Dols
New member
The pros and cons for the use of stainless steel skin fittings and sea-cocks below the waterline. Has anyone any information or experience of this please?
Stainless steel below waterline
- Thread starter Dols
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rivonia
Active member
Tranona
Well-known member
There seems to be little or no advantage in using stainless steel on a GRP or wood boat rather than the more common DZR "bronze".
As far as corrosion goes, the weakness in stainless is crevice corrosion in areas like threads.
edsailing
Member
Stainless Steel requires to be in contact with air to keep it 'stainless'. So, below the waterline it has no benefit.
VicS
Well-known member
The corrosion resistance of stainless steel depends on the maintenance of a protective oxide film on the surface.
This requires a supply of oxygen. Not necessarily air, the normal concentration of dissolved oxygen in water is adequate.
The trouble begins in stagnant areas such as the crevices that might exist between a fitting and the hull material or in threads. The water in these places becomes depleted in oxygen and the oxide film is lost. The metal without its oxide film is then anodic to the adjacent metal which having free access to oxygenated water retains its oxide film. Corrosion then occurs of the anodic areas in a similar way to the galvanic corrosion that occurs between dissimilar metals.
Whenever stainless steel is used where it will be exposed to salt water, be that underwater of even through-deck fastenings, it is vital that sufficient sealing compound is used to completely fill any gaps or crevices that might exist and trap stagnant water.
A4 ( or 316) grade of stainless steel should be used wherever exposed to saltwater in preference to A2 (304)
This requires a supply of oxygen. Not necessarily air, the normal concentration of dissolved oxygen in water is adequate.
The trouble begins in stagnant areas such as the crevices that might exist between a fitting and the hull material or in threads. The water in these places becomes depleted in oxygen and the oxide film is lost. The metal without its oxide film is then anodic to the adjacent metal which having free access to oxygenated water retains its oxide film. Corrosion then occurs of the anodic areas in a similar way to the galvanic corrosion that occurs between dissimilar metals.
Whenever stainless steel is used where it will be exposed to salt water, be that underwater of even through-deck fastenings, it is vital that sufficient sealing compound is used to completely fill any gaps or crevices that might exist and trap stagnant water.
A4 ( or 316) grade of stainless steel should be used wherever exposed to saltwater in preference to A2 (304)
Poignard
Well-known member
Here's one removed from my boat in 2007. I don't know whether it was A2 or A4, or how old it was, but I had owned the boat 10 years then without checking them. (Lesson learned)
The odd thing is that 5 other similar screws holding the same fitting on appeared to be alright. I replaced all six with A4 grade screws, and when I checked them last year they seemed OK.
The odd thing is that 5 other similar screws holding the same fitting on appeared to be alright. I replaced all six with A4 grade screws, and when I checked them last year they seemed OK.
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VicS
Well-known member
Nice example.
When that happens to a U bolt that the standing rigging is attached to it's serious!
Poignard
Well-known member
alahol2
Well-known member
Serious but not necessarily disastrous. It happened to one of my U-bolts holding a lower last season. Luckily one leg was enough to hold the stay. It meant I then had to check all six U-bolts + the bolts in my forestay and backstay attachments. I found no other problems but it was a pain. How quickly can crevice corrosion become a problem? Should I be checking all my fittings every season?
PetiteFleur
Well-known member
I just caught mine before they failed - one was leaking slightly and when I tried tightening the nut it sheared off. When the U bolt was removed it had crevice corrosion as in the photo where the material was hidden in the deck. I promptly renewed all 4 U bolts with Wichard ones which had the same hole centres. On a 1974 Jaguar 27 about 8 years ago.
Dols
New member
Thanks for your advice
Many thanks for all your replies. You have confirmed my fears on the point about crevice corrosion as this was a possibility that has been nagging in the back of my mind, knowing that stainless steel required oxygen to stay "stainless".
By way of background, my concerns in this matter arose from when I was working on a friend's boat when a skin fitting came apart in my hands under only moderate force. Fortunately, that boat was hauled-out at the time, but the thought of the same thing happening on my own boat in mid-Channel is scary! It seems that dezincification was the problem with my friend's fitting, but my worry is that on initial inspection there appeared to be no sign of any corrosion. I do not know if the fitting was DZR or not, but I suspect that it probably was, being the original fitting in a standard production boat (Princess). As Tranona mentions, DZR is the material most commonly used.
