There are many kinds of stainless steel, its main components are nickel and chromium, while nickel, chromium and other elements are vulnerable to strong acid and alkali corrosion. The sweat discharged by human body contains hydrochloric acid. When wearing a watch, if it is often touched by sweat or allowed to accumulate acid, alkali and salt dirt and not cleaned up in time, rust spots will appear slowly in the strap and other places, and black stains will also appear on the wrist. This is mainly due to the contact of nickel and other elements in stainless steel with sweat or vinegar, bleaching agent, paint solution and other chemicals, chemical reagents, cosmetics and other substances in human body. The usual treatment is to dip the strap in a soft toothbrush with a cleaning agent and then dry it with a soft cloth; or to clean it with an ultrasonic instrument at the brand authorized maintenance station.