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Cost Effective Corrosion Treatment of Aluminum for Boat Docks

May 24, 2009

Aluminum is an attractive material for building structural frames for fresh water, recreation duty docks.
Customers and builders alike will want to know the corrosion life of the dock, assuming seasonal use and outdoor winter storage. Considering the regulatory licensing process, and costs of new process equipment at a small startup company, how can we compare A) regulatory "hoops", B) process equipment cost and C) corrosion life for:
1) anodizing
2) chromate
3) phosphoric acid
4) electroless nickel

?
What about in the marine environment?

James Parker
project engineer, wannabe entrepreneur - Wichita, Kansas, United States of America

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May 28, 2009

Hello James,

It isn't perfectly clear whether you would be making the docks AND finishing them, or just finishing them. If you are making them AND finishing them, consider sending the parts out to a job-shop to have the finish applied, and then assembling yourself. It saves you all the headache and substantial cost of setting up a plating line, and goes much further to ensure that you get high quality results from the get go. If you are going to be just finishing them, why limit yourself to just docks?

Assuming you are going to be building the docks, and you are willing to consider sending the work out, discount chromating or using phosphoric acid right off the bat - the results won't be what you need. The corrosion resistance of both (used by themselves) would be pretty low in a wet environment. Of the other two, nickel will give the best corrosion resistance, but the parts will be shinier than regular aluminum; anodize may well give good enough resistance and the parts will look like aluminum. I would bet that the aluminum would give you a pretty decent life expectancy in fresh water, in salt water - with a regular seal rather than sodium dichromate (would give a green yellow color you probably don't want) I would be less certain. Cost wise, the anodize would probably cost you less than the nickel, and can be dyed to give you different colors.

Other technologies you have not mentioned include powder coating and painting (amongst others). Powder coating OVER anodize, for example, give a really fantastically corrosion resistant finish that is often used in or near marine environments. And, you could probably get something designed to be non-slip, which would be nice for a dock.

Your best bet, with regard to evaluating corrosion life, would be to check out what finishes are typically used for aluminum used in those same environments. For example, I don't know what finish is typically applied to aluminum canoes, but I bet their lifespan is a close approximation of what you would like. Another benefit to doing a market comparison like this is that whatever finishes ARE used, the manufacturers of those products have already spent a decent advertising dollar convincing the same consumers you'll be going for that the finish is acceptable - this might make it a lot easier to sell your product.

Good luck!

Jim Gorsich
Compton, California, USA