What is the situation that the stainless steel cover is always black?

It is normal that the stainless steel cover always has black color when washing.

The black thing ground by stainless steel is a chemical polishing material called wax removal, which is a wax product. It is toxic, and long-term intake may cause abnormal cells, listlessness and memory loss. In severe cases, it may even cause cancer. Fortunately, this will gradually fade with continuous use, and a small amount will not be a big problem in the short term. Wipe it clean, it won't wash off, so you must wipe it hard.

Polishing wax is divided into: blue wax, purple wax, blue wax, black wax and yellow wax (sand wax). The main components of polishing wax are stearic acid, palmitic acid, oleic acid and rosin, and abrasives such as feldspar powder, chromium oxide, corundum and iron red are made into different fineness and varieties according to different matrix components and requirements. Generally, different colors are used to represent different kinds of polishing wax to avoid confusion when using. High content of abrasive can accelerate the whole polishing process. This process is that the surface of the polishing wheel moves to make the material smooth. The polishing process is divided into three steps: rough polishing, medium polishing, fine polishing and fine polishing. In order to obtain the best performance, targeted polishing wax must be selected.