N. Boucherit et Ahl. Goff, LOCALIZED CORROSION PROCESSES IN IRON AND STEELS STUDIED BY IN-SITU RAMAN-SPECTROSCOPY, Faraday discussions, (94), 1992, pp. 137-147
The part played by molybdenum in the prevention of pitting corrosion o
f iron and steels in the presence of chloride ions can be explained us
ing Raman spectroscopy. Here, molybdenum introduced as a molybdate in
the electrolyte solution was particularly studied. Different polarizat
ion 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 dif
ferent molybdate anions, in which Mo has an oxidation state of 4+ or 6
+. After the pitting is generalized, there rapidly grows a thick, coll
oidal and unstable green rust layer (iron hydroxychlorure), which was
chemically identified on pure iron using Raman spectroscopy. Beneath t
his layer, which represents a very agressive medium, the corrosion rat
e increases dramatically, and the sample is rapidly destroyed. On stai
nless steels, molybdenum (as well as chromium) can be integrated in th
e green rust, thereby slowing the corrosion rate. When pitting is init
iated, the inner layer is formed by tetravalent molybdate, the passivi
ty breakdown being associated with the change in oxidation state of Mo
from 6+ to 4+.