Galvanic corrosion on aluminium mast

[paul.devalier]

I've just noticed what appears to be galvanic corrosion on the keel stepped aluminium mast, but only above deck.
I've just measured current (while connected to shore power and without and with all electrical connections on and off)
I get 0 volts across keel bolts and mast in all conditions.
I get 0.04V across keel bolts.
The propeller anode was checked and is fine.
The mast below deck is perfect. Photo of below and above.
Any ideas or suggestions for confirming and tracing the problem.
I just noticed this and the only change I made recently was replacing the VHF antenna.
These spots may have always been there but I don't think so. To my best recollection these have appeared since early Spring.








Thanks for any help!

[GordMay]

“Dissimilar Metal Corrosion” [AKA: galvanic Corrosion, or wrongly 'electrolysis'] refers to corrosion damage, induced when two dissimilar materials are coupled, in a corrosive electrolyte. It occurs when two [or more] dissimilar metals [such as aluminum, with copper or stee[ are brought into electrical contact, in the presence of water.
Where aluminium is in contact with a more noble metal, and there is [especially sea] water, then there will be corrosion.
Dissimilar-metal corrosion, is one of the three most common types of corrosion, in aluminium. The others are pitting corrosion, and crevice corrosion.

You can prevent galvanic corrosion through:
★ Electrical insulation. You electrically insulate the metals from each other. The insulation needs to break all contact between the metals.
★ Breaking the electrolytic bridge. Where insulation is difficult, in large constructions for example, an alternative solution is to keep an electrolytic bridge from forming between the metals. Painting can help you with this. I suggest coating the cathode surface, i.e. the most noble metal. You can also use an insulating layer between the metals.
★ Cathodic protection. The most common way to do this is to mount an anode of a less noble material (often zinc) in direct metallic contact with the aluminium object to be protected. By doing this, you sacrifice the less noble material – it corrodes – for the aluminium. We call it a sacrificial anode. Another way to obtain cathodic protection is to connect the aluminium object to the negative pole of an exterior DC voltage source.

[boatpoker]

Originally Posted by paul.devalier

I get 0 volts across keel bolts and mast in all conditions.
I get 0.04V across keel bolts.

AC or DC volts ?

[paul.devalier]

@gordmay Thank you. Does it look like galvanic corrosion? Below deck the mast is perfect.

@boatpoker I measured in DC. Just now I tried AC and I am getting 0.03 volts across keel bolts to mast but 0 volts across fore and aft keel bolts.

[thinwater]

It looks more like it got peppered with grinding swarf. Something that penetrated the anodizing. Did something slap on the outside? It also looks like someone tried to sand some of them out in the past.

[paul.devalier]

Originally Posted by thinwater

It looks more like it got peppered with grinding swarf. Something that penetrated the anodizing. Did something slap on the outside? It also looks like someone tried to sand some of them out in the past.

It's possible. I removed the mast last year to do some keel bolt work, which is when the new antenna was set. The mast was running lengthwise behind three other boats.

You are saying the grinding debris is hot enough to de-anodise where it strikes?

What would be a fool proof electrical current test?