Pitting of 431 SS shafts in sea water 2

[tonylazyz]

We have discovered serious pitting on the main propulsion shafts of a 165’ ocean-going boat that has been in the water for only one year. The spec for the shafts is 431 stainless. We need to replace the pitted shafts, but we don’t know what type of stainless to spec for the replacements. On previous boats w/ shafts exposed to sea water I have had 316 L and Duplex stainless.

There are a couple of issues here:

1) I don’t know the spec for the surface finish of the existing 431 shafts
2) The shaft runs through a tube or sleeve for 23 feet between the hull and the prop. The shaft bearings inside this tube are sea water cooled. While the boat is underway the water is constantly being replenished. But when the boat sits for a long period of time the shaft sits in stagnant sea water. (The manufacturer of the shaft claims this caused the pitting and that the tube needs to be flushed when the boat sits idle)

Can anyone comment on why the shipyard would have used 431 rather than 316 or 316 L or Duplex? Can our problem be because the surface finish was not to the correct specification? What surface finish should we request for the next shafts? 1P/2P ?
Also, is there some other type of stainless that would be more suitable?

Finally, EdStainless if you see this post can you expand on this comment of yours from an earlier thread?
“The other issue that you have though is the use of 316 in seawater. This may be common practice, but there is no reason to expect 316 to stand up to any seawater exposure. It doesn't have enough pitting resistance. Make sure that water cannot be trapped anywhere and keep everything clean.”

Thanks
Tony Schultz

 

[unclesyd]

Here is the information on Aqualoy line of boat shafts offered by Western Branch Metals.
Around here we use Aqualoy 17 (17/4) on boats that are in constant motion and Aqualoy 22 (22-13-5) on boats that have a lot of dock time. These materials are treated as boat shaft materials from the get go.

There brochure has a lot of information and the people there were great when requesting information. They should be able to get you on the right track pretty quick.

There is also data on the finish and straightness, etc.

http://www.wbmetals.com/default.asp

 

[EdStainless]

They probably used the 430 for it higher strength and lower cost. The alloys that Syd mentioned are good options. The 22-13-5 is an austenitic grade that is used in the cold worked condition to give it strength. While it isn't bullet proof it has a good reputation.
316 will only stand up in seawater if it is clean and there is a less corrosion resistant material near to provide some galvanic protection. Long term seawater exposure that involves fouling and stagnant conditions requires a stainless grade with at least 25% Cr and 4% Mo. Look at the alloys required in oil fields and sued by the Navy, they are all 6% Mo grades.

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