Why is stainless steel rusty?

- Sep 18, 2019-

When brown rust spots (dot) appear on the surface of the stainless steel tube, people are surprised that it is not stainless steel, and there may be problems with steel.

In fact, this is a misconception of a stainless steel security window that lacks an understanding of stainless steel. Stainless steel will also rust under certain conditions. Stainless steel has the ability to withstand corrosion in media containing acids, bases, and salts—that is, corrosion resistance. However, its anti-corrosion ability is absolutely excellent in corrosion resistance with its chemical composition, added state, use conditions and dry and clean atmosphere, but it is moved to the seaside area and contains a lot of salt. In the sea fog, the steel pipe performed well. Therefore, not any stainless steel can resist corrosion and rust under any environment.

Stainless steel is formed by a very thin, sturdy and fine-grained layer that continues to infiltrate and continue to oxidize, thereby obtaining the ability to resist rust. Once for some reason, the film is continuously destroyed, air or liquid will continue to infiltrate or iron atoms in the metal will be continuously separated, forming loose oxygen and rusting.

A stainless steel can have good corrosion resistance in many media, but in another medium, it may be corroded due to low chemical stability. Therefore, a stainless steel is unlikely to resist corrosion of all media.

Corrosion of metals can be divided into special corrosion, chemical corrosion and electrochemical corrosion according to the mechanism. Most of the metals are electrochemical corrosion.

The main forms of corrosion of stainless steel are uniform corrosion (surface corrosion), pitting corrosion, crevice corrosion and stress corrosion.

Uniform corrosion

Uniform corrosion refers to the phenomenon that all metal surfaces that contact corrosive media are corroded. According to different requirements for different indicators, they can generally be divided into two categories:

1. Stainless steel refers to steel that is resistant to corrosion in atmospheric and weakly corrosive media. A corrosion rate of less than 0.01 mm is "completely resistant to corrosion"; a corrosion rate of less than 0.1 mm/year is considered "corrosion resistant".

2. Corrosion resistant steel refers to steel that is resistant to corrosion in a variety of strongly corrosive media.

Pitting corrosion

Pitting corrosion refers to the local corrosion that occurs on the surface of metal materials without corrosion or corrosion and is highly dispersed. The size of common etch points is less than 1.00mm, the depth is often larger than the surface aperture, and the lighter ones have shallower etch pits, even serious A perforation is formed.

Crevice corrosion

Crevice corrosion refers to the occurrence of spotted etch pits in the gaps of metal members, which is a type of localized corrosion.

Intergranular corrosion

Intergranular corrosion is a selective corrosion attack that is similar in that the locality of corrosion is microscopically scaled and not necessarily macroscopically.

General corrosion

General corrosion is a term used to describe the corrosion phenomena that occur in a more uniform manner across the entire surface of the alloy. When total corrosion occurs, the village material becomes thinner due to corrosion, and even the material corrosion fails. Stainless steel may exhibit general corrosion in strong acids and bases. The failure problem caused by general corrosion is not very worrying because it can usually be predicted by simple immersion tests or by consulting the literature on corrosion.

Stress corrosion cracking (SCC)

Stress corrosion cracking is a general term for stress-bearing alloys that fail in each other due to the expansion of the striated lines in corrosive environments. 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 are tensile stress (whether residual stress or applied stress, or both) and the presence of specific corrosive media. The formation and expansion of the profile is approximately perpendicular to the direction of the tensile stress. This stress value that causes stress corrosion cracking is much smaller than the stress value required for material fracture without the presence of corrosive media. At the microscopic level, the crack passing through the grain is called a transgranular crack, and the crack along the grain boundary is called an intergranular crack. When the stress corrosion cracking spreads to a depth (here, the load is on the material section) The stress reaches its breaking stress in the air), and the material is broken by normal cracking (in the ductile material, usually by polymerization of microscopic defects). Thus, the section of the part that fails due to stress corrosion cracking will include a characteristic region of stress corrosion cracking and a "dimple" region associated with the polymerization of the microdefect.