Effect of Calotropis Gigantea on Corrosion Inhibition of Mild Steel in Acidic Medium

EFFECT OF CALOTROPIS GIGANTEA ON CORROSION INHIBITION OF MILD STEEL IN ACIDIC MEDIUM

A DISSERTATION SUBMITTED FOR THE PARTIAL FULFILLMEN OF THE REQUIREMENTS FOR THE MASTERS OF SCIENCE DEGREE IN CHEMISTRY

SUBMITTED BY:

Bishnu Bahadur Dawadi Exam Roll No.: 422/072 (T.U. Regd. No.: 5-2-240-679-2011)

SUBMITTED TO:

CENTRAL DEPARTMENT OF CHEMISTRY, INSTITUTE OF SCIENCE AND TECHNOLOGY, TRIBHUVAN UNIVERSITY, KIRTIPUR, KATHMANDU,NEPAL June, 2019

BOARD OF EXAMINER AND CERTIFICATE OF APPROVAL

This dissertation work entitled- “Effect of Calotropis gigantea on corrosion inhibition of mild steel in acid medium” submitted by Mr. Bishnu Bahadur Dawadi (Roll No. 422/072, T.U. Regd. No. 5-2-240-679-2011) has been accepted as a partial fulfillment of the requirements for the Degree of Master of Science in Chemistry.

………………………………. Supervisor Prof. Dr. Amar Prasad Yadav Central Department of Chemistry Tribhuvan University, Kathmandu, Nepal

…………………………………….. …………………. Internal examiner External examiner Prof. Dr. Vinay Kumar Jha Assoc. Prof. Dr. Surendra Kumar Gautam Central Department of Chemistry Department of Chemistry Tribhuvan University, Kirtipur Tri-Chandra Campus Kathmandu, Nepal Tribhuvan University Kathmandu, Nepal

…………………………………….. Prof. Dr. Ram Chandra Basnyat Head of the Department Central Department of Chemistry Tribhuvan University, Kirtipur Kathmandu, Nepal

Date: June 26, 2019

RECOMMENDATION LETTER

This dissertation work entitled- “Effect of Calotropis gigantea on corrosion inhibition of mild steel in acid medium” submitted by Mr.Bishnu Bahadur Dawadi (Roll No. 422/072, T.U. Regd. No. 5-2-240-679-2011) for the Degree of Master of Science in Chemistry of Tribhuvan University was carried out under my supervision in the academic year 2017-2019. During the research period, Mr. Bishnu had performed the dissertation work sincerely and satisfactorily.

………………………. Supervisor Prof. Dr. Amar Prasad Yadav Central Department of Chemistry Tribhuvan University, Kirtipur Kathmandu, Nepal

Date: June 26, 2019

DECLARATION

I, Bishnu Bahadur Dawadi, hereby declare that the present dissertation work is done originally and has not been submitted elsewhere for any degree. Any literature, data or works done by others and cited in this dissertation has been given due acknowledgements and listed in the reference section.

…………………………….. Bishnu Bahadur Dawadi Central Department of Chemistry Tribhuvan University, Kirtipur Kathmandu, Nepal Date: June 26, 2019

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DEDICATED TO

My Father Man Bahadur Dawadi

Mother Saraswoti Dawadi

ACKNOWLEDGEMENTS

I express my sincere gratitude to my respected supervisor Prof. Dr. Amar Prasad Yadav of Central Department of Chemistry, Tribhuvan University, Kirtipur, Kathmandu for their valuable guidance, support and suggestion throughout this dissertation work.

I owe my deep gratitude to Professor Dr. Ram Chandra Basnyat, Head of Central Department of Chemistry, T.U., Kirtipur for providing me necessary research facilities to carry out this dissertation work.

Besides, I would like to thank Assoc. Prof. Nabin Karki Asst. Prof. Dipak Kumar Gupta and Asst. Prof. Sanjaya Singh for their insightful comments, encouragement and consistent help and support. I would always be grateful towards both of them for inlighting me the first glance of research. Further, I am indebted to Associate Prof. Dr. Deepak Raj Pant from Central Department of Botany, Tribhuvan University for this help.

I would like to acknowledge to my senior; Shyam Prasad Shrestha, Basu Dev Poudel, Roshan Lama and Colleagues, Amrit Ojha, Ritu Thakur, Sumnath Khanal and other friends of Central Department of Chemistry for their help to carry out this dissertation work.

Last but not the least, I would like to thank my family: my parents Mr. Man Bahadur Dawadi and Mrs. Saraswoti Dawadi, Sister Shanta Dawadi and brother Krishna Dawadi for supporting me throughout writing this thesis and my life in general.

Bishnu Bahadur Dawadi

LIST OF ABBREVIATIONS

IUPAC International Union of Pure and Applied Chemistry

GNP Gross National Product

GDP Gross Domestic Product

VCI Volatile Corrosion inhibitors

Cdl Double layer Capacitance

Ecorr Corrosion Potential

Cls Corrosion Inhibitors

Icorr Corrosion Current Density

CR Corrosion Rate

IE Inhibitor Efficiency ppm Parts Per Million

OCP Open Circuit Potential

휃 Degree of Surface Coverage

SCE Saturated Calomel Electrode

FTIR Fourier Transform Infrared

ABSTRACT

The inhibition effect of Calotropis gigantea extract on corrosion of mild steel in 1M H2SO4 was investigated using weight loss measurement along with electrochemical techniques viz. open circuit potential and potentiodynamic polarization. Weight loss technique was employed to study various concentration effect of extract along with effects of temperature and time while electrochemical techniques were performed to evaluate both concentration and dipped and non dipped effect of test solution. Inhibition efficiency of as high as 89.23 % for dipped sample and 75.42% percent for non dipped sample were achieved for green extract of Calotropis gigantea for 1000 ppm concentration from Tafel plot. The open circuit potential measurement showed that methanolic extract of Calotropis gigantea as mixed type of inhibitor forming a protective layer on the surface of mild steel by the process of adsorption. The inhibition effect of the plant extract could be accelerated by the presence of functional group such as Carbonyl, Nitro, Hydroxyl along with heteroatom and multiple bonds containing compounds as suggested by FTIR results, which is adsorbed on the surface of mild steel.

Keywords: Corrosion, FTIR, Green Inhibitors, Inhibitors, Mild steel, Polarization, Potentiodynamic, Weight Loss Measurements,

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TABLE OF CONTENTS

Board of Examiners and Certificate of approval i

Recommendation Letter ii

Declaration iii

Dedication iv

Acknowledgements v

List of Abbreviations vi

Abstract vii

Contents viii

List of figures xi

List of tables xii

CHAPTER-1: INTRODUCTION 1-31

1.1 Introduction of Corrosion 1 1.2 Importance of Corrosion study 2 1.3 Corrosion Control 4 1.3.1 Selection of right material of construction 4 1.3.2 Surface coating 4 1.3.3 Design 5 1.3.4 Electrical protection 6 1.4 Inhibitors 7 1.4.1 Anodic Inhibitors 7 1.4.2 Cathodic Inhibitor 9 1.4.3 Mixed Inhibitors 10 1.4.4 Volatile Corrosion Inhibitors (VCI) 10 1.5 Natural products as green Inhibitors 10 1.5.1 Use of green inhibitors 12 1.5.2 Mechanism of corrosion inhibitor 12 1.6 Calotropis gigantea as Green Inhibitor 13

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1.7 Adsorption of Inhibitors 14 1.8 Corrosion Monitoring 16 1.8.1 Importance of corrosion monitoring and others engineering system 16 1.8.2 Types of corrosion monitoring techniques 16

