Application of Small Spot XPS and Static SIMS in the Study of Corrosion Mechanisms in Painted Precoated Automotive Steels

The polymer-metal interface in painted precoated galvanized steel has been investigated using X-Ray Photoelectron Spectroscopy (XPS) and static secondary Ion Mass Spectrometry (SIMS). Small spot XPS was used to identify the type of corrosion product formed in cyclic corrosion testing as 6Zn (OH)₂ .ZnCl₂, as opposed to 4Zn (OH)2 .ZnCl₂ which is formed in atmospheric corrosion conditions. Analysis of the paint-metal interface by means of static SIMS demonstrated adsorption of an organic constituent of the paint system on the metal which was identified as uncured epoxy. This effect seemed to be dependent on the curing conditions and was observed to correlate with the performance of the material in a scab corrosion test. The results demonstrate that XPS and static SIMS are very powerful tools for the study of the chemistry at paint-metal interfaces.