Application of Functional Coating in Delaying the Corrosion of Titanium Alloys: A Review

Abstract

Titanium alloys are utilized in various industries due to their remarkable strength-to-weight ratio, corrosion resistance, and biocompatibility. Nevertheless, their corrosion susceptibility is influenced by temperature, pressure, manufacturing methods, electrolyte properties, mechanical handling, alloy composition, surface passivation, constituent phases, and environmental conditions. Their corrosion resistance is largely determined by the passive film’s density, the chemical composition, and the base metal’s microstructure. These alloys face diverse corrosion challenges in different applications, necessitating the development of effective protective measures. This review examines the types of corrosion, the factors influencing it, and the specific issues encountered in various applications. Furthermore, it provides an overview of using functional coatings to enhance titanium alloys’ corrosion resistance. Based on recent research findings, the review evaluates multiple coatings, including thermal spray coating, chemical vapor deposition, cold spray coating, laser surface engineering, and laser surface alloying. It discusses the protective mechanisms of these coatings, such as barrier formation, passivation, and inhibition of corrosive species. Additionally, challenges related to coating adhesion, durability, and performance under extreme conditions are addressed. This analysis aims to shed light on the current state of functional coatings for titanium alloys and identify potential directions for future research to achieve more robust and durable corrosion protection.

1. Introduction

The science and technology of materials aim to grant access to specialized knowledge in this field while sparking interest in pioneering research that results in the creation of new materials and, consequently, proper protection methods to achieve high economic impact [1]. Metals, such as iron, aluminum, copper, and magnesium, and their alloys are used in many structural, marine, and aircraft applications and cultural heritage, etc. Though valuable for their physical attributes like stiffness and high strength-to-weight ratios, these metals are highly prone to corrosion in harsh conditions. Corrosion is the primary cause of energy and material depletion, with reports indicating that the annual estimated losses from corrosion amount to hundreds of billions of dollars, roughly equating to 2–4% of the gross national product (GNP) [1, 2]. The economic toll of corrosion exceeds $100 billion per year in the United States alone. This encompasses expenses related to protective coating applications, surface inspection and repair, and hazardous waste disposal. A common method of shielding metals from corrosion involves applying protective films or coatings, which can also enhance the desired properties of the substrate, such as mechanical strength, optical appearance, and bioactivity, through chemical modification [1, 2]. As a matter of practical concern, the corrosion of metals is more problematic than that of other materials. Titanium, a lustrous, lightweight, non-magnetic, low-density material with good strength, and excellent corrosion resistance, exhibits a coefficient of thermal expansion relatively lower than that of steel, but somewhat lower than that of aluminum, easily fabricated and reinforced by deformation processing and alloying [3, 4]. Ti-6Al-4 V is the most widely used titanium-based alloy due to its superb combination of strength, corrosion resistance, and toughness and broadly finds application in fields like aerospace, medical implantation, and various industries, and its exceptional resistance to corrosion, strong mechanical properties, minimal elasticity, and enduring thermal stability make it highly desirable [2]. Titanium, known for its high reactivity, develops a thin layer of titanium dioxide film, typically ranging from 1.5 to 10 nanometers thick when it encounters oxygen or moisture. This oxide layer tends to be tightly bonded to the surface and remains stable in diverse conditions, contributing significantly to titanium’s exceptional resistance to corrosion [4]. This oxide layer formed is significantly more protective than stainless steel and often performs better in environments that cause pitting and crevice corrosion, such as seawater, chlorine exposure, organic chlorides, and others, however, exhibits inferior corrosion resistance in acidic and low-oxygen high-temperature environments, favoring the rapid deterioration of titanium in anhydrous conditions, and if the passive oxide film cannot be maintained or regenerated, self-healing becomes difficult, and corrosion occurs [4, 5]. The main types of corrosion detected on these alloys encompass overall corrosion, crevice corrosion, anodic breakdown pitting, galvanic corrosion, hydrogen-induced damage, and stress corrosion cracking (SCC) [6]. To overcome these potential issues, specific alloys containing palladium, nickel, and molybdenum were introduced by previous researchers. These elements facilitate cathodic reactions, aiding the system in achieving a passive state and simultaneously boosting its resistance to reducing acids like sulfuric, hydrochloric, and phosphoric. They also raise the critical temperature threshold for crevice corrosion in seawater. However, incorporating these elements is costly [4], giving rise to another method to enhance the natural thickness of TiO₂ through surface treatments. High-temperature oxidation encourages the formation of a chemically resistant, highly crystalline TiO₂ layer, increasing its thickness and resulting in a primarily amorphous phase. It improves pure titanium’s corrosion resistance to match that of more expensive alloys. Thermal oxidation, chemical oxidation, and ion implantation are other possible techniques [4, 6]. Therefore, finding a way to improve the corrosion resistance of titanium to the attack of fluoride ions in acidic environments is of engineering significance.

