DTU Study: Heat-Treated Zinc Morphology Extends Cathodic Protection of Epoxy Coatings to 70 Days

en.Wedoany.com Reported - A study from the Technical University of Denmark compared the effects of different zinc particle morphologies on the anticorrosion performance of zinc-rich epoxy coatings, finding that heat-treated polyhedral angular zinc particles significantly extend cathodic protection time and reduce rust creepage. Zinc-rich coatings are widely used for heavy-duty corrosion protection of steel, with performance critically dependent on the electrical connectivity between zinc particles and the substrate, as well as the electrochemical activity of zinc itself. Researchers compared coatings formulated with commercial spherical zinc (ZRC-CB) and heat-treated zinc (T-ZRC-CB), the latter synthesized via controlled melt recrystallization at 450°C followed by slow cooling at 2°C/min.

Both formulations contained identical zinc and carbon black content. The heat-treated zinc particles exhibited polygonal and hexagonal geometries with more pronounced edges and corners, contrasting with the smooth spherical shape of the commercial reference. XRD analysis revealed that the treated zinc exposed a higher proportion of electrochemically active crystal planes, particularly (100), (101), (110), and (102) planes, which possess higher surface energy and lower dissolution energy compared to the more stable (002) plane.

Open circuit potential (OCP) measurements showed that the T-ZRC-CB coating reached the cathodic protection threshold immediately upon immersion and maintained cathodic protection for nearly 70 days, far exceeding the 10 days of the reference ZRC-CB coating. Rust creepage evaluation after 30 days of salt spray exposure according to ISO 9227:2022 further confirmed the improved protective performance, with the T-ZRC-CB coating showing only 1.3 mm of rust creepage compared to 2.6 mm for the reference. Electrochemical impedance spectroscopy (EIS) supported these findings, showing an earlier increase in impedance for the T-ZRC-CB coating, attributed to faster zinc dissolution and rapid formation of insulating corrosion products that seal pores and enhance barrier properties. Adhesion tests conducted according to ASTM D3359 and EN ISO 4624:2023 confirmed excellent adhesion of both coatings to grit-blasted steel substrates, with T-ZRC-CB achieving a pull-off strength of approximately 10.2 MPa.

The performance improvement is attributed to two complementary mechanisms. First, greater exposure of electrochemically active crystal planes promotes faster and more efficient sacrificial zinc dissolution. Second, the angular geometry of the treated particles creates multiple contact points between zinc particles and with the carbon black network, improving electrical connectivity compared to the typical single-point contact of spherical particles. Notably, incorporating carbon black prior to heat treatment prevents excessive particle coalescence, maintaining a particle size distribution suitable for coating formulations (D50 = 6 μm). These findings provide a practical pathway to enhance the long-term corrosion protection of zinc-rich epoxy coatings without altering pigment composition.

Source: Aminian, A., et al., Effect of zinc particle morphology on the corrosion protection performance of zinc-rich epoxy coatings. Progress in Organic Coatings (2026). https://doi.org/10.1016/j.porgcoat.2026.110194