Pitting corrosion composition, electrolyte

Both, a higher chloride concentration and a locally lower pH render repassivation of a growing pit more difficult. In addition, certain dissolution products from sulfide inclusions also inhibit repassivation. As the pit grows, the chemical composition of the electrolyte inside becomes thus increasingly aggressive. Pitting corrosion, therefore, is an autocatalytic process once formed, the pit itself creates conditions that favor its further growth. 

Nitrate and perchlorate may act as inhibitors. Figure 7.1 gives an example of the dependence of Ep and E, on the electrolyte composition for Ni in chloride- and nitrate-containing solutions. The gap between the two lines is the potential range of pitting corrosion. Similar results have been obtained for C104 and Fe [2]. A very peculiar property is observed for... 

Depending on the potential, chloride concentration in the electrolyte, structure of the alloy, passive film thickness, and the chemical composition of the corrosion environment, pits may appear in various morphologies as illustrated in Fig. 7.4. 

The details of the mechanisms also depend on the metal or the composition of the alloys as well as on the electrolyte and other environmental conditions. Metallic and norunetallic inclusions often play a decisive role in the start of a corrosion pit. In most cases the presence of aggressive anions is necessary for breakdown of passivity and stable pit growth. The discussion in this chapter explains the effect of these anions by their tendency to form complexes with metal ions. It concentrates on the behavior of some pure metals, such as pure Fe and Ni, in simple electrolyte solutions. Some basic concepts are the center of interest, although it is known fi om the technical applications of the difieient materials that the appearance and the... 

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