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Sensitization Prevention

I am currently in my fourth year at University in Aberdeen Scotland. For my coursework in Offshore Engineering I have to look at a piece of stainless steel (316 + 316l) pipe which carries sea water on an oil rig and discuss how corrosion has occurred and how it may be prevented. I have ascertained that Sensitization occurred but I have became stuck on trying to find ideas on how to prevent sensitization. The sea water was lying in the pipe not moving for 3 month and so I know that one of my recommendations is to keep the sea water moving therefore increasing the amount of oxygen because I know that stainless steels usually have poor corrosion resistance in low-oxygen environments.

James Reid
student - Peterhead, Aberdeenshire, U.K
2004

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2004

Are you sure that the stainless steel has been sensitized? Corrosion can occur for reasons other than sensitization. Either way, the oxygen-content within or outside of the pipe during use has nothing to do with sensitization. Sensitization occurs during fabrication, usually either welding or post welding heat treatment, due to slow cooling through the temperature range of ~ 550-850 °C. If you want to discuss eliminating sensitization, then you need to discuss one or more of the following:

1. Alloy composition (C, Ti, Nb, N, etc.)
2. Joining processes (welding)
3. Heat treating (stress relieving, temperature profile during cooling)

As an example, changing the composition from 316 to 316L (lower carbon) will result in less susceptibility to sensitization. Or changing the joining process from arc welding to mechanical fastening will eliminate the thermal excursion to 550-850 °C that introduces sensitization. You can find a great deal of information on this subject in the ASM HANDBOOKS (Volumes 1 & 13 especially), ASM Metals Handbook Vol. 13 'CORROSION' ⇦[this on on Amazonaffil links], to which your library should have access. Also, the Nickel Development Institute (website is nidi.org) has many technical reports available for free that discuss stainless steels, corrosion, etc.

Good luck.

Toby Padfield
Automotive module supplier - Michigan