Why do electrochemical measurements often give corrosion rates higher than reported in the literature when done by weight loss?
A2. There are a variety of reasons that tend to make weight loss testing measure low and electrochemical tests measure high. This can be minimized by careful design of the test and good lab technique. Weight loss tests can give lower corrosion rates because the corrosion products are difficult to remove especially within crevices and some changes do not result in a net weight loss. Electrochemical tests can give higher corrosion rates if multiple chemical mechanisms are allowed to occur instead of just the corrosion that you want to measure. Wide ranging and slow potentiodynamic scans can drive additional reactions which skew the curves and result in a higher Tafel constant. Poor test cell design can result in significant galvanic corrosion in addition to the chemical attack of the electrolyte which is being measured. Also, if the corrosion rate is calculated from the polarization resistance only, the assumed beta constants of 0.1, are usually not realistic values.
