Effect of alloyed Cu on localized corrosion susceptibility of Al-Cu solid solution alloys - Surface characterization by XPS and STEM

Cu additions to Al alloys decrease pitting susceptibility provided that the Cu is retained solid solution. In this study, a range of electrochemical, surface analytical and electron microscopy characterization experiments were carried out with Al-Cu solid solution alloys to understand the origins of this inhibiting effect. Potential-controlled electrochemical polarization experiments carried out in dilute chloride solutions show that stimulation of alloy dissolution without inducing passive film breakdown significantly ennobles a subsequently measured pitting potential. Similar effects are not observed with high-purity Al or Al-Zn solid solution samples. The ennobling effect is more difficult to detect in large area electrodes due to pit initiation triggered by defects and inclusions that are more regularly encountered in larger electrodes. X-ray photoelectron spectroscopy of Al-Cu solid solution samples reveals no Cu enrichment above detection limits. However, scanning transmission electron microscopy and spectral imaging of specially prepared Al-Cu needle-type samples shows that Cu is enriched at the alloy interface by a range of exposure regimens including those that induce pitting. These results suggest the possibility of local Cu enrichment at incipient pit sites by metastable pitting, which can contribute to an ennoblement of the critical pitting potential under certain circumstances.