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

By Popić, J.P.; Jegdić, B.V.; Bajat, J.B.; Veljović, i.; Stevanović, S.I. & Mišković-Stanković, V.B.
Published in Applied Surface Science NULL 2011

Abstract

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 °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 °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 °C. The phosphate crystals were of laminated and needle-like structures for deposits obtained at temperatures lower than 50 °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.

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