Zinc Phosphate Coating Formation on Multi-Metal Materials: Effects of Bath pH and Temperature

Hisam Mansur, Irfan Purnawan, Athiek Sri Redjeki

Abstract

Multi-metal automotive bodies integrating steel and aluminum require adaptable zinc phosphate conversion coatings to provide corrosion protection. Zinc phosphate is widely applied prior to painting because it enhances both corrosion resistance and paint adhesion. However, comparative studies involving tri-cationic zinc phosphate on aluminum and steel remain limited. This study evaluated the effects of bath pH (2.4–3.6) and temperature (40–60 °C) on the coating performance of steel and aluminum substrates. Corrosion performance was assessed using a 72-hour salt spray test, combined with gravimetric weight-loss measurements, and supported by SEM–EDX characterization and adhesion testing. Zinc phosphate deposition on aluminum occurred at pH 3.3–3.6, forming plate-like crystals with incomplete surface coverage, whereas steel exhibited dense and uniform crystal structures at pH 3.0–3.3. Bath pH mainly influenced crystal nucleation and morphology, while higher temperatures promoted coating formation. The lowest corrosion rate for aluminum was 0.084 mm/year at pH 3.6 and 60 °C, whereas steel showed the lowest corrosion rate of 0.27 mm/year at pH 3.0 and 60 °C. Under these conditions, inhibition efficiencies reached 20.78% for aluminum and 84.49% for steel relative to the uncoated substrates. All coated samples achieved grade 5B adhesion in the tape test. The different optimum conditions observed for steel and aluminum highlight the challenge of using a single phosphating bath for multi-material automotive bodies.

Keywords

References

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DOI: 10.14416/j.asep.2026.09.005

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