Considering these circumstances, significant attention has been devoted to investigating the corrosion performance of titanium alloys, particularly concerning localized corrosion, in recent years. The corrosion challenges encountered by titanium alloys during application predominantly encompass uniform and localized forms, such as galvanic corrosion, crevice corrosion, pitting corrosion, hydrogen-induced cracking, stress corrosion cracking, microbiological corrosion, corrosion fatigue, and corrosion wear, posing a considerable threat to the service life and performance of these alloys.

This paper review aims to examine types of corrosion and various influencing factors that significantly impact the corrosion process of titanium alloys, including temperature, pressure, manufacturing techniques, properties of the surrounding electrolyte, mechanical handling, alloy makeup, surface passivation, constituent phases, environmental conditions, and specific corrosion issues across various applications. Additionally, it provides an extensive overview of surface treatments employed to enhance the corrosion resistance of titanium alloys.

2. Corrosion behavior of titanium alloys

2.1 Uniform corrosion of titanium and its alloys

Uniform corrosion is a type of corrosion attack that is uniformly distributed over the entire exposed surface of a metal, especially in acidic conditions at high temperatures. In strong, hot reducing acids, the oxide layer dissolves, exposing the metal to oxidation and forming soluble trivalent ions (Ti Ti³⁺ + 3e⁻). When hot acid contains dissolved oxygen, Ti³⁺ ions are oxidized to Ti⁴⁺ ions and the latter subsequently hydrolyzes to form insoluble titanium dioxide [2]. This layer may slow down corrosion spread. Without oxygen, severe corrosion in a hot, reducing acid solution leads to a matte silver-gray surface formation due to titanium hydride formation in the layer. The resistance of titanium to corrosion greatly depends on the durability of its protective oxide layer, which can vary based on the surrounding conditions [4] such as seawater, the human body, fluoride-containing environment, chloride-containing solutions, bromide-containing solution, and nitric acid [2, 7].

2.2 Localize corrosion of titanium and its alloys

Common types of corrosion in localized titanium alloys are fretting, galvanic, pitting/crevice, hydrogen-induced cracking, stress corrosion cracking, microbial corrosion, corrosion wear, and corrosion fatigue. Titanium exhibits resistance to various types of localized corrosion, such as pitting (although its resistance is reduced in iodide or bromide solutions), intergranular attack, galvanic corrosion (although it may experience hydrogen absorption if serving as the cathode), and stress corrosion cracking (except dry methanol environment) [8]. This insidious form of corrosion takes place within cracks and crevices stemming from adhering process stream deposits or scales, metal-to-metal joints, gasket-to-metal flange, and other seal joints [6, 8] in the presence of small amounts of multivalent metals such as nickel, copper, or molybdenum, acting as a cathodic depolarizer and tends to drive the corrosion potential of the titanium in a crevice in a positive direction in Figure 1b [9]. A schematic diagram of the processes involved in crevice corrosion on Ti is given in Figure 1a.

Figure 1.

(1a) Schematic depiction of crevice corrosion and the chemical processes involved [8], (1b) crevice corrosion under a deposit [9].

Dissolved oxygen or other oxidizing species in the solution are depleted in a restricted volume of solution in the crevice exterior shown in Figure 1b [9]. Metal dissolution is followed by cation hydrolysis, which results in precipitation of corrosion product deposits and acidification of the crevice interior. Once the crevice anolyte pH is low enough, metal oxidation can also couple to proton reduction within the crevice. This is a self-sustaining reaction, producing more protons by cation hydrolysis while other protons are reduced in driving corrosion [8]. This sets up an electrolytic cell with the metal in the crevice acting as the anode and the metal outside the crevice acting as the cathode. Metal dissolves at the anode under the influence of the resulting current [9]. The process of proton reduction results in the production of hydrogen gas and its absorption into the metal. There is a chance of auto-passivation occurring through Ti(iv) compounds if both active and passive sites are present in the enclosed region [8]. A typical example of crevice corrosion, as depicted in Figure 2, was investigated by [4] following exposure to anaerobic seawater with and without CO₂. They observed that in the absence of CO₂, none of the Ti alloys experienced corrosion at 60°C. However, at 80°C, commercial Grade 2 began to show signs of crevice corrosion, with the number of affected sites increasing with temperature, reaching a maximum penetration depth of 90 μm at 200°C. Ti6AlV Grade 5, on the other hand, resisted crevice corrosion until 200°C, at which point all 40 sites became active, though with a maximum penetration depth of only 30 μm, significantly less than observed with Grade 2 alloy. In contrast, Grade 7 Pd remained immune to crevice corrosion, particularly under a 3 Nm torque, as higher torque is known to increase the risk of such corrosion. Presumably, the presence of Pd serves as an effective catalyst, enhancing the open-circuit potential and allowing for the formation of a thicker and more stable TiO₂ passive film [10].

