INFLUENCE OF THE SURFACE FILM STABILITY ON THE CORROSION RESISTANCE OF Mg IN AQUEOUS SOLUTIONS

Abstract

<p>This thesis presents an investigation into the structure and composition of the surface film formed on Mg in aqueous solutions and describes the influence of film stability on corrosion resistance. The composition and structure of the surface film formed on pure Mg in pure H₂O exposed at E_corr for 48 h was investigated using plan-view SEM-EDS imaging and STEM–EDS analysis of a FIB cross-section. The film formed was duplex in nature, consisting of a thinner, more-porous, nano-crystalline MgO-rich inner layer (50-100 nm), and a thicker, less-porous, Mg(OH)₂-rich outer platelet layer (700 nm). The results were consistent with the theory that a chemical breakdown (hydration) of the bulk inner MgO layer (native oxide) is a necessary precursor step to the corrosion process resulting in a significant thickening of the partially protective outer Mg(OH)₂ layer.</p> <p>The surface film formed on pure Mg exposed in 0.01 M NaCl for 24 h at E_corr was found to be a diffuse bilayer structure similar to pure H₂O, including a thin and porous nano-crystalline MgO-rich inner layer (50-100 nm), and a thicker, more porous Mg(OH)₂-rich outer layer (300-600 nm). The un-pitted film region formed in 0.01 M NaCl solution at +0.1 V above E_b for 0.5 h after 24 h aging at E_corr exhibited a similar duplex structure. However, the thickness of both Mg(OH)₂-rich (~450 nm) and MgO-rich layers (~30-60 nm) appeared to be decreased. Furthermore, the pitted region film formed on an anodically polarized Mg sample exhibited thinner MgO-rich layer (~20-30 nm), decorated with macro-pores (~50 nm), and Mg(OH)₂ rich middle layer (~200-300 nm) buried under a thick, more porous needle-like Mg(OH)₂-rich top layer (~0.2-1 µm). EDS analysis confirmed the presence of Cl within the films formed in 0.01 M NaCl, and Fe containing particles within the pitted region anodic film. These results were consistent with an accelerated hydration of MgO to Mg(OH)₂ at lower pH and in presence of Cl^-, formation of more soluble Mg-hydroxy-chlorides, enhanced Mg dissolution due to Cl^- ingress via increased film porosity, and influence of more noble impurities on the pit initiation and growth.</p> <p>The ~50-150 nm film formed spontaneously on Mg exposed to 1 M NaOH was found to consist mainly of a crystalline MgO layer that has been hydrated to Mg(OH)₂ to a variable degree. Although the film exhibited excellent corrosion resistance, it was not stable. The film tended to experience an irregular breakdown/repair process, which was characterized by large irregular potential drops (about 1 V) under E_corr conditions. The breakdown/repair process is believed to involve the hydration-induced stress-rupture of the MgO film at discrete sites and the subsequent formation of a Mg(OH)₂ self-healing corrosion product nodule (~ 500 nm).</p> <p>To understand the corrosion mechanisms and propose surface film evolution models, EIS behavior of Mg in all environments was investigated. The EIS spectra were compared with the proposed Nyquist behavior based on the film structure results. It was confirmed that the diffuse bilayer films formed in pure H₂O and 0.01 M NaCl possessed porosity, in which the electrolyte ingressed, forming the double layer and facilitating Mg faradic dissolution reaction across the Mg/film interface. Decreased porosity and R_ct along with the absence of a C_film/R_por loop, were respectively consistent with lower corrosion resistance of Mg in 0.01 M NaCl and a severely damaged pitted region film. Furthermore, the film compactness and increased R_ct in 1 M NaOH were consistent with a more stable film with excellent corrosion resistance in alkaline environments. In this film, liberated Mg²⁺ ions at the Mg/film interface have to diffuse through the film to facilitate the Mg(OH)₂ precipitation, thus a thicker Mg(OH)₂ film forms.</p>