Chloramphenicol and hydralazine as corrosion inhibitors for steel

Contents

CHAPTER- 1 INTRODUCTION
1.1 Definition and importance of corrosion
1.2 Economic effects
1.2.1 Cost of corrosion
1.3 Types of corrosion
1.3.1 Dry corrosion
1.3.2 Wet corrosion
1.3.2.1 Uniform corrosion
1.3.2.2 Galvanic corrosion
1.3.2.3 Differential aeration corrosion
1.3.2.4 Crevice corrosion
1.3.2.5 Stress corrosion
1.3.2.6 Intergranular corrosion
1.3.2.7 Erosion corrosion
1.4 Electrochemical theory
1.5 Factors affecting the corrosion rate
1.5.1 Nature of the metal
1.5.1.1 Electrode potential of the metal
1.5.1.2 Purity of the metal
1.5.1.3 Hydrogen over voltage
1.5.1.4 Relative areas of anode and cathode
1.5.1.5 Nature of the corrosive product
1.5.2 Environment surrounding the metal
1.5.2.1 pH
1.5.1.2 Temperature of the corrosive environment
1.5.1.3 Humidity
1.6 Corrosion rate measurements
1.6.1 Weight loss measurements
1.6.2 Electrochemical measurements
1.6.2.1 Electrochemical Tafel polarization measurements
1.6.2.2 Electrochemical impedance Spectroscopy (EIS)
1.6.3 Adsorption isotherm
1.6.4 Theoretical study
1.6.4.1 Quantum chemical measurements
1.6.4.2 Quantum chemical parameters
1.6.4.3 Semi-empirical quantum measurements
1.7 Methods of corrosion control
1.7.1 Selection and Design of material
1.7.2 Protective Coatings
1.7.2.1 Metal coating
1.7.2.2 In-organic coating
1.7.2.3 Organic coating
1.7.3 Cathodic protection
1.7.3.1 Sacrificial anodic method
1.7.3.2 Impressed current method
1.7.4 Corrosion Inhibitors
1.7.4.1 Types of corrosion inhibitors
1.7.4.1.1 Adsorption corrosion inhibitors
1.7.4.1.2 Passivation corrosion inhibitors
1.7.4.1.3 Pickling corrosion inhibitors
1.7.4.1.4 Vapor-phase corrosion inhibitors
1.7.4.1.5 Slashing corrosion inhibitors
1.7.4.2 Toxic effect of inhibitors
1.7.4.3 Friendly corrosion inhibitors
1.7.4.4 Effect of the molecule structure on corrosion inhibition
1.8 Aim and scope of the present work

CHAPTER- 2 MATERIALS AND METHODS
2.1 Materials
2.1.1 Mild Steel
2.1.2 Pre-treatment of electrodes
2.1.3 Chemicals
2.1.3.1 Electrolytic medium
2.1.3.2 Inhibitor solution
2.1.4 Acute toxicity studies
2.2 Electrochemical cell and electrode assembly
2.2.1 Pretreatment of the electrochemical cell
2.3 Instrumental setup
2.4 Corrosion Studies
2.4.1 Chemical method
2.4.1.1 Weight loss measurements
2.4.2 Electrochemical measurements
2.4.2.1 Electrochemical Tafel polarization method
2.4.2.2 Electrochemical Impedance Spectroscopy (EIS)
2.4.3 Adsorption Isotherm
2.4.4 Activation Parameters
2.4.5 Theoretical method
2.4.5.1 Quantum chemical calculation
2.4.6 Surface study
2.4.6.1 Scanning electron microscopy (SEM)

CHAPTER- 3 Chloramphenicol drug as a corrosion inhibitor for mild steel in 1M HCl solution
3.1 Results and Discussions
3.1.1 Weight loss measurements
3.1.2 Electrochemical measurements
3.1.2.1 Tafel polarization measurements
3.1.2.2 Electrochemical impedance spectroscopy (EIS)
3.1.3 Adsorption isotherm and thermodynamic 50 parameters
3.1.4 Activation parameters
3.1.5 Scanning electron microscopy (SEM)
3.3 Mechanism of inhibition
3.4 Conclusions

