ELECTROCHEMICAL OXIDATION OF HIGH-PURITY AND HOMOGENEOUS AL-MG ALLOYSWITH LOW MG CONTENTS

The susceptibility to the electrochemical corrosion of homogeneous Al- Mg alloys containing 0.33%, 0.43%, 0.77% and 0.88% (in weight) Mg in N aCl solutions has been studied by means of ocp measurements, cv, EIS, optical microscopy, SEM and EDX. The breakdown and repassivation poten tials of these alloys were shifted in the negative direction when the Mg content was increased, indicating an increase in their susceptibili ty to localized corrosion in the same direction. The impedance measure ments were interpreted according to suitable equivalent circuits. In t he passive region, two capacitive semicircles were obtained, the first being related with the film capacitance and the second one with a dif fusion process in the oxide film itself. The oxide film capacitances o n the Al-Mg alloys were greater than those found on high purity Al, su ggesting the formation of more defective and hydrated films on these a lloys. Pit nucleation of Al-Mg alloys is discussed considering the for mation of a defective oxide. In the pitting potential region, the Nyqu ist diagrams consisted of a capacitive semicircle followed by one or t wo inductive semicircles. The film capacitances were greater than thos e found for the passive oxide films because of the chloride-rich film formation in pitting conditions. In addition, the film capacitances in creased with Mg content of the alloy because of the increase in the pi tted surface area,confirmed by SEM, in the same direction. (C) 1997 El sevier Science Ltd.