The role of molybdenum in the crevice corrosion of stainless steels

The paper starts with a literature review of Mo as an alloying element in stainless steels, with particular reference to crevice corrosion. Relevant aspects of the chemistry and electrochemistry of Mo are also summarized and used to discuss the possible role of soluble Mo species in corrosion inhibition. The need to examine solutions of low-valent Mo is indicated. Experiments are reported on stainless steels in the active range of corrosion potentials and the influence of soluble Mo^III and Mo^IV. No inhibitive effects are found; but inhibition is produced by films of insoluble Mo^IV oxide deposited on stainless steels. Such films can be produced by reduction of Mo^VI solutions, but not by oxidation of Mo^III. It is concluded that, in the corrosion of alloys, Mo probably passes directly from the metal into the protective film, and that soluble Mo compounds play no part in this process.