Stainless Steels and Alloys: Why They Resist Corrosion
and How They Fail*
The passive film on stainless steels has its own composition
which is strongly enriched in chromium. For an alloy containing 15% Cr
in its composition, the chromium content in the top layer of passive film
can be as high as 60% (Fig.6)! In Fig.6, the chromium enrichment factor
(defined as the ratio of chromium content in the passive film to that of
the alloy) is plotted against the film thickness for two alloys passivated
in 0.1M sulphuric acid at 920 mV (SHE) for 1 hour. The chromium content
was measured using X-ray Photoelectron Spectroscopy (XPS). It is clearly
seen that the 4% molybdenum in alloy Fe15Cr4Mo markedly increased the chromium
content in the passive film! It is also seen from the figure that the chromium
enrichment factor approach unity at a thickness of about 3 nm, which is
considered to be the passive film thickness.
Fig. 6 Chromium enrichment in the passive film as a function
of thickness |
