Toshiro Fukushima

Anodic oxidation of aluminium in sulphuric acid containing aluminium sulphate or magnesium sulphate

The effect of Al2(SO4)3 or MgSO4 in the sulphuric acid electrolyte on the formation and dissolution of anodic films on aluminium has been investigated. When the sulphate concentration was increased, kinetic viscosity of the electrolyte became higher and the anode surface temperature during anodizing rose gradually. Increase in the concentration of the sulphate suppressed chemical dissolution of the anodic film. As the sulphate concentration was increased, hardness of the film generally increased at first and then decreased, while the coefficient of static friction of the film decreased. From measurement of microroughness and electro-microscopic observations, it was found that by the addition of the sulphate, the surface became more microscopically even and the porosity near the outer surface was reduced. The uniformity of the film was decreased with increased in sulphate concentration.

Anodic oxidation of aluminum-foreign metal composites in 13M H2SO4 solutions.

The anodic behavior of copper, carbon steel and Fe-Cr-Ni alloys in 13M H2SO4 solutions has been investigated and the composition and the electrical resistance of corrosion products formed on these metals and alloys were examined. The results obtained suggest the possibility of anodizing composites of aluminum with these metals and alloys without causing severe attack on these metals.Anodic polarization curves show that the breakthrough potential at the beginning of transpassivation were 47.0, 31.5, 30.5 and 26.0V for Cu, carbon steel, Al and Fe-Cr (18%)-Ni (0.2%) stainless steel and that at these potentials, thin oxide films with thicknesses of 420, 240 and 240A are formed on these metals. In the case of Cu and carbon steel, the oxide films are covered with corrosion products, the component of which were identified by X-ray diffraction, thermogravimetric analysis and chemical analysis to be CuSO4⋅3H2O and CuSO4⋅2O for Cu and FeSO4⋅H2O and Fe2 (SO4)3·H2O for carbon steel.In the anodization of composites, partial current passing through aluminum decreased in the order of the Al-Cu, Al-carbon steel and Al-stainless steel. For the Al-stainless steel composites, the thickness of the oxide film on aluminum increased with decreasing area of the stainless steel and the amount of nickel in it.

Electrodeposition of nickel in the pores of anodic oxide films formed on aluminum in a 13M sulfuric acid solution.

Aluminum was anodized for different durations in a 13M H2SO4 solution (20°C) at constant voltages of 30, 35, 40 and 45V.It was possible to deposit nickel uniformly into the pores of the oxide films formed without any pretreatment. Electrodeposition did not take place with films formed in dilute H2SO4 solutions. The behavior and mechanism of the electrodeposition in relation to film structure were examined and the results are summarized as follows:Film structure: Reanodizing behavior of the specimens in a neutral borate solution suggests that the 13M H2SO4 films have numerous micro-voids in the barrier layer. The formation of the microvoids, which probably penetrate through the oxide to the substrate metal, is caused by incorporation of SO42- ions in the oxide and their partial removal when the film is washed with water.Deposition behavior: Electrodeposition was carried out in two steps in a solution of 70g·dm-3 NiSO4⋅7H2O-30g·dm-3 H3BO3 (20°C) by applying a small cathodic current of 0.05A·dm-2 for 15min, followed by a current of 1.45A·dm-2 for 45min.The first step with the small current is essential to form nickel nuclei in the micro-voids, and a uniform deposit was obtained only by the two step procedure. The maximum thickness of the uniform deposit increased with the voltage at which films formed. The distribution of nickel in the pores and the surface of films was observed by XMA.

Local Corrosion of Anode in Low Temperature Hard Anodizing Process of Aluminum

Experiments were carried out to determine the anodizing condition of low temperature hard anodizing process, under which a uniform oxide coating was formed, and also to examine the mechanism of local corrosion of anode.Commercial aluminum sheets such as 99.99% Al, 1100, 2017, 4043 and 5052 were used as specimens. Each sulfuric acid of 1, 10, 20, and 40%, containing aluminum sulfate equivalent to each 1/10 of the acid in concentration, was used as an electrolyte. The current density was 0.1-8.0amp./dm2 and the temperature of the bath was kept constant at 0.5°C for each electrolyte. The surfaces of anodized specimens were observed on microscope, electron microscope and scanning electron microscope, and their roughness was determined. In conclusion, it was preferable that electrolysis would be carried out in the acid of high concentration under low current density for the purpose of preventing local corrosion of the anode.When anodizing was performed under high voltage in a bath, in which solvent attack against the anodic coating is relatively weak, current passed preferentially through weak points of barrier layer of the coating to provoke the propagation of pittings or burnings of the anode.

Current Distribution and Burnings of Anode in Vertical Type Electrolytic Cell

Current distribution was examined by applying direct current in the vertical type electrolytic cell containing sulfuric acid with an anode of a long aluminum sheet. It was carried out by the measurement of thickness of anodic oxide coating and color difference in dyed specimens and by the observation of burnings of anode.Current distribution was more uniform when the concentration of acid was higher, the concentration of aluminum was lower, anodizing temperature was higher, anode current density was lower, and circulation velocity of electrolyte was higher. When the current density was extremely bad in uniformity, current passed through a certain point, at which burnings were produced. Shape and size of burnings would depend upon convection of heat generated on the anode. The two step raise of the current at the early stage of anodizing was effective for preventing the production of burnings. Moreover, studies were also made on the effects of arrangements of the electrodes.