QDP Analysis of Galvanized Steel

Importance of Spectroscopic Analysis for Galvanized Steel

Uniform and controlled zinc based coatings are essential in extending the service life of steel structures in industrial, marine, underground and painted applications. Depth resolved compositional analysis ensures that coating thickness, alloy composition and surface treatments meet corrosion protection and quality requirements.

Objectives and Study Overview

This application note demonstrates the use of quantitative depth profiling by glow discharge spectroscopy to

• Measure coating thickness and weight for various hot dip and electroplated galvanizing processes
• Determine elemental composition of coatings and substrates
• Evaluate surface treatment layers such as chromate films

Methodology and Instrumentation Used

Depth profiles were acquired using a glow discharge spectrometer equipped with a Grimm type lamp under argon at low pressure. A high negative potential on the sample cathode sputters material in a layer by layer fashion. Emitted light from excited atoms is wavelength separated to quantify elements. The Quantitative Depth Profile software automates report generation, including plots of weight percent versus depth and tables of coating and substrate data.

Main Results and Discussion

Hot dip galvanizing with pure zinc and Zn 5% Al alloys yielded coating weights from 90 to 1200 g per square meter and thicknesses around 12 micrometers. Replicate analyses showed relative standard deviations below 6 percent for depth and weight measurements and below 5 percent for alloy composition. Galvannealed coatings containing 8 to 11 percent iron exhibited thinner layers around 8 micrometers and weights near 57 g per square meter. The 55 percent aluminum galvanizing alloy demonstrated improved corrosion resistance at a thickness near 19 micrometers with silicon present to control alloy growth. A chromate surface treatment layer of about 300 nanometers was quantified separately from the underlying zinc coating. Electroplated zinc coatings achieved weights between 50 and 80 g per square meter and thicknesses near 6 micrometers with excellent uniformity, as shown by five replicate profiles with RSD values near 2 percent. Electroplated Zn Ni alloy coatings with 10 to 16 percent nickel were profiled at thicknesses of approximately 2.7 micrometers and weights near 20 g per square meter.

Benefits and Practical Applications

• Quantitative depth profiles enable quality control of coating weight and uniformity across multiple galvanizing processes
• Layer by layer removal separates sampling and excitation, reducing matrix effects and metallurgical history influences
• Automated report generation speeds up laboratory workflows and supports traceable documentation
• Low consumption of reference material and argon gas reduces operating costs

Future Trends and Applications

Advances may include higher resolution sputtering heads for submicron profiling, expanded alloy libraries for calibration, integration into inline production monitoring and machine learning driven data interpretation to predict coating performance under varying environmental conditions.

Conclusion

Glow discharge based quantitative depth profiling provides a robust, precise and efficient method to characterize galvanized steel coatings. It delivers critical information on coating thickness, weight and elemental distribution for a wide range of zinc based processes and surface treatments. This capability supports quality assurance and product development in corrosion protection applications.

Reference

ISO 16962 Performance test methods for metallic and other inorganic coatings Quantitative determination of coating thickness