Corrosion of titanium: Part 2: effects of surface treatments

Introduction

Titanium has outstanding corrosion resistance due to a thin, amorphous, nonstoichiometric TiO₂ protective layer (max. 10-nm thick (1)) that is formed spontaneously on the surface when exposed to an aerated environment. This protective layer is very stable and allows the use of titanium in severe working conditions, such as offshore; in an acid environment; in aerospace (2, 3) and automotive industries; in high-temperature applications; in the chemical and food industries (4-5-6); and in marine hydrometallurgical applications and nuclear fuel waste containment (7-8-9-10), where no other metal can be used.

Nevertheless, commercially pure titanium may suffer different forms of corrosion in very severe environments (11). Generalized corrosion is caused by small quantities of fluoride ions (more than 0.002 M that combine with titanium, forming TiF₄, and destroy the passivity film. Hydrogen embrittlement happens mainly on α and α-β titanium, due to the low hydrogen solubility in α-Ti (12). Stress corrosion cracking can also happen in very specific environments, such as anhydrous methanol, nitrogen tetroxide, red-fuming nitric acid or solid cadmium (13). However, the most critical forms of corrosion of titanium are due to localized breaking of the passive layer, and this is favored by the presence of concentrated halides, such hot salty water (above 200°C) or bromide-containing species (14, 15).

To overcome this weakness, corrosion resistance–enhancing treatments can be completed, or proper alloying elements may be added; the latter is the case with inclusion of elements that promote cathodic reactions, raising the cathodic polarization curve above the critical active anodic curve at higher potentials in the passive region, such as with palladium (16). A well-know treatment is nitration, which involves introducing nitrogen in the first micrometer of the surface to promote the formation of TiN and increase corrosion resistance (17, 18). These methods, acting on the titanium composition, are effective but complicated to perform and expensive, as are other surface coating techniques, such as vacuum plasma spray coating, plasma spraying and chemical vapor deposition (19).

For this reason, treatments that act on the naturally formed passive layer are preferred. The easiest and cheapest treatment to tune the oxide layer is anodic oxidation, which involves applying an anodic polarization of several 10s of volts to the metal, promoting the growth of the natural oxide layer with thicknesses from about 40 nm, with an anodizing potential of 10 V, to about 250 nm at 100 V (20-21-22). At potentials higher than 100 V, the anodizing begins to cause the instauration of microarcs in the insulating oxide layer; in this regime, called anodic spark deposition or plasma electrolytic oxidation (PEO), the oxide grows much more, reaching the thickness of several micrometers but switching from a compact to a porous structure (23).

Due to the promising results with increasing corrosion resistance using anodizing treatments, as described in this review the effect of anodizing potential has been investigated, with values ranging from 10 to 200 V, to define a treatment procedure able to delay or prevent localized corrosion in very severe environments.

Surface treatments are greatly influenced by the condition of the initial surface, in terms of roughness and chemical composition. Prior to any anodizing treatment, the surface has to be degreased, cleaned and have any adsorbed chemicals removed; for this reason, pretreatments such as etching and mechanical polishing are required. Moreover, by controlling the conditions of these pretreatments, it is possible to obtain a similar surface roughness on all titanium samples prior to oxidation. However, to perform chemical etching on titanium, a hydrofluoric acid–containing solution has to be used, and this is reported to cause fluoride surface enrichment (24, 25). To investigate the effect of the presence of bromides on anodized titanium corrosion resistance, the same treatments, in terms of final potential, anodizing solution and anodizing current density, are performed on samples prepared with the 2 different pretreatments. The samples are then tested for corrosion resistance.

The corrosion resistance of each metal strongly depends on its surface behavior when exposed to corrosive agents: The development of a protective passive layer is required, to reach a passive state in which corrosion is still proceeding but at such a low rate as to be negligible. For this reason, barrier layer treatments are very important to increase the corrosion resistance of metals. The most common and promising treatments to increase titanium corrosion resistance are described in the next sections.

Anodizing

Anodizing is one of the easiest techniques for modifying the thickness, composition and morphology of oxides appearing on valve metals, such as aluminum and titanium. The treatment involves anodically polarizing a metal to a fixed voltage in an electrochemical cell containing a counter electrode and an electrolyte providing conduction. Common electrolytes are acids, such as H₂SO₄ and H₃PO₄, and salts, such as (NH₄)₂SO₄, Na₂SO₄ and NH₄BF₄.

