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Hidden threats on metal surfaces: Do you know what pitting corrosion is?
Hidden on the metal surface lies a very destructive corrosion phenomenon called pitting corrosion. This form of corrosion is highly localized and can randomly create small holes, causing severe damage to metal structures. The driving force of pitting corrosion comes from the loss of protection of small areas, which become anodes (oxidation reactions) while the surrounding large areas become cathodes (reduction reactions), resulting in extremely localized electrochemical corrosion. As the corrosion process progresses, the interior of the metal will be affected in a chain reaction, threatening its overall stability.
The development process of pitting corrosion can be briefly described as three steps: the first step is the initialization of pitting corrosion, the second step is the growth of metastable pores, and the third step is the growth of larger and more stable pores.
The formation of pitting can be viewed as a two-step process: first nucleation, followed by growth. When the protective layer on the metal surface is damaged, corrosion begins. This destruction can be due to physical damage or chemical reactions, where destructive anions such as chloride and thiosulfate ions accelerate the process.
In a liquid environment, as corrosion progresses, the anode and cathode regions form small electrochemical cells, allowing oxidation and reduction reactions to occur at different locations.
This phenomenon occurs when metal is immersed in an oxidizing aqueous solution containing sodium chloride. In this process, the oxidation reaction of the metal and the reduction reaction of oxygen proceed at different rates, resulting in the opening of new corrosion areas on the metal surface. Especially under acidic conditions, the rate of corrosion reaction will increase significantly.
What cannot be ignored is that the combination of different alloys and environments will affect the occurrence of pitting corrosion. Metals such as steel will not form a protective oxide film in an environment with a pH value below 10. Once chloride ions are added, it will cause uniform corrosion of the entire surface. In an environment with a pH value greater than 10, it is relatively safe.
Even in a low-oxygen environment, pitting corrosion can still occur, and many reducing substances may increase the chance of dissolution of the protective oxide film.
Interestingly, this corrosion is not just the result of redox reactions. There are many other factors that influence the further development of corrosion, such as microbial activity in the industrial environment and changes in local oxygen concentration. These can lead to changes in corrosion conditions, which are difficult to predict.
Strategies to prevent and manage pitting corrosion involve the use of different corrosion inhibitors such as chromates and nitrites, among others. These chemicals can form a protective film on the metal surface to prevent further corrosion reactions.
Even if corrosion inhibitors are used, if their concentration is too low, local anode formation may occur, which in turn aggravates corrosion.
Cases of engineering failures demonstrate the potential risks of pitting corrosion. For example, the accidental explosion in Guadalajara, Mexico in 1992 was caused by pitting corrosion in a steel gasoline pipeline that led to a leak. For many infrastructures, a single small hole like this could cause huge losses, and the risk is often not easily detected.
For example, if the barrel of a firearm is not cleaned in time after using corrosive ammunition, pitting corrosion is very likely to occur, which will cause rifling deformation and affect shooting accuracy. In laboratories, equipment damage caused by corrosion may also affect its performance and service life. Especially in ventilation systems involving harmful gases, corrosion problems need to be treated with more caution.
In high-end technology fields such as structures and aerospace, the presence of pitting corrosion may invisibly affect the overall safety of the system. As our understanding of material properties deepens, how to effectively prevent and manage pitting corrosion has become a new technical challenge. So, what more effective countermeasures should we take for this threat hidden on the metal surface?
