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Why do some alloys corrode more easily in chlorides? What is the science behind this?
In the world of metal corrosion, pitting corrosion is an extremely localized form of corrosion that often creates small, random holes in the metal surface. The driving force behind this phenomenon comes from the destruction of the passive film protecting the metal surface. This small area becomes the anode and undergoes oxidation reactions, while other areas become the cathode and undergo reduction reactions, resulting in a very localized battery reaction. This causes corrosion to penetrate deep into the metal, while ion diffusion is restricted.
According to Frankel (1998), the development of pitting corrosion can be divided into three consecutive steps: first the destruction of the protective film, followed by the growth of unstable points, and finally the formation of stable large pits.
In the natural environment, chloride and other reactive anions such as sulfate or iodide can accelerate this process. Many alloys, such as stainless steel and nickel alloys, while offering good corrosion resistance under normal conditions, can fail in the presence of chlorides, leading to early pitting corrosion.
Mechanisms of Pitting
The formation of pitting can be viewed as a two-step process: nucleation and growth. The protection between the metal substrate and the corrosive liquid is usually effective in preventing corrosion due to the presence of an oxide layer. However, when the protective film is locally damaged, this area becomes an anode and the surrounding metal surface becomes a cathode. The metal in the anode area begins to oxidize, forming pits.
The growth of the etch is considered to be an autocatalytic process. The separation of the anode and cathode creates a potential gradient that drives reactive anions (such as chloride) into the pits, which the American Metals Society suggests is the root cause of pitting development.
Chloride and the role of environmental factors
Chlorides are one of the main culprits of pitting corrosion in various alloys. When metals (such as stainless steel) are exposed to chloride environments, these anions can penetrate the protective film and weaken its protective effect. In addition, when there is still water with low dissolved oxygen, or aquaculture-active chlorides in the environment, the chance of pitting corrosion is greatly increased.
For example, carbon steel does not form a passive oxide film in an environment with a pH value below 10, and the addition of chlorides will cause uniform corrosion, but this situation disappears at a pH value above 10. .
How to prevent pitting
Commonly used industrial preservatives such as chromates and nitrites can effectively restore the passivity of metal surfaces and reduce the risk of pitting corrosion. By controlling the ratio of chemical components, the corrosion resistance of the alloy can also be improved. However, in the absence of necessary corrosion inhibitors, local anodes may form, aggravating corrosion failures.
Lessons from failed projectsIn engineering projects, the consequences of pitting corrosion can be extremely serious. In 1992, a gasoline leak destroyed several kilometers of streets in Guadalajara, Mexico. The cause of this tragedy was a single point of corrosion in the metal pipe. It can be said that understanding and preventing pitting corrosion of metals is the key to preventing potential disasters.
In an increasingly complex industrial environment, how can we effectively prevent and control the problem of alloys being easily corroded in chlorides?
