Corrosion Behaviour of Alumix 123 P/M Alloy and AA2014-T6 in 3.5wt% NaCl

Abstract

The corrosion behaviour of the commercial aluminum powder metallurgy (P/M) alloy ‘Alumix 123’ and a compositionally similar wrought alloy, AA2014-T6, has been studied in naturally aerated 3.5wt% NaCl electrolyte by a variety of electrochemical methods and the subsequent corrosion morphology characterized by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). The P/M material was sintered in an industrial setting and was studied in the ‘as-sintered’ condition or after a standard sizing operation, denoted by the T1 and T2 temper, respectively. The electrochemical methods employed included open circuit potential (OCP), cathodic potentiodynamic polarization, cyclic potentiodynamic polarization, and potentiostatic polarization. The OCP of Alumix 123-T1 stabilized at potentials near the onset of hydrogen evolution, where corrosion was partially under anodic control and proceeded via cathodic hydrogen evolution. This is postulated to be due to a reduction in cathode area or depassivation induced by propagating crevice corrosion within residual porosity. In this state, most of the anodic current on Alumix 123-T1 was generated from creviced areas within residual porosity which eventually repassivated when the concentration of anodic reaction products exceeded the solubility limit and precipitation occurred. The OCP of Alumix 123-T2 stabilized at the pitting potential, where corrosion was predominantely under cathodic control and proceeded via cathodic oxygen reduction. The sizing operation of Alumix 123-T2 reduced the amount of residual porosity through plastic deformation and sealed surface porosity with sizing fluid so that crevice corrosion did not initiate. In Alumix 123-(T1, T2), pitting was not always associated with copper- and iron-rich intermetallics. This behaviour was attributed to the refractory layer formed on the P/M materials as a result of the sintering process, which retained integrity over these intermetallics. Above the pitting potential of Alumix 123-(T1, T2), crystallographic pitting was scarcely observed and the majority of attack was intergranular in nature. The OCP of AA2014-T6 stabilized at the pitting potential where corrosion was predominantly under cathodic control and proceeded via cathodic oxygen reduction. In AA2014-T6 pitting was associated with copper- and iron-rich intermetallics. SEM observations of AA2014-T6 suggested that copper is redeposited on copper- and iron-rich intermetallics and that cathodic trenching around iron-rich intermetallics may have liberated copper from the matrix in a non-Faradaic process. Above the pitting potential of AA2014-T6, there was extensive attack by crystallographic pitting and intergranular corrosion.