Pitting corrosion aggressive anions

It should be mentioned that passive layers are not protective in all environments. In the presence of so-called aggressive anions, passive layers may break down locally, which leads to the formation of corrosion pits. They grow with a high local dissolution current density into the metal substrate with a serious damage of the metal within very short time. In this sense halides and some pseudo halides like SCN are effective. Chloride is of particular interest due to its presence in many environments. Pitting corrosion starts usually above a critical potential, the so-called pitting potential /i]>j. In the presence of inhibitors an upper limit, the inhibition potential Ej is observed for some metals. Both critical potentials define the potential range in which passivity may break down due to localized corrosion as indicated in Fig. 1. 

The accumulation of corrosion products within the pits suggest that a high concentration of chloride is a necessary condition for a stable growth in their early stage of development. As a consequence, the kinetics of repassivation of small pits may be related to the transport of accumulated aggressive anions from the pit to the bulk electrolyte [19, 29]. If this transport is the rate-determining step, one expects the repassivation time to increase with the depth of a corrosion pit and thus to the distance the chloride has to travel by diffusion. If we simply apply the relation of Einstein-Smoluchowski for the transport time fr out of a pit of radius r (Eq. 14), and if the radius r is given by the local current density ic,p and the lifetime fp of the pit by Eq. (15), we obtain Eq. (16) for the repassivation time fr. 

One may therefore conclude that the rate-determining step for the repassivation is the transport of accumulated aggressive anions out of small pits. This result coincides well with the explanation that localized corrosion is stabilized at its initial stage by the accumulation of aggressive anions, which prevent the formation of a passive layer at the active pit surface. 

Three main mechanisms are being discussed for the processes leading to the breakdown of passivity and the nucleation of corrosion pits [19] (Fig. 15). The penetration mechanism involves the migration of aggressive anions from the electrolyte through the passive layer to the... 

As mentioned already for the discussion of the penetration mechanism, pit nucleation is an extremely fast process of a few ms only for nonstationary conditions of the passive layer. Stepping of the potential in either direction, positive or negative, causes excessive formation of corrosion pits, especially for potentials well above the critical value and in the presence of a high concentration of aggressive anions. Even for stationary conditions, a... 

E7.9. Pitting inhibitor effectiveness depends strongly on type and anion concentration. The thermodynamic (critical) activity, ai, required to inhibit pitting corrosion for a given activity of the aggressive anion is described by [13] ... 

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. ... 

The development of pits starts with a crack or a hole of atomic dimension in the passive film caused, e.g., by tensions or by local chemical dissolution of the fihn. Permanent pitting corrosion can start above a critical potential and a critical concentration of the chloride ions. Above these critical values repassivation is prevented by the adsorption of the aggressive anions in the crack or the hole. The small dimensions of the crack or hole stabilize the large potential drop between active and passive surface. 

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