Research progress on the corrosion behavior of titanium alloys

2.1 Galvanic corrosion

When two or more metals, with a significant potential difference, are used jointly, it is prone to galvanic corrosion (Zhang 2018). The metal with a low corrosion potential will undergo anodic polarization increasing anodic dissolution rate. The metal with a high corrosion potential will undergo cathodic polarization, which slows down its dissolution rate (Cao et al. 2016). During this process, the anodic corrosion acceleration effect and cathodic protection effect exist simultaneously.

The factors of affecting galvanic corrosion are complex and diverse, including the property of the electrode materials, the influence of the external media environments. The main driving force for galvanic corrosion is the potential difference between the cathode and the anode to generate a net current. Although the standard electrode potential of titanium is very negative. It has a more positive corrosion potential than ordinary alloys due to its dense passive film. As a result, titanium alloys act as cathodes to be protected, while other metals act as anodes to be accelerated corrosion. The corrosion degree depends on the corrosion potential difference, the effective anode/cathode area ratio, solution conductivity, temperature, and stability of the passive film, etc. (Yin et al. 2008; Pan et al. 2015; Du et al. 2014; Hasan 2014).

It is well known that the galvanic corrosion risk increases as the potential difference rises for the general galvanic system (Liu et al. 2003), and severe galvanic corrosion will occur when the potential difference exceeds 0.25 V (Qi et al. 2017). However, electron transport during galvanic corrosion can be blocked due to the presence of the dense passive film on the surface of titanium alloys. It can slow down the reduction reaction rate on titanium surface, further inhibit the galvanic corrosion of anode metals. Zhao et al. (2019) recently found that the potential difference between Ti60 and H62 is 0.33 V, but there is no galvanic corrosion (Figure 1). The result also confirms the above view. When the titanium alloys are coupled with a precious metal like Pd, the coupling potential is positive value, and the precious metal remains within its immune range as a cathode.

Figure 1:

SVET of ZTi60-H62 couple in 3.5% NaCl: (a) optical image of the position relationship of the coupled samples; (b) distribution map of galvanic current after 0.5 h; (c) distribution map of galvanic current after 15 days (Zhao et al. 2019).

The electrical conductivity of sea-water and industrial wastewater is high enough to promote galvanic corrosion. Moshrefi et al. (2011) clarified the influence of the anode/cathode area ratios on the corrosion rate. It is shown that the galvanic corrosion rate increases as the anode/cathode area ratios decreases. Therefore, the galvanic corrosion rate is inversely proportional to the anode/cathode area ratios. It can be calculated from average current and can be expressed as:

Some efforts that have been used to reduce galvanic corrosion include chemical conversion, micro-arc oxidation, electrochemical oxidation, plasma surface treatment etc. (Di 2014; Tang et al. 2015). Yao et al. (2005) prepared composite ceramic coatings in NaAlO₂ using micro-acr oxidation technology and testing the galvanic corrosion currents of treated Ti-6Al-4V alloy coupled with LY12 aluminum alloy and H62 brass, respectively. The results showed that the galvanic corrosion resistance of the coated samples was enhanced compared with the substrate. Recently, Kaseem and Choe (2021) used plasma electrolytic oxidation to generate a porous oxide layer with Zn and Al particles on Ti-6Al-4V alloy, acting as physical barriers to inhibit the penetration of corrosion ions into the passivation layer/substrate interface. Chen et al. (2020) investigated the influence of graphene content in the electrolyte on the galvanic corrosion performance of TC4 alloy and S135 steel based on the coatings prepared by micro-arc oxidation. The corrosion degree of S135 steel is found to be the lightest when the graphene content is 3 g/L. The electrochemical parameters showed that the galvanic corrosion rate is affected by both the potential difference and the polarization resistance. Zhao et al. (2021a, b, c) found that organic coatings can also be applied to the surface of anode materials to inhibit galvanic corrosion. The curves of the galvanic current with time after the two treatments are shown in Figure 2. Zhao (2021) found that the protection of anodic metal is better than the protection of cathode titanium alloys to inhibit galvanic corrosion (Figure 3).

Figure 2:

The curve of galvanic current versus time (Zhao et al. 2021a,b,c). (a) ZTi60@MAO-Al2024. (b) ZTi60- Al2024 coated with paint.

Figure 3:

XPS spectra of Ti60: (a) Ti 2p and (b) F 1 s for 9 h immersion in 3.5% NaCl solution containing 20 mM F⁻, (c) Ti 2p and (d) F 1 s for 9 h immersion in 3.5% NaCl solution containing 100 mM F⁻ (Zhao 2021).

2.2 Crevice corrosion

Crevice corrosion occurs frequently on fasteners due to the presence of stagnant solution in the small gaps. The occluded battery is formed within crevice to form an acidic environment. Afterward, the oxide film or passive film will be destroyed, thereby causing local corrosion (Huang et al. 2008), namely crevice corrosion.

