constitute a great economy. From the very early time, it was presumed that the structures made from metals and its alloys are long lasting for hundreds of years. But they deteriorate as time passes. This takes so much dead full and dangerous form to those large‐scale industrial plants, like chemical processing, and electrical power plants shut down as a result of corrosion. All these enhance the problems in economy and lead to losses in many ways.
The percentage of corrosion happened and damage caused by it can be monitored at time before it starts happening by the application of multiple techniques. The main purpose of monitoring corrosion is done because it helps to know the working state of equipment or predicting remaining life of materials and to know locations where defect is occurring, getting good service conditions, specific remedies, and corrosion rates with variables. By knowing all these parameters, we can easily administer corrosion control schemes [10–12]. The key role for corrosion to happen or not can be decided by environments. We can understand the term environment as the integrated surrounding in contact with the metallic structures. Some basic points to keep in mind before describing environments are its physical state (gas, liquid, or solid), chemical composition, constituents, pH, presence of impurities, ions present, temperature, and velocity [13]. So for corrosion to happen, we have to study two components like materials and environments. Further when corrosion is discussed, it is important to think of a combination of a material and an environment. On the other hand, the aggressiveness of an environment cannot be considered without taking metal into consideration. In short, we can surely say that the corrosion performance of the metallic structure can be calculated on to which it is subjected, and the aggressiveness of a surrounding/environment built upon on the material exposed to that environment. Various agencies are engaged to calculate the cost of corrosion in different countries including USA, UK, Australia, Japan, Germany, Finland, Kuwait, Sweden, China, and India. The most common thing among all countries finding is that annually cost of corrosion aligned in between 1 and 5% of the gross national product of each country.
Latest report characterizes direct and indirect corrosion cost of metallic structures in the United States, where total direct (infrastructure – $22.6 billion, utilities – $47.9 billion, transportation – $29.7 billion, production and manufacturing – $17.6 billion, government – $20.1 billion, total – $137.9 billion) and indirect cost (cost of labor attributed to corrosion management activities, cost of the equipment required because of corrosion‐related activities, loss of revenue due to disruption in supply of product, and cost of loss of reliability) including total is estimated at $276 billion per year, which comprise of 3.1% of the 1998 US gross domestic product. The data in terms of cost was determined by scrutinizing 26 industrial sectors, where presence of corrosion was expected, and extrapolating the results for a nationwide estimate [14]. From literature, the word monitoring must not be confused with inspection, as the use of electrical methods can be said under monitoring while measurement via nonelectrical methods such as gravimetric analysis comes under inspection or detection [15, 16].
Multiple definitions of corrosion monitoring have been applied since corrosion inhibition came into effect and dominates in Europe after United States. The most accepted definition by authors is that, “It is the organized measurement of the corrosion or deterioration of assets with the aim of assisting the knowledge of corrosion process and getting report for corrosion control.” This clearly explains how we can get important information relating the assistance procured in the operation of a corrosion monitoring program [17]. In another definition Roth well described, “As the estimation of the deterioration of a material which happens through any factor such as chemical reaction, electrochemical, environmental or biological.” This explanation put forth the fact how corrosion reactions and surrounding are interrelated [18]. The most compact definition comes when any technique if used to know or measure the evolvement of corrosion. This definition although is least explanatory [19].
2.2 Methods and Discussion
2.2.1 Corrosion Monitoring Techniques
The progression of corrosion precepts, i.e. how lengthily any structures made of metals can be safely operated at specific conditions. Monitoring procedures object to know assertive possibilities in order to elongate the life and forbearance of valuables meantime enhancing defense and diminishing restoration costs. Some key points that are observed during corrosion monitoring are as follows:
1 The failure can be predicted on knowing the deteriorating processes.
2 By correlating the changes taking place and their aftermaths on system corrosively.
3 By getting knowledge of particular corrosion problem and its controlling factors such as temperature, pressure, pH, air flow rate, and many more.
A wide variety of corrosion monitoring techniques have been employed, which are divided into two categories:
1 Destructive methodsGravimetric analysisPotentiodynamic polarization techniqueElectrochemical impedance spectroscopyLinear polarization technique
2 Nondestructive methodsRadiographyUltrasonic testingEddy current/magnetic fluxThermography
3 Destructive MethodsGravimetric Analysis The feasibility of the process can be reviewed in literature published by NACE, American Society for Testing Materials (ASTM), and other organizations [20, 21].This method is simplest, inexpensive, and effective method for monitoring the corrosion rate in any suspected system or structure. It is supposed to be accurate and versatile as involves simple measurement. Here the specimen/sample/coupon of material is allowed to expose with environment for a specified duration and then removing the studied sample for further analysis. The basic quantity, which is resolved from corrosion coupons, is loss in weight taking place over the period of exposure to the aggressive surroundings. Expected parameters of single coupon, which have been taken in account for effective corrosion monitoring are presented in Figure 2.1. It provides direct measurement of general corrosion rate [22]. The studied coupons can be exposed to any kind of aggressive environment such as high temperatures, liquid corrosives, different gases, multiple soils, and the atmospheric conditions. The coupons are available in different geometries such as strip, disc, weld, scale, U‐bends, C‐rings, or stressed (Figure 2.2). However, the most common form of coupon is the metal strip used for equipment surfaces. Coupon samples can be exposed in duplicate/triplicate or multiple batches allowing various numbers of coupons made up of different materials at a specified location.Figure 2.1 Parameters of single coupon.This analysis was carried out in a thermos‐stated water bath for different time durations ranging from 4 to 12 hours but generally 6 hours can be considered standard as per ASTM designation G1‐90. Here, metal coupons were freshly prepared, which further can be suspended in 250 ml beakers containing 200–250 ml of aggressive/test solutions and allowed to maintained temperature in the range of 20–100°C. The specimens were immersed in triplicate, and average corrosion rate was calculated. The corrosion rate in mpy was calculated using equation:Figure 2.2 Different shapes of metal coupons. (a) Strip/rectangular shape coupon, (b) rod/cylindrical shape coupon, (c) disc shape coupon, (d) flash disk coupon.In the given equation, “W” is weight loss in mg; “ρ” is the density of metal specimen in g/cm3; “A” is the area of specimen in cm2, and “t” is exposure time in hours [23, 24].Potentiodynamic PolarizationTo carry out these techniques, polarization properties of the metal‐surrounding system of interest are measured [25]. The basic theory behind is that polarization curves are acquired by polarizing a working electrode potential comparative to a reference electrode availing external current supplied by way of a counter electrode in a conventional electrochemical cell arrangement. This causes a big problem in getting selection of reference electrode for the measurement of potential. In this investigation, the Tafel constants, i.e. ba and bc, are obtained from the slopes of the linear portions present in anodic and cathodic (Figure 2.3) theory, which explains the corrosion mechanism. Further, the corrosion rates can