ON THE QUESTION OF ENERGY TRANSFER DURING THE CORROSION REACTION OF ALUMINUM

The corrosion reaction of aluminum has been studied with three different techniques: the impressed polarization technique devised by Engell-Stolica, the adsorption and dissolution test and, finally, scrape potential measurement studies. The Engell-Stolica potentiostatic method has been used for the kinetic study of the breakdown of the passive film on the aluminum surface. A statistical treatment of the induction time has been developed in order to get meaningful data. Pitting initiation time has been found to be potential-independ and the linear increase in the induction time for pitting with the increase of applied potential in the passive range is due to the growth of the oxide film layer on the A1 surface. The value of the critical potential obtained by extrapolation of the induction time vs potential plot to a zero induction time correlates with the value given in the literature. The trend toward a decrease in the kinetic order of the pitting reaction with an increase in the pH value of the aggressive solution has been confirmed. The independence of the kinetic order on the electrode potential indicates that the pitting initiation is controlled by a chemical process with the formation of an intermediate complex and therefore is consistent with the complex-ion theory of corrosion. In the absorption study, attention was focused on the first steps in the pitting process. These steps are the adsorption of aggressive anions and the dissolution of aluminum oxide. The adsorption of chloride on alumina and on aluminum powder are similar and show a saturation plateau at high chloride concentration. The amount of chloride adsorbed is proportional to the weight of alumina. The amount of "free" aluminum ions dissolved from alumina is inversely proportional to the chloride concentration while the amount dissolved from aluminum powder appears to be independent of chloride concentration. Nitrate ion is reduced by aluminum metal but not by alumina. Alumina will greatly reduce the amount of "free" aluminum in solutions of AlCl(,3), Al(NO(,3))(,3), and Al(,2) (SO(,4))(,3). These results support the idea that the first steps in aluminum pitting in aggressive solutions involve adsorption of the anion on the oxide film followed by chemical interaction to form a soluble species resulting in the thinning of the film and making direct contact with the metal surface. The scrape potential measurements have successfully given information about the mechanism of dissolution of bare aluminum. It is observed that there are two different relaxation processes involved during the attack of the aggressive anions on the fresh metal surface. The first relaxation time is on the order of microseconds while the second one is on the order of milliseconds. The large difference in the observed relaxation time has enabled derivation of a simplified rate equation which has led to the calculation of the different specific rate constants. From the Arrhenius plot of the logarithm of the specific rate vs inverse temperature, the activation energy of relaxation processes has been obtained for various solutions. From the results of this work, a potential energy diagram has been tentatively sketched for the first time.