A Study by Solution and Surface Analysis of the Passivation of Stainless Steel.

Surface analyses of passive films have been playing more and more important roles in the elucidation of the corrosion properties of stainless steels. Analyses of the corrosion products in solutions, however, are, by no means less important than analyses of passive films. The effect of alloying elements on the passivation process, the partial dissolution behaviour of individual components and more importantly, the film growth kinetics can only be revealed by monitoring the ions dissolved in solution. When used in conjunction with information from surface analyses of passive films, a more complete understanding of the passivation can be obtained. This thesis describes the work on the study of passivation of three steels, Fe17Cr, Fe15Cr4Mo, Fe19Cr10Ni in 0.1 M sulphuric acid solution, A revolutionary new method of analysis in analytical chemistry, the Inductively Coupled Plasma-Source Mass Spectrometry (ICP-MS), is employed in this study so as to explore the dissolution behaviour of the stainless steels under passsivation. Surface analyses of the passive films are carried out by X-ray Photoelectron Spectroscopy (XPS), Information on depth distribution of alloying components is also obtained by ion beam sputtering. Electrochemical technique shows that the alloying effect of Ni is more pronounced than that of Mo on the passivation of the Fe-Cr binary steel. Solution analyses by ICP-MS reveals selective dissolution of Fe from the steels during passivation, which accounts for the enrichment of Cr and/or Mo in the passive films. The selectivity factor S is defined and used to characterize the dissolution behaviour of individual components. The Ni-bearing steel dissolves in a more complex way than the binary or Ni-free ternary steels. Selective dissolution of nickel from the Ni-bearing steel is initially observed but changes to selective dissolution of iron in the final stage. Corresponding to the selectivity factor S, the surface enrichment factor f is defined and used to characterize the enrichment or depletion of alloying components in passive films, Cr and Mo are found to be significantly enriched in the films, while Ni is enriched markedly in the metallic phase underneath the film. A model based on the selective ionization is proposed to interprete such an enrichment in metallic state. The growth kinetics of passive films are discussed in terms of charge balance. There exists an equilibrium thickness after which the film stops thickening. The equilibrium thickness and the time to reach it are dependent on the steel compositions. For the Fe17Cr binary steel, the film thickening conforms to the direct logarithmic law, in the form of y=klog(1+at), The correlation of the static surface information with the dynamic dissolution behaviour of the steels is established