Galvanic corrosion is what happens when dissimilar metals are in contact within an electrolyte and an electrical current is applied. That’s the technical definition. The practical description is it’s the white stuff that grows around your stainless steel fasteners in your aluminum mast and boom when you are around salt water.
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You can clean away the corrosion that you can see on the outside of parts but don’t be fooled, it’s not just a harmless white powder. It’s what you cannot see that’s the bigger problem. It slowly and invisibly welds fasteners and fittings together to the point that they cannot be loosened without destroying one or the other.
It can be a serious problem, especially on older boats. Following are examples from my 1981 Catalina 22 that were bad enough that they required replacement parts, a Hall of Shame of sorts. Learn from them and prevent corrosion on your boat.
The first example is the boom end fitting shown below. Notice the concentration of corrosion around the screw holes. I needed to remove the screws that attach it to the boom to replace the eye strap with a larger one in my mainsail outhaul project. One screw came out with some extra effort. The other snapped off flush with the casting. The only way to extract what remained was to try to drill it out.
“Try” is the operative word here. Drilling out small diameter fasteners made of hard, brittle metal that are seized in a soft, rounded casting is a gamble at best. Unless you can clamp the casting very solidly, use very sharp, very hard drill bits, and prevent the bit from wandering to the edge of the fastener, it will drift into the softer surrounding metal and the game will be over because you will have lost.
As you can see from the picture below, I lost about half of the thickness of the casting in my attempt and decided that it would be too weak to reuse considering how critical this part is to the rigging. I sometimes use my boom as a gin pole to raise and lower my mast. Having these screws pull out could have disastrous results.
The next example is (gulp) my masthead. I needed to remove it to install an anchor light although I first discovered the extent of the corrosion during my oversize masthead sheaves project. The masthead through-bolt came out easily enough. It’s not a tight fit there. But as you can see in the picture below, the tenon was heavily corroded and I had to hammer it off the end of the mast. I was eventually able to get the sheave pins out but not without more than a little extra effort. However, the forestay and backstay pins were the worst.
If you ever have to do this job, here’s some tips that helped me.
Be patient and try to work the pins out gradually by tapping them back and forth with the least amount of force necessary. If at first they don’t budge, try soaking the masthead at least overnight in a mild acid solution that will dissolve some of the corrosion, like household vinegar. Avoid very strong acids that can eat away the aluminum, for example, muriatic acid . You can also try heating the masthead lightly with a torch but be careful to not get it too hot or you could damage the temper of the casting.
You might need to use a punch to tap the pins all the way out. If they need really forceful persuasion (a bigger hammer), be careful to not deform the ends of the pins. If you flare the end of a pin, it won’t fit through the hole in the masthead and you’ll have to either file the flare away or cut the end off and sacrifice the pin. The cotter pin holes in clevis pins are especially weak. If one collapses shut, you’ll need a new pin. After you get the pins out, clean out any remaining corrosion in the pin through-holes.
Be very careful to not overstress the casting or it will break as in the picture above. This happened after I had successfully removed the three other pins and spent hours trying different methods to loosen the fourth but it hardly moved at all. It is possible to weld a break like this but it will never be as strong as the original. Considering how critical this piece is, it is a bad idea and you should replace the part.
At the time that I wrote this, the masthead casting alone was no longer available from that online Catalina parts dealership even though it appeared on their website. They did accept my order for the entire masthead assembly including sheaves, spacer, and pins. However, they are built to order and require 2 to 12 weeks lead time. In the summer of 2014, I had to wait 15 weeks for mine, a seriously long time to not be able to sail.
Don’t get rid of this inhibition
To prevent this level of corrosion from happening to you, apply a corrosion inhibitor like Marelube TEF-45 to all fasteners in dissimilar metals. Use corrosion inhibitor anywhere a stainless steel fastener is in constant contact with aluminum, including the masthead tenon. Depending on how much your boat is exposed to salt water and sea air, inspect and reapply on a regular basis such as when you’re doing your standing rigging inspection.
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