Characterization and corrosion resistance of electroless black Ni-P coatings of double black layer on carbon steel

Abstract

In this work, the corrosion evaluation of a double black layer, by an acidic bath of double electroless Ni-P coatings, were conducted. A double black layer and a coarse sandblast finishing were intended for increasing the sunlight absorption and its anchoring. The surface preparation process comprises four stages: A) a Ni-P electroless coating, B) an acidic attack for a black layer formation, C) a second Ni-P electroless coating, and D) an acidic attack for a second black layer. The properties of the coating were investigated by X-ray diffraction, scanning electron microscopy, mechanical profilometry, confocal laser scanning microscopy and low-discharge optical emission spectroscopy. The results of these studies indicate that a double layer Ni-P coating with Rp and corrosion rate values of 1170Ωcm2 and 0.4mm/year were obtained, thereby exhibiting an increase in corrosion resistance compared to a one layer Ni-P coating. The GDOES profiles show two Ni-P coatings and two black layers intercalated through small variations of the percentage of nickel and phosphorus, which was associated with the interface between both layers.