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Abstract
Phosphate coatings are one of the ancient methods for improvement the corrosion resistance of steels. Many researchers have investigated the effect of process parameters on the structure and properties of these coatings, but the influence of substrate microstructure has been less studied. In this article, the effect of heat treatment of AISI 4130 steel on the surface morphology and corrosion resistance of Zn–Mn phosphate coating has been investigated. The steel substrate was subjected to different heat treatments before coating application. X-ray diffraction, optical and scanning electron microscopy were used for phase and microstructure analysis respectively. The corrosion resistance of the coatings was studied by means of salt spray, potentiodynamic polarisation and electrochemical impedance spectroscopy methods. The results showed that the corrosion resistance of phosphate coatings is greatly influenced by substrate microstructure. The finer the steel microstructure, the more condensed becomes the coating morphology and the better its corrosion resistance.