So how can one tell if one's skin fittings are sound or liable to disintegrate with the slightest knock? I thought in terms of stainless steel in the belief that at least I could see if it is rusting, but crevice corrosion and a sight of Parsifal's bolt scotches that idea!
My own boat is a very non-standard ferro-cement cutter, but about the same age as my friend's (25 years). So to put my mind at rest I have removed one of the skin fittings to check its condition. Completely sound! Unfortunately I could only get it out by grinding off the mushroom, and the ball-valve attached is a bit "iffy". So the question now is with what to replace them without 'down-grading'. I guess the original fitting must have been good quality bronze, so I had best aim for that for both the fitting and the valve. Any recommendations for sourcing? Also, any comments concerning ball-valves vs gate-valves?
Thanks again for your interest.
Many thanks for all your replies. You have confirmed my fears on the point about crevice corrosion as this was a possibility that has been nagging in the back of my mind, knowing that stainless steel required oxygen to stay "stainless".
By way of background, my concerns in this matter arose from when I was working on a friend's boat when a skin fitting came apart in my hands under only moderate force. Fortunately, that boat was hauled-out at the time, but the thought of the same thing happening on my own boat in mid-Channel is scary! It seems that dezincification was the problem with my friend's fitting, but my worry is that on initial inspection there appeared to be no sign of any corrosion. I do not know if the fitting was DZR or not, but I suspect that it probably was, being the original fitting in a standard production boat (Princess). As Tranona mentions, DZR is the material most commonly used.
So how can one tell if one's skin fittings are sound or liable to disintegrate with the slightest knock? I thought in terms of stainless steel in the belief that at least I could see if it is rusting, but crevice corrosion and a sight of Parsifal's bolt scotches that idea!
My own boat is a very non-standard ferro-cement cutter, but about the same age as my friend's (25 years). So to put my mind at rest I have removed one of the skin fittings to check its condition. Completely sound! Unfortunately I could only get it out by grinding off the mushroom, and the ball-valve attached is a bit "iffy". So the question now is with what to replace them without 'down-grading'. I guess the original fitting must have been good quality bronze, so I had best aim for that for both the fitting and the valve. Any recommendations for sourcing? Also, any comments concerning ball-valves vs gate-valves?
Thanks again for your interest.
Watson47
New member
I find this chart helpful with all metals in a marine environment ...
http://www.classicmarine.co.uk/Articles/metals.htm
http://www.classicmarine.co.uk/Articles/metals.htm
vyv_cox
Well-known member
For conventional DZR or other brasses masquerading as bronzes, dezincification is very obvious. When the zinc departs from the alloy it leaves copper behind, which is quite red by comparison. Clean up the surface and if there is dezincification you will see it. Here's an example of the pink colour.
Gate valves are bad news on a boat. They corrode rapidly as they are normally 60/40 brass intended for fresh water applications. There may be some made in DZR but I have yet to come across any. They fail when the very small actuating pin corrodes through. This normally happens when the valve is closed, so it cannot be opened.
Gate valves are bad news on a boat. They corrode rapidly as they are normally 60/40 brass intended for fresh water applications. There may be some made in DZR but I have yet to come across any. They fail when the very small actuating pin corrodes through. This normally happens when the valve is closed, so it cannot be opened.
prv
Well-known member
The four bolts holding Kindred Spirit's rudder on had undergone a similar process, but were much further gone. There was only the tiniest thread of metal left between the two ends of each bolt. I think the rudder was mostly being held in by friction and the remains of the silicone that had (failed to) seal the fittings.
One of the bolt heads was just intact enough to reveal an "A2" marking. Wrong bolts used as a replacement.
Very glad we followed the surveyor's advice to pull them and check before we ever put her in the water. Under the coats of paint, and with most of the corrosion in the middle rather than the ends, they looked pretty much OK till we pulled them out.
New ones are A4 and well-sealed with Sikaflex.
Pete
Salty John
Active member
The OP could consider Marelon (Xytel) glass reinforced nylon through hulls and seacocks. They would be my choice every time.
AngusMcDoon
Well-known member
And then if the stainless steel survives all this corrosion, it can suffer from fatigue cracking - as I've experienced twice in the last year with stressed components letting go with a loud bang.
It's known as shiny sh*te on my boat.
KellysEye
Active member
Marelon throughulls are the way to go. No corrosion in the fitting or handle, which is often the problem.
