Galvanic corrosion

[marioxp]

I have a lot of problems with galvanic corrosion on my boat. I had to change the housing for the anchor winch a few years ago because corrosion damaged the aluminum housing so much that the screws literally fell out. Now I see that the corrosion is beginning at the new housing. The anchor chain is very rusty, I think that first all the zinc came off the surface and then it rusted. I also saw some minor damage to the rudder shaft. A white powder appears on the window frames. I have sacrifice anodes on the keel and on the saildrive, I regularly change them every year, wear is visible on them but not that they are completely eaten.

I don't have a galvanic isolator installed, do you think a galvanic isolator would prevent this damages.

When I am not on board, the ship is not connected to the electrical grid.

[geoff]

The corrosion of the windlass is universal. Lofrans mount them in a rubber gasket which forms a little lake of salt water . I replaced mine years ago  when the corrosion was worse than yours. I mounted the new one on a flat nylon board [kitchen chopping board] 10 years on and no problem. The rudder shaft may be the result of copper antifouling used too close to the shaft. I cant see a galvanic isolator would help ,BUT they are a good idea anyway. Geoff

[symphony2]

The corrosion on the windlass comes from the use of stainless fastenings in aluminium which causes the galvanic action plus the aluminium itself will corrode if not protected with a coating and in damp conditions. The way to reduce it and even avoid it completely is to paint the bottom face and remove all the fastenings then put back with something like Duralac or Tef Gel which will isolate the stainless from the aluminium. As suggested if your type of windlass has a rubber mounting pad, remove that as it holds moisture. The loss of galvanising is a different matter and is dependent on the quality of galvanising which varies from chain to chain and is difficult to tell whether the galvanising is good or not even on new chain. Chain is subject to abrasion which can also wear the zinc away and once the coating is gone rust starts and spreads. At some point often between 6 and 10 years if you use the anchor regularly you have to either regalvanise or junk the chain. As already suggested the pitting on the rudder shaft is almost certainly electrolysis between the copper in antifouling and the aluminium shaft and can be avoided by using a non copper antifoul like Trilux in a 10cm ring around where the shaft comes out of the hull. The window surround corrosion is again just the aluminium where the coating has failed or where it is not coated but polished has probably been scratched. Difficult to avoid except by keeping clean and removing any powder when it appears.

None of this is anything to do with anodes or the 240v system. Anodes are used when the 2 metals are in contact in an electrolyte like seawater and the only example on a standard Bavaria is the anode between the saildrive housing and the stainless internals and fastenings. Some propellers made with mixed metals may also have anodes. No point in having an anode on the keel as it will not stop rust on an iron keel. However it may well erode because the keel is not pure iron but has tiny amounts of other metals with different potentials in a huge mass of iron. The zinc is eroding in preference to these "bits". You will not notice any difference if you remove the anode on the keel except you have a hole to fill.

Hope this helps.

[marioxp]

Thank you both very much for the answers.

I didn't even say that my windlass is mounted on a stainless steel plate. The original plastic plate was cracked and the repairman installed two stainless steel plates for the former owner, one on the top and the other on the bottom. I have now pasted plastic kitchen plates from IKEA over both stainless steel plates and put heatshrink tubes on the screws to totally electrically isolate the windlass from these stainless steel plates, I hope that will help. I put Duralac on the underside of the windlass.

My only concern is this pool of water you mention that collects in the rubber seal, do you think it is better to remove the rubber seal?

Can the screws be removed from the windlass case? Should Duralac be applied to them as well?