Abstract

Commercially pure titanium has been anodized with the use of a pulsed signal in unipolar regime, with 25 % of anodic contribution at low frequency (20 Hz). This anodizing regime can effectively enhance thickness and crystallinity of the barrier region directly in contact with the metallic substrate. The latter condition will be particularly advantageous for corrosion resistance enhancement in strong reducing acidic environment, as concentrated hot sulfuric acid (10 %v/v at 60 °C). Corrosion response has been investigated through the use of Electrochemical Impedance Spectroscopy (EIS) and results compared with weight-loss tests and Linear Polarization Resistance (LPR). The higher degree of crystallinity of the coating, in the interfacial region, will be found to provide an effective barrier against proton diffusion thus retarding debonding of the oxide promoted by hydrogen evolution reaction (HER).

Introduction

Industrial usage of Plasma Electrolytic Oxidation (PEO) has grown consistently in recent years, thanks to the improved characteristics imparted to the oxide film in terms of surface adhesion, hardness, crystallinity, uniformity, and corrosion resistance. The metallic substrate is not subjected to elevated temperature and the overall equipment complexity is relatively simple, making the technique a good candidate for surface functionalization. In PEO treatments, high voltages are employed (~ 150-750 V 1) allowing for the formation of an insulating, or at least semiconductive, oxide layer that's limits ion transport responsible for the initial coating growth. Beyond the spark voltage (prerequisite the enter the PEO regime) oxidation does not occur only as the result of a continuous flow of ions but rather it takes place after the cooling of a plasma discharge. This makes a PEO coating particularly affected by the heat developed during the plasma emission, allowing to obtain high hardness according to the abundant amount of crystalline phase developed. It is possible to estimate PEO on Ti as a process with a technology readiness level (TRL) ~ 4÷6, indicating the need of several effort to consolidate its industrial application. Ti is abundantly used in many fields like aerospace where the metal is mainly employed in turbine components. A PEO coating may be strategic for this application as the high thickness combined with the relatively high porosity results in enhanced thermal barrier properties. Titanium is also used in many car components: valves, rods and turbocharge rotor are an example of possible applications. Here PEO can be beneficial to enhance overall surface properties like corrosion resistance and anti-galling. From a morphological point of view PEO coatings can be described according to a double layer structure, i.e., a porous outer layer mainly imparting thickness, wear and abrasion properties and a barrier layer responsible for the corrosion resistance and substrate isolation. Recently a detailed stoichiometric investigation2 of PEO barrier layers on Ti was proposed for both PEO in direct current and pulsed regime. It was shown how the application of a DC field allows to retain the peculiar structure assumed by the natural oxide layer, i.e.: a first layer in contact with the metal composed of TiO (Ti2+) followed by Ti2O3 separating the former by a well developed TiO2 (Ti4+) component.

This content is only available via PDF.
You can access this article if you purchase or spend a download.