Corrosion on Steel Parts

Q. We manufacture boat trailers using hot-rolled steel (HRS) and some cold-rolled steel (CRS). Heavy steel parts are blasted before assembly, but light-gauge parts are not blasted. Parts are then welded, assembled and powder coated after fabrication. We use a single-step cleaner/iron phosphate treatment that is applied by a handheld spray wand. We have been having problems with rust breaking out on lighter-gauge parts, especially on horizontal surfaces. Rust is showing through on some parts while still in our outdoor storage yard. What is the likely problem, and what can we do to stop the rust?  –T.D.

A. Corrosion occurs when you have a corrosive element in contact with moisture and oxygen. Corrosive minerals can include ferrous metal or some residual salt on the surface of the steel. Factors that can contribute to corrosion are light coating or voids in the coating.

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First see if there is any rust on the steel before it is coated. A corrosion cell that already exists will establish a low-pressure area and extract moisture from the atmosphere through osmotic pressure. The corrosion cell will expand and create a blister under the coating film, the film will crack and flake off, and the corrosion will further expand as the exposure to moisture and oxygen increases. Next, make sure that the cleaning and phosphating is adequate and the operation of the chemistry and spray process is effective. If you do not get the surface cleaned and phosphated properly, the same corrosion process described above will occur. Finally, check the film thickness and look for thin areas. For outdoor performance over ferrous metal, you should have at least 3 mils of coating.

I expect that after checking these things you will find that you have all three problems: some rust already exists, some residual minerals are being left on the part, and some areas do not have enough coating. The most likely root causes are that your cleaning operation is not adequate for the application and your film build is not thick enough. A single-step hand wand can work for outdoor steel, but you do not have much room for error. You need very good chemical control, a dedicated and skilled operator, and good drainage to avoid deposits of unreacted salts. A multiple cleaning operation with a rinse and rinse/seal would be superior, if you can make that change. You also must have enough coating to protect the surface. A primer would add a lot to performance.

So, test your chemistry, inspect the parts before and after cleaning, check film thickness, and test the parts for corrosion on a regular basis. Consider and upgrade to a multiple-stage process for better cleaning and phosphating.