On-site detection of hexavalent chromium in protective paint primers

Importance of the topic

The presence of hexavalent chromium (Cr6+) compounds such as zinc chromate in protective paint primers poses significant health and environmental risks. Chronic exposure can lead to cytotoxic and carcinogenic effects, driving stringent regulations like OSHA and REACH. In aerospace, maritime, and industrial coatings, rapid on-site identification of Cr6+ is essential for compliance, safety, and forensic analysis.

Aims and overview

This study evaluates the performance of Metrohm Raman’s handheld MIRA XTR DS system for detecting hexavalent chromium in paint primers, overcoming fluorescence interference common with 785 nm Raman spectroscopy. The goals are to establish distinctive Cr–O spectral markers, test authenticity of commercial chromate paints, and demonstrate real-world application on legacy military aircraft primers.

Methodology and instrumentation

Pure hexavalent chromium compounds from standard suppliers were analyzed in glass vials using MIRA XTR DS with Long Working Distance Smart Tip. Commercial paint samples labeled as zinc or strontium chromate were tested through glass and on dried surfaces with Short Working Distance attachments. Comparative measurements with a handheld 1064 nm Raman system assessed fluorescence rejection and sample damage. Automatic Smart Acquire routines optimized laser power, integration time, and averaging for reliable results.

Main results and discussion

XTR processing of 785 nm spectra yielded high-resolution Cr–O stretching peaks in the 800–1000 cm−1 range for pure chromate standards, free of fluorescence background. Commercial paints lacking Cr–O features were revealed as false positives, while a strontium chromate standard confirmed peak assignments. The 1064 nm system produced low-resolution data and caused sample degradation. Field testing on a retired Lockheed T-33 landing gear successfully identified zinc chromate in aged primer layers, demonstrating robust on-site capability.

Benefits and practical applications

The MIRA XTR DS system delivers fast, non-destructive identification of Cr6+ compounds in complex matrices, with seamless workflow, extensive spectral libraries, and fluorescence rejection. Orbital Raster Scan and interchangeable Smart Tips facilitate representative sampling and prevent laser damage. Integration with HazmasterG3® supports actionable intelligence for forensic, QA/QC, and industrial inspections.

Future trends and opportunities

Advancements in fluorescence-free Raman will broaden detection of hazardous and emerging compounds in situ. Expanding reference libraries, incorporating artificial intelligence for spectral interpretation, and adapting to new laser wavelengths can enhance sensitivity and selectivity. As regulations continue to phase out Cr6+ species, developing portable methods for alternative coating verification and environmental monitoring will be critical.

Conclusion

Metrohm Raman’s MIRA XTR DS establishes a new benchmark for handheld Raman spectroscopy by combining 785 nm fluorescence rejection with high spectral resolution and field-ready versatility. It enables reliable on-site detection of hexavalent chromium in protective primers, addressing health, regulatory, and forensic needs across aerospace and industrial sectors.

Instrumental setup

• Handheld Raman analyzer MIRA XTR DS (785 nm excitation)
• Long and Short Working Distance Smart Tip sampling attachments
• Orbital Raster Scan (ORS™) functionality
• Integration with HazmasterG3® software
• Comparative handheld 1064 nm Raman system

Reference

1. Bouali A. et al. Layered double hydroxides as functional materials for corrosion protection of aluminum alloys: A review. Appl Mater Today 2020;21:100857.
2. Feller RL. Artists’ Pigments: A Handbook of Their History and Characteristics. Oxford University Press;1986.
3. Langård S. Role of chemical species in cancer among persons exposed to chromium compounds. Scand J Work Environ Health 1993;19:81–89.
4. Sunderman FW Jr. Nasal toxicity and carcinogenicity of metals. Ann Clin Lab Sci 2001;31:3–24.
5. Xie H. et al. Zinc Chromate Induces Chromosome Instability in Human Lung Cells. Toxicol Appl Pharmacol 2009;234:293–299.
6. Gharbi O. et al. Chromate replacement: what does the future hold? NPJ Mater Degrad 2018;2:1–8.
7. OSHA Cites Northrup Grumman Painting. PaintSquare News. 2015.
8. Reisch MS. Confronting the Looming Hexavalent Chromium Ban. Chem Eng News 2017;95(9).
9. Weinstock N. et al. The vibrational spectra of tetrahedral species including CrO42−. J Chem Phys 1973;59:5063.
10. Ramsey JD. et al. Raman spectroscopic analysis of chromate solutions. Corros Sci 2001;43:1557–1572.
11. Eremin K. et al. Examination of pigments on Thai manuscripts. J Raman Spectrosc 2008;39:1057–1065.
12. Burgio L., Clark RJH. Library of FT-Raman spectra of pigments. Spectrochim Acta A 2001;57:1491–1521.
13. Ormancı Ö, Bakiler M. Complementary Use of Raman and µ-XRF for Oil Painting Analysis. JOTCSA 2021;8:491–500.