Unveiling the growth mechanism of Zn coatings on Sandelin steel: The role of dilute Al (0.1 wt%) addition in the galvanizing bath

The Sandelin effect poses a critical challenge in large-scale industrial hot-dip galvanizing, leading to excessively thick coatings and elevated production costs. This study demonstrates that the addition of trace aluminum (0.1 wt.%) to the zinc bath reduces coating thickness by an order of magnitude, thereby effectively suppressing the Sandelin effect, while also elucidating the underlying mechanism. Characterization was performed using scanning electron microscopy (SEM) equipped with energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), focused ion beam-transmission electron microscopy (FIB-TEM), and complementary analytical techniques. The results confirm that 0.1 wt.%Al effectively suppresses the Sandelin effect through two synergistic mechanisms: first, the Fe/Zn interfacial reaction is hindered by the in-situ formation of a discontinuous η -Fe 2 Al 5 Zn x phase, which acts as a diffusion barrier for Fe/Zn atoms; second, atomic diffusion is limited by the lateral growth of columnar δ Al -FeZn 10 Al y (δ Al ) intermetallic compounds. During the initial stage of galvanizing, discontinuous nano-sized η -Fe 2 Al 5 Zn x particles were preferentially formed on the steel substrate. The δ Al phase was observed to either nucleate concurrently with the η -Fe 2 Al 5 Zn x phase or evolve via the metastable decomposition of η -Fe 2 Al 5 Zn x . Early-stage δ Al presented a flocculent or granular morphology, which progressively transformed into a dense columnar structure. The growth of the δ Al phase slows down and follows parabolic kinetics, ultimately dominating the coating growth process. Coating thickness on Sandelin steel was significantly reduced in the Zn-0.1wt.%Al bath compared to pure zinc. These findings provide theoretical and practical basis for mitigating the Sandelin effect in industrial galvanization.