1.9 Mild steel 20

1.10 Literature Review 22

1.11 Objectives of the Study

1.11.1 General Objectives 27

1.11.2 Specific Objectives 27

CHAPTER-2: MATERIALS AND METHODS 28-32

2.1 Preparation of plane extract and its solution

2.1.1 Selection, specimen collection and plant identification 28

2.1.2 Preparation of power of plant specimen 28

2.1.3 Preparation of Concentrated Methanol Extract 29

2.1.4 Preparation of solution 29

2.2 Phytochemical screening test. 29

2.3 Preparation of Mild Steel Sample 30

2.4 Electrochemical Measurements 30

2.4.1 Open Circuit potential Measurement 30

2.4.2 Potentiodynamic Polarization 31

2.5 Weight Loss Measurements 32

CHAPTER-3: RESULTS AND DISCUSSION 33-48

3.1 Variation of open circuit potential with time 33

3.2 Polarization of mild steel in 1M H2So4 and inhibitor solution 37

3.3 Effect of Concentration of Calotropis gigantea extract 39

3.4 Effects of concentration on inhibition Efficiency 40

3.5 Variation of weight Loss with time of immersion 41

3.6 Variation on inhibition Efficiency with time of immersion 42

3.7 Variation of inhibition Efficiency with concentration of

Calotropis gigantea extract by weight loss method 46

3.8 Variation of inhibition Efficiency of Calotropis gigantea

extract with temperature 46

3.9 FTIR Spectroscopic Analysis 47

CONCLUSIONS 49

REFERENCES 51

APPENDIX A1-A3

LIST OF FIGURES

Fig. 1.1 : Classification of inhibitors 9

Fig. 1.2 : Calotropis gigantea plant 13

Fig.1.3 : Adsorption of negatively charged inhibitor 15

Fig. 1.4 (a) : Positively charged inhibitor molecules 15

Fig.1.4 (b) : Synergistic adsorption of positively charged inhibitor 15

Fig.1.5 : Tafel plot 20

Fig. 2.1 : Google map showing the study site 28

Fig.2.2 : Experimental set up for potentiodynamic polarization 31

Fig. 3.1(a) : Variation of OCP with immersion of as dipped sample 34

Fig. 3.1(b) : Variation of OCP with 1 hr dipped sample 35

Fig. 3.1(c) : Variation of OCP with 24 hr dipped sample 36

Fig. 3.2 : Polarization behavior of mild steel in 1M H2SO4 37

Fig. 3.3 (a) : Effects of concentration with as dipped sample 38

Fig. 3.3 (b) : Effects of concentration with 1 hr dipped sample 38

Fig. 3.3(c) : Effects of concentration with 24 hr dipped sample 39

Fig. 3.4 : Variation of inhibition efficiency with concentration 40

Fig. 3.5 : Variation of weight loss with time for immersion 42

Fig. 3.6 : Variation of inhibition efficiency with time for 43

immersion

Fig. 3.7 : Variation of corrosion rate with time for immersion 43

Fig. 3.7(a) : Variation of corrosion rate with concentration 43

Fig. 3.8 : Variation of inhibition efficiency with temperature 45

Fig.3.9 : FTIR spectrum of 1000 ppm extracts solution 47

LIST OF TABLES

Table No.1 : Inhibition efficiency for the as dipped sample A-1

Table No.2 : Inhibition efficiency for 1 hour dipped sample A-1

Table No.3 : Table for the 24 hours dipped sample A-1

Table No.4 : Table for the weight loss and surface coverage A-2

Table No.5 : Variation of concentration with inhibition efficiency A-2

Table No.6 : Variation of temperature with inhibition efficiency A-2

Table No.7 : Table for the FTIR Spectrum A-3

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CHAPTER ONE: INTRODUCTION

1.1 Introduction of Corrosion

In general gradual deterioration, damage and failure of structures of surfaces of metallic materials as the result of interaction of metal with corrosive environments is called corrosion. In 1960 corrosion is defined as the destructive attack on the surface of metallic materials by chemical and electrochemical reaction with its environment leading to their degradation [1,2].

The spontaneous instability of the metallic substances (i.e., metals or/and alloys) cause the charge-transfer reactions at the electrified interfaces between the metallic substances and their environment causes the corrosion [3]. Now-a-days, corrosion is more precisely defined as an undesirable deterioration of metallic material, usually metals and alloys by chemical or/and electrochemical reactions with their environment that adversely affects those properties of metals and alloys that are to be preserved [4]. On another hand, according to the International Union of Pure and Applied Chemistry (IUPAC), the term corrosion is defined as an irreversible interfacial reaction of materials with their corrosive environment, which results in the utilization of the materials [5]. The IUPAC recommended corrosion definition is very board and not accepted recently by corrosion scientists and material scientists, because the IUPAC recommended corrosion definition includes not only metallic materials but also polymeric materials like plastics, rubbers, etc. and ceramics like concrete, bricks, etc.

The definition by Fontana is simply as that of IUPACs and as the deterioration of materials as a result of interaction with its environment. In recent corrosion is defined as undesirable deterioration of metallic materials (metals and alloys) by electrochemical and chemical reaction with their corrosive environments that adversely affects those properties of metals and alloys to be preserved. This definition is widely accepted by all the scientists of the world [4].

On the other hand, Corrosion is an electrochemical process, usually occurs not by direct chemical reaction of a metal with its environment but rather through the operation of coupled electrochemical half - cell reaction. A half- cell reaction is one of the two electrodes in a galvanic call or simply battery [6].

Anodic reaction:

The loss of electron occurs as an anodic reaction,

M Mn+ +ne- (1.1)

„M‟ stands for a metal. “n” stands for the number of electrons that an atom of the metal will easily release.

Cathodic reaction

The cathodic reaction taking place according to the environment, the reactions are;

- i. 2H+ (aq) + 2e H2 (g) (1.2)

- - ii. 2H2O + 2e H2 + 2OH (1.3)

Hydrogen evolution from neutral water.

1 - - iii. O2 + H2O + 2e 2 OH (1.4) 2

Oxygen reduction in neutral or basic solution

iv. Mn+ + ne- M (1.5)

Metal reduction

1.2 Importance of Corrosion Study

In general, annual corrosion cost of the most developed countries is estimated approximately 3-4% of the total gross national product (GNP). Corrosion is one of the major industries scientific and technological problems, because small carelessness regarding to the corrosion

2 phenomenon at the beginning of implementation of any system plants and process can cause major economic loss during their operation [1]. Nowadays corrosion study is not only the interest of corrosion scientists but also of engineers and industrialists [7-8].

In general, two things are to be significantly taken into mind before the engineering materials are subjected for their applications. Firstly, under what conditions it is used and secondly, how it is processed [4]. Recently, the corrosion scientist tries to develop high corrosion resistance materials.

The economic factor is a very important motivation for much of the current research works in the field of corrosion. Losses from corrosion be many billions of dollars annually. According to study, approximately $276 billion or 3.1 % of the gross domestic product (GDP) lost by the corrosion in the United States of America. The cost of corrosion is approximately 3-4 % of the gross national product of the most industrialized countries [9].

There are mainly three reasons for being the importance of corrosion study; i.e. Economics, safety and conservation [11, 12]. The economic losses are divided into direct and indirect losses. The direct corrosion losses include the costs of replacing corroded structures and machinery or their components, such as condenser tubes, mufflers, pipelines and metal roofing, including labor. The indirect corrosion losses are more difficult to assess and hence it is generally practiced that the indirect corrosion loss should be same as that of the direct losses. Therefore, the total corrosion loss is generally estimated by doubling of the estimated direct corrosion loss [11, 12]. Some examples of indirect corrosion losses are shutdown, loss of product, loss of efficiency, contamination of product and overdesign [11, 12].

Industrial importance of the corrosion study is mainly of three folds:

 Economics: The prime motive of this factor is to reduce the cost associated with material loss.  Safety: To improve the safety of different operating equipment.  Conservation of precious materials: To conserve the precious metallic resource of the world.

Some of the important reasons for corrosion studies are summarized as:

1. To prevent the losses of precious resources of a country, because corrosion destroys the precious metals gradually.

2. To understand the corrosion mechanism of the metallic materials as a result it can be applied an appropriate corrosion control techniques.

3. To preserve valuable natural resources.

4. To protect from contamination of air, water and soil.

5. To design of artificial implants for the human body requires a complete understanding of corrosion science.

1.3 Corrosion Control

There are many method to control corrosion, some of them are:

1. Selection of right materials of construction

2. Coating

3. Design

4. Electrical protection

5. Inhibitors

1.3.1 Selection of Right Material of Construction

The selection of appropriate materials in a given corrosive environments is a key factor for corrosion control strategy. Selected metals or alloys for the suitable environmental conditions prevailing (composition, temperature, velocity) taking in to account mechanical and physical properties, availability, method of fabrication and over cost of structure [4].

Every metal and alloy has inherent and unique corrosion behavior that can range from the high resistance of noble metals. The corrosion resistance of a

4 metal strongly depends on the environment to which it is exposed, like chemical composition, temperature, velocity, and so on [14].

