Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here


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

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

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

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

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

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.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

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

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

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


White Paper: Rapid Manufacturing with FDM
The automotive industry is increasingly turning to additive manufacturing, and fused deposition modeling (FDM) specifically, as a reliable alternative to traditional metal-cutting methods, like milling, turning and boring. Download Now

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close