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 (Ecorr), passive current density (ipass), and pitting potential (Epit) as a function of salt type. In the case of 1D pits evaluated during fast downward scan rates, the saturation potential (Esat) 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 (Esurf) of activated pit surfaces reached Ecorr prior 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 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.
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1 July 2022
Research Article|
May 12 2022
Pitting Propagation Behavior on Low Alloy AISI 4130 (UNS G41300) Steel Exposed to Various Alkali and Alkaline Earth Metal Chlorides
Charles Demarest;
Charles Demarest
‡
*University of Virginia Center for Electrochemical Science and Engineering, The University of Virginia, 395 McCormick Rd, Charlottesville 22903-1738, Virginia.
‡Corresponding author. E-mail: crd4tt@virginia.edu.
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R. Matthew Asmussen;
R. Matthew Asmussen
**Energy and Environment Directorate, Pacific Northwest National Laboratory, Richland, Washington, DC.
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John R. Scully
John R. Scully
*University of Virginia Center for Electrochemical Science and Engineering, The University of Virginia, 395 McCormick Rd, Charlottesville 22903-1738, Virginia.
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CORROSION (2022) 78 (7): 634–649.
Citation
Charles Demarest, R. Matthew Asmussen, John R. Scully; Pitting Propagation Behavior on Low Alloy AISI 4130 (UNS G41300) Steel Exposed to Various Alkali and Alkaline Earth Metal Chlorides. CORROSION 1 July 2022; 78 (7): 634–649. doi: https://doi.org/10.5006/4042
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