Figure 2.

Typical crevice corrosion specimens after exposure to seawater at elevated temperatures for 7 days [10].

The duration of a corroding crevice can be categorized into three distinct phases: initiation, propagation, and re-passivation. Initiation necessitates the presence of chemically aggressive conditions, such as low pH or a low concentration of dissolved oxygen [8], involving the inward diffusion of oxygen in the titanium alloy developed within the crevice, which forms a stable, adhesive, and protective layer mainly consisting of rutile layer (TiO₂) and to some extent is able to protect these alloys against corrosion because of its thermodynamic stability, chemical inertia, and low solubility in the body fluids. However, severe corrosion can occur when this passive oxide layer is mechanically disrupted [7]. This rutile (TiO₂) layer is un-protective and allows oxygen diffusion to the metal at high temperatures. If the passive film stays intact, such that the corrosion rate is low, the corrosion potential of a titanium alloy in an anaerobic environment can be expected to lie on or just below the hydrogen equilibrium potential [10]. Unlike stainless steels, as seen from the Pourbaix diagram for titanium in Figure 3, Ti can potentially suffer crevice corrosion in anaerobic seawater (pH 8.2), i.e., there is a large corrosion regime below the hydrogen equilibrium line [7]. Oxide layer properties depend on composition and layer thickness, temperature, pressure, and oxidation time. The mechanical properties of oxide layers also change with the temperature of oxidation. Oxide layers are an effective corrosion barrier and enhance mechanical properties [8].

Figure 3.

Pourbaix diagram for titanium [10].

From the diagram, the dot-dash line shows the calculated hydrogen equilibrium potentials at 200°C. When combined with the temperature dependence of pK_w, the position of the hydrogen equilibrium potential in seawater moves from point X (solid circle) at 25°C to point Y (open circle) at 200°C. In addition, the solubility of Ti3 + TiO2+ ions increases with temperature, such that the anaerobic corrosion zone will expand to the right and could encompass point Y [10].

Several coating techniques, including plasma electrolytic oxidation (PEO), plasma spraying, laser cladding, surface electro/electroless plating, physical vapor deposition (PVD), chemical vapor deposition (CVD), electrochemical deposition, plasma spray, sol-gel process, anodizing, chemical conversion coatings, gas-phase deposition, laser surface alloying/cladding, organic and inorganic coatings, are employed to improve the high-temperature oxidation resistance, hardness, and wear resistance [11, 12, 13].

3. Surface engineering/modification

These are engineering processes employed for the modification of engineering surfaces for improved service performance; some engineering materials like the Ti-6Al-4 V alloy have surface limitations which include, poor tribology, poor microhardness, and high tribo-corrosion rates, etc., hence the need for processes that can alleviate these surface challenges. Surface engineering methods are technically attractive and economically viable, to improve the superficial layer of the material [14]. Surface engineering techniques are used in several engineering sectors, including but not limited to aerospace, automotive, aircraft, electronic, military, biomedical, petrochemical, and chemical fields. The modified surface can be achieved by metallurgical, chemical, and mechanical means.

The desired properties and characteristics of surface-engineered surfaces include:

• Enhanced surface mechanical properties

• Improved tribological (wear and microhardness) and tribe-corrosion properties.

• Improved oxidation, thermal oxidation, and sulfidation resistance.

• Improved electronic and electrical properties.

• Improved corrosion resistance.

