Crevice corrosion- great article but lost it !!

pcatterall

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Just read a comprehensive and interesting article on crevice corrosion on prop shafts and especially at lip seal sites.
It was by Straight Line Marine ( Fort Lauderdale) but I cannot recover the article despite searching around.
There are several ( scary!) stages to the process after the start up by oxygen depletion.
I can recommend it to anyone with an interest in this sort of thing.

On that ( vexed to me!!) subject I would be interested to be able to determine what grade of stainless was used on my 4 year old prop shaft, could I tell if it was 304 or 316?
If I go for a new one then should I specify one in one of the special steels like aqualloy 22

It seems that in our warm Med waters we should have the engine run (shaft turned) every 2 weeks minimum another idea ( our own expert) is to use the flushing hose to force fresh raw water into the lip seal area ( wonder if I could automate this )


Current strategy plan is to fit a PSS seal
 

vyv_cox

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A couple of years ago I had a long conversation with a man who makes and sells ball valves in every stainless steel grade imaginable. Even Duplex and various high molybdenum alloys etc all suffer crevice corrosion in warm seawater. The only grade he found that does not is a 900 series, very high nickel content. This is the stuff that Rolex watch cases are made from, so no doubt the cost is not low.
 

vyv_cox

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pcatterall

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Not a bad article but it concentrates solely on the chemical aspects and does not mention the metallurgical. There is no explanation of the 'active' and 'passive' condition of stainless steels, and thus the crucial transfer of electrons between them is omitted. In fact crevice corrosion occurs in water that is chloride free, although to a lesser extent.

yes that's the article thanks for being better at searching for stuff than I am!!
One of the photographs was of particular interest to me in that it showed a sort of irregular crater of corrosion. On my own shaft as well as the worn/corroded rings there is a patch that looks just like that! It is inside of the seal and looks just as if a drop of something very corrosive has sat on top of the shaft and eaten its way into it ! The only drop of anything that has got on there is sea water..... very strange!
 
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