Some of the common materials used in constructing a variety of facilities, such as steel and steel-rain forced concrete can be affected by corrosion [15].

The right material of construction should have following properties:

a. High mechanical strength

b. High corrosion resistance

c. Low cost

1.3.2 Coating

Coating or painting is one of the most important methods of corrosion control metallic or polymeric materials that are applied to materials to decrease their resistance to corrosive degradation. The purpose of applying the coating may be decorative, functional or both [4].

Corrosion protection can be classified into metallic, organic and inorganic coating. The coating of high resistivity such as epoxies, vinyl‟s, chlorinated rubber etc., and the flow of electric current to metal surface is impeded. The increased in electrical resistance of the coating offers a good method of corrosion prevention [14].

A. Metallic coating can be carried out by following methods 1. Electroplating 2. Spraying of the molten metal on the work piece. 3. Hot dipping of the work piece in molten metal covered with flux.

B. Non metallic coating is of two types 1. Inorganic coatings a) Oxidation b) Phospating c) Cement coating d) Enamels

2. Organic coating

a) Paints b) Plastics c) Greases d) Coal tar

1.3.3 Design

There is a great deal of information available on the corrosion resistance of metals or alloys in various environments. However in spite of all these information environmental conditions will encounter for which corrosion data is not available. Since the appropriate design of equipment can help reducing the corrosion rate of material. The design can be fabricated such that the following factors which facilitate corrosion are avoided.

1. Features that traps dust, moisture and water. 2. Design that leads to corrosion or cavitations damage. 3. Crevices and situations where deposits can form on the metal surface. 4. Designs that lead to heterogeneities in the metal or in the environment [4]. 5. Stresses in the structures should be avoided [15].

1.3.4 Electrical protection

The devices that are used to protect the power systems from faults are called protection devices. It is one of the most important approaches of the corrosion prevention technique and it can be classified in two methods.

A. Cathodic protection method

Cathodic protection is one of the most important approaches of the corrosion prevention technique. In this technique corrosion damages are reduced virtually to zero by means of an externally applied electric current and metallic substance can be maintained without deterioration or degradation for an infinite time in a corrosive environment. The process in which rate of

6 metal dissolution is dramatically decreased by shifting the potential to more negative direction is known as cathodic protection [4].

There are mainly two types of cathodic protection [15-17].

(a) Impressed current method

In this method negative current is flowed in to the conducting substance to be protected from corrosion. In this method few anodes are needed, voltage may be adjusted to allow for environmental and coating changes, current output can be easily varied to suit requirements etc.

(b) Sacrificial (Galvanic) anode technique

In this method substance or metal to be protected is kept in contact with less noble metallic substance which acts as sacrificial anode. This method of cathodic protection has relatively low risk of over protection. On the other hand, In sometimes sacrificial anodes may require replacement at frequent intervals when current output is high. Sacrificial anode technique for the cathodic protection employs reactive metal or alloy as auxiliary anode that is electrically connected directly to the steel or metallic substance to be protected [ 4].

B. Anodic protection

Anodic protection is a technique to control the corrosion of a metal surface by making it the anode of an electrochemical cell and controlling the electrode potential in a zone where the metal is passive. Anodic protection is mainly based on diffusion barrier protective films formed on the surface of metals and alloys using externally applied anodic current. The majority of the applications of the anodic protection involve the manufacture, storage and transport of sulfuric acid. The anodic protection is most extensively applied to protect equipment used to store and handle sulfuric acid [4].

In this method, the materials to be protected is made the anode in an electrochemical circuit with an appropriate cathode such as mild steel, stainless steel or oxidized graphite. The potential of the anode is maintained

7 in the passive region by a potentiostat using a suitable reference electrode [ 15-17]. Anodic protection method is applicable only to mostly transition metals and alloys which are readily passivated when anodic ally polarized. On the other hand it is not applicable to anodic protection method of Zinc, Magnesium, Cadmium, silver, copper, and copper based alloys [4].

1.3.5 Inhibitors

An inhibitor is a chemical substance that, when added in small concentration to an environment, effectively decrease the corrosion rate [4]. A corrosion inhibitor is a substance which, when added to a corrosive environment, significantly decreased the rate of corrosion attack caused by the environment or prevent the reaction of the metal with the media [18]. Investigation revealed a significant relationship between adsorption of organic compounds and corrosion inhibition process, as corrosion inhibition is a well-known surface phenomenon and adsorption is a function of degree of protection of the metal surface [18].

Inhibitors are used in much system, namely cooling system, refinery units, chemical, oil and gas production unit, boiler, and so forth [18]. Corrosion inhibitors generally control corrosion by forming various types of firms that modify the environment‟s corrosively at the metal surface [18]. In recent years, owing to the growing interest and attention of the world towards the protection of the environment and the hazardous effects of using chemicals on the ecological balance, the traditional approach on corrosion inhibitors has gradually changed. From the economic and environmental viewpoints, plant extracts are an excellent alternative as green corrosion inhibitors, because of their availability, biodegradability and environmental friendly [19].

Corrosion inhibitors are used to protect metals from corrosion, including temporary protection during storage or transport as well as localized protection [20].

They reduce the corrosion rate by

a. Increasing or decreasing the anodic /cathodic reaction b. Decreasing the diffusion rate for reactants to the surface of the metal. c. Decreasing the electrical resistance of the metal surface.

Inhibitors can be classified as

1. Anodic inhibitors 2. Cathodic inhibitors 3. Mixed inhibitors 4. Volatile corrosion inhibitors (VCI)

Classification of inhibitor

Inorganic mixed Environmental conditions

anodic Cathodic Film Former Physical Chemical

Scavengers Biocides

poisons Precipitators

Fig 1.1: Classification of Inhibitors

1. Anodic inhibitors

Anodic inhibitors are usually used in near neutral solutions where sparingly soluble corrosion products such as oxides, hydroxides, or salts, are formed. They form, or facilitate the formation of passivating films that inhibit the anodic metal dissolution reaction [20].

The chemical substance which retards the anodic reaction and performs its action by forming protective oxide film on the surface of corroding material is anodic inhibitor. An example of such inhibitors includes Nitrite,

Phosphate, Chromates, and Molybdates [21]. The anodic inhibitor includes chromates, nitrites, Molybdates and phosphates, etc. Chromates and nitrates are called passivating inhibitors because of their tendency to passivated the metal surface [4].

2. Cathodic inhibitors

The classes of inhibitors which decrease the rate of cathodic reaction in a metal surface are called cathodic inhibitors. It causes the formation of insoluble compounds precipitate on the cathodic sites. At cathode hydrogen and oxygen molecules consume electrons and the alkalinity of the electrolyte at the metal (cathode/electrolyte interface) increases which leads to the precipitation of the cathodic inhibitors on the surface and form a barrier of insoluble precipitates over the metal [20]. Thus, restricts the metal contact with environment, even if it is completely immersed, preventing the occurrence of the corrosion reaction. Due to this, cathodic inhibitors are independent of concentration. The cathodic inhibitors include zinc salts, calcium salts, magnesium salts and polyphosphates, etc [23].

3. Mixed Inhibitors

The film forming compounds which reduce both cathodic and anodic reactions are mixed inhibitors. Phosphates and silicates used in domestic water softeners to prevent the formation of rust layer are the most commonly used mixed inhibitors [23].

The mixed inhibitors protect the metal in three possible ways

a. Chemisorptions b. Physical adsorption c. Film formation [4]

4. Volatile corrosion inhibitors

Volatile inhibitors decrease corrosion in closed spaces (package bags). VI compound is emitted (vaporized) by the material enclosing the space. The vapors condense in the metal surface in form of microscopic crystals, which

10 dissolves in the moisture film present on the surface. The ions of the dissolved VI displace water molecules from the metal surface and form monomolecular insoluble protection film reducing the corrosion rate. Volatile corrosion inhibitors may be added to various package materials, polymer film (e.g. low density polyethylene), paper, foam, powder, oils etc.The volatile inhibitors includes Cyclohexylamine, Dicyclohexylamine, Amino alcohols, etc [24].

1.4 Natural products as Green Inhibitors

The term “Green inhibitor” or “eco friendly inhibitor” refers to the substances that have biocompatibility in nature. Plants are the sources of naturally occurring compounds which are used as corrosion inhibitors because they are environmentally acceptable, low-cost, easily available, and biodegradable and so on [4, 20].