CHAPTER- 4 Hydralazine drug as a corrosion inhibitor for mild Steel in acidic medium
4.1 Results and Discussions
4.1.1 Weight loss measurements
4.1.2 Electrochemical measurements
4.1.2.1 Tafel polarization measurements
4.1.2.2 Electrochemical Impedance Spectroscopy (EIS)
4.1.3 Adsorption isotherm and thermodynamic parameters
4.1.4 Activation parameters
4.1.5 Quantum chemical studies
4.1.6 Scanning electron microscopy (SEM) analysis
4.3 Conclusions

CHAPTER- 5 Summary and main conclusions

References

ACKNOWLEDGMENTS

A special and sincere thanks to Dr. Praveen, B M, my guide and Head of Chemistry Department, Srinivas School of Engineering, Mukka, Mangalore who greatly enriched my knowledge with valuable comments and constantly inspired me and who had to spare many precious hours of his, to supervise my research work. I wish to express my deep sense of gratitude to my guide for his inspiration, support and help.

I am grateful to Dr. Manjunatha T S, Principal, Jain Institute of Technology, Davanagere for his inspiration and kind support for this book. Along with that I extend my gratitude to management of JIT, Davanagere for moral support to finish this book.

I also extend my earnest gratitude to Dr. P Shivakeshava Kumar, HOD, Civil Departement, JIT, Davanagere for his scholarly suggestions and support throughout the course of this study. I will always remember throughout my life Dr. M R Jagadeesh Associate professor, Department of Physics, JIT, Davanagere for his continuous support from the beginning of research work and has helped me timely in all my hardship.

This research work would not have been completed successfully without my my wife Prema C.M. and my kid B.M. Pratham cooperation at all levels with moral and social support, due to which I could accomplish this work. Finally I accept the best wishes from my parent’s blessings of Mrs. B.M. Gangamma & Mr. B.M. Kotraiah wishes of my sister B.M. Geethanjali and my brother in law Mr. Praneethkumar for their support.

Dr. B.M. Prasanna

Chapter 1 Introduction

1.1 Definition and Importance of Corrosion

Metals and their alloys are widely used in structural and industrial applications rather than plastics, ceramics, wood, rubber etc., due to its excellent physical, chemical and thermal properties. These metals and their alloys undergo corrosion because of their exposure to corrosive environment. In this process, there is a transition of metals from their primary state to oxide state. The oxide form of the metals has lower energy and is thermodynamically stable. Metals are extracted from their stable oxidized state through metallurgical processes. Hence, metals are in unstable state due to higher energy. However, metals exhibit a natural tendency to retain their steady state either by chemical or electrochemical reactions to form a corrosive product on the metal surface. This process is referred to as corrosion.

“Corrosion is a chemical process of gradual degradation or deterioration of metals and their alloys through chemical or electrochemical reaction with their environment.”

Eg. Rusting of iron, green deposit on copper, etc.

Corrosion is a serious problem and it is a challenge to scientists to protect the metals from the attack of corrosion. To understand the corrosion process, it requires an electrochemistry background. Corrosion scientists study the mechanism and kinetics of corrosion for identifying its reasons in various applications. Thus, to decide the most convenient method to prevent or minimize the metal damage caused by corrosion attack.

1.2 Economic Effects

Corrosion mainly affects the economy of a nation. These economic effects are caused due to the following reasons such as,

- The replacement of corroding structures and machinery spares such as mufflers, pipes and metal roofs etc., causes extra expense for the maintenance of the structures.
- The shutdown of entire unit instead of replacement of a corroded structure might be highly expensive. eg. A corroded pipe system can undergo leakage of oil, gases and water.
- The market value of a product largely depends on its purity. In some cases, a slight corrosion at the time of transpotation and storage can introduce metal ions into the product. This may cause catalytic decomposition of the product leading to its contamination. eg. Hydrogen peroxide gets contaminated because of the metallic container used for storage and transportation¹.

1.2.1 Cost of corrosion

Corrosion causes a tremendous economic loss due to metal dissolution. National Association of Corrosion Engineers (NACE) has estimated the damage occurred due to corrosion and it is provided in Table 1.1.

Table 1.1 Annual costs of corrosion for various countries [2-4].

illustration not visible in this excerpt

In India, the corrosion problem is more severe than cold countries because of its tropical climate. In a recent report (2012), the estimated cost of corrosion loss was $45 billion [5, 6].

1.3 Types of Corrosion

Metals gradually undergo degradation in weight due to corrosion. Based on the nature of metal and its corrosive environment, corrosion is broadly classified into two types.

