Coating Aluminum

Mad Dog · Post by **Mad Dog** » Tue Nov 13, 2007 4:27 pm

I have an aluminum fuel tank under tha sole of my AQ 22-2. Well actually I have removed it from the boat. It has a couple of places where crevice corrosion has set in. None has eroded completely through the aluminum.

My question is, is it safe and sound to coat the outside of the tank with S3 primer and top coating to build up the corroded patches and prevent any further decay?

I would rather be safe than sorry but a new tank is going to run almost $600. The new one would come with and epoxy coating so I am wondering if I can save the one I own.

Thanks.

Boomer · Post by **Boomer** » Tue Nov 13, 2007 5:19 pm

Coal tar epoxies are sort of the new "traditional" coating for metal tanks, due to the flexibility. Do a search and study the results.

I would be very leery of painting the aluminum. It will take special preparation and priming.

Cracker Larry · Post by **Cracker Larry** » Tue Nov 13, 2007 5:28 pm

What Boomer said. Coal tar epoxy.

ks8 · Post by **ks8** » Tue Nov 13, 2007 6:05 pm

Is it crevice corrosion, or is it metal fatigue from vibration? Aluminum hardens with vibration and then becomes more brittle, eventually beginning to crack from the brittle metal being subject to yet more stress and more vibration. Crevice corrosion, I think, is more often associated with stainless steel, when oxygen is somehow frequently restricted from properly oxidizing the chromium in the steel, for building and maintaining the *stainless* protective layer. If it is aluminum metal fatigue, I think the $600 is a whole lot safer alternative, beings that it is fuel you're storing in that tank. Just a thought... but I'm not a professional in this industry.

Cracker Larry · Post by **Cracker Larry** » Tue Nov 13, 2007 6:53 pm

Not worth a risk if there is any doubt to it's integrity. That I know

Once about 20 years ago I had an old 1966 28' Chris Craft Commander, with an inboard gas engine, a Chrysler 360. My mate and I got to the boat early one morning for a charter and smelled gas real bad when we stepped on the boat. Normally one of us would have a lit cigarette in hand, but not today. When we stepped aboard the bilge pump kicked on, which was not normal either. I unlocked the cabin door and the smell was overpowering. I lifted a hatch cover and the bilge was full of gas

The old galvanized 75 gallon tank in the transom had ruptured. I turned off the pump and we got the heck off the boat to think.

Well, we had a real serious problem. The boat was at a large public marina with liveaboards and mega-yachts. Our clients were due. This thing was a floating bomb. I thought about cutting it loose, I thought about pumping it out, I thought of just going home, but I decided to call the fire dept, and sent my mate to wake up the liveaboards and get them away from the boat while I did that. No cell phones back then. So he started evacuating the marina at 0500 on a Saturday, while I went up to a pay phone and called the fire dept and the police, who called the Coast Guard and county Marine Patrol. Then I called my partner to arrange for another boat for my fishing party.

Everybody showed up about the same time, along with the local news crews. It was a very embarrassing and expensive day

and one I never want to repeat. Not to save a mere $600. Just the charter cost me that. Hard to put a price on the rest of it, but it was a whole lot better than it could have been. Nobody got hurt. Don't risk anything relating to fuel to save a few dollars.

That was the most expensive bilge cleaning I ever had, but I tell you what, that bilge was clean

PJPiercey · Post by **PJPiercey** » Tue Nov 13, 2007 6:54 pm

Crevice corrosion is a very real problem with aluminum. Our Bristol Bay, Alaska, commercial fishing fleet of aluminum vessels is suffering from a large number if crevice corrosion cases. The cause of the corrosion in the commercial vessels is due to moisture getting between the foam insulation and the aluminum plating of the fish holds (which are integral to the bottom and sides of the hull). When small amounts of water sit against aluminum for extended periods of time a chemical process develops and a molecule of oxygen is remove from H2O resulting in HO. HO is an acid.

Paul

Boomer · Post by **Boomer** » Tue Nov 13, 2007 7:02 pm

HO is an acid.

Boy I hate being the resident pedantic, but I just can't let all those years of chemistry go to waste.

OH is your basic alkali (or postive pH) ion. Aluminum is much more reactive with alkalies than acids. You can etch it with common household laundry detergent, and a standard paint prep is to clean/etch it with TSP (strong detergent) and then neutralize that with a weak acid wash, such as vinegar.

Mad Dog · Post by **Mad Dog** » Tue Nov 13, 2007 8:19 pm

Boomer wrote: Boy I hate being the resident pedantic,

I had to look that word up so I figure a few others might like to know what it means.

peÂ·danÂ·tic /pÉ™ËˆdÃ¦ntÉªk/[puh-dan-tik]
â€“adjective
1. ostentatious in one's learning.
2. overly concerned with minute details or formalisms, esp. in teaching.

It's amazing what you learn on this site.

Mad Dog · Post by **Mad Dog** » Tue Nov 13, 2007 8:28 pm

Yep! Crevice Corrosion! That's what the fabricator of my new tank called it. No cracks or fractures but pitting and oxidation. It was caused by that infamous flotation foam someone used to cushion the tank.

Anyone need a used 60 gallon fuel tank? I know where you can find one, cheap.

TomW · Post by **TomW** » Tue Nov 13, 2007 9:05 pm

Yep most tanks available are .090 5250 marine aluminum acid etched. They may or not be coated. If you coat them with the coal tar epoxy they will probably last a life time.

As far as an old tank you need to determine the debth of the corrosion/etching if it is minor then coat it with the coal tar epoxy and it should be fine. If it is deep replace the tank.

Tom

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