The extracts are generally obtained from cheap solvents which are widely available, at a low cost and with low toxicity; the aqueous extract is more relieved, but due to the low solubility of many natural products in water, common ethanol extracts are also obtained. These extracts contain variety of natural products such as essential oils, tannins, pigments, steroids, terpenes, flavones and flavonoids, among other well-known active substances used as CIs. In general, these compounds present conjugated aromatic structures, long aliphatic chains such as nitrogen, sulfur, and oxygen heteroatom with free electron pairs that are available to form bonds with the metal surface in most cases; they act synergistically to exhibit good efficiency regarding the corrosion protection. Much number of papers has been published with the intention of developing an environment friendly corrosion inhibitors and a lot of researches has been performed for this development of “green” corrosion inhibitors [25].

Green inhibitors are more eco-friendly, has low cost, low toxicity and inhibition efficiency. In recent years, plant sources have become major source for the green corrosion inhibitors for the metals and alloys [20]. Currently, corrosion research is focused on “green corrosion inhibitors”, that

11 show good inhibition efficiency with low risk of environment pollution. The term “green inhibitor” or “eco-friendly inhibitor” refers to substance that is biocompatible such as plant extracts since they are of biological origin. Thus, natural products are being studied for their corrosion inhibition potential as they are showing good corrosion protection and are more environment friendly [26].

In recent years, owing to the growing interest and attention of the world towards the protection of the environment and the hazardous effects of using chemicals on the ecological balance, the traditional approach on corrosion inhibitors has gradually changed. From the economic and environmental viewpoints, plant extracts are an excellent alternative as green corrosion inhibitors, because of their availability, biodegradability and environmental friendly. These extracts can be obtained in a simple way and purification methods are not required [19].

1.4.1 Use of green inhibitors

Natural products are a good source of green corrosion inhibitors, where most of their extracts containing the necessary elements such as O,C,N and S, where are active in organic in organic compounds, assist in adsorption of these compounds on metals or alloys to form a film that protects the surface and hinders corrosion.

The use of the green corrosion inhibitors is one of the best options of protecting metallic materials against corrosion damages. The environmental toxicity of the synthetic organic corrosion inhibitors has prompted the search for the green corrosion inhibitors as they are generally biodegradable, do not contain heavy metals or other toxic compounds. In addition to being environmentally friendly and ecologically acceptable inexpensive, readily available and renewable, investigations of corrosion inhibiting abilities of tannins, alkaloids, organic, amino acids and organic dyes of plant origin are of interest. Some research groups had reported the successful use of naturally occurring substances as the corrosion inhibitors in acidic and alkaline environments [19].

1.4.2 Mechanism of corrosion inhibitor

In general, the mechanism of the green corrosion inhibitor towards the metallic materials is follows [19].

1. The inhibitor is chemically adsorbed (chemisorptions) on the surface of the metallic substances and forms a protective thin film with inhibitor effect or by combination between inhibitors and metallic surface.

2. The inhibitor leads a formation of a film by oxide protection of the base metal.

3. The inhibitor reacts with a potential corrosive component present in aqueous media and the product is a complex [19].

1.5 Calotropis gigantea as a Green Inhibitor

Among various medicinal plants, Calotropis gigantea is one of the common plants. It belongs to the family Apocynaceae and genus Calotropis locally called as “Aank” in Nepali language, is a shrub having the maximum height 4m. Being medicinally and economically very important plant, it has extensively used for various medicinal purposes such as antibacterial, antipyretic, wound healing, anti-diarrhoea activities etc. It is found in variety of environments, such as; agricultural areas, forest margins, riparian zones, grass lands, secondary forest and beach fronts [26].

Calotropis is a plant. Many people use the bark and root bark for medicine purpose. It is widely used for digestive disorders including diarrhoea, constipation and stomach ulcers for painful conditions including toothache, cramps and joint pain. Some people use Calotropis for syphilis, boils, inflammation epilepsy, hysteria, fever, muscular spasm, warts leprosy, gout snake bites and cancer. It can also be used for inhalation therapy; smoke from bark is inhaled for coughs, asthma [26]

Taxonomical Classification:

Kingdom: Plantae

Phylum: Tracheophyta

Class: Magnoliopsida

Order: Gentianales

Family: Apocynaceae Fig 1.2: Calotropis gigantea plant Genus: Calotropis

Species: Calotropis gigantea

Conditions for the green corrosion inhibitor selection

Following are the important conditions for selecting green corrosion inhibitor

 Cost and availability of inhibitors  Toxicity and pollution problems of inhibitors  Its long range effectiveness

1.6 Adsorption of Inhibitors

Green inhibitors are the organic inhibitors which cannot be designed as anodic, cathodic, or act as mixed inhibitors [20]. Organic inhibitors react by adsorption on a metallic surface. Cationic inhibitor like amines or anionic inhibitors like sulphonates are preferentially adsorbed depending on the charge of metal or alloy surface. At zero charge point, there is no particular preference for an anodic and cathodic inhibitor. In such a situation a combination of an inhibitor which could be strongly adsorbed at more negative potentials along with cathodic protection would provide a greater degree of inhibition than either applying cathodic protection [4].

Green inhibitor which are of mixed type shows particularly 3 different ways of protecting the metal surface:

1. Chemical adsorption (chemisorptions) 2. Physical adsorption (electrostatic adsorption) 3. Film formation

Chemisorptions take place more slowly than physical adsorption. As temperature increases, adsorption and inhibition also increase. Chemisorptions is specific and is not completely reversible. Adsorbed inhibitor molecules may undergo surface reactions, producing polymeric films. Corrosion protection increases markedly as the films grow from nearly two-dimensional adsorbed layers to three-dimension films up to several hundred angstroms thick. Inhibition is effective only when the films are adherent, are not soluble, and prevent access of the solution to the metal. Protective films may be no conducting (sometimes called ohmic inhibitors because they increase the resistance of the circuit, thereby inhibiting the corrosion process) or conducting (self-healing films) [20].

Fig. 1.3: Adsorption of negatively charged inhibitor on a positively charged metal surface.

Fig. 1.4 (a): Positively charged inhibitor molecules do not interact with positively charge surface

Fig. 1.4(b): Synergistic adsorption of positively charged inhibitor and anion on a positively charged metal surface [20].

1.6.1 Important consideration in selection of inhibitors

Following are the important factors under consideration in selection of inhibitors [4].

1. Effect of temperature and concentration on the performance of inhibitor.

2. Effect of inhibitor on heat transfer characteristics.

3. Toxicity and pollution problems of inhibitors.

4. Magnitude of supervision of uniform and localized corrosion.

1.7 Corrosion Monitoring

Corrosion monitoring refers to corrosion measurements performed under different industrial and practical operating conditions. Corrosion monitoring is more complex than the monitoring of most other processes because there is no single measurement technique that will detect all of these various conditions. General corrosion rates may vary substantially, even over relatively short distances.

1.7.1 Importance of corrosion Monitoring

a. Improved system. b. Reduced down time c. Reduced maintenance costs d. Reduced pollution and contamination risk. e. Reduced operating costs.

1.7.2 Types of corrosion monitoring techniques

There are several methods of corrosion monitoring, they are

Direct Corrosion Monitoring Methods

a. Weight loss method b. Faraday‟s equation for monitoring corrosion rate c. Tafel extrapolation method d. Linear polarization resistance method e. Electric resistance method.

Indirect Corrosion Monitoring Methods

i. Corrosion (open circuit) potential method. ii. Hydrogen monitoring method.

Among these some important methods are summarized below

(A) Weight loss method

The weight loss method is the best known and simplest of all known corrosion monitoring technique. In this method, a sample specimen is exposed to corrosive environments for a specific period of time and subsequently removed for the weight loss method [4]. The corrosion rate is given by

퐾 △푊 Corrosion rate = (1.6) 퐴푇퐷

Where, K =8.76×104 (constant) ; W= weight loss in gm ; A= area in cm2

D = density in g/cm3 ; T= time in hour

푊1 Inhibition efficiency (%) = 1- ×100% (1.7) 푊2

푊1 Surface coverage = 1 − (1.8) 푊2

Where, W1 and W2 are the weight losses for mild steel in presence and absence of inhibitor respectively.

