Mooring chain corrosion

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gpascoe

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My Mooring chains corrode rather quickly (10mm down to 3mm in 3 years). I'm attached to ground blocks fore and aft in a rather crowded but sheltered quay (Clyde Quay Wellington, NZ) and there's a lot of electricity about. Still, rather the chain degrade than the hull fastenings.

I was wondering, if I take a 6 inch length of a strong pipe, or solid metal tubing, put a great big anode on it, then shackle it to the chain as a link, would that help?
 
Don't think so, my shackles attaching the chain to to the ropelines look to be galvanised (I inherited these with the mooring so not sure). The blocks are railway wheels so that should be fine too.

A guy on the woodenboat forum thinks this is about the right rate of degrading. Unfortunate.
 
Aberdovey on the west Welsh Coast was an old shipyard, and there is lots of copper waste around on the foreshore. This means chains detriorate very fast - 10mm can burn through in a few months apparently. They solved the problem by attaching anodes to all the chains.
 
I would think that an anode would be a good idea, but I don't like the idea of putting anything else, such as your length of pipe, in the actual line. I also wonder about the use of some of these 'wonder ropes' such as Spectra, instead of chain? Perhaps one of these ropes threaded throughout the length of the chain as a backup in case of failure of the chain? The ideal would be a non-metallic chain, such as a carbon fibre-reinforced one. It just requires a company to see the economic value in producing such a chain.
Peter.
 
That's a nice idea about the spectra. Will look into it. I've only seen anodes that fit to a prop shaft or screwed directly to the hull. How else might you find one that gets a good fit to chain? Will contact Aberdovey
 
Then there's the question of just how good is the electrical conductivity between one rusty link and the next? Surely each link would need a separate anode, clamped tightly on to bright metal?
 
It is possible to buy a section of chain with a zinc anode cast around a few links - I have seen them advertised, but cannot find a link now.....
Correction, just found one by M G Duff, here at http://www.marinemegastore.com/product.asp?pf_id=ECS_5550&jump=12&cat_id=MAIANO003
Although they are rather expensive.....

I have used shaft anodes clamped on to the lower ring on the mooring buoy, and on to the heavy ground chain links, and they do seem to help. The main problem is the electrical contact betwen the chain and the anode.
 
Heavy corrosion in the aerated zone at the surface is not uncommom, you have a corrosive substance, sea water, with a lot of oxygen.

One solution can be to use a fibre riser for the last 10 feet or so thus keeping most of the chain in less oxygenated water.
 
I find it is the bit at the bottom affected by the rise and fall of tide that is worst. That is raised and lowered twice a day and gets sand or mud rubbed in as an abrasive for good measure. That and alternating between anaerobic conditions (in the mud) and more aerobic conditions in the water.

I found that 10mm was too small a diameter for a mooring and used to use 15mm, which went down to about 6mm at the wear points in the links in about four years. I say "used to" as I am now on a trot mooring which is serviced by a contractor and I have no idea what is actually down there, only that I am assured that it is really heavy stuff. It certainly feels it when the mooring gets fankled and I need to full up a few feet of ground chain to get the loop on deck.
 
I would agree that the section of chain where it is lifting off the bottom tends to wear the most - especially so where you have a very small tidal range like here (0.5 m. on springs).

We have 15' of studlink ships anchor chain on our mooring where the chain is lifting off the bottom. The end connected to the riser chain is now (after 10 years) considerably smaller than the other end (which doesnt move much). And the studs in the first 5' or so of the links at the riser end have all dropped out (worn away?) as a result.
The ground chain going to the 2 x 40 lb anchors is 3/8", and still original (and in good condition!) as it stays buried in sand most of the time.
Whereas we are now on the third riser chain to the buoy - previously I used 3/8', but it is now 1/2".
 
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