AUTOCATALYTIC DISSOLUTION OF PB IN HNO3

The reaction between Pb and HNO3 has been investigated using the thermometric technique. Weight-loss measurements on the reaction were also obtained. As the HNO3 concentration is increased from 5 x 10(-2) to 4 mol l-1, the corrosion rate increases. This is shown thermometrically by a substantial increase in the maximum temperature attained, T(m), as well as a decrease in the time, t, required for reaching T(m). Dissolution of Pb in HNO3 is proposed to take place according to an autocatalytic mechanism. Passivation sets were detected in solutions greater-than-or-equal-to 11 mol l-1 HNO3 . A parallel indication between the thermometric technique and weight-loss measurements was obtained. The rate-determining step of the autocatalytic process involves HNO2 in dissolution of Pb in HNO3. This is supported by the results of addition of hydrazine to the solution. This additive raises the maximum measured temperature, without affecting the corresponding time necessary to reach it. The effect of addition of NaNO2, NaNO3, NaCl, Na2SO4, NaH2PO4 and NaClO4 on the reaction number, RN, of Pb in 4 mol l-1 HNO3 was examined. Only NaNO2 accelerates the dissolution reaction while the other salts show as inhibition effect. It was found that these additives inhibit dissolution due to the displacement of some cathodic depolarizing components, as NO2, from the active sites on the metal surface. The effect of addition of HCl, H2SO4, HClO4 and H3PO4 on the reaction number, RN, of Pb in 4 mol l-1 HNO3 was also investigated. The observed acceleration and retardation of the dissolution of Pb was found to be dependent on both the concentration and nature of anions of the extra acids added.