedia.org/wiki/Niobium_carbidehttp://en.wikipedia.org/wiki/Tantalum_carbidehttp://en.wikipedia.org/wiki/Tantalum_carbidehttp://en.wikipedia.org/wiki/Carbonhttp://en.wikipedia.org/wiki/Quenchinghttp://en.wikipedia.org/wiki/Carbidehttp://en.wikipedia.org/wiki/Weldinghttp://en.wikipedia.org/wiki/Heat_treatmenthttp://en.wikipedia.org/wiki/Titaniumhttp://en.wikipedia.org/wiki/Niobiumhttp://en.wikipedia.org/wiki/Tantalumhttp://en.wikipedia.org/wiki/Titanium_carbidehttp://en.wikipedia.org/wiki/Niobium_carbidehttp://en.wikipedia.org/wiki/Tantalum_carbidehttp://en.wikipedia.org/wiki/Tantalum_carbidehttp://en.wikipedia.org/wiki/Carbonhttp://en.wikipedia.org/wiki/Quenchinghttp://en.wikipedia.org/wiki/Carbide

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This affects only a thin zone several millimeters wide inthe very vicinity of the weld, making it difficult to spot

and increasing the corrosion speed.

Structures made of such steels have to be heated in awhole to about 1950 F, when the chromium carbidedissolves and niobium carbide forms.

The cooling rate after this treatment is not important, as

the carbon that would otherwise pose risk of formationof chromium carbide is already sequestered as niobiumcarbide.

The photos above show the microstructure of a type304 stainless steel. The figure on the left is thenormalized microstructure and the one on the right is

the "sensitized" structure and is susceptible tointergranular corrosion or intergranular stress corrosioncracking.

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Figure : Intergranular corrosion of a failed aircraftcomponent made of 7075-T6 aluminum

Many aluminum base alloys are susceptible tointergranular corrosion on account of either phases

anodic to aluminum being present along grainboundaries or due to depleted zones of copper adjacentto grain boundaries in copper-containing alloys.

High strength aluminium alloys, especially whenextruded or otherwise subjected to high degree ofworking, can undergo exfoliation corrosion.

Here the corrosion products build up between the flat,elongated grains and separate them, resulting in liftingor leafing effect and often propagating from edges ofthe material through its entire structure.

Intergranular corrosion is a concern especially for alloyswith high content ofcopper.

http://en.wikipedia.org/wiki/Exfoliation_corrosionhttp://en.wikipedia.org/wiki/Copperhttp://en.wikipedia.org/wiki/Exfoliation_corrosionhttp://en.wikipedia.org/wiki/Copper

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Other kinds of alloys can undergo exfoliation as well;the sensitivity ofcupronickel increases together with itsnickel content. A broader term for this class of corrosionis lamellar corrosion.

Alloys ofiron are susceptible to lamellar corrosion, asthe volume ofiron oxides is about seven times higherthan the volume of original metal, leading to formationof internal tensile stresses tearing the material apart.

Similar effect leads to formation of lamellae in stainlesssteels, due to the difference of thermal expansion of theoxides and the metal.

Copper-based alloys become sensitive when depletionof copper content in the grain boundaries occurs.

Anisotropic alloys, where extrusion or heavy workingleads to formation of long, flat grains, are especiallyprone to intergranular corrosion.

Intergranular corrosion induced by environmentalstresses is termed as stress corrosion cracking.

Intergranular corrosion can be detected by ultrasonicand eddy current methods.

SENSITIZATION EFFECT

http://en.wikipedia.org/wiki/Cupronickelhttp://en.wikipedia.org/wiki/Ironhttp://en.wikipedia.org/wiki/Iron_oxidehttp://en.wikipedia.org/wiki/Tensile_stresshttp://en.wikipedia.org/wiki/Anisotropichttp://en.wikipedia.org/wiki/Extrusionhttp://en.wikipedia.org/wiki/Stress_corrosion_crackinghttp://en.wikipedia.org/wiki/Cupronickelhttp://en.wikipedia.org/wiki/Ironhttp://en.wikipedia.org/wiki/Iron_oxidehttp://en.wikipedia.org/wiki/Tensile_stresshttp://en.wikipedia.org/wiki/Anisotropichttp://en.wikipedia.org/wiki/Extrusionhttp://en.wikipedia.org/wiki/Stress_corrosion_cracking

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

Certain alloys when exposed to a temperaturecharacterized as a sensitizing temperature becomeparticularly susceptible tointergranular corrosion.

