A Comparison Between the Properties of Anodic Oxide Films on Aluminum Obtained in Different Alkali Nitrate Melts at Various Temperatures

Abstract

Crystalline form, hardness, kinetic parameters, coating ratio, and resistance to corrosion of the films produced in lithium‐containing melts are reported and compared with results obtained in false(normalNa,Kfalse)NO3 fused mixtures at higher temperatures.

Determination of Current Efficiency of Anodic Film Formation in the Molten Melt by ICP-AES

“…The efficiencies were determined by anodizing at least three samples at each temperature. The results of the current efficiencies are summarized in table I and II, where it is evident that the current efficiencies obtained from the theoretical expression [1] are slightly higher than those which are based on the detection of the dissolved aluminum in the melt by ICP-AES technique. Generally, an efficiency of film formation of 100 % is not expected where alternative anodic processes, such as pitting and gas evolution arise.…”

“…Expressions [1] and [4] have been used to calculate the current efficiency in the present study, in which, the density of the anodic film can be obtained from the energy spectrum of backscattered α particles using RBS; Q d can be obtained by ICP-AES measurements; the thickness of the anodic film can be obtained from transmission electron micrographs of ultramicrotomed sections.…”

“…no chemical dissolution of the outer film region proceeded and no alternative anodic processes consumed charge. The thickness, X, at 100% efficiency, can be estimated from the equation [1] below: X = (mol · wt · Al 2 O 3 / ρ · A · Z · F) · i · t [6] where i · t is the charge passed, A is the exposed surface area (m -2 ), Z is the charge number required to form one molecule of alumina and F is the Faraday's constant. Expression [6] was used to calculate the current efficiency, in which, i can be obtained directly from the readings of the digital ammeter attached to the anodizing circuit; t can be obtained from the quotient of the distance between the commencement and termination positions of the pen in the potentiometric chart recorder divided by the present chart paper speed during the run.…”

“…where i is the current density, t is the anodizing time and A is the area of the specimen exposed to the anodizing electrolyte. Q f in expression [1] is the electric charge required to form the anodic film, estimated by…”

“…Anodizing of aluminum in essentially molten melts provides an interesting opportunity to examine ionic transport processes at relatively high temperatures in essentially nonaqueous situations. Importantly, although the subject is not new, having been examined for capacitor applications [1] and for aluminum pretreatment to improve stress corrosion cracking performance in aerospace applications [2], little is known of the processes contributing to film growth. Generally, for anodizing in bisulphate melts, Tajima et al [3] showed that the films are barrier-type provided the voltage is restricted to avoid dielectric breakdown.…”