Introductory Chapter: Corrosion

1. Introduction

Corrosion can also be termed “metallic cancer” because it is an irreversible process that leads to the loss of billions of dollars. It involves the deterioration and consequently loss of solid metallic structure due to the chemical (dry gases, moisture, liquid, ionic solutions, microbes, etc.) or electrochemical (micro-cell formation like Daniel cell) reactions resulting from the potential difference in the structure and presence of a suitable electrolyte (salt-water) [1]. The process of corrosion is a surface phenomenon whose rate depends upon various factors such as temperature, the presence of harmful chemicals in the environment, ionic species, as well as humidity. Once the surface is breached the process of deterioration continues within, thereby making the structure vulnerable to stress and load. The combination with acid in the environment generated via., possible chemical reactions speed up the process resulting in the reduction of the life span of metal structure [2]. Corrosion can be sub-divided into many forms such as pitting, galvanic, intergranular, stress, and others. All these can affect the structural integrity by weakening the metallic units within any construction by corroding rods/wires, water pipelines (leakages), metallic bases, electrical units, etc. Few of these examples are shown in Figure 1.

Figure 1.

Effect of corrosion on different metallic structures.

These corrosive reactions change the microstructure of metal which results in the loss of its elasticity, mechanical and tensile strength converting them into flaky and brittle units. The losses incurred due to corrosion directly effects the gross domestic production (GDP) of all the countries. In 2018, the corrosion protection industry was estimated to be around 2.5 trillion (USD) and is expected to cross 3 trillion (USD) by the end of this year [3]. Proper use of corrosion prevention measures can help to refrain from such severe damages and can partially avoid the loss in economy, environmental pollution, direct and other indirect losses associated with corrosion [4].

The most popular remedies involved in corrosion protection are the use of cathodic protection, anodic protection, corrosion inhibitors, and protective coatings. Among them, utilization of corrosion inhibitors (chemical barrier) and protective coatings (physical barrier) is mostly focused on by a larger population around the world [5]. Coatings provide a physical barrier between the harmful environment and the metal surface which enhances the lifespan of metallic structures. A number of coatings that have been developed for corrosion protection include metallic coatings, chemical conversion coatings, organic and inorganic coatings, nanocomposite coatings [6]. However, such coatings have limited applications in complex design and high-rise infrastructures. Thereby coating with rapid curing ability, strong adhesive abilities, and stability against the harsh environment (exposure to atmospheric moisture, UV radiations, and other pollutants) are being researched. On the other hand, corrosion inhibitors are chemical additives that are added in low concentration to the destructive media, and timely they tend to suppress the corrosion progress [7]. These chemicals are classified as cathodic, anodic, or mixed inhibitors depending on the inhibition of the type of corrosion. The use of renewable resources in the field of corrosion prevention in the form of coatings and inhibitors is gaining significant importance. The chapter describes the types of corrosion, mechanism, and its control through synthetic/renewable resources based on synthetic coatings, micro to nano-coating, and corrosion inhibitors.

2. Corrosion

Corrosion of steel is a spontaneous electrochemical process that takes place in the presence of a solution containing dissolved oxygen. This process includes the delocalization of metal ions through oxidation into the solution at the anode (active area) and mobilization of electrons through reduction of metal to an acceptor such as oxidizing agents, or oxygen or hydrogen ions at cathode (less active area) [8].

The mechanism of corrosion involves the release of electrons from the metallic surface into the electrolyte in the presence of oxygen (Figure 2). This process occurs due to the tendency of metals to return to their natural oxidation state. The reactions occurring at both the electrodes can be expressed as follows:

Figure 2.

Mechanism of corrosion.

At anode:

$$\mathrm{Fe}\to {\mathrm{Fe}}^{+2}+{\text{2e}}^{-}$$E1

$${\mathrm{Fe}}^{+2}\to {\mathrm{Fe}}^{+3}+{\mathrm{e}}^{-}$$E2

At cathode:

$$\frac{1}{2}{\mathrm{O}}_2+{\mathrm{H}}_2\mathrm{O}+{\text{2e}}^{-}\to {\text{2OH}}^{-}$$E3

The deterioration of metallic surfaces depend upon the different forms of corrosion, which is caused due to different type of corrosive environment (Figure 3). Dry corrosion, also known as chemical corrosion, is the type of corrosion that occurs in the absence of moisture or water and metal oxidizes only due to the atmosphere. Therefore, this may also be referred to as, an oxidation process sustained by atmospheric oxygen without a liquid solution. At encompassing temperatures, most metals have slow oxidation rates [9]. Galvanic corrosion (GC), also known as bimetallic corrosion, is an electrochemical process where two different metallic materials are connected electrically in a corrosive environment. In this type of corrosion, one metal (the anode) corrodes preferentially while the other metal (the cathode) remains protected [10]. Pitting corrosion (PC) is the most destructive type of corrosion in which the attack of corrosive ions is localized that results in the formation of pits. This type of corrosion results in the failure of machines without much of a weight loss. The process of pitting takes a longer period of time to initiate, however, once the pitting is initiated it penetrates into the section at an accelerated rate [11].

