Closure to “Discussion of ‘The Valency of Aluminum Ions and the Anodic Disintegration of the Metal’ [M. E. Straumanis and K. Poush (pp. 1185–1188, Vol. 112, No. 12)]”

128

Data have been obtained for electrode reaction rates on bare aluminum-7 weight percent magnesium immersed in aqueous solutions. The reactions investigated were metal dissolution, proton reduction, and oxide growth in sulfate and chloride containing solutions in the pH range 1.0-6.5. The bare metal surface was produced by rapidly and lightly scratching a potentiostatically controlled rotating disk electrode. The resulting current transients were measured under various conditions of electrode potential, electrolyte composition, temperature, and electrode rotation rate. Current densities of up to 8 Acm -2 on the scratch were measured in this way. The rates of metal dissolution and proton reduction are increased by many orders of magnitude by removing the high impedance surface oxide. Dissolution of the bare surface occurs with the following rate-determining step: A1 + H20 --> AIOHads + I-I+ + e-, the rate being independent of pH and sulfate (0.5M) or chloride (I-4M) content. Proton reduction occurs with the discharge step being rate controlling. The rate of nucleation and growth of the oxide film follows the empirical law /at ~ /am exp(--~tn). Initially n ----I, but at longer times changes to 0.5, the changeover occurring earlier with increase in anodic polarization and by the presence of C1-.The generalized theory of stress-corrosion cracking, originally suggested by Mears, Brown, and Dix (1) and expanded upon by others (2-4) has long been accepted for aluminum alloys since it is capable of explaining qualitatively the variation of cracking rate with second-phase morphology and, possibly, with elementary changes in electrochemical conditions at the crack edge. In essence this theory proposes that crack propagation progresses from a nucleating notch (usually formed by intergranular attack associated with discrete secondphase particles) when sufficient stress concentration exists to cause ductile tearing at the notch edge. Accelerated dissolution then occurs preferentially at the resultant film-free regions and continues until the dissolution rate is decreased by oxide formation or the intergranular penetration is halted at a grain boundary of unfavorable orientation. Thereupon the cycle of slow intergranular corrosion followed by rapid growth of the oxide film starts again.Recently, however, there is increasing evidence (5, 6) that an alternative mechanism involving hydrogen embrittlement may be valid in humid and aqueous environments. This evidence is based primarily on the similarity between the potential dependence of stresscorrosion susceptibility and of hydrogen permeation (7,8), the fact that the tensile ductility may be reversibly * Electrochemical Society Active Member. Present address" ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 130.237.29.138 Downloaded on 2015-03-09 to IP ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 130.237.29.138 Downloaded o...

Section: Discussionmentioning

confidence: 99%

Bare Surface Reaction Rates and Their Relation to Environment Controlled Cracking of Aluminum Alloys: I . Bare Surface Reaction Rates on Aluminum‐7 Weight Percent Magnesium in Aqueous Solutions

Ford

Burstein

Hoar

1980

128

“…Electron probe microanalysis detected no impurity elements within the particles. Their exact nature is unknown, but they may be a secondary oxide phase or a localized dispersion of fine metallic aluminum particles (15). Oxide NHG and all composite oxides were always clear and free of blemishes.…”

Section: Properties Of the Hydrated Oxide--bernard Andmentioning

confidence: 99%

The Anodic Oxidation of Aluminum in the Presence of a Hydrated Oxide

Alwitt¹

1967

The effect of a hydrated oxide on the kinetics of anodic oxidation of aluminum has been studied and the properties of the resulting composite oxides compared with those of purely anodic oxides. The presence of a hydrate layer reduced the amount of anodic oxide needed to support a given voltage and increased the current efficiency to as much as 100%. Water was lost from the oxide during anodization, probably as the result of transformation of boehmite to ~-A1203, which was incorporated into the barrier layer. The dielectric strength was greater for a composite oxide than for an oxide produced in the absence of a hydrate, though the dielectric constants were probably not very different. The impedance characteristics of a composite oxide indicated fewer microfissures than in a purely anodic oxide.The formation of an oxide film on aluminum by the sequential processes of reaction in hot water followed by anodic oxidation produces a composite oxide with some unique properties. This process has been of technological importance for some time in the manufacture of electrolytic capacitors (1, 2), but has received scant attention in the scientific literature. Altenpohl (3) briefly mentioned that the anodic oxide grows underneath the hydrate and that the latter is partially consumed by the growing film. He suggested that water is eliminated from the consumed portion of the hydrate whenever this portion crystallizes into ~/-A1203 under the influence of a high electric field. Unfortunately, no experimental evidence accompanied this discussion. Burger and Cheseldine (4, 5) also advanced the idea that the barrier portion of the composite oxide consists of both anodic oxide and converted hydrate, but their data were insufficient to confirm this.The lack of information on the composite oxide prompted us to perform the investigation reported here of the effect of a hydrated oxide on the kinetics of anodic oxidation, and the properties of the resultant composite oxide. Our data support the hypothesis that some hydrate is dehydrated during anodization and becomes converted to barrier oxide. Moreover, details of this process are presented that have not been available previously. Experimental ProceduresCommercially etched 99.97% pure aluminum foil was used throughout. The increased surface area improved the sensitivity of weight measurements and the "corrugations" increased the rigidity and strength of the isolated oxide, greatly simplifying handling procedures. Any change in surface area over the range of oxide thickness studied was sufficiently small to have no effect on the interpretation of results. Sample size was 6.4 x 7.6 cm for weight and charge measurements and smaller samples, 2.6 x 3.6 cm, were used for impedance measurements. The foil was not treated in any way prior to hydration in boiling water at atmospheric pressure2 Anodic oxidation was at 2 ma/cm 2 in 100 g/1 boric acid at 90~The boric acid was electronic grade, and distilled deionized water with a resistivity ca. 504),000 ohm-cm was used throughout. Foils were rinsed...

“…The results can be read from Fig. Since In goes into solution as In 8+ (12,13), Sb must do the same. 3.…”

Section: March 1971mentioning

confidence: 99%

The Anodic Dissolution Reaction of InSb: Etch Patterns, Electron Number, Anodic Disintegration, and Film Formation

Straumanis

1971

Self Cite

The etching behavior of the inverse {111} planes of undoped, semiconducting, n-type, InSb single crystals was explored. Depending upon the etchant, including anodic dissolution, various etch patterns were obtained on the inverse planes. In general the etch pits on the In{lll} plane were round, and the face was shiny, whereas the face of the inverse plane was dark and rough. The rates of dissolution in the electrolytes used were very low, especially in absence of oxidizers. The components dissolve as In 3 + and Sb 3+. At current densities above 40 or 60 mA cm -2 (on Sb{-1]]} or In{ill}), growth of a black, collodial film of Sb405C12 containing very fine metallic Sb particles occurs on both planes. The Sb particles result from the partial disintegration of InSb. Upon heating the film in vacuum, recrystallization occurs and the Sb aggregates to form larger particles. An explanation is offered for the different behaviors of the inverse {111} planes.