Etching of copper by anodically oxidized graphite suspension in sulfuric acid solution was investigated with special attention to the effects of anode material, amount of suspension, size of graphite particles, temperature and pH of the solution. The anodic current-potential curves for Pt, graphite and PbO2 (Pb) anodes depend only upon the amount of graphite in the solution as far as the potential is below the gas evolution potential. The quantity of electricity required to oxidize graphite particles per unit surface area of graphite particles increases with increasing temperature. As graphite particles are oxidized at the anode, the potential of the suspension approaches near the anode potential. The etching velocity of copper increases with increasing amount of graphite and with decreasing diameter of graphite. The velocity also increases with increasing temperature and lowering pH. The current efficiency for the dissolution of copper are over 100%. This is probably due to the reaction with dissolved oxygen. The dissolved copper is able to recover simultaneously as the deposit at the cathode.
The effect of surface area of graphite particles on the etching rate of copper was mainly investigated and based upon the experimental results the etching mechanism of copper is briefly discussed. The potential of the graphite suspension corresponds to the redox potential of a system consisting of graphite and its oxidized species formed at the anode. The suspension-potential is mainly controlled by the quantity of electricity accumulated per unit surface area of graphite particles by anodizing. The etching velocity of copper in the suspension increases with the increase in the total surface area of graphite particles striking the copper plate per unit time. The etching rate divided by the apparent surface area of graphite in the unit volume of the suspension increases with increasing concentration of graphite, but decreases with decreasing diameter of graphite particles. The etching of copper takes Place according to the local cell reaction consisting of Cu→Cu2++2e as anodic reaction and (C-OH)+H++e→C+H2O and partly O2+4H++4e→2H2O as cathodic reactions.
Effect of some pretreatments for direct-on porcelain enameling, especially anodic etching, on the adhesion of the enamel has been investigated on steel sheets. It was found that in anodic etching of steel in electrolytes flowing fast, direct-on enamel with excellent adhesion was obtained even in an etching weight loss of 0.1lg/dm2 of steel (In the conventional pretreatment, removal of a surface layer of 0.2g/dm2 or more in sulfuric acid is required for direct-on porcelain enameling of low carbon steel). Most of the experiments were conducted with rotating anodes. Suitable direct-on enameling was found to be made under the conditions: (1) lower H2SO4 concentration (10, 20g/l etc) and higher FeSO4 concentration (100-500g FeSO4⋅7H2O/l) (2) higher rotational speed of anodes (e. g. 400rpm for a cylindrical sheet of 100mm∅in diameter and with distance of 30mm between the anode and a cathode) (3) higher rate of flow of electrolyte and (4) higher etching temperature (e. g. 80°C) Electron microscopic observations showed that under these experimental conditions, a great number of etch pits on electron microscopic scale appeared on the anode steel surface. It is suggested that the etch pits give an anchor effect on the adhesion of the enamel after firing.
Aluminum strip was electrolytically etched by applying alternating current (1∅, 3∅) in two different acidic electrolytes; hydrochloric and nitric acids. Morphology and distribution of pits, formed on aluminum was studied by using scanning electron microscopy and the stylus traverse method. The pit pattern was essentially a function of the electrolytes used and was independent of current density. On the other hand, surface roughness was primarily a function of current density and was less affected by the electrolytes. It was found that more uniform hemispherical arrays of pits were formed in nitric acid electrolytes than in the hydrochloric acid. From these facts the mechanism of etching procesesses is discussed. Etched surfaces were covered with thick layers containing anodic dissolution products and materials deposited from the electrolyte. During the cathodic process in which hydrogen gas evolves the layer is incompletely destroyed and at the same time some materials might be deposited from the electrolyte. Alternatively, when aluminum is made andodic over pitting potential in the electrolytes that attack aluminum, bared or thinned pit sites are dissolved anodically and is also covered by anodic dissolution products. The simultaneous film formation and incomplete destruction of the layer tend to roughen the surface and thus etched pits are produced. In order to obtain uniform pit pattern at will, it's necessary to controll the layer formed during etching processes.