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402F stainless steel rusts, 303S doesn't. Why?



I would like to know why the SUS402F stainless steel more easy been rust compare to stainless steel SUS303S. What is the differential of chemical composition for both of stainless steel? After both of the parts go through washing and drying process, there are a certain quantity of SUS402F rusty parts been found. Recently, we found a lot of yellow stain parts when received the parts from the supplier. I would like to know what is the process that will leave the stain on the parts.

Lily Tan
electronic parts - Seberang Perai , Selangor, Malaysia


As far as I know, all of the 400 series stainless steels are martensitic, having 10 to 13% Chromium and less than 1% Nickel,are heat treatable like the normal steels, carbon and alloy etc, and behave like these steels in a corrosive or wet environment.

The 300 series are austenitic with much higher content of 16 to 18% Chromium and 8 to 12% Nickel. Therefore these are the 'True' Stainless steels and have much improved corrosion resistance over the 400 series. They are behaving differently because they are different.

The staining is probably rust - oxides of iron etc.


Martin Rich
DML - Plymouth, UK


400 series SS is mostly iron with a percentage of chrome for strength. 300 series SS has a variable amount of nickel as well as the chrome which makes it more corrosion resistant. Some 300 series add moly for additional resistance. Most people will use 400 series because it is heat treatable and has enough corrosion resistance to "get by". Also it is normally cheaper.

James Watts
- Navarre, Florida

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