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Cause of Discoloration of stainless steel pump seal?

Q. We are investigating a mechanical seal failure on a large pump in which a couple of the components of the seal are stainless steel sleeves of the following material:

1) ASTM A351 Gr. CF3 (cast 18-8 LC)
2) ASTM A351 Gr. CF3 w/ LC-4 Cr Oxide Coating (cast 18-8 LC)

These two sleeves are separated by a very small gap (I don't have the dimension handy). We are seeing a multi-colored hue (straw/yellow, brown, blue, etc.) on the OD of the inside sleeve and the ID of the outer sleeve. The only material that should be in the environment is a very small amount of water. Furthermore, this water is not expected to be above 200 °F, and our calculations show that it is not reasonable to assume that it should ever get above 300 °F based on the flow of water (nominally 0.0022 gpm).

So the question is, what's causing the discoloration? Is it heat related? Chemically induced? If so, what do we look for in an EDX/XRD examination? Is there a different analytical technique we should use, such as Auger spectroscopy? What temperatures would be required to cause it if it is due to heat? Etc.

Thanks for any insights.

Dan Lyons
- San Clemente, California, USA
2003