Towards developing self-repairing oxides to protect zinc
thesisposted on 28.02.2017 by Thomas, Sebastian
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The long term protection of zinc is nominally achieved by its own corrosion products, collectively known as the zinc patina. This dissertation undertakes a mechanistic study of the role played by the patina, in protecting zinc. It is focused towards improving the protection imbibed by the zinc patina and also to enhance its ability to self-repair. The zinc patina formed on exposure to a single 1 µl droplet, was firstly characterised along its cross-section, by both electron microscopy and XPS. Initially (after about 30 minutes of exposure) a thin layer of porous oxides is formed on the metal, with the pores serving as electrolyte channels to attack the metal. After about 6 hours, the patina evolves into a multilayered structure on zinc, with a continous inner compact oxide, an inner dense precipitated layer and an outer porous precipitated layer. These inner oxide layers serve as an efficient electrochemical/ diffusion barrier on zinc. There is also an absence of electrolyte channels in these inner layers which suggest that the patina intrinsically has the potential to naturally self-repair itself. The aqueous stability of the precipitated layer depends on the electrolyte pH. The corrosion of zinc as a function of pH was thus studied by potentiodynamic polarisation. The electrochemical data was then compared/contrasted with thermodynamic data which depicts the most stable zinc phases which may form at a particular pH. In general, the corrosion rate of zinc is lowest at pH values where ZnO is stable (and hence can precipitate upon zinc) in solution. At highly alkaline pH (>13), and acidic pH (<4), since ZnO is unstable in solution, the corrosion rate of zinc is higher. Steady state passive currents are manifest when zinc is exposed to a solution of pH 12-13, with electrode potential around -1.1 VSCE. The precipitated layer (ZnO/Zn(OH)2 formed on zinc in a pH 13 solution is unstable, as it tends to dissolve into zinc hydroxycomplexes such as Zn(OH)3- and Zn(OH)42-. This results in high passive currents at this pH. At pH 12, since the precipitated layer is stable, it serves as an anodic barrier protecting zinc, resulting in lower passive currents. Therefore, a stable precipitated layer serves to lower zinc corrosion and thus, zinc passivation is punctuated by the presence of both a compact oxide and a stable precipitated layer. The protective oxides formed on zinc at pH 12 cause a polarity reversal of the passive currents during anodic polarisation, indicating that zinc oxides support the oxygen reduction reaction (ORR) on their surface at significant rates. The oxygen consumption on zinc, by ORR, measured using a Pt microelectrode during passivation is significant, when compared with bare/actively corroding zinc. Therefore, in zinc systems, ORR may readily take place on the surface of zinc oxides. It is proposed that ORR on the surface of the zinc oxides may itself catalyse the growth of the zinc patina. ORR may take place at the oxide/solution interface driving the complementary anodic processes at the metal surface. The zinc ions ejected into solution by the anodic processes may precipitate back as zinc compounds on the metal surface, causing a gradual growth of the zinc patina with time. In order to improve the barrier properties of the zinc patina, and also enhance its self-repair, firstly the stability of the precipitated layer needs to be enhanced. Secondly, ORR on the surface of the zinc patina needs to be lowered to protect zinc from self-corrosion in the longer term.