The main parameters affecting the characteristics of the resulting oxides are as follows (19, 26, 27):

Increasing the current density of anodization accelerates the oxidation rate and increases the amount of oxide converted from an amorphous state to anatase or even rutile crystalline structure (28), as shown in Figure 1.

Titanium anodized up to 120 V shows the appearance of a surface color, which is due to interference between light and the oxide layer and is dependent on the oxide thickness, in the range of 10s of 100s of nanometers (29). For this reason, anodizing techniques are distinguished between color anodizing, also known as traditional anodizing, with voltages below 100-120 V, and anodic spark deposition or PEO anodizing, which will be further described in the section “Anodic spark deposition or plasma electrolytic oxidation.”

Thermal oxidation

Another method to grow titanium oxides is to thermally treat titanium in an oxygen-containing atmosphere. The passive layer formed on titanium with exposure to air is some nanometers thick and composed of amorphous or poorly crystallized nonstoichiometric TiO₂. This layer is highly stable and provides protection against aggressive environments but can be disrupted at very low shear stresses, even by rubbing against soft tissue (54). The structure of thermally grown titanium oxides can be either anatase or rutile, depending on treatment temperatures (55).

Such oxides show higher chemical resistance than those grown anodically, as well as those produced through chemical oxidation: In fact, the latter can be removed by an acid pickle, while the removal of thermally grown oxides often requires sandblasting or a caustic descaling bath (55).

With thermal oxidation, surface morphology changes from a thin adherent surface layer at 500°C to the formation of a small grain structure at 650°C, until there is a complete coverage with grains oriented perpendicular to the substrate, at 800°C. Corrosion resistance increases with increasing treatment temperature, with lower anodic current densities at higher temperature (Fig. 4) (54). This trend is explained by the increase in oxide thickness with temperature, passing from 10s of nanometers at 500°C-600°C to hundreds of nanometers at around 700°C (55, 56). Temperatures higher than 800°C and longer times are reported to cause spallation of the oxide layer and worsening of the properties (54).

Concerning treatment time, Kumar et al (57) performed thermal treatment at 650°C with a duration of from 8 to 48 hours. Their results showed the presence of oxide scales throughout the surface without any spalling, irrespective of treatment time. At 800°C longer treatment caused oxide spallation. X-ray diffraction (XRD) on oxidized samples with different durations reveals the formation of rutile, TiO and α-Ti phase, as reported in Figure 5.

The advantages of thermal oxidation compared with anodic oxidation are well reported in the literature (55, 58).

Birch and Burleigh report the effect of anodization at low voltages (up to a few volts), and of thermal treatment at different temperatures with different durations, on free corrosion potential (55). They performed potentiodynamic tests and electrochemical impedance spectroscopy (EIS) tests, measuring anodic current densities in the passive range and free corrosion potentials of all samples prepared with those treatments.

Higher-temperature oxides show progressively lower passive current density and higher corrosion potential, which implies that the films are more protective and inert. The oxide formed at 700°C for 30 minutes shows the highest free corrosion potential and the lowest current density. Both anodization and thermal oxidation provide better corrosion resistance compared with nontreated metal. The difference in corrosion behaviors of the tested oxidizing techniques is due to the crystal structure of the oxide: In Figure 6, a summary of oxides formed according to the difference techniques is shown.

Chemical oxidation

Thermal oxidation is simple, but its reproducibility is poor and should be preceded by acid pickling to remove embedded surface contamination, such as iron. In addition, the temperature has to be higher than 500°C to build a film with good corrosion resistance. It is not practical to heat a large vessel at 500°C. Anodic oxidation, instead, is more consistent and can be effective as a single step if it is applied for a sufficient time to dissolve surface contamination. However, sometimes anodizing at high voltage is time consuming and laborious with potential safety problems, and applying current to a large vessel is not practical. In these cases, another technique can be used – that is, chemical oxidation (8).

Heating up titanium in a H₂O₂ or NaOH environment at temperatures of 60°C-90°C results in oxidation of the surface with no external contamination. Acids like H₂SO₄ and HCl can be added to the treatment solution to remove surface contaminants (59).

Oxide thickness increases almost linearly with treatment duration. Oxide growth with this technique is comparable to that produced with anodization or thermal oxidation.

