Because of its excellent mechanical properties, high hardness and wear resistance, stainless steel has been widely used in the military parts processing field. Their surface treatment is carried out after heat treatment, solid solution and aging, because the heat treatment process of those thicker oxide skin and material itself has some particularity, resulting in pickling passivation processing is very difficult.
Pickling passivation process
The process of stainless steel surface treatment is not complicated. However, in the actual processing, the pickling passivation process is very difficult, mainly for two points. It is a stainless steel material can not rust according to conventional methods to remove oxidation surface skin, followed by stainless steel after the passivation processing technology will not meet air corrosion, will not damage the workpiece dimensions and surface finish, is actually in the process must form a passivation film surface passivation with strong oxidizing acid wash this agent to the parts, so as to obtain the corrosion resistance.
The main elements in stainless steel with titanium, chromium, nickel, silicon, manganese and molybdenum oxide, their skin is relatively thick and dense in the heat treatment process, and compact oxide film consisting of iron, chromium, nickel and oxygen with spinel structure (FeCrO4), its chemical properties are very stable. This oxide film is not easy to remove.
Parts for low temperature solidification and aging
Low temperature solid aging stainless steel parts are thin, usually light purple, and it is easier to handle the oxide skin of such stainless steel parts. After pickling, the surface oxide can be basically clean, and it will not corrode itself. The stainless steel parts are not covered with ash and can be directly passivated.
For high temperature solid fusion aging parts
High temperature solid aged stainless steel parts have relatively thick oxide scales and are usually black purple or even black. Because the surface of stainless steel parts will produce thick, dense and complex oxide skin, it is difficult to remove the oxide scale of stainless steel parts by pickling. Removal of such oxide should be carried out according to the process of loosening oxide scale, pickling and removing dust. Because of the stainless steel plate after the loose oxide process, the surface of stainless steel parts will generally attach a layer of ash, need to remove this layer of ash and then passivation treatment.
Removal of the operation process of hanging ash is more important, mainly passivation process, after pickling parts in the air corrosion resistance is poor, if exposed to the air, stainless steel parts will rust surface. In order to enhance its corrosion resistance, it is necessary to produce a compact and corrosion resistant passivation film on its surface, so as to achieve the purpose of corrosion resistance.
Source: China Pipe Fitting Manufacturer – Yaang Pipe Industry Co., Limited (www.yaang.com)