Q. Can you tell me whether caustic soda will dissolve the zinc coating on zinc-coated steel? Regards. T.C.
A. The caustic soda solution will dissolve the zinc coating to some extent, but will be a function of time, temperature and concentration of the caustic soda in contact with the surface. If your primary concern is whether there still will be adequate zinc coating in place to protect the steel, the answer is yes. The function of the zinc on the steel is to provide corrosion resistance. To provide protection from various environments, increasing levels of zinc coating are available. While there will be some dissolution of zinc at the surface, the majority of it still will be left intact to provide the corrosion resistance as intended.
However, if you have concerns regarding trace levels of dissolution and the effect on something like a waste stream, then you should realize that there will be some zinc dissolution occurring and that it will be present in the liquid and, to a lesser extent, in the subsequent rinses. While zinc will tend to precipitate as a hydroxide salt in an alkaline solution, the caustic solution has too high a pH and will likely remain in solution.