- Dry corrosion
- Wet corrosion

1.3.1 Dry corrosion

Dry corrosion of metals arises due to the direct attack of metals from non- metallic elements such as oxygen, sulfur, hydrogen sulfide, sulfur dioxide and halogens in gas or vapor form in the absence of moisture. In such reactions, the metal undergoes initial oxidation and the attacking non-metallic species undergo reduction to form a thin film of a compound at the metal/non-metal interface as a corrosive product. Due to high volatility, further layers form continuously, which leads to the increase in corrosion rate. The chemical corrosion reaction is explained as follows,

illustration not visible in this excerpt

Where, M and X are the divalent metal and halogen respectively.

eg.

illustration not visible in this excerpt

1.3.2 Wet corrosion

Wet corrosion of metals takes place in the presence of an aqueous environment. Electrochemical reaction can explain this type of corrosion. Oxidation of metals and reduction of the species takes place. In the process, electrons get transferred through the metal from the anode (the metal that corrodes) to cathode (species that gets reduced)⁷. eg. Rusting of iron, green deposit on the surface of the copper etc. Wet corrosion can further be classified as follows,

- Uniform corrosion  Galvanic corrosion
- Differential aeration corrosion  Crevice corrosion
- Stress corrosion
- Intergranular corrosion  Erosion corrosion

1.3.2.1 Uniform corrosion

Uniform corrosion arises where all areas of the metal corrode at the same rate when the entire metal is exposed to the corrosive environment. A highly reactive metal such as Zinc, Aluminum, Iron etc., undergoes this type of corrosion usually in acid medium. eg. Metal dissolution in acids. A pictorial representation of uniform corrosion of metal sheets after immersion in acids is shown in figure 1.1.

illustration not visible in this excerpt

Figure 1.1 Uniform corrosion of metal in acids

1.3.2.2 Galvanic corrosion

When two dissimilar metals are in contact with each other, the reactive metal acts as an anode which suffered from corrosion. Another less reactive metal serves as a cathode. A potential difference develops in between those two metals which drives the corrosion process. The difference in their potential decides the extent of corrosion rate which depends on the selection of different materials in the galvanic series. Several investigations have shown that galvanic

corrosion is directly proportional to the area ratio of the cathodic metal to the anodic metal ⁸. Galvanic corrosion of a boat hull is shown in Figure 1.2.

illustration not visible in this excerpt

Figure 1.2 Galvanic corrosion of boat hull

1.3.2.3 Differential aeration corrosion

Differential aeration corrosion arises, when two different surface areas of the same metal is exposed to different concentrations of oxygen. In this case, the part of the metal which is less exposed to oxygen acts as an anode and suffers corrosion. Other part which is more exposed to oxygen serves as a cathode. Differential aeration corrosion can be further classified in the following two categories,

- Waterline corrosion
- Pitting corrosion

Waterline corrosion

Waterline corrosion arises due to different concentration of air/oxygen exposed to two different surface areas of the same metal. Hence, by developing a potential difference which causes the corrosion. This type of corrosion is quite common in metal tanks used for storing water, ships etc.,

When water is stored in a metallic tank, the surface area below the water level is less exposed to the air/oxygen. This area which behaves as an anode

and suffers the attack of corrosion. The other surface area which is above the waterline is more exposed to air/oxygen. This acts as a cathode. This remains unaffected by the corrosion. Therefore, the metal corrodes below the waterline rather than that of above the waterline. This waterline corrosion can be controlled by painting the metallic surface below the waterline, with high quality paint.

illustration not visible in this excerpt

Figure 1.3 The waterline corrosion in metal pipes

Pitting corrosion

Pitting corrosion arises due to the attack of dust and mud particles on the metal surface. The dust attack, leaves the metal surface into two distinguished areas. One is below the dust particles with less exposure to air/oxygen, which acts as an anode and gets attacked by corrosion. As a result, it forms small pinholes called pits. Therefore, this corrosion is referred to as pitting corrosion⁹. The rest of the surface of the metal away from the dust particles (i.e. cleaned surface) acts as a cathode, which is unaffected by the corrosion processes. The pitting corrosion in metallic pipes is shown in Figure 1.4.

