Correlation Between Microstructure, Electrochemical Degradation and Hydrogen Permeation behaviour of Electrodeposited Cobalt Based Coatings

Abstract

The application of cobalt and cobalt based coatings is limited by their electrochemical degradation in corrosive environments. In the present work, corrosion and hydrogen permeation behavior of electrodeposited Co and Co-based coatings (Co-W, Co-Mo) has been studied by examining the corrosion products, distribution of phases, morphology, micro-texture, and coating strain. Key observations made in this work are (ⅰ) in the work on the electrochemical behaviour of cobalt coatings electrodeposited at different deposition current densities (10, 20, 30, 40, 60 mA/cm2) over mild steel substrate, it was observed that the coating deposited at 30 mA/cm2 exhibited the highest corrosion resistance due to higher fraction of low energy grain boundaries and special boundaries. The coatings electrodeposited at 60 mA/cm2 exhibited lowest corrosion resistance due to higher fraction of high energy grain boundary constitution (ii) in the work on Co coatings electrodeposited using surfactants with different polarities (CTAB: cationic surfactant; SLS: anionic surfactant; Triton X-100: non-ionic surfactant), it was noted that the coatings deposited using the Triton X-100 surfactant showed highest corrosion rate due to the presence of higher fraction of high angle grain boundaries (HAGBs). Whereas the coating with CTAB surfactant, exhibited the lowest corrosion rate due to lowest fraction of HAGBs with highest number of low energy special boundaries along [011 ̅0] and [112 ̅0] axis and highest fraction of CSL boundaries, (ⅲ) in the work on the correlation between corrosion behavior and micro-texture of electrodeposited cobalt coatings containing different volume fractions of carbon nanotubes (CNTs), it was noted that the coating with the optimum CNT concentration showed highest corrosion resistance due to lesser fraction of HAGBs as well as higher fraction of low energy special boundaries like STGBs (32°/ [2(11) ̅0], 62°/[2(11) ̅0], 75°/[2(11) ̅0], angle axis pair), (ⅳ) in the work on the electrochemical behavior and hydrogen permeation of electrodeposited Co-W coatings with varying W compositions, it was noted that the coating with highest W content (13wt%) exhibited W-enriched nanoclusters, promoting galvanic coupling and reducing corrosion resistance. The tensile strain around these coherent clusters provided pathways for hydrogen to escape, increasing permeation rates. In contrast, the Co-6wt% W coating exhibited superior corrosion resistance due to a favorable low-energy texture and evolution of stable oxides in corrosive environment. In this coating, tungsten atom in solid solution of cobalt matrix created compressive stress (due to larger size of tungsten atoms) hindering hydrogen permeation, (ⅴ) in the work on the, electrochemical corrosion and hydrogen permeation of electrodeposited Co-XMo (X= 0,1.5,4,8,11) coatings, it was observed that incorporation of Mo enhanced the coating corrosion resistance with Co-4wt% Mo exhibiting the highest and pristine Co coating exhibiting the lowest corrosion resistance. The highest corrosion resistance of Co-4wt% Mo coating was due to less coating strain and formation of stable passive oxides. The Co-11wt% Mo coating showed the highest resistance to hydrogen permeation due to the compressive strain of Co-Mo solids solution, owing to larger size of molybdenum atoms, which hindered hydrogen diffusion though the interstitials.