The sensitivity of different metals to crevice corrosion differs, and those with a strong self-passivation ability are more sensitive. Well-known examples include titanium alloys. Hence crevice corrosion is an essential factor limiting the application of titanium and its alloys. The corrosion process is divided into the incubation period and active dissolution period. The dissolution of titanium occurs in the crevice, the oxygen concentration gradually decreases with the progress of corrosion, and the oxygen reduction reaction gradually shifts outward. Meanwhile, the passive film on the surface of the titanium alloys cracks. The Ti³⁺ formed by the dissolution of Ti is hydrolyzed to obtain Ti(OH)₂⁺. The H⁺ generated by the reaction causes the acidity of the corrosive media and continues to activate the surface of the titanium alloys, further accelerating their corrosion. Therefore, the crevice corrosion of titanium alloys present autocatalytic property.

In the cases of low oxygen content and high temperature, like deep-sea oil wellheads, the passive film on the surface of the titanium alloys is prone to damage. Simultaneously, film formed by re-passivation has poor corrosion resistance. This can also be understood with the help of a Pourbaix diagram (Pourbaix 1967). Titanium has a large corrosion area state below the hydrogen balance line. The solubility of Ti³⁺/TiO₂⁺ increases with the increase of the temperature, which expands the anaerobic corrosion area in the Pourbaix diagram and significantly increases the possibility of crevice corrosion. This also theoretically confirms the above-mentioned. Studies have found that the critical temperature for crevice corrosion of titanium is generally above 80 °C. Crevice corrosion is exacerbated by the lack of oxygen during application. The oxygen content below 1 ppm and the temperature exceeding 120 °C will cause the loss of the passive film in the crevices. The active corrosive media in the crevices makes it difficult for the passive film to repair itself and causes crevice corrosion (Rajendran and Nishimura 2007). Pang and Blackwood (2016) studied the corrosion behavior of the industrial pure titanium, Ti-6Al-4V and the grade 7 titanium in an environment with very low oxygen content (below 1 ppm). It was found that the crevice corrosion of industrial pure titanium and Ti-6Al-4V increased with the increase of temperature, and no crevice corrosion occurred in grade 7 titanium at any temperature, which is mainly due to the presence of Pd in it.

2.3 Pitting corrosion

A unique feature of titanium is its high pitting potential. Many environments can increase the potential of titanium into the passive region. But under certain conditions such as concentrated hydrochloric acid and concentrated hydrofluoric acid, the oxide film could not be retained and titanium alloys are susceptible to pitting attack. Golvano et al. (2015) analyzed the effect of pH on corrosion behavior and proved that the stability and protection of the passive film are poor at acidic pH (pH = 3.5). In such an aggressive environment, the broken passive film of the titanium alloys causes corrosion pits on the underlying bare metal. It is generally considered as the failure mechanism of these alloys (Brossia and Cragnolino 2004; Kolotyrkin 1961; Leckie and Uhlig 1966).

During the pitting corrosion process of titanium alloys, Ti+2H2O→TiO2+4H++4e− reaction causes local acidification at the corrosion area together with the migration and movement of halides on the corrosion pits. Therefore, the probability of pitting nucleation and growth rises with the increase of the halide concentrations (Brossia and Cragnolino 2004). Chen et al. (2020) investigated the pitting corrosion behavior of cpTi in different concentrations of NaF solutions. The results showed that the damage of the passive film becomes more serious as F⁻ concentrations increase. The re-passivation process of the Ti surface is hindered, reducing the corrosion resistance. Zhao (2021) recently demonstrated that as the F⁻ concentrations increase, the more aggressive to titanium alloy passivation film is (Figure 4). Casillas et al. (1994) studied the pitting potential of titanium in Cl⁻ and Br⁻ containing solutions and observed that pitting corrosion occurs at a lower potential in Br⁻ containing solution. This indicates that Br⁻ is more likely to cause pitting corrosion of titanium alloys than Cl⁻ because Br⁻ is easily adsorbed on the impurity (e.g. Al, Si and Fe) parts of the surface passive film to destroy the oxide film there (Garfias-Mesias et al.1998). Moreover, Seo and Lee (2020) used the Langmuir isotherm to predict the equilibrium adsorption constant after testing the pitting potential of Ti-6Al-4V in Cl⁻ and Br⁻ containing solutions. The result showed that the equilibrium adsorption constant of Br⁻ is higher, which further verifies the above viewpoint.

Figure 4:

Open circuit potential curves of TC4 in 3.5% NaCl solutions with different F⁻ concentrations (Zhao 2021).

Apart from external factors, pitting corrosion is also affected by the factors of titanium alloys themselves. The unsaturated metal bonds on the surface of titanium alloys will lead to the occurrence of point defects, thus forming activation centers. Ionization of the activated center is the initiation of pitting.

