Characterization of Electrochemically Formed Coloured Passive Layers on Titanium and Zirconium: Optical, Surface and Corrosion Properties
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Electrochemically formed passive layers on titanium and zirconium, and their optical, surface and corrosion properties are presented. With the use of higher applied AC voltages, the passive layers become thicker and more protective of the underlying metal, as determined from thickness measurements by scanning electron microscopy and focused ion beam instruments, as well as passive layer corrosion resistance measurements by electrochemical polarization curve and inductively coupled plasma mass spectrometry experiments. The surface morphology of all samples was studied with atomic force microscopy, profilometry, visible light microscopy, transmission electron microscopy, and finally electron backscattered diffraction experiments. The passive layers were found to be uniform in their surface structure with no cracks or fractures. However, the samples prepared at VAC = 60-80 V showed defects, which were increasingly prevalent at higher VAC. These defects are thought to be attributed to a process of localized electrochemical breakdown. The aforementioned increase in corrosion resistance is in spite of the defects, which were observed for the samples prepared at VAC = 60-80 V. Surface morphology, in relation to optical properties, was also investigated through reflectance spectroscopy measurements. A correlation between grain colouration and light reflected from the sample surface was observed through measurements using visible light microscopy as well as near infrared ultraviolet visible reflectance spectroscopy. These experiments showed a red-shift of wavelength maxima (λmax) values as voltages, and therefore thickness, were increased. This is the reason that different colours are seen for different thicknesses; the red shift causes different wavelengths to be reflected constructively and destructively. Overall, these passive layers are protective of an already remarkable metal and, with greater knowledge of their properties, their potential may be observed in a wide range of applications.