Steady‐State Anodic Leakage Current in Barrier‐Type Aluminum Oxide Films

Abstract

When aluminum is polarized potentiostatically in a barrier-forming electrolyte, such as aqueous ,ammonium tartrate, the current passed diminishes steadily to a low value commonly termed the leakage current. This value is not stable, but increases gradually, attaining a constant value after many hours. The dependence of this steady-state current on potential has been studied for aluminum electrodes in 1M aqueous ammonium tartrate at pH 7 and test temperatures of 30 ~ 50 ~ and 70~ In the overpotential range of approximately 50-80V, current is independent of overpotential, while exponential dependence of the anodic current on overpotential is seen at lower overpotentials, with a significant reverse contribution below approximately 20V. The values of both forward and reverse exponential coefficients are extremely anomalous in comparison to normal Tafel slopes. Above approximately 80V, current increases rapidly with potential until breakdown occurs.During the potentiostatic formation of barrier-type anodic oxide films on "valve" metals the anodic current is seen to decrease monotonically to a minimum value, commonly termed the "leakage current." For aluminum anodized in an electrolyte conducive to barrier film formation, such as neutral aqueous ammonium tartrate, this minimum is reached in a matter of minutes. However, this leakage current is not stable, but gradually increases, attaining a stable value only after many hours. A similar effect is seen if the value of the applied potential is reduced after the leakage current has stabilized. There is then a marked reduction in the anodic current, with a new minimum value being observed after a period of several minutes. Again, however, this value is not stable, but increases at an accelerating rate with a higher, apparently stable value being attained after many hours. This is apparently a distinct phenomenon from that observed by several previous investigators (1-8), in which a transient current response is seen following an abrupt change in applied potential or an interruption in the application of an anodic potential. Such transients have durations of the order of minutes, two to three orders of magnitude shorter than the time required for establishment of a stable leakage current, and have been attributed to a relaxation process involving the conducting ions in the oxide (9, 10).Research on valve metal electrodes has been extensively reviewed (9-17); therefore, it is necessary only to refer to those aspects of previous work 'that directly pertain to the establishment of a stable leakage current through the anodic oxides formed on such electrodes. Much earlier work was stimulated by the rectifying properties of valve metal electrodes. Van Geel (18, 19) and Taylor and Haring (20) attributed this behavior to the occurrence of a p-n junction with-* Electrochemical Society Active Member.Key words: electronic current, polarization~ transients.in the anodic oxide, while Saski proposed a p-i-n junction (21). The latter observation is somewhat supported by the resul...

The Role of Defects in the Fracture of Oxide Films on Metals

“…1. A series of calibration measurements on small samples of known area showed that -c --KSA [1] where 8A is the area of metal being anodized, and K, for our experimental arrangement, had a value of 300 mse c/cm 2.…”

“…From these relationships it is easily shown that for our specimen geometry 8A = 0.57 ex [4] A Combining Eq. [1] and [4], the value of T/A --170 el msec/cm 2. Thus for a tensile strain of 6 X 10 -3 , 9 /A = 10.2 msec/cm 2.…”

“…Various properties of oxide films on metals are often ascribed to the presence of imperfections or flaws. For example, defects in the oxide are held responsible for: (i) the finite conductivity (leakage currents) of these oxide films (1,2); (ii) the occurrence of pitting in a corrosive environment (3) ; and (iii) the initiation of mechanical failure of the film during deformation (4,5).…”

“…The precise nature of the defects is usually indeterminate, particularly for thin oxide films. However, the development of pores in aluminum oxide has been well documented by electron microscopy and demonstrated to be responsible for an increase of leakage current (1,2,6). On the other hand, defects are often attributed to surface contamination of the metal or the presence of inclusions in the metal.…”

Material characteristics and capacitive properties of aluminum anodic oxides formed in various electrolytes

“…Fundamental research on the features of the anodic barrier films formed in the bases of boric acid, [13][14][15][16][17] citric acid, 18,19 tartaric acid, 1,[20][21][22] phosphoric acid, 23,24 and ammonium adipate electrolytes, 25,26 and so forth, are comprehensive. Katoh 27 summarized the performance of the aluminum oxide films formed in various electrolytes.…”