The properties of the anodized films on aluminum or aluminum-iron composite sheet were in vestigated in alkaline baths containig hydrogen peroxide. Anodizing of aluminum-iron composite sheet (the area ratio of Al:Fe=100:1) in sodium hydroxide bath contaning hydrogen peroxide provided thick and uniform films. Electron micrographs of the section of the anodized films showed that the pores were branched. This structure is much different from the vertical and regular structure proposed by Keller and others. It was found that the pores on the surface increased in size and joint to each other in proportion to anodizing time and that the columns in the wall were widened with a rise in electrolysis voltage. Binding energies between Al and O in the anodized films formed at various electrolytic conditions were measured by ESCA. Metallic aluminum and aluminum (III) were observed in the films which anodized for 3 seconds but only aluminum (III) was observed after anodized for additional few seconds in the same anodizing bath at 40 voltage. Electrolytic multicoloring of the anodized films by applying A. C, current was investigated in aqueous amine solutions containing transition metal salts and boric acid. Green colored films were obtained in a copper sulfate-boric acid-triethanol amine bath. Blue, yellow, purple, and green films were obtained in a nickel sulfate-boric acid-triethanol amine bath. Brown colored films were obtained in a cobalt sulfate-boric acid-triethanol amine bath.
Photoluminescent centers and their distribution in the anodic alumina films formed in the mixed electrolyte of sulfosalicylic acid (SSA) and sulfuric acid were investigated; the behaviors of incorporations of the SSA anions into the oxide during anodization were discussed. It was found that the photoluminescent centers were the SSA anions locating on the surface region within-30Å of the cell walls of the oxide and no photoluminescent centers were detected in the inner region of the cell walls. These results suggest that SSA anions cannot enter into the oxide during anodization. This must be the reason why no electroluminescence appears during anodization when the SSA electrolyte is employed. Furthermore, self-coloring and electron spin resonance (ESR) characteristics were observed in the inner region of the cell walls of oxide which contained no photoluminescent centers. Thus it was clarified that photoluminescent centers were different from the colored materials and ESR centers concerning the anodic alumina films formed in the electrolyte mentioned above.
The purpose of this study is to improve the characteristics of the anodic coatings on aluminum alloys, 2014, 2017, 2024 and KS-21 (free cutting alloy) by ‘Current inversion method’. Anodic oxidation was carried out with DC-rectangle inverter (cycle: 0.033-500Hz) in a bath of 35wt% H2SO4 containing 10g/l H2C2O4 2H2O. The peak current density was fixed at 4A/dm2 and the duty was varied from 50 to 100%. It was found that the hardness, coating ratio and thickness of coatings were improved remarkably by current inversion method with 10-20Hz and higher duty. The hardness of coatings obtained from the electrolyte of 35wt% H2SO4 were larger than that from 15wt% H2SO4, and the addition of oxalic acid to sulfuric acid baths improved the properties of coatings. The hardness of coatings on high strength and free cutting alloys decreased with increasing Cu content in alloys.
Aluminum was anodically oxidized to form barrier type oxide films at 5.0mA/m2 and 20°C in adipate, borate, citrate, oxalate, phosphate, and tartrate solutions of pH=7 at different concentrations between 10-3 and 1.0M. From measurements of the rate of increase in anode potential, dEa/dta, and dissolution rate of Al3+ ions, dWd/dta, the oxide formation current, If, oxide dissolution current, id, and electronic current, ie, were determined as a function of electrolyte concentration. It was found that with increasing concentration, if reaches a maximum between 10-2 and 10-1M after which it decreases considerably, and id reaches a minimum in the same concentration range. The oxide formation current, if, in the 10-1M solutions decreases in the following order: Adipate>Phosphate>Citrate>Tartrate>Oxalate>Borate. The electronic current, ie, is appreciable at 10-3M, while it is negligibly small at concentrations higher than 10-2M. The role of the electrolyte anions on the formation of barrier type oxide films is discussed in terms of the pH-buffering ability and complexing ability of the electrolyte anions.
