Heterocyclic Organic Corrosion Inhibitors: Principles and Applications 0128185589, 9780128185582

Table of contents :
Cover
Heterocyclic Organic Corrosion Inhibitors: Principles and Applications
Copyright
List of abbreviations and symbols
Preface
Acknowledgment
1. Heterocyclic corrosion inhibitors
1.1 Introduction
1.2 Heterocyclic compounds
1.3 Important heterocyclic systems
1.3.1 Azoles
1.3.2 Indoles
1.3.3 Pyridines
1.3.4 Diazines
1.3.5 Quinolines
1.4 Nomenclature of heterocyclic compounds
1.4.1 Common or trivial names
1.4.2 Hantzsch–Widman nomenclature
1.4.3 The replacement nomenclature
1.5 Heterocyclic systems as corrosion inhibitors
1.5.1 Five-membered heterocycles
1.5.2 Six-membered heterocycles
1.5.3 Macrocyclic compounds
1.6 Effect of substituents on corrosion inhibition efficiency
Suggested reading
References
2. Experimental methods of inhibitor evaluation
2.1 Introduction
2.2 Gravimetric method
2.2.1 Effect of concentration
2.2.2 Effect of temperature and activation parameters
2.3 Adsorption parameters
2.3.1 Adsorption isotherms
2.3.2 Adsorption energy
2.4 Electrochemical methods
2.4.1 Open circuit potential vs. time
2.4.2 Electrochemical impedance spectroscopy
2.4.3 Potentiodynamic polarization
2.4.4 Electrochemical frequency modulation
2.4.5 Linear polarization resistance
2.5 Surface analytical techniques
2.5.1 Water contact angle
2.5.2 Scanning electron microscopy
2.5.3 Energy-dispersive X-ray spectroscopy
2.5.4 Atomic force microscopy
2.5.5 X-ray diffraction
2.5.6 Fourier transform infrared spectroscopy
2.5.7 X-ray photoelectron spectroscopy
2.5.8 Time-of-flight secondary ion mass spectrometry
Suggested reading
References
3. Computational methods of inhibitor evaluation
3.1 Introduction
3.2 Density functional theory
3.2.1 Theoretical basis
3.2.2 Functionals
3.2.3 Basis sets
3.3 DFT-based quantum chemical parameters
3.3.1 Frontier molecular orbitals
3.3.2 Frontier orbital energies
3.3.3 Electronegativity and the electronic chemical potential
3.3.4 Global hardness and softness
3.3.5 Electrophilicity and nucleophilicity indices
3.3.6 Fraction of electrons transferred
3.3.7 Energy change for donation and back donation of charges
3.3.8 Dipole moment
3.3.9 Proton affinity
3.3.10 Molecular electrostatic potential
3.3.11 Fukui indices
3.4 pKa analysis
3.5 Atomistic simulations
3.5.1 Ensemble
3.5.2 Molecular dynamics simulations
3.5.3 Monte Carlo simulations
3.5.4 Force fields
3.5.5 Boundary conditions
3.6 Application of atomistic simulation to corrosion inhibition studies
3.6.1 Total energy
3.6.2 Interaction energy
3.6.3 Binding energy
3.6.4 Solvation energy
3.6.5 Radial distribution function
3.6.6 Mean square displacement and diffusion coefficient
Suggested reading
References
4. Heterocyclic corrosion inhibitors for acid environments
4.1 Introduction
4.2 Acid pickling and acidizing processes
4.2.1 Acid pickling
4.2.2 Oil well acidizing
4.3 Corrosion and its inhibition in acid solutions
4.3.1 Corrosion mechanism
4.3.2 Mechanism of corrosion inhibition
4.4 Heterocyclic corrosion inhibitors used for acid environments
4.4.1 Inhibitors for acid pickling
4.4.1.1 Azoles
4.4.1.2 Imidazolines and related compounds
4.4.1.3 Pyridines and diazines
4.4.1.4 Quinolines, quinolones, quinoxalines, and quinazolines
4.4.1.5 Triazines and tetrazines
4.4.1.6 Macrocyclic compounds
4.4.2 Inhibitors for acidizing
4.5 Schemes for synthesis of heterocyclic corrosion inhibitors for acid environment
References
5. Heterocyclic corrosion inhibitors for sweet and sour environments
5.1 Introduction
5.2 Sweet corrosion
5.3 Sour corrosion
5.4 Heterocyclic inhibitors for sweet and sour environments
5.5 Schemes for synthesis of heterocyclic corrosion inhibitors for sweet/sour environment
References
6. Heterocyclic corrosion inhibitors for neutral environments
6.1 Introduction
6.2 Metallic corrosion and its inhibition in neutral environment
6.3 Heterocyclic corrosion inhibitors for neutral environments
6.3.1 Inhibitors for iron and alloys
6.3.2 Inhibitors for aluminum
6.3.3 Inhibitors for copper
6.3.3.1 Pyrazoles
6.3.3.2 Imidazoles
6.3.3.3 Triazoles
6.3.3.4 O- and S-containing azoles and tetrazoles
6.3.3.5 Other heterocycles
6.4 Schemes for the synthesis of heterocyclic corrosion inhibitors for neutral environments
References
7. Heterocyclic corrosion inhibitors for alkaline environments
7.1 Introduction
7.2 Mechanism of corrosion in alkaline medium
7.3 Heterocyclic corrosion inhibitors for alkaline environments
References
8. Heterocyclic corrosion inhibitors for vapor-phase environments
8.1 Introduction
8.2 Mechanism and evaluation of corrosion inhibition using VCIs
8.3 Heterocyclic VCIs for ferrous and non-ferrous metals
References
9. Environmentally benign heterocyclic corrosion inhibitors
9.1 Introduction
9.2 Criteria for green corrosion inhibitors
9.2.1 Toxicity
9.2.2 Biodegradation
9.2.3 Bioaccumulation
9.3 Application of green chemistry metrics in the development of corrosion inhibitors
9.4 Application of greener techniques in the development of corrosion inhibitors
9.4.1 Multicomponent reactions
9.4.2 Ultrasound-assisted synthesis
9.4.3 Microwave-assisted synthesis
9.5 Types of environmentally benign corrosion inhibitors
9.5.1 Ionic liquids
9.5.2 Macrocyclic compounds
9.5.3 Drugs
9.5.3.1 Expired drugs as corrosion inhibitors
9.5.4 Plant extracts
9.5.5 Carbohydrates and other biopolymers
9.5.6 Chemically modified nanomaterials
9.6 Schemes for synthesis of heterocyclic ionic liquid based corrosion inhibitors
9.7 Schemes for chemical modification of chitosan for synthesis of corrosion inhibitors
9.8 Schemes for synthesis of heterocyclic corrosion inhibitors using MCRs
References
Index
A
B
C
D
E
F
G
H
I
K
L
M
N
O
P
Q
R
S
T
U
V
W
X
Back Cover

Citation preview

Heterocyclic Organic Corrosion Inhibitors Principles and Applications Mumtaz A. Quraishi Dheeraj S. Chauhan Viswanathan S. Saji

Elsevier Radarweg 29, PO Box 211, 1000 AE Amsterdam, Netherlands The Boulevard, Langford Lane, Kidlington, Oxford OX5 1GB, United Kingdom 50 Hampshire Street, 5th Floor, Cambridge, MA 02139, United States Copyright © 2020 Elsevier Inc. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Details on how to seek permission, further information about the Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/permissions. This book and the individual contributions contained in it are protected under copyright by the Publisher (other than as may be noted herein). Notices Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary. Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility. To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors, assume any liability for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein. Library of Congress Cataloging-in-Publication Data A catalog record for this book is available from the Library of Congress British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library ISBN: 978-0-12-818558-2 For information on all Elsevier publications visit our website at https://www.elsevier.com/books-and-journals

Publisher: Susan Dennis Acquisition Editor: Anita A Koch Editorial Project Manager: Sara Valentino Production Project Manager: Joy Christel Neumarin Honest Thangiah Cover Designer: Mark Rogers Typeset by TNQ Technologies

List of abbreviations and symbols Abbreviation

Full form

WL OCP EIS CPE DEIS EFM CF LPR PDP CV SEM EDX AFM FTIR NMR XRD XPS SECM ToF-SIMS DFT HSAB B3LYP HOMO LUMO MC MD MS CS SAM

Weight loss Open circuit potential Electrochemical impedance spectroscopy Constant phase element Dynamic electrochemical impedance spectroscopy Electrochemical frequency modulation Causality factor Linear polarization resistance Potentiodynamic polarization Cyclic voltammogram/voltammetry Scanning electron microscopy Energy dispersive X-ray spectroscopy Atomic force microscopy Fourier-transform infrared spectroscopy Nuclear magnetic resonance X-ray diffraction X-ray photoelectron spectroscopy Scanning electrochemical microscopy Time of flight secondary ion mass spectrometry Density functional theory Hard and soft acid and base principle Becke 3-parameter LeeeYangeParr Highest occupied molecular orbital Lowest unoccupied molecular orbital Monte Carlo Molecular dynamics Mild steel Carbon steel Self-assembled monolayer

xiii

xiv

List of abbreviations and symbols

Symbol

Meaning

CR q h% R T Ea DH DS Kads DGoads Qads EOCP Rct Rs Rp Cdl Ecorr icorr ba bc EHOMO ELUMO DE c m h u ε DN fk

Corrosion rate Surface coverage Corrosion inhibition efficiency Universal gas constant Temperature Energy of activation Enthalpy of activation Entropy of activation Equilibrium constant for adsorption Standard free energy of adsorption Heat of adsorption Open circuit potential Charge transfer resistance Solution resistance Polarization resistance Double layer capacitance Corrosion potential Corrosion current density Anodic Tafel slope Cathodic Tafel slope Energy of the highest occupied molecular orbital Energy of the lowest unoccupied molecular orbital Energy gap Electronegativity Chemical potential Global hardness Electrophilicity index/Angular frequency Nucleophilicity index Fraction of electrons transferred Fukui function

