Critical analysis of some electrochemical techniques including polarization resistance, for the study of zinc coating performance in near neutral chloride solutions

Abstract

Corrosion rate determination in near neutral solutions by Tafel slope back extrapolation of polarization curves can lead to dubious results due to mass transfer effects. With polarization resistance measurements, attainment of the steady-state following polarization takes considerable time, up to 10 min, and experiments become lengthy, demanding constant corrosion potentials. Results are discussed for reducing experimental times by (a) transient linear polarization, and (b) obtaining charge-transfer resistance instead of polarization resistance. Under the present conditions contributions from diffusion of anodic reaction product must be eliminated from the experimental measurement if corrosion rates are to be accurately calculated from the Stern-Geary equation. The linear range for polarization curves of potential vs. current density near the corrosion potential is only ± 1·3 mV for the zinc system. Results taken in excess of this range can lead to inaccurate corrosion rates. Results are discussed for determining Tafel slopes, which are obtained to convert polarization resistance or charge-transfer resistance to corrosion rate, by the Barnartt low polarization method.