Indirect Measurements for Corrosion Detection

Indirect methods of corrosion rate measurement involve aspects of the electrochemical process other than metal dissolution. These measurements involve cathodic reactions, such as the evolution of hydrogen, or consider current-potential relationships, such as polarization curves or polarization resistance values. 

Hydrogen evolution can be used where reduction of hydrogen ions is the cathodic reaction, e.g. in acidic solutions. The method can be cumbersome, because the solubility of hydrogen in the solution and hydrogen absorption into metals must be considered. The method is most practical at high rates of corrosion in acids, but is not too commonly used. In one case, the technique has been used for rapid screening of acid inhibitors. 

The environment may be either acidic or basic. A technique using the eifects of diffused hydrogen on current flow between electrodes in a vacuum has been used to study corrosion mechanisms and should be valuable for inhibitor studies, because the sensitivity of this method is considerably better than that of the pressure build-up technique. 

Current-voltage relationships are commonly used as measurements of corrosion and are consequently of value in evaluating inhibitors. The first determines the potential versus current curves for both the anodic and cathodic reactions. Data are plotted on a semi-logarithniic current scale and are extrapolated backward toward the low current direction until the anodic and cathodic curves intersect, the current density at that point representing the rate of corrosion. 

The second method uses polarization resistance, which is the slope of the polarization curves at the point of corrosion. This method has practical use both in the laboratory and in the field. 

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