The Rate of Solution of Highest Purity Aluminum in Sodium Hydroxide Solutions

A series of dissolution experiments on highest purity aluminum (99.998%) in NaOH solutions ranging from 0.1 to 5 N has been carried out. The rate of solution in parallel experiments shows deviations up to ±30 per cent from the average value. Homogenization attempts did not improve the results. The rate of solution in NaOH (up to 0.5 or 12V) is proportional to the cube root of the concentration of NaOH . In the region between approximately 0.5 and 3 N the rate increases proportional to the concentration. Then follows a sudden jump and at concentrations above 4 N the rate increases again, probably proportional to the concentration. At concentrations higher than 5 N , the dissolution process is very irregular and deep pits are formed. Upon reducing the concentration below 5 N NaOH (the Al plates were washed and the dissolution continued in fresh NaOH of lower concentration) the rate of solution drops only gradually, remaining considerably higher than when working with increasing NaOH concentrations. When the corroded surface was removed mechanically, the previous velocity could be observed within the limits of reproductibility.