In the high temperature hydrogenation equipment and the equipment with water H2S solution, there will be the process of adding hydrogen and precipitating hydrogen. Hydrogen can cause hydrogen damage, hydrogen embrittlement, hydrogen blistering, surface decarburization and hydrogen corrosion.
a. The Hydrogen embrittlement
In high-strength steel, the metal lattice is badly deformed, and when the hydrogen atoms enter the metal, it will make the lattice more strain, thus reducing the ductility and ductility of the metal, the phenomenon of embrittlement is called hydrogen embrittlement.
Hydrogen embrittlement mainly occurs at low temperature and is irreversible in stress corrosion. At present, the main protective measures for hydrogen embrittlement: to avoid using high strength steel which is sensitive to hydrogen embrittlement, and to choose alloy steel containing Ni and Mo.
b. Hydrogen blistering
It means that when the hydrogen atom is spread in steel, the hydrogen molecules are synthesized in the hole of the steel. When the hydrogen molecule is not able to spread, it forms internal pressure in some parts of the metal, and it will cause the bubbling of the steel surface and even break. In the light oil zone, sulfur exists mainly in the form of H2S. Hydrogen gas is introduced into the metal by H2S and H2O, as well as the welding process, the hydrogen atoms that enter the metal are concentrated at the defect of the material to form the hydrogen, and it will cause the pressure to increase and cause the hydrogen to crack. At the weld, the "trap" of trapping hydrogen atoms is increased by the existence of weld defects such as stomata and inclusions. For example, the high H2O content in the top of the initial distillation tower and the normal pressure tower is high, resulting in the crack of weld seam of the shell and pipe of the top cooler.
The phenomenon is often found in low-strength steel, especially in low-strength steel containing large amounts of impurities. For this kind of corrosion, it can be used to replace the cavitated steel with fewer holes. Austenitic stainless steel with better penetration resistance to hydrogen, or nickel, rubber, plastic, protective layer, ceramic tile, etc., is used as lining material of the equipment and corrosion inhibitor is added.
c. Hydrogen corrosion
It refers to the corrosion when the temperature is above 200 DEG C and the hydrogen pressure is >0.15 Mpa. The hydrogen dissolved in the steel is chemically reactive with the carbon that is not stable in the steel at high temperature, and it forms methane bubbles along the grain boundary. The result of hydrogen corrosion leads to the decrease of steel strength, plasticity and fracture toughness of steel. Hydrogen erosion is a common damage method in high temperature and high pressure hydrogenation equipment.
There are many ways to prevent hydrogen corrosion in steel. For example, Cr and Cr-Mo alloy steel can be used. Because these alloy elements can produce stable carbonates with certain components of steel, thus reducing methane generation, they can withstand high temperature and hydrogen partial pressure. The Austenitic stainless steel can also be resistant to high temperature hydrogen corrosion.