The main local corrosion forms of stainless steel pipes include intergranular corrosion and point corrosion two. What are the reasons for their formation? How can the corrosion be avoided effectively? The following will be described in detail.
Intergranular corrosion of stainless steel pipe
The preferential corrosion is along the grain boundary. This is one of the most common forms of local corrosion. But now it is decreasing. Sensitized intergranular corrosion is due to the stainless steel pipe by 450 ~ 850 DEG C temperature sensitization (post weld heat affected zone), along the steel grain boundary will precipitate chromium carbides, which can lead to grain boundary chromium depletion. Even if the ultra-low carbon stainless steel containing 0.03% carbon is used, it will lead to chromium depletion if it stays for a long time at the sensitive temperature. This effect is the same as that of the previous alloy elements. The solid-state intergranular corrosion is due to the segregation of silicon, phosphorus, sulfur and other elements in the crystal boundary of the stainless steel pipe. The corrosion form of chromium nickel austenitic stainless steel is easily produced in the high temperature and high pressure urea plant in the HNO3 containing Cr6+.
How to prevent the intergranular corrosion? The first thing to do is to select ultra-low carbon stainless steel with carbon content below 0.03% when welding purposes, but also to avoid staying at a long time at the temperature of sensitization. If conditions are available, it can also choose the brand whose carbon is less than 0.02%. Use of various kinds of stainless steel containing the stabilizing elements of titanium, niobium, such as 0Cr18Ni11Ti, 0Cr18Ni11Nb and so on. For the environment that is easy to form solid intercrystalline corrosion, urea class, nitric acid grade stainless steel and so on can be used. In addition to the necessary welding process, the stainless steel material should be prevented from sensitizing temperature during the manufacture and processing of stainless steel.
Corrosion phenomenon of stainless steel pipe point
Most of the corrosion is produced at a certain point on the surface of stainless steel pipeline, and then penetrates further and deeper. The pitting on the outer surface of the stainless steel pipeline is generated by using the process. Pitting corrosion of stainless steel pipes are usually shown in aqueous solutions containing halogen ions, such as Cl-, Br-, F- and so on. Point corrosion is caused by the destruction of the passivation film on the weak part of the passivation film on the surface of the stainless steel pipe. If the surface of the stainless steel contains iron particles, dust and dirt and other attachments, and the inclusions of MnS and some other intermetallic compounds, pitting can also be formed.
The corrosion prevention measures, need to pay attention to regular cleaning and maintenance of stainless steel pipe surface; improving steel purity, reducing manganese and sulfur content of nonmetallic inclusions, using the pitting resistance equivalent value of high stainless steel pipe is high chromium molybdenum and chromium, molybdenum, high nitrogen stainless steel. In the sea water containing Cl- and who is medium, increase flow rate is larger than 1.5m/s, stainless steel pipe surface attachment to avoid sediment and sea creatures.
Source: China Stainless Steel Pipe Manufacturer – Yaang Pipe Industry Co., Limited (www.yaang.com)