Letter 13000

Rusty SUS303 

We have been machining some SUS 303 parts, which under normal conditions should not rust. But recently we had been seeing some of our parts with some brownish stains which resemble rust, after about 1 to 2 weeks. We are using a CNC with water based coolant for machining. Any reason for the parts to turn out like this?

Padmavathy Subbrayan
- Penang, Malaysia

Mr. Subbrayan:

What you are seeing COULD be rust, but it is probably sulfides and sulfates from the sulfur in the 303. You can tell which it is by testing in a 5% NaOH solution overnight (it probably will take only an hour or two). If the yellow stains go away, it was sulfur caused. If they do not go away, soak the parts in a citric acid based product (contact us if you need this). If the discoloration goes away, it was rust. Let us know if we can help.

Lee Kremer Stellar Solutions, Inc. Algonquin, IL, USA

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