The determination of Cr(vi) in corrosion protection coatings by speciated isotope dilution ICP-MS

Abstract

Chromium conversion coatings are used as decorative finishes and to improve the corrosion protection and strengthen the wear resistance of metallic surfaces. Chromium electroplating frequently involves the use of hexavalent chromium (Cr). To reduce environmental impacts, several EU directives restricted its use to threshold values of 0.1% Cr(VI) by weight per homogeneous material in vehicles, and 1000 mg kg⁻¹ of Cr(VI) in electronic and electrical equipment. There are few analytical procedures reported that comply with legislative demands, therefore, in our work a selective, quantitative and sensitive analytical procedure for determination of Cr(VI) in the corrosion protection coatings was developed. The investigation was performed on metallic plates homogeneously treated by chromium conversion or hard chrome coatings on copper or zinc electroplated steel surfaces. An alkaline solution (pH 12) was used for ultrasonic extraction of Cr(VI). Speciation analysis was performed by anion-exchange HPLC-ICP-MS. Species interconversions during the analytical procedure were followed using enriched isotopic solutions of ⁵⁰Cr(VI) and ⁵³Cr(III). To prevent Cr(III) oxidation, Tris, EDTA or MgCl₂ was added along with a double isotopically enriched spike and extractions were performed over different time periods. Under optimal conditions that prevent any species interconversion (30 min ultrasonic extraction at 70 °C using 2% NaOH + 3% Na₂CO₃ + MgCl₂ as an extraction agent), six consecutive extractions were necessary to quantitatively extract Cr(VI) from the protective layers; its content was determined by speciated isotope dilution ICP-MS. Concentrations in the samples investigated ranged from 2 to 7 ng of Cr(VI) per mm². The accuracy of the HPLC-ICP-MS determinations was checked by analysis of the certified reference material CRM 545, Cr(VI) in welding dust, using both external calibration and speciated ID-ICP-MS. Good agreement was obtained between the determined and the reported certified values (±0.7% for external calibration and ±0.2% for speciated ID-ICP-MS). The high sensitivity of the procedure developed (LOQ 0.0107 ng Cr(VI) per mm² on a surface of 250 mm²) and the possibility to use external calibration for quantification of separated Cr(VI) instead of ID-ICP-MS allows its application to be extended to the routine laboratory use.