dissimilar metals corrosion

cagey

cagey

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My electric anchor winch(buried at the back of anchor well) is always sprayed at the end and beginning of each season with corrosion guard but today as I'm getting boat ready I found in 2 places a significant jelly like growth. It appears to be at the boundary between Steel motor case, Stainless shaft and Aluminium(possibly)Gypsy housing.
I haven't disturbed anything yet but what suggestions do you all have to; treat and eradicate this corrosion and second how can I prevent its reappearence. Taking the winch out and dissassembling will be a bugger so please a very easy and lazy chemical wash answer is what I'd like.
Thanks
Keith
 
I bet if you stuck some multi meter leads into the goo and to the metal you will have registered a small DC voltage as it is was actually electrolyte caused by the dissimilar metals.

ps Sorry no idea how to stop it, other than regular cleaning and oiling.
 
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The fundementals have to be sorted first, it isn't a quick fix but for long term protection, try and find some alocrom and coat the aluminium in the alocrom, it will take on an orangey colour but will be protected. http://www.theanodisingco.co.uk/16280.html

You also need a jointing compound like this http://www.ppg.com/coatings/aerospace/sealants1/jc5a.pdf

Apply it to any areas of dissimilar metals where they have mating faces. JC5A is designed not to go off but it will seal and protect the two faces. Maybe the jelly is a form of jointing compound?

This is how it is commonly carried out on aircraft.
 
corrosion

Any corrosion between dissimilar metals will require that the dissimilar metals be parted. You can then clean up the corrosion treat the faying surfaces and reassemble. Sorry there is no way any treatment can get into the tight gap between the metals. (but I think you know that already). Duralc is a common insulating and separating paste for such cases. good luck olewill
 
You'll get about 0.1V between aluminium and stainless steel in sea water (i'm actually experimenting on it at the mo for my uni dissertation :)). The only way you'll stop it is by either a physical barrier between the metals like a rubber pad, or a coating. You would think the manufacturers would have been slightly more enlightend and known about galvanic corrosion, especially with a piece of equipement that gets very wet!
 
ChrisChase said:
You'll get about 0.1V between aluminium and stainless steel in sea water (i'm actually experimenting on it at the mo for my uni dissertation :)). The only way you'll stop it is by either a physical barrier between the metals like a rubber pad, or a coating. You would think the manufacturers would have been slightly more enlightend and known about galvanic corrosion, especially with a piece of equipement that gets very wet!
Click to expand...

But you need to sell spares..... too cynical.... maybe :D
 
ChrisChase said:
You'll get about 0.1V between aluminium and stainless steel in sea water (i'm actually experimenting on it at the mo for my uni dissertation :)). The only way you'll stop it is by either a physical barrier between the metals like a rubber pad, or a coating. You would think the manufacturers would have been slightly more enlightend and known about galvanic corrosion, especially with a piece of equipement that gets very wet!
Click to expand...

According to commonly published data on the "galvanic series" there is rather more than 0.1 volts between aluminium and stainless steels .

Depending upon the alloys and whether the stainless steel is passive or active the difference can be anything from 0.2 volts to 1.0 volts.

See for example http://www.corrosionsource.com/FreeContent/1/Galvanic+Series

Some more thought needed perhaps in the way you are measuring the difference ??
 
VicS said:
According to commonly published data on the "galvanic series" there is rather more than 0.1 volts between aluminium and stainless steels .

Depending upon the alloys and whether the stainless steel is passive or active the difference can be anything from 0.2 volts to 1.0 volts.

See for example http://www.corrosionsource.com/FreeContent/1/Galvanic+Series

Some more thought needed perhaps in the way you are measuring the difference ??
Click to expand...

Yes you are correct, a voltage of around 0.8v will be produced if you use the galvanic series to predict. But this is pretty useless without knowing what the metal series are due to the large differnces in voltage between metal types.
Anyway this is off the point and is not helping the matter in how to prevent the corrosion. I'd recomend some of the metal coatings on the market, as this seems the only way forward. It won't stop the corrosion but will slow its progress. Hope this helps!
 
ChrisChase said:
You would think the manufacturers would have been slightly more enlightend and known about galvanic corrosion, especially with a piece of equipement that gets very wet!
Click to expand...

I think these are wise words, Chris. I had a similar problem with an alluminium alloy section of my factory fitted windlass. It seems to have one of its parts made from every metal in the Periodic Table. The alloy had turned to dust.

I now strip all of the external parts twice per year and reassemble with liberal coating of grease.

Apart from the cynical explanation, I wonder why they seem to use so many different materials...........
 
Keeping the salt water away is the best defence. See if you can arrange a deflection plate so that the salt water from the chain retrieval does not pour over the winch. Then lots of grease and wax corrosion guard.
 
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