# Colored Anodic Titania Thin Layers Involving Various Deep Eutectic Solvent Formulations—Evaluation of Corrosion Behavior

## TL;DR

This study shows that using deep eutectic solvents to anodize titanium can create colorful, corrosion-resistant coatings suitable for harsh environments.

## Contribution

The paper introduces new DES formulations for anodizing titanium that produce a wide color range and excellent corrosion resistance.

## Key findings

- Anodic oxidation using DES formulations produced a wide color palette with growth rates of 1.6–2.1 nm/V.
- Layers made in choline–chloride–oxalic acid and choline dihydrogen citrate–oxalic acid–ethylene glycol showed best corrosion resistance in NaCl and Hank’s solution.
- The color and corrosion performance depend on the applied voltage and the specific DES formulation used.

## Abstract

What are the main findings?
Anodic oxidation of Ti at constant applied voltage using various deep eutectic solvent formulations successfully produced a wide color palette.Growth rates between 1.6 and 2.1 nm/V have been determined, with good linear regression coefficients.Anodic layers synthesized in choline–chloride–oxalic acid and choline dihydrogen citrate–oxalic acid–ethylene glycol eutectic mixtures at 30 V for 3 min showed the best corrosion performance during long-term exposure in 0.5 M NaCl for 240 h and Hank’s biological solution for 96 h.

Anodic oxidation of Ti at constant applied voltage using various deep eutectic solvent formulations successfully produced a wide color palette.

Growth rates between 1.6 and 2.1 nm/V have been determined, with good linear regression coefficients.

Anodic layers synthesized in choline–chloride–oxalic acid and choline dihydrogen citrate–oxalic acid–ethylene glycol eutectic mixtures at 30 V for 3 min showed the best corrosion performance during long-term exposure in 0.5 M NaCl for 240 h and Hank’s biological solution for 96 h.

What are the implications of the main findings?
Deep eutectic solvents may represent an attractive, environmentally friendly route to anodically color Ti substrates.The selected DES-based electrolytes can be applied for short durations (i.e., 3 min) at room temperature.The colored anodic titanium oxide thin layers could provide adequate corrosion protection during long-term exposure in various aggressive environments.

Deep eutectic solvents may represent an attractive, environmentally friendly route to anodically color Ti substrates.

The selected DES-based electrolytes can be applied for short durations (i.e., 3 min) at room temperature.

The colored anodic titanium oxide thin layers could provide adequate corrosion protection during long-term exposure in various aggressive environments.

This paper reports initial experimental results related to the preparation of colored anodic titania thin layers using various deep eutectic solvent (DES)-based formulations. Electrolytes based on choline dihydrogen citrate–oxalic acid–ethylene glycol (1:1:1 molar ratio), choline chloride–oxalic acid (1:1 molar ratio) and choline chloride–lactic acid (1:2 molar ratio) eutectic mixtures were investigated. The anodization has been performed at constant voltage in a range of 10–100 V for various periods of time between 1 and 5 min at room temperature under mild stirring. A brief description of anodization procedures, as well as of some characteristics, from appearance and morphological viewpoints, is presented. A quantitative analysis of color characteristics in relation to the DES-based electrolyte and applied voltage using the CIELAB system is also discussed. The achieved chromatic scale follows this order of colors: golden—blue—light blue—light blue/green—pink—violet. This depends on the applied potential and the DES-based electrolyte. The films present a relatively high brightness and color saturation. The hue vs. anodization voltage diagrams suggest an almost linear dependence of the oxide growth measured against the applied voltage. The corrosion performance has been assessed through continuous immersion tests in (i) 0.5 M NaCl for 240 h and (ii) Hank’s biological solution for 96 h with intermediate visual examinations and recording corrosion potential, as well as potentiodynamic polarization curves and impedance spectra at open circuit potential. Different corrosion performances are discussed considering the aggressive medium involved and the used DES-based systems.