illustration not visible in this excerpt

Figure 1.4 Pitting corrosion in metallic pipes

1.3.2.4 Crevice corrosion

This type of corrosion occurs due to the presence of crevices created at the joints, rivets, bolts and gaskets of the metallic structures. These regions are shielded and lack of air/oxygen causes the corrosion. eg. Crevice formed at the two materials‟ joints by gaskets or washers as shown in Figure 1.5.

illustration not visible in this excerpt

Figure 1.5 Crevice corrosion of metal

1.3.2.5 Stress corrosion

The metals and their alloys develop stress due to various mechanical operations such as hammering, bending, rolling, annealing etc. Due to this, localized zone develops with lower electrode potential that acts as an anode and undergoes corrosion. The unstressed portion of the metal gets higher reduction potential acting as a cathode which is immune to attack of corrosion. Caustic embrittlement of steel is the best example of stress corrosion. A pictorial representation of stress corrosion of metal is shown in the Figure1.6.

illustration not visible in this excerpt

Figure 1.6 Stress corrosion in metal

1.3.2.6 Intergranular corrosion

Intergranular corrosion arises in between the grains or crystals adjacent to the grain boundary of metal. This type of corrosion is also called intercrystalline corrosion and it occurs due to the presence of impurities at the grain boundaries that are highly reactive. Grain boundaries in metals are more susceptible to the attack of corrosion. eg. Corrosion attacks on knife liner.

Alloys quickly suffer from this intergranular corrosion. eg. During the welding of steel, chromium carbide in the steel alloy gets precipitated at the grain boundaries which deplete the adjacent region of chromium. Therefore, that area becomes more anodic and suffers corrosion. In the case of nickel based alloys, when these are exposed to sulfur bearing environment, nickel sulfide can form and cause catastrophic failures¹⁰. The intergranular corrosion at the grain boundary is shown in Figure 1.7.

illustration not visible in this excerpt

Figure 1.7 Intergranular corrosion at the grain boundary in metal

1.3.2.7 Erosion corrosion

This type of corrosion arises due to the flow of electrolyte or gases along with the metal surface and the rubbing action of material on the metallic surface. In both cases, protective layer of the metal surface get destroyed by various mechanical operations and is prone to corrosion. Sand and suspended materials enhance the erosion-corrosion. eg. Erosion in pump impellers, agitators and elbows.

illustration not visible in this excerpt

Figure 1.8 Fretting corrosion of metal by sliding operation

1.4 Electrochemical Theory

Different approaches such as acid theory, chemical theory, colloidal theory, biological theory and electrochemical theory have been used to understand corrosion. Among these, the electrochemical theory is the most convenient and acceptable to explain the mechanism of corrosion. The scientist Whitney introduced this in 19thcentury.

According to the electrochemical theory, when a metal is exposed to the corrosive environment (oxygen and moisture), minute galvanic cells are produced over the metal surface as shown in Figure 1.9.The metal surface gets distinguished into two layers such as anodic and cathodic area and results in corrosion. At the anodic coating, the metal undergoes oxidation to yield its metal ions with the liberation of electrons as an anodic ion.

illustration not visible in this excerpt

Cathodic layer undergoes reduction by gain of electrons from the anodic layer to liberate either hydrogen (H2) or hydroxyl ion (OH-) as cathodic ions.

illustration not visible in this excerpt

Finally, there is an interaction between the anodic ion (Fe+² )and one of the cathodic ion (OH-) to form an insoluble and unstable Fe (OH) 2 over the metal surface.

illustration not visible in this excerpt

Figure 1.9 The electrochemical cell developed on the metal surface

illustration not visible in this excerpt

Unstable Fe (OH) 2 undergoes further oxidation in the presence of oxygen and moisture to form a stable hydrated ferric oxide [Fe2O3.3H2O] as a corrosion product.

illustration not visible in this excerpt

1.5 Factors affecting the corrosion rate

The rate and extent of corrosion reaction of metal are related to the following major factors

1.5.1 Nature of the metal

In relation to corrosion, the nature of the metal depends on properties such as electrode potential, purity, physical property, hydrogen overvoltage, relative areas of anode and cathode etc.

