Corrosion and Protection of Stainless Steel thick-walled Pipes
In petrochemical equipment, thick-walled stainless steel pipes are widely used because of their good thermoplasticity and corrosion resistance, but they may be corroded and damaged under certain media conditions, such as pitting corrosion, intergranular corrosion, and stress corrosion.
Stress corrosion cracking is a form of corrosion damage that occurs without warning, and the damage is fast and the damage is more serious. Due to the unpredictability of brittle fracture, it poses a serious threat to the safe long-term operation of chemical equipment. Therefore, it is necessary to study the causes of stress corrosion of thick-walled stainless steel pipes and take effective protective measures.
1 Characteristic and mechanism of stress corrosion
Thick-walled stainless steel pipes are corrosion cracking under the combined action of specific corrosion factors and tensile stress, and the stress during cracking is lower than the strength limit of the material itself, and the macroscopic morphology of the fracture is a brittle fracture. It is generally believed that under the action of tensile stress, the passivation film attached to the surface of the metal material ruptures, forming pits and crack sources, and exposing the metal material to a corrosive environment.
Under the repeated action of tensile stress, the newly generated passivation film continues to rupture. , the crack continues to expand along the direction of tensile stress, and hydrogen is generated in the occlusion area of the crack tip, and the hydrogen diffuses into the metal to cause catalysis, and brittle fracture occurs under the action of tensile stress. Brittle cracks continue to propagate along the depth until the metal material breaks and fail.
2 Influencing factors of stress corrosion
2.1 Corrosive medium
There are many media that cause stress corrosion to thick-walled stainless steel pipes. The common ones in the petrochemical industry are the following: chloride ion, lye, polysulfuric acid, hydrogen sulfide aqueous solution, sulfate radical, nitrate radical, fluoride ion aqueous solution, etc. There are many stress corrosion accidents of thick-walled stainless steel pipes caused by chloride ions. Chloride ion stress corrosion must satisfy that the medium is a neutral environment and contains oxidants or dissolved oxygen. The rate of stress corrosion increases with the increase of chloride ion concentration.
When the corrosive medium is At 50 to 200°C, the tendency for stress corrosion to occur is the greatest. The lye stress corrosion also needs to occur under aerobic conditions. When the lye temperature reaches its boiling point, it will cause stress corrosion cracking of metal materials. The stress corrosion rate slows down as the lye concentration decreases. When the lye concentration is lower than 50%, stress corrosion almost no longer occurs.
Lientosulfuric acid is mostly produced by the occurrence of iron sulfide on the metal surface and oxygen and water in the air in a humid environment during equipment shutdown and maintenance. When the thick-walled stainless steel tube is in a sensitized state and there is tensile stress, the metal material may cause stress corrosion after contacting polythionate. Polysulfuric acid stress corrosion is also related to pH value. When the pH value is greater than 5, the probability of stress corrosion occurrence is small, and stress corrosion accelerates with the decrease of pH value.
2.2 Stress
The stress causing stress corrosion of thick-walled stainless steel pipes mainly comes from the working load, residual stress during equipment processing, thermal stress, and assembly stress. Among them, the damage caused by residual stress accounts for the largest proportion of stress corrosion, accounting for about 80%. Residual stress is unavoidable in the process of equipment processing and installation.
The most common residual stress is welding residual stress, residual stress caused by cold working and bending, and residual stress caused by expansion pipe. The greater the residual stress, the easier it is to cause stress corrosion cracking.