Stainless Steel Valves Corrosion Resistance Of Stainless Steel Materials

Generally speaking, Lined Ball Valve is steel that is not easy to rust. In fact, some stainless steels are both rust-proof and acid-resistant (corrosion-resistant). The rust-proof and corrosion-resistant properties of stainless steel are due to the formation of a chromium-rich oxide film (passivation film) on its surface. This rust-proof and corrosion-resistant property is relative. Tests have shown that the corrosion resistance of steel in weak media such as the atmosphere and water and in oxidizing media such as nitric acid increases with the increase of the chromium content in the steel. When the chromium content reaches a certain percentage, the corrosion resistance of the steel changes suddenly, from easy to rust to not easy to rust, and from not corrosion-resistant to corrosion-resistant. There are many ways to classify stainless steel. According to the organizational structure at room temperature, there are martensitic, austenitic, ferritic and duplex stainless steels; according to the main chemical composition, it can basically be divided into two major systems: chromium stainless steel and chromium-nickel stainless steel; according to the purpose, there are nitric acid-resistant stainless steel, sulfuric acid-resistant stainless steel, seawater-resistant stainless steel, etc. According to the corrosion resistance type, it can be divided into pitting-resistant stainless steel, stress-corrosion-resistant stainless steel, intergranular corrosion-resistant stainless steel, etc.; according to the functional characteristics, it can be divided into non-magnetic stainless steel, easy-to-cut stainless steel, low-temperature stainless steel, high-strength stainless steel, etc. Because stainless steel has excellent corrosion resistance, formability, compatibility, and toughness in a wide temperature range, it is widely used in heavy industry, light industry, daily necessities industry, and architectural decoration industries.
A stainless steel can have good corrosion resistance in many media, but in another medium, it may corrode due to low chemical stability. Therefore, it is impossible for a stainless steel to be corrosion-resistant to all media. In many industrial uses, stainless steel can provide satisfactory corrosion resistance. According to the experience of use, in addition to mechanical failure, the corrosion of stainless steel is mainly manifested in: a serious form of corrosion of stainless steel is local corrosion (i.e. stress corrosion cracking, pitting corrosion, intergranular corrosion, corrosion fatigue and crevice corrosion). Failure cases caused by these local corrosion account for almost more than half of the failure cases. In fact, many failure accidents can be avoided by reasonable material selection.
Metal corrosion can be divided into three types according to the mechanism: special corrosion, chemical corrosion and electrochemical corrosion. Most of the metal corrosion in real life and engineering practice belongs to electrochemical corrosion.
Stress corrosion cracking (SCC): It is a general term for the mutual failure of alloys under stress due to the expansion of cracks in a corrosive environment. Stress corrosion cracking has a brittle fracture morphology, but it can also occur in materials with high toughness. The necessary conditions for stress corrosion cracking to occur are the presence of tensile stress (whether residual stress or external stress, or both) and a specific corrosive medium. The formation and expansion of the cracks are roughly perpendicular to the direction of tensile stress. The stress value that causes stress corrosion cracking is much smaller than the stress value required for material fracture when there is no corrosive medium. Microscopically, cracks that pass through grains are called transgranular cracks, and cracks that expand along grain boundaries are called intergranular cracks. When stress corrosion cracking extends to a certain depth (here, the stress on the cross-section of the material under load reaches its fracture stress in the air), the material breaks according to normal cracks (in ductile materials, usually through the aggregation of microscopic defects). Therefore, the cross-section of parts that fail due to stress corrosion cracking will contain characteristic areas of stress corrosion cracking and "tough dimple" areas associated with the aggregation of microscopic defects.
Pitting corrosion: Pitting corrosion refers to highly localized corrosion that occurs in a dispersed manner on the surface of a metal material with no corrosion or slight corrosion. The size of common corrosion spots is less than 1.00 mm, and the depth is often greater than the surface aperture. In mild cases, there are shallow pits, and in severe cases, perforations are even formed.
Intergranular corrosion: Grain boundaries are disordered and mismatched boundaries between grains with different crystallographic orientations. Therefore, they are favorable areas for the segregation of various solute elements in steel or the precipitation of metal compounds (such as carbides and δ phases). Therefore, it is not surprising that grain boundaries may be corroded first in some corrosive media. This type of corrosion is called intergranular corrosion, and most metals and alloys may exhibit intergranular corrosion in specific corrosive media. Intergranular corrosion is a selective corrosion damage. It differs from general selective corrosion in that the locality of corrosion is microscopic, but not necessarily localized macroscopically.
Crevice corrosion: refers to the occurrence of spot-like or ulcer-shaped macroscopic pits in the crevices of metal components. It is a form of localized corrosion that may occur in crevices where the solution is stagnant or in shielded surfaces. Such crevices can be formed at the joints of metal and metal or metal and non-metal, for example, at the joints with rivets, bolts, gaskets, valve seats, loose surface deposits, and marine organisms.
General corrosion: is a term used to describe the corrosion phenomenon that occurs in a relatively uniform manner on the entire alloy surface. When general corrosion occurs, the material gradually becomes thinner due to corrosion, and even the material fails due to corrosion. Stainless steel may show general corrosion in strong acids and strong alkalis. The failure problem caused by general corrosion is not very worrying, because this kind of corrosion can usually be predicted by a simple immersion test or by consulting corrosion literature.
Uniform corrosion: refers to the phenomenon that all metal surfaces in contact with corrosive media are corroded. Different indicators are required for corrosion resistance according to different usage conditions, and it can generally be divided into two categories:
1. Stainless steel refers to steel that is resistant to corrosion in the atmosphere and weakly corrosive media. A corrosion rate of less than 0.01mm/year is considered to be completely corrosion-resistant; a corrosion rate of less than 0.1mm/year is considered to be corrosion-resistant.
2. Corrosion-resistant steel refers to steel that can resist corrosion in various strongly corrosive media.

