All metals react with oxygen in the atmosphere to form an oxide film on the surface. Unfortunately, the iron oxide that forms on ordinary carbon steel continues to oxidize, allowing the rust to expand and eventually to form holes. Carbon steel surfaces can be protected by electroplating with paint or oxidation-resistant metals (eg, zinc, nickel, and chromium), but, as is known, this protection is only a thin film. If the protective layer is damaged, the underlying steel begins to corrode.
Steel that is resistant to weak corrosive media such as air, steam, and water, and chemically corrosive media such as acid, alkali, and salt. Also known as stainless acid-resistant steel. In practical applications, the steel that is resistant to weak corrosive media is often called stainless steel, and the steel that is resistant to chemical media corrosion is called acid-resistant steel. Due to the difference in chemical composition between the two, the former is not necessarily resistant to chemical medium corrosion, while the latter is generally stainless. The corrosion resistance of stainless steel depends on the alloying elements contained in the steel.
Chromium is the basic element for stainless steel to obtain corrosion resistance. When the chromium content in steel reaches about 12%, chromium and oxygen in the corrosive medium act to form a thin oxide film (self-passivation film) on the steel surface. , which can prevent further corrosion of the steel matrix. In addition to chromium, the commonly used alloying elements are nickel, molybdenum, titanium, niobium, copper, nitrogen, etc., to meet the requirements of various uses for the structure and properties of stainless steel.
