The corrosion resistance of stainless steel is mainly due to the surface covered with a very thin (about 1 nm) dense passivation film. This film 1n is corrosive to the media and is the basic barrier for stainless steel protection. Passivation of stainless steel has a dynamic feature and should not be seen as complete stoppage of corrosion, but rather as a barrier to diffusion, which greatly reduces the rate of anode reaction. The film tends to be destroyed in the presence of a reducing agent (e.g., chloride ion) and can be retained or repaired in the presence of an oxidizing agent (e.g., air).
Stainless steel parts placed in the air will form an oxide film, but the protection of this film is not perfect. It is usually necessary to conduct a thorough cleaning, including alkali washing and pickling, and passivation with an oxidizing agent to ensure the integrity and stability of the passivation film. One of the purposes of pickling is to create favorable conditions for passivation and to ensure the formation of a good passivation film. Because the surface of the stainless steel surface on the average 10 μm thick is etched away by pickling, the chemical activity of the acid solution causes the dissolution rate of the defect site to be higher than that of other parts of the surface. Therefore, the pickling causes the entire surface to be evenly balanced. Corrosion hazards are removed. But more importantly, through passivation by pickling, the oxides of iron and iron are preferentially dissolved compared to the oxides of chromium and chromium, and the chromium-depleted layer is removed, resulting in the enrichment of chromium on the surface of stainless steel. This chromium-rich passivation film The potential is up to +1.0V (SCE), which is close to the noble metal's potential and improves corrosion stability. Different passivation treatments also affect the composition and structure of the film, and thus affect the stainless properties. For example, the electrochemical modification treatment can make the passivation film have a multilayer structure, forming CrO3 or Cr2O3 in the barrier layer, or forming a glassy state. The oxide film enables stainless steel to exert its maximum corrosion resistance.
Degreasing and Degreasing - Flowing Water Rinsing - Activated - Flowing Water Rinsing - Passivation - Flowing Water Rinsing - Neutralization - Flowing Water Rinsing Clean --- too pure water ---- drying packaging
Process description:
1. Degreasing, if you are sure that the surface of the workpiece is free of any oil, you can omit this step.
2. Activation, soaking time is 2-5 minutes, if you are sure that the workpiece surface without any rust, welding spots and other impurities (such as freshly polished parts), this step can be omitted.
3. Passivation must be processed. The soaking time is 30 minutes, and the longer the passivation film is, the more complete it is.
4. Neutralization, soaking time is 3 minutes, if the surface is simple in shape, no ridges, no blind holes, this step can be omitted.
5. Over pure water must be processed for 3 minutes.