Advanced Corrosion and High-Temperature Protection through Surface Modification and Coatings

Coatings and surface modifications have long been applied in a broad variety of areas including petrochemical, automotive, power generation, aerospace and even in the medical industry. The origins of the field date back to early society and the knowledge in this discipline has been built owing to the efforts of multidisciplinary professionals. Nowadays, surface engineering has evolved to a level of significant commercial maturity leading to a wide range of cost-effective technologies [1]. Examples of well-established techniques include anodizing, thermal spraying, carburizing, nitriding and many others [2,3]. Worldwide surface engineering industry today is a billionaire market with multiple players which foresees steady annual growth and vast possibilities for innovation in strategic sectors.

Engineering environments are normally complex and severe, usually combining a state of mechanical loading with a chemical and physical degradation component which may readily compromise the microstructure and properties of metallic alloys. Coatings and surface modifications can provide solutions to surface-related phenomena, such as corrosion, i.e., wet conditions and high temperature, different forms of wear and also fatigue. The fundamental idea behind the application of a surface engineering method is to enable an improved performance of materials in situations where the surface is exposed to environments that promote some type of damage. The goals of applying a coating and/or a surface treatment are numerous and may range from simply extending the life of a component and therefore reducing costs to promoting a specific interaction between a surface and its surroundings.

Some types of coatings naturally form as a consequence of a spontaneous reaction between the metal and its surrounding medium. Passivation in stainless steels and nickel alloys is an example of a naturally occurring thin-oxide film that provides protection in specific environments. A stable, dense and defect-free surface oxide layer may yield protection even against high temperature corrosion, acting as a diffusion barrier and therefore hindering the degradation progress. In this sense, minor additions of reactive elements, i.e., Hf, Ta, Nb, Ce, Y and others, may further improve the high-temperature response of an alloy [4,5,6].

The present Special Issue highlights important aspects related to the corrosion and protection of metals through the application of surface modification and coatings. Significant advances in the field of aerospace, oil and gas, biomaterials and modeling are addressed and successful examples of materials, coating technologies and strategies are additionally presented. Clearly, the purpose here does not cover all industries and applications where coatings are applied. For example, in energy production industries, as nuclear [7] or solid oxide fuel cells [8], coatings can optimize material performance as well.

The aerospace industry is a very important sector in our modern society that has continuously evolved and pushed the boundaries of science and technology. In the aerospace industry, the application of surface treatments and coatings has been practiced for more than five decades. Nowadays, many aircraft structures and components rely on surface treatment technologies in order to ensure protection against a variety of degradation mechanisms. As an example, in order to increase the efficiency of a turbine, the operation temperature should be as high as possible. The so-called thermal barrier coatings have experienced important evolution across the decades allowing significant improvements in turbine efficiency. These refractory-oxide ceramic coatings protect the metallic turbine blades by insulating and therefore removing the metal from the high heat flux regions [9,10].

Another family of materials that has prompted significant research interest is the aluminides, including mainly iron, nickel and titanium aluminides [11]. Research interests in intermetallic compounds have been motivated by their advantageous properties, such as outstanding high temperature oxidation and corrosion resistance, wear resistance, relatively low density and cost effectiveness. Recent strategies have managed to overcome its low ductility and brittle failure characteristics which are critical for processing bulk forms of this unique class of metallic materials [12]. Additionally, aluminides may be grown/deposited as a surface coating applied via a variety of methods [13,14], yielding high-temperature corrosion resistance in a range of hostile environments [15].

In recent years, a number of changes have occurred in the field of materials science, mostly triggered by a dynamic technological development resulting in new processes, new classes of materials, and new manufacturing methods [16]. The field of high-entropy alloys (HEAs) has emerged in the past two decades, bringing new alloying strategies. These new strategies have expanded the compositional space allowing for the discovery of novel functional properties [17]. Refractory HEAs present excellent mechanical properties at high temperature; however, the oxidation resistance is a limitation for most of the refractory elements [18]. Despite this, rutile-type oxides (Cr,Ta,Ti)O₂ seem to protect refractory alloys at temperatures as high as 1500 °C, considerably surpassing the application temperatures of chromia-forming superalloys [19]. Regarding manufacturing processes, additive manufacturing technologies took shape and developed rapidly as a commercial tool in the last decades. As opposed to traditional subtractive manufacturing, additive manufacturing accounts for a controlled layer upon layer deposition of materials to produce a solid three-dimensional component. The processes are particularly valuable for complex geometries and may be applied to a wide range of industry sectors, enabling increased energy efficiency and a lower environmental impact [20,21]. Most of the systems applied for metallic materials utilize metal powder/wire as an input associated with a point-wise energy source. Recently, some additive manufacturing methods have been adapted as a tool for obtaining coatings and surface modification [22].

