The effect of pickling and passivation during prefabrication, welding, testing, and heat treatment will cause iron oxide, welding slag, grease, and other dirt to accumulate on the surface of the pipe (carbon steel pipe, carbon copper pipe, stainless steel pipe), which will reduce the corrosion resistance of the steel pipe. Variety. Pickling is a chemical rust removal method: dilute acid rust removal mainly removes metal oxides on the surface of stainless steel pipes with oxygen. For ferrous metals, it mainly refers to iron oxide, which chemically reacts with these metal oxides and dissolves them in the acid to achieve the purpose of rust removal. Before pickling and rust removal, the grease on the wall of the stainless steel pipe should be removed first, because the presence of grease prevents the pickling liquid from contacting the wall of the steel pipe. Affects the rust removal effect. Oil-free pipelines (such as stainless steel pipelines for oxygen) must be degreased first. Pickling refers to using pickling solutions such as sulfuric acid to wash away the oxide layer and dust on the workpiece. Phosphating is a treatment method for cleaning the surface.