Passivity of Metals Studied by Surface Analytical Methods, a Review

Anodic oxide and hydroxide films on metals are very important for corrosion protection and they determine the electrochemical properties of electrode surfaces. For a better understanding of the leading corrosion mechanisms the chemical composition as well as the structure and electronic properties of passive layers are of decisive importance. They have been investigated with numerous ex situ and in situ methods. In this review special attention is given in a first part to the application of X-ray Photoelectron Spectroscopy (XPS), Ion Scattering Spectroscopy (ISS) and Rutherford Backscattering (RBS) as ex situ methods, which provide detailed information on the chemical composition and the multilayer structure of these films. The second part concentrates on the application of Scanning Tunneling Microscopy (STM) to obtain information on the atomic structure of these layers and their changes with environmental parameters. Passive layers are usually complicated multilayer structures. Their protecting properties are determined by the chemical characteristics of the metals and their anodic oxides as well as the environmental factors as the potential and the composition of the contacting electrolyte. The investigation of some metals and alloys like Cu, Fe, Co, Al/Cu and Fe/Cr are presented in detail to give insight to the application of electrochemical and surface analytical methods. It is the aim of this review to show, that one has to apply many methods in combination to understand the mechanisms of the leading corrosion processes and the nature of the passive behavior. References to more detailed reviews and original papers are given.