The use of anion exchange pigments to inhibit the filiform corrosion of zinc-aluminium-magnesium coated steel

Highlights: • In-coating carbonate exchanged hydrotalcite reduces filaments initiated on Zn-Al-Mg. • Filiform corroded area decreases with increase in hydrotalcite volume fraction. • Filament propagation rate decreases with increased hydrotalcite volume fraction. • Decrease in filament width at reduced rates of filament propagation. • Hydrotalcite thought to sequester anions or neutralise head electrolyte. In-coating natural (carbonate exchanged) hydrotalcite is shown to significantly reduce the extent of filiform corrosion (FFC) on zinc-aluminium-magnesium (ZAM) coated steels. The hydrotalcite pigment is dispersed within the model poly vinyl butyral (PVB) coating, which is then applied to ZAM. Acetic acid is applied to an artificial scribe penetrative PVB coating defect to initiate corrosion. Filiform corroded area and filament length are shown to decrease with increase in pigment volume fraction (ϕ). Tapering of filaments is also observed. Findings are consistent with hydrotalcite acting to sequester anions or neutralise the filiform head electrolyte.