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The mysterious destruction of metal protective films: Do you understand the starting process of pitting
Pigment corrosion is a very challenging problem in the lifespan of metals.It is a highly localized corrosion phenomenon that randomly creates small holes on the metal surface, and this visually almost undetectable corrosion often threatens the safety of the facility.According to Frankel's research, the development process of pitting can be divided into three basic steps: starting, growing and stabilizing.
The initiation process of pitting corrosion is caused by the destruction of the protective film, which makes small areas anode and wide areas a cathode, which leads to local electrochemical corrosion.
The development and dynamics of pitting
According to Frankel's (1998), the development of pitting corrosion is mainly divided into three steps: the first step is the rupture of the protective film, which causes the metal to no longer be protected by oxidation; the second step is the growth of the substitute stability point , after these points grow to the micrometer scale, they will be reconciled by the protective film again; the third step is the formation of larger and stable holes.The density of points during this change takes an S-shaped curve over time, and the development process is like the logic of Latin America, showing quite typical progress characteristics.
Formation mechanism
Pipe etch formation can be basically considered a two-step process: first nucleation, then growth.The passage of the plastic layer is the beginning of the pitting nucleation process.This process usually involves physical or mechanical damage, which locally interferes with the protective layer, thereby contributing to corrosion.
The damage of the protective film can be affected by chemical composition, the properties of steel fused materials and many other factors.
Growth of Pitted Eclipse
More common explanation is that pitting is an autocatalytic process driven by small electrochemical cells that form randomly.When the metal is exposed to an aqueous oxidation solution containing sodium chloride, the metal will be oxidized locally and then oxygen reduction will be performed in the cathode area.
The locally generated areas of the metal anode and cathode form an electrochemical cell, which locality promotes the acceleration of corrosion.
Effects of Environment and Alloys
Pipe corrosion generally occurs in alloys with good protective oxide films, such as stainless steel, nickel alloys and aluminum alloys, most commonly in environments containing chloride or thiazoles.Some alloys cannot form a protective film in an environment with a pH value of less than 10, resulting in a holistic attack.In an environment with a pH value greater than 10, the formed protective film will prevent pitting.
Steel corrosion inhibitor
Different types of corrosion inhibitors can provide protection to metals.Oxidants such as chromate and nitrite can help rebuild the protective oxide layer of the metal.These inhibitors can be converted into non-soluble trivalent iron, thereby rebuilding the passive state of the metal, especially in the volatile environment of concrete.
Failed cases in the project
In many cases, pitting corrosion can lead to major engineering failures.An explosion in Guadalajara, Mexico in 1992 was caused by a small hole formed by corrosion that leaked gasoline, which eventually triggered a large-scale explosion, reminding people of the importance of metal protective films.
Although the number of studies on pitting corrosion is gradually increasing, the randomness and locality of its occurrence makes it difficult to predict in many cases.This hidden corrosion phenomenon often causes damage to many facilities to occur quietly, making us wonder how to effectively prevent the potential threat of this metal protective film in the future?
