THE APPLICATION OF X-RAY TECHNIQUES TO STUDIES OF CORROSION AND PROTECTION .2. EXPERIMENTAL X-RAY-DIFFRACTION MEASUREMENTS

The level of sophistication of X-ray diffraction (XRD) experiments is considered with respect to the sample environment and the control of electrode potential. In the simplest case, a sample may be scraped from a surface, followed by grinding and ex-situ powder diffraction using a classical scanning powder diffractometer. On the other hand, more advanced techniques are being developed which permit the surface to be examined in-situ i.e. in the presence of electrolyte and with the facility for potential control. Such techniques allow electrochemical methods to be coupled with XRD measurements. The use of XRD is illustrated by laboratory research and industrial case studies. Ex-situ powder diffraction of debris from a marine, white-metal bearing surface allows the identification of the phase composition of the bearing metal as well as that of pitting corrosion products (SnO2). Energy dispersive measurements using synchrotron radiation reveal the presence of SnO following a controlled potential during anodic oxidation of tin in NaOH. In-situ, potential modulation XRD is used to investigate the reversible formation of oxides (CuO, Cu2O) on copper in NaOH. Information on the phase composition and preferred orientation of phosphated layers on automotive steel sheet may be obtained by scanning powder diffraction using synchrotron radiation.