This method is applicable to all environments solid, liquids and gases. Similarly corrosion deposit observed and analyzed, visual inspection is the added advantages of this method. Since corrosion rate can be easily calculated.

(B) Potentiodynamic polarization method

The polarization measurement was made to evaluate the corrosion current, corrosion potential, Tafel slope. Potentiodynamic polarization involves the characterization of the sample by its current- potential relationship. A three- electrode corrosion system is used to polarize the electrode of interest. The current response is measured as the potential which is shifted away from the corrosion potential. When a potential, more positive than the corrosion potential is applied on the sample through an external source, otherwise known as anodic polarization, then the anodic current predominates over the cathodic current. As the anodic polarization increases, the cathodic current becomes negligible with respect to the anodic current. Conversely, on cathodic polarization of the sample, the cathodic current predominates and the anodic current becomes negligible [27].

Potentiodynamic polarization measurements are generally used in aqueous systems. The basic difference from the linear polarization technique is that the applied potentials for polarization are normally stepped up to levels of several hundred mill volts. When the potential more positive than Ecorr is applied on WE through an external source anodic current predominates and as the polarization increases, the cathodic current becomes conversely, cathodic current predominates while anodic current becomes negligible in

18 cathodic polarization. This technique is very useful to detect the localized corrosion for passivating alloys such as stainless steels, nickel-based alloys containing chromium and other alloys. These polarization levels facilitate the determination of kinetic parameters such as the general corrosion rate and the Tafel constants. The formation of passive films and the onset of pitting corrosion can also be identified at characteristic potentials, which can assist in assessing the overall corrosion risk [28].

This method is based on mixed potential theory. Both anodic and cathodic reactions are components of the mixed electrodes involved in the corrosion process and the intersecting points of the anodic and the cathodic reactions corresponds to corrosion current density (icorr) and corrosion potential (∅corr). Plotting the logarithms of current density (logi) vs potential and extrapolating the current densities in the two tafel regions gives the ∅corr and icorr, by knowing icorr, the corrosion rate can be calculated.

In other hand, any electrochemical reaction can be divided into two or more oxidation and reduction reactions without any accumulation of the electric charge. In corrosion system, oxidation of metal (corrosion) and reduction of some species in solution takes place at the same rate and the net measured current is zero [29].

Im = ir – io= 0 (1.9)

When a metal is in contact with a solution, the metal will assume a potential that is independent of the metal and the nature of the solution. [62]. electrochemically, corrosion rate measurement is based on the determination of the oxidation current at the corrosion potential. This oxidation current is known as corrosion current and “open circuit potential” is called corrosion potential [29].

Im = ir – io= 0 at Ecorr (1.10)

The Tafel extrapolation method was introduced to illustrate the application of mixed potential theory to aqueous solution. When a corroding specimen is polarized by applying current in both the anodic and cathodic directions, the experimental polarization curve originates at Ecorr and at high current densities becomes linear on a semi-logarithmic plot. This linear portion coincides with the extended oxidation and reduction curves as shown in the figure (1.5). De-aeration of the solution restricts the cathodic reaction to hydrogen evolution, rather than also including the cathodic reduction of oxygen. In de-aerated acid solution, oxide films initially present on the metal surface are dissolved by the acid solution to attainment of the steady-state, open-circuit potential [29].

Fig. 1.5: Tafel plot

1.8 Mild Steel

Mild steel contains approximately 0.05-0.25% carbon making it malleable and ductile. It has a relatively low tensile strength, but it is cheap and easy to form, surface hardness can be increased through carburizing. Steel is less malleable and harder than mild steel. Mild steel can be further strengthened through the addition of carbon. Mild steel has wide application, it is susceptible to corrosion due to its thermodynamics instability especially in

20 acid medium and hence, the study of mild steel corrosion phenomena has become important particularly in acidic medium. Among the acids, sulfuric acid is one of the most widely used agents in several industries for the removal of undesirable oxide films and corrosion product of the surface metal [7]. Mild steel is a materials of choice due to its characteristics and find wide application in most of the chemical industries [8].

Mild steel has wide spectrum of applications and are used almost everywhere ranging from constructions, pipelines, mining, chemical processing, nuclear power plants ,fossil fuel power plant ,metal processing equipment and so on [9]. Since mild steel forms the backbone of manufacturing and civil infrastructures, when it starts to corrode the financial impacts are substantial. Though inevitable like natural disaster, there cost could however be decreased by the help proper control method. Use of inhibitors which are green is one of the suitable practices for minimizing the cost associated with loss of such important material, “mild steel” [30].

Fig. 1.6: Mild steel before exposing in inhibitor

Fig. 1.7: Mild steel after exposing in inhibitor

1.9 Literature Review

The watermelon (citrullus lanatus) extract was inspected for its efficiency as green corrosion inhibitor for mild steel in acidic media by odewunmi et al. where effective capacitance value obtained by utilizing the impedance parameters were in the range of double layer capacitance (Cdl).This decrease in Cdl resulted from the increase in double layer thickness that will lead to decrease in dielectric constant as a result of adsorption of extract on to the surface of mild steel by displacing the adsorbed water molecules ;there by protecting the metal from the acid attack. Corrosion inhibition effect was thus attributed to adsorption of constituents of studied plant extract on to the mild steel surface which was then approximated by Temkin adsorption isotherm model [34].

The inhibition effects of the aqueous extract of musa paradisica (banana) peels on mild steel corrosion in 0.5 M H2SO4 as well as change in inhibition efficiency during ripening of the peels have been investigated by atomic force microscopy techniques Tafel polarization, weight loss measurement, electrochemical impedance spectroscopy. It was reported that inhibition ability of the extracts was decreased with the maturity stages but increased with the extracts concentration. Similarly, it was reported that many drugs and antibacterial are used as corrosion inhibitors. It was reported that benzimidazole, thiourea, ethyl amine were found to be quite effective corrosion inhibitors to mild steel in acidic medium [35].

It was reported that pomegranate peel extract was reported to increase with increasing its concentration and acted as good corrosion inhibitor for mild steel in Hydrochloric acid solution. Similarly; it was also reported that phyllanthus amarus plant extracts were used as green corrosion inhibitors for mild steel in acid medium. Different parts of Phyllanthus amarus plant were used to control corrosion for mild steel in HCl and H2SO4 solutions using weight loss and polarization techniques. It was reported that inhibition efficiency was increased with increasing the extract concentration. Most of the plant extracts are the rich sources of ingredients which may have very high inhibition efficiency. The use of plant products as corrosion inhibitors

22 are justified by Phytochemical compounds present there with in molecular and electronic structures bearing close similarity to those of conventional organic inhibitor molecules [36].

Desai et al. studied effect of temperature of calotropis Gigantea leaves on aluminum corrosion in HCl using weight loss measurement. Studied showed the inhibition efficiency increases with increasing concentration and corrosion rate decreases [37].

In study carried out by Desai et. al., (2015) , inhibitory action of extract of calotropis Gigantea leaves on mild steel corrosion in HCl solution using weight loss measurement and electrochemical techniques which showed that inhibition efficiency increases with increasing concentration .Leaves extract of calotropis gigantea behaved as mixed type inhibitors by potentiodynamic polarization [38].

Peter C. okafor et al. (2008), studied Azadirachta indica extracts as corrosion inhibitors for mild steel in acid medium by weight loss measurements and gasometric techniques. The study revealed that plant extract acted as good inhibitors and inhibition efficiency increases with increasing concentration. Popular major crop barley (Hordeum Vulgare) is rich source of chemical constituent like alanine, glycine, leucine, valine, tyrosine with various number of functional group which is able to chelate metal cations for the same reason in paper published in (2015), Saadawy presents evidences for inhibition of acid corrosion of steel up to 94% by extract of barley [39].

Raja et al., (2013)., studied corrosion inhibition property of alkaloids extracts of ochrosia oppositifolia leaves (OOL) and isoreserpiline as the major alkaloid isolated from OOL against mild steel in acidic medium where these inhibitors decreased the corrosion current densities by mixed mode mechanism thereby reducing the corrosion rate [43] Another paper published in the same year 2013 by Raja et al., they established Neolamarckia cadamba alkaloids as ecofriendly corrosion inhibitor for mild steel in acidic medium [40].

Quraishi et al., (2010) studied corrosion inhibition of mild steel in acidic medium by the extracts of Murraya Koenigii leaves and showed that both

HCl and H2SO4 media were successfully inhibited [41].

