In-depth analysis of stress corrosion and hydrogen cracking problems

Studies have shown that the effect of H2S concentration on stress corrosion is obvious, wet H2S-induced cracking is not only hydrogen sulfide stress corrosion (SSCC), hydrogen-induced (HIC) and stress-oriented hydrogen cracking (SOHIC) and hydrogen bulging (HB), etc., and its damage sensitivity increases with the increase in the concentration of H2S, and reaches a maximum value in saturated wet hydrogen sulfide.
Liquid medium hydrogen sulfide concentration of mild steel, when the H2S concentration in solution from 2PPm to 150PPm, the corrosion rate increases faster, but as long as it is less than 50PPm, the destruction of a long time, the H2S concentration increased by 1600PPm, the corrosion rate decreases rapidly.
When higher than 1600PPm ~ 2420PPm when the corrosion rate is basically unchanged, which indicates that the high concentration of hydrogen sulfide corrosion is not more serious than the low concentration of hydrogen sulfide corrosion; but for low alloy high-strength steel, even if a very low concentration of hydrogen sulfide, can still cause rapid destruction.
Therefore, in the wetting of hydrogen corrosion environment, the choice of equipment for the pressurised components of the material will be very important, especially when the hydrogen sulphide contains water, to determine the degree of corrosion is the partial pressure of hydrogen sulphide, rather than the concentration of hydrogen sulphide.
At present, the domestic petrochemical industry will be 0.00035Mpa (absolute) as a control value, when the partial pressure of hydrogen sulfide in the gas medium is greater than or equal to this control value, it should be from the design, manufacture or use of all aspects of the measures taken and the choice of new materials in order to try to avoid and reduce the hydrogen sulfide corrosion of carbon steel equipment.
From the chemical composition of the material, the main chemical elements in the steel affecting hydrogen sulfide corrosion is manganese and sulfur, manganese in the equipment welding process, resulting in martensite, bainite high strength, low toughness micro-metallurgical organisation, showing a very high degree of hardness, which is extremely unfavourable to the equipment anti-SSCC, sulfur in the formation of MnS, FeS non-metallic inclusions in the steel, resulting in the local microstructure laxity, in the wet hydrogen sulfide HIC or SOHIC is induced in wet hydrogen sulphur environment.
Therefore, for pressure vessel steels used in wet hydrogen sulphide environments, their manganese and sulphur contents and non-metallic inclusions levels should be paid great attention to and not allowed to exceed the standards. In order to improve the resistance of steel to wet hydrogen sulphide performance, the French pressure vessel standard CODAP-90 Appendix MA3 put forward the following recommendations: (1) Reduce inclusions, limit the sulphur content of steel, so that the S ≤ 0.002%, if you can achieve ≤ 0.001% is better. (2) Limit the oxygen content in steel to ≤ 0.002%. (3) Limit the phosphorus content in steel, try to make it ≤ 0.008%. (4) Limit the nickel content of steel. (5) In order to meet the mechanical properties of steel plate conditions, the carbon content of steel should be reduced as far as possible. Of course, at present, domestic materials are also working in this direction. 16MnR due to its Mn content of up to 1.20 ~ 1.60, more sensitive to sulfides. Domestic usually used in wet hydrogen sulphide environment is limited to 50PPm below, or try not to use.
12CrMoR, 15CrMoR, 1.25Cr1Mo and other materials have good resistance to hydrogen corrosion and a certain degree of sulphur resistance, but for wet hydrogen sulphide corrosion, is still not ideal. General austenitic stainless steel is not resistant to wet hydrogen sulphide and chloride ion stress corrosion. 20R, Q235, 20-gauge steel, etc., in the wet hydrogen sulphide environment, the corrosion rate is faster than the above low alloy steel materials. Applications
At present, it has been cooperated with PetroChina, Sinopec, Dai Steel and many other enterprises Welcome to visit us. The following items can be stamped CNAS, CMA.
Applied in petrochemical, aerospace, marine, natural gas, nuclear power and other fields and exported abroad to meet the requirements of the relevant standards HIC test involves pressure vessels, welding process evaluation, material acceptance standards GB/T8650-2015, NACE TM0284-2016 Acceptance indicators hydrogen bulge Another is the length ratio, width ratio and area ratio of CLR, CTR, CSR cracks The SSC test involves the acceptance of pressure vessels, welding process evaluation, and materials GB/T4157-2017 and NACE TM0177-2016.
Commonly used methods are four-point bending method and tensile method, the result is to see whether there is EC crack, four-point bending loading according to the yield strength and modulus of elasticity, constant load is based on the yield strength. Generally the domestic is about 80% of the yield limit or 247Mpa, Japan and the United States requirements will be 100% higher or even higher. Can also do the SSC test in simulated working conditions (all loading methods can be done)
SSC test A method specimen diagram
SSC test national standard is GB/T4157-2017 the latest standard of four-point bending loading to join the standard E method. There is also NACE TM0177-2016, the main thing is EC crack, four-point bending loading according to the yield strength and modulus of elasticity, constant load is based on the yield strength. Generally the domestic is about 80% of the yield limit, the Japanese requirements will be high to 100% or even higher.