Preface The science and technology of corrosion inhibitors is one of the most important areas in the field of corrosion. Among the different class of inhibitors, the heterocyclic organic inhibitors have attracted a great deal of attention due to the ease of synthesis and high corrosion inhibition attributes. Through the lone pair of electrons present in the heteroatoms such as O, N, S and P, they can be effectively chemisorbed on the metal surface. This books aims to provide a comprehensive overview of heterocyclic corrosion inhibitors in different applications. Followed by the first chapter on basics of heterocyclic compounds, we have provided two chapters on various experimental and computational methods used to study corrosion inhibition. Chapters 4e7 highlight the heterocyclic corrosion inhibitors with reference to their aqueous/industrial applications, viz. acid pickling/acidizing, sweet/sour corrosion, and corrosion in neutral pH and alkaline environments. The last two chapters, respectively, explain heterocyclic vapor phase inhibitors and green inhibitors. Chapter 1 provides an overview of the fundamentals of heterocyclic compounds with a view to their applications in corrosion inhibition. The chapter also describes the major heterocyclic corrosion inhibitors and the influence of various substituent groups on corrosion inhibition. Chapter 2 entirely focuses on various experimental methods which are used for evaluation of the inhibitor performance. The methods described include gravimetric weight loss, electrochemical techniques, surface characterization techniques, and adsorption isotherms. Chapter 3 gives a concise description of different computational approaches used in corrosion inhibition studies. Techniques such as density functional theory, Monte Carlo, and molecular dynamics simulations are described with emphasis on the different reactivity parameters that are calculated using these techniques. Chapter 4 explains in detail the reported heterocyclic corrosion inhibitors for acidic environments. The severe/harsh corrosive environment created by the use of concentrated acids requires the use of thermally stable efficient corrosion inhibitors in applications such as acid pickling and acidization processes. Chapter 5 is specific for heterocyclic corrosion inhibitors for sweet and sour corrosion environment. The corrosion due to H2S and CO2 are very important in various industrial sectors such as oil and gas. Chapter 6 provides a good description on heterocyclic corrosion inhibitors that are being used in neutral environments, whereas Chapter 7 details the heterocyclic corrosion inhibitors for alkaline environments. Chapter 8 gives a concise description on heterocyclic volatile corrosion inhibitors. The last chapter (Chapter 9) highlights the important features of environmentally benign heterocyclic corrosion inhibitors. The criteria for classification of corrosion inhibitors according to PARCOM guidelines, the different green chemistry metrics, and the type of environmentally benign heterocyclic inhibitors are outlined. The environmental awareness and strict legislations related to the use of toxic corrosion inhibitors discussed. With each chapter, several

ix

x Preface

tables and schemes are provided on the nature and type of inhibitors and their synthesis approaches. We hope that the book will be a handy reference tool for students and researchers working in the field of corrosion inhibition. M.A. Quraishi, D.S. Chauhan, V.S. Saji

Acknowledgment We would like to express our gratitude to the Deanship of Scientific Research, King Fahd University of Petroleum and Minerals (KFUPM), Saudi Arabia, for providing funds on book writing project (BW 181003). We would also like to express our gratitude to all those who granted us the copyright permissions for reproducing illustrations. Our sincere thanks for the Elsevier team in evolving this book into its final shape.

xi

1 Heterocyclic corrosion inhibitors Chapter outline 1.1 Introduction ..................................................................................................................................... 1 1.2 Heterocyclic compounds................................................................................................................. 2 1.3 Important heterocyclic systems..................................................................................................... 4 1.3.1 Azoles ..................................................................................................................................... 4 1.3.2 Indoles .................................................................................................................................... 4 1.3.3 Pyridines ................................................................................................................................. 4 1.3.4 Diazines .................................................................................................................................. 5 1.3.5 Quinolines .............................................................................................................................. 5 1.4 Nomenclature of heterocyclic compounds................................................................................... 6 1.4.1 Common or trivial names..................................................................................................... 6 1.4.2 HantzscheWidman nomenclature....................................................................................... 7 1.4.3 The replacement nomenclature .......................................................................................... 9 1.5 Heterocyclic systems as corrosion inhibitors ............................................................................. 10 1.5.1 Five-membered heterocycles.............................................................................................. 10 1.5.2 Six-membered heterocycles................................................................................................ 12 1.5.3 Macrocyclic compounds ...................................................................................................... 13 1.6 Effect of substituents on corrosion inhibition efficiency......................................................... 13 Suggested reading ............................................................................................................................... 17 References............................................................................................................................................. 18

1.1 Introduction The term “corrosion” is usually referred to as the deterioration of metallic materials by its surrounding. In general, corrosion can be defined as a chemical or electrochemical reaction between a metal and its environment, which results in its deterioration. The serious consequences of corrosion have become a problem with global implication. Corrosion loss causes a substantial economic and ecological impact on entire global infrastructure and consumes 3%e4% of the gross domestic product of industrialized countries [1e5]. Heterocyclic Organic Corrosion Inhibitors. https://doi.org/10.1016/B978-0-12-818558-2.00001-1 Copyright © 2020 Elsevier Inc. All rights reserved.

1

2 Heterocyclic Organic Corrosion Inhibitors

Selection and application of suitable corrosion prevention methods are hence highly essential for the protection and efficient use of metallic structures. Most of the industries including oil and gas, water desalination, and chemical industries are suffering from various corrosion issues resulting in enormous economic loss. The virtue is that by the adoption of suitable corrosion prevention strategies, a significant extent of the loss can be avoided. Among the various corrosion control strategies, the use of corrosion inhibitors is perhaps the simplest, economic, and effective approach that is in routine use in industries. A corrosion inhibitor can be defined as a substance that when added in suitable quantity to a corrosive environment lowers the corrosion rate significantly [1,2]. One way of classification of inhibitors is (i) inorganic and (ii) organic inhibitors. When compared to conventional surface passivating inorganic inhibitors, organic inhibitors (adsorption-type) in general are attractive due to their high efficiency and environmental friendliness. Organic inhibitors are widely employed in various industries for various aggressive environments. Their inhibition performance is correlated with their chemical structure and physicochemical properties such as the nature of functional groups, electron density at donor atoms, p-orbital character, and the molecular electronic structure. The inhibition is mainly attributed to adsorption and formation of a protective barrier film [1,2]. Among the various organic inhibitors, the best available class is perhaps the heterocyclic compounds. Organic compounds having heteroatoms such as O, N, and S are found to have higher basicity and electron density and thus act as better inhibitors. The first part of the chapter provides a concise description of the fundamentals of heterocyclic compounds (types, structure, and nomenclature) with a view of their application in inhibition technology that is followed by an account on the basics of heterocyclic corrosion inhibitors. For more details on fundamentals of corrosion and corrosion inhibitors, the reader is referred to bonafide textbooks available [1e5].

1.2 Heterocyclic compounds An organic compound containing the carbon atoms arranged in the form of a ring is called a carbocyclic compound. However, if an atom of a different element replaces one of the carbon atoms, then this compound is referred to as a heterocyclic compound [6,7]. Nitrogen (N), sulfur (S), and oxygen (O) are the most commonly occurring heteroatoms, although heteroaromatic rings containing other heteroatoms are also well known. It is noteworthy to mention that the heterocyclic compounds constitute the building blocks of many drugs. The phytochemicals found in different parts of a plant such as the root, stem, leaf, flower, seed, fruit etc., are also composed of complex heterocycles. In addition, several essential/nonessential amino acids, carbohydrates, proteins etc., are also made of heterocycles [7e10]. By definition, any atom other than carbon can be designated as a heteroatom, and the organic ring can be termed as a heterocycle. Among the heterocyclics, the compounds having N, S, O, and P are the most common (Fig. 1.1).

Chapter 1
Heterocyclic corrosion inhibitors

Carbocycle

Pyridine

Pyrrole

3

Heterocycle

Thiophene

Furan

FIGURE 1.1 Examples of heterocyclic compounds.

In a heterocyclic corrosion inhibitor, the lone pair of electrons in the heteroatoms is readily available for donation to the targeted metal and that in effect can result in an effective chemical adsorption of the inhibitor molecule. Further, the heteroatoms when present in acid/alkaline media may undergo protonation/deprotonation resulting in the development of positive/negative charge on the atoms. This can promote either acceptance of electrons from the metal atoms via back donation or electron donation to the metal surface [6,7]. The following section describes an overview of the different types of heterocyclic compounds: (1) Heterocycloalkanes: In these compounds, the ring is saturated, for example, dioxane, thiane, dithiane, piperidine, and piperazine (Fig. 1.2A).

(A)

Cyclohexane

X = NH (Piperidine) X = S (Thiane) X = O (Oxane)

X = NH (Piperazine) X = S (1,4-dithiane) X = O (1,4-dioxane)

X = NH X=S X=O

X = NH X=S X=O

X=N X = S+ X = O+

X = NH (Pyrrole) X = S (Thiophene) X = O (Furan)

X = N (Pyridine) X = S (Thiinium ion) X = O (Pyrylium ion)

X = N (Imidazol)

(B)

Cyclohexene

(C)

Benzene

X = N (Pyrimidine)

FIGURE 1.2 Saturated and unsaturated heterocyclic compounds. Reproduced with permission from T. Eicher, S. Hauptmann, A. Speicher, The Chemistry of Heterocycles: Structures, Reactions, Synthesis, and Applications, Wiley, New Jersey, 2013; Copyright 2013 © John Wiley and Sons.