1.5.1.1 Electrode potential of the metal

The electrode potential or its position in galvanic series decides the tendency of metal to undergo corrosion. Metals with lesser electrode potential in top position in the galvanic series are more susceptible to corrosion than less reactive metals, e.g., Zn, Al, Mgand Li etc. The metals which have smaller electrode potential and are more reactivity in galvanic series readily undergo corrosion. The nobel metals like Au, Pt and Ag are less liable to corrosion.

1.5.1.2 Purity of the metal

Metals which have greater purity have higher resistance to corrosion. Therefore, presence of impurities in metal forms tiny galvanic cells which ease the corrosion. eg. Zinc with 99.95 % purity readily undergoes corrosion than 99.99 % purity.

1.5.1.3 Hydrogen overvoltage

Hydrogen over voltage is the capacity of the working electrode to liberate hydrogen. Metal with lesser hydrogen overvoltage on its surface undergoes corrosion. In this case, hydrogen gas is evolved readily and thus the cathodic reaction rate is fast. This increases the anodic reaction as well also faster, thereby promoting overall corrosion reaction. When the hydrogen overvoltage on the metal surface is high, the cathodic reaction is slower and corrosion of the metal also becomes slow.

1.5.1.4 Relative areas of anode and cathode

The rate of corrosion is influenced by the relative surface areas of anode and cathode. The rate of corrosion is more for the metal having a large cathodic area and a small anodic area. This is because the demand for electrons at the larger cathodic area from the small anodic area is high leading to higher current density in the anodic area. Therefore, the anodic reaction takes place at maximum rate, thus increasing the corrosion. Conversely the larger anodic area and smaller cathodic area, causes slow corrosion at the anodic area.

1.5.1.5 Nature of the corrosive product

The corrosive product on the surface of metal may or may not act as a protective film. If the deposited corrosion product is insoluble, stable, uniform and nonporous, it serves as a protective film and thus, preventing further corrosion of metal.eg. In case of lead, this forms insoluble lead sulfate in the presence of sulfuric acid. Conversely, if the corrosion product is soluble, unstable, porous and non uniform in nature, the corrosion continues. In such cases, the fresh metal surface is continuously exposed to the corrosion environment and corrosion of the metal takes place consistently. eg. iron and molybdenum.

1.5.2 Environment surrounding the metal

Environmental factors surrounding the metal also influence the rate of corrosion. The most significant environmental factors which affect the rate of corrosion include pH, temperature and humidity of the surrounding medium.

1.5.2.1 pH

The rate of corrosion decreases with increase in pH of the corrosive environment, as in iron. At higher pH (i.e. greater than 10), corrosion retards due to the formation of a passive layer of hydroxide of metal. Acids (i.e. lower pH) are more aggressive for corrosion than alkaline medium. The amphoteric metals like Aluminum, Zincand Lead, dissolve in alkaline solution in the form of complex ions. The corrosion rate of iron is much faster in oxygen-free water (i.e.,.pH<5).At pH<4, the corrosion of iron is stimulated by the oxidation of Fe+² to Fe+³ by the dissolved oxygen and the subsequent reduction of Fe+³ to Fe+² at the cathodic region. In less acidic solution, an excess of OH-ions reacts with the metal ion (Fe+² ) to form unstable Fe(OH)2, which undergoes further oxidation to form rust. Zinc readily corrodes in weakly acidic solution such as carbonic acid. The minimum pH for corrosion of various metals is reported in Table 1.2.

Table 1.2 pH for the minimum corrosion attack for the various metals

illustration not visible in this excerpt

1.5.2.2 Temperature of the corrosive environment

An increase in temperature increases the conductance of the corrosion medium, which contributes to enhance the corrosion rate.

1.5.2.3 Humidity

The corrosion rate increases in the presence of moisture, as the conductance of the corrosive medium increases which influences the corrosion of metal.

1.6 Corrosion Rate Measurements

The corrosion rate is measured by various chemical and electrochemical methods. In the present work, following methods were used to study the corrosion.

1.6.1 Weight loss measurements

The weight loss analysis is the simple, most convenient and practical technique to measure the corrosion rate of metals. In this measurement, the weight loss of metal is determined whenever exposed to a corrosive environment with time. The corrosion rate is the weight loss of metal in unit time over unit surface area. The corrosion rate (ρ) is calculated using an expression as

illustration not visible in this excerpt

Where [illustration not visible in this excerpt] is the corrosion rate, [illustration not visible in this excerpt] is the loss in weight of metal before and after the exposure to a corrosive environment. S and t are the exposed surface area and time duration to corrosive environment respectively.

