Investigations on the Use of Radiofrequency Glow Discharge Optical Emission Spectrometry for In-depth Profile Analysis of Painted Coatings

Abstract

The use of a radiofrequency glow discharge (rf GD) for in-depth profile analysis of painted steel materials by optical emission spectrometry (OES) was investigated. The types of sample selected as models were coated steel films currently in use for the manufacture of corrosion-resistant car bodies. The study was focused on three critical aspects: evaluation of the risk of melting of the painted samples, optimization of the craters obtained (both for the paints and for the metallic base) and possibilities of conversion of ‘intensityversus time’ profiles into ‘concentration versus depth’ in such samples. A suitable system for refrigeration of the source was first studied in order to avoid melting of the painted samples. In addition, with the aim of obtaining flat crater profiles, the discharge operation conditions and design of the source were investigated. Results showed that after optimization of the conditions and appropriate design of the source, very flat crater profiles, without accumulation of redeposited material in the area of the crater rims, could be obtained for both metallic films and non-conductive coatings, while problems with melting of painted layers were eliminated. Finally, the search for a method for in-depth quantification was addressed. The in-depth profile measurements were carried out at constant pressure. Significant changes in the excitation efficiency were observed after the discharge reached the interface between the non-conductive and metallic layers. However, the use of an emission line of argon (plasma gas) as internal standard proved to be a promising approach to quantitative depth-profile analysis, an unsolved problem in rf GD-OES.