Let’s talk about pitting corrosion
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Pitting is a highly localized form of corrosion. Most of the metal surface does not corrode or only experiences slight corrosion, with one or several holes appearing only in localized areas. Small, deep holes can penetrate metal sheets, leading to accidents such as fluid leakage, fires, and explosions; it is one of the most destructive and hazardous forms of corrosion. Pitting occurs in metals that are prone to passivation, such as stainless steel and titanium-aluminum alloys, because their surfaces are covered with a strongly protective passivation layer, which limits corrosion. However, due to local defects on the surface such as exposed bolt dislocations, scratches, or non-metallic inclusions, and the presence of active ions in the solution (e.g., Cl⁻, Br⁻) that can break down this passivation layer, the passivation layer is damaged locally. The metal exposed at these small cracks becomes the anode of a battery, while the larger area of the passivation layer surrounding it acts as the cathode. The high current density at the anode causes corrosion to progress rapidly inward, resulting in the formation of pits. After the pore is formed, its exterior is blocked by corrosion products, which hinders convection and diffusion both inside and outside the pore. A unique occluded region (also known as an occluded cell) forms within the pore; the oxygen there is rapidly depleted, leaving only the anodic reaction of metal corrosion, while the cathodic reaction involving oxygen ions takes place entirely on the outside of the pore. As a result, positively charged metal ions accumulate rapidly within the pore. To maintain electrical neutrality, negatively charged Cl‑ ions migrate from outside the pore into it, leading to an increase in Cl‑ concentration. The hydrolysis of metal ions produces H+, causing the pH value inside the pore to decrease. H+ and Cl- form highly corrosive hydrochloric acid, as shown in the following equation: M+ + Cl- + H2O = MOH↓ + H+ + Cl-. The composition of the solution in the blocked area (H+, Cl-) is very different from that outside the area. When the pH value in the area drops to a certain critical value, the corrosion rate increases suddenly, leading to accelerated corrosion; a cathodic hydrogen evolution reaction occurs within the pores, and pitting is controlled by the acidic cell in the blocked areas. After pit formation, whether it will progress further until perforation is still difficult to predict at present due to the complexity of influencing factors. Generally, when there are few pores, the current concentrates, increasing the likelihood of further development ； If the pore is also shallower and the degree of obstruction is low, the risk is also smaller.
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Pitting is a severe and localized form of metal corrosion that occurs primarily in metals such as stainless steel, whose surfaces are protected by a passivation film. Pitting is usually triggered by surface defects and harmful ions (such as chloride ions), leading to the local breakdown of the protective film. This cracking exposes the internal metal as the anode, while the passivation film over the surrounding larger area serves as the cathode. In such an environment, the corrosion reaction at the anode accelerates, resulting in the formation of deep holes. The internal environment of the pores forms blocked areas due to the accumulation of corrosion products, which further exacerbates the acidic corrosion reactions within those pores and can lead to more severe equipment damage or safety incidents. The increase in depth and severity depends on various complex factors, making it difficult to predict the development of pitting corrosion with precision at present. .
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