Correlation between Morphology, Microstructure and Corrosion Behaviour of Nickel-Phosphorous (Ni-P) Based Electrodeposited Coatings

Abstract

The Ni-P alloy coatings are widely studied due to their superior mechanical and tribological properties. Ni-P coatings are also considered to be a viable alternative to the chromium (Cr) coatings which utilize environmentally hazardous and toxic carcinogenic electrolytic solutions. The current work focuses on strategies to enhance the corrosion resistance performance of electrodeposited Ni-P coatings primarily by incorporation of foreign additives (carbon nanotubes (CNTs) and graphene) and by engineering of the Ni-P micro-texture and phase fraction (crystalline and amorphous phases). Nickel-phosphorus (Ni-P) coatings were electrodeposited over mild steel substrate using DC power source in conventional two electrode electrochemical setup. As-deposited Ni-P coatings were subjected to phase, microstructural and morphological characterizations using x-ray diffraction, electron microscopy and electron backscatter diffraction techniques. The corrosion analysis was accomplished by using electrochemical impedance spectroscopy (EIS) and potentiodynamic polarisation techniques (Tafel plot) in 3.5 wt.% NaCl solution. Key observations are: (a) in the work on the incorporation of CNTs and Graphene in Ni-P coatings, it was observed that an optimum volume fraction of the additives yielded high corrosion resistance performance. This was essentially due to the smooth compact and defect free surface morphology, (b) in the work on the correlation between micro-texture and corrosion behaviour of Ni-P coatings as a function of phosphorous content, it was observed that the phosphorous concentration range where the nano-crystalline region (along with the minor amorphous phase fraction) was dominant a slight alteration in texture determines the corrosion rate. With increase in the amorphous region, the galvanic coupling between the anodic amorphous phase and cathodic crystalline phase determined the corrosion behaviour. A mixture of amorphous and crystalline phases with lower fraction of the amorphous phase enhanced the corrosion rate due to increased galvanic coupling. For higher addition of phosphorus, large fraction of amorphous phase evolved which significantly reduced the galvanic coupling leading to higher corrosion resistance behaviour, (c) in the work on the effect of deposition temperature (bath temperature of 15˚C, 20˚C, 25˚C, 35˚C) on the evolution of correlation between texture and corrosion behaviour of Ni-P coatings, it was observed that the coating deposited at 15°C and 25°C yielded the maximum and minimum corrosion rate respectively. Analysis of the coating texture revealed that the higher corrosion rate for the 15°C coating was due to low fraction of low energy low angle grain boundaries (LAGBs), higher strain within the grains, and (101) growth texture. Lower corrosion rate, on the other hand, for the 25°C coating was due to low energy (001) growth texture, low average strain within the grains, and high fraction of LAGBs, (d) in the work on the effect of deposition current density on the evolution of correlation between texture and corrosion behaviour of Ni-P coatings, it was observed that the Ni-P coating (deposited using 60 mA.cm-2) that exhibited the lowest corrosion rate was characterized by the presence of lower energy surface texture, lower grain size, narrow grain size distribution and a relatively higher fraction of low energy Σ3 coherent twin boundaries. A higher corrosion rate for coating deposited using 5 mA.cm-2 was due to higher energy surface texture and larger grain size distribution.