A Safer Way to Remove Mill Scale
Awhile ago, I published a post regarding removing mill scale from hot rolled steel with muriatic acid. Although it is a quick method, there several safety issues when handling muriatic acid. One being that the acid is very strong and quite corrosive to many surfaces including human flesh. Another safety issue is fumes and vapors. Even when dilute with three times the volume with clean water, the odor of the acid can be oppresive. Oh, let us not forget that, like other strong inorganic acids, muriatic acid is a poison.
Out of curiosity, I had to try a safer alternative. One day I wanted to descale some steel indoors because it was cold out and there was a lot of snow. I decided citric acid as a mill scale remover. Although skeptical, I realized that citric acid was allowing me to remove rust stains from my blue jeans. Citric acid is a weak organic acid but it has a strong affinity for iron. Citric acid is a chelating agent which forms water soluble complexes with iron. I used about 1.5 ounces of crystalline citric acid dissolved in 2 cups of near boiling water for this test.
The hotter temperatures speed up the reaction considerably. Instead of waiting for 24 hours, it takes less than two hours at very hot solutions. At first, it looked as if the mill scale was still there. But on rubbing the metal, it was coming off as a blackish smut leaving an uninspired dull grey finish. When you lightly sand or wire brush the metal, it shines up nicely.
Citric acid is excellent for rust removal also. It is less aggresive than other rust cleaners or converters. The thickness of the rust determines soak time.
There are several blogs and forums discussing passivation of stainless steel with citric acid. It removes iron from the surface of the alloy preventing rust stains.