Both pickling and passivation are chemical treatments applied to the surface of stainless steel castings to remove contaminants and assist the formation of a continuous chromium-oxide, passive film.
What is Pickling？
Pickling can effectively remove oxide scale and rust on the surface of stainless steel castings by using acid solution. It is a way to clean the surface of stainless steel castings, and is usually carried out together with passivating treatment.
Pickling Process: hanging stainless steel casting parts → chemical degreasing (conventional alkaline chemical degreasing or surfactant degreasing) → hot water washing → running water washing → first pickling → running water washing → second pickling → running water Washing → transfer to the next process (such as: chemical coloring → recycling → running water washing → hardening treatment → running water washing → sealing treatment → running water washing → drying → finished product).
What is Passivation?
Passivation is the treatment on stainless steel castings with a mild oxidant with intent to remove free iron and other foreign matter, for the purpose of improving corrosion resistance by enhancing the formation and structure of the material’s naturally occurring, corrosion-resistant passive film. Passivation prepares the alloy to perform to its engineered capabilities, in a corrosive environment. The chemicals used in the passivation process are not aggressive enough to dissolve surface oxides, thus, pickling may be used prior to, or in lieu of, passivation.
Why Pickling & Passivation on Stainless Steel Castings?
The main purpose of pickling is to remove oxide scale on the surface of stainless steel castings. Therefore, stainless steel castings do not have to be pickled. It is only needed when there is oxide scale on the surface. For example, stainless steel castings annealed with protective gases such as pure hydrogen and ammonia decomposition do not require pickling.
Pickling is usually associated with passivation when operating. Stainless steel castings that need pickling are generally passivated, because there is no passivation film formed on the surface after pickling or the thickness of the passivation film is very thin, which has no effect. A follow-up passivation process must be added to form and perfect a passivation film on the surface of stainless steel. This film is the key for stainless steel to not rust and corrode in general media.
The advantages of passivation are as follows:
● Compared with the traditional physical sealing method, the passivation treatment has the characteristics of absolutely no increase in the thickness of stainless steel castings and no change in color.
● Since the passivation process is carried out in a non-reactive state, the passivation agent can be added and used repeatedly, so the life is longer and the cost is more economical.
● Passivation promotes the formation of the oxygen molecular structure passivation film on the surface of stainless steel castings. Compared with the traditional method of applying anti-rust oil, The passivation film is more stable and corrosion resistant.
Pickling and passivation is one of the surface finish ways that usually operated after stainless steel casting. Beside Pickling & passivation, we can also supply other surface finishes, such as shot blasting, electro-polishing, mirror polishing, etc.