Growth and passivation of tunnels within Al foil by on-off controlling DC etching in 6 wt.% HC1 solution has been investigated.It was found that,in a given etchant solution at a special temperature,the longest tunnel length was only a function of the t-on interval of DC.The potential of AI foil broke at on-off controlling DC by the result from anode polarization curves and potential-time (E-t) responding curves.When DC was switched on,the potential increased abruptly over pitting potential,leading to nucleation of pits at the surface and the growth of tunnels at special length.When DC was switched off,the potential decreased rapidly to a passive value,leading to stoppage of nucleation and death of tunnels.By this way,the longest tunnel length can be controlled and a non-piercing layer can be obtained.Hence,etching of Al foil at this current is beneficial for maintaining a good mechanical strength.