Figure 3.

Different types of corrosion.

Intergranular corrosion (IC) is a localized form of corrosive attack that is preferentially along the grain boundaries or areas adjacent to them. The corrosion activity occurs at the grain boundary area since it is electrochemically different from the bulk. This process is mainly observed in stainless steel [12]. Waterline corrosion (WC) is an oxidation process that occurs when one part of the metal is submerged in water and another part is in contact with air. Water tanks are often prone to this type of corrosion [13]. Stress corrosion (SC) occurs when tensile stress and corrosive environment work together, often at elevated temperatures. In this type of corrosion stressed area of metal is anodic in respect to the unstressed area of the metal. This corrosion is not visible prior to fracture; therefore, it results in catastrophic failure [14]. Microbiologically induced corrosion (MIC) is a type of corrosion in which metal deteriorates through the metabolic activity of microorganisms. The common bacteria that cause MIC are acid-producing bacteria, sulfate-producing bacteria, and iron-reducing bacteria [15].

2.1 Corrosion protective coatings: combating mechanism

Corrosion cannot be completely eradicated but overcame by using protective coatings and corrosion inhibitors. Protective coatings are developed to retard the corrosion rate to protect metallic substrates [16]. These coatings are applied using several techniques such as roller, moving belt, or brush technique. They are functionalized with the help of various organic, inorganic, hybrid, or metallic layers to enhance their performance [17]. Highly protective coatings provide an impenetrable barrier protecting the substrate from the aggressive environment (Figure 4).

Figure 4.

Corrosion protection mechanism of polymeric and modified polymeric coatings.

Modified polymeric coatings provide enhanced corrosion protection as compared to the simple polymeric coatings [18]. Various types of coating utilized for protection purposes are shown in Figure 5. Metallic coating means protecting the metals with the help of metal-coating. These coatings are applied to the substrates for several reasons but among all corrosion protection is major [19]. Various methods are used to apply these coatings on a substrate such as metalizing, electroplating, vapor deposition, hot dipping, cladding, etc. [20]. Depending upon the metal used to coat the substrate, metallic coatings can be divided into two categories: Anodic and cathodic coatings. In anodic coatings, anodes are made up of the alloys that are electrochemically more active than the base metal as a result of which the anodic metal depletes at a faster rate. These anodic coatings act as a physical obstruction between the corrosive environment and the base metal thereby protecting the base metal. These anodic metals can also be called “Sacrificial anodes”. Zinc, magnesium, and cadmium are well non-sacrificial anodic metals that provide protection to steel [21]. While in cathodic coatings, the coating metal is selected in such a manner that it remains electropositive with respect to the base metal. For example, copper is used to coat steel.

Figure 5.

Types of coatings used to combat corrosion.

Chemical Conversion coatings, also known as surface passivation, are produced through the chemical and electrochemical reaction of metal. Through these coatings, the surface of a metal is modified such that it possesses desired porosity. These types of coatings are more adhesive as there is a chemical bond and intermediate layer between underlying metal and coating [22]. They are formed by immersing a metallic substrate in a chemical solution. Various types of conversion coatings are available. Phosphate coatings are produced on steels by dipping them in an appropriate phosphate solution. The thickness of these coatings depends upon the porosity of the coatings as it forms. These coatings increase corrosion resistance, absorb lubricant, promote adhesion, and enhance the appearance of the substrate. There are three types of phosphate coatings a) Iron phosphates, b) zinc phosphate and c) manganese chromium phosphate [21]. Chromate conversion coatings (CCCs) are generally formed by chemical or electrochemical treatment of metals and their alloys in a solution containing hexavalent chromium [Cr (VI)] and trivalent chromium [Cr(III)] ions with other components. These coatings form a complex chromate film over the entire surface of the metal. They are used on aluminum, magnesium, zinc, copper, cadmium, etc., [23]. Anodized coatings are formed by converting the workpiece of metal into an anode. This is usually done in order to form an oxide coating to increase the performance of the surface [24]. Polymeric coatings are widely applied for decorating as well protecting purposes. They act as a corrosion barrier between the underlying metal and corrosive media. These coatings consist of pigment, polymer, corrosion inhibitors, additives, etc., [25]. The protection provided by these coatings depends upon their ability to form highly resistant pathways between the cathodic and anodic areas on the surface of metal [26]. Acrylic, vinylic, epoxy, polyurethane, alkyd (oil-based) coatings are some of the examples of organic coatings used in corrosion protection [27]. With the advancement of technology, the need to introduce specialized coating with highly advanced functioning continues to increase. Various types of speciality coating are being investigated these days, such as Flame retardant coatings, nano-coatings, nanocomposite coatings, organic–inorganic hybrid coatings, etc. All these coatings are tailored according to their end-use and the type of environment they will be applied in [28].

