There are several methods for chemical surface treatment of a material. Their purpose is to obtain a clean, metallic surface.
1. Pickling
Pickling aims to remove oxides, primarily the colored scale that forms as a result of welding the material, as well as other discolorations or corrosion deposits. The result of pickling is a clean metallic surface, which enables the initiation of the natural, spontaneous passivation of stainless steel on the metal surface. The most common pickling agents contain nitric acid (HNO3) and hydrofluoric acid (HF). The time required to carry out proper pickling depends on the solution temperature and its concentration.
2. Passivation
Natural (spontaneous) passivation of stainless steel occurs in the presence of oxygen contained in water or in the air. However, the process under natural conditions is lengthy and can take even several days before the passive layer reaches an adequate thickness. Chemical passivation aims to accelerate the formation of the passive layer and guarantees its rapid growth until optimal strength is achieved. Passivation is carried out in a solution of diluted nitric acid, and the treatment time ranges from several minutes to one hour.
3. Acid cleaning or decontamination
This process removes iron particles that corrode when they are on the surface of stainless steel. This situation occurs when stainless steel parts are exposed to contamination, e.g. iron (grinding dust, filings, corroded iron parts, etc.)
4. Electropolishing
Electropolishing is used to clean metallic surfaces, reduce roughness, and achieve many other desired effects. The process utilizes the phenomenon of electrolysis, so it requires an electric current and the use of an electrolyte; most often this is a solution of sulfuric and orthophosphoric acids. Electropolishing usually takes from a few to a dozen minutes.
