Improvement of the Polarization Resistance Method

Katoh Masayoshi, Koyama Masaaki

doi:10.3323/jcorr1974.34.4_215

Polarization resistance to obtain an instantaneous corrosion rate has been measured from the steady state potential reached after charging of the double layer when a constant current was applied. In neutral solution, however, the potential never attain to that state but gradually increase owing to the presence of the rust formed on the surface. This makes the measurement inaccurate. The present method consists of a short time polarization (0.5sec) and a determination of the steady state potential in accordance with the following theoretical transient equation. ln[i(2R_p+R_sol)]-E=ln(2iR_p)-t/C_pR_p where the term, i(2R_p+R_sol), corresponds to the steady state potential and this was determined by curve fitting. Two sheets of mild steel were immersed in tap water in parallel, apart 1cm. A constant current of 5μA/cm² was applied between the two and the potential build-up curve was recorded with a transient converter. This method was found to have advantages over the conventional method in that 1) Solution resistance between the specimens (R_sol) and pseudo capacity (C_p) can also be measured as well as polarization resistance (R_p). 2) Reference electrode is not necessary and simple two electrodes method is applicable due to the accurate determination of R_sol. 3) Short-time polarization ensures no disturbance of natural corrosion state. 4) Measuring apparatus are simple compared with other non-steady state methods. Another interesting information was also derived through this research in that abnormally large value of C_p was due to the continuous electrochemical change in rust oxidation state during the polarization.

Improvement of the Polarization Resistance Method

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