2.4 Hydrogen-induced cracking

When titanium alloys are in contact with the hydrogen-containing environments, part of the hydrogen atoms adsorbed on their surfaces can form molecules to escape in the form of bubbles, and the others can be dissolved in the titanium base as atom to cause hydrogen-induced cracking when its concentration reaches a certain value (Thompson and Moody 2013). The critical hydrogen content of hydrogen-induced plastic loss in some titanium alloys is shown in Table 1 (Wang et al. 2020). The factors affecting the hydrogen absorption content are considered to be the microstructure of titanium alloys, surface stress level, and temperature (Tal-Gutelmacher and Eliezer 2004). The microstructure of titanium alloys plays a decisive role in the interaction between hydrogen and titanium alloys. This is mainly due to the different behavior of hydrogen in the α and β phases of titanium, which vary significantly in sensitivity to various forms and conditions of hydrogen embrittlement (Tal-Gutelmacher and Eliezer 2004). Studies have shown that β-Ti has a relatively open BCC structure compared with the HCP lattice of α-Ti. The BBC structure allows hydrogen to have high solubility and a rapid diffusion rate in β-Ti. This makes the β titanium alloys insensitive to hydrogen-induced cracking. The solubility of hydrogen is very small in α phases, hence titanium hydrides are formed at a very low hydrogen concentration. Generally, its formation or cracking usually occurs in α phases, or at the interface between α phases and β phases (Gutelmacher and Eliezer 2005).

The formation and fracture of hydrogen embrittlement phases are the main mechanisms of hydrogen-induced cracking in titanium alloys. Ti is dissolved as shown in Equations (2)–(4). Afterward, the reaction produces H⁺ which is reduced to proton H in Equation (5). As shown in Equation (6), the reduced protons will be diffused to titanium base to form TiH_x and cause hydrogen-induced cracking. Pazhanivel et al. (2021) studied the corrosion behavior of Ti-6Al-4V in NaCl solution and proved that the complete reduction of hydrogen will produce more hydrides and accelerate the failure of the sample.

2.5 Stress corrosion crack (SCC)

Stress corrosion crack is divided into two stages. Crack initiation is first driven by surface chemistry, and then the crack propagation depends on corrosion kinetics, fracture mechanics, and material properties (Joseph et al. 2018). It could be hardly found at the early stage of its occurrence, but once stress corrosion cracking occurs, it can lead to serious safety accidents.

In the past few years, most studies were focused on two titanium alloys, Ti-6Al-4V and Ti-8Al-1Mo-1V (Martin 1966; Piper and Fager 1966; Pustode et al. 2014). Studies have shown that the stress fracture of titanium alloys is transgranular and the stress corrosion sensitivity is affected by their microstructure and grain size of the titanium alloys. For example, Cao et al. (2017) found that SCC is more likely to occur in titanium alloys containing more than 6 wt% Al. The authors suggested that Al promotes the orderly precipitation of Ti₃Al and specific positions are occupied by Ti and Al atoms in the hexagonal close-packed structure. The ordered α phases cause local plane slip (Banerjee and Williams 1986), and the shift in slip mode increases the sensitivity of SCC. The crystal orientation result of the SCC crack demonstrates that the crack propagates in the micro-textured region. The effective slip length is increased and the SCC crack propagation speed is improved by an order of magnitude. The locations are blocked by grain boundaries in the absence of micro-texture. In this case, the grain size limits the slip length. Therefore, the micro-texture has a greater impact on the SCC of the titanium alloys compared with the grain size (Cao et al. 2017; Uta et al. 2009).

In addition to the factors of titanium alloys, the sensitivity of SCC is affected by the corrosive medium to a certain extent. The stress corrosion sensitivity rises as the halide concentrations increase in the halogen-containing aqueous solutions (Mahoney and Tetelman 1976). Pustode et al. (2014) further studied the relationship between SCC and temperature of titanium alloys exposed to a hot-salt environment. The result shows that titanium alloys are not sensitive to SCC at 250 °C. The sample fractured at 300 °C and the number of fractures increased with the rise of temperature, implying that SCC has a strong dependence on temperature.

2.6 Microbiological corrosion

Titanium and its alloys show good biocompatibility. Microbes like bacteria and fungi adhering to the surface of titanium alloys could form biofilms to increase their corrosion rate (Fu et al. 2021; Wu et al. 2016; Yuan and Pehkonen 2007). This electrochemical process is called microbiologically influenced corrosion (MIC). It is usually unpredictable and could cause major economic and safety issues. Nowadays, the main areas where microbial corrosion has been found are ship bottoms and bone/dental implants.

