this months PBO - Stainless Steel

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davehu

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In this months edition advice on Stainlees steel suggests that A2 is not for use in a harsh marine environment and that A4 is not suitable for use if immersed in saewater. I thought A4 was OK underwater. If not what do you use. Forum opinions please
 
I think that the point Nigel Warren was trying to make was that A4 (316) has superior resistance to corrosion in the marine environment compared to A2 (304) but that neither is wholly acceptable for use underwater.

If you had seen the state of the shroud U bolts we removed after one failed you would be wondering about the suitability above the waterline in critical situations. I do not doubt that they were A2 but I wonder if A4 is more resistant to crevice corrosion,or not. That was almost certainly the cause of the failure and near failures we experienced.

Nigel Warren does suggest the use of "lots of bedding compund". Presumably this is to eliminate the sites that can suffer from crevice corrosion. Certainly in our case those fittings which had been removed and rebedded some years previously were still in good condition (visually) whereas those which had not, and which leaked slightly, were the ones which failed or were close to that stage of deterioration.
 
I have not read the article yet but the fact remains that very many drive shafts and other underwater components are made from stainless steel. In general they don't corrode but the point at which attack does take place most commonly is beneath the cutless bearing and at the prop-shaft joint. Both are, of course, crevices where the oxygen level in the seawater is depleted, thus setting up a cell. The additional 2% molybdenum that is added to A4 (316) by comparison with A2 (304) gives the alloy better resistance to crevice corrosion as well as pitting corrosion but it does not make it immune.

I would say that I have a lot of respect for Nigel Warren and very rarely take issue with his contributions.

As VicS says, eliminate the crevice and the crevice corrosion is eliminated!
 
A cautionary tale! My stripper is clamped onto the P bracket by two steel plates held in place by six nuts and bolts. Every winter I remove the stripper for cleaning and replace the nuts and bolts. Last year I bought new nuts and bolts from a local DIY instead of the chandlery. On taking the boat out this autumn I found that the nuts had completely rotted away and the plates were rattling around, only held in place by the plastic retaining ring of the nuts. I now assume that the nuts were not A2, so it is back to the chandlery next spring.
 
There are more grades of stainless steel than you could shake a stick at, including the sort of stuff we used to make for jet engines and razor blades. The 2 types commonly available as fasteners are the straight 18/8 stainless steel ( ie type 301,302,304,303) and the 18/10 with 3% moly (316 grades) - respectively A2 and A4 if you use the German nomenclature. No reason to do so, incidentally - not only was stainless discovered in the UK but we also made the worlds best stainless until recently and exported much of it worldwide including to Sweden, Germany and Japan. Sadly now gone along with the rest of UK manufacturing.

Anyway, thats a different story. A2 grade is not really stainless in the sense that it readily stains particularly in salty environments. Usually, that is only surface discoloration and can easily be polished off. A4 has a much more tenacious oxide coat because of the added moly, so it doesnt discolor anything like so much.

Neither grade fares well in situations where it sits in stagnant oxygen free water. Wet wood is one example, so is the skeg of your rudder. Personally I would never use stainless in wood on a boat except possibly on a dry deck for cosmetic fastenings. I have used A4 below the water line on the skegs of my Prout but only after ensuring they were thoroughly bedded into a sealant. In other words they were bone dry where they went through the grp. I was surprised at the erosion of the 10 year old bolts I took out and which had been put in by Prout. None were re-useable and several came out in bits.

Most monos have a shaft made of stainless and some of them are of A2. How many shafts have you heard of rotting away? The reason is that they are always in well oxygenated water.

One final point. Never replace high tensile steel bolts with the stainless you get from the chandlers. You can get high tensile stainless, but thats by special order from a fastener supplier
 
Our company deploys positioing beacons in depths of water up to 7000m. These beacons have a stainless steel guard that is made from A4 Stainless and I've seen these last for years but in saying that we do fit a number of zinc anodes around and onto the beacon /forums/images/graemlins/smirk.gif
 
I recently extracted the A4 bolts from one of my rudders and even those under the waterline showed no corrosion other than some surface discolouration (well over 20 years old, maybe 25)

In fact some of the metal on the rudders was A2, also in good to excellent condition.

There is one bolt that continually stains the rudder with a rust type weep, but there is no sign of rust on the bolt at all, very odd.

I personally believe SS A4 is the best option even if not perfect for underwater fixings. Far stronger than bronze, my old props were eaten away yet the shafts are perfect. Ho hum
 
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