CORROSION RESISTANCE OF AUSTENITIC AND LOW-NICKEL DUPLEX STAINLESS STEEL BARS

摘要

The use of stainless steel bars in reinforced concrete structures can be considered as an effective method to prevent corrosion in aggressive environments, especially in the presence of chlorides. The cost of this preventive technique often limits its use. A selective use of the stainless steel bars, i.e. limited to the more vulnerable parts of the structure, can significantly reduce costs of using stainless steel. This is possible since, as demonstrated by Pedeferri et al., the effect of the galvanic coupling between conventional carbon steel bars and stainless steel bars is negligible. A designer of reinforced concrete structures can decide, according to the required service life and to the environmental aggressiveness, where it could be convenient to use stainless steel bars. For an estimation of the service life of structures where stainless steel bars are used, the chloride threshold for stainless steel bars should be defined, and the influence of chemical composition and metallurgical factors that may affect the corrosion resistance (strengthening, welding, surface finishing, etc.) should be assessed. The number of types of stainless steels proposed on the market has increased in recent years, due to the increase and fluctuation of cost of traditional austenitic stainless steels (1.4301 and 1.4401), as a consequence of variations in the cost of alloying elements, especially nickel. Duplex stainless steels with low nickel content have been proposed as an alternative to traditional steels to reduce the cost of the bars; nevertheless, few results are available regarding their corrosion performance in chloride contaminated concrete.This paper summarises the results of a research aimed at studying the corrosion resistance of two low-nickel austeno-ferritic stainless steels compared to traditional austenitic stainless steels in chloride bearing concrete. Tests were carried out on ribbed bars of austenitic (1.4311 and 1.4406) and duplex (1.4362 and 1.4162) stainless steels. Stainless steel bars were embedded in concrete specimens contaminated by chloride (up to 8% by mass of cement) and corrosion rate and potential were measured at 20°C and 90% R.H. Potentiodynamic and potentiostatic polarisation tests were also carried out in neutral and alkaline solutions with different chloride concentrations (up to 10% by mass), to evaluate the pitting potential and the critical chloride content to corrosion initiation of the stainless steel bars.The paper discusses the difference in the corrosion resistance between traditional austenitic and low-nickel duplex stainless steel bars as a function of the chloride content, and analyses the effects of the chemical composition. An attempt to define a chloride threshold for the different stainless steels is made.