The influence of the sodium chloride concentration on the corrosion behavior of a duplex stainless steel

摘要

The corrosion behavior of duplex stainless steel 0.08C-22.5Cr-8Ni-2.5Mo-3Cu in cast and quenched states was investigated in aqueous solutions of sodium chloride at various concentrations ranging from 0.03 M to 4 M. Measurements were carried out by means of the linear sweep technique with a sweep rate of 0.2 mV/s. Anodic polarization curves were recorded and the potentials at which a sharp rise in the current occurred, called the pitting potentials, were determined. Three such potential values were observed: the first, corresponding to the beginning of non-metallic inclusions dissolution, the second, corresponding to pit initiation due to preferential ferrite dissolution at the ferrite-austenite phase boundaries and the third, corresponding to the onset of general dissolution of the specimen. It was observed that only the pitting potential of preferential ferrite dissolution and the pit size depend on the salt concentration. This pitting potential becomes more negative as the concentration increases (by about 80 mV per tenfold increase in the salt concentration). Yet, the pit size and the current of general dissolution become larger. The total number of pits, however, does not change with the salt concentration. Optical microscopy observations, performed during and after anodic potentiodynamic dissolution of the specimen, showed that local dissolution of the steel starts at boundaries between non-metallic inclusions and the base metal as well as between austenite and ferrite phases. Quenched specimen showed much better corrosion resistance then cast specimen. In particular, the anodic current is by more than one order of magnitude larger for the cast specimen than for the quenched one.