Raja and Sethuraman have given a review on intense version of green compounds which has potential of being applicable rust inhibitor. They discussed the function of inhibitor over the metal surface and application and limitation of natural corrosion inhibitor with the conclusion natural substances rise as impressive rust inhibitor in nearby future owing to their advantages as easy accessibility, ecofriendly in earth and non-toxicant, with the remark era of green corrosion inhibitors has already begun [42].

Mohd and Ishak analyzed Piper nigrum extract as an attractive alternative to prevent corrosion for they had found great inhibition efficiency of the studied plant for mild steel in corrosive medium [43].

Okafor et al., 2008 studied the inhibitive action of leaves and seeds extract of the Phyllanthus amarus on mild steel corrosion in acidic media using weight loss method and gasometric techniques. It was found that the extract functioned as a good inhibitor in the acidic medium [44].

Baldwin (British patent 2327) gave a first patent in corrosion inhibitors. There is the evidence of the use of inhibitor since the 19th century. In 1960, Baldwin et al., performed the first research in the field of corrosion inhibition by using molasses and vegetable oil as inhibitors for steel sheets in acid pickling process [45].

Biodiesel is a one of the environmental friendly alternative source of energy. It was conducted transesterification of a vegetable oil as early as 1853, four decades before the first diesel engine became functional. It was reported that first time, the biodiesel from peanut oil was used as engine fuel on August 10 in 1883 and hence up to date 10 August of every year is celebrating the “International Biodiesel Day” [46].

Prevention of corrosion is one of the challenging task when metallic substances in contact with biodiesel and its blends. Use of corrosion

24 inhibitors is one of the widely accepted to retard the corrosion of metallic materials. It was reported that the common corrosion inhibitors used in oil and gas are imidazole, primary amines, diamines, amino-amines, oxyalkylated amines, naphthanetic acid, phosphate esters, and dodecyl benzene sulfonic [47].

Mathur et al. (2015) studied Corrosion of Mild steel in various concentrations of hydrochloric acid and sulfuric acid by Mass Loss method in the absence and presence of the extracts of seeds plant Pennisetum glaucum. In this paper, the mass loss equation concludes that corrosion inhibition increased to proportionate Concentration of the extract. It has been found that seed extract of plant Pennisetum glaucum is the effective and have high corrosion inhibition efficiency [48].

Khadraoui et al. (2013) tested the leaves extract of Mentharot undifolia as a corrosion inhibitors of steel in 1M HCl. Result obtained reveal that inhibitor tested differently decrease the kinetics of the corrosion process of steel and inhibition efficiency increases with the concentration, and act as a good inhibitor at 35% showing 92.87% efficiency [49].

It was reported that leave of Punica granatum extract had fairly good inhibiting properties for mild steel corrosion in 1M HCl solution, with inhibition efficiency of around 94% at a concentration of 1 g/L solution using X-ray diffraction confirmed the role of leaf of Punicagranatum extract as an effective corrosion inhibitor for mild steel in acidic medium [50].

Rochaa et al., (2014) investigated the aqueous extracts of mango and orange peels were shown to be good corrosion inhibitors for carbon steel in a 1 M HCl solution. In the presence of 400 mg of mango and orange peel extracts, the weight loss measurements was performed, result showed an increase in the inhibition efficiency with immersion time, after 24 hour of immersion the efficiency of 97% and 95% respectively were analyzed [51].There are many such publications on corrosion studies of plant extracts. Many sources of new and efficient green corrosion inhibitor are still unidentified. In future, effective inhibitor will be derived from naturally occurring substances.

1.10 OBJECTIVES

1.10.1 General objective

The thesis on this topic aims to develop eco-friendly corrosion inhibitor from natural products of Nepal.

1.10.2 Specific objectives

The main aims to study the effect of Calotropis gigantea as green corrosion inhibitors for mild steel in acidic media and to test the synergistic effect of rare metal ion on such natural products so as to enhance its inhibition efficiency.

The specific objectives are highlighted as

 Study the potentiodynamic polarization behavior of mild steel in presence and absence of inhibitor (Calotropis gigantea)  Study the effect of inhibitor concentration and temperature on the inhibition efficiency by weight loss method.  Study the relationship between weight loss and time of exposure.  Analysis of extracts and its component that gets adsorbed onto the surface of the sample resulting corrosion inhibition by FTIR.

CHAPTER 2: MATERIALS AND METHODS

2.1 Preparation of plant extract and its solution

2.1.1 Selection, Specimen collection and plant identification

The plant extract under investigation as a potential green corrosion inhibitor for mild steel in acidic medium is selected on the basis of available literature survey. The leaves of plant Calotropis gigantea (L.) Dryand, were collected from Tikauli forest of Bharatpur Nepal at altitude of about 415 m from sea level during April 2018 and identified by Assistant professor Dr. Deepak Raj Pant a taxonomist from Central Department of Botany, Tribhuvan University , Kirtipur, Kathmandu, Nepal.

Fig. 2.1: Google map showing study site

2.1.2 Preparation of powder of plant specimen

Only leaves of the plant were used for the preparation of the sample during the study. Plant was shade dried for a month and dried leaves were broken in to small pieces manually and then finally ground into power form using grinder.

2.1.3 Preparation of concentrated methanol extract

Extraction of alkaloids was done through cold percolation method. 1000 gram of dried sample was dipped in to the 2 liter of hexane and mixture was allowed to 24 hours at room temperature. The residue was obtained through filtration method. Then after the residue was mixed with 400 mL of methanol and the mixture was well stirred and allowed to stand for 7 days. So that compound of salt of alkaloids was obtained by the filtration of methanol extract solution and acidified by 5% tartaric acid and PH was maintained by addition of ammonia solution .The alkaloid extract solution was kept in the separating funnel with equal amount of dichloromethane. As a result two distinct aqueous and organic layer was separated where as in the organic layer 1º, 2º, 3º alkaloid was obtained which was further concentrated by using rotary evaporator. Finally, concentrated solution was collected and dried by using water bath at 40 ºC .After evaporation of solvent finally dried form of solid extract of Calotropis gigantea was obtained for further use.

2.1.4 Preparation of solution

1M H2SO4: 55.6 mL of concentrated 17.98 M H2SO4 was taken in 1000 mL volumetric flask and it was diluted up to the mark using distilled water.

Inhibitor solution: 1 litre of inhibitor solution was prepared by dissolving 1 g of plant extract in 1M H2SO4. Mixture was filtered to removed undissolved extract from the stock solution and prepared solution was marked concentration of 1000 ppm. So dilution of different concentrated were prepared by using 1M H2SO4 as solvent.

2.1.5 Phytochemical screening

Test of alkaloids

1. Mayer’s Test Small amount of extract was treated with Mayer‟s reagent

(KI+HgCl2) appearance of yellow precipitate indicates the presence of alkaloids.

2. Wagner’s Test

In crude extract, Wagner‟s reagent (2 g of I2 + 6 g KI in 100 mL distilled water) was added and appearance of reddish-brown precipitate indicates the presence of alkaloids.

2.2 Preparation of mild steel sample

Mild steel coupons used for this study were prepared from mild steel sheets available from different mild steel suppliers of kirtipur Kathmandu Nepal and cut into the desire dimension (4 cm× 4 cm). And, Cut sample was used in weight loss and potentio-dynamic polarization. The coupon acted as working electrode. Prior to each experiment, the mild steel samples were mechanically abraded with different grade of SiC paper (100-1200 grade) and stored in moisture free desiccators. Before the experiment, samples were sonicated in ethanol and dried. After sonication reproducible surface was obtained for each coupon after the removing of air formed oxide film and other dirt and immediately used for the weight loss method.

2.3 Electrochemical Measurement

2.3.1 Open circuit potential (OCP)

The OCP is used to measure the open circuit voltage of an electrochemical cell. The measurement of OPC was conducted for a better understanding of the corrosion behavior of the mild steel sheet in 1M HCl solution at room temperature in absence and presence of plant extract as corrosion inhibitor. The measurement was carried out using Hokuto Denko Potentiostat (HA- 151). The mild steel coupon was used as a working electrode, graphite as a counter electrode and a saturated calomel electrode was used as a reference electrode. The OCP was measured in different concentrations of inhibitor solutions for 30 minutes, immediately after the mild steel sample place in the inhibitor solution; similarly after the immersion of sample for 24 hours. The tafel plots of cathodic and anodic curves were plotted to corrosion potential to obtain corrosion current densities (Icorr) values using equation.

