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Stainless steel reinforcing bars - reason for their high pitting corrosion resistance

Elsener, B. / Addari, D. / Coray, S. / Rossi, A.

This combined electrochemical and XPS (X-ray photoelectron spectroscopy) surface analytical work links the corrosion behavior, especially the pitting potentials, of conventional and new nickel-free stainless steels to the surface chemistry of the protective films formed in alkaline solutions. The following conclusions can be drawn: High pitting corrosion resistance is obtained by a high Cr(III)oxyhydroxide content in the passive film, a marked enrichment of nickel underneath the passive film and a high Mo(VI) content in the film. The duplex steel DIN 1.4462 has the highest pitting resistance owing to a combination of all three influencing factors. Despite that no nickel enrichment is possible below its passive film, the new nickel-free stainless steel DIN 1.4456 shows high pitting corrosion resistance owing to the high Cr(III)oxyhydroxide and molybdenum(VI) content in the passive film. The new nickel-free stainless steel DIN 1.4456 (18 % chromium, 18 % manganese, 2 % molybdenum) with its high pitting corrosion resistance is a cost-effective alternative to the nickel-containing austenitic stainless steels for reinforcing bars in concrete. Experiments in solutions give conservative results in terms of resistance to chloride-induced corrosion in structures. In concrete, where chloride ions reach the steel after years or even longer time, the beneficial effect of ageing (transformation of the passive film to a less Fe(2+) containing, more hydrated film) will lead to higher pitting potentials. The implications of the large scatter observed in the pitting potential in critical conditions on the determination of a 'critical chloride content' for service life design has to be further analyzed on the basis of the stochastic nature of pitting corrosion.