4 Heterocyclic Organic Corrosion Inhibitors

(2) Heterocycloalkenes (partially unsaturated systems): In these compounds, p bonds are present in ring (Fig. 1.2B). The heteroatom present in the ring can also be the part of a double bond inside the heterocycle. In the case of X ¼ Oþ, the compounds act as oxenium salts, X ¼ Sþ, sulfenium salts, and X ¼ N, imines. (3) Heteroaromatic systems: These are heterocyclic compounds that follow the Hu¨ckel’s Rule, i.e., the rings possessing (4n þ 2) p-electrons. Prominent examples are furan, thiophene, pyrrole, pyridine, imidazole, and pyrimidine (Fig. 1.2C).

1.3 Important heterocyclic systems In this section, we have given some of the important types of heterocyclic systems that constitute the major corrosion inhibitors.

1.3.1

Azoles

Heterocyclic compounds containing one N atom and at least one other noncarbon atoms (e.g., N, S, O) arranged in a five-membered ring are known as azoles. These compounds constitute a wide range of pharmaceutical compounds. Examples include pyrazoles, imidazoles, benzimidazole, benzotriazole, etc.

Pyrazole

1.3.2

Indoles

Indole is a bicyclic structure, in which the benzene ring is fused to a five-membered pyrrole ring. These compounds show a wide range of biological activity and constitute an integral part of many drugs.

Indole

1.3.3

Pyridines

These are six-membered heterocycles with structure similar to that of benzene where one CH is replaced by N atom.

Pyridine

Chapter 1
Heterocyclic corrosion inhibitors

1.3.4

5

Diazines

Diazines are a class of organic compounds having molecular formula C4H4N2, that is, each diazine ring contains a benzene ring where N atoms replace two CH groups. Three isomers of diazines are given below:

Pyridazine (1,2-diazine)

1.3.5

Pyrimidine (1,3-diazine)

Pyrazine (1,4-diazine)

Quinolines

These are heterocyclic compounds having chemical formula C9H7N, where the benzene ring is fused with pyridine. The isomer of quinoline is isoquinoline.

Quinoline

Isoquinoline

Fig. 1.3AeD shows examples of well-known heterocyclic organic compounds. For an easy understanding, the heterocycles are shown in red color throughout the book.

Adenine

Nicotine

Indole-3-acetic acid

FIGURE 1.3A Examples of natural products based on heterocyclic compounds.

Histidine

Histamine

Tryptophan

FIGURE 1.3B Examples of heterocyclic amino acids.

6 Heterocyclic Organic Corrosion Inhibitors

Streptomycin

Carbamazepine

Quinine

FIGURE 1.3C Examples of heterocyclic drugs.

Riboflavin (Vitamin B2)

Niacin (Vitamin B3)

Pyridoxine (Vitamin (B6)

FIGURE 1.3D Examples of heterocyclic vitamins.

1.4 Nomenclature of heterocyclic compounds There are three important systems by which heterocyclic compounds are named: (i) common names, (ii) HantzscheWidman system, and (iii) replacement nomenclature.

1.4.1

Common or trivial names

The common names of heterocyclic compounds are based on the following guidelines: (1) The names are based on occurrence, origin, and special properties. Examples: furan, thiophene, pyrrole, pyridine, indole, quinolone, etc. (2) In a heterocyclic ring, numbering preferably commences at a saturated rather than at an unsaturated heteroatom (Fig. 1.4). (3) If more than one type of the heteroatoms is present, the ring is numbered from the heteroatom of the higher priority (O > S > N). The numbering is done in such a way so that heteroatom gets the smallest value.

Imidazole

Isoxazole

4-bromo-5-methylisoxazole

1,2-dihydropyridine

FIGURE 1.4 Numbering in the heterocyclic rings.

Chapter 1
Heterocyclic corrosion inhibitors

Benzofuran

7

Benzopyrrole

FIGURE 1.5A Naming the heterocyclic ring according to the parent molecule.

Pyridine

1,4dihydropyridine

2H-pyran 2-dihydropyran

4H-pyran 4-dihydropyran

FIGURE 1.5B Nomenclature of partially hydrogenated systems.

(4) Name of the heterocyclic ring is chosen as the parent compound, and the names of the fused ring are attached as a prefix. The prefix in such names have the ending “o,” e.g., benzo, naptho, and so on (Fig. 1.5A). (5) In partially hydrogenated systems, the terms dihydro, trihydro, etc. are used. The number indicates the location of hydrogenation. For example, 2-dihydropyran (2H-pyran), 4-dihydroydropyran (4H-pyran), 1,4-dihydropyridine, 2,3dihydropyridine, etc. (Fig. 1.5B).

1.4.2

HantzscheWidman nomenclature

The HantzscheWidman nomenclature is based on the type of heteroatom, the ring size (n), and the nature of the ring, whether it is saturated or unsaturated. This nomenclature is based on the following guidelines: (1) Type of heteroatom: The type of heteroatom is designated by a prefix, e.g., “aza” for N, “thia” for S, “oxa” for O, and “phospha” for P. (2) Ring size: The ring size of the saturated/unsaturated systems is indicated by appropriate suffixes using Latin numerals as given in Table 1.1. The ending Table 1.1 cles [9].

The ring size and degree of unsaturation of heterocy-

Ring size

Unsaturated

Saturated

Saturated with N

3 4 5 6 7 8

-irene -ete -ole -ine -epine -ocine

-irane -etane -olane -inane -epane -ocane

-iridine -etidine -olidine

8 Heterocyclic Organic Corrosion Inhibitors

indicates the degree of unsaturation in the ring. For example, three-membered saturated and unsaturated heterocycles are named as irane and irine, respectively. Suffix “ir” represents the ring size. If N is present, the name will be iridine. (3) Monocyclic systems: The compound having the highest number of noncumulative double bonds is considered as the parent molecule. The nomenclature is done by linking one or more prefixes with a suffix [7].
Monocyclic systems with one heteroatom: The numbering starts from heteroatom (Fig. 1.6).
Monocyclic systems with two or more identical heteroatoms: The prefixes di-, tri-, tetra-, etc., are used. The numbering is done in such a way that heteroatoms get the smallest number (Fig. 1.7A).
Monocyclic systems with two or more different heteroatoms: For different kinds of heteroatoms, prefixes are used, for example, the preference is as follows: S > N > O. The heteroatom highest in Table 2.1 is assigned the 1-position in the ring, and the left over heteroatoms are allocated the smallest possible set of number locants (Fig. 1.7B):
Identical systems linked by a single bond: In such cases, prefixes bi-, ter-, quater-, etc., are used (Fig. 1.7C) [7]. The above structures in where two or more heterocyclic rings are separated by single bonds are known as isolated heterocyclic compounds. (4) Heterocyclic systems fused with benzene rings: In this case, carbocyclic ring is designated as benzo and trivial name is given to heterocycle (Fig. 1.8).

Pyrrole

Pyridine

Azepine

FIGURE 1.6 Numbering in monocyclic systems with single heteroatom.

(A)

1,2,4-triazole

(B)

Isothiazole (1,2thiazole)

(C)

2,2':4',3''-terthiophene

FIGURE 1.7 Numbering in monocyclic heterocycles (A) having more than one heteroatom, (B) having more than one heteroatom of different types, and (C) identical systems connected by a single bond.

Chapter 1
Heterocyclic corrosion inhibitors

Indole

Quinoline

(benzo[b]pyrole)

(benzo[b]pyridine)

9

FIGURE 1.8 Some of the common fused ring heterocycles.

pyrido[2,3-d]pyrimidine

pyrido[3,2-d]pyrimidine

FIGURE 1.9 Numbering in more than one heterocycles fused together. Reproduced with permission from T. Eicher, S. Hauptmann, A. Speicher, The Chemistry of Heterocycles: Structures, Reactions, Synthesis, and Applications, Wiley, New Jersey, 2013; Copyright 2013 © John Wiley and Sons.

(5) Heterocyclic systems fused with other heterocyclic rings: This category contains compounds where one heterocyclic ring is fused to one or more heterocyclic rings (Fig. 1.9). Here, firstly, the system is divided into its components. The name of the fused component, by replacing the terminal ‘‘e’’ with ‘‘o’’ is added prior to base component’s name. Numbers and letters in square brackets describe the atoms common to both rings, where the order of the numbers must agree to the direction of the lettering of the base component [7].

1.4.3

The replacement nomenclature

In replacement nomenclature, the parent compound is named as carbocyclic compound and the heteroatom as prefix “aza”, “oxa”, and “thia” for N, O, and S ring atom, respectively. The heterocyclic rings are numbered so that the heteroatom has the lowest possible number. (1) Monocyclic systems: The type of heteroatom is indicated by a prefix (Table 1.1). The location and prefix of heteroatoms are written in front of the corresponding hydrocarbon name. The order and numbering of the heteroatoms follows the guidelines as discussed above.

10 Heterocyclic Organic Corrosion Inhibitors

Phenanthrene

3,9-diazaphenanthrene

Bicyclo[2.2.1]heptane

7-oxabicyclo[2.2.1]heptane

FIGURE 1.10 Replacement nomenclature of heterocyclic compounds. Reproduced with permission from T. Eicher, S. Hauptmann, A. Speicher, The Chemistry of Heterocycles: Structures, Reactions, Synthesis, and Applications, Wiley, New Jersey, 2013; Copyright 2013 © John Wiley and Sons.

(2) Bi- and polycyclic systems: Here also the location and prefix are put in front of the hydrocarbon name, with the numbering retained as such (Fig. 1.10). A detailed discussion of the systematic nomenclature for polycyclic systems, in which several aromatic or heteroaromatic rings are fused together, is beyond the scope of this chapter. More details about structure and nomenclature of these compounds are available in Refs. [7,8].

