Corrosion behaviour of Duplex stainless steels in organic acid aqueous solutions

This paper reports on mass loss and electrochemical polarization behaviour of two duplex stainless steels, SAF2205 and SAF2507, in aqueous solutions of acetic acid, formic acid and their mixtures, in the presence of specific contaminant, e.g. sulphuric acid. The effect of temperature and redox couples was also investigated.Both steels withstand corrosion in acetic acid solutions in a wide temperature and concentration range. The corrosion rate is higher in the more dissociated formic acid at the same concentration. Uniform corrosion with selective dissolution of the austenitic phase was observed in all tested solutions, being more pronounced with increasing solution aggressiveness.The presence of sulphuric acid is severely detrimental to the corrosion resistance. The corrosion rate increases almost linearly with H2SO4 concentration. The addition of few tens of ppm of oxidant species such as hydrogen peroxide and Fe3+ or Cu2+ improves the corrosion performance markedly, leading to corrosion rates close to zero. In particular, the addition of H2O2 increases the corrosion potential up to the passivity of the steels owing to the possible formation of peracids.