Galvanising chain

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Aja
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Re: Galvanising chain

Postby Aja » Tue Sep 03, 2013 9:03 pm

Resurection.....

I'm going to have my 50 odd metres of chain re galvanized this winter. My 35lb CQR is also showing a bit of rust and discolouration around the fluke edges. Question is - is it worth getting it re-galvanized or will it just get worn with use?


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Donald

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mm5aho
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Re: Galvanising chain

Postby mm5aho » Wed Sep 04, 2013 7:48 pm

"will it get worn with use?"

Galvanizing is a sacrificial coating, its designed to get worn with use.
Corrosion is prevented / reduce by two methods - a barrier coating (paint is an example) or electrolytically.
With a barrier coating it is effective as long as the barrier is not perforated. You'd be doing well to not perforate a paint coating on a chain used for anchoring, so that's probably not worth considering.
Galvanizing does form a barrier, but more importantly it (zinc) is electolytically more active than iron, and so the corrosion set up in an electrolytic cell on the chain takes zinc first and reacts with it forming zinc oxide, zinc carbonate and some other zinc compounds, and keeps doing that until all the zinc is depleted. In an anchor chain on a recreational boat that might be between 3 and 6 years. When the zinc is gone, the chain starts rusting, and it too gets consumed, but at a faster rate than the zinc.
The reason its faster is that iron doesn't form resistant oxides on its surface, it forms rust which accellerates the oxidation. Zinc foms oxides that slow down the oxidation.

So, in not re-galvanizing the chain, (and assuming the current zinc is nearly or completely depleted), then the chain itself is dissapearing, and getting weaker, lighter, thinner etc.

The economics are pretty simple.
Galvanizing is bought by the weight of the item being coated. The price per kg varys with quantity and with material type. Ignoring minimum order issues, chain is about 50-60p/kg.
8mm chain is about 1.35 to 1.45 kg/metre. So that's about 90p/metre or something like.
(The chain is weighed for a price. It gains about 6-7% in weight due to the zinc, so take that into account.)
Then compare against new chain. Typical price £5/metre (calibrated 8mm)

Doing an anchor at the same time is common. I'll be doing my own chain and anchor again this winter.

If you want more info, or a yachtsmans eye on it, be in contact.
Geoff.
"Contender" Rival 32: Roseneath this winter, Gourock in summer.

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Aja
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Re: Galvanising chain

Postby Aja » Wed Sep 04, 2013 8:01 pm

Geoff

So much clearer. The working end of the chain is now rusty and the intention was to end-to-end and get it re-galvanised. I was just musing whether it was worth re-doing the anchor bearing in mind the weight and whether one of the zinc sprays would give the same life? It's not a money saving thing - just whether it was the best way forward.

Chain is 10mm and can take it into the factory at the end of the season and weigh it. So I'll be in touch at the tail end of the year, if I may.

Edit: does the chain end rust quicker if attached to a stainless swivel?

Regards

Donald

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mm5aho
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Re: Galvanising chain

Postby mm5aho » Thu Sep 05, 2013 9:15 pm

I thought I'd replied to this earlier, but it didn't post???

Stainless and galvanised? Theoretically should not go together, but practically you'll never notice the slightly quicker consumption of zinc from the chain.

Anchors suffer from corrosion and abrasion.
Galvanizing is a 2 layer coating. There's a hard dense alloy layer next to the steel, overcoated with pure zinc. The alloy is harder than mild steel, the zinc softer. The combination gives better abrasion resistance than mild steel. But practically again, the galv will wear off the abraded edges faster than is corrodes off in "normal" use. (whatever that might be)

Most galvanizers have a minimum charge, typically about £50+VAT. For that you'd get about 100kg. Not all do this, and not for every transaction. :-)
Geoff.
"Contender" Rival 32: Roseneath this winter, Gourock in summer.

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Aja
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Re: Galvanising chain

Postby Aja » Fri Sep 06, 2013 7:47 pm

Geoff

Considering it's for a boat £50+VAT seems remarkably reasonable (I hesitate to say "cheap"!!) :)

Regards

Donald


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