Growth Rates of Aluminum Etch Tunnels

Growth rates of aluminum etch tunnels, formed by anodic dissolution in hot aqueous chloride solutions, have been measured by a new technique in which ac signals of about 1 Hz were superimposed on the dc applied current. SEM photomicrographs indicated that the superimposed current caused the tunnel widths to oscillate during etching. Local metal dissolution rates were thus obtained by dividing the wavelength of the width oscillations by the period of the cur-rent signal. It was found that growth velocities were constant with increasing tunnel ength, and were independent of the frequency of the signal. An Arrhenius dependence of the growth rate on temperature, with an activation energy of 15 kcal/ mol, was obtained. Information on the kinetics of metal dissolution during localized corrosion is important for understanding the controlling mechanisms ofthese reactions. Attempts to in-terpret current measurements, however, are complicated by the large number of pits, the continuous nucleation and death of pits, and simultaneous cathodic reactions. Also, corrosion rate measurements in large-scale artificial sys