Article Cite This: J. Phys. Chem. C 2018, 122, 20691−20700
pubs.acs.org/JPCC
Mitigation of the Surface Oxidation of Titanium by Hydrogen Ying Zhang,*,†,‡ Zhigang Zak Fang,*,† Lei Xu,† Pei Sun,† Brian Van Devener,† Shili Zheng,‡ Yang Xia,† Ping Li,‡ and Yang Zhang‡ †
Department of Metallurgical Engineering, University of Utah, Salt Lake City, Utah 84112, United States Key Laboratory of Green Process and Engineering, National Engineering Laboratory for Hydrometallurgical Cleaner Production Technology, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China
‡
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ABSTRACT: As a reactive metal, Ti is prone to surface oxidation spontaneously when exposed to environment containing oxygena phenomenon also known as surface passivation. It has also been known that titanium hydride (TiH2) is impervious to oxygen. However, it is not clear to date and there is little published report on how and why hydrogen affects the oxidation of Ti. “Impervious” may be an overstatement because TiH2 does oxidize. Because surface oxygen is also a part of the total oxygen in titanium in addition to bulk oxygen and the passivation film affects the properties of titanium, understanding the surface oxidation behavior of titanium and the effects of hydrogen is thus of considerable interest from both fundamental and practical perspectives. This article studies the effect of hydrogen on the surface passivation of titanium from different aspects including (I) the comparison of oxygen contents in α-Ti and TiH2 powders when exposed to air, (II) the oxidation states, their relative fractions, and the thickness of the oxidized layers as a function of hydrogen contents, and (III) the characterization of the passivation layer on the surface by high-resolution transmission electron microscope. The experimental data showed that the presence of hydrogen can indeed make titanium metal less prone to oxidation. The alloying of titanium with hydrogen can result in reduced thickness and the relative fraction of titanium in the form of TiIV in the passivated surface, effectively minimizing surface oxidation. The fundamental reason for the effect of hydrogen on the surface oxidation of titanium is discussed and attributed to the difference in oxidation behavior of α and δ phases.
1. INTRODUCTION Oxygen content in and oxidation of reactive metals are important topics for understanding their behavior and applications. For example, on the one hand, the oxygen content in titanium alloys is generally considered detrimental to the mechanical properties,1 and therefore, it must be carefully controlled, albeit in some special case, oxygen has been found to be beneficial to the mechanical properties of Ti alloys owing to its solid solution strengthening effect.2 On the other hand, oxidation on the surface of Ti is essential because of its almost unrivaled corrosion resistance.3,4 Indeed, controlled surface oxidation processes are utilized to engineer the corrosion resistance,5−8 biocompatibility,9,10 and hightemperature stability of Ti.11 In the case of Ti alloys, the oxygen content is generally limited to 0.2% by weight by industry for structural alloys.12 However, because of its strong affinity to Ti,13 controlling oxygen content in Ti to below 0.2 wt % is difficult, thus it is a major factor contributing to the high cost of Ti materials. Ti is known to form an oxide on its surface spontaneously as soon as it is exposed to air.14 This is an especially challenging issue when Ti or Ti alloy powders are used. Fine powders with high © 2018 American Chemical Society
specific surface areas can contribute significantly to the total oxygen content of the powder purely because of the formation of surface oxide layers. The oxide layers can also dissolve into the Ti matrix at moderately higher temperatures. The total oxygen content is the sum of the surface oxygen and the oxygen dissolved in the bulk of the metal. To control and minimize the oxygen content in Ti, a number of approaches have been used within industry including vacuum arc re-melting, electron beam melting, and acid pickling to remove surface oxide layers. A unique approach for handling Ti powder is to introduce hydrogen and utilize TiH2. TiH2 is described as being impervious to oxygen,12 and in terms of safety, it is widely known in practice that handling TiH2 powder is safer than handling Ti metal powder. These facts suggest that the chemistry between TiH2 and O is different from that between Ti and O. In other words, H can affect the chemical affinity of Ti to O or the stability of the Ti−O bond. Recently, it was reported by the present Received: May 16, 2018 Revised: August 14, 2018 Published: August 14, 2018 20691
DOI: 10.1021/acs.jpcc.8b04684 J. Phys. Chem. C 2018, 122, 20691−20700
Article
The Journal of Physical Chemistry C
