Development and Evaluation of Point Defect Models for the Growth of Passive Films on Single Crystal, Polycrystalline, and Amorphous Metal Surfaces

摘要

The principal task of this program was to develop a better understanding of: (1) the mechanisms of the growth of passive films on metal surfaces, (2) the role of space charge in determining the properties of the film, and (3) passivity breakdown phenomena. Experimental data for the ac impedance and passivity breakdown properties of passive films on nickel are presented in Sections 2 and 3. Also a theoretical development of the point defect model is presented in Section 4. A comparison between the theoretical and experimental data for the growth of passive films on nickel and iron is included. In addition, the role of minor alloying elements (such as molybdenum) on pitting and passivity breakdown is considered theoretically, and the model is compared with experimental data for 18% Cr ferritic stainless steel. The results of this study indicate that the point defect model although at present in a crude state of development, is a reasonable first-approximation theory for explaining some of the basic phenomna related to the growth and breakdown of anodic passive films on metal surfaces. On the basis of data developed in this program it is recommended that further experiments be conducted to further refine the model.