Abstract
The potentiodynamic polarization method was found to be unsuitable for the evaluation of inhibitors and antifreeze coolants for localized corrosion of aluminum because of non-reproducibility and high scan rate sensitivity of the results. Current-controlled methods were found to have several advantages over potential-controlled methods in monitoring the break-down in the passive layer. The single point galvanostatic method was found useful for rapid screening and especially for probing the passivation/depassivation processes occurring at the aluminum surface. The final screening of inhibitors was done using the galvanostep method. Long term potentiostatic tests (not point by point) showed that both pitting and crevice corrosion occurred at potentials >Ep (protection potential). Therefore Ep appears to be the valid parameter for the evaluation of inhibitors and coolant formulations for localized corrosion of aluminum. Nitrate was the only "preventive inhibitor" that could protect against localized corrosion of aluminum by penetrating the thick passive layer and oxidatively maintaining the barrier layer. Chromate is a "remedial inhibitor" that can heal any breaches in the passive layer but it cannot inhibit localized corrosion of aluminum because it is unable to reach the barrier layer when the passive layer is intact. A pitting/crevice domain is defined to help interpret some of our observations.
