Various corrosion and measures of valves are as follows:
1. Stress corrosion
Stress corrosion is generated for stainless steel in a corrosive medium environment containing oxygen and chloride ions. The proportion of the failure caused by stress corrosion is as high as about 45%.
Commonly used protective measures for stress corrosion
Reasonable selection of materials: the selection of stress corrosion resistant materials mainly include austenitic chromium-nickel steel with high purity, austenitic chromium-nickel steel containing much silicon, ferritic steel containing much chromium and ferritic-austenitic dual-phase steel. Among them, the ferrite-austenite dual-phase steel has the best stress corrosion resistance. The methods of controlling stress: when assembling, try to reduce stress concentration, and make the part in contact with the medium have the smallest residual stress; prevent scratches, and strictly abide by the specifications of the welding process.
Strictly follow the operating procedures: strictly control the raw material's composition, flow rate, medium temperature, pressure, pH value and others. Add corrosion inhibitors within the allowable range of process conditions. When chromium-nickel stainless steel is used in dissolved oxygen chloride, the mass fraction of oxygen should be reduced to 1.0×10-6 or less. Practice has proved that in water containing chloride ions with a mass fraction of 50.0 × 10-6, it is only necessary to add a mixture of nitrate with a mass fraction of 150.0 × 10-6 and sodium sulfite with a mass fraction of 0.5 × 10-6 and good results can be obtained.
2. Holes and its preventive measures
Small holes generally occur easily in static media. The holes usually develop along the direction of gravity or the lateral direction. Once a hole is formed, it will automatically accelerate to the depths. The oxide film on the surface of stainless steel is dissolved in an aqueous solution containing chloride ions. As a result, small holes with a diameter of 20 μm to 30 μm are formed on the base metal. These small holes are pitting nuclei. As long as the medium contains a certain amount of chloride ions, nuclei may develop into pits.
Common preventive measures: add molybdenum, nitrogen, silicon and other elements to stainless steel or increase the chromium content while adding these elements. Reduce the content of chloride ions in the medium. Add corrosion inhibitors. Increasing the stability of the passivation film may help the damaged passivation film to be passivated again. Use external cathodic current protection to suppress pitting.
3. Pitting
As any metal contains non-metallic inclusions to varying degrees, these non-metallic compounds will quickly form pitting under the corrosive action of Cl ions. The Cl ions outside the pit will move to the pit due to the closed battery effect. The positively charged metal ions in the pit will move to outside of the pit. Stainless steel with Mo has better pitting corrosion resistance than that without Mo. The more Mo is added, the better the pit corrosion resistance becomes.
4. Crevice corrosion
The mechanism of crevice corrosion is the same as that of pit corrosion. It is corrosion that occurs due to the closed battery in the crevice, which leads to the gathering of Cl ions. This type of corrosion generally occurs in the gaps among flange gaskets, lap joints, bolts and nuts, and the gaps between heat exchange tubes and tube plate holes. Crevice corrosion is greatly related to the concentration of a static solution in the gap. Once there is a crevice corrosion environment, the probability of inducing stress corrosion is very high.
1. Stress corrosion
Stress corrosion is generated for stainless steel in a corrosive medium environment containing oxygen and chloride ions. The proportion of the failure caused by stress corrosion is as high as about 45%.
Commonly used protective measures for stress corrosion
Reasonable selection of materials: the selection of stress corrosion resistant materials mainly include austenitic chromium-nickel steel with high purity, austenitic chromium-nickel steel containing much silicon, ferritic steel containing much chromium and ferritic-austenitic dual-phase steel. Among them, the ferrite-austenite dual-phase steel has the best stress corrosion resistance. The methods of controlling stress: when assembling, try to reduce stress concentration, and make the part in contact with the medium have the smallest residual stress; prevent scratches, and strictly abide by the specifications of the welding process.
Strictly follow the operating procedures: strictly control the raw material's composition, flow rate, medium temperature, pressure, pH value and others. Add corrosion inhibitors within the allowable range of process conditions. When chromium-nickel stainless steel is used in dissolved oxygen chloride, the mass fraction of oxygen should be reduced to 1.0×10-6 or less. Practice has proved that in water containing chloride ions with a mass fraction of 50.0 × 10-6, it is only necessary to add a mixture of nitrate with a mass fraction of 150.0 × 10-6 and sodium sulfite with a mass fraction of 0.5 × 10-6 and good results can be obtained.
2. Holes and its preventive measures
Small holes generally occur easily in static media. The holes usually develop along the direction of gravity or the lateral direction. Once a hole is formed, it will automatically accelerate to the depths. The oxide film on the surface of stainless steel is dissolved in an aqueous solution containing chloride ions. As a result, small holes with a diameter of 20 μm to 30 μm are formed on the base metal. These small holes are pitting nuclei. As long as the medium contains a certain amount of chloride ions, nuclei may develop into pits.
Common preventive measures: add molybdenum, nitrogen, silicon and other elements to stainless steel or increase the chromium content while adding these elements. Reduce the content of chloride ions in the medium. Add corrosion inhibitors. Increasing the stability of the passivation film may help the damaged passivation film to be passivated again. Use external cathodic current protection to suppress pitting.
3. Pitting
As any metal contains non-metallic inclusions to varying degrees, these non-metallic compounds will quickly form pitting under the corrosive action of Cl ions. The Cl ions outside the pit will move to the pit due to the closed battery effect. The positively charged metal ions in the pit will move to outside of the pit. Stainless steel with Mo has better pitting corrosion resistance than that without Mo. The more Mo is added, the better the pit corrosion resistance becomes.
4. Crevice corrosion
The mechanism of crevice corrosion is the same as that of pit corrosion. It is corrosion that occurs due to the closed battery in the crevice, which leads to the gathering of Cl ions. This type of corrosion generally occurs in the gaps among flange gaskets, lap joints, bolts and nuts, and the gaps between heat exchange tubes and tube plate holes. Crevice corrosion is greatly related to the concentration of a static solution in the gap. Once there is a crevice corrosion environment, the probability of inducing stress corrosion is very high.
Next: Stainless Steel in Water Solutions Containing Chlorine
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