Corrosion Performance of Epoxy Powder-Coated DD12 Steel with Iron Phosphate and Zirconium-Based Conversion Coating Pretreatments

Year 2025, Volume: 12 Issue: 4, 966 - 978, 31.12.2025

Mustafa Yiğit Dayınlarlı , Abdülkerim Gök

https://doi.org/10.54287/gujsa.1760258

Abstract

Corrosion poses a significant challenge in many industries. Among the various methods for corrosion protection, conversion coatings and powder coatings are widely adopted due to their effectiveness and cost-efficiency. Conversion coatings not only enhance corrosion resistance but also prepare the surface for subsequent finishing processes. In this study, two types of conversion coatings, iron phosphate and zirconium-based, were applied to DD12 steel substrates. Following the coating process, samples were finished with electrostatic powder coating and subjected to two industry-standard corrosion tests: the ISO 9227 neutral salt spray test (up to 400 hours) and the VDA 233-102 cyclic corrosion test (up to three weeks). Samples treated with iron phosphate coatings had thinner and less uniform paint layers compared to those with zirconium-based treatments. Scanning electron microscopy revealed that iron phosphate coatings form larger, irregular grains, while the zirconium-based coatings produced finer and more evenly distributed grains. These structural differences influenced performance: samples with iron phosphate coatings showed better corrosion resistance and paint adhesion during the salt spray test but performed worse in the cyclic test. In contrast, samples with zirconium-based coatings exhibited superior performance in the VDA test but were less effective in the NSS test, suggesting a lower ability to maintain protective qualities under constant salt exposure. Contact angle measurements indicated that zirconium-based coatings led to lower angles, pointing to higher surface energy and potentially stronger adhesion between the powder coating and the metal surface. Cross-sectional SEM images reinforced these results, showing more significant cracking and corrosion in the iron phosphate-coated samples, whereas the zirconium-treated samples had far fewer such defects.

Keywords

Conversion Coatings , Corrosion Testing , Corrosion Resistance , Iron Phosphate Coating , Zirconium-Based Coating

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