weld seam corrosion

[AGVAS]

I have recently encountered a problem with weld seam corrosion. I read that the ferrite enriched-zone is more susceptible to corrosion but I really don't understand why and I would like to. Does it have something to do with the low carbon content in ferrite?

 

[mcprofusion]

Diffusion rates of carbon and chromium are much higher in b.c.c (ferrite) than f.c.c. (austenite) lattice.

In ferritic SS rapid cooling from above 925C does not really supress the precipitation of chromium carbide at the grain boundaries. For the same reason, lowering the carbon content does not effectively prevent chromium carbide from precipitating, unless the carbon content is extremely low (i.e. 0.002% in 446 SS)


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