Determination of "True" Polarization Curves for Corrosion Rate Measurements of Steel in NaCl Stagnant Solutions

Determination of corrosion rate for carbon steel in stagnant NaCl solutions was attempted by the linear polarization method, but none of the polarization data obtained by conventional procedures (potentiostatic or potentiodynamic) gave satisfactory results. It was shown that the value of calculated corrosion rate is affected by the direction in which the polarization is undertaken, by the scanning rate for curves determined by potentiodynamic technique, and by the time interval between consecutive steps for curves determined by potentiostatic technique. A new method for obtaining polarization data was derived from chronoamperometric investigations. This method is similar to the potentiostatic one, with two differences: (1) The polarization potential is switched off for some minutes between two consecutive steps so as to allow the corroding system to recover, at least partially, the conditions observed initially, and (2) the current is determined by extrapolating the data of the current decay curves to infinite time. It was observed that in those curves, the current holds an exponential relationship to the inverse of the square root of time. The data generated by this method is in good agreement with weight loss and corrosion potential measurements not only in terms of corrosion rate but also in terms of Tafel slopes.