Corrosion resistance of new nickel free stainless steel inalkaline solutions containing chloride ions

Corrosion of the reinforcing steel in concrete can be due to the ingress of chloride ions or due to carbonation (neutralization of the concrete due to reaction with CO2). The use of carbon steel, even in concrete with coverdepth and quality according to the codes of practice, does not always provide sufficient resistance to corrosion, leading to deterioration of reinforced concrete structures. Corrosion can be avoided by replacing carbon steel with corrosion-resistant alloys such as stainless steels. As most of the commercial alloys contain about 10 – 12% nickel, stainless steel is up to 10 times more expensive than carbon steel. This is the main reason why industry developed nickel-free, austenitic stainless steels where nickel has been replaced by manganese. The knowledge about the electrochemical and corrosion behaviour of these new stainless steels is comparatively low. In this work, a nickel-free stainless steel, DIN 1.4456, containing 18% of manganese and 18% of chromium, has been investigated. The steel was first characterized with optical micrographs and with SEM and EDX analysis. The main part of this work were electrochemical measurements in solutions of pH 13 simulating the environment of alkaline concrete with addition of up to 5 M chloride ions. The results of dynamic polarization curves (scan rate 0.2 mV/s) showed that this stainless steel remains passive up to a chloride concentration of 4 M, pitting (or crevice corrosion) occurs at higher chloride concentrations. Long term immersion tests and polarization resistance measurements in chloride free and 5 M chloride containing solution at pH 13 showed that under open circuit condition the nickel-free stainless steel 1.4456 remains passive even in a 5 M chloride solution. The polarization resistance value indicates a clearly higher dissolution rate in alkaline solution with 5 M chlorides.
Comparing these results with other, nickel containing stainless steels such as DIN 1.4301 (18/8 CrNi) or 1.4401 (18/8/2 CrNiMo) it can be noted that the nickel-free stainless steel 1.4456 at chloride concentrations from 0 to 5 M has a similar corrosion resistance as DIN 1.4301.
This new nickel-free stainless steel can be recommended as more economic corrosion-resistant material in alkaline reinforced concrete structures with moderately severe exposure to chloride ions.