Mechanisms of Pitting Corrosion

Pitting corrosion occurs at passivated metal surfaces during the access of so-called aggressive anions. Halides very effectively attack passivating thin oxide layers, leading to an intense localized dissolution of the metal surface, which is otherwise protected by the passive layer against general dissolution. Chlorides cause the most serious problems due to their presence in many environments such as seawater and salt on roads, in food, and in the chemical industry. Many metals and their alloys are subject to this type of corrosion (e.g., iron, nickel, copper, aluminium, steels), whereas chromium is one of the few exceptions that resists pitting in aggressive environments. The restriction of the dissolution to pits within a large passivated metal surface, which may serve as a large cathode for the reduction of oxidants such as dissolved oxygen, leads to fast perforation of the metal, which weakens the construction and thus causes large economic losses and safety problems. Pitting at passivated metal surfaces is a complex process with a sequence of steps. For each stage of the development and growth of a corrosion pit, one has to study the mechanistic details in order to understand the process as a whole. For a detailed mechanistic discussion the following stages are usually distinguished: