Stainless Steel underwater

[Twister_Ken]

I have a s/steel rudder fixing which is permanently underwater. One of the s/s machine screws holding it in place is corroding. Rather than replace like with like, for the problem to recur, can anyone suggest an altermative metal to use for both the fitting and the fixings. BTW, at the moment this fitting is not connected with the zinc anode.

A couple of pix exist at:
<A target="_blank" HREF=http://uk.photos.yahoo.com/bc/kenjmunn/lst?.dir=/indigo&.src=ph&.view=>http://uk.photos.yahoo.com/bc/kenjmunn/lst?.dir=/indigo&.src=ph&.view=</A>

Thanx in advance

 

[jamesjermain]

Nasty bit of pitting there, Ken. How long has it taken for this corrosion to develope?

I'm sure most people would think you had the best option already. The only alternative I can think of which might be better would be bronze. I think it's usually recommended that bronze shouldn't be electrically linked but it is a bit of a problem with alluminium

JJ

 

[AndrewB]

I am really surprised that is a screw, surely a through bolt would be preferable for that application. I would replace it with stainless, though phosphor-bronze is an option.

The risk to stainless steel underwater is that it suffers from crevice corrosion, not rusting. Some rust can occur alongside crevice corrosion, but is rarely as heavy as it looks in pic 2. Is it possible the screw/bolt is electro-plated, not stainless? Are you screwing into wood - if so these marks might signify electrolytic damage or rot in the wood and would need investigating. Could the screw have gone right through the skin, causing weeping and damage from inside?

 

[Twister_Ken]

>How long has it taken for this corrosion to develop?<

Wasn't there last winter. A man who knows suggests it is 'crevice corrosion' something which s/s is apparently susceptible to in the absence of oxygen.

 

[mickshep]

Seem to recall that when Iwas tracing new bolts etc for my seacocks, was told not to use phosphor bronze as some grades little better than brass, Can't remember who said it but he said to only use silicon bronze below water line. (just to make things a little more confusing), Mike.

 

[Twister_Ken]

There is a mystery as to what it is screwed into. I'll know more when I (attempt to?) remove it next weekend. One suggestion is that it is just tapped into GRP. It's certainly not through bolted, because there are machine screws on the stbd side as well. The fittings are not visible from inside the boat, so I hope they are into a solid area, not into a void.

 

[AndrewB]

I believe you are right that silicon-bronze is regarded as the best for underwater use. Phosphor-bronze though is also satisfactory, at least in my experience. These alloys are of similar strength and both are reasonably electrolytically compatible with stainless steel.

The thing to avoid is 'commercial' bronze (sometimes just described as bronze) and the stuff sculptors use. This has a high zinc content and as you say is little better than brass.

 

[davehu]

I would think that there is a stainless plate inside the moulding which is tapped to take the screw thread. If the other 3 machine screws are OK then perhaps the one which is corroding is A2 and not A4. A2 will suffer corrision underwater and is slightly magnetic whereas A4 is non-magnetic. I would want to undo the other bolts to see what they are like.

 

[jamesjermain]

Crevice corrosion it certainly is. For this to happen in a year suggessts something has happened to cause it. Could it be that stray currents have developed? Has your anode been unexpectedly eaten away this year as well? A check for currents in the wiring with everything switched off might be revealing - but then again it might not.

If the other screws are all corroded then the likelyhood is there is some galvanic action. If it is only that screw then suspect the quality of the stainless steel. It might be worth drawing all the screws to check the hidden parts as well.

JJ

 

[Twister_Ken]

Roger, wilco.

 

[jamesjermain]

Manganese bronze

...is the culprit here, I believe - galvanically it behaves like brass.

JJ

 

[Jaguar]

Stainless steel tends to suffer from chloride attack (usually localised pitting). When under stress (e.g. machine screw), a phenomomen known as stress-corrosion-cracking (CCC) can also occur. The susceptibility of stainless steels to salt water corrosion depends largely on the chemical composition of the metal. SS with higher alloy levels of molybdenum and nickle will be more resistant. SS type 316/316L is generally taken as the mimimum grade to protect against chloride (sea water) attack, but the choice depends on other factors as well, i.e. temperature, chloride level, galvanic effects etc. Without knowing the grade of stainless steel, the type of corrosion suffered, or whether or not there is any galvanic attack, it is difficult to recommend a solution. Higher grades of SS (such as duplex 2205) will be difficult to get hold of - possibly through specailised fastener suppliers. Alternatives may include the bronzes (depending on bolt loading). The correct solution needs a proper engineering evaluation.

JJ

 

[AndrewB]

Re: Manganese bronze

Yes, not surprising really as it also is a copper-zinc alloy containing one third or more of zinc, with just trace quantities of tin, iron and manganese. I don't recall seeing bolts or screws made out of manganese-bronze though. It is often used for castings, such as propellors.

 

[30boat]

Monel,perhaps?

 

[TheBoatman]

A4 below water
A2 above

 

[Twister_Ken]

And M3/M27 to get to the water in the first place!

 

[charles_reed]

Seriously - you use A2 (which is what you have supplied in the chandlers as 316 stainless) above the waterline and A4 below.
You'll have to go to a fixings specialist, because most chandleries don't know the difference.

An A2 nut on my rudder was completely eroded by 6 months in the water, it's A4 replacement has been there for 2 years. Decent quality nuts and bolts, certainly above 8mm, have the type stamped on them.