reversed on exposure to environment (oxygen and water). On the exposed metal, oxides, sulfates, and carbonates exist [3–4]. Corrosion science as a subject has been around for many years in the textbooks, and surely its relevance has increased now. Education of corrosion and corrosion mitigation makes the environment safer and more sustainable.
1.2 Economic and Social Aspect of Corrosion
The incurred monetary losses and negative effects on environment geared the current ad on‐going researches in the field of corrosion. To sum up the total monetary loss due to corrosion, cost studies have been carried out in several countries. The first significant work on cost of corrosion was presented as a report by Uhlig in 1949, estimating the annual cost of corrosion as US$5.5 billion [5]. However, comprehensively the first study on losses incurred due to corrosion was conducted in the United States in late 1970s. In the year 1978, US$70 billion were wasted, equivalent to approximately 5% of gross national product (GNP) of that year [6]. The US Federal Highway Administration (FHWA) published a breakthrough study back in 2002, estimating the direct corrosion cost associated with USA’s industrial sector. The study was conducted by NACE International initiated the study as part of Transportation Equity Act for the 21st Century (TEA‐21), having a Congress mandate. The estimated direct cost of corrosion annually is $276 billion, which implies GNP’s 3.1% [7]. This estimation is solely inclusive of the direct costs pertaining to maintenance. Other expenditures after production loss, negative environmental effect, disrupted transports, fatalities, and injuries were computed to be as much as the direct costs. Similarly, some countries conducted corrosion cost studies. These countries were Australia, United Kingdom, Japan, Germany, Kuwait, Finland, India, China, and Sweden. It was inferred that annual corrosion costs was 1–5% of the national GNPs. The recently published material relates the global economic losses due to corrosion, summed up by NACE International in 2016 as $2.5 trillion, which is 3.5% of global GDP [8–10]. The Central Electrochemical Research Institute calculated the cost of corrosion in India by NBS input/output economic model for 2011–2012. The direct cost was US$26.1 billion or 2.4% GDP. The cost avoidable was US$9.3 billion or 35% direct cost of corrosion. The indirect cost was US$39.8 billion or 3.6% of IGDP [11]. NACE International according to the latest global studies estimated Indian cost of corrosion to be GDP’s 4.2% [12]. Beyond the cost of corrosion financially are the indirect costs like loss of opportunities and natural resources, potential hazards, etc. A project constructed using building material unable to withstand its environment for the estimated design life, the l resources are being needlessly consumed at later stages for maintenance and repair. Wasting the already depleting natural resources is a direct opposition to the increasing emphasis and demand for sustainable development in order to safeguard for future generations. Along with the wastage of natural resources, weak constructed structures pose threat to lives and well‐being. Huge safety concerns have been established in regards with the accidents that might happen in case of corroding structures. A single pipeline that fails, a bridge that collapses, a derailed train compartment due to corroded track, or other accidents is one among numerous that cause enormous indirect losses and huge public outcry. According to the market sector considered, the indirect losses might make up to 5–10 times the direct loss.
1.3 The Corrosion Mechanism
Corrosion occurs by formation of an electrochemical/corrosion cell (Figure 1.2).
This particular electrochemical cell comprises of five parts.
1 Anodic zones
2 Cathodic zones
3 Electrical contact between these zones
4 An electrolyte
5 A cathodic reactant
Inside this electrochemical cell, electrons depart from anodic to cathodic sites. The charged particles, ions, move across the conducting solution to balance the electrons flow. Anions (from cathodic reactions) move toward the anode and cations (from the anode itself) drift toward the cathode. Resultantly, anode corrodes and the cathode does not. There also exists a voltage/potential difference amidst anode and cathode. Numerous discrete micro cells develop on the metal surfaces, due to the constitutional phase difference, from stress variations, coatings, and imperfection levels like dislocations, grain boundaries, kink sites, or from ionic conductivity alterations or compositional changes in the conducting solution. The corrosion process is chemically spontaneous oxidation of the metal on reaction with the cathodic reactant. Every similar cell reaction results from a pair of simultaneous anodic and cathodic reactions going on at identical rates on the surface of metal.
Figure 1.2 An electrochemical cell.
1.3.1 Anodic Reaction
At the anode, the metal corrodes. The anodic reaction is the oxidation of a metal to its ionic form when the electric charge difference exists at the solid–liquid interface. Generally, anodic reaction is an oxidation reaction of a metal to its metal ions, which passes into conductive solution:
(1.1)
where “n” is the metallic valence, e− is the electron, M is metal, and Mn+ its metalion.
1.3.2 Cathodic Reactions
The cathodic reaction involves the environment and can be represented by the following reaction:
(1.2)
where R+ is the positive ion present in the electrolyte, e− is the metallic electron, and R0 is the reduced species. Based on the environment, many cathodic reactions and electron consuming reactions are possible. The main reactions are as follows.
The anaerobic acidic aqueous environment
(1.3)
In the anaerobic alkaline aqueous environment
(1.4)
In the aerobic acidic aqueous environment
(1.5)
In the aerobic alkaline aqueous environment
(1.6)
