Microstructure and corrosion performance of plasma electrolytic oxidation coatings on the surface of conventional and selective laser melted Ti-6Al-4V alloy

Abstract

Abstract Plasma electrolytic oxidation was carried out on conventional and selective laser melted Ti-6Al-4V alloy in electrolytes with different concentrations of sodium hexametaphosphate. Microstructure and chemical analysis of the surface and cross section of the samples were studied by scanning electron microscopy. X-ray diffraction was carried out to evaluate the phase analysis of the specimens. Corrosion behavior of the specimens was investigated by electrochemical impedance spectroscopy. The results showed that the coatings were mainly composed of titanium dioxide. When sodium hexametaphosphate was added to the electrolyte, phosphorus was incorporated into the coatings and the morphology of the coatings was altered to a non-uniform one with large pores and cracks. Corrosion studies confirmed that the coatings on the conventional alloy show higher corrosion resistance, mainly due to the formation of denser coatings with lower porosities. Addition of 1 and 2 g l−1 sodium hexamethaphosphate to the electrolyte lead to the change in impedance modulus of the coated conventionally produced samples from 83 KΩ.cm2 (for the sample coated without sodium hexametaphosphate) to 85 and 146 KΩ.cm2, respectively. On the other hand, these values for the coated selective melted specimens were 80 (for the sample coated without sodium hexametaphosphate) to 23 and 1.5 KΩ.cm2, respectively. Ate the same time, the values of charge transter resistance for the coated conventional samples were 105, 102, and 201 KΩ.cm2, while the equivalent values for the selective laser melted specimens were 135, 51, and 4.1 KΩ.cm2.