1.6.2 Electrochemical measurements

The corrosion behavior of metals is attributed to the electrochemical reactions. Hence, they are governed by the laws of electrochemical theory. Therefore, it is vital to study the electrochemical characteristics of metals during corrosion tests to understand the mechanism and rate of the corrosion process [11-14].

1.6.2.1 Electrochemical Tafel polarization measurements

Polarization method provides a scientific insight to charge transfer reaction in an electrochemical corrosion reaction. An electrical current is passed through an electrochemical cell, which causes deviation of the potential of the working electrode from the equilibrium potential and is termed as the polarization. To study the corrosion behaviors, its parameters are computed by the Tafel extrapolation.

The potential difference between the polarized (working) electrode and unpolarized (equilibrium) electrodes is known as overpotential (ŋ). Figure 1.10 shows a stern diagram for electrochemical polarization curve showing Tafel extrapolation, which relates to the kinetic parameters. This graphical representation is helpful to understand the electrochemical behavior of polarized working electrode in an electrolyte containing hydrogen (H+) ions.

Corrosion parameters are determined by extrapolation of the linear portion of the anodic and cathodic lines in Tafel polarization curves. Through this polarization, behavior can be explained experimentally by using potentiodynamic methods.

Cathodic Tafel slope (βc) and anodic Tafel slope (βa) are measured from smaller linear parts of the cathodic and anodic curves respectively. The points of intersection of both the curves are termed as corrosion current density (icorr) and corrosion potential (Ecorr), which is steady state potential.

illustration not visible in this excerpt

Figure 1.10 stern diagrams for electrochemical Tafel extrapolation

This figure is a hypothetical electrochemical behavior of a metal immersed in an oxidizer containing electrolyte (HCl, H2SO4 etc).This potentiodynamic non-linear Tafel curve is divided into two parts,

(i) If E > E corr, the upper curve represents the anodic polarization behavior

due to the oxidation of the metal.

(ii) If E < E corr, the lower curve represents the cathodic polarization due to the hydrogen evoluation.

Due to the corrosion process, the anodic and cathodic reactions are combined on the metal electrode surface at a current density known as corrosion current density (icorr).At the time of polarization from the steady state corrosion potential (Ecorr) on anodic and cathodic current density. The potential changes from steady state corrosion potential (Ecorr) for the anodic and cathodic current densities are expressed as,

illustration not visible in this excerpt

Let us assume that the applied current density is,

illustration not visible in this excerpt

Substitute the expressions of i and i to yield Buttler-Volmer equation, which explains the kinetics of the electrochemical corrosion.

illustration not visible in this excerpt

The corrosion current is directly related to the corrosion rate (CR) regarding mpy, through the following reaction,

illustration not visible in this excerpt

Where E.W.= equivalent weight of the corroding species in grams. d = density of the corroding species, g/cm².

i = corrosion current density, μA/cm²

1.6.2.2 Electrochemical impedance spectroscopy (EIS)

Electrochemical impedance spectroscopy method is used to study the characteristics of a metal electrode. This technique is a response of an equivalent AC circuit for electrode / electrolyte interface. The transfer functions obtained by potential excitation of varying AC signals with small amplitude are investigated¹⁵.

The characterstic behavior of corroded metal includes polarization resistance (Rp) and corrosion mechanism. These parameters can be measured by modeling the AC signals to an equivalent circuit.

The magnitude of the impedance containing elements (Resistors, Capacitorsand Inductors) in an equivalent circuit termed as Z. In modeling of an electrochemical circuit, a potential waveform is applied to the circuit and current response to the AC signals is observed. This produces impedance data related to phase shift angle, potential variationand current amplitudes. The potential excitation and its current responses are as shown in Figure.1.11.

illustration not visible in this excerpt

Figure 1.11 The plot of potential excitation and its current responses

The equation of circle for charge control mechanism whenever a potential sinusoidal excitation is applied across the electrode/solution interface is as follows¹⁶,

illustration not visible in this excerpt

The graphical representation of the above equation with a radius of RP /2 is referred Nyquist plot as shown in Figure 1.12.