The method of electrodeposition of Ni or Zn into pores of anodic film on aluminum was investigated with the objective of hardening the film. Anodization of A1050 aluminum was carried out in 98g/dm3 H2SO4 at 30°C and 20V for 30min. Then, this was followed by DC deposition of Ni from a Watts-type bath. Spalling of the film occurred at 0.5A/dm2 (0-20V) because of H2 evolution. Nickel was deposited ununiformly in the film at 0.6V (0.02-0.14 A/dm2). The same results were obtained in the Ni deposition in films colored by AC electrolysis. The barrier layer of the film was thinned by decreasing anodizing voltage from 20 to 1-0.05V for 3 to 4min. Then, the current was stopped and the barrier layer was subjected to galvanic dissolution in the H2SO4 anodizing bath for 10 to 20min. These treatments led to the uniform deposition of Ni, Zn or Zn-Ni alloy into the pores of thick films (27μm). The hardness of films after the electrodeposition was higher by Hv 40 to 100 than that of as-anodized specimen. Ni-deposited or Zn-Ni alloy-deposited specimen was corroded by less than 24h salt spray, whereas Zn-deposited specimen was not corroded by 240h salt spray.
Porous anodic films on aluminum were formed in 20ml/l phosphoric acid solution at 21V with direct current. Electrolytic multi-coloring treatment of the films was carried out appling alternating current of 15-16 volts in a aqueous solutions of sulfate of nickel, copper, or tin. Pore diameter, thickness of barrier layers, barrier layers/deposits interface, and size and morphology of deposits in the films were observed using TEM and SEM. The results obtained are as follows; Thickness of barrier layers varies from cell to cell, and the variation of the thickness nearly obeys a normal distribution. During the electrolytic multi-coloring treatment in the tin solution, pore-filling phenomenon is found at the interface between the barrier layers and the tin deposits. This phenomenon is more remarkable when the barrier layers become thinner. Consequently, adhesion of the tin deposits to the barrier layers is improved. The nickel and the copper deposits are composed of primary fine particles, while no primary fine particles are found in the tin deposits. Therefore, the nickel and the copper deposits do not show primary color, but the tin deposits show various primary colors with variation of their sizes.
Porous oxide films anodically formed on Al at a constant c.d. in an oxalic acid solution were immersed in hot water at 99.5°C and time-variation in the degree of hydration was followed by weight measurements. The oxide films were then dissolved in a chromic acid-phosphoric acid solution to measure the weight loss-time characteristics. The results of the experiments were analyzed as a function of the initial film thickness (or anodizing time) and the time of hot water treatment. Conclusions we obtained regarding the hydration and dissolution behavior are as follows: 1) In initial periods, the hydration proceeds at the entire pore-wall surface leading to a decrease in pore diameter. The density of the formed hydrous oxides is estimated to be 2.5-2.7, while that of the original oxide is around 2.9. The rate of hydration decreases as the hydrous oxides fill up the pores; this is more pronounced for thicker films. 2) For the oxide films hydrated for a very short time (th<3min) the dissolution occurres in a way to widen the pores so that the time required to dissolve out the oxide does not depend on the initial film thickness. When the pores are filled up with the hydrous oxides, the dissolution proceeds at a considerably slower rate because of a decrease in the area of the surface exposed to the acid solution. Thus, the time needed to dissolve out the film increases as the initial film thickness increases. After prolonged hot water treatment (th>30min), the outmost part of the hydrous oxide becomes very resistant to acid dissolution.
The structural changes produced by a sudden voltage drop from E1 to E2 during porous film formation in an oxalic acid solution have been studied using electron microscopy and ‘pore filling’ methods. Porous structures formed during the current recovery were strongly dependent on the ratios of voltage drop (E2/E1). When the ratio was 2/3rd or more, even and vertical growth of new films having smaller cell dimensions was observed. If the voltage was drop to 1/2nd or less, new films nucleated only at the bottoms of pores which had been dissolved selectively, and grew radially accompanied with pore branching. The difference in structural changes is due to the uneven thinning behavior of the barrier layer. Namely, the barrier layer is dissolved uniformly near its outer layer but uneven dissolution proceeds in the middle. It was recognized that new pores nucleated when the barrier layer was thinned to 1.4-1.5 times of the thickness corresponding to the second voltage (E2).