Films obtained in solutions containing HCl are thicker. XRD analyses of the same samples show that the addition of SO4^- ions promotes the formation of pure anatase, while the addition of Cl^- ions favors the formation of pure rutile phase.

An evolution of this oxide growing method has involved annealing a chemically oxidized surface. Wang et al (60) studied the effect of thermally treating the sample at 350°C, 400°C, 500°C and 600°C after chemical oxidation at 80°C for 6 hours in H₂O_2. They found large differences in surface wettability: with average water contact angles of 29.5°, 130.5°, 115.6° and 30.6°, respectively. This effect influences corrosion resistance, as can be seen from the potentiodynamic tests reported in Figure 7, due to the great changes of surface structure of the film after thermal treatment; scanning electron microscopy (SEM) images of samples treated at the same 4 temperatures are shown in Figure 8.

A different trend is reported by Krupa et al (61) for chemical oxidation carried out in a 10 M NaOH solution at 60°C for 24 hours and then heated at 500°C, 600°C and 700°C. In this case, the last sample, calcined at 700°C, showed the best corrosion resistance according to the polarization resistance of the oxide and the potentiodynamic behavior. Corrosion resistance follows the order: 700°C > 500° > 600° > untreated. The explanation proposed is related to the sodium titanate hydrogel present in the oxide. At 500°C, the sodium titanate hydrogel surface layer is dehydrated and transformed into amorphous sodium titanate. At 600°C, the hydrogel layer is partially transformed into crystalline sodium titanate and rutile, but remains amorphous. From 600°C, a densification process begins, resulting in the pores being closed. The transformation of the amorphous structure into crystalline is observed to occur above 700°C. The higher corrosion resistance achieved at 500°C compared with that measured on samples heated at 600°C may result from the better structural homogeneity of the former samples. The structure of samples heated at 600°C is partially amorphous and partially crystalline, and this morphology negatively affects corrosion resistance (61).

Ion implantation

The ion implantation technique has been exploited to enrich the surface of titanium with certain amounts of selected elements; this permits the production of surfaces that are more biocompatible (62). Thus, the majority of the studies reported in the literature available regarding this technique refer to attempts to enhance biocompatibility. Another requirement for devices in the human body is the resistance to corrosion, and ion implanted samples are usually tested for this purpose in simulated body fluid (SBF).

According to EIS analyses, implantation of calcium at a dose of 10¹⁷ Ca⁺/cm² with ion energy 25 keV produces an increment in uniform corrosion resistance of titanium in an SBF environment. However, the formation of pits is reported in corrosion tests after this treatment. The mechanism of the initiation of the pits on the surface of implanted titanium has not yet been explained, though it is suggested that the alteration of chemical composition of the oxide layer results in a weakening of the oxide toward localized corrosion (63).

The same test performed on samples implanted with 10¹⁵-3 × 10¹⁷ P⁺/cm² shows that for doses higher than 10⁶, a new TiP phase appears in the oxide (64). Phosphorous implantation causes an increase in corrosion resistance performance – a conclusion derived from both potentiodynamic and EIS analyses – and the resistance to polarization of the implanted layer is 2.5 times larger than that of the untreated oxide, which is due to the amorphous structure of the TiO₂ surface during the implantation. Phosphorous is known to stabilize amorphous structures (28, 65). The authors claim that the amorphous layer made up of the native titanium oxide does not contain defects, such as grain boundaries and contaminant segregation, which are characteristic of the crystalline state; this fact can be considered the reason for the increased corrosion resistance of amorphous oxides compared with crystalline oxide with the same thickness. The formation of the TiP phase, which itself has a high corrosion resistance, probably helps the increase in resistance (64).

The effect of both calcium and phosphorous ion implantation is to avoid the pitting corrosion observed in the case of implantation of Ca alone (66). The corrosion resistance of the treatments can be classified as follows: Ca + P implanted titanium > P implanted titanium > Ca implanted titanium > untreated titanium (67).

Oxygen implantation in concentrations from 10¹³ to 10¹⁵ ions/cm² has been exploited by Ningshen et al (68). Unfortunately, this treatment was found to be detrimental for titanium corrosion resistance in highly aggressive environments.

