Metal corrosion can be generally divided into two categories: chemical corrosion and electrochemical corrosion.
Chemical corrosion is caused by the chemical interaction between the metal surface and the medium, and its characteristic is that no current is generated during the action . Chemical corrosion can be divided into the following two categories.
(1) Gas corrosion: Corrosion . Gas corrosion generally refers to the corrosion of metals at high temperatures, such as oxide scales generated during steel rolling, and burnout of internal combustion engine pistons.
(2) Corrosion in non-electrolyte solution) Corrosion of metals in non-conductive liquids. For example, the corrosion of metals in organic liquids (such as ethanol, petroleum, etc.).
Electrochemical corrosion is caused by the electrochemical action between the metal surface and the medium. During the action process, there are cathode and anode areas, and current flows between the metal and the medium. Electrochemical action sometimes causes , and sometimes mechanical action and biological action jointly produce corrosion. Electrochemical corrosion can be divided into the following categories .
(1) Atmospheric corrosion: Corrosion of metals in humid air.
(2) Soil corrosion: corrosion caused by metal structures buried underground (such as pipes, cables, etc.) .
(3) Corrosion in electrolyte solution: This is a very broad type of corrosion. The corrosion of metal structures by natural water and most aqueous solutions (such as corrosion in seawater and aqueous acids, alkalis, and salts) belongs to this category.
(4) Corrosion in molten salt: Corrosion of metals in molten salt. For example, in the heat treatment workshop, the corrosion of the electrode and the metal to be processed in the molten salt heating furnace,
(5) Stress corrosion cracking: it is metal corrosion damage under the action of tensile stress and corrosive medium. Stress comes from residual stress or external stress during hot and cold processing. The cracks of cracking are perpendicular to the tensile stress, sometimes cracking along the grain boundaries of metal crystals, and sometimes transgranular cracking.
(6) Corrosion fatigue: Under the combined action of alternating stress (or pulse stress) and corrosive medium , the fatigue limit of the metal is greatly reduced, resulting in premature rupture (such as propeller shaft, pump shaft) corrosion.
(7) Wear corrosion (abrasion): Corrosion and mechanical wear exist at the same time, and the two accelerate each other. Wear and tear can take many forms.
① Impact corrosion (erosion): It is caused by liquid turbulence or impact.
② Cavitation corrosion (cavitation erosion): High-speed flowing liquid, due to irregular flow, produces so-called cavitation. There is only a little water vapor or low-pressure air in this cavity. Due to the frequent changes in , the cavity will periodically appear and disappear . The metal surface of the cavitation produces the so-called "water hammer effect", which often destroys the protective film on the metal surface and makes the corrosion continue to deepen.
③ Fretting wear and corrosion is the wear caused by the mutual vibration of two adjacent surfaces, which destroys the protective film of the metal and accelerates corrosion.
(8) Microbial corrosion: The life activities of some microorganisms can promote the electrochemical reaction in the anode or cathode area , or can weaken the corrosion resistance of the metal surface film, or can produce corrosive substances, thereby accelerating electrochemical corrosion. Such as sulfate reducing bacteria and iron bacteria on metal corrosion.
(9) Corrosion of marine organisms. Many marine organisms (animals or plants) can adhere to the metal surface and reproduce continuously. During their metabolism, they can produce corrosive substances, or cause oxygen due to their uneven coverage on the metal surface. Concentration battery, thereby accelerating electrochemical corrosion.