Hryha et al.23 investigated the thickness of the homogeneous titanium oxide layer in the as-atomized titanium and titanium alloy powders, which were ∼2.9 nm for Ti, ∼3.2 nm for Ti6Al4V and ∼4.2 nm for NiTi powders, respectively. Qin et al.20 compared the spontaneously formed passive film on CP titanium in methanol without or with water, and it was predominantly TiO2 and contains small amounts of Ti2O3 in methanol, compared with that of only TiO2 when the amount of water was increased to 0.2%. In addition, a two-layer model19 for the oxide film was applied to interpret the electrochemical impedance spectroscopy spectra, that is, a thin barrier inner layer and a porous outer layer, and the thickness of the barrier inner layer is no more than 3.6 nm which also decreases with the increasing water content in methanol solution. Paulin et al.24 confirmed the existence of oxide layers on TiH2 particles by using atomic emission spectroscopy, Xray photoelectron spectroscopy (XPS), and energy-dispersive X-ray spectrometry cross section line scans and determined the thickness of TiO2 layers in the range of 130 nm from the results of the XPS depth profiles. Ivasishin et al.25 compared the surface contamination of Ti and TiH2 using XPS with Al and Ag X-ray sources and summarized that they both had a core−shell structure (covered with TiO2 scale) and the thickness of the scale was lower for Ti powder than for TiH2. However, previous results presented by the present authors26 showed that TiH2 powder had an apparently thinner passive layer than does the Ti metal powder (2.1 nm vs 4.1 nm), and obviously more Ti2O3 suboxide was seen in the XPS spectra for TiH2, which are critical factors to weaken the surface passivation. Because there are contradictory reports, it is important that a systematic study be conducted to understand the effect of hydrogen on the surface oxidation of titanium. In this research, the surface passivation of Ti with and without hydrogen were examined and compared based on experimental results, including the change of oxygen content when exposed to air, the oxidation states, their relative fractions and the thickness of the oxide films on titanium as determined by XPS, as well as the analysis of the near-surface oxide film using a transmission electron microscope (TEM). Through these techniques, the effect of hydrogen on the surface oxidation of Ti is investigated.
authors that H can destabilize Ti−O solid solutions making it possible to reduce oxygen content in Ti−O solid solutions to very low levels using Mg.15 However, there seems to be very few published reports on the oxidation behavior of TiH2 or how hydrogen affects the oxidation behavior of Ti metal, which is the focus of this work. As mentioned above, the surface of Ti can spontaneously oxidize and form an oxide layer. This is also called surface passivation, which is a phenomenon that allows reactive metals to protect their bulk composition from corrosion including further oxidation. Oxidation layers can also be deliberately prepared to control the thickness and composition of the layer. Depending on how the oxide film is formed, its thickness varies from a few nanometers to more than 1 μm.4,10,16 The properties of the oxide film on titanium including stability depend strongly on the structure, composition, and thickness. To enhance the stability, sometimes high-temperature oxidation is utilized, which tends to promote the formation of the chemically stable, highly crystalline form of rutile TiO2.8 The results by Hristova et al.14 demonstrated that the thickness of the thermal films on pure titanium surfaces grew with the heating time and increased from initially a few nanometers to more than a hundred nanometers and the structure transformed from amorphous to rutile at 500 °C. Anodic oxidation is another way to deliberately passivate the titanium surface.7 Leitner et al.17 passivated titanium in a wide range of potentials and suggested the formation of different types of films from noncrystalline (probably amorphous) to an intermediate and to crystalline oxides with increasing potential. Pouilleau et al.4 studied the structure of Ti oxide films obtained via different techniques including thermal oxidation, anodization, stabilization, aging, and the combinations thereof. He reported that the oxide layer can be viewed as an amorphous TiO2 outer layer and an intermediate TiOx layer. The thickness of the intermediate layer is 10−40 nm, and the outer layer is in the range of 10−20 nm, varying with the oxidation conditions. There have been numerous research reports8,18−20 regarding the character of the anodic film on titanium. However, because the characteristics of the passive layer rely on the preparation method and associated parameters, there is no agreement on how the method of preparation affects the structure and thickness of the oxide layers. As mentioned earlier, hydrogen affects the oxidation of Ti. It is logical to hypothesize that hydrogen can also affect the structure and thickness of the oxidized surface layer on Ti and Ti alloys. From a practical perspective, understanding the effects of hydrogen can have a significant impact on the titanium industry. Since the total oxygen in titanium comes both from the bulk and the surface and the oxygen from the bulk can be effectively removed by reacting Ti with alkaline metals or electrons,21 limiting the oxidation of the surface layer becomes a key to reducing the total oxygen content in Ti. It has been estimated that for a spherical titanium particle of 2 μm diameter, assuming the thickness of the surface oxide layer is 3 nm, the surface contribution to the oxygen content would be 3900 ppm,22 which is higher than what is required based on the ASTM specification for titanium sponge. On the basis of the reported results, we can also hypothesize that the surface oxygen content can be reduced by reducing the thickness of the spontaneously formed passive film, while adjusting the phase composition of the film to have more intermediate TiOx(x
