Effects of Laser Glazed Surface on Pitting Corrosion of Nickel Alloy 718 in Acidic and Seawater Environments

Abstract

Studies on the effects of laser glazed and unglazed sample surfaces on the pitting corrosion resistance of nickel alloy 718 have been carried out. Both sample surfaces of nickel alloy 718 were cut to different dimensions for pitting corrosion tests using potentiostatic polarization process. The alloy samples used for electrochemical testing were connected to a flexible wire joined by spot weld process. The alloy samples were coated with low viscous polymeric wax leaving the surface areas to be tested exposed to seawater and one mole concentration of hydrochloric acid environments. These samples which are working electrodes have electrical connection between reference electrode and counter electrode and the entire system were connected to computer with PSTrace software that stands as potentiostat. Pre and post morphological examination of the sample surfaces were done using confocal laser scanning microscopy and scanning electron microscopy respectively. The results of pitting corrosion tests showed that the glazed and unglazed alloy sample surfaces immersed in seawater environment suffered from pitting corrosion as a result of the presence of pit initiation sites developed during surface laser treatment and also the presence of some aggressive anions like Cl^-, SO₄^- etc in the environment which migrated into the pits due to electrostatic balance and escalated pitting corrosion; while the samples of both surfaces immersed in 1MHCl suffered more from general corrosion. Post pitting corrosion examinations revealed that corrosion oxide films formed on the sample surfaces in acidic and alkaline environments were not protective as they flaked and exposed the sample surfaces to more corrosion attack.