Organic Corrosion Inhibitors

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].

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 be extracted by extrapolating the linear portions of the obtained curves to intersect at the natural corrosion potential.These Tafel plots can be made by giving a scan in the range of 250 mV below Ecorr to 250 mV above Ecorr with scan rate of 0.1 mV/sec. In the curve, applied potential is present on Y‐axis, while logarithm of measured current density along X‐axis ( Figure 2.3). In next step, a straight line is allowed to cap along the linear portion of the anodic and the cathodic curves and further it is extrapolated to Ecorr. The intersection point can be named as corrosion current (icorr) [26]. Figure 2.3 Presentation of anodic and cathodic Tafel curves and their extrapolation.The % IE was calculated from the measured Icorr values using the relationship:Electrochemical Impedance SpectroscopyThis special technique was designed to dodge severe depreciation of the bared surface of the structure studied and was widely used for examining the corrosion of a working electrode [27]. The monitoring process involves application of frequencies with low amplitude sinusoidal voltage wave to outcome disturbance signals from working electrode. The percentage of corrosion can be analyzed by current response of the frequency or voltages. Specifically, it is generally monitored by giving AC potential to an electrochemical setup and alternatively getting current value via cell.As we are knowing the concept of electrical resistance, it can be defined as the ability of a circuit element to resist the flow of electrical current. So as per Ohm’s law, electrical resistance can be defined as the ratio between voltage, E, and current, I. Consider on application of sinusoidal potential excitation, we can get an AC current signal. This obtained current signal can be summed up as sinusoidal functions or Fourier series. The electrochemical impedance is specifically measured applying a modest excitation signal to get cell’s response in a pseudo‐linear manner as shown in Figure 2.4.If the excitation signal is expressed as a function of time (t), the equation takes form like,Here, in equation, Et is the obtained potential at time t, Eo is the amplitude of the signal, and ω is designated as the radial frequency. The interrelationship between radial frequency (ω) who is with unit of radians/second and frequency (f), which is expressed in hertz, is:For a linear system, the response signal for current, It, is shifted in phase (Φ) and has a different amplitude than Io,An expression resembling to Ohm’s Law permits to determine the impedance of the system as follows:where, K is sin (ω t)/sin (ωt+ Φ).So impedance can be measured in two forms: magnitude (Zo) and phase shift (Φ). Also in addition, the plot between Et (X‐axis) vs It (Y‐axis) was made and oval named as “Lissajous Figure” was obtained as resultant ( Figure 2.5). The figure obtained was than analyzed as impedance measurement before the application of modern electrochemical instrumentations. Figure 2.4 The sinusoidal current response in a linear system on application of potential. Figure 2.5 The making of Lissajous figure.From Euler’s theorem, the expression of impedance can be written in complex form as:So the potential can be best described as:While the current response can be recorded as,Putting these values in impedance formula, the complex equation is formed as,The complex equation is composed of a real and imaginary components, who are plotted on X‐axis and Y‐axis in graph forming “Nyquist Plot” ( Figure 2.6). The impedance can be best presented as an arrow or vector with dimensions of |Z|, whereas the angle made in between this vector and X‐axis, commonly known as “Phase Angle.”The most common equivalent circuit, which has been in use to sculpt corrosion of exposed metal in liquiform electrolyte, is called Randles circuit as presented in Figure 2.7.Where RΩ is the solution resistance, due to the presence of the electrolyte between the reference and working electrodes, polarization resistance Rp and Cdl or CPEdl is the double‐layer capacitance or double‐layer constant phase element (CPE). Another plot to mark presentation of impedance is the Bode Plot ( Figure 2.8), where the impedance is plotted with log frequency on the X‐axis and both the absolute values of the impedance (|Z| = Zo) and the phase‐shift on the Y‐axis. This plot can give frequency information. Figure 2.6 Typical Nyquist plots for a Randles equivalent circuit with Cdl CPEdl with N = 0.8 (red dots). Figure 2.7 A typical Randles plot.For the recent analysis, the impedance is mainly measured with amplifiers or frequency‐response analyzers, which are consider being faster and are also more convenient than impedance bridges. The basic principle involves interpretation of the equivalent resistance and capacitance standards in provision of interfacial aspect. This technique is precise and intermittently in usage for evaluating amalgamate charge transfer criterion to get knowledge of double‐layer arrangement. The mathematical value of Rp obtained from electrochemical impedance spectroscopy can be considered more perfect compared to other monitoring techniques. The graph obtained such as Nyquist and Bode gives better understanding of corrosion procedure happening.Linear Polarization TechniqueThis electrochemical technique, which commonly also known as linear polarization resistance, is the only corrosion monitoring technique in its type method that permits measurement of corrosion rates directly across real time. So, it is fast and a nonintrusive method that needs an association in between metal reinforcement to get assess of on‐going corrosion in structures [28, 29]. One disadvantage of it is that it is only confined to electrolytically conducting liquids. While its response time and data quality are far more superior compared to other corrosion monitoring techniques. It can be harvested to get current–potential (i–E) domain, which can be monitored by applying polarization resistance at a very small voltage differences generally less than 30 mV, above and below its corrosion potential [30]. The obtained current response is linear over narrow range of corrosion potential. So the slope of this current–potential curve is defined as polarization resistance (Rp) whose value is constant. As per Stern and Geary in 1957, current value can be obtained by given equation, where Rp is inversely proportional to the instantaneous corrosion rate, at some conditions [31]. Figure 2.8 Typical Bode plot. β a and βb are obtained Tafel constants from Tafel plots, polarization measurements for the studied system. In the last phase, the corrosion rate of the structure can be calculated via Icorr.The numerical value of ΔE/ΔI is known as the polarization resistance. This variable can be conveniently measured by putting another electrode/auxiliary in the liquid, and in turn connecting it to the working/corroding/test electrode via external power supply. The whole set‐up is given in circuit provided ( Figure 2.9).Here, Rp is polarization resistance, Rs is solution resistance, while Ce is electrode capacitance. An important advantage of using LPR is that it does not take more than half an hour to provide a conclusion with preliminary apparatus adjustments, balancing of readings, calculating process, and computation of the polarization resistance Rp. Figure 2.9 Circuit for linear polarization resistance.