Effect of molybdenum partitioning induced lattice strain and surface oxide chemistry on hydrogen permeation and corrosion behavior of Co-Mo coatings

Abstract

Co-XMo (X = 0, 1.5, 4, 8, 11) coatings were electrodeposited onto mild steel. Analysis of the electrochemical corrosion and hydrogen permeation behavior was conducted and correlated with the coating microstructure. All the Co-Mo coatings exhibited higher corrosion resistance than the pure Co coating. The corrosion resistance of the Co-Mo coatings, however, exhibited a non-monotonic trend with increase in molybdenum concentration, and at an optimum concentration of 4wt.% Mo, highest corrosion resistance was obtained. The polarization resistance values exhibited by pure Co, Co-4wt.% Mo, and Co-11wt.% Mo were 1491, 4968, and 2929 Ω cm², respectively. The highest corrosion resistance of Co-4wt.% Mo coating was due to lower coating strain and relatively stable passive oxide corrosion products (CoO and MoO₃). Conversely, the lowest corrosion resistance of pristine Co coating was due to higher coating strain and higher fraction of relatively less stable oxides (Co(OH)₂) after exposure to the corrosive media. The lowest and highest hydrogen permeation currents exhibited by Co-11wt.% Mo and Co-4wt.% Mo coatings were 5.6 and 10.3 μA/cm², respectively. The lowest hydrogen permeation through the Co-11wt.% Mo coating was due to compressive strain in the Co-Mo solids solution matrix (owing to the larger size of molybdenum atoms), which hindered hydrogen diffusion through the matrix. The higher hydrogen permeation through the Co-4wt.%Mo coating was due to the formation of relatively Mo-enriched clusters in the Co-Mo solid solution matrix, which produced tensile strain in the solid solution matrix near the matrix/cluster interface, thus providing a short-circuit path for hydrogen diffusion.