Suscettibilità all'innesco di morfologie di corrosione localizzata dell'acciaio inossidabile duplex SAF 2205 in soluzioni neutre contenenti ioni bromuro

The susceptibility to localized corrosion of duplex stainless steel SAF 2205 in neutral aqueous solutions containing bromide ions was investigated by means of potentiodynamic cyclic polarization curves. SAF 2205 were previously subjected to a solubilization heat treatment at 1080 °C for 30 minutes to allow the relaxation of grains. Tests were performed at 25 °C under nitrogen atmosphere in NaBr solutions at different concentrations. The pitting potential Epit was found to vary linearly with the logarithm of the concentration of bromide ions. At bromide content higher than 0.01 M a marked hysteresis was observed. Pit distribution was strictly related to bromide content in solution and increased with concentration. The behaviour of SAF 2205 in bromide environment is different than in chloride. In chloride, the material showed a complete passive behaviour also at temperature higher than room temperature, with no corrosion damages except for light crevice corrosion. In bromide the behaviour was completely different and it was a function of ion concentration and material microstructure. To elucidate the effect of ferrite/austenite ratio on the pitting initiation and propagation, the SAF 2205 was subjected to solubilization heat treatments at different times and temperatures. The corrosion attack started preferentially in the ferritic grains. However this specificity decreased with the increase of solubilization temperature and the material showed an improved resistance to localized corrosion. The performance of SAF 2205 was also evaluated in comparison with two austenitic steels (AISI 304 e AISI 304L) and a first generation duplex (AISI 329). In spite of difference in microstructure, the two austenitic steel and AISI 329 showed a similar behaviour, with a pitting potential around 0.3 V. As a general rule, SAF 2205 showed higher pitting potential and better performances towards localized corrosion.