**1. Introduction**

Corrosion of metal is a significant problem, costing worldwide industries more than \$300 billion annually. The inhibitors minimize the rate of corrosion by forming a thin adsorbed film on metal. In the last decades, much attention has been focused on the need to design and develop new and emerging materials for corrosion protection. As an example, nanomaterials, biomaterials, corrosion inhibitors, sol-gel coatings, self-healing and smart materials. Out of these, self-healing coating and corrosion inhibitors are an emerging and broad field.

A self-healing system is inspired from biological systems that have inherent ability to repair damage via healing mechanisms and is categorized into three types; namely, microencapsulation, vascular based and intrinsic materials [1]. These systems have the ability to repair the damage caused due to mechanical stress or energy and to recover their functionality using resources inherently available to them. On the other hand, corrosion can be inhibited or controlled by introducing a stable protective layer of inert metals, conductive polymers, inorganic compound or monolayers of graphitic or heterocyclic structure between a metal and a corrosive environment. A corrosion inhibitor is a chemical constituent which, when added in small amount to the metal environment, diminishes or controls and prevents corrosion. In the oil and chemical industry, inhibitors are considered as the first line of defense against corrosion.

In simpler words, corrosion can be defined as failing of materials by chemical process. Among them the most significant is electrochemical corrosion of metals, in which oxidation process (*<sup>M</sup>* <sup>→</sup> *<sup>M</sup>n*<sup>+</sup> <sup>+</sup> *ne*−) is helped by the presence of a suitable electron acceptor, sometimes referred to in corrosion science as depolarizer. In general, corrosion is a two-step electrochemical process having both anodic and cathodic sites, with flow of charges (electrons and ions), it is conventional in both wet and dry conditions. Wet corrosion is a major problem to tackle; it is a dominating corrosion at or near room temperature and in presence of an electrolyte, or even in presence of water.

Since corrosion process is a surface reaction, addition of corrosion inhibitor in very small concentration to an interfacial layer can prevent or reduce the corrosion rate of a metal exposed in aggressive environment. Generally there are three mechanisms of the corrosion inhibition as given below [2,3]:


Based on the above mechanism of corrosion inhibitors, they can be classified to three different types; cathodic, anodic, and mixed or adsorption type inhibitors. Corrosion inhibitors that cause the delay in the cathodic reaction are known as cathodic inhibitors. Similarly, the anodic inhibitors slow down the anodic reaction. Those inhibitors that affect both the cathodic and the anodic reactions are known as mixed inhibitors, and these inhibitors generally work by an adsorption mechanism and known as adsorption inhibitors. In general, inorganic inhibitors have either cathodic or anodic actions, while organic inhibitors have both cathodic and anodic actions [2] (Figure 1).

**Figure 1.** Classification of corrosion inhibitors [2].

Due to the toxicity of inorganic inhibitors, a variety of organic compounds have been used as corrosion inhibitors for the protection of steel specifically in acid medium [4]. In general, organic corrosion inhibitors are more effective than inorganic compounds for protection of steels in acid media. Organic inhibitors work by an adsorption mechanism in which the adsorption of the inhibitor molecule at the metal-solution interface results in formation of a film of inhibitor molecules to protect the surface from the corrosive environment either by physically blocking or by delaying the electrochemical processes [5]. Organic inhibitors generally contain heteroatoms (S, O, or N) and their efficiency is related to the presence of these atoms in the molecule as well as heterocyclic compounds and π electrons [6,7]. This is due to the fact that O, N, and S are found to have higher basicity and electron density and are the key active centres for the adsorption process on the metal surface.

Adsorption inhibitors protect the metal following three possible ways: (1) physical adsorption, (2) chemical adsorption and (3) film formation (Figure 1). Physical (or electrostatic) adsorption is a result of electrostatic attraction between the inhibitor and the metal surface. Physically adsorbed inhibitors interact rapidly, but they are also easily removed from the surface. The most effective inhibitors are those that chemically adsorbed (chemisorbed). Chemisorption occurs as a result of charge sharing or charge transfer between the inhibitor molecules and the metal surface. However, chemisorption is slower than physical adsorption process and is not completely reversible [8].

Film formation mechanism is based on the surface reactions of inhibitor molecules and formation of thin film on the surface with blocking both anodic and cathodic areas. Organic inhibitors are able to form a protective hydrophobic film, adsorbed on the metal surface. In fact, the polar group of the organic molecule is directly attached to metal and the nonpolar end is oriented in a vertical direction to the metal surface. Thus, they can prevent diffusion of corrosive species and establish a barrier against chemical and electrochemical attack [6].

Most organic inhibitors contain at least one functional group. The strength of adsorption of organic inhibitors relies on the charge of this group rather on the hetero atom present in the organic molecule. The structure of the rest of the molecule influences the charge density on the functional group [6]. Most common organic inhibitors belonging to different chemical families such as fatty amides [9,10], pyridines [11,12], imidazolines [13,14], other 1,3-azoles [15,16] and polymers [17] have showed excellent performance as corrosion inhibitors.

While a variety of different inorganic and organic compounds can be used as inhibitors, however, the practical application of many of those inhibitors poses risk for environmental protection standards, cost and toxicity. Thus, there is a strong need to develop efficient and environmentally friendly corrosion inhibitors. Among various classes of compounds, ionic liquids (ILs) have attracted considerable attention in recent years as "green material," because of their attractive properties such as chemical and thermal stability, nonflammability, very low or negligible vapour pressure, high ionic conductivity, a wide electrochemical potential window. They can be used as potential inhibitors whose specific interactions with metal can be tailored through choice of their amphiphilic structures or using them in various forms such as microcapsule, gel, emulsion, nanoparticles or using them in synergistic combinations. Due to high sensitivity of the metal-IL interactions, careful design and tailoring of ionic liquid materials play a crucial role for successful corrosion inhibition application.

Another promising material, graphene a single-atom-thick sheet [18], a flat monolayer of carbon atoms tightly packed into hexagonal honey comb lattice in which carbon atom is *sp*<sup>2</sup> hybridised has been identified as a next generation inhibitor material for shielding of metal from corrosion as it possesses matchless properties such as excellent thermal and chemical stability, high strength, chemical inertness, permeability to molecules and gases, extremely high aspect ratio, high theoretical specific surface area. From the point of permeability, the hexagonal network of carbon atoms in graphene is so dense that no known material can penetrate through it. However, there are a number of critical challenges related to application of graphene on various metals, which needs significant attention. To date, graphene coatings on metals have been employed using chemical vapour deposition (CVD) or transfer techniques involving high energy consumption, special expensive tools, high temperatures, careful treatments and multistep processes. Such techniques (CVD or transfer) are cumbersome, uneconomical and not very practical for large scale application. On the other hand, there are significant advantages if graphene can be deposited from preformed ink, which is reproducible, and can be used to coat objects of any dimensions as in conventional paints or coatings.

Thus, the key focus of this review is to present and discuss some important results on the physico-chemical properties of the emerging corrosion inhibitors based on IL and graphene to advanced coating applications. Before the presentation of these results, a precise description of the synthesis, characterisation, structure-properties relationship and performance of IL and graphene based inhibitors suitable for anticorrosion applications is given below.
