Pitting corrosion passive film breakdown

As mentioned, corrosion is complexly affected by the material itself and the environment, producing various kinds of surface films, e.g., oxide or hydroxide film. In the above reactions, both active sites for anodic and cathodic reactions are uniformly distributed over the metal surface, so that corrosion proceeds homogeneously on the surface. On the other hand, if those reaction sites are localized at particular places, metal dissolution does not take place uniformly, but develops only at specialized places. This is called local corrosion, pitting corrosion through passive-film breakdown on a metal surface is a typical example. 

The test method ASTM F7464 covers the determination of the resistance to either pitting or crevice corrosion of passive metals and alloys from which surgical implants are produced. The resistance of surgical implants to localized corrosion is carried out in dilute sodium chloride solution under specific conditions of potentiodynamic test method. Typical transient decay curves under potentiostatic polarization should monitor susceptibility to localized corrosion. Alloys are ranked in terms of the critical potential for pitting, the higher (more noble) this potential, the more resistant is to passive film breakdown and to localized corrosion. (Sprowls)14... 

Corrosion, especially pitting corrosion, is a typical heterogeneous reaction composed of several processes. Usually, it is reduced to each elemental phenomenon, such as breakdown of passive film and substrate dissolution, which are treated separately to establish the theoretical and experimental bases of corrosion. 

Pitting corrosion (Table 4.8) involves pit initiation (breakdown of passive film) followed by pit growth. The chloride ion induces pitting corrosion. Type 304 steel undergoes pitting more readily than Type 316 steel. The molybdenum in 316 steel is responsible for its reduced susceptibility to pitting corrosion. Type 316L steels contains... 

FIGURE 22.26 Schematic polarization curves for anodic metal dissolution, passivation, passivity breakdown, pitting corrosion, and transpassivation Eb = film-breakdown potential and Ep]l — pitting potential. 

Initiation of pitting corrosion takes place when the chloride content at the surface of the reinforcement reaches a threshold value (or critical chloride content). A certain time is required from the breakdown of the passive film and the formation of the first pit, according to the mechanism of corrosion described above. From a practical point of view, the initiation time can be considered as the time when the reinforcement, in concrete that contains substantial moisture and oxygen, is characterized by an averaged sustained corrosion rate higher than 2 mA/m [8], The chloride threshold of a specific structure can be defined as the chloride content required to reach this condition of corrosion. 

Alloys, used at high temperatures, obtain their protection from a dense and adherent oxide layer formed on the metal surface. The corrosive attack of metals and alloys in molten salts is due to the solubility of oxide scales by basic and acidic dissolution. This breakdown of the passive film gives rise to accelerated metal consumption by enhanced oxidation (Hot Corrosion). The phenomenon is closely related to pitting corrosion of metals and alloys in aqueous solutions. 

Among metals there are differences in composition and stoichiometry of the oxide films. Halides such as chlorides play an important role in the growth and breakdown of passive films. Borates help stabilize the oxide film. Chloride ions cause severe localized corrosion such as pitting. Well-developed pits have high chloride ion concentration and low pH. Pitting can be random and amenable to stochastic (statistical) theory and very sensitive to experimental parameters such as induction time and electrochemical properties, which are difficult to reproduce. Electrochemical noise (EN) can clarify the initial conditions for pit initiation (24). 

Iron and steel in the presence of aggressive anions like chloride ions show the phenomenon of local breakdown of the passive film. On pipes, vessels, etc., semi-spherical pits develop on the surface, which penetrate the walls and destroy the parts with time. The omnipresence of chloride ions makes pitting corrosion a very general and dangerous phenomenon. The process has been intensively investigated. ... 

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