Forming Hard Anodic Films on Aluminum by Anodization in Oxalic Acid and Alcohol

Abstract

Porous anodic films have attracted wide attention because of their potential applications in various industrial fields. Reducing their porosity is expected to improve the hardness. This study focused on evaluating the effect of the electrolyte composition on the hardness of anodic alumina films formed on aluminum by anodization under mild conditions. In particular, the effect of adding alcohol (i.e., ethanol and ethylene glycol) to an oxalic acid–based electrolyte was considered. The voltage–time curve, coating ratio (i.e., film formation efficiency), morphology, and Martens hardness of the resulting anodic alumina films were clearly affected by the type and amount of added alcohol. Adding alcohol effectively suppressed the chemical dissolution of the anodic film during anodization without needing to lower the electrolyte temperature, which formed high-strength films with low porosity and thick pore walls. The Martens hardness of the formed films increased by 18% with the addition of 50 vol% ethanol and increased by 25% with the addition of 50 vol% ethylene glycol.

Influence of Ethanol on Porous Anodic Alumina Growth in Etidronic Acid Solutions at Various Temperatures

“…This result, together with the observed smaller porosity in the PAAs produced in the 1:3 v / v EtOH:H 2 O solution ( Figure 7 c), strongly suggests that the PAA formation efficiency increases at T > 10 °C in the etidronic electrolyte with the largest amount of ethanol. The higher efficiency of the oxide formation upon the addition of the alcohol was also found in the oxalic and sulfuric electrolytes [ 29 , 30 , 31 ]. As indicated in previous works (e.g., [ 12 , 27 ]), ethanol is a cooling agent that evaporates at the bottom of pores due to the high effumability taking away the heat generated at the reaction spot and preventing the increase of the localized temperature.…”

“…Apart from the cooling effect, it was demonstrated that the addition of modifiers into an electrolyte can significantly change the process and influence the growth of PAA membranes [ 28 ]. The addition of ethanol was effective in suppressing the chemical dissolution of alumina into the electrolyte, thus improving the efficiency of the film formation (the ratio of the formed Al 2 O 3 to the consumed Al) under a constant charge [ 29 , 30 , 31 ]. Thanks to the lower dissociation power of the modified electrolyte, the maximal attainable PAA thickness was increased.…”

“…For 50 vol% ethylene glycol added to sulfuric electrolyte, PAA with less than 10% porosity was formed. Furthermore, no influence of ethanol on PAA growth rate was found in these works [ 29 , 30 ]. In contrast to those results, Qin et al [ 32 ] have shown a strong impact of ethanol on PAA growth rate.…”

On-Wafer Wide-Pore Anodic Aluminum Oxide

“…The addition of the ethanol facilitates cooling of the electrolyte. 7,[62][63][64][65] The substrate was cooled via a Peltier stage until it was stable below <4 °C. Additional electrolyte cooling was provided via a custom-made ¼' stainlesssteel tube coil connected to an external cooler.…”

“…Depending on the specific anodization conditions, domains of close-packed pore arrays form, with diameters ranging from ∼20 nm -∼650 nm, lengths from ∼100 nm to >50 μm, with interpore spacings (D s ) from ∼50 nm to ∼500 nm, have been reported. [5][6][7][8][9][10][11][12][13] Initially led by Matsuda et al, AAO has attracted significant attention as a template for the fabrication of various nanostructured materials and devices. 14 These include, but are not limited to; sensors, [15][16][17][18][19] filters, [20][21][22][23] optics, [24][25][26][27][28][29] piezoelectrics, [30][31][32][33][34] catalysis, [35][36][37][38][39][40] and energy storage devices, [41][42][43][44][45][46] where it has played multiple roles.…”