The morphologies of tunnel tips in diferent stages for aluminum foils during DC etching in 1.5 mol/L HCl solution at 90 C were observed by field emission scanning electron microscopy(FE-SEM).A novel model was proposed to describe the morphological evolution of tunnel tips throughout the growth processes.In the pit nucleation stage,the pits vary from the hemispherical to half-cubic shapes due to the activation of pit tips from the center to the edge.During the tunnel growth stage,the pits dissolve toward the depth direction and develop into the tunnels,and their tips remain flat.In the tip passivation stage,as the passivation of tunnel tips speeds up from the edge to the center,the tunnel tips change from flat shapes to three-dimensional protrusions.The mechanism may be attributed to the order of activation or passivation on the tunnel tips changed in diferent stages. The morphologies of tunnel tips in diferent stages for aluminum foils during DC etching in 1.5 mol / L HCl solution at 90 C were observed by field emission scanning electron microscopy (FE-SEM). A novel model was proposed to describe the morphological evolution of tunnel tips throughout the growth processes.In the pit nucleation stage, the pits vary from the hemispherical to half-cubic shapes due to the activation of pit tips from the center to the edge. During the tunnel growth stage, the pits dissolve toward the depth direction and develop into the tunnels, and their tips remain flat. In the tip passivation stage, as the passivation of tunnel tips speeds up from the edge to the center, the tunnel tips change from flat shapes to three-dimensional protrusions. The mechanism may be attributed to the order of activation or passivation on the tunnel tips changed in diferent stages.