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Intergranular Corrosion (IGC) (Mechanism (IGC of Iron-Chromium Stainless…
Intergranular Corrosion (IGC)
Formation
Localized attack at or near grain boundaries.
Microscopic level
Factor
Impurities at grain boundaries area.
Enrichment of alloying elements at grain boundaries area.
Depletion of alloying elements at grain boundaries area.
Control Methods
Intergranular Corrosion (IGC)
Alter environment condition
Lower acidity
Reduce oxidizing condition
Solution annealing
Heat above 815˚C to redissolve chromium carbide, then perform rapid cooling.
Reduce carbon content to less than or equal to 0.03%.
Use stabilized stainless steel.
Knife Line Attack (KLA)
Heat above 815˚C to dissolve chromium carbide and form Nb/Ti carbide.
IGC of Iron-Chromium Stainless Steel
Soak at 800˚C or slow cool from 700˚C-900˚C to replenish Cr-depleted grain boundaries by diffusion to surrounding grains.
Mechanism
IGC of Austenitic Stainless Steel
At temperature from 425˚C-815˚C, carbide precipitate along the grain boundaries.
Carbide precipitate consumes chromium atoms.
Forms Cr depleted zone (less corrosion resistant) around precipitated carbide.
Form large cathodic area to small anodic area.
Result in dissolution of carbide, microscopic localized galvanic attack occur, stainless steel become sensitive to IGC (sensitized stainless steel).
Knife Line Attack (KLA) - in Stabilized Austenitic Stainless Steel (Type 321 and 347)
Occurs only a few grain diameters adjacent to weld beads.
When material is heated over 1230˚C during welding, it is then rapid cooled to remain carbon atoms near weld beads as solid solution.
The material is reheated to critical temperature.
Localized sensitization takes place.
IGC occur in the narrow sensitized region.
IGC of Iron-Chromium Stainless Steel
Material is sensitized only after being heated above 925˚C.
Low solubility of interstitials in ferrite sensitizes the material faster and at lower temperature.
Duplex stainless steel is resistant to IGC
Insufficient carbon remains to precipitate carbide at austenite grain boundaries.
IGC of Other Alloys
Aluminium alloys depend on precipitation phases for sthrengthening but are susceptible to IGC.
When solution-quenched, resistant towards IGC increased but strength is reduced.
Die cast zinc alloys containing aluminium is not resistant towards IGC under steam and marine atmosphere.