Features of electrochemical noise generated during pitting of inhibited A516-70 carbon steel in chloride solutions

The features of electrochemical noise generated during pitting of A516-70 carbon steel in chloride solutions containing Na2CrO4 were investigated. Polarization curves showed that the threshold Cl- concentration in 0.01 M Na2CrO4 solution to initiate pitting and the threshold CrO42- concentration in 0.1 M NaCl solution to maintain passivation for A516-70 carbon steel were approximately 0.1 M and 0.01 M, respectively. Noise measurements indicated that above the critical Cl- concentration, the stochastic current fluctuations appeared after 5 h of immersion, which was an indication of pit initiation. Increasing CrO42- concentration to 0.05 M led to the production of symmetrical current spikes at the start of immersion. The noise pattern changed to frequent fluctuations with small amplitude during immersion. Noise analysis in the frequency domain showed that pitting initiation was characterized by the slope of the PSD plot approaching -2. Extending the immersion time decreased the pitting initiation rate, as indicated by the decreased PSD slope and increased PSD frequency.