Zinc morphology boosts corrosion protection in epoxy primers

Zinc-rich coatings (ZRCs) are among the most widely used protective coatings for heavy-duty corrosion protection of steel. Their performance depends critically on the electrical connectivity between zinc particles and the substrate, as well as on the electrochemical activity of the zinc itself. A study conducted at the Technical University of Denmark investigated how zinc particle morphology and crystallographic orientation influence the performance of zinc-rich epoxy coatings, comparing coatings formulated with commercial spherical zinc (ZRC-CB) and thermally treated zinc (T-ZRC-CB) synthesised via a controlled melt-recrystallisation process at 450 °C followed by slow cooling at 2 °C/min.

Both formulations contained identical zinc and carbon black contents. The heat treatment produced zinc particles with polygonal and hexagonal geometries featuring more pronounced edges and corners, in contrast to the smooth spherical shape of the commercial reference. XRD analysis revealed that the treated zinc exhibited a higher proportion of exposed active crystallographic planes, particularly the (100), (101), (110) and (102) planes, which possess higher surface energies and lower dissolution energies than the more stable (002) plane.

Extended cathodic protection and reduced rust creep

Open-circuit potential (OCP) measurements demonstrated that the T-ZRC-CB coating reached the cathodic protection threshold immediately upon immersion and maintained cathodic protection for nearly 70 days, substantially longer than the 10 days achieved by the reference ZRC-CB coating. Rust creep 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 rust creep of only 1.3 mm compared with 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 the rapid formation of insulating corrosion products that seal pores and enhance barrier performance. Adhesion testing according to ASTM D3359 and EN ISO 4624:2023 confirmed excellent adhesion to the sandblasted steel substrate for both coatings, with pull-off strengths reaching approximately 10.2 MPa for T-ZRC-CB.

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Combined activity and connectivity effects

The improved performance is attributed to two complementary mechanisms. Firstly, the greater exposure of electrochemically active crystal planes promotes faster and more efficient sacrificial dissolution of zinc. Secondly, the angular geometry of the treated particles creates multiple contact points between zinc particles and with the carbon black network, improving electrical connectivity compared with the single-point contacts typical of spherical particles. Notably, incorporating carbon black prior to the thermal treatment prevented excessive particle merging, keeping the particle size distribution suitable for coating formulation (D50 = 6 μm). The findings offer a practical route to enhancing the long-term corrosion protection of zinc-rich epoxy coatings without altering their 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

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