Corrosion Behavior of Ni Steels in Aerated 3.5-wt.% NaCl Solution at 25ºC by Potentiodynamic Method

This work studied the corrosion behaviors of 15Ni, 23Ni, 31Ni and 40Ni steels in aerated 3.5% NaCl solutions at pH 2, 7, 10 and temperature of 25ºC. The observed microstructures of 15 and 23 wt.% Ni steels were partially martensite, but those of 31 and 40 wt.% Ni steels were fully austenite. Polarization curves of the nickel steels were measured by potentiodynamic technique. The corrosion potential (Ecorr), corrosion current density (Icorr), from which the corrosion rate (Rmpy) was calculated, pitting potential (Ep), primary passive potential (Epp) and passive current density (Ip) were evaluated form the polarization curves. The results showed that increasing nickel content increased the corrosion potentials but decreased the corrosion current density or the corrosion rate. Only pH 10, the passive characteristics of the 23Ni, 31Ni and 40Ni steels were observed. The pitting potentials and primary passive potentials of those three steels increased with nickel contents, but the passive current density decreased. Nickel improved general corrosion and pitting corrosion resistances of the tested steels in basic, neutral and acid 3.5% NaCl solutions. The effects of nickel and solution pH on general corrosion resistances of nickel steels were discussed.