illustration not visible in this excerpt

Figure1.12 Nyquist plot

This graph consists of a depressed high-frequency semicircle. In this plot, the diameter of the semicircle is termed as polarization resistance (Rp).This value increases with increasing inhibitor concentration on the blank solution due to the inhibition effect of the inhibitor. Thus, the corrosion rate (CR) of metal also decreases because of the increasing thickness of the electrical double layer (Cdl). This value can be calculated from the following expression¹⁷.

illustration not visible in this excerpt

1.6.3 Adsorption isotherm

The adsorption isotherm explains the interaction between the surface coverage ( ) of the metal surface with the adsorbed inhibitor molecules. The Obtained experimental data fits into various adsorptions isotherm models. It is an interrelationship between the surface covered by the inhibitor molecules and inhibitor concentration in the bulk of the solution. The formulation of various adsorption isotherms is reported in Table 1.3.

Table 1.3 Adsorption isotherm models for thermodynamic study of corrosion inhibition process¹⁸

illustration not visible in this excerpt

1.6.4 Theoretical study

1.6.4.1 Quantum chemical measurements

A quantum chemical method has already proved to be very useful for elucidating the electronic structure and its reactivity¹⁹. It is used to study the corrosion inhibition by quantum chemical calculations. It can also co-relate the electronic structure with inhibition efficiency (q ). In this, many of researchers have reported that the inhibition efficiency of the inhibitor molecule depends on the physical, chemical and electronic characteristics. These are related to the functional groups and electronic density of the donor atoms of the inhibitor molecule [20, 21].

1.6.4.2 Quantum chemical parameters

Quantum chemical study is a theoretical method and molecular modeling technique. The parameters obtained by this method are entirely different from parameters obtained experimentally. These are,

Atomic charges: Chemical interactions are either electrostatic or orbital. The electrostatic interactions mainly depend on the electric charges on the molecules. Thus, the atomic charge is commonly used to measure the chemical reactivity of the molecule. Various methods are used to estimate the atomic charges on the molecule. Mulliken charge analysis²² is the most commonly utilized method for the calculation of atomic charge distribution in the molecule. These parameters facilitate the quantitative calculation of the structure and its reactivity with the inhibitor molecule²³.

Molecular orbital energy: The energies of highest occupied molecular orbital (E )and lowest unoccupied molecular orbital ( E )are the quantum HOMO LUMO chemical parameters. The energy of HOMO is an indication of the capability of donating electrons. For LUMO, it is the ability of accepting electrons by the molecule. According to the frontier molecular orbital theory, the formation of a transition state is due to the interaction of the frontier molecular orbital (HOMO and LUMO) of the reactants²⁴. The difference between the HOMO and LUMO energy gap[illustration not visible in this excerpt] influences inhibition. The molecule with lower values of ΔE gives higher inhibition efficiency because the excitation energy gap is more polarizableand related with chemical reactivity of the molecule²⁵.

Dipole moment ( [illustration not visible in this excerpt] ): Dipole moment is a widely used parameter to explain the polarity of molecules. It is a measure of the polarity of a covalent bond.The total dipole moment of the molecule is one of the critical quantum chemical parameters which decide the adsorption of the inhibitor onto the metal surface and thus increasing the inhibition efficiency.

Energy: The total energy is a sum of internal, potential and kinetic energy. The total energy of a system which includes that of the many body effects of electrons in the presence of latent external potential is a unique function of the charge density²⁶.

1.6.4.3 Semi-empirical quantum measurements

Semiempirical quantum chemical method is an efficient technique to estimate the quantum chemical parameters quantitatively. The calculations for some of the parameters are explained below.

MNDO (Modified neglect of differential overlap) AM1 (Austin model 1)

PM3 (parametric model 3)

MNDO: It is based on NDDO (neglect of diatomic differential overlap) approximation. NDDO is an improved method of INDO (Intermediate neglect of differential overlap)²⁷.

AM1: It is based on NDDOand is used to improve the MNDO model by reducing the repulsion of atoms by addition of off-center attraction. This method was developed by Michel Devar in 1985²⁸.

PM3: This method is the most widely used semi-empirical method for structure elucidation and co-relation with inhibition efficiency. It has same equations similar to the AM1 method. In this approach, two Gaussian functions are utilized for the core repulsion function. The numerical parameters are entirely different from AM1 method. The value obtained by this approach is treated as optimized. Stewart developed this method in 1989²⁹.

