|
·
Stainless
steel with a chromium content of more than 11% is corrosion resistant due
to the formation of a few monolayers of chromium oxide on the surface of
the material. Austenitic 304 stainless steel alloy has the approximate
composition:
Fe - 0.08% C -18% Cr - 10% Ni - 1% Si.
· In
an oxygen environment, the Cr reacts to form ions and free electrons in
an anodic reaction. The electrons react with the oxygen and Cr ions in
the cathodic region of the material forming a protective chromium oxide
layer, Cr2O3, on the metal surface. This film reduces
electron transfer to the electrolyte and inhibits further electrochemical
reactions. In this state the material is said to be "passivated."
·
The
passivating surface layer may be destroyed in a reducing environment (no
oxygen present). The alloy then becomes active and is readily corroded,
an "activated" condition. In evaluating a material for use in a corrosive
environment, the possibility of this condition occurring must be taken
into account.
·
An
alloy with the composition: Fe - 60% Ni
- 20% Cr is passivated for both the oxidizing
and reducing electrolytic ambient. |
