Determination of chromium in the surface nano-layer covering the Zn-based anti corrosion coating of steel sheets using a laser-induced breakdown spectrometry

A method was proposed for the determination of Cr in a thin surface nanolayer deposited on top of a micrometrical Zn-based anticorrosive coating of steel sheets using laser-induced breakdown spectrometry (LIBS). Optimization of the LIBS parameters was performed with respect to the statistical parameters of regression, these being the coefficient of determination (R ²), akaike information criterion and mean-squared prediction error. These were calculated for curves describing the relationship between the Cr surface concentration and the intensity of LIBS signal. The most critical parameter of analysis appears to be the focal spot diameter. When its value was 200 μm and corresponding energy density (fluence) had value of 413.8 J/cm², the intensity–concentration relationship revealed a negative slope. This phenomenon was caused by the difference in total ablated volume for samples with a different content of Cr in the surface layer. This phenomenon was not observed for higher values of the focal spot diameter (400 and 500 μm) and lower values of fluence (103.5 and 66.2 J/cm²). A range of calibration obtained under optimal conditions (focal spot diameter of 400 μm; single pulse mode, laser pulse energy of 130 mJ) was 11–21 mg/m² and the limit of detection was 0.7 mg/m². The recovery values calculated from the results of the proposed LIBS method and the standard ED XRF method ranged from 99.2 to 101%.