Mechanism of zinc phosphate conversion coating formation on iron-based substrates

This study investigates the formation mechanism of zinc phosphate conversion coatings on pearlitic steel and ferritic iron with emphasis on the impact of microstructure and substrate corrosion. Deposition parameters, including pH (2 and 2.5) and temperature (50°C and 70°C), were explored. Open circuit potential (OCP) monitoring was used to identify different stages of coating formation. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) showed that higher temperatures accelerated the growth of phosphate crystals, composed of hopeite (Zn₃(PO₄)₂) and phosphophyllite (Zn₂Fe(PO₄)₂) on both substrates. Deposition at pH 2.5 led to bulk-solution precipitation on substrates, while at pH 2, coatings were growing from the substrate surface. Electrochemical impedance (EIS) and inductively coupled plasma mass spectrometry (ICP-MS) measurements revealed that iron corroded around 2.5 times slower than steel. As identified by energy dispersive X-ray spectroscopy (EDS) in SEM, iron phosphate particles were formed on both substrates. These particles accumulated in higher amounts on steel, while iron exhibited minimal corrosion product accumulation. Different phosphating stages were then studied using SEM and time-of-flight – secondary ion mass spectrometry (ToF-SIMS). ToF-SIMS depth profiles highlighted a thicker iron phosphate layer on steel at early phosphating stages, compared to iron. Focused ion beam (FIB) cross-sections of fully phosphated steel showed a porous interlayer, mainly composed of iron-phosphate, at the coating/steel interface. Zinc phosphate crystals were nucleated on this porous layer or formed by near-surface solution precipitation. Iron substrates did not show this porous interlayer, and had lower phosphatability with only near-surface solution precipitation of zinc phosphate crystals.