Thread Content
Put simply, stainless steel is steel that does not rust easily; in fact, some types of stainless steel possess both rust resistance and acid resistance (corrosion resistance). The rust resistance and corrosion resistance of stainless steel are due to the formation of a chromium-rich oxide film (passivation film) on its surface. This film isolates the metal from external substances, preventing further corrosion of the metal; it also has the ability to repair itself – if damaged, the chromium in the steel reacts with oxygen in the surrounding environment to re-form the passivation film, thereby continuing to provide protection. This rust resistance and corrosion resistance are relative. Tests have shown that in weak media such as air and water, as well as in oxidizing media such as nitric acid, the corrosion resistance of steel increases as the chromium content in the steel rises. When the chromium content reaches a certain percentage, there is a sudden change in the steel’s corrosion resistance: it goes from being prone to rusting to less prone to rusting, and from being non-corrosion-resistant to corrosion-resistant. The rust resistance of stainless steel is also related to the environment in which it is used; different environments require stainless steel with varying chromium contents. The chromium content is the fundamental factor that determines the properties of stainless steel. It is reported that standards in countries such as Europe and the United States require that the chromium content be no less than 10.5%, Japan sets the requirement at 11%, while China’s standard is 12%. Classification of stainless steel: There are five basic types of stainless steel: austenitic, ferritic, martensitic, duplex stainless steel, and precipitation-hardening stainless steel. (1) Austenitic stainless steels are non-magnetic. Representative grades of these steels contain 18% chromium and a certain amount of nickel to enhance their corrosion resistance, and they are widely used. (2) Ferrite is magnetic, with chromium being its main component at a concentration of 17%; this material possesses excellent oxidation resistance. (3) Martensitic stainless steels are also magnetic; they typically contain 13% chromium and a suitable amount of carbon, and can be hardened through quenching and tempering. (4) Duplex stainless steels have a mixed structure of ferrite and austenite; their chromium content ranges from 18% to 28%, while their nickel content is between 4.5% and 8%. They offer excellent resistance to chloride corrosion. (5) The conventional content of chromium in precipitation-type stainless steel is 17, with certain amounts of nickel, copper, and niobium added; these can be hardened through precipitation and aging. Based on their microstructure, they can be classified as: (1) Ferritic stainless steels (400 series), which are chromium-based stainless steels; the main representatives include Gr13, G17, Gr27-30 ； (2) Austenitic stainless steels (300 series), chromium-nickel stainless steels; the main representatives include 304, 316, 321, etc ； (3) Martensitic stainless steels (200 series), chromium-manganese stainless steels with a high carbon content; representative grades include 1Gr13, etc. Why do stainless steels rust? Reasons for rusting: Chromium is key. There may be several reasons for stainless steel to rust: (1) The presence of chloride ions in the environment. Chloride ions are widespread, such as in table salt/sweat/seawater/seaside air/soil, etc. Stainless steel corrodes rapidly in environments containing chloride ions, even faster than ordinary low-carbon steel. Therefore, there are requirements regarding the environment in which stainless steel is used, and it needs to be wiped regularly to remove dust and kept clean and dry. (That way, we can label it as “improper use”). ) There is an example in the United States: a company used an oak container to hold a solution containing chloride ions; this container had been in use for over a hundred years. In the 1990s, it was planned to replace it, and since oak is not a modern material, stainless steel was used as a replacement. However, 16 days after the replacement, the container leaked due to corrosion. (2) Without solution treatment, the alloying elements do not dissolve into the matrix, resulting in a low alloy content in the matrix structure and poor corrosion resistance. (3) Inherent intergranular corrosion: These titanium- and niobium-free materials have a tendency to suffer from intergranular corrosion. Adding titanium and niobium, along with stabilization treatment, can reduce intergranular corrosion. A high-alloy steel that can resist corrosion in air or chemical corrosive media. Stainless steel features an attractive surface and excellent corrosion resistance; it does not require any surface treatment such as plating, allowing it to exhibit its inherent surface properties. It is a type