With the increasing health requirements for drinking water, thin-walled stainless steel water supply pipes have been widely used in public buildings and urban residences, while countries such as the Netherlands, Germany, Spain, and Italy began using them as early as 60 years ago. Its application can effectively prevent secondary pollution of drinking water during transportation, establish a barrier for people's health, and is a major transformation of drinking water supply pipelines.

The occurrence of corrosion damage to stainless steel can be roughly divided into two categories: large-scale corrosion and local corrosion.large areaCorrosion refers to the corrosion that is evenly or unevenly distributed on the surface of the entire component. This corrosion is easily detected by people and is easier to take countermeasures. Local corrosion refers to the phenomenon where corrosion occurs selectively in a small area, leading to local damage to the component. Local corrosion is relatively concealed and can cause sudden accidents without warning. Local corrosion is an important form of corrosion damage in stainless steel components during use.

The common forms of corrosion damage in stainless steel include spot corrosion, crevice corrosion, intergranular corrosion, stress corrosion, wear corrosion, cavitation corrosion, galvanic corrosion, and high-temperature corrosion.

Common forms of corrosion damage to stainless steel include:

1. Spot corrosion:It is a common form of obvious corrosion of stainless steel. Corrosion is concentrated in certain specific point areas of stainless steel, where small pits of corrosion develop deeper, and the corroded areas turn black or dark brown. In most severely corrosive environments, the number and depth of pitting increases, giving the surface a corroded appearance. Under weak corrosion conditions, if the pitting surface is not obvious, the rust spots may seep out and cause the surrounding area to lose its luster.

2. Gap corrosion:It refers to the destruction of the passivation layer at the gaps of metal components, resulting in spotted or ulcerated corrosion pits, which is a form of local corrosion. Such gaps can be formed at the junction of metal to metal, metal to non-metal, when encountering factors such as rainwater, tap water, or non-metallic cleaning agents, leading to the formation of crevice corrosion. If formed at the contact of rivets, bolts, welding, and loose surface deposits (sand particles, dirt, welding slag, and rust layer). Gap corrosion is prone to occur when oxygen is insufficient, especially in areas where cracks are very small and oxygen is difficult to penetrate.

3. Stress Corrosion (SCC):There are two possible situations where stress corrosion cracking may occur. Firstly, stainless steel may undergo stress corrosion cracking when exposed to chloride aqueous solutions; Secondly, stainless steel is prone to stress corrosion without reducing the metal molecular structure that is damaged after processing or deep processing. For example, in a sea fog environment, the temperature exceeds the normal ambient temperature (exceedingAt 60 ℃ or lower temperatures, stainless steel is subjected to high tensile stress, which can cause stress corrosion cracking.

4. Intergranular corrosion:Carbon in stainless steelWhen combined with chromium (usually containing 0.08%), grain boundaries precipitate during heat treatment or welding, resulting in chromium deficiency at the grain boundaries. After heating, the carbon content of stainless steel increases, leading to a decrease in corrosion resistance. If the surface of steel is further contaminated by grease, paint, and paint, intergranular corrosion is also prone to occur.

5. Electrochemical corrosion:This corrosion may occur when two metals with a significant difference in electrochemical potential energy come into contact with each other. When carbon steel and stainless steel come into contact, driven by electrolytic voltage or stray current, the two metals generate a current circuit, and the electrolytic current between the two components can cause electrochemical corrosion. If the corrosion rate increases under the continuous action of current, it can corrode the components in a short period of time.

Common corrosion phenomena and analysis of stainless steel water supply pipes during installation

There are many pipelines in a building, including water supply and drainage pipes, cable pipes, gas supply pipes, fire protection pipes, etc., interwoven together, and most of them are in the basement and between the pipelines. The ventilation is poor, the space humidity is high, the usage environment is poor, and there are many pipeline corrosion phenomena. However, for stainless steel water supply pipelines, it is crucial to prevent corrosion of stainless steel fittings due to environmental factors and improper installation. Below, we will focus on introducingThin walled stainless steel pipelineThere are several forms of corrosion that are prone to occur during installation and use:

1large areaCorrosion phenomenon:Stainless steel pipe fittings are widely present in environments with chloride ions and corrosive media, causing the passivation film to be damaged, leading to the formation of corrosion pits. In addition, some inclusions, chromium rich areas, grain boundaries, dislocations, etc. on the surface of stainless steel are prone to corrosion, causing rust spots and loss of luster around them, resulting in corrosion. So try to improve the usage environment, reduce the content of chloride ions, keep the space clean and dry, and reduce dust adhesion during use. For example, in coastal cities, humid spaces, and areas with high chloride ions, choose corrosion-resistant materials with good corrosion resistance316 or 316L material, reducing the erosion of chloride media.

2. Crack corrosion phenomenon.The conditions that cause crevice corrosion in stainless steel are quite extensive, and almost all corrosive media will cause crevice corrosion in stainless steel pipe fittings. The occurrence of crevice corrosion can be caused by water vapor containing chloride ions in rainwater and condensate, especially in areas where cracks are very small and oxygen is difficult to penetrate. For example, crevice corrosion is prone to occur in flange plates, screw connections, and connections with other different materials of water supply pipes. Therefore, special attention should be paid to crevice corrosion in project design. In environments and areas prone to encountering water vapor and corrosive media, efforts should be made to avoid crevice formation. A clamping type connection method can be considered to reduce crevice corrosion.

3. Electrochemical corrosion phenomenon.The above are two types of metals with significant electrochemical potential differences, such as carbon steel and stainless steel, which may exhibit potential differences in contact and are prone to corrosion. We also need to emphasize here that when carbon metal or grinding wheel cutting foam, residual welding oxides, carbon steel grinding powder layer and other corrosive media remain in stainless steel pipes, and encounter the effect of voltage and current (or stray current) in the surrounding environment, unexpected electrochemical corrosion will occur. This phenomenon is not very common and is often overlooked by everyone. If it occurs, the rapid corrosion rate will have a certain impact on construction.

stray current The current flowing outside the designed or specified circuit is called stray current. It is mainly divided into DC stray current, AC stray current, and earth (natural) stray current.

Generation of stray current:AC stray current mainly comes from the transmission, induction, and grounding of industrial electricity. There are strong AC stray currents near substations, radar stations, carrier phones, and wired broadcasting stations. DC stray current is mainly caused by electrified railways, DC motors, DC electrical appliances, and various grounding and lightning protection devices.

Characteristics of stray current corrosion:（1) Rapid corrosion; (2) More concentrated in the local area; (3) When there is an external anti-corrosion layer, focus on the defective parts of the anti-corrosion layer (insulation material with carbon ions). Causes pitting of stainless steel pipes in a short period of time.

In addition, for the purpose of insulation, a layer of black rubber plastic foam material is coated on the outer surface of the steel pipe at the construction site. This packaging material contains carbon black, which is a good conductor, making the insulation material also have a certain degree of conductivity. The two fit together, forming countless contacts between the stainless steel pipe and the rubber plastic insulation layer, causing similar corrosion shallow pits to appear on the surface of the stainless steel pipe.

The phenomenon of stray current corrosion often exists in the middle and lower parts of the horizontal pipe, and the reason is that the corrosive medium precipitated in the steel pipe encounters stray current and causes stray current corrosion. However, this phenomenon rarely occurs in the installation of vertical water pipes.

The corrosion principle of stainless steel pipes is:Due to external factors, there is stray current on the basement wall of the project. Stainless steel pipes are not insulated during installation, and pipe clamps, brackets, etc. come into contact with the wall or other pipes, forming a closed circuit of current and causing stray current corrosion.