| Abstract: | Mechanism of passivation and breakdown of passive layers on Al and Al-Ta alloys has been examined in pH 8.4, 11 and 12. The methods used include a study of the H/T separation factor; radiotracer examination of Cl- and SO4[^2-] adsorption and absorption; XPS depth analysis; both negative and positive SIMS; Nomarski interferometry; SEM, EDS and STM. The separation factor is 7 - 4 depending on potential and pH. In solutions of pH [greater than or equal to] 12 only adsorption takes place, while at pH = 8.4, adsorption and absorption occur. Absorption succeeds adsorption at θ[Cl-] > 1 and precedes the breakdown. Distribution of ionic species in the film shows non-stoichiometry of [about] 8% and a high built-in electric field, [about] 10^7 V cm^-1. Ta species at the surface are only c. 16 %, but cause reduction in corrosion rate more than 50 times. SIMS shows OH-/O to decrease with distance from the solid-solution interface but remain [about] 0.05 through the interior of the film. It declines with Cl- entry. The effect of Cl- is radically different for pure Al, and for Al-Ta films. EDS gives an Al/Ta ratio of < 9 % at the surface (pH = 8.4). STM shows the topography of the dissolving film and indicates "boulders" of Al ([about] 500x200 A in dimension). The mechanism for H.E.R. on Al is slow discharge-fast electrochemical desorption. Time dependence of adsorption reveals that mass transport controls the rate of adsorption of inorganic ions. From the concentration dependence of adsorption on Al oxide, Temkin parameter, r, was calculated from the heterogeneity contribution parameter f - 26. Solubility product considerations showed that θ[Cl-] [to] 1 can be interpreted as adsorption of Cl- on porous layer of Al(OH)3... |
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