Electrochemical differentiation between aluminised and galvanised steels in aerated chloride environments

Abstract

The potential measurements in NaCl solutions of varying concentrations carried out separately on galvanised steel and aluminised steel, each coupled galvanically with mild steel, have shown continuous rise of potential with increase in the concentration of NaCl in both the cases. For the aluminised steel-mild steel couple, the protective potential of -760 mV is reached close to the NaCl concentration of 2%, while the potential in the case of galvanised steel-mild steel couple has always been above the protective potential, right from the beginning, within the concentration range of NaCl (upto 3%) studied. The current and potential measurements in 0.1% NaCl solution have shown that the resistance of the surface film on aluminised steel grows from 1.037 ×103 ohm to 5.500×103 ohm within the test duration ranging from 20 hours to 100 hours and that on the galvanised steel increases from 0.90×103 ohm to 2.307×103 ohm within the test span ranging from 30 hours to 100 hours. The low current found to have been associated with the aluminised steel-mild steel couple is not sufficient to bring the potential at the required protective level for the protection, while about 4 times higher current measured in the galvanised-mild steel couple helps it in attaining the protective potential.