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Corrosion SUS 303 versus SUS 400 series

2006

I am a student that are interested in studying the stainless steel corrosion.

Based on my understanding so far, SUS 303 or the 300 series can withstand corrosion especially in a salted environment due to the existence of Nickel in its composition compared to the SUS 400 series.

But recently, I discovered that the SUS 303 suffered corrosion in a salt spray test and so I tried the test again in a similar environment with a 400 series stainless steel and found that it resist corrosion.

Therefore, I was wondering how could this happen? And is there any solutions to it?

Vincent Valentine
student - Taiping, Perak, Malaysia

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First of three simultaneous responses --

Hi Vincent,
There are tens of thousands of publications on the corrosion of stainless steel. Many are freely available online. See letter #43011. Also, search for PREN, a measure of resistance to pitting corrosion. The PREN increases with Cr, Mo and N (nitrogen) content.

303 is a free machining stainless steel containing sulfides, which aid machinability but worsen corrosion resistance. Such stainless steels should be given a modified passivation procedure, as described by Carpenter Technology: HOW TO PASSIVATE STAINLESS STEEL PARTS http://crswnew.cartech.com/wnew/techarticles/TA00042.html

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.
2006

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Second of three simultaneous responses --

303 will not fail very fast in a salt spray test IF it does not have any iron in the surface from machine operations. Passivation or electropolishing is required. 400 series will fail very fast once the rusting starts.
If you want a guaranteed failure, just put a rotary brush on the part, even a SS brush, as it is 400 series SS. You can not passivate it enough to get it to pass.

James Watts
- Navarre, Florida
2006

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Third of three simultaneous responses --

All stainless steel will eventually fail when subjected to a salt environment for long enough. 303 has an additional problem, it is a high sulfur alloy, so if it has not been properly treated you can see sulfide blooms on the surface, which appear very much like rust to the naked eye.

Ray Kremer
Stellar Solutions, Inc.

McHenry, Illinois

2006