Studies have shown that about half of MIC are caused by sulfate-reducing bacteria (SRB) (Li et al. 2010). SRB commonly found in marine corrosive environments has been reported to accelerate sacrificial anode corrosion (Liu et al. 2014) by converting SO₄²⁻ into S²⁻ (Javaherdashti 1999). S²⁻ reacts with Ti to form TiS₂ with low oxygen content like seawater. The presence of TiS₂ causes micro-pits on the surface of the titanium alloy, thus inducing the pitting corrosion (Rao et al. 2005; Anandkumar et al. 2013). Apart from SRB, the researchers (Cournet et al. 2010; Hashemi et al. 2018) also studied the effects of bacteria like Pseudomonas aeruginosa and Staphylococcus aureus on the corrosion resistance of titanium alloys. It has been reported that the presence of the above bacteria accelerates cathodic oxygen reduction and changes the passive film composition. For example, Khan et al. (2019) studied the effect of P. aeruginosa on the corrosion behavior of Ti. It is shown that the composition of the passive film is composed of TiO₂ and a small amount of Ti₂O₃. The presence of Ti₂O₃ decrease the corrosion resistance and integrity of the passivation layer. Moreover, the re-passivation behavior of titanium alloys is hindered by the anaerobic bacteria. Furthermore, the biofilms formed by bacteria changes the environment at the interface, thus causing local acidification of the alloy’s surface and leading to local corrosion (Anandkumar et al. 2017). Some aerobic bacteria will work with corrosive media to aggravate pitting corrosion like Shewanella, electrically active marine bacteria, and other seaweed organisms. Li et al. (2021) tested the effect of algae on the corrosion resistance of TC4 in a solution containing NaCl. They found that no obvious corrosion pits were found in TC4 alloy under aseptic conditions. In the Shewanella algae medium, pits appear on the titanium alloy with a depth of about 1.5 μm, mainly because algae could breathe by absorbing electrons in the titanium alloy matrix.

2.7 Corrosion fatigue

Corrosion fatigue is subjected to a combined interaction of cyclic load and corrosive media. Small damage on the surface of the material destroys the oxide film and causes cracks of the material through local stress concentration, thus resulting in a decrease of the fatigue life. Observed from only the impact of the corrosive environment, titanium alloys show good corrosion fatigue resistance due to the existence of the passive film compared with traditional steel materials (Dimah et al. 2012; Vilhena et al. 2019). As for practical application, most metal parts have certain gaps. But titanium alloys have a larger stress concentration factor and their fatigue sensitivity factor will be greater than steel materials.

As an essential parameter in corrosion fatigue, fatigue load will cause different degrees of damage to materials and even fatigue damage. Materials are mainly affected by a high load to cause corrosion fatigue. Fatigue life decreases with the increase of the load. However, they are more susceptible to environmental influences under low load. Lee et al. (1999) studied the effect of load on corrosion fatigue of Ti-6Al-4V. The results also confirmed the above statement. Fleck and Eifler (2010) indicated that the fatigue life of titanium alloys in an oxygen-free environment is lower than the fatigue life of those in an oxygen-containing environment. In addition, the fatigue strength of the sample decreases significantly with the increase of cycle life. Given the above, the combined action of mechanical load and corrosion could promote the corrosion fatigue of titanium alloys. Researchers discuss the effects on the corrosion fatigue of titanium alloys. Azevedo et al. (2015) found that the fatigue damage of the titanium alloys is mainly caused by the stress corrosion cracking mechanism at high stress levels. Zhou et al. (2019) proved that corrosion fatigue is affected by strong electrochemical reactions at low stress levels.

Many research results have shown that corrosion fatigue is also affected by the corrosiveness of the solution. In a corrosive media, the crack growth rate increases significantly. Jesus et al. (2020) recently compared the crack growth performance of Ti-6Al-4V alloy in air, artificial saliva, Ringers solution and 3.5 wt% NaCl solution, respectively. The function of crack growth rate and crack tip stress intensity factor ΔK, namely da/dn-ΔK, is obtained based on the Paris formula. The result shows that the crack growth rate increases significantly in NaCl solution, which was about 3.3 times higher than that in air. Further research on the surface morphology of the fracture found that the local defects are formed on the surface of the titanium alloys due to pitting corrosion and crevice corrosion in the NaCl solution. Afterward, the increase in local stress could lead to fatigue fracture. Apart from the above factors, fatigue performance is related to the surface condition of the titanium alloys (Leon and Aghion 2017). Chen et al. (2018) studied the implantation of Cr and Zr into TC18 by ion implantation. After implantation, the fatigue strength decreases with the increase of roughness.

2.8 Corrosion wear

The interaction between material corrosion and wear has aroused the research interest since it was first proposed by Zelder in 1949. In recent years, a series of studies on the corrosion and wear behavior are carried out in different service environments (Graves et al. 2021; Hua et al. 2021). The results have indicated that the wear performance of titanium alloys is not ideal due to the destruction of the passive film (Budinski 1991). This complicates the corrosion behavior of titanium alloys. When titanium alloys are affected by the dual effects of corrosion and wear, the interaction between the two tends to further intensify them, thereby accelerating the damage to the material (Hussein et al. 2015; Kunčická et al. 2017). Some biomedical applications like implants, especially artificial joints, will cause the stress in the contact area to increase. The passive film of the titanium alloys is destroyed and the matrix is exposed to a corrosive environment under the action of stress. Meanwhile, plastic deformation and the formation of slip bands lead to the removal of surface oxides and deteriorate wear resistance. Nakai et al. (2021) explored the effect of slip distance on the corrosion and wear behavior of TC64 titanium alloy and found that the wear amount of TC64 grows with the increase in slip distance.

