Under what circumstances will intergranular corrosion occur in 304 stainless steel pipe

Under what circumstances will intergranular corrosion occur in 304 stainless steel pipes? 

Intergranular corrosion of 304 stainless steel is mainly influenced by the material's own properties, as well as the surrounding environment and processing conditions. The specific circumstances are as follows: 

I. Material Factors

1. High carbon content: In 304 stainless steel, the carbon element has a strong affinity with the chromium element. When the carbon content is high, within a specific temperature range, the carbon will combine with chromium to form chromium carbide (Cr₂₃C₆) and precipitate at the grain boundaries. This results in a decrease in chromium content near the grain boundaries, creating a chromium-poor area. When corrosive media are present, the chromium-poor area is prone to intergranular corrosion. 

2. Inappropriate alloy element ratio: The ratio of alloy elements such as chromium, nickel, and molybdenum in 304 stainless steel is crucial for its intergranular corrosion resistance. If the chromium content is insufficient, or the contents of elements like nickel and molybdenum deviate from the normal range, it will affect the stability of the passivation film and corrosion resistance of the stainless steel, and increase the sensitivity to intergranular corrosion. 

II. Processing Factors

1. Welding Process: When welding 304 stainless steel pipes, the weld seam and the heat-affected zone will undergo a rapid heating and cooling process. During this process, the welding thermal cycle may cause the stainless steel to remain in the temperature range of 450 - 850°C for a certain period of time. This is precisely the temperature range where chromium carbide is prone to precipitate, thereby resulting in chromium deficiency at the grain boundaries, making the welded area prone to intergranular corrosion. This phenomenon is also known as welding corrosion. 

2. Cold processing deformation: When 304 stainless steel pipes undergo cold processing, such as bending and stretching, internal stress and lattice distortion will be generated in the material. These defects will increase the activity of the grain boundaries, accelerating the diffusion of chromium atoms at the grain boundaries. Under certain conditions, this promotes the precipitation of chromium carbide, thereby increasing the tendency of intergranular corrosion. 

III. Environmental Factors

1. Specific Corrosive Medium: When 304 stainless steel pipes are placed in a corrosive medium containing specific ions, intergranular corrosion is prone to occur. For instance, in environments with chloride, sulfate, etc., these ions will destroy the passive film on the surface of the stainless steel, especially in the chromium-poor regions at the grain boundaries. The repair ability of the passive film is weaker in these areas, making it more susceptible to erosion and thus triggering intergranular corrosion. 

2. Temperature and Humidity: Temperature and humidity also have significant effects on intergranular corrosion. Generally speaking, in environments with relatively high temperatures and humidity, the activity of the corrosive medium increases, and the diffusion speed of ions accelerates, which will accelerate the process of intergranular corrosion. Especially when the temperature is within the sensitive range of 450 - 850℃ and there is corrosive medium present, the possibility of intergranular corrosion will increase significantly. 

To prevent intergranular corrosion of 304 stainless steel pipes, one can choose 304L stainless steel with a lower carbon content or stainless steel containing stabilizing elements such as titanium and niobium. During welding, appropriate welding techniques and materials should be adopted, the welding heat input should be controlled, and stress-relieving annealing treatment should be carried out on the stainless steel pipes after cold processing. At the same time, efforts should be made to avoid the stainless steel pipes being exposed to strong corrosive media environments.