Interfacial voids as possible precursor site for pore formation and pitting in aluminum

The nucleation and growth of nanoscale voids at the passive oxide - aluminum interface under benign electrochemical conditions provides an opportunity to explore the relationship between nanostructure and the initiation of pit and pore formation. Additional knowledge is necessary to generate controlled void populations that can be electrochemically interrogated. We identify open circuit equilibration time and electrolyte composition as two controlling factors of void nucleation and growth. Equilibration time is critical because initially anhydrous films slowly equilibrate to a limiting thickness. Void formation is correlated with mass transport through the passive oxide, so rapid growth for the unequilibrated oxide has a large impact on the void population. The addition of an alternate anion like borate also impacts the void population by altering the potential dependence of mass flux through the passive oxide. Data is presented to show that this regulation is a result of interfacial Cl concentration and resulting defect activity within the oxide.