Passivity and local breakdown

Figure 1 shows a current-potential plot of DIN 1.4301 stainless steel in 0.01 M sulfuric acid and of iron in a boron buffer solution (at pH 8.4). As seen, the current density resulting from corrosion attacks drops by several orders of magnitude for a certain potential value (E = -550 mV and E = -300 mV respectively).

Fig. 1. Current density vs. potential plots of (a) iron in a boron buffer solution and (b) DIN 1.4301 stainless steel in 0.01 M sulfuric acid. A clear drop in the current density is observed at the passivation potential.

This is explained by the formation of an oxide layer, the so-called passive layer, which forms on the metallic surface. However, the exact composition of this passive layer is not known and descriptions in the literature are very contradictory. Photoelectrochemical measurements [1] lead to the following model describing the formation of a passive layer on stainless steel and iron.

The oxide layer on iron in a boron buffer solution mainly consists of Fe2C>3. Fe2+ ions in the film act as misfits. They disturb bondings within the film and lead to dangling, bonds. These misfits strongly influence the electronic behavior of the semiconducting passive layer.

The oxide layer on stainless steels in 0.01 M sulfuric acid mainly consists of Cr2C>3. Depending on the concentration of alloying elements in the bulk material, the layer shows a varying concentration of Fe and, sometimes, Mo. Again Fe2+ ions act as misfits and influence the electronic behavior of the semiconducting passive layer.

Yet the corrosion resistance of these metallic materials is usually limited by the occurrence of localized corrosion processes such as pitting and crevice corrosion. Due to the manufacturing process, every kind of steel possesses a certain amount of inclusions such as manganese sulfide or chromium carbide. These inclusions may drastically influence the electrochemical behavior of highly alloyed stainless steels, especially having an influence, on pitting the behavior [2-5]. This is the reason why it is necessary to obtain accurate knowledge on the shape, size distribution, and electrochemical behavior of these inclusions. Furthermore it is from this basis that further investigations of local processes on and at the interface of inclusion may be started. In order to understand the mechanisms of initiation and propagation of localized corrosion, the processes should also be studied in the micro- and nanometer ranges.

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