In-depth study of the dry-anodizing process on Ti6Al4V alloys: Effect of the acid content and electrical parameters

Abstract

Titanium (Ti) surface has the capability of changing color due to the oxides formed when it meets oxygen under acidic solutions and current application. Additionally, this passive oxide film formed on the Ti surface is responsible for its high corrosion resistance that is needed in several industries. Consequently, titanium has a wide range of applications in the aerospace, automotive, and biomedical industries. This study introduces a never-used dry-anodizing process utilizing DryLyte® Technology. In this sense, the effects of electrical parameters (voltage, and current density) and the acid content of the dry electrolyte on the performance of the new dry-anodizing process are evaluated using different advanced characterization techniques. Direct and indirect coating thickness measurement of the anodic layer by using the Focused Ion Beam and reflectometric techniques, respectively, revealed that the anodic layer thickness does not depend on the acid content, yielding a homogeneous value of around 60 nm. Additionally, the study demonstrated that the application of low-frequency pulses with a duty cycle of 20 %, reduce the heating during the process and allows to increase further the anodizing voltage compared with direct current anodizing, thereby expanding the available color range. Finally, the experimental results were adjusted to a polynomial fitting to predict the TiO2 anodizing layer thickness induced along the process. From all the aforementioned information, it has been stablished that the anodizing quality of the dry electrolyte decreases under service-like working conditions, which is mainly attributed to two different effects related to the electrical current that passes through the polymeric particles and that takes place at the same time: (1) dehydration of the particle and (2) degradation of the core-shell structure, caused by Joule's effect during the dry-anodizing process.