The corrosion of stainless steel pipe is mainly caused by the material properties, surface conditions, medium properties, pH value, temperature, flow rate and time. In addition, the factors affecting the properties of stainless steel include composition, crystal structure and passivation. Membrane and so on.
The composition and crystal structure of the stainless steel pipe and the contained impurities determine the corrosion resistance of the material. For example, the addition of niobium and titanium in a stainless steel pipe can effectively prevent the formation of chromium carbide, which enhances the grain boundary corrosion resistance of the material. The proper combination of molybdenum and chromium can effectively stabilize the passive film even when chloride is present in the environmental media.
Many grain boundary corrosions are caused by heat treatment, and stainless steel pipes heated to a certain temperature during the welding process will generate a deposition of chromium carbide on the grain boundary. Therefore, chromium is lost in the area close to the chromium carbide. Increase the liveliness of chromium in the area. If there is an aqueous solution condition, a primary cell with bare chromium as the anode and stainless steel as the cathode will be generated. Larger cathode area will form anode control, so the corrosion is very serious, resulting in intergranular cracking or pitting. This is called intergranular corrosion of the welded joint. This type of stainless steel is also an activated steel. Using low-carbon austenitic stainless steel can reduce this problem.
Passivation film is the main barrier to protect stainless steel, but on the other hand, passivation characteristics of the metal or alloy, the stronger the passivation ability is, the higher the sensitivity to pitting corrosion, stainless steel is more prone to pitting corrosion than carbon steel. .
The occurrence of pitting corrosion is related to the presence of active anions or oxidizing cations in the medium. Most of the pitting events occur in chloride- or chloride-containing media. Experiments have shown that, under anodic polarization conditions, pitting corrosion can occur in the metal as long as it contains chloride ions. Therefore, the chloride ion is also called the “exciting agent” of the pitting corrosion, and with the increase of the chloride ion concentration in the medium, the pitting potential decreases, so that the pitting corrosion easily occurs, and then it is easy to accelerate.
The pitting corrosion rate of the metal in the stationary state of the medium is greater than when the medium is in the flowing state. The flow rate of the medium plays a dual role in slowing down the hole erosion. Increasing the flow rate on the one hand is conducive to the transport of dissolved oxygen to the metal surface, making the passivation film easier to form; on the other hand, it can reduce the chance of sediment deposition on the metal surface, thereby reducing the occurrence of Hole erosion opportunities. The induction period for the occurrence of pitting corrosion is usually from a few months to a year depending on the circumstances.
Source: China Stainless Steel Pipe Manufacturer – Yaang Pipe Industry Co., Limited (www.yaang.com)