Passivating 3

[TJR]

Can someone explain to me exactly what passivating does to metal? We currently passivate thin steel (0.020" thick), then handle it (no gloves), machine it, perforate it, etc. I thought it was a cleaning process. Handling it after the fact does not make sense to me, but perhaps I have the wrong idea about what it actually does.

 

[butelja]

Passivating stainless steel causes the formation of a chromium oxide layer on the surface. This oxide layer is what gives stainless steel its corrosion resistance. The areas that are machined after passivation would have less protection against corrosion.

 

[TJR]

Thanks - I thought so. What about handling the material. Do oils from the fingers matter?

Tom

 

[arunmrao]

Please visit corrosionsource.com. There is a good article on passivation and methods of passivation in discussion forum on manufacturing.

 

[grayseal]

Passivating stainless steel involves removing embedded iron particles from the surface. These particles are what could cause the stainless to corrode. The chromium oxide layer forms naturally in air. It is best to passivate as the last step in the manufacturing process. Also, oils and salts from the hands can effect corrosion resistance. Passivating will not necessarily remove them. Another cleaning process would be recommended.

 

[rustbuster1]

"Passivation" of stainless steel (some water treatment companies use this term for enhancing oxide formation on carbon steel too) involves using a chloride-free, oxidizing solution - 10% nitric acid is common - to produce a "pure" passive (oxide) film on the surface. This contrasts with simply immersing the SS in a service environment that might contain chlorides, in which case the chlorides are incorporated into the passive film and make it more vulnerable to breakdown.
Other replies also correctly point out that nitric acid passivating solution will remove iron contamination, rust from which also a compromises the passive film uniformity. Passivation does not remove heat tint or welding scales - pickling is required.
You can handle passivated stainless steel reasonably normally without any downside - think of it as a rugged, very thin ceramic coating that can be broken/scratched off mechanically.

 

[wildturkey]

The many response are correct. Sum up the responses and you have the solution.

The best source is ASTM A 967 & ASTM A 380.