Corrosion resistance of various stainless steels
304 is a general-purpose stainless steel that is widely used in the manufacture of equipment and parts that require good comprehensive properties (corrosion resistance and formability).

301 stainless steel exhibits obvious work hardening when deformed and is used in various occasions that require higher strength.

302 stainless steel is essentially a variant of 304 stainless steel with a higher carbon content, which can be made to have higher strength through cold rolling.

302B is a stainless steel with a higher silicon content, which has higher resistance to high-temperature oxidation.

303 and 303Se are free-cutting stainless steels containing sulfur and selenium respectively, which are used in occasions where free cutting and high surface finish are mainly required. 303Se stainless steel is also used to make parts that require hot upsetting, because under such conditions, this stainless steel has good hot workability.

304L is a variant of 304 stainless steel with a lower carbon content, which is used in occasions that require welding. The lower carbon content minimizes the precipitation of carbides in the heat-affected zone near the weld, which may cause intergranular corrosion (weld erosion) of stainless steel in certain environments.

304N is a nitrogen-containing stainless steel, and nitrogen is added to increase the strength of the steel.

305 and 384 stainless steels contain higher nickel content and have lower work hardening rates, making them suitable for various occasions that require high cold formability.

308 stainless steel is used to make welding rods.

309, 310, 314 and 330 stainless steels all have higher nickel and chromium contents in order to improve the oxidation resistance and creep strength of the steel at high temperatures. 30S5 and 310S are variants of 309 and 310 stainless steels, the only difference being that the carbon content is lower in order to minimize the precipitation of carbides near the weld. 330 stainless steel has particularly high resistance to carburization and thermal shock.

316 and 317 stainless steels contain aluminum, so their resistance to pitting corrosion in marine and chemical industrial environments is much better than that of 304 stainless steel. Among them, 316 stainless steel has variants including low-carbon stainless steel 316L, high-strength stainless steel 316N containing nitrogen, and free-cutting stainless steel 316F with a high sulfur content.

321, 347 and 348 are stainless steels stabilized with titanium, niobium and tantalum, and niobium, respectively, suitable for welding components used at high temperatures. 348 is a stainless steel suitable for the nuclear power industry, and has certain restrictions on the amount of tantalum and cobalt.