Formation and Breakdown of Anodic Oxide Films on Aluminum in Boric Acid/Borate Solutions

Abstract

Highly pure aluminum was anodized at a constant current density of 25Am−2 at 293 Kin 0.5 M boric acid/0, 0.005, or 0.05 M sodium tetraborate solutions, to examine the effect of sodium tetraborate concentration on the formation and breakdown characteristics of barrier oxide films by using inductively coupled plasma atomic emission spectrometry, electroluminescence/photoluminescence measurements, scanning electron microscopy, transmission electron microscopy, and electrochemical impedance spectroscopy. In boric acid/borate solutions, a crystalline alumina formed locally in the middle of the amorphous oxide film. Above the crystalline alumina, a void is formed and may lead to a breakdown of the oxide film at 420 to 540 V. In boric acid solution, an amorphous oxide film grew until 1180 V with the formation and development of imperfections and with enhancement of electroluminescence and gas evolution. At imperfections, the oxide/solution interface was convex and the oxide/metal interface curved in the opposite direction. This deformation is attributed to high‐pressure O2 evolved in the pores of imperfections and to the local formation and dissolution of oxide. The breakdown of the oxide film started when the O2 evolution and oxide dissolution at imperfections become predominant. The mechanism of formation and breakdown of the anodic film in the boric acid/borate solutions is discussed in terms of pH buffering of the anodizing solution, and the electronic structure of anodic oxide films is correlated with electroluminescence and photoluminescence spectrum results.

Growth of anodic TiO2 nanotubes in mixed electrolytes and novel method to extend nanotube diameter

“…9a) obtained at constant current anodization in the mixed electrolyte, the uniform oxide film thickening is terminated eventually until the oxide dielectric breakdown [54,57], with visible sparking observed frequently and oxygen gas releasing over the film surface [26,58]. When the oxide dielectric breakdown occurs, the voltage reaches a maximum value ($230 V) and no longer increases.…”

“…Therefore, the anodizing current J T is composed of the J ion and the J e , i.e., J T % J ion + J e , then oxide forming efficiency u % J ion /J T . In fact, oxide growth stops at the breakdown stage, thus the J ion and the forming efficiency u drop to zero [26,58]. At the moment, the J T would all transform into avalanche electronic current ($5 mA/cm 2 ).…”

Growth behavior of anodic oxide formed by aluminum anodizing in glutaric and its derivative acid electrolytes

“…Subsequent anodizing causes an increase in voltage and film thickening, and a linear relation, with a proportional constant of approximately 1.2-1.3 nmV -1 , between the anodizing voltage and the thickness of the barrier oxide under constant current anodizing has been reported [1,45]. The growth of the barrier oxide film then stops abruptly due to dielectric breakdown, with visible sparking on the specimen.…”

Fabrication of a novel aluminum surface covered by numerous high-aspect-ratio anodic alumina nanofibers

“…In general, anodic aluminum oxide can be classified into two different types as follows: barrier and porous oxides [1][2][3]. Aluminum anodizing in neutral solutions results in the formation of a thin dense barrier oxide that can be as thick as 1 µm on the aluminum substrate [4,5]. Barrier oxides have been widely used for electrolytic capacitor applications due to their high dielectric property [6][7][8].…”