Getting the Stain Out of Stainless

If you’ve messed around with boats for very long you will know there are all sorts of grades and qualities of “stainless” steel. Sometimes good vendors will supply you with poor quality materials, resulting in a continuing battle with rust stains.On Wind Horse our stainless vendor in Auckland supplied a high quality of stainless, and even after four months of salt water washing, with never a wipe down, it was still shiny – despite the fact that there was so much salt stuck on that we had to use a “Lime Away” type of product to get it off.

We were not so lucky with the heater exhaust sections from Kabola. Within a few weeks they began to run with rust; when we got back to California, Kabola replaced these with a second set. Granted, a heater exhaust is a tough play for the stainless. The combination of heat and salt is much worse than salt by itself. Still, really good quality stainless should have been OK.

electro polishing and passivation to battle rusty stainless on boats

Our new set of exterior exhaust fittings looked fine for the first month or so – and then, as you can see above, they started to rust just like the previous exhaust, with the stains running down the topside.

There are several possible reasons for this. It could be iron deposits from the welding process, or surface contamination from some of the forming equipment.

Whatever the cause of the problem, the solution is an electrochemical process known as “electro polishing”. We took these the rusty exhaust fittings – along with a few other small stainless parts supplied by Stateside vendors – to Alliance Finishing and Manufacturing in Oxnard, California for clean up.

In this process the stainless parts are first dipped in a bath of phosphate/sulfuric acid, with a small DC current (3 amps at 9-12 volts DC per square inch) running through the parts. This process enriches the surface in chromium and adds to the passivity of the stainless while removing surface contaminates.

The second process is called passivation. Passivation is done with a bath of nitric or citric acid.

electro polishing and passivation to battle rusty stainless on boats

The result of these two processes are shiny, clean parts, which should stay rust-free. The rust stains on the aluminum were cleaned off with a mild solution of Ospho (phosphoric acid) applied by brush, and then rinsed with fresh water.

Posted by Steve Dashew  (November 13, 2006)

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