In a corrosive atmosphere, the grain interfaces of thesesensitized alloys become very reactive andintergranular corrosion results.

This is characterized by a localized attack at anadjacent to grain boundaries with relativelylittle corrosion of the grains themselves.

The alloy disintegrates (grains fall out) and/or loses itsstrength.

Intergranular corrosion is generally considered to becaused by the segregation of impurities at the grainboundaries or by enrichment or depletion of one of thealloying elements in the grain boundary areas.

Thus in certain aluminium alloys, small amountsofiron have been shown to segregate in the grainboundaries and cause intergranular corrosion.

Also, it has been shown that the zinc content ofa brass is higher at the grain boundaries and subject tosuch corrosion.

http://en.wikipedia.org/wiki/Galvanic_corrosionhttp://en.wikipedia.org/wiki/Galvanic_corrosionhttp://en.wikipedia.org/wiki/Microstructurehttp://en.wikipedia.org/wiki/Alloyhttp://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/Galvanic_corrosionhttp://en.wikipedia.org/wiki/Galvanic_corrosionhttp://en.wikipedia.org/wiki/Microstructurehttp://en.wikipedia.org/wiki/Alloyhttp://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/Brass

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High-strength aluminium alloys such as the Duralumin-type alloys (Al-Cu) which depend upon precipitatedphases for strengthening are susceptible tointergranular corrosion following sensitization at

temperatures of about 120C.

Nickel-rich alloys such as Inconel 600 and Incoloy 800show similar susceptibility.

Die-castzinc alloys containing aluminum exhibitintergranular corrosion by steam in a marine

atmosphere.

Cr-Mn and Cr-Mn-Ni steels are also susceptible tointergranular corrosion following sensitization in thetemperature range of 400-850C.

In the case of the austeniticstainless steels, when

these steels are sensitized by being heated in thetemperature range of about 500 to 800C, depletion ofchromium in the grain boundary region occurs, resultingin susceptibility to intergranular corrosion.

Such sensitization of austenitic stainless steels canreadily occur because of temperature servicerequirements, as in steam generators, or as a result of

subsequent welding of the formed structure.

Several methods have been used to control or minimizethe intergranular corrosion of susceptible alloys,particularly of the austenitic stainless steels.

http://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/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/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/Austenitichttp://en.wikipedia.org/wiki/Stainless_steelhttp://en.wikipedia.org/wiki/Steam_generatorhttp://en.wikipedia.org/wiki/Weldinghttp://en.wikipedia.org/wiki/Stainless_steel

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Thus a high-temperature solutionheat 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 largeassemblies, and also ineffective where welding issubsequently used for making repairs or for attachingother structures.

Another control technique for preventing intergranularcorrosion involves incorporating strong carbide formers

or stabilizing elements such as niobium ortitanium inthe stainless steels.

Such elements have a much greater affinityforcarbon than does chromium; carbide formation withthese elements reduces the carbon available in thealloy for formation ofchromium carbides.

Or the stainless steel may initially be reduced in carboncontent below 0.03 percent so that insufficient carbon isprovided for carbide formation.

http://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/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_carbide

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These techniques are expensive and only partiallyeffective since sensitization may occur with time.

The low-carbon steels also frequently exhibit lowerstrengths at high temperatures.

REFERENCES:

1. en.wikipedia.org/wiki/Intergranular_corrosion2. corrosion-doctors.org/Forms-

Intergranular/intergranular.htm3. www.corrosionclinic.com

http://en.wikipedia.org/wiki/Low-carbon_steelhttp://en.wikipedia.org/wiki/Low-carbon_steel