A mathematical model for the growth of aluminum etch tunnels

A simulation of the growth of pits on aluminum during anodic etching in hot chloride solutions was developed. The simulation is based on equations for mass transport and for the potential-controlled removal of chloride ions f rom the dissolving surface. The latter process initiates oxide passivation. Etch pits are found to transform into tunnels which at first maintain para llel sidewalls and then begin to taper. The predicted tunnel shapes agree q uantitatively with those measured experimentally. Tunnel formation is possi ble only when the potential at the tunnel entrance during etching is within 20-30 mV of the repassivation potential; as a result, the size of the diss olving surface is nearly constant during pit growth. In the tapered-width r egime of tunnel growth, the AlCl3 concentration at the end of the tunnel is near saturation, despite the absence of precipitation from the model equat ions. The model shows that this condition derives from the low conductivity of the concentrated solution, coupled with the sensitivity of the rate of surface chloride removal to changes in the potential at the dissolving surf ace. (C) 2001 The Electrochemical Society.