Pitting Propagation Behavior on Low Alloy AISI 4130 (UNS G41300) Steel Exposed to Various Alkaline Earth Metal Chlorides Using the 1-D Pit Method

Abstract

This study examined pit propagation to elucidate whether alkali and alkaline earth metal chloride saltssuch as RbCl affect pitting behavior in distinctly different manner than NaCl. Pit propagation studies were conducted on a low alloy steel using one-dimensional (1-D) pit method over pit depths from 300-1000 µm. LSV and EIS of planar electrodes of 4130 in a range of Cl- solutions were conducted and revealed no differences in impedance, open circuit, corrosion potential (Ecorr), passive current density (ipass), and pitting potential (Epit) as a function of salt type. In the case of one-dimensional pits during fast downward scan rates, the saturation potential (Esat) varied as a function of salt type and pit depth when pits were shallow. Mass transported limited current density also differed with salt type in shallow pits when other alkali metal and alkaline metal cations where present. The pit surface potential (Esurf) of activated pit surfaces reached Ecorrprior to establishing a condition where the pit electrolyte surface concentration (Csurf) was less than the critical concentration for active acidified pitting (Csurf<Ccrit) in this marginally passivating steel. For various Esurf and pit current density (ipit) combinations at constant Csurf where Ccrit< Csurf < Csat, E-log(i) plots were constructed using the method of Li, Tianshu to create 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 reexamined and slight differences in Tafel behavior were found. Differences in metal cations exert no effect on passive planar electrodes and only effect pit propagation stage in shallow pits.