Icorr uninhibited − Icorr (inhibited ) I.E. (%)= ×100 % (2.1) Icorr (Uninhibited )

(A) Polarization using concentrated (1000ppm) inhibitor solution and

1M H2SO4 as electrolyte.

The mild steel coupon polished was used for cathodic polarization using 1M

H2SO4 solution as electrolyte. Again, another sample was used for anodic polarization using same electrolyte. The metal sample was subjected for cathodic polarization using inhibitor solution as electrolyte. Again, another sample was used for anodic polarization using same electrode. Two polished mild steel coupon were immersed for 24 hours in inhibitor solution and used as a sample for polarization. One sample was used for cathodic polarization using inhibitor solution as electrolyte. Again, next sample was used for anodic polarization using same electrolyte.

(B) Polarization using different concentration of inhibitor solutions

The similar method was followed to carry out potentiodynamic polarization of different concentration of (200, 400, 600, 800 and 1000 ppm) and corrosion inhibition efficiency was calculated using above equation.

Fig 2.2: Experimental set up for potentiodynamic polarization of mild steel

2.4 Weight Loss Measurements

The weight loss measurement were performed in mild steel sample by immersing the mild steel coupons into the acid solution (150 mL) in absence and presence of different concentrations of inhibitor. Prior to the experiment, the surface of each sample was polished on both side using the silicon carbide paper grade numbers (100-1200). Then, the specimen were

30 measured by using the Vernier caliper and sonicated with the ethanol and dried with air blower to remove the oxide film formed on the surface of the grill sheet. Then the initial weight of samples was weighted by an electronic balance, and then placed in the acid solution (150 mL) of 1M H2SO4 for (3, 6, 9, 12, and 24 hours) separately.

Similarly the metal sample was immersed in 150 mL of 1000 ppm inhibitor solution for same hour as acid separately to measure the time effect. Finally, the samples were taken out, washed thoroughly with distillated water, dried with air blower and their final weights were noted again in order to calculate inhibition efficiency and corrosion rate. From the initial and final weights of the sample, the loss in weight was calculated.

Similarly, the effect of different concentration was studied, the initial weight of the specimens were noted and immersed in the 150 mL of 1M H2SO4 solution with the different concentrated inhibitor solutions of (200, 400, 600, 800 and 1000 ppm) separately for a 6 hours at room temperature. After the 6 hours immersion, the specimens were taken out and washed, dried and final weight was taken. Again, the temperature effect was also studied, the initial weight of the specimens was noted and the specimens were immersed in 1M

H2SO4 solution and 1000 ppm of inhibitor solution separately. The test was performed in Clifton Unstirred water bath model No. NE2-4D, it was set to temperature 25ºC for 6 hours, then the specimen were taken out washed, dried and their final weight were noted. Similar tests were performed for in 35, 45, 55 and 65ºC respectively. From the initial and final weights of the specimen, the loss in weight was calculated. The corrosion rate (mm/yr), inhibitor efficiency and surface coverage are calculated using the formula mentioned in equations (1.6), (1.7) and (1.8).

CHAPTER 3: RESULTS AND DISSCUSSION

Polarization measurement were carried out in order to have knowledge of Polarization measurement were carried inhibitory effect of methanolic extracts of Calotropis gigantea on mild steel in 1M H2SO4 solution. The research was done by using potentiodynamic polarization technique, weight loss method.

3.1 Variation of Open Circuit Potential with Time

The open circuit potential (OCP) measurements of mild steel in 1 M H2SO4 was studied by monitoring changes in corrosion potential (Ecorr) with time. The change in OCP of mild steel in absence and presence of inhibitor was analyzed. The OCP were measured at various concentrations of inhibitor (200, 400, 600, 800, 1000 ppm) solutions. The change in OCP of mild steel in absence and presence of inhibitor were measured for 30 minutes at room temperature. The measurement where carried out in three electrode system.

OCP measurement for non dipped mild steel sample for various times

Figure 3.1(a) shows the effects of inhibitor on the change of OCP of the non- dipped mild steel sample with the added green inhibitor solution open to air as a function of immersion time. It was observed that potential shifted towards more positive sharply at first and then decreased slowly and became almost constant with the passage of time for acid but for plant extract it was observed that potential shifted to more negative value than that of for acid which reveals that sample was cathodically protected i.e. it is due to cathodic inhibition and later again potential becomes constant almost constant. Therefore, it reveals that extract was mixed type of inhibitor.

-0.42 -0.43 -0.44 -0.45 -0.46 1M1M H2so4H2SO4 -0.47 200 PPM 400 ppm -0.48 600 ppm

Potential/V vs SCE Potential/Vvs -0.49 800 ppm -0.5 1000 ppm -0.51 -0.52 0 5 10 15 20 25 30 35 Time/minute

Fig. 3.1(a): Variation of OCP against time of immersion of mild steel in different concentration of inhibitor solution, when the OCP was measured immediately.

OCP measurement for 1 hr dipped mild steel sample for various time.

Figure 3.1(b) shows the effects of inhibitor on the change of OCP of the dipped mild steel sample with added green inhibitor solution for 1 hours open to air as a function of immersion time along with blank solution of acid. It also reveals the similar trend where shift of OCP towards more negative potential as compared to the blank 1 M H2SO4 solution is obtained which indicates retardation of cathodic reaction. This proves the extracts of Calotropis gigantea acts as mixed type of corrosion inhibitor. In both the cases, however the OCP of mild steel for different concentrations of inhibitor solutions varies without a distinct trend which can be accounted for the process of adsorption and desorption while moving from lower concentration to higher concentration.

-0.42

-0.43

-0.44

-0.45 1M HH2so42SO4 200 PPM -0.46 400 PPM

Potential/VvsSCE -0.47 600 PPM 800 PPM -0.48 1000 PPM -0.49 0 10 20 30 40 Time/minute

Fig. 3.1(b): variation of OCP against time of immersion of mild steel in different concentration of inhibitor solution, when the OCP was measured after 1 hour of immersion.

OCP measurement for 24 hours dipped mild steel for various time

Figure 3.1(c) shows the effects of inhibitor on the change of OCP of the dipped mild steel sample with added green inhibitor solution for 24 hours open to air as a function of immersion time along with blank solution of acid. It also reveals the similar trend where shift of OCP towards more negative potential as compared to the blank 1 M H2SO4 solution is obtained which indicates retardation of cathodic reaction. This proves the extracts of Calotropis gigantea acts as mixed type of corrosion inhibitor. In the above cases, however the OCP of mild steel for different concentrations of inhibitor solutions varies without a distinct trend which can be accounted for the process of adsorption and desorption while moving from lower concentration to higher concentration.

-0.42

-0.43

-0.44

1M1M HH2so42SO4 -0.45 200 PPM

-0.46 400 PPM 600 PPM -SCE Potential/Vvs 0.47 800 PPM 1000 PPM -0.48

-0.49 0 10 20 30 40

Time/minute

Fig. 3.1(C): variation of OCP against time of immersion of mild steel in different concentration of inhibitor solution when the OCP was measured after 24 hour of immersion

3.2 Polarization of Mild steel coupon in 1M H2SO4 and Inhibitor Solution Methanol Extract of Calotropis gigantea

The current response was measured as the potential was shifted away from the open circuit potential of mild steel in acid solution as well as for inhibitor solution. In all cases, scan rate in 30 mV/ minute was applied in both anodic and cathodic polarization after 30 minutes of OCP measurement before the beginning polarization measurement. Both anodic and cathodic polarization were carried for mild steel using electrolytic solutions of blank

1 M H2SO4 acid only and 1000 ppm inhibitor solutions. Further, polarization was also performed after the immersion of sample in inhibitor solution for 1 hour and 24 hours.

0.1 2 - 0.01

1M H SO 0.001 1M H2SO42 4 Non dipped sample

0.0001 1 hr dipped sample Current/mAcm 24 hr dipped sample 0.00001

0.000001 -2 -1.5 -1 -0.5 0 Potential/V vs SCE

Fig 3.2: Polarization behavior of mild steel in 1M H2SO4 and in presence of inhibitor solution for both dipped and as-dipped cases

The plot between potential and current shows the decrease in both cathodic and anodic current density for both dipped and non dipped sample. This specifies the mixed type of inhibitor for corrosion of mild steel. However, for dipped sample decrease in cathodic current density was more significant than for non-dipped sample while decrease in anodic current density exhibited similar in both cases. This can be justified by the fact that the immersion period facilitated sufficient time for formation of protective layer on the mild steel surface. Based on this observation, inhibitor solution proves to be more effective when sample is immersed for 1 hour and 24 hours in same inhibitor solution.