As a key variable of friction corrosion, the slip rate has a significant effect on the passivation rate and the growth of passive film on titanium alloys. It was found that the worn titanium alloy part was passivated again under the sliding rate of 1 Hz and 2.5 Hz. But the worn surface could not be passivated at higher rates (10 Hz and 15 Hz) (Namus et al. 2021). Nakai et al. (2021) lately found that the friction film widely exists on the surface of 2.5 Hz but much less on the surface of 5 Hz. The increase of speed increases the interface temperature, reduce the viscosity of the lubricant, inhibit the formation of the friction film, and increase the corrosion and wear. Ultra-fine grains have an adverse effect on the wear resistance of most metals (Kucukomeroglu 2010; Wang et al. 2011), but have an opposite effect on titanium alloys. Li et al. (2014) and Ralston et al. (2010) pointed out that the existence of ultra-fine grains in titanium alloys improves the wear resistance. This is because the fine structure in the passive material is the center of the oxide nucleus, which is conducive to the rapid formation of the passive film. This clearly illustrates the interaction of chemical composition and microstructure on the friction and corrosion behavior of titanium alloys.

Apart from the above mutual promotion effect, wear corrosion also has negative interaction. It has been observed in both titanium alloys and stainless steel. Generally, this phenomenon occurs when the media is slightly corrosive, the loss due to corrosion is small, and the material loss is mainly abrasion.

Among the above corrosion forms, stress corrosion is a very harmful failure mode, which has caused huge losses. In addition, titanium and its alloys are very sensitive to hydrogen, and a considerable amount of hydrogen can be diffused into the titanium substrates. A small amount of hydrogen absorption can lead to embrittlement of titanium alloys. Once hydrogen embrittlement occurs, it will cause severe engineering accidents and lead to unpredictable losses, which seriously affect the application and development of titanium as a structural material.

3.1 Chemical composition of titanium alloys

The chemical composition of the titanium alloys also affects the performance of the passive film, which in turn affects corrosion resistance (Mandry and Rosenblatt 1972). The elemental content of several common titanium alloys is listed in Table 2 (Leyens and Peters 2003). Al, Cr, Mo, Mn, V are often added to titanium alloys. Among them, Al is the most commonly added α-phase stabilizing element, and almost all titanium alloys are added with Al. Due to the solid solution strengthening effect of Al, the tensile strength of titanium alloys is significantly improved with the increase of Al content. At the same time, Al improves the solubility of H in α-Ti and reduces the sensitivity of titanium alloy to hydrogen embrittlement (Mirza et al. 2008). But the Al content should not exceed 7%, otherwise the plasticity and toughness of the alloy will be greatly reduced. V is the most commonly added β-phase stabilizing element, but the harmful substance V ions will be released during service. Therefore, researchers have expected to replace V ions with other β-phase stable elements, such as Mo, Cr, Mn, Fe and other elements. Among them, Mo can increase the stability of the passivation film and increase the corrosion resistance of the samples. Wang et al. (2016) proved that the inhibitory effect of Mo element on the anode improves the corrosion resistance of titanium alloys, especially the crevice corrosion resistance of titanium alloys in halide solutions. Fe increases the potential difference between the β phase and the α phase, reducing the crevice corrosion resistance. Mareci et al. (2007). replaced V with Mo and Fe without affecting the corrosion resistance of Ti6Al4V. Furthermore, the cathode depolarization can be promoted by adding beneficial alloying elements, and the potential of the titanium alloys is transferred to the position (Nakagawa et al. 2002). Some noble metals, such as Pd, act as alloying elements to catalyze the reduction of oxygen and water, thereby improving cathode efficiency (Blackwood et al. 2000). The potential is shifted toward the positive direction. Blackwood et al. (1989) demonstrated that the passive film thickness is proportional to the potential drop at the titanium/solution interface. Therefore, the passive film is thickened with the increase of the potential.

In addition, the microstructure plays an important role in titanium alloys. Lu et al. (2017) found that in HCl solutions, Ti-1300 is more severely corroded than Ti-6Al-4V. This is due to the α phases and α + β phases are dissolved preferentially. Corrosion is accelerated by the galvanic cells formed by α phases and β phases.

