Synergistic corrosion protection for galvanized steel by phosphating and sodium silicate post-sealing

To improve the corrosion resistance of phosphate coatings, the phosphated hot-dip galvanized (HDG) sheets were post-sealed with sodium silicate (water glass) solutions. The morphology and chemical composition of the composite coatings was analyzed using scanning electron microscopy (SEM) and energy...

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Bibliographic Details
Published in:Surface & coatings technology 2008-02, Vol.202 (9), p.1831-1838
Main Authors: Lin, Bi-lan, Lu, Jin-tang, Kong, Gang
Format: Article
Language:English
Subjects:
Applied sciences
Corrosion
Corrosion environments
Corrosion prevention
Cross-disciplinary physics: materials science
rheology
EIS
Exact sciences and technology
Galvanized steel
Heat treatment
Materials science
Metals. Metallurgy
Phosphate coatings
Physics
Polarization curves
Production techniques
Silicate
Surface treatments
Thermochemical treatment and diffusion treatment
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Summary:To improve the corrosion resistance of phosphate coatings, the phosphated hot-dip galvanized (HDG) sheets were post-sealed with sodium silicate (water glass) solutions. The morphology and chemical composition of the composite coatings was analyzed using scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The effect of sodium silicate post-sealing treatment on the corrosion behaviors of phosphate coatings was investigated by neutral spray salt (NSS) tests and electrochemical measurements. The results show that after the silicate post-treatment the pores among zinc phosphate crystals are sealed with the films containing Si, P, O and Zn, leading to the formation of the continuous composite coatings on the surface of HDG steel. The corrosion resistance of the composite coatings depending on concentration of sodium silicate and post-sealing time is greatly improved by the silicate post-treatment. The optimum concentration of silicate and post-sealing time are 5 g/L and 10 min, respectively. Both the anodic and cathodic processes of zinc corrosion on the samples are suppressed conspicuously, and the synergistic protection effect of the single phosphate coatings and the single silicate films is evident. Moreover, the low frequency inductive loop in electrochemical impedance spectroscopy (EIS) is disappeared and the electrochemical impedance values are increased for more than one order of magnitude. The corrosion protection of the composite coatings is comparable to that provided by the chromic acid post-treatment.
ISSN:0257-8972
1879-3347
DOI:10.1016/j.surfcoat.2007.08.001