Cape Cod sailboat anodized aluminum corrosion failure

Tom's first Cape Cod consulting visit. A 40-foot sailboat owner had aluminum rails pitting because the anodizer failed to seal the porous coating. Illustration of the trade-off between anodizing-for-bonding (leave porous) and anodizing-for-corrosion-resistance (seal in boiling water).

The first time I was ever down on Cape Cod, I went down there in the middle of a 6-inch snowstorm because some guy here in Boston had bought a nice new 40-foot sailboat. It had a nice wooden gunnel with anodized aluminum rails up above the wooden gunnel — the sides of the rail of the boat. The aluminum was pitting. It was only a year old, and it was going to cost a fortune because you had to remove all that beautiful mahogany woodwork in order to lift the aluminum rail off. He wanted to know why it was pitting. I had to go down there in the middle of the snowstorm and look at it, and I determined that whoever had anodized it forgot to seal it. They didn't know that if you want corrosion resistance for your aluminum oxide you have to continue the process after you form the porous coating. You have to seal it by making continuous coating. But if you're going to use it for an adhesive-bonded prep surface, you don't want to seal it. You want to leave it nice and porous. So it's mechanical interlocking.