Hydrogen damage
Hydrogen damage refers to the mechanical damage caused by hydrogen atoms contained in or reacted with hydrogen in the metal, including hydrogen bubbling, hydrogen embrittlement and hydrogen corrosion. Hydrogen atoms are mainly derived from high temperature and humid atmosphere, corrosion or the cathodic process of electrolysis. When the metal is corroded by acid or pickled, the reduction reaction of hydrogen ions can occur on the surface. The hydrogen atoms generated during the reaction are adsorbed on the surface of the metal. Due to the small radius of the hydrogen atoms, they can diffuse into the metal. However, once the hydrogen atoms are combined into hydrogen in the metal or react with other substances to form gaseous substances, they cannot continue to diffuse in the metal.
Hydrogen bubbling means that during the use of the metal, due to the diffusion of hydrogen atoms into the metal, it gradually recombines into high-pressure hydrogen in the cavity of the metal, causing the material to rupture. The hydrogen pressure in the cavity can be as high as 1.0X103MPa. Hydrogen embrittlement is due to the diffusion of atomic hydrogen into the metal, which makes the metal brittle, and the metal undergoes brittle fracture or fracture under the action of tensile stress. Hydrogen corrosion is due to the interaction of diffused hydrogen atoms with the second phase (inclusions, alloy additives, etc.) in the metal to generate high-pressure gas, causing brittle fracture of the metal. For example, at high temperature, hydrogen atoms in steel can react with Fe3C in steel to generate methane gas [Fe3C+4H→3Fe+CH4 (gaseous)], resulting in decarburization of steel and destruction of metallographic structure. The generated methane gas is heated and expanded, and a large stress is generated inside the metal, and its value can reach 1.8X103MPa, which makes the metal brittle. When the metal is not thinned, the tube burst occurs. For thermal equipment, hydrogen embrittlement may occur in carbon steel furnace tubes during boiler pickling or acid corrosion of boilers.
Pitting
Pitting corrosion, also known as pitting corrosion, is a typical localized corrosion. Its characteristic is that the corrosion is mainly concentrated in the local area (active point) of the metal surface, and develops deep into the metal. Usually, the depth of pitting corrosion is much larger than its hole diameter, and the equipment can be perforated in severe cases. Stainless steel often exhibits this form of damage in solutions containing a certain concentration of chloride ions. The pitting corrosion of thermal equipment mainly occurs on stainless steel components, such as the contact of the water side wall of the condenser stainless steel tube with the cooling water containing chloride ions, which may lead to pitting corrosion of the stainless steel tube under certain conditions; if the steam turbine is not properly protected when it is out of service, pitting corrosion may occur on the stainless steel blades, and these corrosion points may induce corrosion fatigue of the blades during operation.
Crevice corrosion
The corrosion damage that occurs in the crevice of the metal structure is called crevice corrosion, and crevice corrosion usually occurs in the crevice with a seam width of 0.025~0.1mm. The main sources of gaps on the metal surface are: structural gaps, such as flange connection surfaces, nut pressing surfaces, etc.; cracks formed by solid deposits (silt, corrosion products, etc.); gaps formed between the protective film on the metal surface (such as enamel, varnish, phosphating layer, metal coating) and the metal substrate.
In thermal equipment, a gap can be formed between the condenser cooling tube and the tube sheet, and corrosion products, inorganic scale layers, sediment, bio-slime, etc. can be deposited or attached to the surface of the metal (such as condenser stainless steel pipes or copper alloy pipes) to form gaps. Serious crevice corrosion can occur in corrosive media containing chloride ions.
