When we use stainless steel castings, white spots often appear after anodizing. The reason for this situation is that the stainless steel casting has a loose structure, high porosity, and contains a variety of metal and non-metallic impurities. Therefore, the quality of the oxide film is often difficult to guarantee, but we can also take other methods to reduce the pollution rate of stainless steel castings:
1. High voltage and high current density impact method. That is, high-voltage electric shock is used in the early stage of oxidation of stainless steel castings, so that the already oxidized parts are connected to the sheet by electric shock, and the surface is polished. This method is to fill the gap left by the oxidized part with the polished aluminum mill, so that the gaps produced are connected together and act as a bridge.
2. Surface shot peening. This method is to hit the gap with a round-headed hammer and close it by tapping to achieve the purpose of connecting pieces. We should choose the corresponding method according to the degree of contamination of stainless steel castings. According to relevant information, before the casting is cleaned, the pit is filled with slag material. Lithofacies analysis shows that iron silicate, manganese silicate and chromium silicate are present in the slag material at the defect. Electron diffraction results show that the black pits are composed of magnetite (Fe3O4) and iron chromium spinel (FeO · Cr2O3). As a result of spectral analysis, the silicon content in the metal components increased at the defects, while the manganese content was extremely small. After the casting is shot blasted and blast cleaned, the surface of the casting will have gray and black pits. The pit defects often appear in the local thick section, corners and inner holes of the casting, or even on the entire surface of the casting.