3.2 Temperature and pressure

There are two factors for the effect of temperature on the corrosion rate. Firstly, higher temperature increases the dynamics of electrochemical reactions, thereby promoting corrosion (Hua et al. 2005). Secondly, the micro-structure of titanium alloys is changed by temperature and transformed into a more stable structure. Corrosion is only promoted with increasing temperature at low temperatures. When the temperature reaches a certain value, the corrosion is affected by two factors together, and the second aspect plays a leading role. Therefore, the corrosion rate is likely to have a maximum value at a certain temperature. For example, Gurrappa and Reddy (2005) tested the corrosion potential of titanium alloys at different temperatures. It is found that the potential shifted positively at 25 °C and a protective film was formed on the surface. The passive film is dissolved and cracked with the fluctuation of corrosion potential at 50 °C. Blasco-Tamarit et al. (2009) showed that an increase in temperate increases the possibility of galvanic corrosion of TA2 and welding titanium in LiBr solutions, and the maximum current density is reached at 50 °C. The passive film structure changes from anatase to rutile at 50–70 °C (Hua et al. 2005), which is the most stable structure of TiO₂ at high temperature. The rutile structure is denser and corrosion resistant than anatase. The corrosion rate is reduced due to this transition.

High pressure reduces the protection of the passive film on the titanium alloy’s surface, greatly increasing the possibility of pitting corrosion, which results in stress corrosion cracking (Liu et al. 2020). Liu et al. (2021) studied the corrosion potential of Ti-6Al-4V alloy at pressures of 0.1 MPa and 20 MPa, and found that the breakdown points are quite different. V element in the titanium alloy is dissolved, causing more point defects at 20 MPa. Moreover, the content of TiO₂ in the passive film is reduced and the stability of the passive film is reduced. Hydrostatic pressure breaks down the passive film, but also inhibits the secondary passivation of the titanium alloys. Hu et al. (2020) observed that the corrosion surface of Cu-Ni alloy became rough during the galvanic corrosion process and large corrosion pits appeared at 3.5 MPa. This result indicates that an increase in hydrostatic pressure promotes the local corrosion of the coupling metal.

3.3 Types and concentrations of halide ions

Apart from the effect of temperature and pressure, the types of ions in corrosive media have an influence on corrosion resistance. Halide anions migrate into the passive film, forming a titanium halide (Burstein et al. 2005). Once the molar volume of titanium halide becomes greater than TiO₂, the passive film is destroyed due to the increase of internal pressure (Ilevbare and Burstein 2001).

Different halides have different effects on the breakdown behavior of the titanium alloy’s passive film. Dugdale et al. found that the critical pitting potential in Cl⁻ containing solution is approximately 12 V, while the pitting potential in the F⁻ containing solution is 0.9 V. The negative shift of the pitting potential indicates that the corrosion resistance has decreased (Dugdale and Cotton 1964). It is noteworthy that for most passive metals, the most well-known corrosive ion Cl⁻ does not significantly corrode titanium alloys. To the contrary, F⁻ is the biggest corrosive threat. The main reason for this phenomenon is that halogen ions have a synergistic effect on the passivation behavior of titanium during the corrosion process (Yang et al. 2018). Cl⁻ ions have no effect on the chemical composition of the film. But the radius of F⁻ is smaller and it is easier to adsorb on the titanium alloy, and the oxygen vacancies in the passive film are occupied by F⁻. Especially, it greatly accelerates the anodic dissolution rate and cathodic reduction reaction rate of titanium alloys under the synergistic effect of F⁻ and H⁺ due to the formation of TiF₆²⁻ and TiF₆³⁻ complexes when pH is below a critical value (Mandry and Rosenblatt 1972; Zhao et al. 2021a,b,c). The critical pH value for corrosion titanium alloys increases as the concentration of F⁻ increases. For example, for the F⁻ concentration of 900 mg/L, the critical pH value is 4, and when the F⁻ concentration increases to 9000 mg/L, the critical pH value will correspondingly increase to 6.5. Imani and Asselin (2021) recently observed the corrosion degree on titanium alloy surfaces under different F⁻ concentrations. The study found that at the F⁻ concentration of 0.0025 mol/L, the passivation layer is basically stable, without any obvious corrosion phenomenon. The state of the passive film is also reflected in the OCP. It can be seen from Figure 4 that when the F⁻ concentrations are greater than 50 mM, the negative shift of the initial OCP indicates that the passive film is damaged (Zhao 2021).

In order to slow down the corrosion of titanium alloys by F⁻, Zhao et al. proposed to add Fe³⁺ as a corrosion inhibitor to the corrosive medium, and form the complexation of Fe³⁺ and F⁻ to reduce the concentration of F⁻ and slow down the destructive effect of F⁻ on the passive film (Zhao et al. 2021a,b,c).

