Intergranular corrosion Flashcards
What are the recognizing features of IGC?
> grain structure is visible under microscope
> some cases the grains fall as loose powder
What grades of steel have less susceptibility?
304 and 316
Why are GB more prone to corrosion?
> high energy of planar defects
> differences in local alloy composition (due to segregation, precipitation, sensitization)
IGC of Austenitic SS
> heating of austentitic SS in range of 425-815C, resulting in sensitizatoin
improper heat treatment, welding, sintering, processing
noble chromium carbides (Cr23C6) precipitate at the GB
matrix region next to the GB (solute depleted zone) becomes depleted in Cr
corrosion attack adjacent to the GB
IGC of the less noble chormium depleted regions sensitized in SS will occur in acidic solutions
depends on temp and time
Mo-containing intermetallic phases (sigma and mu) can also cause GB depletion and sensitization
How do you reduce IGC susceptibility of austenitic SS?
> solution anneal > 815C
reduce the carbon content < 0.02wt% (304L, 316L)
add Cb (Nb) or Ti, so that Ti or Nb carbides form
Ti at 5wt% C is 321 SS
Nb/Cb at 10wt% is 347 SS
lower the acidity and less oxidizing conditions
What is a knifeline attack?
> results from dissolution of Ti or Nb carbides BACK into SS at the high weld temp, a rapid quench to low temp, then reheating into temp range where Cr carbides precipitate
What is reactivation during single-loop EPR test?
> it’s a result of the preferential breakdown of the passive film covering the Cr deplete regions
results in a large loop (active region) in the potential vs current curve
What causes IGC of Al alloys?
the direct attack of a GB precipitate or solute depleted zone that’s more active than the matrix
Al-Mg alloys (5XXX)
Mg2Al8 is active relative to the Al-rich matrix and corrodes at the GB
When is IGC a problem?
when the intermetallic phase is a continuous phase at the GB
Al-Mg-Zn alloys
(ex 7030) MgZn2 is anodic to the matrix and is preferentially attacked
Al-Cu alloys
(ex 2024) the Cu deplete zone next to the GB is anodically attacked
How does IGC of nickel occur?
hydrogen embrittlement
What are the contrlolling parameters of IGC?
> alloy chemistry > stability of alloy components > local variations in comp > environment > metallurgical treatment
What is prevention and control of IGC?
> high temp soln. heat treatment > addition of stability elements > lower interstitial content of alloys > stabilized welding rod, and post weld heat treatment > material selection
