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DETECTING SUSCEPTIBILITY

TO

INTERGRANULAR CORROSION



SEMINAR ON:

DETECTING SUSCEPTIBILITY

TO


NAME: NASHIKKAR TRUSHIT K.

ROLL NO. : 913

B.E. – III

METALLURGICAL AND MATERIALS ENGINEERINGFACULTY OF TECHNOLOGY AND ENGINEERING

M. S. UNIVERSITY

VADODARA

NOVEMBER 2011

PREPARED BY:



LAYOUT

INTRODUCTION TO INTERGRANULAR CORROSION

MECHANISM

KNIFELINE ATTACK

EXFOLIATION CORROSION

LAMELLAR CORROSION

SENSITIZATION EFFECT

REFERENCES



INTRODUCTION


MICROSTRUCTURE OF METALS AND ALLOYS:

DEFINITION:

-Intergranular corrosion is a localized attack along the grain boundariesor immediately adjacent to the grain boundaries, while bulk of thegrains remain largely unaffected.

-It is associated with chemical segregation effects or specific phasesprecipitated along the grain boundaries.

-Such precipitation produces zones of reduced corrosion resistance in theimmediate vicinity.



Microscope view of a polished cross section of a materialattacked by intergranular corrosion



MECHANISM

What causes intergranular corrosion?

-Local differences in composition such as coring (alloy castings).

-e.g. chromium carbide precipitation at the grain

boundaries in stainless steel. (cause)

-Consumption of chromium from a narrow band along thegrain boundary so this zone becomes anodic w.r.t. theunaffected grains. (effect)

-Hence this chromium depleted zone becomes apreferential path for corrosion attack and crack propogation if under the tensile stress.



CHROMIUM PROFILE ACROSS GRAIN:



In nickel alloys and austenitic stainless steels, chromium isadded for corrosion resistance.

Around 12% chromium is minimally required to ensure

passivation, mechanism by which a thin invisible layerforms at the surface of stainless steels.

This layer protects the metal from corrosive environmentsand it is thus stainless.

But here the mechanism involved is formation of chromiumcarbide at the grain boundaries, forming chromiumdepleted zones and so intergranular corrosion occurs.



Selective leaching often involve grain boundary depletionmechanisms.

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

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

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



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



Many aluminum base alloys are susceptible to intergranularcorrosion on account of either phases anodic to aluminum being present along grain boundaries or due to depletedzones of copper adjacent to grain boundaries in copper-

containing alloys.

Intergranular corrosion is a concern especially for alloys with high content of copper.

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



Anisotropic alloys, where extrusion or heavy working leadsto formation of long, flat grains, are especially prone tointergranular corrosion.

Intergranular corrosion induced by environmental stressesis termed as stress corrosion cracking.

Intergranular corrosion can be detected by ultrasonic and

eddy current methods.



KNIFELINE ATTACK

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

Titanium, niobium, and their carbides dissolve in steel at very high temperatures.

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

This affects only a thin zone several millimeters wide in the very vicinity of the weld, making it difficult to spot andincreasing the corrosion speed.



EXFOLIATION CORROSION

High strength aluminium alloys, especially when extrudedor otherwise subjected to high degree of working, canundergo exfoliation corrosion.

Here the corrosion products build up between the flat,elongated grains and separate them, resulting in lifting orleafing effect and often propagating from edges of thematerial through its entire structure.



LAMELLAR CORROSION

The sensitivity of cupronickel alloy increases together withits nickel content. A broader term for this class of corrosionis lamellar corrosion.

Alloys of iron are susceptible to lamellar corrosion, as the volume of iron oxides is about seven times higher than the volume of original metal, leading to formation of internaltensile 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.



SENSITIZATION EFFECT

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

Certain alloys when exposed to a temperature characterizedas a sensitizing temperature become particularly

susceptible to intergranular corrosion. In a corrosive atmosphere, the grain interfaces of these

sensitized alloys become very reactive and intergranularcorrosion results.

This is characterized by a localized attack at an adjacentto grain boundaries with relatively little corrosion of thegrains themselves.



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

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 elementsin the grain boundary areas.

Thus in certain aluminium alloys, small amountsof iron 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 ishigher at the grain boundaries and subject to suchcorrosion.



High-strength aluminium alloys such as the Duralumintype alloys (Al-Cu) which depend upon precipitated phasesfor strengthening are susceptible to intergranular corrosionfollowing sensitization at temperatures of about 120°C.

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

Die-cast zinc 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°-850°C.



In the case of the austenitic stainless steels, when thesesteels are sensitized by being heated in the temperaturerange of about 500° to 800°C, depletion of chromium in thegrain boundary region occurs, resulting in susceptibility to

intergranular corrosion.

Such sensitization of austenitic stainless steels can readily occur because of temperature service requirements, asin steam generators, or as a result of subsequent welding of the formed structure.



The photos above show the microstructure of a type304

stainless steel. The figure on the left is the normalizedmicrostructure and the one on the right is the sensitizedstructure and is susceptible to intergranular corrosion orintergranular stress corrosion cracking.



REFERENCES

en.wikipedia.org/wiki/Intergranular_corrosion

corrosion-doctors.org/Forms-Intergranular/intergranular.htm

www.corrosionclinic.com



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