1.5 Heterocyclic systems as corrosion inhibitors There are large families of heterocyclic compounds containing 3, 4, 5, 6, 7, and even larger ring sizes. When compared to 5 or 6-membered heterocyclic compounds, the 3- or 4- membered heterocycles are considerably less stable due to the ring strain [7,8]. Therefore, considering the applicability in corrosion inhibition, only the molecules having five or more ring members will be considered in this book. Then, the benzenefused ring systems are covered followed by the condensed systems having more than two fused heterocycles in the same group.

1.5.1

Five-membered heterocycles

Among the five-membered heterocycles, the N-containing corrosion inhibitors especially those having two or more N atoms are of the greatest importance. Examples are the derivatives of pyrazoles, imidazoles, triazoles, tetrazoles, etc. Next the inhibitors containing O or S atoms in addition to N constitute the important types. Examples are the derivatives of oxazoles, thiazoles, thiadiazoles, etc. Among the fused ringebased heterocycles, there are derivatives of indole, benzimidazole, benzotriazole, benzoxazole, etc. Some of the common five-membered heterocycles with single and fused rings are shown in Figs. 1.11 and 1.12, respectively. Fig. 1.13 shows the structures of some heterocyclic corrosion inhibitors based on fivemembered heterocycles.

Chapter 1
Heterocyclic corrosion inhibitors

Pyrrole

Furan

Thiophene

1H-pyrazole

1H-imidazole

Imidazoline

Imidazolidine

Oxazole

Isoxazole

Thiazole

Isothiazole

1,2,3-triazole

1,2,4-triazole

1,3,4-oxadiazole

1,3,4-thiadiazole

Tetrazole FIGURE 1.11 Five-membered heterocycles.

Indole

Isatin

Benzimidazole

Benzothiazole

Benzoxazole

Benzotriazole

FIGURE 1.12 Five-membered heterocycles with fused rings.

11

12 Heterocyclic Organic Corrosion Inhibitors

2-mercapto-benzimidazole

Imidazole derivative

4-amino-3-hydrazino-5-thio-1,2,4triazole

3-alkyl-5-mercapto-salicylidene 4-amino-1,2,4-triazole

2-aminobenzothiazole

2-salicylideneamino-6-chlorobenzothiazole

2-amino-4-phenylthiazole

2-cinnamalideneamino-4phenylthiazole

2,2’-diamino (1,3,4-thidiazol-2-yl) methane

5,5’- (1,2-phenylene) bis(1,3,4-thidiazol-2-amine)

1-furfurylidene-3thiocarbohydrazide

N, N´-bis (isatin)-1,2-diaminoethane

N, N´-bis (isatin) thiocarbohydrazone

N, N´-bis (isatin)-o-phenylene diamine

FIGURE 1.13 Structures of some heterocyclic corrosion inhibitors based on five-membered heterocycles.

1.5.2

Six-membered heterocycles

Ring strain in six-membered heterocycles is the least [7]. Therefore, they constitute major class of corrosion inhibitors. The key corrosion inhibitors in this group come again from the N-based heterocycles, i.e., pyridine, diazines (pyrazine, pyridazine and

Chapter 1
Heterocyclic corrosion inhibitors

Pyridine

Pyridazine

4H-Pyran

Piperidine

Piperazine

Pyrimidine

Pyrazine

Morpholine

13

FIGURE 1.14 Six-membered heterocycles.

Quinoline

Isoquinoline

Cinnoline

Quinazoline

Quinoxaline

FIGURE 1.15 Six-membered heterocycles with fused rings.

pyrimidine) (Fig. 1.14), triazines, tetrazines, and their derivatives. Among fused rings, the most common inhibitors come from the category of quinoline derivatives (Fig. 1.15). Structures of some of the corrosion inhibitors based on six-membered heterocyclic rings are shown in Fig. 1.16.

1.5.3

Macrocyclic compounds

Macrocycles (large ring compounds) are commonly described as molecules and ions containing 12 or more membered ring. Well-known examples are the crown ethers, calixarenes, porphyrins, and cyclodextrins. The macrocyclic compounds define large, mature area of chemistry, and some of the members that have been used as corrosion inhibitors are shown in Fig. 1.17.

1.6 Effect of substituents on corrosion inhibition efficiency Electron-donating and electron-withdrawing substituents significantly affect the inhibition efficiency of the organic inhibitors as they affect electron density over the active sites. Effect of substituents on the substituted aromatic and aliphatic amines can be

14 Heterocyclic Organic Corrosion Inhibitors

Pyridine

2-mercaptopyrimidine

Acridine & their n-hexadecyl derivatives

1,1’-ethylene-3,3’dimethyl-bispyridinium-iodide

2,2’-Biquinoline

1,10-phenanthroline

7-nitroso-8-hydroxyquinoline

5-acetyl-6-methyl-4-(3-nitro phenyl)-3, 4-dihydro-pyrimidin2(1H)-one

Phenothiazine & vinylpyridine polymers

5-(2-hydroxybenzylidene) pyrimidine-2, 4, 6-trione

N-(morpholino methyl) isatin-3isonicotinoyl hydrazone

N-(2-thiobenzimidazolyl methyl) isatin-3-isonicotinoyl hydrazine

N-(piperadino methyl) isatin-3isonicotinyl hydrazone

N-(morpholino methyl) isatin-3thiocarbohydrazone

7-amino-5-(4-nitrophenyl)-2, 4dioxo-2, 3, 4, 5-tetrahydro-1Hpyrano [2, 3-d] pyrimidine-6carbonitrile

FIGURE 1.16 Structures of some heterocyclic corrosion inhibitors based on six-membered heterocycles.

Chapter 1
Heterocyclic corrosion inhibitors

Tetraphenyl-dithiaoctaazacyclotetradecane hexaene

Tetraphenyl-dithiahexaazacyclobidecane hexaene

Tetraphenyl-dioxo-hexaazacyclo bidecane hexaene

1,2,5,10,13-tetraoxo-1,6,9,14tetraazacyclohexadecane

7,8:15,16-dibenzo-2,5,10,13tetraoxo-1,6,9,14-tetraazacyclohexadecane

7,14-dimethyl-5,12-dioxo-1,4,8,11tetaazacyclotetradeca-1,7-diene

2,3:9,10-dibenzo-7,14-dimethyl5,12-dioxo-1,4,8,11tetraazacyclotetradeca-1,7-diene

3,4:11,12-dibenzo 2,5,10,13tetraoxo-1,6,9,14-tetraazacyclohexaxecane

15

FIGURE 1.17 Macrocyclic compounds reported as corrosion inhibitors. Modified after M. Quraishi, J. Rawat, A review on macrocyclics as corrosion inhibitors, Corrosion Reviews 19 (2001) 273e299.

described with the help of Hammett substituent constants (s). The simplified forms of Hammett equations are given below [2,12e15]: KR ¼ rs KH

(1.1)

1  h%R ¼ rs 1  h%H

(1.2)

h%R qR ¼ rs  log h%H qH

(1.3)

log

log

log

where KH and KR denote the equilibrium constants for adsorptionedesorption process of inhibitor molecule without and with substituent eR, respectively. h%, q, and r correspond to percentage inhibition efficiency, surface coverage, and reaction parameter. Value of the r represents the total electronic effect of the substituent(s) at the active sites

16 Heterocyclic Organic Corrosion Inhibitors

Table 1.2 [12,16].

Values of some Hammett substituent constants (s)

S. No.

Substituent

sm

sp

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

-H -F -Cl -Br -I -NH2 -NO2 -OH -OCH3 -SH -CN -COOH -CHO -CONH2 -CF3 -CH3 -CH2CH3 -CH(CH3)2 -C(CH3)3

0.00 þ0.34 þ0.37 þ0.39 þ0.35 0.16 þ0.71 þ0.12 þ0.12 þ0.25 þ0.56 þ0.36 þ0.36 þ0.28 þ0.43 0.07 0.07 0.07 0.10

0.00 þ0.06 þ0.23 þ0.23 þ0.28 0.66 þ0.78 0.37 0.22 þ0.15 þ0.66 þ0.43 þ0.22 þ0.36 þ0.54 0.17 0.15 0.15 0.20

sm and sp denote the Hammett constant value of substituents present at meta- and paraposition, respectively.

of the inhibitor molecule(s). Table 1.2 denotes the values of Hammett substituent constant for several common substituents present at meta- (m-) and para- (p-) position of the aromatic rings. Generally, the occurrence of electron-releasing substituents such as eOH, eOCH3, eNH2, eNHMe, eNMe2, and eCH3 augments the electron density at the donor sites, which results in the improvement of the inhibition efficiency of the organic inhibitors, whereas electron-withdrawing substituents such as eNO2, eCOOH, and eCOOC2H5 reduce the electron density at the donor sites, thereby decreasing the inhibition efficiency [12,17e22]. Normally, halogens also act as electron-withdrawing (due to negative inductive effect) substituents; therefore, they decrease the protection efficiency of the inhibitors. However, in many cases, halogens can enhance the inhibition efficiency because of their resonance effect [12,20,22e26]. In general, an inhibitor having hydrophobic chain(s) shows better protection efficiency than the inhibitor without the hydrophobic chain. The increasing carbon chain length also enhances inhibition performance by increasing hydrophobic nature of the inhibitors [12,27]. Fig. 1.18 shows the variation of corrosion rates with s for several organic inhibitors [12,28e30]. The corrosion inhibitor having the highest negative value of s displayed the lowest corrosion rate and thus the highest inhibition efficiency that could be credited to the highest capability of electron donation. The presence of electron-withdrawing

Chapter 1
Heterocyclic corrosion inhibitors

17

FIGURE 1.18 Variation in corrosion rates of (A) 5-arylazothiazole, (B) 1-methyl-4[4](-X)-styryl pyridinium iodides, and (C) 1-benzoyl-4-phenyl-3-thiosemicarbazide derivatives having different substituents with their Hammett constant (s) values. Reproduced with permission from C. Verma, L. Olasunkanmi, E.E. Ebenso, M. Quraishi, Substituents effect on corrosion inhibition performance of organic compounds in aggressive ionic solutions: a review, Journal of Molecular Liquids, 251 (2018) 100e118; Copyright 2018 © Elsevier.

groups (having a positive value of s) resulted in a rise in the corrosion rate and a diminution in the efficiency. These observations support the abovementioned discussion on the relation between s and h%. Detailed descriptions on various heterocyclic compounds can be found in Chapters 4e7 based on their applications in acidic, sweet and sour, neutral, and alkaline media. Vapor phase and green corrosion inhibitors are provided in Chapters 8 and 9, respectively.