3.1 Chemical vapor deposition

Chemical vapor deposition (CVD) is a powerful technology for producing high-quality solid thin films and coatings. Although widely used in modern industries, it is continuously being developed as it is adapted to new materials [15]. The conventional approaches to making Ti films are physical vapor deposition (PVD) and chemical vapor deposition (CVD). The former mainly uses ultra-pure Ti as a target material and employs sputtering to deposit Ti on the substrate. This approach is highly costly and only suitable for the relatively planar substrates of simple geometry. In contrast, CVD utilizes the chemical reaction of low-cost gaseous Ti-containing precursors to form a solid Ti deposit on the substrate [16]. Chemical vapor deposition (CVD) is a versatile process widely utilized across various applications including the production of coating, powders, fibers, and consistent components. It enables the deposition of metals and composites comprising non-metallic substances like carbon, silicon, carbide, nitride, oxides, and intermetallic compounds. Thin film and coating synthesis find the primary application of CVD, and this method has effectively addressed several challenges. In the CVD process, a precursor gas is introduced into a chamber and flows over a heated substrate, leading to the chemical reaction in the vapor phase and subsequent deposition of thin films onto the surface [14]. Today, CVD synthesis is being pushed to new heights with the precise manufacturing of both inorganic thin films of 2D materials and high-purity polymeric thin films that can be conformally deposited on various substrates [15]. Ref. [16] performed chemical deposition (CVD) of metallic titanium film at low temperatures using a new precursor. The results revealed that at 620°C, the deposited Ti film consists of individual Ti nanoparticles, as depicted in Figure 4a. As the temperature is raised to 640°C, these nanoparticles gradually merge, forming a continuous Ti film with a thickness of 1.1 μm, as shown in Figure 4b and b′. Further increasing the temperature to 700°C results in the formation of a denser Ti film, as illustrated in Figure 4c. The thickness of this dense Ti film notably increases to 3.1 μm at 800°C, as seen in Figure 4e and e′. Therefore, it is determined that the minimum temperature required to produce a dense Ti film should not be below 700°C.

Figure 4.

SEM images of the surface and cross-section of the Ti films deposited on molybdenum wires at different temperatures: a, a’) 620°C; b, b’) 640°C; c, c’) 700°C; d, d’) 750°C; e, e’) 800°C. EDS spectrums on the surface of the Ti films deposited on molybdenum wires at different temperatures: a”) 620°C; b”) 640°C; c”) 700°C; d”) 750°C; e”) 800°C [16].

3.2 Thermal spray coating

Thermal spray encompasses a range of coating techniques where small particles, metallic or non-metallic are propelled onto a surface to create a layered coating. As these particles hit the substrate, they adhere through mechanical interlocking. This method accommodates various materials, including ceramics, metals, polymers, and composites, due to its flexibility in processing temperatures and speeds [17]. The molten or partially molten feedstock material, propelled by a flame, is dispersed onto the substrate, typically in forms such as wire, rod, powder, or solution/suspension. Figure 5 provides a diagrammatic depiction of this process. Upon collision with the substrate, the fully or partially melted particles flatten and create splats. Through mechanical impact and subsequent solidification, the coated material becomes interlocked with the substrate. Thermal spray methods facilitate the deposition of various materials and enable swift recoating of worn sections [18]. These coating processes alter the surface microstructure of the substrate without incurring a relatively high development cost. Thermal spray coatings encompass several categories of related manufacturing methods but have one thing in common, which is the size of the feedstock material that is heated and sprayed on the substrate, that is approximately 100 nm to 1 μm. Depending on the specific method of thermal spray that is used, particle temperatures and velocities of 4000 k and 1000 m/s, respectively, could be achieved [19]. The thermal spray process is one of the most efficient and cost-effective methods of enhancing the surface structures of engineering materials. “Several Thermal spray processes have been developed in the last decade” (high-velocity oxygen fuel spraying (HVOF), high-velocity air fuel spraying (HVAF), atmospheric plasma spraying (APS), vacuum plasma spraying (VPS), detonation gun spraying (DGS), wire arc spraying (WAS) [20], and they are usually categorized according to their energy source as shown in Figure 6. The molten material, in the form of droplets, appends on a substrate and solidifies at a rapid rate to form a thin “splat” [21]. Splat networks consist of a matrix of splats created by the molten material’s high-velocity impact on the substrate’s surface. Splat networks can consist of voids, un-melted particles, and large and small splats.

Figure 5.

Formation of splat networks on thermal coatings methodologies [18].

Figure 6.

Classification of the thermal spray process [18].

