Seeking help regarding stainless steel corrosion
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While reviewing the documents, it was found that for some stainless steel equipment or those with stainless steel components, the specified limit for CL ion content during the manufacturer’s hydrostatic testing is less than 25 ppm, and this is to prevent corrosion caused by Cl. However, in actual production, the CL ion content is quite high, often exceeding several hundred ppm. Why is this? Are there any special circumstances regarding hydrostatic testing?
Such water needs to be treated; following the regulations should not pose any problems. If those regulations are not followed, things can get very complicated if issues arise.
The corrosion sensitivity of austenitic stainless steels to Cl ions cannot be ignored. Hydraulic testing has specific requirements, and strict standards should also be applied during operation. During hydrostatic testing, there is a high-stress condition, and during operation, the material is exposed to corrosive agents over a long period of time; in both cases, damage to the material is possible. Attention should be paid not only to the inside of the pipeline but also to its outside; for example, I recently issued a standard, an ASTM standard, regarding the requirements for external insulation materials for austenitic stainless steel.
Controlling the CL ion content below 25 ppm during hydrostatic testing is intended to prevent intergranular corrosion in stainless steel. It differs from ordinary metal corrosion. Ordinary metal corrosion progresses from the surface to deeper layers, and can be measured ； Intergranular corrosion occurs along the grain boundaries; CL ions destroy the chemical bonds in the lattice, reducing the interatomic forces or even eliminating them, thereby causing stainless steel equipment to fail.
Stainless steel is not as useful as carbon steel in this regard!
In pressure vessels made of austenitic stainless steel, stress corrosion can also occur in the presence of chloride solutions. This is because the chloride ions in the solution damage the passivation film on the surface of stainless steel; under tensile stress, cracks appear in the areas where the passivation film has been damaged, which become the anodic regions of corrosion cells. Continuous electrochemical corrosion can ultimately lead to the fracture of the metal. This type of corrosion is not greatly dependent on the concentration of chloride ions; even trace amounts of chloride ions can cause stress corrosion. In actual production, some equipment is not corroded and damaged under normal operating conditions, but rather develops stress corrosion cracks during shutdown due to low-concentration (5%) chloride condensate remaining in the containers. Stress corrosion can also occur as a result of using water with a high chloride ion concentration for pressure testing, which leads to the concentration of water remaining in the container. Stress corrosion of austenitic stainless steels caused by chloride ions typically features transgranular cracks, and most of these are branched cracks. Most corrosion cracks occur near the welds, which clearly indicates that welding residual stress is an important factor.
Is the hydrostatic test a high-stress condition, while operation involves being exposed to corrosive media over a long period of time? I just don’t understand this; I think this must be why the standards during testing are higher than those during operation! Why is a hydrostatic test a high-stress condition? Is its test pressure much higher than its operating pressure? Is it the stress?
The hydrostatic test is generally carried out at 1.5 times the design pressure; for equipment pipelines, the stress exerted on the steel plates is clearly much higher than the operating pressure. But do not misunderstand this as the strict basis for controlling chloride ions. Several factors contributing to corrosion include stress levels (for stress corrosion), the material’s susceptibility to corrosion, the corrosive environment, and time. When equipment pipelines are in operation, the stress levels may be lower, but the factor of time cannot be ignored. Therefore, whether it is a hydrostatic test or equipment operation, the chloride content in the medium must be strictly controlled
Controlling the external insulation material is also aimed at regulating chloride ion levels; it is also necessary to ensure that the insulation layer does not absorb water or accumulate moisture, especially in acidic chemical environments. I have seen cases of external stress corrosion occur.
Who has corrosion data for stainless steel? :(
Stainless steel is inferior to carbon steel in this regard~
Pressurize with soft water, or use water whose CL level has been reduced to below 25 ppm through treatment
Yes, the corrosion problem of stainless steel affects Cl ions as well as certain double salts.
In the past, the water used for pressure testing was determined based on the chlorine content in municipal tap water; in some areas, the poor quality of water posed difficulties for manufacturers and construction sites as a result. Foreign standards all specify 50ppm; therefore, the new regulations, in line with international standards, also set 50ppm. At 50°C, stainless steel welds have very low susceptibility to chloride stress corrosion, but there is a risk of pitting if residual water remains for an extended period; therefore, the water must be drained after pressure testing. Article 89(1) of TSG DZ001-2008 \"Regulations on Safety Technical Inspections of Pressure Piping – Industrial Piping\": Generally, clean water should be used. When conducting hydrostatic tests on austenitic stainless steel pipes, or on pipes connected to such pipes or equipment, the chloride content in the water must not exceed 0.005%. If water has an adverse effect on the pipes or the process, other suitable non-toxic liquids can be used. When flammable liquid media are used for testing, their flash point must be no lower than 50℃ ；