Suggested reading M.G. Fontana, Corrosion Engineering, Tata McGraw-Hill Education, 2005. V.S. Sastri, Corrosion inhibitors: Principles and applications, Wiley, 1998.

18 Heterocyclic Organic Corrosion Inhibitors

T. Eicher, S. Hauptmann, A. Speicher, The Chemistry of Heterocycles: Structures, Reactions, Synthesis, and Applications, John Wiley and Sons, 2013. J.A. Joule, K. Mills, Heterocyclic Chemistry at a Glance, John Wiley and Sons, 2012. J. Alvarez-Builla, J.J. Vaquero, J. Barluenga, Modern Heterocyclic Chemistry, John Wiley and Sons, 2011. D.T. Davies, Aromatic Heterocyclic Chemistry, Oxford University Press, 1992.

References [1] V.S. Sastri, Corrosion Inhibitors: Principles and Applications, Wiley, New York, 1998. [2] V.S. Sastri, Green Corrosion Inhibitors: Theory and Practice, Wiley, New Jersey, 2012. [3] M.G. Fontana, Corrosion Engineering, Tata McGraw-Hill Education, New Delhi, 2005. [4] R.W. Revie, Uhlig’s Corrosion Handbook, Wiley, New Jersey, 2011. [5] R. Cottis, M. Graham, R. Lindsay, S. Lyon, T. Richardson, D. Scantlebury, H. Stott, Shreir’s Corrosion, Elsevier, Amsterdam, 2010. [6] D.T. Davies, Aromatic Heterocyclic Chemistry, Oxford University Press, New York, 1992. [7] T. Eicher, S. Hauptmann, A. Speicher, The Chemistry of Heterocycles: Structures, Reactions, Synthesis, and Applications, Wiley, New Jersey, 2013. [8] J.A. Joule, K. Mills, Heterocyclic Chemistry at a Glance, Wiley, New Jersey, 2012. [9] S. Derese, Nomenclature of Heterocyclic Compounds, Lecture Notes, University of Nairobi, Kenya. [10] S. Kirchhecker, M. Antonietti, D. Esposito, Hydrothermal decarboxylation of amino acid derived imidazolium zwitterions: a sustainable approach towards ionic liquids, Green Chemistry 16 (2014) 3705e3709. [11] M. Quraishi, J. Rawat, A review on macrocyclics as corrosion inhibitors, Corrosion Reviews 19 (2001) 273e299. [12] C. Verma, L. Olasunkanmi, E.E. Ebenso, M. Quraishi, Substituents effect on corrosion inhibition performance of organic compounds in aggressive ionic solutions: a review, Journal of Molecular Liquids 251 (2018) 100e118. [13] J. Leffler, E. Grunwald, Rates and Equilibria of Organic Reactions, Wiley, New York, 1963. [14] F.M. Donahue, K. Nobe, Theory of organic corrosion inhibitors adsorption and linear free energy relationships, Journal of the Electrochemical Society 112 (1965) 886e891. [15] Z. Szklarska-Smialowska, M. Kaminski, Effect of various substituents in thiophene on the inhibitor efficiency, Corrosion Science 13 (1973) 1e10. [16] E. McCafferty, Introduction to Corrosion Science, Springer Science & Business Media, New York, 2010. [17] S.K. Saha, A. Dutta, P. Ghosh, D. Sukul, P. Banerjee, Novel Schiff-base molecules as efficient corrosion inhibitors for mild steel surface in 1 M HCl medium: experimental and theoretical approach, Physical Chemistry Chemical Physics 18 (2016) 17898e17911. [18] S. Deng, X. Li, H. Fu, Two pyrazine derivatives as inhibitors of the cold rolled steel corrosion in hydrochloric acid solution, Corrosion Science 53 (2011) 822e828. [19] M. Yadav, L. Gope, N. Kumari, P. Yadav, Corrosion inhibition performance of pyranopyrazole derivatives for mild steel in HCl solution: gravimetric, electrochemical and DFT studies, Journal of Molecular Liquids 216 (2016) 78e86.

Chapter 1
Heterocyclic corrosion inhibitors

19

[20] A. Fouda, A. Mohamed, H. Mostafa, Inhibition of corrosion of copper in nitric acid solution by some arylmethylene cyanothioacetamide derivatives, Journal de Chimie Physique et de Physico-Chimie Biologique 95 (1998) 45e55. [21] N. Soltani, M. Behpour, E. Oguzie, M. Mahluji, M. Ghasemzadeh, Pyrimidine-2-thione derivatives as corrosion inhibitors for mild steel in acidic environments, RSC Advances 5 (2015) 11145e11162. [22] H. Hamani, T. Douadi, M. Al-Noaimi, S. Issaadi, D. Daoud, S. Chafaa, Electrochemical and quantum chemical studies of some azomethine compounds as corrosion inhibitors for mild steel in 1 M hydrochloric acid, Corrosion Science 88 (2014) 234e245. [23] I. Ahamad, R. Prasad, M. Quraishi, Adsorption and inhibitive properties of some new Mannich bases of Isatin derivatives on corrosion of mild steel in acidic media, Corrosion Science 52 (2010) 1472e1481. [24] M. Hegazy, H. Ahmed, A. El-Tabei, Investigation of the inhibitive effect of p-substituted 4-(N, N, Ndimethyldodecylammonium bromide) benzylidene-benzene-2-yl-amine on corrosion of carbon steel pipelines in acidic medium, Corrosion Science 53 (2011) 671e678. [25] M. Bahrami, S. Hosseini, P. Pilvar, Experimental and theoretical investigation of organic compounds as inhibitors for mild steel corrosion in sulfuric acid medium, Corrosion Science 52 (2010) 2793e2803. ¨ zmen, M. Kabasakalog ¨. O lu, Investigation of some Schiff bases as acidic corrosion of [26] A. Aytac, U alloy AA3102, Materials Chemistry and Physics 89 (2005) 176e181. [27] J. De Damborenea, J. Bastidas, A. Vazquez, Adsorption and inhibitive properties of four primary aliphatic amines on mild steel in 2 M hydrochloric acid, Electrochimica Acta 42 (1997) 455e459. [28] M. Abdallah, H. Al-Tass, B.A. Jahdaly, A. Fouda, Inhibition properties and adsorption behavior of 5arylazothiazole derivatives on 1018 carbon steel in 0.5 M H2SO4 solution, Journal of Molecular Liquids 216 (2016) 590e597. [29] E.A. Noor, A.H. Al-Moubaraki, Thermodynamic study of metal corrosion and inhibitor adsorption processes in mild steel/1-methyl-4 [40 (-X)-styryl pyridinium iodides/hydrochloric acid systems, Materials Chemistry and Physics 110 (2008) 145e154. [30] H. Mostafa, S.A. El-Maksoud, M. Moussa, Effect of 1-benzoyl-4-phenyl-3-thiosemicarbazide derivatives on the corrosion inhibition of copper in nitric acid, Portugalia Electrochimica Acta 19 (2001) 109e120.

2 Experimental methods of inhibitor evaluation Chapter outline 2.1 Introduction ................................................................................................................................... 22 2.2 Gravimetric method...................................................................................................................... 22 2.2.1 Effect of concentration....................................................................................................... 23 2.2.2 Effect of temperature and activation parameters .......................................................... 24 2.3 Adsorption parameters ................................................................................................................ 26 2.3.1 Adsorption isotherms.......................................................................................................... 26 2.3.2 Adsorption energy .............................................................................................................. 30 2.4 Electrochemical methods ............................................................................................................. 30 2.4.1 Open circuit potential vs. time .......................................................................................... 30 2.4.2 Electrochemical impedance spectroscopy......................................................................... 31 2.4.3 Potentiodynamic polarization............................................................................................ 35 2.4.4 Electrochemical frequency modulation ............................................................................ 37 2.4.5 Linear polarization resistance ............................................................................................ 40 2.5 Surface analytical techniques ...................................................................................................... 40 2.5.1 Water contact angle ........................................................................................................... 41 2.5.2 Scanning electron microscopy............................................................................................ 42 2.5.3 Energy-dispersive X-ray spectroscopy................................................................................ 42 2.5.4 Atomic force microscopy .................................................................................................... 44 2.5.5 X-ray diffraction .................................................................................................................. 45 2.5.6 Fourier transform infrared spectroscopy .......................................................................... 46 2.5.7 X-ray photoelectron spectroscopy..................................................................................... 47 2.5.8 Time-of-flight secondary ion mass spectrometry............................................................. 49 Suggested reading ............................................................................................................................... 51 References............................................................................................................................................. 52

Heterocyclic Organic Corrosion Inhibitors. https://doi.org/10.1016/B978-0-12-818558-2.00002-3 Copyright © 2020 Elsevier Inc. All rights reserved.