3.4 Methods of fabrication

Generally, the structure obtained by ingot metallurgy is coarse, and a series of forging processes are required to obtain titanium alloy parts. The cost and production cycle are increased, which limits the large-scale popularization and use of titanium alloys (Tamirisakandala et al. 2005). The powder metallurgy method can effectively reduce the production cost of titanium alloys because of its near-net shape characteristics, and the prepared titanium alloys have better composition uniformity. Bolzoni et al. (2014) used titanium powder, Al powder and V powder as raw materials to prepare powder metallurgy Ti-3A1-2.5V titanium alloy. Compared with traditional ingot metallurgy, its strength is increased by nearly 200 MPa, but the elongation at break is only 6%. Dai et al. (2016a,b) used selective laser cladding (SLM) to prepare Ti6Al4V and found that the alloy has more α phases and less β phases. However, it was found that the corrosion resistance of the prepared titanium alloy in NaCl and 1 mol/L HCl solution was worse than ordinary titanium alloy materials in the subsequent research (Dai et al. 2016). Recent studies have found that the corrosion dissolution rate of Ti6Al4V titanium alloy prepared by powder metallurgy during long-term HCl immersion is much higher than that prepared by traditional methods. When soaking for more than 200 h, the difference between the two losses is as high as 50–70 g/m² (Pohrelyuk et al. 2021). The residual porosity of the sintered material is larger than that of conventional casting. This is most likely the reason for the poor corrosion resistance of powder metallurgy. Therefore, titanium alloys prepared by powder metallurgy are not suitable for service in strong corrosive environments. And its excellent performance in terms of strength and wear resistance can be considered as an alternative material for implants. For example, under the same wear conditions, the wear rate of Ti35Zr28Nb alloy prepared by powder metallurgy is close to Ti6Al4V, which has been widely used now. And it can be spontaneously passivated in a simulating body fluids environment, showing good corrosion resistance (Xu et al. 2020). Replacing Ti6Al4V with Ti35Zr28Nb alloy can also effectively solve the harm caused by V to the human body during the corrosion process. The research on new preparation methods of titanium alloys mostly focuses on the microstructure and mechanical properties. The powder metallurgy technology has indeed made great progress in these aspects. However, due to the large pores of titanium alloys prepared by powder metallurgy, the problem of corrosion resistance needs to be considered. At present, it is also necessary to increase the research on the electrochemical corrosion behavior of titanium alloys, especially the difference between the corrosion behavior of titanium alloys prepared by traditional methods.

4.1 Aerospace

The primary reasons to use titanium alloys in aircraft is its high specific strength and excellent overall performance. Those performance reduce the structural weight and improve flight efficiency (Zhou 2016). For example, titanium is 45% lighter than steel of the same strength, and is twice the strength and weighs only 60% of aluminum. Also noteworthy is the high temperature resistance of titanium alloys (Ma et al. 2012; Zhang et al. 2002), which enables them to be used instead of aluminum at temperatures above 130 °C. Therefore, titanium alloys are considered to be another new functional material after steels and aluminum alloys in aerospace field (Boter 1996; Mi et al. 2013).

The continuous progress and development of aerospace technology in recent years have created a much more complicated and severe service environment for aircraft parts, with temperatures potentially reaching 1400 °C (Tan et al. 2019). However, titanium begins to absorb hydrogen significantly at around 250 °C. And the creep resistance and high temperature oxidation resistance drop sharply above 600 °C in existing titanium alloys. Hydrogen embrittlement is the main form of corrosion that needs to be considered. Based on the above situation, titanium alloys used in high temperature environment are required to show creep resistance, good thermal stability and good fatigue resistance at room temperature and high temperature. It has high lasting strength and sufficient plasticity in the whole working temperature range. Titanium alloys that meet high temperature work environment are solid solution strengthened α + β type and near α type titanium alloys, such as TC4 (Ti-6Al-4V), Ti2AlNb. In recent years, researchers have carried out a series of studies on the above alloys. Dai et al. (2021) studied the corrosion resistance of Ti₂AlNb under high temperature conditions, which is most applicable to the aerospace field, and found that the titanium alloy still maintains high temperature resistance performance at 923 K, but the oxidation becomes more intense and the TiO₂ surface becomes loose and porous as the temperature reaches 1023 K. The oxide film began to fall off at a higher stress. Obvious corrosion cracks appeared on the surface, and the corrosion resistance decreased. The current solution to this problem is to add alloying elements such as Nb and Mo, or use surface technology to prepare high-temperature and oxidation-resistant thermal protective coatings on the titanium alloys surface (Boter 1996). Wang et al. (2005) prepared a Co-Ni coating on the titanium alloy surface and found that passive film can be formed in an acidic medium due to the slow dissolution rate of Ni. The high temperature corrosion resistance can be improved due to the addition of Co. Furthermore, the work by Chai et al. (2016) reported the influence of the microstructure in the Co-Ni coating on corrosion resistance and found that the grain refinement and corrosion resistance are enhanced after the addition of Co and Ni. On the basis of these studies, Adesina et al. (2020) used laser cladding technology to apply Ti-Co-Ni ternary alloy coatings to the surface of Ti-6Al-4V alloy. They explored the influence of the synthesis process on corrosion resistance. It is found that at a scan rate of 1.2 m/min resulted in a sample with the lowest corrosion current density and the best corrosion resistance.

