The effect of deposition temperature on the surface coverage and morphology of iron-phosphate coatings on low carbon steel

The influence of deposition temperature and concentration of NaNO2 in the phosphating bath on the surface morphology and coverage of iron-phosphate coatings on low carbon steel was investigated. The phosphate coatings were chemically deposited on steel from phosphate bath at different temperatures (30-70 deg. C) and with the addition of different amounts of accelerator, NaNO2 (0.1, 0.5 and 1.0 g dm-3). The morphology of phosphate coatings was investigated using scanning electron microscopy (SEM) and atomic force microscopy (AFM). The composition of iron-phosphate coatings was determined using energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD). Surface coverage was evaluated by the voltammetric anodic dissolution (VAD) technique. It was shown that the increase in temperature of the NaNO2-free phosphating bath up to 70 deg. C caused an increase in surface coverage. The addition of NaNO2 in the phosphating bath significantly increased the surface coverage of phosphate coatings deposited at temperatures lower than 50 deg. C. The phosphate crystals were of laminated and needle-like structures for deposits obtained at temperatures lower than 50 deg. C, while at higher temperatures needle-like structure was transformed to laminated structure. The increase in NaNO2 concentration in the phosphating bath from 0.1 to 1.0 g dm-3 did not significantly increase the surface coverage, but decreased the crystals size, consequently favouring the phosphate nucleation and better packing of the crystals.