Stainless steel corrosion.

[burgundyben]

There was some evidence inside the transom on Playtime that the exhaust outlet bolts had been leaking a little, I pulled the bolts in order to re-seal them. These have been in place for about 9 years.

The exhaust outlet has been bonded to the anode all the time, it seems that is not enough to prevent crevice corrsion.

Certainly stainless should not be considered a fit and forget material....

 

[epervier]

It's just the newness worn off Ben, looks like only 50% gone, good for another 9

 

[VicS]

Almost certainly crevice corrosion.

If stainless steel fasteners are not properly sealed where they pass though deck, hull etc. the water that seeps into the joint becomes depleted in oxygen and a corrosion cell is set up.

Vital therefore to ensure that stainless fasteners are thoroughly sealed to completely exclude water from the joint.

Regard any rust stains as suspicious and deal with any minor leaks promptly.

We were lucky not to lose the mast and rig due to this afew years ago. We had ignored a tiny leak because the water drained it the galley sink and was not itself any problem.

Fortunately a keen eyed crew-member spotted a little bit of threaded stainless steel and a couple of nuts on the cabin floor.

 

[Tranona]

An anode won't do anything for crevice corrosion. It is caused by the threads being wet, but oxygen starved. Solution is to keep the fastenings dry - not always easy. Does not have to be in threads, although that is the most common place - have also seen it on propshafts that have not moved for a long time and the bearing filled up with deposits, excluding oxygenated water.

 

[Firefly625]

Almost certainly crevice corrosion.

If stainless steel fasteners are not properly sealed where they pass though deck, hull etc. the water that seeps into the joint becomes depleted in oxygen and a corrosion cell is set up.

Vital therefore to ensure that stainless fasteners are thoroughly sealed to completely exclude water from the joint....

Very interesting indeed, and certainly a lesson to me... Must say on boat fittings I do always add a liberal application of sealant but as a leak preventer...but had not considered this to be a corrosion cell preventer... my ignorance!

Thanks for posting!

We were lucky not to lose the mast and rig due to this afew years ago. We had ignored a tiny leak because the water drained it the galley sink and was not itself any problem.

Fortunately a keen eyed crew-member spotted a little bit of threaded stainless steel and a couple of nuts on the cabin floor.

 

[boatmike]

An anode won't do anything for crevice corrosion. It is caused by the threads being wet, but oxygen starved. Solution is to keep the fastenings dry - not always easy. Does not have to be in threads, although that is the most common place - have also seen it on propshafts that have not moved for a long time and the bearing filled up with deposits, excluding oxygenated water.

Vic and Trenona absolutely correct. It's a common misconception that this is caused by normal electrolytic corrosion and can be prevented by anodes. Stainless steel is OK with either a free flow of water over it or totally sealed. Otherwise oxygen depletion plus water equals crevice corrosion.

 

[Poignard]

Here's one I found on my boat.

 

[vyv_cox]

There is a page on it on my website http://coxengineering.sharepoint.com/Pages/Crevice.aspx with some more nice examples.

 

[Poignard]

There is a page on it on my website http://coxengineering.sharepoint.com/Pages/Crevice.aspx with some more nice examples.

You can add mine to it if you want. There's no copyright!

 

[vyv_cox]

You can add mine to it if you want. There's no copyright!

Thanks, may do. Does that apply to burgundyben also? Really nice picture.

 

[jfm]

BB, thanks, nice pic. Can you explain exactly the assembly in which this screw was sitting, with guessed dims? I'm intrgued as to why the crevice corrosion isn't present until 20mm or so down from the head. And at the tail end of the bolt there is plenty of length without cc. Was this becuase it was an over-long screw with redundant thread?Thanks

 

[burgundyben]

Vyv - feel free to use the pic.

JFM - Fitting a stainless steel exhaust outlet into a wood transom, the machine screw is a M6 by 50mm, the transom about 25mm thick so as you noted, the last 20mm of so of the thread was exposed to air under the cockpit sole. It was fitted with sikaflex polyurethan sealer about 7 years ago, bought from a Hamble Swindlery, I can't say if its A2 or A4. I would say the corrosion corresponds to the inner 12mm or so of the wood.

