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303 Stainless Steel Passivation

adv.    koslow passivation test kit

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I have some 303 stainless steel 3/8" pins that will see salt water and fresh water environments. These parts were passivated. I subjected them to 500 hours of salt fog testing and they rusted mostly at the ends where they were machined/cut. I also ran 304 pins at the same time and they did not rust. I was told that 303 should not rust if they are properly passivated. How can I tell if they are properly passivated in the future? Is there a test I can run?

Sid Tryzbiak
- Tulsa,OK


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Most of the 303 that I saw at two shops was "free cutting" and was normally 303Se. This proved to be much harder to passivate than plain 304. Salt spray of a larger sample would be my approach to telling if it were properly passivated. Copper sulfate testing can be done fairly easily "in house" also. Ferroxyl is a far more discriminating test, but it is more sensitive than most peoples requirements. You might look into doing a citric acid passivate in house with a proprietary product such as Stellar. For the ultimate, in my opinion, have them electropolished.

James Watts
- Navarre, Florida


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Sid, dump the 303. Use 304 or 316, or some other stainless that isn't free machining. The small manganese sulfide inclusions that give 303 its easy machinability mess with the corrosion resistance.

If you're stuck with 303, try passivating it with a Type VIII solution, rather than the type II your vendor is likely using. We've had better luck with VIII.

Lee Gearhart
metallurgist
East Aurora, New York


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