Pitting Propagation Behavior on Low Alloy AISI 4130 (UNS G41300) Steel Exposed to Various Alkali and Alkaline Earth Metal Chlorides

This study examined pit propagation to elucidate whether alkali and alkaline earth metal chloride salts such as RbCl affect pitting in some manner previously not expected compared to NaCl. Pit propagation studies were conducted on low alloy steel using one-dimensional (1D) pit method over pit depths from 300 µm to 1,000 µm. Linear sweep voltammetry and electrochemical impedance spectroscopy on planar 4130 electrodes over a range of Cl⁻ concentrations revealed no differences in impedance, open circuit, corrosion potential (E_corr), passive current density (i_pass), and pitting potential (E_pit) as a function of salt type. In the case of 1D pits evaluated during fast downward scan rates, the saturation potential (E_sat) varied as a function of salt type and at shallow pit depths. Mass transported limited current density also varied with salt type in shallow pits when other alkali metal and alkaline metal cations where present. The potential (E_surf) of activated pit surfaces reached E_corr prior to establishing a condition where the pit electrolyte surface concentration (C_surf) was less than the critical concentration for active acidified pitting (C_surf < C_crit) in this marginally passivating steel. For various E_surf and pit current density (i_pit) combinations at constant C_surf where C_crit < C_surf < C_sat, E-log(i) plots were constructed using the method of Li Tianshu to unmask IR  ohmic voltage corrected Tafel plots at fixed pit solution concentrations. Under these conditions, the influence of salt identity on charge-transfer-controlled kinetics was re-examined and slight differences in Tafel behavior were found. Differences in metal cations have little effect on passive planar electrodes and only affect pit propagation stage in shallow pits.