Rusting (staining) of stainless steel

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

  • 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.

File: stainless steel rust

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