Controlling limiting length of tunnels on Al foil electroetched in HCl-H2SO4 solution

The limiting length of tunnels, l_lim, of Al foil electro-etched in HCl-H₂SO₄ solution and the corresponding anodic polarization curves in the same solution were measured. It is found that there is a dependence of l_lim on the potential difference, Δφ, between the pitting potential, φ_pit, and the corrosion potential, φ_corr, of Al foil, when the temperature and H₂SO₄ concentration of HCl-H₂SO₄ electrolyte are changed. The dynamic equation on the tunnel growing and the linear equation between l_lim and Δφ were deduced by analyzing the relationship among the over-potential on Al foil surface, the transport over-potential in tunnel solution and the over-potential at tunnel tip during the electro-etching. The results show that the growing velocity of tunnels decreases with their extending in length and the changing trend of l_lim can be judged by measuring Δφ, which supplies a convenient access to explore new kinds of etchants.