Stainless Steel Knowledge

All metals react with oxygen in the atmosphere, forming an oxide film on the surface. Unfortunately, the iron oxide formed on ordinary carbon steel continues to oxidize, causing rust to expand and eventually form holes. Paint or oxidation-resistant metals (such as zinc, nickel, and chromium) can be used for electroplating to protect the carbon steel surface, but as is well known, this protection is only a thin film. If the protective layer is damaged, the underlying steel begins to rust.

Steel that resists corrosion from weak corrosive media such as air, steam, water, as well as chemically aggressive media such as acids, alkalis, and salts, is called stainless steel or acid-resistant steel. In practical applications, steel that resists corrosion from weak corrosive media is often referred to as stainless steel, while steel that resists chemical media corrosion is called acid-resistant steel. Due to differences in chemical composition, the former is not necessarily resistant to chemical media corrosion, whereas the latter generally possesses stainless properties. The corrosion resistance of stainless steel depends on the alloying elements contained in the steel. Chromium is the basic element that gives stainless steel its corrosion resistance. When the chromium content in the steel reaches about 12%, chromium reacts with oxygen in the corrosive medium to form a very thin oxide film (self-passivation film) on the steel surface, which prevents further corrosion of the steel matrix. In addition to chromium, commonly used alloying elements include nickel, molybdenum, titanium, niobium, copper, and nitrogen to meet various requirements for the structure and properties of stainless steel in different applications.

Definition of Stainless Steel

A high-alloy steel that can resist corrosion in air or chemically corrosive media. Stainless steel has an aesthetic surface and good corrosion resistance. It does not require surface treatments such as plating or coloring to bring out its inherent surface properties. It is a type of steel used in many applications and is commonly referred to as stainless steel. Representative types include 13-chromium steel, 18-8 chromium-nickel steel, and other high-alloy steels. From a metallographic perspective, because stainless steel contains chromium, a very thin chromium film forms on the surface. This film isolates the steel from intruding oxygen and provides corrosion resistance. To maintain the inherent corrosion resistance of stainless steel, the steel must contain more than 12% chromium.

Types of Stainless Steel

Stainless steel can be broadly classified by application, chemical composition, and metallographic structure. Austenitic steels are basically composed of 18% chromium and 8% nickel. Variations in the amounts of each added element lead to the development of various steel grades for different applications.

1. Classification by chemical composition:

① CR series: Ferritic series, Martensitic series
② CR-NI series: Austenitic series, Anomalous series, Precipitation hardening series

2. Classification by metallographic structure:

① Austenitic stainless steel
② Ferritic stainless steel
③ Martensitic stainless steel
④ Duplex stainless steel
⑤ Precipitation hardening stainless steel

3. Surface finishes of stainless steel

With the development of stainless steel, its properties such as corrosion resistance, appearance, workability, and strength have far surpassed those of other materials. Moreover, many surface treatment methods for stainless steel can achieve a wide variety of colors and shapes, which greatly contributes to the development of stainless steel.