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Hydrogen sulfide in high-sulfur and high-acid crude oil can cause severe corrosion to equipment, especially wet H2S stress corrosion cracking and hydrogen-induced cracking, which may cause sudden and catastrophic accidents. hydrogen-induced cracking （HIC） and sulfide?stress?corrosion?cracking （SCC）testing is a basic requirement for oil-related pipeline steel and pressure vessel steel，they are specified in National Association of Corrosion Engineer(NACE).

The corrosion of steel in wet hydrogen sulfide environment depends on the performance of steel, the environment and other factors. Hydrogen-induced cracking is the occurrence of cracks along the direction of rolling in pipeline steel, pressure vessel steel and welding seams. Cracks in a plane tend to connect with cracks in the adjacent layer, resulting in ladder-shaped cracks along the direction of thickness. These cracks reduce the effective wall thickness to service or failure of pipeline steel or pressure vessel steel under excessive stress. A large number of studies show that HIC has little influence on the conventional strength index of steel, but has a great influence on the toughness index, which will increase the brittleness tendency of steel.

Under the action of hydrogen pressure, adjacent hydrogen bubble cracks on different sides are connected to each other, and the internal cracks that form the stepped characteristic are called hydrogen-induced cracking. These cracks can sometimes extend to the metal surface. The occurrence of HIC requires no external stress, and is generally related to the large plane inclusions of high density or the irregular microstructure of alloying elements segregation in steel. Hydrogen induced cracking (HIC) is now widely used to describe crack types and is adopted by NACE international. NACE TM 0284 is a common standards for evaluation of Pipeline and Pressure Vessel Steels for Resistance to Hydrogen-Induced Cracking.

Sulfide Stress Corrosion Cracking (SSCC/SSC) is a special type of Stress Corrosion. The content of H2S in the natural gas transmission can be as high as 400 ~ 500mg/m3, and the partial pressure is up to 0.0003mpa or higher, which has the conditions for the occurrence of SSC. The cracking is caused by hydrogen, and the sulfur ion or hydrogen sulfide promotes the diffusion of hydrogen into the material. For long distance pipelines of oil and natural gas, sulfide stress corrosion cracking of high pressure and high strength pipelines will cause major safety accidents, so it is necessary to evaluate the Anti-SSC performance of pipeline steel to ensure the safety of pipelines.

The low alloy high strength steel and carbon steel material ASTM A516 Gr60 or ASTM A516 GR70(JIS SB450; EN P235GH /P355GH is a HIC tested steel commonly used to manufacture pressure vessels for petrochemical, gas separation and gas storage and transportation equipment. It is mainly used in petrochemical, chemical, electric power, automobile and other industries that require sufficient strength and toughness, good welding performance and cold and hot processing performance such as reactor, heat exchanger, separator, air storage tank, nuclear reactor pressure vessel, liquefied oil gas bottle, spiral case, such as the steam turbine cylinder, the corrosive medium complex large equipment such as: water tower, the second transform furnace, coke tower gas waste heat boiler, desulfurization trough, transformation, methanation furnace, regenerator, hydrogenation reactor, the transformation of methanation heater, gas steam generator and other equipment and components.

There are mainly three acceptance indexes for HIC tests: the percentage of crack length (CLR), the percentage of crack thickness (CTR), the percentage of crack sensitivity (CSR). No stress is required, and the test time is 96h; the common standard is NACE MR 0284, and the special requirements can be based on the standard NACE TM 0103 stress-resistant hydrogen induced cracking test. Several important factors affecting the HIC and SSC resistance test of materials:

C: The higher the carbon content, the higher the HIC sensitivity;

Mn: Easy to form hard brittle banding segregation, increasing the sensitivity of HIC, especially CLR；

S: Harmful elements, forming MnS and other inclusions that increase material HIC sensitivity;

P: Easy segregation, Ferrite - Pearlite band structure;

P: Segregation, increasing HIC sensitivity;

Cu: Environment with PH greater than 4.5 can effectively reduce HIC sensitivity;

Mo: Inhibits the formation of block ferrite and reduces HIC sensitivity;

Cr: The austenite grain can be refined, which is a medium to strong carbides forming element. The dispersed carbides can reduce the enrichment of hydrogen in steel, and the passivation film formed by chromium on the steel surface can also prevent the invasion of hydrogen, both of which are beneficial to reduce the HIC sensitivity of steel.

V, Nb, Ti, etc., are strong carbon/nitride forming elements, and dispersed carbon/nitride reduces hydrogen enrichment. In addition，the shape and distribution of inclusions affect the HIC resistance of steel plate, mainly the inclusion of class A such as MnS. The higher the content, the higher the HIC sensitivity. Finally, grain refinement can significantly reduce HIC sensitivity4. Microstructure. For low alloy pipeline steel/steel: banded Pearlite - Ferrite structure > Acicular Ferrite (AF) structure.