Regarding the oil and gas industry, it has constantly posed countless challenges to traditional engineering alloys. The strategies for mitigating external and internal pipeline corrosion are vast, such as cathodic protection, drying, corrosion inhibitors, biocides and coatings [23]. Coatings for mild steel based on more noble metals such as aluminum and nickel are used [24], but the most promising technology is related to superhydrophobic polymer coatings [25]. They can be considered as bio-inspired coatings and have recently received great attention. Nature is a source of inspiration for scientists and has provided several smart multifunctional coatings with characteristics such as self-healing, self-cleaning, superhydrophobicity, toughness and adhesion [25,26]. This is a highly interdisciplinary field which can provide solutions to diverse corrosion problems.

Bio-inspired coatings have also been conceived in the biomaterials field. One successful example is the lipo-coat, a coating that mimics the cell’s phospholipid bilayer conferring appropriate biocompatibility to the treated surface [27]. As already mentioned, coatings and surface treatments have a proven potential to alter the biomaterial response as well. The human body is composed of different tissues which impose very complex environments. When a synthetic material is implanted in the biological environment, a cascade of events is then triggered and the surface of the device guides the course of this interaction. In this sense, the surface plays a vital role in guiding the host response. The precise course of reactions occurring upon the implantation of different classes of biomaterials is not well understood. Degradation products, i.e., corrosion and/or wear debris, may incite an inadequate immune response, leading to chronic inflammation which may result in implant failure [28,29,30].

In recent years, a range of techniques have been developed to alter the surface of implant materials and promote tissue-specific interactions [31]. Plasma electrolytic oxidation (PEO) applied to titanium and its alloys is an example of an electrochemical method of surface modification frequently used to improve biocompatibility. PEO-treated titanium alloys develop a patterned surface with appropriate roughness that provides optimum sites for osteoblast proliferation, yielding improved integration to the bone tissue [31,32]. Biomaterials also benefit from calcium phosphate (CaP) coatings, a material naturally present in the human body. In this case, interesting developments regarding CaP surface modification bring new functionalities for implants as well as a better inflammatory response and osseointegration [33,34,35].

Although the challenges imposed by different industries are substantial, the accelerated growth of information technology has recently created potential for the modeling and prediction of material behavior as well. Currently, computational tools are applied to nearly all fields of science. Currently, a number of commercial software packages for thermodynamics and kinetics calculations are available [16]. Important advancements have been made in method development to predict the degradation behavior of materials and coatings in a wide range of environments at high temperatures [16,36]. However, such tasks are usually challenging due to limited thermodynamic and kinetic data, encouraging the continued development and maintenance of such databases for improved accuracy, for the description of complex processes, and, ultimately, lifetime prediction [37].

Many aspects must be considered for the proper prediction of high-temperature behavior of coated and uncoated alloys in complex mixed gas environments, including parameters related to the environment, oxide scale, presence of a coating, interdiffusion zone and the substrate material [16]. Additionally, the lack of consistent data may render the modeling results implausible. Aimed at future progress, it is expected that a combination of experiments, computational power and increased availability/reliability of databases and mobility data should allow for a more realistic description of corrosion-related phenomena for multicomponent systems and coatings, microstructure evolution and other important high-temperature phenomena [16,37].

Knowledge on environmental and material properties allows engineers, researchers and specialized professionals to design and propose tailored coatings or engineered surfaces that match the required properties in specific applications. Therefore, the usage of coatings and surface treatments will continue to prosper by combining chemistry, physics, mechanical engineering and even medicine with materials science and metallurgy to provide smart solutions for surface-related phenomena.

In a quest for a sustainable future with a more circular economy, natural resources must be efficiently applied [38]. Additionally, as the global economy moves towards decarbonization, it is expected that some surface engineering methods will experience a downfall while new technologies may arise. In this sense, the present Special Issue intends to collect the latest developments in the field and to stimulate fruitful discussions regarding new methods, routes and/or techniques that allow for an improved response of surface-engineered materials and components.