The part played
by molybdenum in the prevention of pitting corrosion of iron and steels
in the presence of chloride ions can be explained using Raman spectroscopy.
Here, molybdenum introduced as a molybdate in the electrolyte solution
was particularly studied. Different polarization methods were used (potentiostatic
polarization before or after the pitting potential, or voltammetric cycling),
leading to different pit distributions and sizes. Around the pit, one
can discriminate the different molybdates anions, in which Mo has an oxidation
state of 4+ or 6+. After the pitting is generalized, there rapidly grows
a thick, colloidal and unstable green rust layer (iron hydroxychlorure),
which was chemically identified on pure iron using Raman spectroscopy.
Beneath this layer, which represents a very agressive medium, the corrosion
rate increases dramatically, and the sample is rapidly destroyed. On stainless
steels, molybdenum (as well as chromium) can be integrated in the green
rust, thereby slowing the corrosion rate. When pitting is initiated, the
inner layer is formed by tetravalent molybdate, the passivity breakdown
being associated with the change in oxidation state of Mo from 6+ to 4+.
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