3.2.1 High-velocity oxygen fuel spraying (HVOF)

The HVOF technique operates by containing combustion gases and particles within a chamber under high pressure to generate a fast-moving stream. Combustible gases like propane, acetylene, and oxygen are introduced into a combustion chamber where they are ignited [17]. The HVOF method harnesses the combustion of fuel with oxygen to generate heat and kinetic energy for heating and propelling feedstock powder (coating material). In this process, fuel types like propane, kerosene, or acetylene, along with oxygen, are introduced into the combustion chamber where they blend, ignite, and create intense heat and pressure, enabling a rapid gas flow through the nozzle [7]. The feedstock powders are injected radially or axially into the jet by a carrier gas (usually argon or nitrogen) and accelerated through the barrel to deposit onto a substrate [17]. After heating and acceleration, the particles collide with the substrate in a molten, semi-molten, or solid-state and undergo plastic deformation, eventually forming a well-adhered and dense coating [7]. This process results in higher coating densities and improved adhesions; this is due to relatively high in-flight particle velocity and low melt or semi-melt temperatures. Titanium coatings deposited via HVOF under optimized conditions typically exhibit around 4% porosity and contain approximately 14% oxide content. Enhanced HVOF systems have been devised to minimize titanium coating oxidation. These adapted HVOF systems, also known as warm sprays, are widely discussed in the literature. A gas shroud mechanism is incorporated into these systems to diminish particle oxidation by introducing an inert gas, thereby limiting the presence of entrained air and oxygen available for particle reactions [17]. Typically, traditional single-stage HVOF setups consist of a high-pressure combustion chamber paired with a converging–diverging nozzle. Feedstock particles are introduced into this system, heated, and propelled by both subsonic and supersonic gas flows and then made to impact on the substrate which is usually around 300 mm from the nozzle exit as shown in Figure 7, [21]. HVOF processes can produce coatings containing a smaller amount of oxide inclusions, high hardness, and high amorphicity when compared to the APS and TWAS processes [22]. The industry considers the HVOF process as one of the most efficient means of high-quality hard metal and carbide-based cermet coatings with desired bond strengths. Further improvement and modifications in the spray methods have led to further enhanced surface microstructures and properties [23]. When compared to plasma spraying and arc spraying, coatings created through HVOF spraying technology exhibit reduced susceptibility to phase alteration, oxidation, and particle decomposition [7].

Figure 7.

HVOF spray process schematic view [19].

3.2.2 Atmospheric plasma spraying (APS)

The APS process is a thermal coating method that utilizes a thermal plasma to achieve high impact velocities and very high temperatures (> 10,000 K), and the APS process achieves high deposition rates and can produce coatings with varying thicknesses. The high energy density and the high temperature of plasma flow make it possible to deposit high refractory materials on the surface of substrates, which would have been difficult to achieve using other methods [24]. The APS method is commonly employed because of its extensive applications and versatility in diverse technologies such as aero engines, particularly for combustion chambers, a coating of stainless-steel components, pump applications, and a coating of metal structures. The APS involves injecting feedstock powder particles into a plasma jet using a carrier gas and spreading it over the substrate surface as “SPLATS” [25] , shown in Figure 8. Numerous research endeavors have employed APS for depositing pure titanium feedstock, resulting in coatings with porosity levels reaching up to 10.2%. Additionally, both intra-splat cracks and elongated cracks along the interface between the coating and substrate were observed [17] during deposition via APS and results in titanium aluminide coatings undergoing phase and compositional alterations, yielding titanium oxide and titanium nitride. Despite this, these coatings exhibit greater density and superior bond strength when compared to those deposited from pure titanium feedstock, largely due to the aluminum phase filling cracks and pores. The APS technique has been utilized for depositing composite coatings of Ti-6Al-4 V and hydroxyapatite. These composite coatings aim to combine the mechanical robustness and resilience of titanium alloys with the biocompatibility of hydroxyapatite [17].

Figure 8.

Plasma spraying setup [25].

Ref. [26] characterized plasma-sprayed titanium coating on stainless steel and concluded that titanium coating modified by post-heat treatment created a more cohesive microstructure and formation of the second phase subjected to limited oxidation during plasma spraying, resulting in the adhesion of titanium coating to the stainless steel. As the annealed samples underwent greater oxidation, the titanium coating layers primarily comprised pure titanium along with minor quantities of titanium oxides. The findings suggest that heat treatment of titanium coatings on implants could enhance adhesion for clinical applications, but it requires optimization to prevent excessive oxidation and the formation of a porous layer (Figure 9).

Figure 9.

SEM image of a cross-sectioned sample of the titanium coating (T) on stainless steel (S) [26].

