Effect of Chloride on Growth of an Anodic Film

Abstract

A1 was anodized in 2.4M H2SO4 at 5 mA/cm 2 and electronmicroscopic studies made of breakdown and formation of pits beneath the film after various surface treatments or after additions of up to 0.05M Cl-to the acid and selection of those specimens which had remained in the film-growth stage before stable pits grew. An electron-opacity technique was developed for studying film growth. Even in the absence of Cl-, there was a continued incidence of breakdown and a substantial time-dependent direct-dissolution current. With CI-, cell voltage tended to be lower. Film formation efficiency and geometrical structure were correlated directly with average cell voltage and not directly with C1-concentration. Cl-was interpreted as having only an indirect effect on film growth by stabilizing concurrent pit growth, wl~ch reduced the filmforming current. Possible modes of pitting beneath the film are discussed.In a recent paper (1), the authors presented electronmicroscopic evidence that during galvanostatic anodizing of aluminum in sulfuric acid many small breakdown-repair (B-R) or breakdown-pitting-repair (B-P-R) events occurred at the base of the film concurrently with porous film growth and that these events were not generally associated with substrate lattice defects or impurity clusters. As part of a program concerned with the mechanism of pit initiation, this was followed in the present work by studies of: (i) the possible influence of the surface preparation on the incidence of breakdown and (ii) the effects of C1-additions to the anodizing bath. The previous results and those of Part (i) directed attention not to the production by C1-of breakdown, which occurred in its absence, but to its reduction of the effectiveness of repair. This might be due to an effect of C1-on film growth rate or structure. Conditions favorable for examining this question were obtained by adjusting the C1-concentration in the bath to the highest level at which film growth would not be immediately short-circuited by growth of optically visible stable pits. Observations had been made identifying the transition in behavior with increasing C1-concentration (2).The surface treatment before anodizing was of concern specifically because with the perchloric acidethanoi electropolishing bath employed a residual film was known to be present after rinsing and this film presumably contained C1, which has been detected by Auger analysis on surfaces polished in this bath (3). Such surfaces have been reported to yield films of low ionic resistance after forming (4).The processes occurring in this system were such that the microtopographic features most accessible to observation were not those of growing pits but those of repair zones in the film and of the pore structure at various stages of growth. Comparisons were effectively made by examining the film after anodizing times at which it contained evidence of the previous occurrence of a high density of small (~0.1 ~m) B-P-R or B-R "events," and the nature of the geometry deduced is illustrated schematically ...

Structure and Stability of Anodic Films Formed on Aluminum Containing Dispersed Al3Fe Phase

“…If the growth of the pit around the margin of the particle is continuous and slow, this cannot be active-mode pitting at low potential with no film. The process might then be a type of high potential pitting perhaps similar to the one suggested earlier by the authors in which dissolution occurs through a soluble film (referred to in a general way as an electro~olishing-type film) (1). The presence of some film over the dissolving surface is suggested in Fig.…”

“…However, questions of local film character and its relation to heterogeneities in the substrate and to breakdown processes will require high magnification microscopy and related tools. The authors have employed electron microscopy to study film stability and breakdown models during anodie film growth on single phase 99.999% A1 in sulfuric acid (1). In the present work, this approach is applied to alloys containing dispersed A1sFe phase.…”

“…This constituent was of interest for a first study in part because there is some disagreement in previous work as to whether it dissolves, anodizes, or passes into the film unchanged. An attractive feature of the A1-Fe system is that there is limited solid solubility so that the extensive results on high purity aluminum (1) should be applicable to the behavior of the matrix in the alloy.…”

A‐C Impedance and Corrosion Studies on Implantable Anode Materials

“…There is difference of opinion as to the morphology and initiation (14,22) of pitting of A1 in solutions containing C1-ions. According to Kaesche (15) and Wood et al (16), A1 polarized to or below the pitting potential Ep shows crystallographic pitting which turns to electropolished pits as the potential is raised (more WeV); whereas Galvele et al (17) claim that brightened hemispherical pits are not a general rule.…”

Photogalvanic Cells: II . Current‐Voltage and Power Characteristics

“…The current is normalized with respect to aD and Xtr -1 as the diffusionlimited currents in the cell are usually proportional to these parameters 0 = ],hP[ --I,(PL --PD) I [13] where PL and PD are the dimensionless potentials of the illuminated and dark electrodes. The power, W, produced by the cell is given by W = oADaDRT Xtr -1 [14] The fractional conversion achieved by the photochemistry is described by ], where an --aD(1 --f) [15] bL ~--bD -{-JaD ~-~ laD [16] YL ----YD ~-lad [17] ZL ----ZD --lad [18] We have assumed that the concentration of B generated photochemically is much larger than its concentration in the dark. The relationship between I and g" is readily found by substitution into the equation governing the composition of the photostationary state…”