Stainless corrosion when submerged

There's a fellow trying to tell me that stainless steel (no specific variety) will corrode as quickly as mild steel when submerged in fresh
water.
This doesn't sound right to me, but is it?
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On Fri, 29 Sep 2006 12:21:51 GMT, snipped-for-privacy@NOSPAM.hfx.andara.com wrote:

Ask him why do they make kitchen sinks out of stainless steel.
Regards,
Boris Mohar
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Stainless steel is subject to corrosion when it is deprived of oxygen. It needs oxygen to react with the chromium on the surface. If the water is stagnant and totally devoid of oxygen it is possible for the protective chromium oxide to decay in very small spots and start the metal staining. Eventually small pits will form. Because there is even less oxygen inside the pits the corrosion speeds up and forms crevasse. This usually happens in tight restricted places like between a bolt and a washer but it takes years to do much damage. In normal fresh water that is not a problem because the oxygen constantly renews the chromium oxide. OTOH, with mild steel the effect is opposite. In oxygen rich water mild steel quickly starts turning into iron oxide (rust).
In any case the stainless will last far longer in fresh water than mild steel.
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We had some 316 stainless bodied instruments held together with small s/steel screws in fresh water - we found out about crevice corrosion! The screws were M3 and only engaged 4 or 5 threads, it was like the threads had been cleanly stripped.
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snipped-for-privacy@NOSPAM.hfx.andara.com wrote:

Besides the crevice corrosion problem, there is also the problem of passivation. If Stainless steels are welded or come into contact with a lot of mild steel, there will be small disassociated iron particles in the surface. These will rust and as they rust, form pits and crevices on their own.
In fresh water it isn't as much or a problem unless the water is acidic from rainfall. In Salt water it is a greater problem.
To prevent this from happening, stainless steel pieces should be passivated in an acid bath to remove these particles.
This is required for all food service work as well.
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