3.2.3 Vacuum plasma spraying (VPS)

The vacuum plasma spraying is quite like the atmospheric plasma method, but in the VPS process, the plasma spray is done in a vacuum and features a traditional plasma spray torch, the nozzle of which has been adapted to endure the high-pressure expansion resulting from the plasma jet entering the low-pressure spray chamber [17]. VPS provides an inert atmosphere which could prevent the oxidation of feedstocks through a closed chamber with reduced pressure, this is why it is most suitable for the fabrication of refractory-metal disilicide coatings [27]. The VPS processes are done in inert atmospheres with a pressure of less than 1 atm. Spray forming using VPS has been employed in the production of Ti-6Al-4 V alloy components. Traditionally, components from this alloy are manufactured through methods like casting, forging, and powder metallurgy, which have drawbacks due to the reactivity of titanium alloys. The VPS forming process integrates melting, rapid solidification, consolidation, and welding into a single step, thus obviating the need to repeatedly subject reactive titanium alloys to high temperatures. Furthermore, VPS forming offers several advantages over conventional techniques, including a finer-scale microstructure, reduced segregation, and the capacity to fabricate intricate shapes with layered or graded structures [17]. Ref. [28] the researchers examined the microstructure and mechanical properties at room temperature of both as-sprayed and heat-treated samples of the Ti-6Al-4 V alloy. They noted that the sprayed structure is comprised of distinct lamellae (splats) alongside splat boundaries and pores. Additionally, they observed that the microstructure displayed needle-like martensite and a fine lamellar or plate-like α phase, which formed during rapid cooling within the N2 atmosphere of the atomization chamber shown in Figure 10. The composition of the coating remains unchanged from the original compositions [29]. There is a massive reduction between the plasma jet and oxidative environment; this enables the VPS to process a more controlled process with greater uniformity and less contamination in the surface coatings [30]. Because of the controlled atmosphere of the VPC technique, it is more expensive than the APS system [31].

Figure 10.

Micrograph of a cross-section of the plasma-atomized Ti-6Al-4 V powder showing a mixture of martensite and fine α plate-like structures [28].

3.2.4 Detonation gun spraying (DGS)

Detonation gun spraying (DGS) makes use of the detonation of hydrocarbon fuel to heat and accelerate powder particles [28]. The DGS process is well known for fabricating coatings of high hardness, low porosity, and outstanding adhesive strength with compressive residual stress [29, 30]. During the DGS process, the feedstock powder is highly plasticized and accelerated with high velocities, impacting upon the surface of the substrate and solidifying rapidly to form a slat, and the coatings on the substrate are created as a result of the impact and the deformation of fine grains of the powder [31]. Contrasting with alternative thermal spraying methods, the DGS process involves propelling particles accelerated by the detonation wave, which then impact the substrate surface at elevated velocities ranging from 800 to 1200 m/s. The prevailing notion is that such high particle velocities lead to the formation of a uniform and compact coating [32]. Ref. [23] the tribological properties of Ti-6Al-4 V were enhanced using detonation spray coatings, and tungsten carbide-cobalt (WC-Co) ceramic coatings were deposited on the surface of the substrate. The results indicated an enhanced surface with superior tribological and hardness properties (Figure 11) [34].

Figure 11.

Basic concept of the shot control method [33].

3.2.5 Wire arc spraying (WAS)

Wire arc spraying (WAS) is a process that has been used in a variety of applications such as wear-resistance coatings, anticorrosive coatings, and dimensional restorations. An electric arc is formed at the point of interception between two electrodes, this leads to their melting, and a compressed air jet causes the impaction of the atomized droplets on the surface of the substrate [35]. In the WAS process, the atomized droplet spray is created by the formation of a direct current arc between two consumable conductive wires. High-temperature and atomized gas breaks up the molten material on the wire tips and projects in the form of droplets [36]. The process of WAS is an in-expensive thermal spray deposition method, low operational costs, and high spray efficiency making it a choice process for spraying large areas (Figure 12) [38].

Figure 12.

Wire spray process [37].

3.3 Cold spray coating (CSC)

The cold spray (CS) coating process impacts solid powders with high velocity on the surface of substrates. Particles adhere to the surface through plastic deformation when the impact velocity exceeds a threshold value. Unlike other thermal processes (HVOF, WAS, and VPS), the cold spray process does not make use of thermal energy to adhere particles on the surface of substrates, and in this process, powders (1 to 50 μm) are impacted on the substrate surface with a speed of about 500–1200 m/s. With high kinetic energy, the coating powder particles adhere to the substrate through plastic deformation via an adiabatic shearing process, thus forming a dense coating [33]. CS is vigorously used to repair metallic structures, it has been noted that the use of CS to restore a damaged or a corrosion resistance layer application reduces the potential for corrosion damage, and this is because coating with very low porosity, low oxygen content, and fine-grained microstructure is obtained (Figure 13) [40].

