aluminum corrosion

[HAWAIIAN]

my boat have some white powder on the bottom of the step mast i know this is aluminum corrosion, can someone tell me how to protect and preserve aluminum mast and how to clean it .

thank you

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[richardandtracy]

There've been quite a few posts about aluminium corrosion, and it would be a good idea to look through some of them - I'd suggest you use the search facility.

The way to avoid aluminium corrosion is basically to prevent water and/or air getting to any part of the metal, particularly crevices between two parts. In general this will boil down to Anodising, Paint or grease/ mastic etc. dependant on the applicability of where the corrosion is.

If the anodising had been breached, it's possible to use Alochrom to repair the anodizing - but it is pretty nasty stuff and really should be confined to industrial uses.
Paint - the best type is epoxy as the diffusion rate of air and water through an epoxy film is the lowest you'll get for a paint.
Finally grease/mastic etc. The use of these should be limited to areas where paint/ anodising won't work as it's really only a temporary solution. This is really places like threaded holes, rivets and so on.

A final, and usually significant, cause of corrosion is having metals other than aluminium attached electrically to the aluminium (SS rivets, galvanised rigging - that sort of thing). If all these items are electrically isolated from the aluminium (say by wet assembling with mastic), then a fair proportion of the corrosion problems should disappear without paint or anodising.

Regards

Richard.


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[AndrewB]

There was a time when yachts were regularly built with aluminum masts stepped on stainless deck plates used to spread the load. As these easily trap sea-water, electrolytic action is inevitable and chews into the bottom of the mast. If you haven't already, its worth taking a look at the condition of the mast heel when the mast is next down. If its bad, cutting an inch off the bottom of the mast is easy enough.

I never heard of a mast coming down because of this happening, but it would still be a good idea to isolate the metals as Richardandtracy suggests.

A power wire-brush is the most effective way of cleaning unanodised aluminium (be sure to wear eye protection). I'm with Richardandtracy that epoxy paint will give the best finish, but if you have areas that rub, like the mast heel, it may help to insert a thin piece of plastic to keep dissimilar metals apart.

 

[maxxi]

One other thing to look at, frequently present but not too often discovered.
Look for stray electrical currents that may go between, say, the mast step & a nearby cablegland or other fitting, particularly when the deck is wet. Tiny currents can result from a pinched wire or a chafed cable in the mast, and can eat away the aluminium alarmingly quickly.
You will need a good multimeter and a thorough system of checking, with and without electrical power switched on ( dont forget the domestic radio if you are using the rigging as an aerial).
Good luck

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[mfhmfh]

As a preventative, especially were in contact with stainless steel try using Lancote.
A natural Lanolin product (a million + sheep can't be wrong) and very effective. Available from Davey's.

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