Electrochemical noise analysis to examine the corrosion behavior of Ni-P deposit on AM60B alloy plated by Zr pretreatment

Abstract

Electroless Ni-P coating was applied on AM60B magnesium alloy using eco-friendly zirconium pretreatment. Firstly, the alloy surface was entirely covered by a network-like cracked Zr conversion film after appropriate pretreatment time which was determined by Continuous Noise Resistance Calculation (CNRC). Energy Dispersive X-ray Spectroscopy (EDS) was used to elemental analysis of the conversion coating. Next, the pretreated surface was plated by dense, pore-free, and uniform electroless Ni-P coating with fine cauliflower-like nodules. Elemental analysis revealed that Ni-P coating has moderate amount of phosphorus corresponding to a mixed crystalline-amorphous structure which was confirmed by X-ray Diffraction (XRD). CNRC technique was also used to analyze corrosion protection capacity of the electroless layer in acidified 3.5 wt% NaCl solution. Variation of noise resistance versus time for the bare and Ni-P coating revealed that the corrosion resistance remarkably improved by the electroless plating. The noise resistance was initially decreased because of slow diffusion of the corrosive species into intergranular spaces and then increased due to the filling of the solution pathways by corrosion products. Finally, the noise resistance continuously decreased as a result of corrosion products dissolution. There was no numerical agreement between the results of EIS and CNRC techniques due to localization of the corrosion which was confirmed by SEM. Also, Continuous Localization Index (CLI) was utilized to show the localization of the corrosion in Ni-P/Mg system. Microhardness and adhesion of the obtained coating were also evaluated.