of steel used in various applications, and is commonly referred to as stainless steel. High-alloy steels such as 13 chromium steel and 18-8 chromium-nickel steel represent high performance. From a metallographic perspective, stainless steel contains chromium, which forms a very thin chromium layer on its surface; this layer acts as a barrier, preventing corrosion caused by oxygen that penetrates into the steel. To maintain the inherent corrosion resistance of stainless steel, the steel must contain more than 12% chromium. Used in applications that require welding. The lower carbon content minimizes the precipitation of carbides in the heat-affected zone near the weld; such carbide precipitation can lead to intergranular corrosion (weld erosion) of stainless steel in certain environments. Because it damages the stainless steel surface and causes rust to form due to the accumulation of iron particles. In daily life, we sometimes observe that the stainless steel used in various facilities such as street flagpoles, bus shelters, and light boxes shows signs of rusting. Since stainless steel is supposed to be resistant to rust, why does it still get rusty? There are two reasons for these situations: first, the chromium content in the material is low, indicating that it is low-quality stainless steel. Second, it isn’t stainless steel at all; instead, users are deceived with electroplating. It is understood that many decorative materials these days have their surfaces treated using this electroplating process. Since the material is ordinary steel, it will naturally rust once the electroplated layer peels off. Stainless steel has the ability to resist oxidation in the atmosphere—that is, its rust resistance—and it also possesses the ability to resist corrosion in media containing acids, alkalis, and salts—that is, its corrosion resistance. However, its corrosion resistance varies depending on the chemical composition of the steel itself, its microstructure, operating conditions, and the type of environmental medium. For example, 304 steel pipes have excellent corrosion resistance in a dry and clean atmosphere, but when placed in coastal areas exposed to sea fog containing high levels of salt, they rust very quickly ； The 316 steel pipe, on the other hand, performed well. Therefore, not all stainless steels are resistant to corrosion and do not rust in any environment. Stainless steel gains its corrosion resistance thanks to an extremely thin, strong, dense, and stable chromium-rich oxide film (protective layer) that forms on its surface, preventing further penetration of oxygen atoms and continued oxidation. Once, for some reason, this film is continuously damaged, oxygen atoms from the air or liquid will keep penetrating in, or iron atoms from the metal will continue to separate, forming loose iron oxide; as a result, the metal surface suffers from continuous rusting. There are many ways in which this protective film on the surface can be damaged; the most common ones in daily life are as follows: 1. Dust containing other metal elements or particles of different metals accumulate on the surface of stainless steel. In humid air, the condensation water between these deposits and the stainless steel forms a microbattery, triggering an electrochemical reaction that damages the protective film – this phenomenon is known as electrochemical corrosion. 2. Organic substances such as juices from fruits and vegetables, noodle soup, sputum, etc., adhere to the stainless steel surface. In the presence of water and oxygen, these substances form organic acids, which over time cause corrosion of the metal surface. 3. The surface of stainless steel can become adhered to substances containing acids, alkalis, and salts (such as alkaline solutions or lime water used in wall decoration), leading to localized corrosion. 4. In polluted air (such as air containing large amounts of sulfides, carbon monoxide, and nitrogen oxides), when it comes into contact with condensed water, sulfuric acid, nitric acid, and acetic acid are formed, leading to chemical corrosion. All of the above situations can damage the protective film on the stainless steel surface, leading to rusting. Therefore, to ensure that the metal surface remains bright permanently and is free from rusting, the following recommendations are given: 1. It is necessary to regularly clean and scrub the surface of decorative stainless steel to remove any adhering substances and eliminate external factors that could cause damage. 2. Stainless steel of grade 316 should be used in coastal areas, as this grade can resist corrosion caused by seawater. 3. In the market, the chemical composition of some stainless steel pipes fails to meet the corresponding **standards; they do not satisfy the requirements for 304-grade material. This can also lead to rusting, which requires users to carefully choose products from reputable manufacturers.