Electrochemical etching model in aluminum foil for capacitor

A novel tunnel growth model is proposed to reveal how hydrogen is transported out of tunnels and explain the phenomena during etching process of aluminum foil for capacitor. Experimental results indicated that tunnel density increased and tunnel width decreased with temperature increasing at the temperature range of 70–80 °C. The pressure in electrolyte had an effect on the tunnel density. The tunnel density after etching at 0.2 atm was larger than that at 1 atm. A pulse electrochemical etching process is described according to the novel tunnel growth model. It considers that hydrogen bubble of nanometer dimension is absorbed on the tunnel's walls surface during the pulse electrochemical etching process, and the saturated hydrogen at the end of the tunnel is accumulated to be a large bubble before hydrogen is transported out of the tunnel. The large bubble will emanate from the end of tunnel when the pressure in the bubble is equal to that outside. The wall surface passivation phenomena is explained by this model; the naturally corrugated texture with ripples of about 0.1 μm in tunnel's walls surface is regarded to be produced by the potential periodical changes, which are caused by the large hydrogen bubble at the end of the tunnel. At the same time, the effect of temperature and pressure on the morphology of the tunnel is also investigated by use of the model. The pressure of the large hydrogen bubble in the tunnel is calculated according to the date in other references when the period of the pulse electrochemical etching is inferred to be 3 ms at their experimental conditions. The proposed process of pulse electrochemical etching could explain well the calculation results and the SEM images of etched tunnels under the same experimental conditions.