Abstract:
Stainless steels differ from iron in that chromium content allows for the
formation of a passive iron-chromium oxide film which is only nanometres
in thickness, offering protection from the environment. While the
composition of this oxide layer has been established, the mechanism of
its formation is not well understood. In particular, the threshold level of
chromium for oxide formation is significantly lower then the chromium
content of the alloy itself. We present a Cahn Hilliard type analytical
model that relates the onset of passivation to an instability which leads
to a phase segregating current above 17% Cr in a bulk alloy. Proposing
that this current could lead to Cr enrichment at a surface, we compare
atomistic simulations with and without a surface driven Cr current. We
implemented a kinetic Monte Carlo algorithm with extensions to allow
for vacancy assisted nearest neighbour migration in a body centered cubic
alloy, tracking a surface, dissolution and surface passivation. We compare
the time evolution of Fe dissolution rates, Cr surface enrichment and the
threshold for passive film formation and find that the Cr current has a
significant impact on each of these properties.