Corrosion prevention coatings highly utilize petro-based products which are high cost, toxic, and constantly depleting. Constant research is being carried out to formulate better strategies that can meet the environmental and economical requirements. Renewable resources are environment friendly, less expensive, and naturally available. Renewable resource-based coatings are called “green”.Figure 6 shows the different polymeric coating materials transformed from renewable resources.

Figure 6.

Polymers based on various renewable resource.

These green materials are highly employed in the field of corrosion protection in the form of corrosion-resistant coatings, inhibitors, pigments, composites, etc. Starch, cellulose, lignin, tannic acid, vegetable oils, and Cashew Nutshell Liquid (CNSL) are most commonly used for the synthesis of green coatings (Figure 7) [29].

Figure 7.

CNSL derived polymers.

Apart from using solely the polymeric coatings and relying on their chemical protective abilities, the formulations of nanostructures within the coating units have resulted in materials with higher corrosion protection efficiency [30, 31, 32]. These nanostructures can be formulated via., simple chemical reactions or by technologically advanced techniques like lithographical techniques [33]. The addition of inorganic or organic nanoparticles or units to improve corrosion protective performances has also been employed. Especially the inorganic nanoparticle dispersed within the polymeric matrix enhances the electrochemical stability, toughness, strength, and provides a tortuous pathway to the corrosive ions. The nanostructured surface coatings also tend to have higher surface hydrophobicity and scratch hardness [34]. Further, the in-situ synthesis of nanostructured components is also followed by many industries. The in-situ synthesis provides additional connectivity between the inorganic unit and organic unit, thereby generating well cross-linked high-density coatings [35, 36]. Due to all these value-added properties, nanocomposite coatings are getting more and more attention. In order to further enhance the efficacy of nanocomposite coatings, inhibitor-incorporated hybrid coatings are also considered innovative corrosion remediation systems [37].

2.2 Corrosion inhibitors

Pure metals and their alloys tend to react chemically and electrochemically with the corrosive environment to produce a stable compound. In this process, metals lose their mechanical strength and elasticity thereby becoming weak. Several strategies have been investigated which can retard or minimize or completely stop the cathodic or anodic or both reactions. Among them, utilization of corrosion inhibitors is a famous technique [7]. The chemical substances which when added in a small amount to the corrosive environment slow down/reduce the corrosion rate are called corrosion inhibitors. Inhibitors function through the adsorption of ions/molecules on the surface of the metal. They can drastically reduce the reaction rate either by decreasing/increasing the anodic and/or cathodic reactions or by decreasing the diffusion rate of reactants or decreasing the electrical resistance of the metallic surface. They can be easily applied and offer numerous advantages owing to their in-situ application without disturbing the process [38].

3. Corrosion testing methods

Two types of corrosion-resistant tests are generally used: a) salt spray test, b) Electrochemical impedance spectroscopy (EIS). Salt spray test (ASTM B117) is a standardized testing method conducted for the evaluation of the extent of corrosion resistance or protective coating. This test is generally carried out for 8-3000 hrs depending upon the coating in presence of 5% NaCl solution with pH between 6.5–7.2 [39]. EIS technique (ASTM G106) measures the impedance of the coating with the help of small amplitude, alternating current (AC). This AC signal is scanned at different frequencies to generate the spectrum for the electrochemical cell (the specimen) under the test [40].

4. Computational fitting and programming

Computational approaches like density functional theory (DFT), classical molecular dynamics (MD), Monte Carlo (MC) simulations, and others are becoming more and more preferred for corrosion studies. In order to improve the quality of the results, it is mostly combined with the experimental data. Particularly to verify the concept of corrosion inhibitors before performing the multiple experiments these studies were performed to find the thermodynamics, kinetics, energy levels (HOMO, LUMO, and Frontier molecular orbitals), bandgap energies, active sites, etc. All these theoretical calculation methodologies could be employed by artificial intelligence in the future to provide the platform which can create new and advanced corrosion inhibitors [41]. The complementary data with the experimental results puts the potential ground for the inhibitor to be explored further for corrosion inhibition applications. Even the small variations in the chemical structure which can result in better inhibition results can be evaluated through these computational approaches. Better pictorial view with the knowledge of possible interaction site, adsorption strength, and other quantum parameters proved to provide in-depth insight about the inhibition mechanism as done by different researchers in the area [42, 43].

5. Conclusion

Corrosion is an electrochemical process that causes metals to degrade at an increasing rate. Various types of corrosion can never be stopped completely but various methods can be applied to decrease the rate of corrosion. These methods include the utilization of coatings as well as corrosion inhibitors or a combination of which tends to protect metals from the attack of the corrosive environment thereby increasing the lifetime of metals. A lot of work is going on to build corrosion-resistant materials but more discussions over the topic and other advanced technologies which can improve the performance of the materials should be encouraged. The involvement of computational aided technologies can further help in reducing the search or designing of effective inhibitors without doing wet-lab experiments. The combined results of computational simulations and traditional electrochemical characterization techniques (impedance spectroscopy, variation in potential, corrosion current, rate of corrosion, and coating capacitance values) may further improve the understanding of this natural but unwanted phenomenon.