Galvanising underwater steel parts…?

MYStargazer

MYStargazer

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While looking at possible coatings for my soon-to-be shot-blasted mild steel engine beds, I discovered that galvanising is much cheaper than I had imagined (about 60 pence per Kg) and can be carried out by the same place which is doing the blasting.

So… might my steel p-brackets also benefit from galvanising?

A quick Google throws up alternating views on this…

http://www.ybw.com/forums/showthread.php?18423-Mooring-Chain
http://www.gaa.com.au/index.php?page=performance

Here’s the p-bracket…

PBracket1.jpg
 
The bottom line is that galvanizing lays down a zinc coating which is a sacrificial layer, and works just the same as zinc anodes. It also excludes air from the steel. Galvanizing things that are infrequently exposed to salt water, and where exclusion of air from the steel will help prevent rusting, is valuable - hence its use for chain and anchors and above water fittings on more traditional craft. Below the water line, it will do no harm but the zinc will disappear very quickly. Even a tiny scratch in the surface will expose steel, and create an efficient electrochemical circuit that will erode the zinc quickly.

I'd say don't bother - it will do nothing that your anodes don't do, and at best will prolong the life of your anodes by a few weeks.
 
Worth doing if you can get them really clean. As AP says, downside is that galvanising can get damaged so prime and antifoul to protect the coating. The galvanised plate bilge keels on my old boat are now 50 years old and still sound with no serious rusting.

No need to use any anodes if you use galvanised fastenings to the hull and phenolic shell cutless bearings (or composite) as they will not be in contact with any other metals.
 
Tranona said:
Worth doing if you can get them really clean. As AP says, downside is that galvanising can get damaged so prime and antifoul to protect the coating. The galvanised plate bilge keels on my old boat are now 50 years old and still sound with no serious rusting.

No need to use any anodes if you use galvanised fastenings to the hull and phenolic shell cutless bearings (or composite) as they will not be in contact with any other metals.
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This is exactly what I'm leaning towards. They'd be scraped, chipped, blasted, galvanised, epoxy primed, epoxy top coated, held in with galvanised fastenings, and electrically isolated.

But then... coppercoated as with the rest of the hull?
 
Fitzroy said:
This is exactly what I'm leaning towards. They'd be scraped, chipped, blasted, galvanised, epoxy primed, epoxy top coated, held in with galvanised fastenings, and electrically isolated.

But then... coppercoated as with the rest of the hull?
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Yes; the draw-back I highlighted (that the zinc erodes sacrificially) would be avoided if there was a strong, impervious coating. Copper-coat recommend a zinc-rich primer on metal surfaces, so I guess that a galvanized surface would be an excellent base! Then, the galvanizing would protect the metal in the event of a minor scratch, and allow a much simpler touch-up job than if it wasn't galvanized.
 
I guess bronze replacements will make your wallet spontaneously combust, but what about getting an S/S fab to replicate them? They would eventually succumb to crevice corrosion, but if built heavily enough that might be a long time hence.
 
penfold said:
I guess bronze replacements will make your wallet spontaneously combust, but what about getting an S/S fab to replicate them? They would eventually succumb to crevice corrosion, but if built heavily enough that might be a long time hence.
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Indeed they would!! Thought about S/S (am going to buy a TIG welder and teach myself as it looks satisfying to get right) but while (if!) there's plenty of meat left on the original steel ones, it seems worth protecting them and putting them back...
 
Yup , then the zinc will get eaten by electrolysis an hey presto back to bare steel ,if the coating survives you dont need the galvanising , get my point , tongue in cheek it may have been ;)
 
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But suppose the paint fails more at one point than another - which is the most likely scenario. Capillary action will take the water under the paint. This is when you'll need the zinc protection. Otherwise the oxidation will go on to expand the volume of the steel and increase the breach in the paint coating.
 
Whilst in your case it's likely, it's not inevitable that all underwater galvanizing will get electrolytically eaten. It needs to form an electric circuit to something else in the water. Some of the pintles and gudgeons on Amulet have almost 12 years behind them. The backstay chain plates, whose tails are immersed, have intact galvanizing - they are original since her launch in 1964!
 
Amulet said:
Whilst in your case it's likely, it's not inevitable that all underwater galvanizing will get electrolytically eaten. It needs to form an electric circuit to something else in the water. Some of the pintles and gudgeons on Amulet have almost 12 years behind them. The backstay chain plates, whose tails are immersed, have intact galvanizing - they are original since her launch in 1964!
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Why is it likely? When covered by paint, the zinc coating would be out of line-of-sight with any dissimilar metals other than the mild steel it covers, so there couldn't be any galvanic action...??
 
Fitzroy said:
Why is it likely? When covered by paint, the zinc coating would be out of line-of-sight with any dissimilar metals other than the mild steel it covers, so there couldn't be any galvanic action...??
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It is in contact with steel, so any scratch that penetrates the galvanizing will result in electrolytic corrosion of the zinc. That's how galvanizing works to protect the iron or steel it is applied to.
 
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