Figure 13.

Illustration of the cold spray process [39].

The first cold spray principle that came into use was a low-pressure cold spray, which suffered some setbacks due to voids and porosity on the deposited layers. The underlined shortcomings of the low-pressure cold spray system gave birth to the high-pressure cold spray system (HPCS). The HPCS system mitigated the problem of voids and high porosity levels on substrate surfaces, enabling the deposition of highly consolidated layers with enhanced mechanical properties. Also, the HPSC systems produce high strain rates which generate ultrafine grain structures at the particle boundaries [41]. Regarding size, cost of available commercial equipment, achievable impact velocities, and temperatures, there is a considerable difference between the two processes. The scope of the LPCS is hence limited. The LPCS has a pressure range of 0.3 to 1.0 Mp while the HPCS has a pressure range of greater than 1 Mp. Since coating formation in the cold spray process only relies on the plastic deformation of powders, only metallic feedstock is suitable for the process [42]. Ref. [43] carried out cold spray coatings on Ti-6Al-4 V, and silicon carbide (SiC) cermet was deposited on the surface of the substrate. Results indicated no decomposition, decarburization of SiC, and phase transformations. In conclusion, a Ti-6Al-4 V surface with enhanced micro-hardness value was achieved.

3.4 Laser surface engineering

Laser surface engineering processes are a group of techniques that make use of lasers in the modification of engineering surfaces. Lasers are potent devices used in the modification of engineering materials to improve their micro-hardness, tribological, thermal, and corrosion-resistant properties. Lasers are ideal for the modification of engineering surfaces because they can perform many surface treatments with high precision. Lasers can be focused on the substrate surface to produce a wide range of treatments which includes laser heat treating, laser melting, laser surface texturing, and laser surface alloying. In laser surface modification, the rate of solidification is rapid and is typically between 10³ and 10⁵ K/s, and this helps in eliminating the formation of unwanted equilibrium intermetallic compounds between incompatible metals/alloys [39]. Laser surface processing operations can be controlled by a combination of power density and interaction time; process parameters play essential roles in the engineering material’s design and final surface finish [44].

3.4.1 Laser surface heat treatment

They are processes that make use of lasers in the heat treatment of engineering materials; heat treatment is the controlled heating and cooling of metallic engineering materials to alter their mechanical, physical, and metallurgical properties without changing the shape of the material or melting the material. Heat treatment is often associated with increasing the strength of materials. Laser beam power, laser beam diameter, heat intensity distribution across the beam, the absorptivity of the beam energy (material surface), scanning velocity of the laser, and the thermal properties of the treated material are the technological parameters that affect laser surface heat treatment. Because of the small nature of the beam diameter, laser heat treatment (LSHT) is most suitable for treating a localized area of materials. The process is usually achieved without the means of external quenching [45].

3.4.2 Laser melting

The laser melting method’s primary objective in surface treatment is to enhance characteristics like wear, erosion, and corrosion resistance by creating hard, uniform, and extremely fine microstructures on the material surface. This is achieved without altering the chemical compositions. Laser melting of metallic materials is widely applied in the industry because of their precision of operation and a resultant fusion zone with high depth/width ratios. The absorption of the high-energy beam from lasers results in a rapid temperature rise in the surface of the substrate; the high beam intensity can exceed the melting point of the material, thereby forming a melt pool on the surface. Metallurgical changes occur on the surface of the treated substrates in the form of grain refinement, supersaturated solid solutions, and fine dispersion of particles [45]. Ref. [46] improved the tribology and hardness properties of Ti-6Al-4 V by laser melting under ultrasonic vibrations.

3.4.3 Laser surface alloying

These are methods that introduce external alloying elements on the surface of substrates; this is done using a high-energy laser beam as the thermal source. In laser surface alloying (LSA), an alloyed zone is formed on the top surface of the substrate by melting the alloying elements. Rapid solidification leads to the formation of excellent metallurgical bonded coatings with fine microstructures [47]. LSA processes have been employed in the improvement of various surface properties like hardness, wear, corrosion, tribo-corrosion, and thermal oxidation. In LSA processing, laser radiations in the form of thermal energy interact with the material surface, resulting in several complex phenomena such as melting, intermixing of components, solidification, and microstructure evolutions [48]. The enhancement of the metallurgical and mechanical properties of engineering surfaces by LSA is usually associated with highly concentrated solid solutions; nano-sized phases and intermetallic compounds are found on those surfaces. Intermetallic compounds are highly advantageous when synthesized on the surface of materials because they retain their structures up to their melting point [48]. LSA is considered one of the best processes in surface alloying because of its ability to obtain excellent surface properties without affecting the bulk substrate (Figure 14) [48].

