Influence of surfactant polarity on the evolution of micro-texture, grain boundary constitution and corrosion behavior of electrodeposited Zn coatings

Abstract

Corrosion behavior, morphology and micro-texture of Zn coatings, electrodeposited from electrolyte bath without and with surfactants of different polarity, were investigated and correlated. Polarization resistance (Rp) value which is directly proportional to the corrosion resistance behavior, was determined from the electrochemical impedance spectroscopy technique. Rp value was highest for the coating with Cetyltrimethylammonium bromide [CTAB (cationic surfactant)], followed by coatings with Sodium lauryl sulphate [SLS (anionic surfactant)], and Triton X-100 (non-ionic surfactant) surfactant. Surfactant-free coating showed the lowest Rp value, indicating lowest corrosion resistance. Surfactant-free coating and coating with SLS showed large-sized columnar grains, whereas coatings with CTAB and Triton X-100 contained smaller-sized grains throughout the coating thickness. XRD and EBSD suggested low energy near basal micro-texture on the coating surface with SLS as surfactant. Whereas other coatings with CTAB and Triton X-100 showed higher energy micro-texture. SLS containing coatings and surfactant-free coatings exhibited higher percentage (greater than 60%) of high angle grain boundaries (HAGBs). Zn coatings with CTAB showed lower fraction of HAGBs followed by coatings with Triton X-100. With similar micro-texture and grain boundary constitutions, in coatings electrodeposited using CTAB and Triton X-100, higher corrosion resistance in the case of CTAB was due to more compact and relatively less defective surface morphology. Between the coatings, produced without surfactant and using SLS surfactant, both of which exhibited similar columnar grains and percentage of high angle grain boundaries, the higher corrosion resistance in the case of SLS surfactant coating was due to near basal micro-texture along coating growth direction and high fraction of low energy special boundaries (90°/211¯0 angle/axis pair).