Montmorillonite Nanosheets as an Accelerator for Low-TemperatureZinc Phosphate Coatings with Improved Corrosion Resistance

Traditional zinc phosphating requires high temperatures and toxic nitrite accelerators. In this study, montmorillonite (MMT) nanosheets, a low-cost two-dimensional layered silicate, are introduced as a green nucleating agent to accelerate low-temperature (40 ± 2 °C) phosphating of Q235 steel. And the effect of MMT concentration on coating properties is systematically examined. XRD and SEM show that MMT does not alter the coating’s phase composition but significantly refines the crystal structure. At the optimal concentration of 1.0 g/L, the average crystal size decreases from 50–80 μm to 20–40 μm, yielding a denser and more complete coating. Electrochemical tests confirm that MMT-modified coatings provide an effective physical barrier. Notably, the coating prepared with 0.5 g/L MMT exhibits a charge transfer resistance (Rct) of 9.2 × 108 Ω·cm2, more than 5 orders of magnitude higher than the unmodified coating, alongside a significantly reduced corrosion current density and a 32.3% decrease in the corrosion rate. This work offers a practical, low-cost, and environmentally friendlier route to enhance the corrosion resistance of low-temperature zinc phosphate coatings, compared to conventional nitrite-accelerated processes.