21

22 Heterocyclic Organic Corrosion Inhibitors

2.1 Introduction A corrosion inhibitor, when introduced to the corrosive solution, undergoes adsorption over the target metal surface. Therefore, to properly understand the adsorption and the corrosion inhibition efficiency of the adsorbed layer, it is a prerequisite to perform specific tests. These include inhibitor efficiency evaluating tests, mechanism elucidating tests, adsorption tests, surface characterization tests, and so on. These tests, based on the operating principle and the data obtained, can be divided into many categories. ASTM standards are a primary reference for the test procedures [1]. The first and the foremost test is the immersion test in which the information such as the concentration of inhibitor required for optimum performance is obtained [2e4]. In spite of the numerous advancements in the testing technology and equipment, the weight loss method remains the most simple yet most reliable method of inhibitor evaluation. The next category of tests is the electrochemical investigation of the metal sample, which is carried out using suitable electroanalytical techniques. Some of the methods viz. impedance spectroscopy, electrochemical frequency modulation (EFM), polarization resistance etc., are nondestructive and provide real-time information of the electrochemical behavior [5e9]. Other techniques such as the potentiodynamic polarization (PDP) are destructive methods after the use of which no further test can be performed on the studied metal surface. These methods and the obtained electrochemical parameters provide quantitative information of the inhibitor adsorption and the electrochemical behavior of the metal surface in the corrosive solution. The third category is the direct examination of the surface using techniques of microscopic imaging, spectroscopy, crystallinity, wettability, etc., which provide information of the chemical structure and elemental composition of the metal surface upon inhibitor adsorption [2,9,10]. These techniques also help in elucidating the mechanism of inhibitor adsorption on the metal surface and the metaleinhibitor interaction. In this chapter, we have described all the typical experimental methods used frequently to study corrosion inhibitors. A typical case of the behavior of a corrosion inhibitor on metal surface in a corrosive acidic medium is taken depending upon the descriptions of the experimental and characterization tests provided.

2.2 Gravimetric method The weight loss test provides the simplest, cost-effective, and the most common means for evaluating the behavior of an inhibitor in terms of the corrosion inhibition efficiency. Usually, the first step is to cut the metal sample under investigation into small-sized coupons having known dimensions followed by degreasing, cleaning, and polishing of the samples. Subsequently, the samples are immersed in the corrosive media and then removed after a known amount of time typically 6, 12, or 24 h and cleaned following ASTM standard protocols [11e14]. The difference between the mass before and after immersion provides the values of the average weight loss [15].

Chapter 2  Experimental methods of inhibitor evaluation

23

The corrosion rate (CR) can be defined as the speed at which the deterioration takes place in a specific environment. It can also be described as the amount of corrosion loss per year in thickness. The (CR; mg cm
2 h
1) is estimated by the following relationship: CR ¼

w At

(2.1)

where w symbolizes the average weight loss (mg), A denotes the area of the sample (cm2), and t denotes the time of exposure (h). The surface coverage (q) and the corrosion inhibition efficiency (h%) can be determined from the values of the CR [2,5,8,16]: q¼ h% ¼

CR
CRðiÞ CR

CR
CRðiÞ  100 CR

(2.2)

(2.3)

where CR and CR(i) denote the corrosion rates obtained in the absence and presence of a corrosion inhibitor correspondingly. Alternatively, the CR can be expressed into its standard unit (mm y
1 or mmpy) using the following relationship [17]: CR ¼

87:6w Atd

(2.4)

where d represents the density of the metal sample in mg cm
3.

2.2.1

Effect of concentration

The inhibition behavior of an organic compound can be studied using the weight loss method by adding varying concentrations of a corrosion inhibitor in increasing order (Fig. 2.1) [2,18,19]. The corrosion inhibition efficiency (h%) first rises with the

FIGURE 2.1 Effect of concentration for the adsorption of chitosan (CS) cross-linked with thiosemicarbazide (TS) and thiocarbohydrazide (TCH) on mild steel in 1M HCl. Reproduced with permission from D.S. Chauhan, K. Ansari, A. Sorour, M. Quraishi, H. Lgaz, R. Salghi, Thiosemicarbazide and thiocarbohydrazide functionalized chitosan as ecofriendly corrosion inhibitors for carbon steel in hydrochloric acid solution, International Journal of Biological Macromolecules 107 (2018) 1747e1757; Copyright 2018 © Elsevier.

24 Heterocyclic Organic Corrosion Inhibitors

concentration and then reaches a constant value beyond which the addition of inhibitor either does not produce any further increase in efficiency or causes a slight reduction in the efficiency. This concentration can be termed as the optimum inhibitor concentration. The gradual reduction in the CR with rise in inhibitor concentration indicates the development of a protective surface film, which provides a greater surface coverage and reduces the extent of active sites available for corrosion. This behavior advocates that the inhibitor acts by adsorption at the metal/solution interface. The durability of inhibitor film can be analyzed by performing the weight loss experiment for longer immersion times using the optimum concentration of the inhibitor. If the inhibition efficiency remains almost constant or shows only a slight decline with the increase in immersion time, the studied molecule can be understood as an effective corrosion inhibitor.

2.2.2

Effect of temperature and activation parameters

Temperature is a main kinetic factor that can strongly affect the corrosion behavior of metals and cause a variation in the strength of inhibitor adsorption. The weak physical interactions (electrostatic attraction) responsible for the inhibitor adsorption can disappear with elevation in temperatures [20]. The increase in temperature raises the kinetic energy of the molecules and causes difficulty in physical adsorption, resulting in the shift of the adsorption equilibrium toward the desorption process, which leads to a lowering in the surface coverage and a reduction in the inhibition efficiency [21]. On the other hand, if the efficiency does not change or increases slightly with temperature, this suggests that the inhibitor undergoes chemical adsorption by forming strong chemical bonds with the metal surface. However, it should be noted that this is the description of the typical behavior of a corrosion inhibitor in acid solutions, and depending upon the variation in experimental parameters, this trend may vary. An interesting case is the sweet corrosion of steel in which a rise in temperature first increases the corrosion rate, and beyond a certain limit, the rise in temperature actually helps in reducing the corrosion rate that further depends upon the conditions of pH, pCO2, etc., as discussed in Chapter 5. The reliance of corrosion rate on temperature can be computed from the Arrhenius and the transition state equations as shown below [2,22,23]: CR ¼ A exp

CR ¼



Ea RT



 RT DS DH  exp
exp R RT Nh

(2.5)

(2.6)

where Ea denotes the energy of activation, A denotes the Arrhenius preexponential factor, R and T denote the universal gas constant and the absolute temperature, respectively, N and h symbolize the Avogadro number and the Plank’s constant, respectively, and DS and DH  correspond to the entropy and enthalpy of activation,

Chapter 2  Experimental methods of inhibitor evaluation

25

FIGURE 2.2 (A) Plots of the corrosion rate (CR) of mild steel without and with the optimum concentration of isatin bis-Schiff bases in 1M HCl. (B) Corresponding transition state plots. Reproduced with permission from K. Ansari, M. Quraishi, Bis-Schiff bases of isatin as new and environmentally benign corrosion inhibitor for mild steel, Journal of Industrial and Engineering Chemistry 20 (2014) 2819e2829; Copyright 2014 © Elsevier.

respectively. A graph of log CR vs. 1=T is called Arrhenius plot, which provides a straight line with a slope of Ea =2:303R (Fig. 2.2A). A plot of log CR =T vs. 1=T provides a straight line with slope of DH  =2:303R and intercept of log ðR =NhÞ þ ðDS=2:303RÞ (Fig. 2.2B) known as the transition state plot, which allows the calculation of DS and DH  . A greater value of Ea in the existence of inhibitor indicates an upsurge in the double layer thickness, which increases the activation energy barrier to be overcome for the process of corrosion to take place [24,25]. The increase in the energy of activation in the presence of an inhibitor is commonly ascribed to the physical adsorption, which takes place as the first step of adsorption [26]. Contrariwise, a lesser value of Ea in the presence of an inhibitor indicates chemical adsorption. However, the type of adsorption cannot be determined only based on the trends observed in the Ea values because of the competitive adsorption taking place with the water molecules that are preadsorbed on the steel surface whose removal also requires some activation energy [27]. A positive DH  value suggests a slower metal dissolution [23,24]. A rise in the value of DH  in the occurrence of an inhibitor indicates a rise in the energy barrier for corrosion. Similarly, a rise in the DS value in the existence of an inhibitor can be ascribed to the rise in randomness on moving from the reactants to the activated complex attributed to the inhibitor adsorption on the metal surface [28]. The inhibitor adsorption is accompanied by the simultaneous desorption of water molecules. Therefore, a rise in the solvent entropy is reflected as a rise in the DS value [23,28].

26 Heterocyclic Organic Corrosion Inhibitors

2.3 Adsorption parameters 2.3.1

Adsorption isotherms

The nature of interaction taking place between a metal surface and the corrosion inhibitor molecules can be understood by employing different adsorption isotherm models [17,29]. The choice of an appropriate adsorption isotherm has a key role in the understanding of adsorption process. This knowledge is useful for determining the standard adsorption free energy and its relationship with the surface coverage, the nature of inhibitor film at the metal/solution interface, and the interaction of the inhibitor molecules with the surface atoms of metal samples [4,30]. Herein, some of the commonly used adsorption isotherms for steel corrosion are discussed briefly: (1) Langmuir isotherm: This model assumes that the adsorbed inhibitor film has the thickness of a monolayer, and the adsorption takes place on a fixed number of similar localized equilibrium sites. There is no interaction among the sites. This isotherm can be given as [2,31,32] C 1 ¼ þC q Kads

(2.7)

where Kads symbolizes the equilibrium constant of adsorption, C denotes the inhibitor concentration in mol L
1, and q represents the surface coverage (Fig. 2.3). (2) Temkin isotherm: This model can be given as expð
2aqÞ ¼ Kads C

(2.8)

where C denotes the inhibitor concentration, and a is the parameter of interaction between the adsorbing species and the surface [33,34]. On plotting the surface

FIGURE 2.3 Plot of Langmuir isotherm for adsorption of chitosan derivatives corresponding to Fig. 2.1. Reproduced with permission from D.S. Chauhan, K. Ansari, A. Sorour, M. Quraishi, H. Lgaz, R. Salghi, Thiosemicarbazide and thiocarbohydrazide functionalized chitosan as ecofriendly corrosion inhibitors for carbon steel in hydrochloric acid solution, International Journal of Biological Macromolecules 107 (2018) 1747e1757; Copyright 2018 © Elsevier.

