Surface Changes during A.C. Etching of Aluminum

Abstract

Electrochemical etching of Al with alternating current produces a high density of cubic etch pits covered with a film of oxide or hydroxide. Micrographs of the metal and film structures are presented and a model is proposed to explain the observations. Calculations demonstrate the feasibility of film deposition from acid solution when a high cathodic current density is applied. A critical feature is that the high aluminum salt concentration near the metal surface hinders proton mobility.

Fabrication of superoleophobic hierarchical surfaces for low-surface-tension liquids

“…[22][23][24][25] In contrast, AC etching in the similar HCl solutions produces a rough etched layer comprising interconnected cubic pits. [26][27][28][29] The size of the cubic pits is usually submicrometers. In this study, more simple chemical etching has been used.…”

Importance of Etch Film Formation During AC Controlled Pitting of Aluminium

“…electrolytic graining or electrograining is a process used for the creation of a uniformly pitted morphology on aluminium surfaces. Electrograining can increase the specific area of a smooth aluminium foil by up to a factor of 200 [Ono & Habazaki, 2011, Dyer & Alwitt 1981. The process is industrially applied in lithographic printing plates, where a larger surface area is necessary in order to enhance the water retentive properties of the aluminium and to improve the adhesion of the photosensitive coating which is deposited after electrograining and anodising of the aluminium substrate.…”

“…1. Applied alternative AC current waveform during one period During the anodic half cycle oxidation of aluminium to Al +3 ions occurs; dissolved aluminium is removed to the bulk of the solution and cubic pits are formed [Dyer & Alwitt, 1981]. The reactions taking place during the positive polarisation are:…”

“…The previously formed Al 3+ ions which were not yet removed from the surface, precipitate as aluminium hydroxide Al(OH) 3 (reaction 4). Aluminium hydroxide is insoluble in water and precipitates as a white gel [Vargel et al, (2004), Dyer & Alwitt, 1981]. The latter together with aluminium particles, water and ions from the electrolyte create an amorphous etch film, called smut layer, formed in a colloidal phase which masks the pits and causes an increase in the potential.…”

“…The density and the number of pits are proportional to the electrolyte aggressiveness and the substrate conditions, whereas the pit size is related to the local anodic charge passed [Laevers et al, 1993, Terryn, 1988, Dyer & Alwitt, 1981, Dowell, 1979. The use of Hydrofluoric Acid leads to the formation of surfaces that are made of significantly smaller, shallower and more uniform pits with a much higher pit density as it is illustrated in Figure 4 [Wilson, 2006].…”