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Rusting of 303 stainless vs. 416 stainless

I work for a small medical device company and we have been seeing rapid rusting in 416 ss. parts which have a thin layer of Tungsten Carbide applied to them. We also do the same thing with the same parts except they are made of 303 steel.Both types of parts aren't passivated. My question is ,do 416 parts rust more or is it a process problem?

Thanks for any help anyone could provide

Mike Caruso
Medical device manufacturer - Islandia, N.Y. USA
2007

Yes, 416 is less corrosion resistant than 303. 416 also has better corrosion resistance, in my experience, when it is fully hardened. Passivation helps too, of course.

Lee Gearhart
metallurgist - E. Aurora, New York

The composition of 416 SS (lower chromium, no nickel, higher carbon and sulfur) makes it much more rust prone than 303 SS. Also, the carbon content makes 416 more rust prone if annealed rather than quenched & tempered.

Both 416 & 303 should be passivated per ASTM A967 and then handled carefully (e.g., clean cotton gloves) prior to carbide coating. Follow all passivation steps applicable to free-machining stainlesses to avoid 'frosting' the parts.

Ken Vlach [deceased]
- Goleta, California

Finishing.com honored Ken for his countless carefully researched responses. He passed away May 14, 2015.
Rest in peace, Ken. Thank you for your hard work which the finishing world, and we at finishing.com, continue to benefit from.

Rust is iron oxide. apply a magnet to 416 and it clamps on readily, where it does not on 300 series SS. The reason is the makeup of the alloy 416 is 12-14 % chrome, less than 1% other material anc balance iron.
303 is not the best grade of SS and it is frequently free cutting which changes its corrosion resistance, but is
17 - 19% chrome and 8 - 10 % nickel with about 1% of other materials and balance is iron. The extra chrome and especially the nickel give 300 series relatively good corrosion resistance with 316 or 321 being significantly better.

James Watts
- Navarre, Florida

Nickel, as an alloy, gives more corrosion resistance than chromium. 300 series SS are more corrosion resistant than 400 series for they have a good amount of nickel and chromium.

Guillermo Marrufo
Monterrey, NL, Mexico

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