Carbon implantation was found to have a much greater effect on titanium corrosion resistance. Table II reports the free corrosion potential E_corr and resistance of the oxide layers of samples implanted with carbon at doses from 5 × 10¹⁵ to 2 × 10¹⁷ ions/cm² (69). The best improvement was obtained with an implantation dose of 2 × 10¹⁷ ions/cm². The effect of carbon in the oxide is based on the formation of a continuous, solid and nanocrystalline TiC layer.

A similar study was conducted on nitrogen implantation, with doses from 10¹⁶ to 10¹⁸ ions/cm². Table III reports the free corrosion potential and polarization resistance for all of the tested implantation conditions (70). The maximum increase in corrosion resistance was found in the correspondence of a dose of 1 × 10¹⁷ ions/cm². Nitrogen in the lattice formed nanocrystalline TiN precipitates that were coherent with the basic titanium lattice and uniformly distributed throughout the surface layer.

Grain refinement

In recent years, a variety of techniques to obtain ultrafine grain (UFG) size on metals were developed (71). A comparison between corrosion resistance of UFG titanium with that of coarse grain titanium was proposed from Balyanov et al (72). UFGs were obtained using the equal channel angular pressing (ECAP) technique, reaching an average grain size of 300 nm. Table IV reports the difference in free corrosion potential and corrosion rate of coarse grain and UFG titanium, for HCl and H₂SO₄ solutions at 1 M, 3 M and 5 M concentrations. Lower corrosion rates and more positive free corrosion potentials were associated with UFG titanium.

Kim et al (73) carried out a similar study on UFG titanium produced using 3 different methods. The first 2 methods involved rolling with a single pass at different speed ratios of 3 and 5, and producing a thickness reduction of 61% and 77% (W3 and W5), respectively. The third method involved cold rolling to produce a thickness reduction of 63% by a single rolling pass (C3). Table V reports the current density and corrosion potential for treated titanium that was immersed in 1 M HCl and 1 M H₂SO₄.

The treatment C3 produced the best result, with the corrosion rate decreasing by 40.6% and 48.2% in HCl and H₂SO₄ solutions, respectively. The corrosion rate decreased linearly with the inverse square root of grain size. This effect was due to the higher energy of the grain boundaries. Corrosion tends to take place in grain-boundary regions, and a large extent of grain boundaries in nanocrystalline materials enhances passivation kinetics, leading to rapid formation of a stable passive layer.

Alloying with palladium

Among the variety of titanium alloys, 3 of them were developed to enhance titanium corrosion resistance: titanium grade 7, grade 12 and grade 16. The best alloying elements to enhance corrosion resistance are palladium, molybdenum and nickel (12, 74). Palladium and nickel have the effect of accelerating cathodic hydrogen evolution reactions (HERs) on titanium, due to their higher exchange current density for HER, while molybdenum is used to enhance the stability of the oxide.

Among the 3 elements, palladium is the one that gives higher corrosion resistance to titanium to the point that Ti-Pd alloy has been proposed for the manufacture of high-level nuclear waste containers (75). Nakagawa et al (76) showed that alloying with palladium altered the cathodic process and resulted in lower overvoltages for HER: This may shift the working potential in the passive region of the alloy. They also found that palladium adsorbs hydrogen to form a protective layer of titanium hydride.

Brossia and Cragnolino (77) report a comparison between Ti grade 2 and 7 pitting resistance, in a 1 M solution of Cl^- at 95°C. Pitting potential increases from 1.08 V vs. saturated calomel reference electrode (SCE) to 7.69 V vs. SCE, in passing from Ti grade 2 to Ti grade 7 (with 0.2% of palladium). They also pointed out the effect of palladium alloying on the cathodic polarization curve, as visible in Figure 9, where the reduced overvoltage of HER is shown together with the higher free corrosion potential of grade 7.

Conclusions

The excellent corrosion resistance of titanium in many aggressive environments can be further enhanced by tuning its surface properties and/or bulk composition. In the second part of this review concerning the corrosion of titanium, some of the most important treatments to increase corrosion resistance are described, in relation to the forms of corrosion described in Part 1. The effects of the variation of treatment parameters have been described, with particular attention to those techniques that enable the modification of natural titanium oxide in terms of thickness and morphology.

However, knowledge of even these kinds of treatment is far from being exhaustive, and further research will likely be conducted into this important topic.