I'd be interested to know if A4 (ie 316) would have fasired better.

I guess titanium would work too.

 

[vyv_cox]

Thanks.

316 contains an additional 2% molybdenum over 304, principally to improve pitting resistance. Crevice corrosion and pitting have exactly the same mechanism, so yes, 316 is better.

 

[VicS]

Thanks.

316 contains an additional 2% molybdenum over 304, principally to improve pitting resistance. Crevice corrosion and pitting have exactly the same mechanism, so yes, 316 is better.

I thought 316 was developed as a better grade for machining. improved pitting resistance being a bonus ??

 

[boatmike]

I thought 316 was developed as a better grade for machining. improved pitting resistance being a bonus ??

It's immaterial really. Neither resist crevice corrosion very well but it is true that 316 is better for underwater use generally. If you want to avoid CC you need either bronze or monel although the mechanical strength of monel can be a problem in some applications. There are also duplex alloys which are considerably better than 316 but a lot more expensive. In difficult applications I have used duplex to replace 316 for propshafts.
On some commercial applications propshafts can be quite long and there is a flooded shaft tunnel that is fed by pressurised water from the engine coolant pump which is fine when the engine is running but it's often missed that when the engine is not running there is no water flow and the shaft bearings can pit for the same reason. Duplex can solve this problem more cheaply than the expense of bronze.....

 

[burgundyben]

It's immaterial really. Neither resist crevice corrosion very well but it is true that 316 is better for underwater use generally. If you want to avoid CC you need either bronze or monel although the mechanical strength of monel can be a problem in some applications. There are also duplex alloys which are considerably better than 316 but a lot more expensive. In difficult applications I have used duplex to replace 316 for propshafts.
On some commercial applications propshafts can be quite long and there is a flooded shaft tunnel that is fed by pressurised water from the engine coolant pump which is fine when the engine is running but it's often missed that when the engine is not running there is no water flow and the shaft bearings can pit for the same reason. Duplex can solve this problem more cheaply than the expense of bronze.....

That makes sense, I have Duplex shafts.

 

[Scarron]

Ok so how about Titanium then?

A serious question as I am machining up some new billet trim tabs in Delrin engineering plastic ( I want to coppercoat them and paint doesn't stay on stainless ) and need to through bolt them on to the hull.

IIRC titanium is very corrosion resistant and is the high performance material of choice on oil rigs and stuff. It is now more widely available at a reasonable price. What do the metallurgists think?

 

[Tranona]

Vyv - feel free to use the pic.

JFM - Fitting a stainless steel exhaust outlet into a wood transom, the machine screw is a M6 by 50mm, the transom about 25mm thick so as you noted, the last 20mm of so of the thread was exposed to air under the cockpit sole. It was fitted with sikaflex polyurethan sealer about 7 years ago, bought from a Hamble Swindlery, I can't say if its A2 or A4. I would say the corrosion corresponds to the inner 12mm or so of the wood.

I'd be interested to know if A4 (ie 316) would have fasired better.

I guess titanium would work too.

The root cause will be the moisture level of the surrounding wood. Sealer will stop water getting in from either end - as your example shows, but can do little for the bit that is in the wood. Two possible solutions. First is to use bolts with no threads in the bit that is in the wood, and second, probably more effective, dry the bolt holes out and then saturate with epoxy and redrill to size. Re-assemble with sealer over the whole length of the fastening so none of it is in contact with the wood.

 

[vyv_cox]

Titanium has excellent resistance to corrosion by seawater. Bolts made from it would be ideal for sub-sea use. The only case I know in which the metal corroded, on a ball valve in seawater service, occurred when the electric actuator failed, passing current through the valve.

 

[vyv_cox]

I thought 316 was developed as a better grade for machining. improved pitting resistance being a bonus ??

Are you sure you mean molybdenum? Various sulphides are incorporated to provide solid lubrication for high speed machining, manganese is the one I am most familiar with. Sulphides would be kept very low in most normal 300 series. AFAIK molybdenum is only there for pitting resistance. In carbon steels it is also a strengthening agent.