ON THE MECHANISM OF THE PASSIVITY OF ALUMINUM AND ALUMINUM-ALLOYS

The mechanism of passivation and breakdown of passive layers on Al and Al-Ta alloy has been examined at pH 8.4, 11 and 12. The methods used include: a study of the hydrogen to tritium separation factor to obtain the mechanism of the cathodic partner reaction; radiotracer examination of Cl- and SO42- adsorption and absorption: X-ray photoelectron spectroscopy depth analysis; both negative and some positive secondary ion mass spectrometry (SIMS); Nomarski interferometry; scanning electron microscopy; energy dispersive X-ray spectroscopy (EDS); and scanning tunneling microscopy (STM). The separation factor H/T is 7-4 depending on potential and pH. Absorption succeeds adsorption at theta(Cl-) > 1 and is the forerunner of breakdown. Distribution of ionic species in the film shows non-stoichiometry of ca 8%. This gives a built-in electric field which is surprisingly high, 10(6)-10(7) V cm-1. Ta species at the surface are 2% -6% depending on pH but cause a reduction in corrosion rate of ca. 10(2) times. SIMS shows the OH-:O ratio to decrease with depth from the solution/oxide interface but to remain > 0.1 through the contact film to the surface. It declines with Cl- entry. The latter behaves radically differently for pure Al, and 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, around 500 angstrom across. Passive layers on Al for pH 8-11 involve a porous pre-layer of Al(OH)3 and Al2O3 mixed crystal. The contact layer consists largely of Al2O3 but a critical component of it (the reason for its protectivity) is AlO(OH), in fibril form. Breakdown involves penetration of these fibrils by Cl-, which displaces the OH- therein and diffuses out again, in the form of an AlClnm+ complex. Interpretation of the reaction order of 2 with respect to Cl- suggests m = 1 and n = 2. The Ta protects by blocking entry to the fibrils.