Paint Bubbles on Mast

[RobertMartin]

My Mast is painted black, some area's around the cleats and a few other spots are bubberling ( spelling?? ) under the paint, should this just be sanded back and painted or is the something else that should be done ( and will probably cost a bloody fortune ), oh ya, its an aluminium mast...

Bobby aka Seawolf..
Freedom is the song of your soul..

 

[ccscott49]

Most likely cause is corrosion, scrape off paint, snad down nicely, apply chromate primer or another primer especially for ally or non ferrous metals and repaint black, with black smoothrite or similar.

 

[AndrewB]

Yes, that's going to be the cause. Stainless screws/bolts in aluminium again, the mastmaker's answer to planned obsolescence. The aluminium dissolves into a white goo that attacks the paint from underneath. Why the paint in the first place, I wonder?

If you just clean it up and repaint around, it'll be back again within a couple of years, max. You have to prevent any moisture getting in to restart the reaction, so the cleats need to be taken off, the area underneath and the holes cleaned up and painted, and then remounted. It is a pain remounting bolted-on cleats, since the only access is via the mast foot. Another approach is to mount them on backing plates and rivet them to the mast with monel rivets.

 

[ccscott49]

If you want to stop the corrosion reappearing, use monel or bronze screws, monel is horrendously expensive, but bronze isn't especially down there, it comes in from the Philipines or Thailand.

 

[AndrewB]

Bronze screws.

If bronze screws are the answer, why is stainless invariably used? Most bronzes are at a similar point in the galvanic scale to 300 series stainless, so why would they not cause the same corrosion in aluminium?

Bronze screws are not easy to find in the UK, and I've no experience of using them. Brass screws are commonplace - but they are quickly weakened by dezincing if used where seawater can reach them. I've never come across monel screws.

 

[ccscott49]

Re: Bronze screws.

Bronze (silicon, aluminium) does not have zinc in it, so will not de-zincify, thats brass. Anglia stainless steel in suffolk have a good range of bronze screws bolts etc. Bronze is more expensive than stainless. Thats why it's not commonly used, in production boats, but very common in quality wooden boats. Monel is a non magnetic alloy, developed for the oil industry for use in drilling equipment used with magnetic surveying equipment, drill collars etc. It is also used in arctic conditions, where its high nickel content resists thermal cracking, common in steel alloys. It is also very resistant to corrosion and is used in the food industry aswell. Copper berrilium is also used in the oilfiled for the same reasons, but is horrendously expensive. I've just noticed, you didnt say bronze would de-zincify, so ignore that bit of the message, sorry, should read better! But bronze is a lot more common than you think.

<P ID="edit"><FONT SIZE=-1>Edited by ccscott49 on Thu Mar 14 10:51:21 2002 (server time).</FONT></P>

 

[Guest]

Corrosion may not be Galvanic... It is likely to be filliform corrosion a type of crevice corrosion that can happen at any exposed edge of paint/ aluminium. What happens is moisture and oxygen gets between the paint and the aluminium and forms an active Hydrochloric acid 'head'. Providing this head is fed with a supply of water and oxygen it will creep under the paint evidenced by the tell-tail 'fingers' of corrosion creeping from the hole/ crack/ damage etc...

This can sometimes be associated with a Galvanic corrosion cell, but is not caused by the galvanic cell... The answer is to clean it up, removing all evidence of the corrosion (back to clean metal), prime with etching chromate primer and several coats of top coat. Then, most importantly seal the hole with Mastic Frame sealer (B&Q) ensuring that there is no chance of water and air getting to the edge of the paint... p.s. any fittings that are put on the mast must be bedded down with mastic. Mastic Frame Sealant is best since it stays soft and lasts for years and allows you to remove the fitting in the future... silicone doesn’t come off so easy!

p.s. Monel rivets are the best solution, they should be coated in Zinc which reduces the galvanic potential difference.

Andrew