The influence of electrode polarization on corrosion beneath paint

A Zn–Fe bi-electrode coated with different clear lacquers was exposed to NaCl solution, so that a well-defined galvanic cell was established with the current directly measured by a zero-resistance ammeter (ZRA). Electrochemical impedance spectroscopy (EIS) was periodically used to monitor the two coated electrodes individually or coupled together. Transient potentiostatic current measurements were also made after applying potential steps of different amplitudes to the individual metals, from which polarization curves were plotted. The ohmic potential drop in the paint was corrected using film resistance values determined by EIS. Evans diagrams were thus plotted from the iR-corrected polarization curves to make the first experimental determination of Evans diagrams as used by Mayne 1 to illustrate his resistance inhibition model.

► Theory of resistance inhibition tested for the first time.
► Electrochemistry of reactions beneath organic coatings.
► Direct measurements of galvanic currents in corrosion cell beneath paint.