Q - We bought replacement casing for our CF8M, type 316, stainless steel pump that has severe corrosion damage. During inspection we noticed the casing has rusts (stains) in some spots. We informed our vendor and they said it is "normal" for stainless steel to have some discoloration from atmospheric exposure. They gave us the heat number of the casting to show the casing was poured with the right metallurgy.
We thought stainless steel is supposed to be stain-free. Any comments?
A - As base metal, stainless steel does not rust, or stain, because it has the property to form a continuous protective passive surface that protects it from corrosion, and gives it its luster look.
However, there are instances where surface contamination from iron oxides will prevent the stainless steel from forming its protective, chromium-rich, passive film. Preventing this passive film from forming, or damaging it, can lead to localized corrosion that makes it look rusty, or stained, in some areas.
The iron oxide contamination of stainless steel can occur through:
casting cleaning by abrasive blasting
casting heat treatment
contamination of ferrous particles from iron or steel tooling during machining operations
exposure to other air-borne contaminants and moisture
In most applications surface rust, or stain, is not a problem. But in food, drug, or high purity service, rust, or stain, can become highly undesirable. To preserve its stainless quality and luster, stainless steel must be kept clean, dry, and free of surface contamination.
Some methods to prevent surface rusting, or staining, of stainless steel surface include:
Abrasive blasting such as shot and sand blasting - will remove most but not all of iron oxide. Remaining oxide will rust when exposed to moisture.
Pickling - acid cleaning of a surface in a nitric and HF acid bath. The acids remove the oxide and iron particles embedded on a surface. Pickling is not done as part of standard cleaning procedure for stainless steel because of its added cost; it must be specified if needed in high purity service.
For more on stainless steel refer to ASTM A380 standard.