Calculation of Seawater pH at Polarized Metal Surfaces in the Presence of Surface Films

A model was developed for calculating the pH at a cathodically polarized metal surface. The model accounts for the buffering capacity and ionic strength of seawater, and it is valid in the presence of uniformly distributed calcareous, biological, or corrosion-product-type surface films. The model does not yet account for hydrogen evolution, and it cannot directly handle chemical reactions or the metabolic activities of microorganisms within the film. Calculated pH values are dependent on applied current density, film thickness, and the various diffusion coefficients. A diffusion correction factor is used to modify diffusion coefficients within the film. The model predicts increasing pH with increasing applied current density and film thickness, reaching a maximum of 9.9 at the limiting diffusion current for oxygen under quiescent flow conditions with no hydrogen evolution. Direct measurements of pH by micro-electrode techniques were in agreement with model predictions at an applied current of 20 μA/cm2. Measured pH values were higher than calculated at 100 μA/cm2, the difference due to hydrogen evolution.