Figure 14.

Illustration of the laser alloying process [48].

4. Conclusion

This review has comprehensively examined the current advancements in the application of functional coatings for delaying the corrosion of titanium alloys. Titanium alloys, widely utilized in aerospace, biomedical, and marine industries, suffer from corrosion-related challenges that can significantly impair their performance and longevity. The application of functional coatings has emerged as a promising solution to mitigate these issues, offering enhanced protection against corrosive environments. Various types of functional coatings, including chemical vapor deposition, thermal spray coating methods such as high-velocity oxygen fuel (HVOF), atmospheric plasma spraying (APS), cold spray coating, vacuum plasma spraying (VPS), detonation gun spraying (DGS), wire arc spraying (WAS), and laser surface engineering methods such as laser surface heat heating, laser melting, and laser alloying are considered. Chemical vapor deposition, unlike highly costly physical vapor deposition that makes use of ultra-pure Ti as a target material and employs sputtering to deposit Ti on the substrate, utilizes the chemical reaction of low-cost gaseous Ti-containing precursors to form a solid Ti deposit on the substrate and find applications in the production of coating, powders, fibers, and consistent components. The thermal spray techniques offer means to create coatings or components with precise control over phase composition and microstructure. Characteristics of the feedstock material, such as morphology and particle size distribution, significantly influence the resulting coating microstructure. For instance, APS tends to produce coatings with noticeable secondary phases, making it more suited for depositing titanium aluminide or composites rather than pure titanium. While VPS generates coatings devoid of secondary phases but at a higher cost, necessitating low-pressure conditions. This method has been utilized for fabricating spray-formed, near-net-shape products. Wire arc spray, on the other hand, yields titanium-based coatings with higher purity compared to powder spraying due to the wire’s lower specific surface area per volume. This process stands out from others as the feedstock is melted before being propelled toward the substrate rather than being melted after acceleration offering a cost-effective, versatile, and efficient alternative to APS and VPS processes for many coating applications, particularly those requiring high deposition rates, material efficiency, and ease of operation. Incorporating a gas shroud into APS and wire arc spray serves as an economical alternative to VPS, facilitating the attainment of high-purity titanium coatings. HVOF and detonation spraying yield titanium-based coatings with a comparable microstructure, characterized by density and some secondary phases, albeit less pronounced than APS coating due to high-velocity processing. Warm spray, a modified HVOF process, allows the formation of secondary phases by introducing inert gas into the system to reduce air entrainment or lower the combustion gas temperature, offering a more versatile, cost-effective, and environmentally friendly alternative to HVOF for certain coating applications, particularly those involving heat-sensitive materials or where reduced oxidation is critical for coating quality. Unlike other thermal processes (HVOF, WAS, VPS), the cold spray process does not make use of thermal energy to adhere particles on the surface of substrates, and in this process, powders (1 to 50 μm) are impacted on the substrate surface with a speed of about 500–1200 m/s. Laser surface engineering such as laser heating, laser melting, and laser alloying is used in the modification of engineering materials to improve their micro-hardness, tribological, thermal, and corrosion-resistant properties. Laser heating alters the material’s properties without changing or melting the shape of the materials, laser melting creates hard, uniform, and extremely fine microstructures on the material surface without altering the chemical compositions, and laser alloying introduces external alloying elements on the surface of substrates through high-energy laser beam as the thermal source with rapid solidification leading to the formation of excellent metallurgical bonded coatings with fine microstructures. Despite these advancements, challenges remain in achieving long-term stability and uniformity of the coatings, particularly under dynamic operational conditions. Future research should focus on optimizing coating formulations, exploring novel materials, and developing more efficient application techniques. Additionally, there is a need for standardized testing methods to assess the performance of these coatings in real-world environments. Conclusively, functional coatings represent a vital strategy for enhancing the corrosion resistance of titanium alloys. Continued research and development in this field hold the potential to significantly extend the service life of titanium-based components, thereby contributing to the sustainability and economic efficiency of various industrial sectors.