In the process of processing stainless steel pipes, the surface of the pipes will always produce yellow or black oxide scales, which are not only beautiful but also seriously affect the corrosion resistance during use. In order to solve these problems, the stainless steel pipe must be processed after the corresponding acid Wash passivation treatment.
The purpose of pickling and passivation is to remove the oxide scale generated after welding, high temperature treatment and other processing steps, so that the surface of the stainless steel tube restores its original silver luster, and a layer of chromium is also formed on the surface of the treated stainless steel tube. The oxide film of the material can prevent the stainless steel tube from being corroded by oxidation again so as to achieve the purpose of passivation. This can enhance the corrosion resistance of the surface of the stainless steel product and prolong the service life of the stainless steel tube equipment in disguise.
From the effect of the above passivation, the corrosion resistance of stainless steel is mainly due to the fact that the surface is covered with a dense passivation film with a thickness of 1-15 nm. This film can completely separate the corrosive medium and is protected by stainless steel tubes. The basic barrier to external corrosion. In addition, the passivation film of stainless steel has obvious dynamic characteristics, because corrosion is a dynamic process that changes constantly, and the passivation film also forms a continuously spreading barrier, which greatly reduces the degree of anode reaction. Generally, in the case of external reducing agents (for example, chloride ions), the film tends to be destroyed, and in the presence of an oxidizing agent (such as air), the film is constantly maintained and repaired.
The passivation treatment of stainless steel tubes is generally thoroughly cleaned first, which includes both alkali washing and pickling, and then passivation using an oxidant, so that it is possible to ensure good integrity and stability of the passivation film. One purpose of pickling is to create the right conditions for the passivation process and ensure the formation of a good passivation film. Since the surface of the stainless steel plate is corroded by an average of 10 μm thick on the surface of the stainless steel, the chemical activity of the acid solution causes the dissolution rate of the defect site to be higher than that of the normal surface. Therefore, the pickling process can cause the entire surface of the stainless steel pipe to be inclined. It is evenly balanced and some of the hidden dangers that are likely to cause corrosion are also removed.
But more critical is that passivation by pickling will make the oxides of iron and iron preferentially dissolve away from the oxides of chromium and chromium. The removal of chromium-depleted layers will lead to the enrichment of chromium on the surface of stainless steel tubes. The potential of the chromium-rich passive film can reach +1.0V (SCE), which is close to the noble metal’s potential, which enhances the stability of corrosion resistance. Different passivation processes affect the composition and structure of the passivation film, which affects the stainless properties of the material. If the electrochemical modification treatment is used, the passivation film can be provided with a multilayer structure, and chromium trioxide can be formed on the barrier layer. Or chromium trioxide may also generate a glassy oxide film. From this point of view, the corrosion resistance of the stainless steel tube can be fully utilized.
Source: China Stainless Steel Pipes Manufacturer – Yaang Pipe Industry Co., Limited (www.yaang.com)