Chapter 2  Experimental methods of inhibitor evaluation

27

coverage values vs. logarithm of inhibitor concentration (Fig. 2.4), a slope of 2.303/a suggests that the inhibitor adsorption follows the Temkin isotherm. (3) Frumkin isotherm: The Frumkin adsorption isotherm can be shown as (Fig. 2.5) [18] KC ¼

q
f q e 1
q

(2.9)

FIGURE 2.4 Plot of Temkin isotherm for adsorption of methionine on mild steel surface in 0.5M H2SO4. Reproduced with permission from E. Oguzie, Y. Li, F. Wang, Corrosion inhibition and adsorption behavior of methionine on mild steel in sulfuric acid and synergistic effect of iodide ion, Journal of Colloid and Interface Science 310 (2007) 90e98; Copyright 2007 © Elsevier.

FIGURE 2.5 Frumkin isotherms for the adsorption of diazoles on mild steel in 1 M HCl. Reproduced with permission from A. Popova, M. Christov, S. Raicheva, E. Sokolova, Adsorption and inhibitive properties of benzimidazole derivatives in acid mild steel corrosion, Corrosion Science 46 (2004) 1333e1350; Copyright 2004 © Elsevier.

28 Heterocyclic Organic Corrosion Inhibitors

FIGURE 2.6 Freundlich isotherm for adsorption of Morinda tinctoria extract on copper in 0.5 M HCl. Reproduced with permission from K. Krishnaveni, J. Ravichandran, Influence of aqueous extract of leaves of Morinda tinctoria on copper corrosion in HCl medium, Journal of Electroanalytical Chemistry 735 (2014) 24e31; Copyright 2014 © Elsevier.

where f denotes the parameter for lateral interaction between the adsorbed molecules of inhibitor, and f >0 represents attraction and f unity, which implies that each molecule of inhibitor occupies >1 active sites on the Cu surface (Fig. 2.7). (7) FloryeHuggins isotherm: This model can be given as follows (Fig. 2.8) [38,39]: q ¼ Kads C xð1
qÞx

(2.13)

FIGURE 2.7 The El-Awady isotherm for adsorption of 8-hydroxyquinoline on copper in 0.1M HCl. Reproduced with permission from H. Gerengi, M. Mielniczek, G.k. Gece, M.M. Solomon, Experimental and quantum chemical evaluation of 8-hydroxyquinoline as a corrosion inhibitor for copper in 0.1 M HCl, Industrial & Engineering Chemistry Research 55 (2016) 9614e9624; Copyright 2016 © American Chemical Society.

FIGURE 2.8 FloryeHuggins isotherm for pantoprazole sodium adsorption in different acid solutions on Cu surface. Reproduced with permission from M. Saadawy, Inhibitive effect of pantoprazole sodium on the corrosion of copper in acidic solutions, Arabian Journal for Science and Engineering 41 (2016) 177e190; Copyright 2016 © King Fahd University of Petroleum and Minerals.

30 Heterocyclic Organic Corrosion Inhibitors

The FloryeHuggins model can also be expressed as log½q=xð1
qÞx  ¼ log K 0 þ y log C

(2.14)

As can be observed in the above equation, the y parameter provides the number of inhibitor molecules wherein each occupies an active site. K 0 value can be computed from Eq. (2.15) relating K 0 and y: K ¼K0

2.3.2

ð1=yÞ

(2.15)

Adsorption energy

The standard free energy of adsorption can be obtained from adsorption models as shown in Eq. (2.16). The equilibrium constant for adsorption Kads can be determined from the abovementioned isotherm plots and is correlated to the adsorption free energy [8,23]: o DGads ¼
RT lnð55:5Kads Þ

(2.16)

where the value 55 represents the concentration of water in molar, which otherwise can be 1000 g L
1 depending upon the unit of the inhibitor concentration. Generally, the o values of DGads bear a negative sign, which indicates the spontaneity of the adsorption o process. A value of DGads w
20 kJ mol
1 or lower negative values indicate physical adsorption and w
40 kJmol
1 or higher negative values indicate chemical adsorption [8,23]. An intermediate value suggests the occurrence of both physical and chemical adsorption. Other parameters such as the enthalpy and the entropy of adsorption can also be calculated [40]. The heat of adsorption of a corrosion inhibitor on a metal surface can be computed using Eq. (2.17): 

 
q2 q1 T1 T2
log  Qads ¼ 2:303R log 1
q2 1
q1 T2
T1

(2.17)

where q1 and q2 are the surface coverages at temperatures T1 and T2, respectively. A positive value of Qads suggests chemical adsorption [41].

2.4 Electrochemical methods 2.4.1

Open circuit potential vs. time

The open circuit potential (OCP) or EOCP represents the potential of a working electrode with respect to a reference electrode in the absence of an external potential or current. Prior to performing the electrochemical measurements, it is necessary to achieve a stable OCP. The variation of the OCP of a mild steel electrode immersed in 1 M HCl without and with varying concentrations of a pyranopyrazole derivative is shown in Fig. 2.9 [42]. The results display that the addition of the inhibitors to 1 M

Chapter 2  Experimental methods of inhibitor evaluation

31

FIGURE 2.9 OCP curves obtained in 1M HCl for adsorption of pyranopyrazole derivative in 1 M HCl. Reproduced with permission from D.K. Yadav, M. Quraishi, Electrochemical investigation of substituted pyranopyrazoles adsorption on mild steel in acid solution, Industrial & Engineering Chemistry Research 51 (2012) 8194e8210; Copyright 2012 © American Chemical Society.

HCl provides a shift in the OCP to higher negative values, which indicates the formation of a protective inhibitor film.

2.4.2

Electrochemical impedance spectroscopy

Impedance means the opposition to the flow of alternating current (AC) in a complex system. The electrochemical impedance spectroscopy (EIS) is a nondestructive measurement that reveals the frequency response of an electrochemical system including the energy storage and dissipation. The major advantages of this technique are it (i) is applicable to low conductivity systems; (ii) provides mechanistic information; and (iii) allows the measurement of solution resistance. Usually, a small amplitude AC signal (w10 mV) and the frequency range of 100 kHz to 10 mHz is employed. The obtained electrochemical data are fitted using nonlinear least square fit method to a suitable equivalent circuit to explain the complex response. The data are displayed in the form of Nyquist (Zreal vs.
Zim), Bode (logf vs. jZj), and phase angle plots (logf vs. a0) [2]. The Zreal symbolizes the real part of impedance (coming from the pure resistors), Zim denotes the complex impedance (coming from any capacitors or inductors in the circuit), f is the frequency, jZj is the impedance modulus, and a0 is the phase angle. The Nyquist plot is the most commonly used graphical representation of EIS data, which allows easy understanding of the electrochemical behavior of the corroding system. It also offers some easy prediction of the equivalent circuit elements as depicted in Fig. 2.10. The response of an electrochemical system as a function of small amplitude of perturbation frequency gives information about the internal kinetics of the corroding system. Generally, for a metal corroding in aqueous solution (acid/neutral/alkaline), the equivalent circuit consists of a parallel combination of capacitance and resistance

32 Heterocyclic Organic Corrosion Inhibitors

FIGURE 2.10 Typical Nyquist plot for a metal surface undergoing corrosion in acid solution. Inset shows the modified Randles circuit containing CPE in place of Cdl. Reproduced with permission from D.K. Yadav, D. Chauhan, I. Ahamad, M. Quraishi, Electrochemical behavior of steel/acid interface: adsorption and inhibition effect of oligomeric aniline, RSC Advances 3 (2013) 632e646; Copyright 2013 © Royal Society of Chemistry.

(which represent the corroding interface) in series with the second (uncompensated) resistance. The values of various circuit components can be calculated from the series of the equivalent circuit. In the given circuit, the RU or Rs represents the resistance of the electrolyte caused in the flow of electric charge. The parallel resistance Rct, which determines the rate of corrosion, is known as the charge transfer resistance. The capacitance occurring at metal/electrolyte interface is generally represented by the double layer capacitance, Cdl. In corrosion inhibition studies, a higher value of Rct reflects the creation of a barrier that needs to be overcome to promote the charge transfer during corrosion, indicating a decrease in the corrosion rate [2]. Generally, the Rct is used to symbolize the difference in the real part of the resistance between the lowest and the highest frequencies; however, in case where a metal surface is immersed in an aggressive medium and is undergoing uniform corrosion, additional resistance parameters become significant (e.g., film resistance Rf , diffuse layer resistance Rd , and the resistance from other accumulations Ra ). The collection of these resistance values is designated as the polarization resistance ðRP Þ. This convention of the usage of RP instead of Rct is recommended in a number of studies [2,7,43,44].