4.2 Marine environment

Seawater compared with other corrosive environments is more complicated due to high salt content. Metals are more susceptible to corrosion in marine environment. Titanium and its alloys are less prone to corrosion in the deep-sea environment when compared to iron-based alloys (Venkatesan et al. 2004). Study by Cheng et al. (2017) further compared the electrochemical behavior of the Ti-46Al-2Cr-2Nb alloy and industrial Al 5083 in artificial seawater. The result validates that the corrosion current and corrosion rate of the Ti-46Al-2Cr-2Nb alloy were nearly 20 times lower than that of Al 5083, which indicates that the TiAl alloy has better corrosion resistance.

However, the deep-sea environment is characterized by low dissolved oxygen content, high hydrostatic pressure and fast sea water velocity. The corrosion behavior of titanium alloys is significantly different from ordinary corrosive environment (Guo et al. 2017). The biggest characteristic of deep-sea environments is the hydrostatic pressure, which increases by 1 atm with each 10 m increase in depth. The increase of seawater static pressure causes an increase in Cl⁻ activity. It promotes dissolution of the passive film on titanium alloys, changes the pitting potential, and accelerates the corrosion rate and structural failure (Ma et al. 2019). Moreover, the corrosion resistance of titanium alloys is mainly due to the dense passive film on the surface. Therefore, titanium alloys are required to have deformation resistance and Cl⁻ corrosion resistance in the marine environment. At present, alloys such as Ti31, Ti75, Ti80 and TC4 are widely used in marine engineering. Yang et al. (2018) discussed the changes in the corrosion process of the passive film on titanium alloy in a simulated seawater environment and found that the sulfide reacts with Ti to form TiS₂ and TiOS in seawater. The formation of the passive film is affected by the above reaction. The TiO₂ content in the passive film reaches its peaks and corrosion resistance is the strongest at the sulfide concentration of 2 mm/L. Self-repairing of the passive film is difficult after the film is damaged in deep-sea hypoxic environments. Titanium alloy equipments are affected by the high hydrostatic pressure, and the stress corrosion sensitivity of high-strength titanium alloys becomes higher than other types of titanium alloys. The stress corrosion of titanium alloys cause the structural failure, resulting in sudden fractures and risks of serious accidents.

In conclusion, pitting corrosion and stress corrosion crack should be the focus of attention in high chloride and hypoxic marine environments.

4.3 Biomedicine

Titanium and its alloys have a wide range of applications in the field of biomedicine (Van 1987). For example, the Ti-6Al-4V and NiTi shape memory alloys have good biocompatibility. Artificial joint prostheses, fracture internal fixators and orthodontic appliances made of those alloys are widely used in clinical practice.

However, the corrosion resistance of titanium alloy materials has not yet reached the ideal level through clinical finding. Corrosion occurs in body fluids after implantation in the human body. For the nickel-titanium shape memory alloys used in orthodontics and stents, an oxide film with crystal structures is formed on the surface after heat treatment, and pitting and crevice corrosion occurs after implanting in the body (Hegmann et al. 2007). This alloy contains high Ni content, around 50%. The occurrence of corrosion reaction after long-term implantation causes a large amount of nickel ions to be released in the body, which affects its biological performance. The Ti-6Al-4V alloy contains Al, V and other elements, and the occurrence of corrosion not only reduces its mechanical properties, but also causes problems such as implant fractures (Mirza et al. 2008). Additionally, corrosion fatigue may also occur when the alloy is used in conjunction with non-metallic materials (Riue et al.1992).

4.4 Chemical industry

Metallic equipment must operate in higher temperature, higher pressure, and more corrosive environments with the increasing amount of oil extraction. This service environment requires more corrosion-resistant materials to provide protection. Currently, the application of stainless-steel pipes is restricted. While titanium alloys, especially industrial pure titanium and TC10 (Ti-6Al-6V-2Sn), have good creep resistance and chlorine-based organic corrosion properties in high temperature environments, and are widely used alloys in the petrochemical industry.

The HCl, H₂S and H₂O gases produced during the fractionation of crude oil are highly corrosive. These gases cause different degrees of corrosion to metals, showing serious safety risks. Among them, the corrosion of sulfides on processing equipment has become a widespread topic in recent years (Houda et al. 2018).Kai et al. (2001) studied the effect of H₂/H₂S/H₂O mixed gas on the corrosion resistance of titanium alloys. It is found that there is a small amount of Ti_1-xS in TiO₂. The x value increases along with the increase in temperature, which increases the defect rate in the passive film, promotes the migration of ions, and accelerates the corrosion rate. Therefore, the main corrosion forms are pitting corrosion and stress corrosion crack caused by the destruction of the passive film.