On-site detection of hexavalent chromium in protective paint primers

Importance of the Topic

Hexavalent chromium compounds such as zinc chromate have been used for decades in anti-rust and anti-corrosion primers in aerospace and marine industries. However, Cr6+ poses serious health risks, including cytotoxicity and carcinogenicity, leading to strict regulations and phased bans worldwide. Rapid, on-site detection of hexavalent chromium in complex paint matrices is therefore critical for environmental safety, regulatory compliance, and forensic investigation.

Objectives and Study Overview

This study evaluates a handheld Raman analyzer equipped with fluorescence-rejection capabilities for the selective detection of hexavalent chromium compounds in protective paint primers. The goals are to demonstrate that fluorescence-free spectra can be obtained with 785 nm excitation, to compare performance against traditional 785 nm and 1064 nm systems, and to validate real-world applicability through analysis of historical military aircraft primers.

Methodology

Pure standards of various Cr6+ compounds were analyzed in glass vials using non-contact Smart Tip sampling. Multiple commercial paint primers labeled as chromates, as well as a strontium chromate reference, were tested through glass and on exposed surfaces. The study compared three approaches:

• 785 nm handheld Raman with fluorescence rejection algorithms (XTR)
• Standard 785 nm Raman without XTR
• Handheld 1064 nm Raman

A field demonstration was performed on the landing gear of a retired Lockheed T-33 jet to confirm in situ identification of zinc chromate primer.

Instrumentation

The core device is the Metrohm Raman MIRA XTR DS, featuring:

• 785 nm diode laser with eXTRactive fluorescence rejection
• Smart Tip sampling attachments (Long Working Distance, Short Working Distance, intelligent Universal Attachment)
• Orbital Raster Scan (ORS) to prevent sample burning and improve representativeness
• Library integration through HazmasterG3® for automated material identification

Main Results and Discussion

XTR processing at 785 nm yielded high-resolution Cr–O stretching peaks around 872 cm⁻¹ for pure hexavalent chromium compounds. Commercial paints labeled as chromate showed no detectable Cr6+ signals except for the strontium chromate standard, calling into question their authenticity. Standard 785 nm acquisitions were overwhelmed by fluorescence, while 1064 nm analysis required high power, long acquisition times, caused sample damage, and reduced battery life.

In the field test on the T-33 aircraft, MIRA XTR DS identified characteristic Cr–O peaks matching those of known zinc chromate and strontium chromate references, confirming the presence of hexavalent chromium primer on aged metal surfaces without any sample contact or preparation.

Benefits and Practical Applications

The fluorescence-free handheld Raman approach offers:

• Rapid, non-destructive on-site screening of paint and coating samples
• Reliable identification of hazardous hexavalent chromium in complex matrices
• Elimination of lengthy lab procedures and hazardous sample handling
• Compact, battery-powered form factor suitable for field and forensic operations

Future Trends and Potential Applications

Advancements may include expanded spectral libraries for emerging Cr6+ alternatives, integration with AI-driven decision support, miniaturized detectors for lower cost, and broader deployment in environmental monitoring, industrial quality control, and heritage conservation. Further work could extend fluorescence-free detection to other regulated metal compounds and advanced polymeric coatings.

Conclusion

The MIRA XTR DS handheld Raman system overcomes fluorescence interference at 785 nm, enabling sensitive and specific detection of hexavalent chromium in protective paint primers. Its robust performance in both laboratory and field settings demonstrates a powerful tool for regulatory compliance, worker safety, and forensic investigation of Cr6+-containing materials.

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