There are few published results of etch tunnel growth in aluminum at temperatures below 60 degrees C because a high tunnel density is not achieved at these temperatures. We find that etching high purity capacitor foil with its as-received surface oxide produces a low but sufficient density of tunnels so steady state tunnel growth rates can be measured at temperatures down to 28 degrees C. These rates follow the same Arrhenius temperature dependence as rates measured at high temperatures. It is concluded that the mechanism of steady state tunnel growth is the same from 28 to 97 degrees C. It is found at lower temperatures that there is a transition period of a few seconds between crystallographic pit growth and steady state tunnel growth during which tunnels grow at a rate that is slower than for pit growth but faster than steady state tunnel growth. The activation energy for this rapid tunnel growth is 7.2 kcal/mol, the same activation energy as for pit growth but one-half that for steady state tunnel growth. This transition period grows shorter with increasing temperature and is not seen at 70 degrees C or above. Tunnel segments grown during this period have a different appearance than steady state tunnels, indicative of a different growth mechanism. (c) 2006 The Electrochemical Society. All rights reserved.