Stainless steel equipment will inevitably damage the original surface state in the manufacturing process (cold processing and hot processing), for example, contact with iron products may cause iron ion pollution; Because of the heat input, the welding process will form the tempering color, and so on. Therefore, subsequent surface treatment is very necessary. For surface treatment, pickling and passivation are often mentioned. Pickling and passivation are different surface treatment processes.
Pickling is to remove any high-temperature oxide scale and adjacent chromium-poor layer from the stainless steel surface by chemical means and the tempering color formed by welding. When stainless steel is heated to a tempered color by welding, heat treatment, or other methods, a chromium-poor layer will be formed under the surface layer. Low chromium content will reduce corrosion resistance. To restore the best corrosion resistance, the damaged metal layer must be removed to expose the fully alloy stainless steel surface. Mechanical removal may leave abrasive or other particles embedded (interfering with corrosion properties) or, therefore, chemical means are usually used. Generally, the mixed acid of nitric acid (HNO3) and hydrofluoric acid (HF) is used to remove the oxide layer and chromium-poor layer to restore corrosion resistance. At the same time, it can also remove iron ion pollution. The chloride ion in hydrochloric acid (HCl) will cause pitting corrosion of stainless steel, so hydrochloric acid is not used for mixed acid. Of course, a citric acid lotion is also used in industrial applications.
Pickling solution (paste) usually contains mixed acid and its inert carrier, which is sprayed (coated) on the area to be treated. After a certain time, it is reused with clean water and then dried.
The pickling process removes trace metals.
Passivation is to treat the stainless steel surface with an acid solution (high) to remove pollutants and repair the damaged passivation film (such as through grinding, machining, or other mechanical damage). Common treatment media include nitric acid (HNO3) solution or paste containing nitric acid. After passivation, completely remove all acid by flushing, and it may be advantageous to neutralize the acid with alkali before the flushing step.
Generally, the passivation treatment is helpful to the rapid formation of the passivation film and will not cause significant changes in the surface appearance.