Chapter 2  Experimental methods of inhibitor evaluation

33

In the representative Nyquist impedance diagram shown in Fig. 2.10, a depressed semicircle can be observed with the center lying under the real x-axis. This is a characteristic behavior of solid electrodes and can be commonly referred as the frequency dispersion that can be ascribed to roughness and nonhomogeneity of the surface, grain boundaries, impurities, and the active site distribution. Therefore, a constant phase element (CPE) instead of a pure double layer capacitor (Cdl) is commonly applied to obtain a more accurate fit of experimental data. The impedance of the CPE can be given as [25,45] ZCPE ¼ Yo
1 ðjuÞ
n

(2.18)

where Y0 denotes the magnitude of CPE, j is the square root of
1, and n is the phase shift, which provides a measure of the surface heterogeneity and u symbolizes the angular frequency (u ¼ 2pfmax, where fmax denotes the AC frequency maximum) [8]. The CPE can be expressed in the form of the classical lumped elements for n ¼
1 (Y ¼ L), n ¼ 0 (Y ¼ R), n ¼ 0.5 (Y ¼ W), and n ¼ 1 (Y ¼ C), where L, R, W, and C represents inductance, resistance, Warburg impedance, and capacitance, respectively. The increase in the diameter of the Nyquist plots upon adding an inhibitor can be attributed to the inhibitor adsorption at the metal/solution interface. The quantitative values of Rct can be used for computing the corrosion inhibition efficiency: hEIS % ¼

Rinh ct
Rct  100 Rinh ct

(2.19)

where the symbols Rct and Rinh ct denote the respective Rct values in the absence and the presence of the inhibitor. The precision of the fitted EIS data can be estimated by the “goodness of fit” chi-squared (c2) values, wherein small values (often in the order of 10
3 to 10
4) support the suitability of the circuit for accurately simulating the EIS data. The values of the double layer capacitance ðCdl Þ can be estimated as [2,46] Cdl ¼ Yo ðumax Þn
1

(2.20)

where umax denotes the frequency (rad s
1) at which the imaginary impedance attains the maximum value. The Cdl value can be associated with the thickness of the protective film of the inhibitor molecules ðdorg Þ using Helhmoltz model [19,47]: Cdl ¼

ε0 εr dorg

(2.21)

where ε0 and εr denote the vacuum and the relative dielectric constants, respectively. Water molecules possess a higher dielectric constant than organic inhibitor molecules. The displacement of water molecules by the inhibitor molecules causes an overall lowering of the dielectric constant. Thus, the inhibitor adsorption causes an apparent lowering of the Cdl values, which supports the inhibition behavior [48,49]. The relaxation time constant ðsÞ is given according to the dielectric theory as s¼

1 2pfmax

(2.22)

34 Heterocyclic Organic Corrosion Inhibitors

This is the required time for the return of the charge distribution to the equilibrium following an electrical perturbation in the absence of any distributed element for replacing the double layer capacitance: s ¼ Cdl Rct

(2.23)

The Nyquist plots are the most commonly used representations of the EIS data. However, there is a major limitation that from these plots, the impedance behavior at particular scanning frequency is not identified. For this purpose, the Bode and the phase angle plots provide useful information. The Nyquist, Bode, and the phase angle plots for carbon steel immersed in 1M HCl in the presence of different concentrations of chitosan-aminomercaptotriazole (CS-AMT) inhibitor are shown in Fig. 2.11 [50]. Fig. 2.11 shows the Nyquist plots where the diameter rises with increase in the inhibitor (CS-AMT) concentration showing the protection effect. This behavior may vary depending upon the metal under study and the corrosive solution used (e.g., acid, neutral, alkaline, etc.). The higher frequency regions are associated with the solution resistance [51,52]. Linearity between the log jZj versus logf with a slope close to 0.9 and the phase angle close to
70 degrees suggests the capacitive behavior at the intermediate frequency values. The phase angles close to the ideal capacitor (slope ¼
1; phase angle ¼
90 degrees) are ascribed to a lowering in the rate of dissolution, which supports the inhibition by CS-AMT [50]. In addition to the above discussion, it is noteworthy to mention that the EIS technique also allows the determination of surface charge developed on metal substrates immersed in the corrosive electrolytes [43,44,53]. Furthermore, in addition to the

FIGURE 2.11 (A) Nyquist diagrams for carbon steel obtained in the absence and the presence of a chitosan derivative in 1M HCl. (B) Corresponding Bode (logf vs. log jZj) and phase angle (logf vs. a0) plots. Reproduced with permission from D.S. Chauhan, M. Quraishi, A. Sorour, S.K. Saha, P. Banerjee, Triazole-modified chitosan: a biomacromolecule as a new environmentally benign corrosion inhibitor for carbon steel in a hydrochloric acid solution, RSC Advances 9 (2019) 14990e15003; Copyright 2019 © Royal Society of Chemistry under CC-BY.

Chapter 2  Experimental methods of inhibitor evaluation

35

potentiostatic EIS measurements, the potentiodynamic EIS (also called the dynamic EIS or simply DEIS) technique allows determining the evolution of the Rct over a wide range of time [41,54].

2.4.3

Potentiodynamic polarization

Here, after attaining the steady-state OCP, the potential is scanned usually within 250 mV vs. OCP, and the response is recorded in the form of a graphical representation of log current density (logi) vs. potential (E) [8,55,56]. PDP is carried out at the last in the electrochemical experiments because unlike the EIS, this is a destructive technique that changes the electrode surface. The term corrosion potential (Ecorr) is generally reserved to define the potential at which there is no net current flow, as determined by fitting the current vs. potential data. In an ideal case, the values of EOCP (OCP) and Ecorr will be identical. The observed difference in the values of the two may be attributed to the changes in the electrode surface occurring during a potential sweep. If the potential of a working electrode is taken far enough in the positive direction with respect to EOCP, the current due to the cathodic reduction will be negligible, and the observed current response will contain only the anodic contribution. On the other hand, at high negative potentials, the cathodic current will dominate the net current response. An electrochemical process under the condition of kinetic control obeys the Tafel equation: 0 i ¼ i0 eð2:303ðE
E Þ=bÞ

(2.24) 0

where i is the current resulting from the reaction, i is called the exchange current, which is a reaction-dependent constant, E is the electrode potential, E0 is the equilibrium potential, which is constant for a given reaction, and b is the Tafel slope, which is also constant for a given reaction and has units of V/decade. The Tafel equations for the anodic and cathodic reactions can be combined to obtain the ButlereVolmer equation: i ¼ i0 eð2:303ðE
Ecorr Þ=ba Þ
e ð2:303ðE
Ecorr Þ=bc Þ

(2.25)

where i is the measured cell current, icorr is the corrosion current, Ecorr is the corrosion potential, ba and bc are the anodic and the cathodic Tafel slopes, respectively, in V/decade. According to the above equation, at Ecorr, each exponential term equals to one. The net cell current is therefore zero. Close to Ecorr, the contribution from both the exponential terms provides the overall current. As the potential is swept far from Ecorr, one exponential term predominates and the other becomes negligible. In this condition, a plot of log current vs. potential provides a straight line. A logi vs E plot is generally called a Tafel plot (Fig. 2.12), which is directly obtained from the ButlereVolmer equation.

36 Heterocyclic Organic Corrosion Inhibitors

FIGURE 2.12 Experimentally obtained polarization curves and an Evans diagram for Fe corrosion in deaerated acid solution. Reproduced with permission from Y.M. Tan, R.W. Revie, Heterogeneous Electrode Processes and Localized Corrosion, John Wiley & Sons 2012; Copyright 2012 © John Wiley and Sons.

The Tafel extrapolation technique is used to find out the corrosion rate, when the dissolution of metal is under activation control [57,58]. For a metal immersed in a deaerated acid solution, the anodic and cathodic reactions can be given as M / M nþ þ ne


(2.26)

2H þ þ 2e
/H2

(2.27)

In the deaerated condition, the hydrogen evolution alone occurs as cathodic reaction rather than the cathodic reduction of oxygen. In acid solution, the oxide layer primarily present on metal surface dissolves due to contact with acid solution, which is in the route to achieve the steady-state OCP. Therefore, the anodic reaction solely represents the dissolution of the bare metal surface. Experimentally determined polarization curves for corrosion of iron in acid solution are shown in Fig. 2.12 together with a icorr comparison with the Evans diagram [58]. The Evans diagram and the PDP curve can both be employed to obtain a description of the mixed electrochemical processes of metallic corrosion. For more details on the Evans diagram and the mixed potential theory, readers can consult bonafide books referred in Chapter 1. The Evans diagrams shown in Fig. 2.12 are simplified curves of the anodic and cathodic reactions of an electrochemical system. On the other hand, the PDP plots are obtained experimentally by applying an external current or potential. It is obvious that

Chapter 2  Experimental methods of inhibitor evaluation

37

the anodic and cathodic polarization curves are merged with the Evans diagram under an area of relatively high polarization (usually referred to as the Tafel region). This suggests that the corrosion potential (Ecorr) and the corrosion current (icorr) could be obtained by extrapolating the polarization curve. The Tafel extrapolation technique has some limitations such as the follows: (i) For the determination of polarization curves, the anodic and the cathodic reaction that occurs at the Ecorr is the only reaction that can take place. It means that the changes occurring in electrode potential should not promote other electrochemical processes in either anodic or cathodic directions. (ii) A clearly defined anodic and cathodic Tafel region should be present (on at least one decade of current). However, according to McCafferty [57], the corrosion rate can be assessed by extrapolating either of the Tafel regions, wherein the cathodic part of polarization curves generally displays a better-defined Tafel plot. (iii) Both the anodic and cathodic polarization curves are under activation control. From extrapolation of the Tafel curves up to the point of their intersection, the electrochemical parameters viz. the icorr , Ecorr , ba , bc , and the hPDP % can be obtained [2,5,8]: hPDP % ¼

inh icorr
icorr  100 icorr

(2.28)

inh where icorr and icorr , respectively, represent the corrosion current densities without and with the corrosion inhibitor. A shift in the Ecorr in the existence of inhibitor of magnitude >85 mV in positive direction compared to blank suggests anodic-type inhibition and a shift >85 mV in the negative direction indicates cathodic-type inhibition. A shift in either direction