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Pitting corrosion is a localized form of corrosion that leads to the creation of small holes, or ‘pits,’ in a metal. It is one of the most destructive forms because it is difficult to detect, predict, and design against. Pitting can cause catastrophic failure of a structure with only a small percentage of total mass loss.
Pitting is an autocatalytic process. It typically initiates at small surface defects, inclusions, or scratches, especially on passive metals like stainless steel or aluminum. The process begins with the local breakdown of the passive film, often triggered by aggressive anions, most notably chloride ([latex]Cl^-[/latex]). Once the film is breached, an active corrosion cell is established. The small area of exposed metal becomes the anode, and the large surrounding passive surface becomes the cathode.
Inside the pit, metal dissolution occurs, for example [latex]Fe \\rightarrow Fe^{2+} + 2e^-[/latex]. The resulting positive metal ions attract negative chloride ions from the solution into the pit to maintain charge neutrality. This leads to the formation of metal chlorides, such as [latex]FeCl_2[/latex], which then hydrolyze with water: [latex]FeCl_2 + 2H_2O \\rightarrow Fe(OH)_2 + 2HCl[/latex]. This reaction produces hydrochloric acid, which dramatically lowers the pH inside the pit, making the local environment highly acidic and aggressive. This increased acidity further accelerates metal dissolution and prevents the passive film from reforming, creating a self-sustaining cycle that causes the pit to grow deeper. The exterior surface remains protected, masking the severe damage occurring beneath.
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