The screw maintenance layer shall meet the requirements:

1. Corrosion resistance, wear resistance, high hardness
2. The structure is tight and intact with small pores.
3. It is firmly separated from the base metal and has good adhesion.
4. It is evenly distributed and has a certain thickness.
The maintenance layer is usually divided into metal coating and non-metallic coating. Metal coating refers to the maintenance layer formed by metal or alloy with strong corrosion resistance on the surface of easily corroded metal. This coating is also called coating. There are many methods and varieties for producing metal coatings, the most common of which is electroplating, followed by molten metal immersion (hot dip) and chemical surface treatment. Non-metallic coating refers to the use of organic polymer materials such as paint, and inorganic materials such as ceramics in metal equipment or parts