Why do Mo and N help the pitting resistance of stainless steel? 4

[mcguire]

The passive film is a semi-conductor about 20 Angstrom thick. In very well passivated or electropolished stainless, it is a P-type semi-conductor. This means the current, in particular the corrosion current, must be carried by the very slow substitutional diffusion of cations, e.g. Fe. In air passivated stainless there is excess iron and the semi-conductivity is N-type, meaning the corrosion process can proceed via the rapid interstitial diffusion of anions. The density of electronic defects in the passive film determine how large the large the corrosion current can be.

Molybdenum and nitrogen can negate defects in the film by supplying sinks for the excess negative charge of the n-type defects. Each element has a larger "valence" than the Fe or Cr. Thus Mo and N repair the passive film if there is inadequate inherent alloy content or iron removal by pickling during passivation. Mn has a contrary effect and is deleterious.



Michael McGuire

 

[EdStainless]

The study of re-passivation is an interesting field. In most applications there will eventually be a breakdown of the passive film, and what will actually control corrosion is re-passivation.

There has been work done recently comparing the electro-chemical pitting on alloys with various passivations. Even for highly corrosion resistant grades (6% Mo superaustenitics) there is a difference when materials are allowed to sit before testing. The samples were mechanically prepared, and then some were chemically passivated and others not. Either way the results of the corrosion testing were more uniform and reproducible if samples sat in open air for at least three days. The values for the best results stayed the same, but the scatter was greatly reduced.

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P.E. Metallurgy, Plymouth Tube

 

[mcguire]

It shouldn't be a surprise that aging helps. Acid passivation removes iron from the surface whereas air passivation leaves iron in situ until diffusion of Cr into the passive film can do more slowly what the acid does quickly. Good observation in any event.

Michael McGuire