In the present work, Quantum chemical calculation of corrosion inhibition for mild steel is investigated by various organic inhibitors using a PM3 method in the gas phase.

1.7 Methods of Corrosion Control

1.7.1 Selection and design of material

It is better to prevent corrosion by proper choice of material and design for various industrial and constructional applications. Thus selecting content that should be less reactive or having high reduction potential is important. Few methods are mentioned below.

The metal should have low absorption and retention of moisture. There should not be gap in between the metals, to avoid incorporation of air or liquids to control the corrosion. Along with this, materials should have smooth surface and free from stress.

1.7.2 Protective coatings

Protective coatings are passive layer on the surface of metal. These protect it against the attack of corrosion. The following class of protective coatings is used to control the corrosion of metals.

- Metal coating
- Inorganic coating
- Organic coating

1.7.2.1 Metal coating

Deposition of protective metal over the surface of the base metal is called metal coating. Hot dipping, spraying, cladding and electroplating techniques are used for metallic coating. eg. Galvanized steel sheet. In this case highly reactive Zinc acts as an anode and readily undergoes corrosion to protect the steel.

1.7.2.2 Inorganic coating

Coating the metal either by surface conversion or by chemical conversion is called inorganic coating. The thin oxide film of metal formed by anodization serves as a protective layer for highly reactive metals such as Al/Mn/Mg. Similarly thin metal phosphate layer in a phosphate bath acts as a protective coating for iron based alloys such as steel. Glass surface and ceramic coating also comes under this classification.

1.7.2.3 Organic coating

Organic coating provides a protective barrier at the metal/solution interface. It gives decorative and artistic appearance to the metal surface as well. Paints, chromates, polyvinyl chloride; epoxy resin coatings are termed as organic layers. This organic coating on the metal surface is used as protection against the attack of corrosion.

1.7.3 Cathodic protection

Cathodic protection is a technique to protect the metal from corrosion by converting it completely into a cathode. Metals and their alloys are protected from corrosion by this method³⁰. The following methods can achieve cathodic protection.

- Sacrificial anodic method (Galvanic protection)
- Impressed current method

1.7.3.1 Sacrificial anodic method (Galvanic protection)

A reactive metal is connected to the base metal structure to be protected. Hence, all the corrosion is concentrated at the more active metal and thus protects the metal structure from corrosion. In general, highly reactive metals such as Mg, Zn, Al and their alloys are used as sacrificial anodes to protect the metals such as Fe, Cu and Cd etc. Galvanic protection of underground steel pipeline by connecting aluminium as a sacrificial anode is as shown in the Figure 1.13.

illustration not visible in this excerpt

Figure 1.13 Galvanic cathodic protection of underground steel tank

1.7.3.2 Impressed current method

In the impressed current protection, an impressed current on the metal surface nullifies the corrosion current and converts the corroding metal from anode to cathode. This protection is shown in the Figure 1.14.

illustration not visible in this excerpt

Figure 1.14 The impressed current method of cathodic protection

1.7.4 Corrosion Inhibitors

Among the various corrosion controlling methods, the use of corrosion inhibitor is most convenient. It is a practical technique to control corrosion by inhibition mechanism³¹. Corrosion inhibitors are chemical substances which are to be added to a corrosive environment to protect the metals from an attack of corrosion. These corrosion inhibitors are widely accepted in the field of industrial systems and commercial applications. eg. Cooling system refinery units, pipelines, chemicals, oil and gas production units, boilers, water processing units etc.³².

Inhibitors may be dyes, plant extracts and heterocyclic organic compounds etc. The organic compounds, which contain electron rich heteroatoms such as N, O and S are effective corrosion inhibitors in aggressive corrosive media [33-38]. The other class of organic inhibitors are fatty amides [39,40] pyridines [41-43], imidazolines [44-46] and 1,3-azoles,[47-49]. These have shown excellent performance as corrosion inhibitors for metals. These compounds contain electron donating groups which decrease the corrosion rate by increasing the hydrogen overvoltage on the corroding metal⁵⁰.Corrosion inhibitor retards the corrosion process by film formation on the corroding metal surface via adsorption process. The adsorbed film acts as a protective layer that cuts off the interaction at metal/solution interface. The inhibition effect of corrosion inhibitor depends on the ability of adsorption on the surface of metal through the replacement of water molecule at metal/solution interface⁵¹.

[...]