A comparative research on corrosion behavior of a standard, crack-free and duplex hard chromium coatings

Abstract

In this study, the single and duplex layers of standard and crack-free hard chromium coatings were prepared by using direct current (DC) and pulse current (PC) electroplating processes on mild steel substrates. The coatings are studied from microstructure, microhardness and corrosion resistance viewpoints. The coating's corrosion resistances have been compared through electrochemical polarization in a 3.5% NaCl solution and standard salt spray test. Before and after corrosion tests microstructural characterization tests were done by optical microscopy (OM) and scanning electron microscopy (SEM). The micro-crack density, in the hard chromium microstructure, could be limited by using the pulse current electrodeposition. Crack-free hard chromium coatings, which are deposited by PC-electroplating, were excellent in terms of corrosion resistance. The thickness ratios in duplex coatings with crack-free sub-layer and hard chromium top coat, were also prepared by PC- and DC-electroplating processes respectively in the standard chromium plating bath. Standard hard chromium coating deposited by DC-electroplating had rust stains on the surfaces and edges. The coatings with crack-free sub-layer coatings exhibited excellent results in corrosion resistance, with no corrosion product on the surfaces and edges of the samples. Their icorr in the 3.5% NaCl solution are lower than the standard Cr-deposited by DC-electroplating. All of the coatings are passivated in both 3.5% NaCl solution and salt spray test environments. In addition, between the duplex layer coatings, the coating with a ratio of a 75% crack-free sub-layer (25% cracked hard top layer) showed the best corrosion resistance.