Compositional Depth Profile Analysis of Galvanized Steel

Significance of the Topic

Zinc and zinc-alloy coatings provide essential corrosion protection for steel components by combining sacrificial (galvanic) and barrier mechanisms. These coatings are critical in sectors ranging from automotive and construction to marine and underground infrastructure, where durability and longevity are paramount.

Objectives and Study Overview

This study demonstrates the application of Compositional Depth Profile (CDP) analysis using the LECO GDS950 instrument to characterize various galvanized steel coatings. The work aligns with ISO 16962 standards and aims to evaluate coating thickness, weight, elemental composition, and uniformity across hot-dip, diffusion-alloyed, aluminum–zinc, and electroplated systems.

Methodology

CDP analysis was performed under a direct-current glow discharge with a 30 mA current and 700 V potential. Sputtering times ranged from 60 to 900 s, depending on coating type and conductivity. Depth-resolved emission signals for Zn, Al, Fe, Si, and Ni were recorded to generate compositional profiles.

Used Instrumentation

The LECO GDS950 glow discharge optical spectrometer was employed, configured for both DC and RF operation to accommodate conductive and non-conductive coatings. The system is ISO-9001:2015 certified, ensuring reliable depth profiling and quantitative analysis.

Main Results and Discussion

Key findings include:

• Hot-Dip Galvanized (GI): Average thickness ~7.6 µm, coating weight ~51 g/m2, ~0.42 % Al; repeatability RSD ≤ 3 %. Profiles show a uniform Zn layer with minor Al admixture.
• Galvannealed (GA): Thickness ~7.5 µm, weight ~53.5 g/m2, ~10 % Fe alloyed into Zn, ~0.18 % Al; RSD ≤ 2 %. The diffusion-alloyed layer exhibits consistent Fe distribution.
• 55 % Aluminum-Zinc Alloy (GL): Thickness ~20 µm, weight ~87 g/m2, ~55 % Al, ~42 % Zn, ~2.4 % Si; RSD < 1 %. Silicon aids alloy growth control and adhesion.
• Organic-Coated 55 % Al-Zn (AFP): RF mode required for non-conductive layer. Thickness ~24.6 µm, weight ~95 g/m2, ~55 % Al, ~43 % Zn, ~1.6 % Si; RSD ≤ 2 %. Demonstrates capability for surface treatments.
• Electroplated Zinc: Certified 6.4 µm thickness standard measured at 6.40 µm and ~44 g/m2; RSD ~1.5 %. Highlights excellent uniformity and surface finish.
• Electroplated Zinc-Nickel: Coating ~3.2 µm thick, ~22 g/m2, ~12.4 % Ni; RSD ≤ 3 %. Confirms precise alloy deposition control.

Benefits and Practical Applications

CDP on the GDS950 offers a rapid, single-method solution for quantifying thickness, weight, and composition of diverse Zn-based coatings. The technique supports quality control, failure analysis, and R&D in industrial environments, reducing analysis time to under 15 minutes per sample and accommodating both conductive and non-conductive layers.

Future Trends and Applications

Advancements may include higher depth resolution, integration with microscopic imaging, and automated data interpretation using machine learning. Emerging coating materials and multilayer systems will benefit from expanded spectral libraries and in situ monitoring capabilities.

Conclusion

The LECO GDS950 CDP method delivers reliable, standardized analysis of various galvanized steel coatings, balancing speed and precision. Its adaptability to multiple coating types, including organic and alloyed layers, makes it a versatile tool for industrial and research laboratories.

References

• LECO Corporation. Compositional Depth Profile Analysis of Galvanized Steel Application Note, GDS950, Form No. 203-821-675, 2023.
• ISO 16962. Determination of coating thickness and composition by glow discharge spectrometric methods, 2015.