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

In harsh chloride bearing environments stainless steel reinforcing bars offer excellent corrosion resistance and very long service life for concrete structures, but the high costs limit a more widespread use. Manganese bearing nickel-free stainless steels could be a cost-effective alternative. Whereas the corrosion behavior of stainless steels in alkaline solutions, mortar and concrete is quite well established, only few information on the reasons for the high pitting resistance are available. This work reports the results of pitting potential measurements in solutions simulating alkaline and carbonated concrete on black steel, stainless steel DIN 1.4301, duplex steel DIN 1.4462 and nickel-free stainless steel DIN 1.4456. Duplex and nickel-free stainless steels are fully resistant even in 5 M NaCl solutions with pH 13 or higher, the lower grade DIN 1.4301 shows a wide scatter between fully resistant and pitting potentials as low as +0.2 V SCE. In carbonated solutions with pH 9 the nickel-free DIN 1.4456 shows pitting corrosion at chloride concentrations ≥ 3 M. This ranking of the pitting resistance can be rationalized based on XPS surface analysis results: both the increase of the Cr(III)oxy-hydroxide and Mo(VI) contents in the passive film and a marked nickel enrichment beneath the film improve the pitting resistance. The duplex DIN 1.4464 shows the highest pitting resistance, which can be attributed to the very high Cr(III)oxy-hydroxide, to a medium Mo(VI) content in the film and to a nickel enrichment beneath the film. Upon time, the protective properties of the surface film improve. This beneficial effect of ageing (transformation of the passive film to a less Fe2+ containing, more hydrated film) will lead to higher pitting potentials. It can be concluded that short-term solution experiments give conservative results in terms of resistance to chloride-induced corrosion in reinforced concrete structures