STRUCTURE, COMPOSITION AND PERFORMANCE OF SURFACE FILMS ON AZ ALLOYS AS A FUNCTION OF pH AND ALLOYED ALUMINUM CONCENTRATION

Abstract

<p>This thesis presents an investigation into the structure, composition and performance of naturally formed surface films on AZ alloys as a function of pH and alloyed Al concentration. STEM verified the film structure was bi-layer, consisting of an inner barrier layer, which was visibly deteriorated, and an outer porous layer. EDS SmartMaps™ coupled with the Inca™ software package determined the inner barrier layer was predominantly composed of MgO, whereas the outer layer was primarily Mg(OH)₂. However, both layers appeared to posses mixed oxide/hydroxide components according to ToF-SIMS analysis.</p> <p>Environmental pH had the largest effect on the structure and composition of the surface film. The near-neutral sample showed significant breakdown within the inner layer, which was attributed to natural hydration of MgO to Mg(OH)₂. This favourable hydration reaction is slower in alkaline environments and as such, the stability of the inner barrier layers of the pH 14 samples were noticeably improved. The effect of alloyed Al concentration was less significant however; increased enrichment of Al into the surface film structure appeared to cause a reduction in the thickness of the corrosion film itself.</p> <p>Drastic differences in corrosion performance were observed between the near-neutral and alkaline environments. Significantly better corrosion resistance to anodic dissolution was present in the alkaline environment coupled with a noticeably lower corrosion rate. The absence of breakdown potentials along with the presence of mass transport controlled anodic kinetics signified that the improved stability of the inner barrier layer was responsible for improved corrosion performance. In contrast, severe pitting and a narrow range of anodic stability were present for the near-neutral samples where the inner barrier layer was significantly compromised. This deterioration was deemed responsible for accelerated cathodic kinetics as well as minimal impedance to aggressive Cl^- ions from initiating wide scale electrochemical breakdown of the surface film.</p>