Effect of phosphating time on protection properties of hurealite coating: Differences between ground and shot peened HSLA steel surface

Shot peening is a standard surface treatment primarily used for enhancement of fatigue properties of metallic materials. However, its influence on surface properties is so radical, that subsequent techniques of surface treatment used on the shot peened surface need to be reconsidered in order to minimize the possible negative side effects of shot peening. In this case, the differences between hurealite formation on the ground and shot peened surfaces of modern high strength low alloy (HSLA) steel were evaluated by morphological and electrochemical corrosion observations and measurements. The surface morphologies of the formed coatings were observed by the scanning electron microscopy (SEM) and analysed by energy-dispersive X-ray analysis (EDX). The corrosion resistance of the HSLA steel with hurealite coating was evaluated using electrochemical impedance spectroscopy (EIS) by the analysis of Nyquist plots obtained in 0.1 M NaCl solution after various phosphating times on both types of pre-treated surfaces (ground and shot peened). The results showed that used technique of shot peening negatively influenced the phosphating process by prolonging the phosphating process in order to reach the coating with maximal protection properties in tested medium. Moreover, shot peened surface caused significantly enlarged size of the hurealite crystals and greater thickness of the coating compared to the surface pre-treated by grinding on the tested HSLA steel at the expense of a lower compactness and corrosion protection.