3.3 Effect of Concentration of Calotropis gigantea extract on the Polarization Behavior of Mild Steel.

In order to study the concentration effect of plant extract in acidic solution for corrosion protection of mild steel, various concentrations (200, 400, 600, 800 and 1000 ppm) of plant extract solutions were used as electrolyte in polarization measurement. The effect of such different extract concentrations were shown in the form of plot between potential and current.

Polarization of non-dipped Mild steel samples using different concentration of extract solutions

0.1

1m1M H2SO4H2SO4 2 - 0.01 200 ppm 0.001 400 ppm

0.0001 600 PPM

Current/m 0.00001Current/m A cm 800 PPM

0.000001 1000 PPM

0.000000 -2 -1.5 -1 -0.5 0 Potential /V vS SCE

Fig. 3.3 (a): Effects of concentration of inhibitor solution on the polarization behavior of non-dipped mild steel sample.

Polarization of dipped mild steel sample using different concentrations of extract solutions

0.1

2 0.01 -

0.001 1m1M H2SO4H2SO4 0.0001 200 ppm 400 ppm Current/m Current/m A cm 0.00001 600 ppm 0.000001 800 ppm

0.000000 1000 ppm -1.2 -1 -0.8 -0.6 -0.4 -0.2 0

Potential/V vs SCE

Fig. 3.3 (b): Effects of concentration of inhibitor solution on the polarization behavior of dipped mild steel sample in respective inhibitor for 1 hour.

Polarization of dipped mild steel sample using different concentrations of extract solutions

1000ppm24hrs 0.1 dip 200ppm24hrs 0.01 dip 400ppm24hrs 0.001

2 dip - 600ppm24hrs 0.0001 dip 800ppm24hrs 1E-05 dip

Current/mACm 1M Acid 1E-06 -1.5 -1 -0.5 0

Potential/V vs SCE

Fig. 3.3 (c) Effects of concentration of inhibitor solution on the polarization behavior of dipped mild steel sample in respective inhibitor for 24 hour.

The Fig. 3.3(a), 3.3(b) and 3.3(c) demonstrates the effect of various extract concentrations on polarization of mild steel which shows the shift of corrosion potential (ϕcorr) on both the sides towards positive and negative indicating the mixed type of behavior of the inhibitor.

Fig. 3.3(b) and 3.3(c) illustrates the polarization curves of mild steel in the presence of Calotropis extract of different concentration (200, 400, 600,800 and 1000 ppm) in 1M H2SO4, after the immersion for 1 hour and 24 hour. The result showed that the corrosion potential was shifted in both directions which imply that inhibitor was mixed type. On the basis of curve, it was clearly notice that decreased in the corrosion current density with concentration of the inhibitors solution. Therefore the decreased current density with concentrations of inhibitor solutions suggests the retardation of corrosive reaction due to increased surface coverage by the inhibitor molecules on mild steel sample surface Thus, it suggested that the Calotropis gigantea extract as a good effective inhibitor for corrosion of mild steel, when immersed in inhibitor solution for 1 hour and 24 hour.

3.4 Effects of Concentration of Inhibition Efficiency

Different concentrations of methanolic extract solution were studied by using potentiodynamic polarization measurements. Cathodic and anodic polarization curves of mild steel in 1 M H2SO4 solution, in the absence and presence of various concentrations of inhibitor solutions are shown in Fig. 3.3. The Tafel polarization curves were studied from cathodic potential to an anodic potential to study the effect of inhibitor on mild steel. The linear Tafel segments of anodic and cathodic curves were extrapolated to obtain corrosion current densities (Icorr). The inhibition efficiency was then calculated from the measured (Icorr) values using equation (2.1). A plot of inhibition efficiency of the extracts vs concentrations of the inhibitors solutions is then made as follows:

100 90 80 70 60

50 undipped sample 40 1 hr dipped

30 24 hr dipped Inhibition efficiency(%) Inhibition 20 10 0 0 200 400 600 800 1000 1200 Concentration/ppm

Fig. 3.4: Variation of inhibition efficiency as a function of concentration of inhibitor solutions for the both dipped and non-dipped mild steel samples.

Figure (3.4) shows the inhibitor efficiency for mild steel specimens for different concentrations of inhibitor solutions used for both non-dipped and dipped sample at room temperature. The results show that the corrosion rate decreased with increased efficiency as the concentration of plant extract increased up to approximately 1000 ppm. This behavior is attributed to higher adsorption level of active inhibitor molecules from the extract on the metal surface. For Calotropis gigantea extract concentrations higher than 1000 ppm, the corrosion rates remained approximately constant. This could be an indication that the 1000 ppm concentrations are approximately the optimum inhibitor concentration. Higher extract concentrations did not show any additional significant protection under these particular lab test conditions and hence it has to be evaluated for its cost performance when applied under real industrial applications. There existed a linear relation with increase in the extract concentration. Inhibition efficiency as high as 75.43% was achieved for test solutions contained 1000 ppm of plant extracts. However, inhibition efficiencies of 83.77% and 89.23 % were achieved for the same test solution containing 1000 ppm plant extract when sample was dipped in same inhibitor solution for 1 hour and 24 hours respectively. The

40 result shows that inhibition efficiency increases with increase in immersion time.

3.5 Variation of Weight Loss with time of Immersion

The weight loss method of monitoring corrosion rate and inhibition efficiency is useful because of its simple application and high reliability. Following equation was used to calculate weight loss per unit area of the mild steel coupon:

푊1−푤2 Weight loss (△w) = (1.11) 퐴

WhereW1 and W2 are the average weights of the mild steel before after immersion into the inhibitor solution. And, A is the surface area of the mild steel.

The weight loss measurement showed that weight loss of mild steel decreased as compared to 1 M sulfuric acid on using the inhibitor extract solution for different time periods. This effect of inhibition is due to adsorption of inhibitor constituents on the surface of mild steel thus forming a protective layer. There is increase in corrosion loss of mild steel with increase in immersion period for both blank solutions of 1M H2SO4 as well as for inhibitor solution of Calotropis gigantea. Such increase in weight loss is probably due to exposure of sample to corrosive environment for longer time period thereby increasing the corrosion attack. With extended exposure inhibitor competes feebly with corrosion attack exhibiting desorption of adsorbed constituents as a result there is increase in weight loss for longer time periods as shown in Fig. 3.5.

2.5

1.5 inhibitor

1 acid Weight loss/ loss/ Weight (gm/cm2)

0.5

0 0 5 10 15 20 25 30 Time/h

Fig. 3.5: Variation of weight loss with time for immersion of mild steel in presence and absence of inhibitor Calotropis gigantea.

3.6 Variation of Inhibition Efficiency with time of Immersion

The variation of the inhibition efficiency with immersion period is determined from weight loss datas. Optimum concentration of plant extracts; 1000 ppm used for protection from acid attack at various time intervals is shown in Fig. (3.6) which shows inhibitor being still efficient for immersion period up to 9 hours. However with prolonged immersion periods there is decrease of efficiency in 12 hours and then increases in 24 hours. Such observations indicate instability of the inhibitor film on metal surface. This decrease in efficiency can be attributed to dynamic desorption/absorption of inhibitor constituents at metal/ solution interface.

100 95 90 85 80 75 70 65

Inhibition Efficiency Inhibition % 60 55 50 0 3 6 9 12 15 18 21 24 27 30 Time/ h

Fig. 3.6: Variation of inhibition efficiency with time for immersion of mild steel in presence and absence of inhibitor Calotropis gigantea.

0.0009

0.0008

0.0007

0.0006

0.0005

0.0004

CorrosionRate 0.0003

0.0002

0.0001

0 0 5 10 15 20 25 30 Time/h

Fig. 3.6(a): Variation of corrosion rate with time for immersion of mild steel in presence of inhibitor Calotropis gigantea.

3.7 Variation of Inhibition Efficiency with Concentration of Calotropis gigantea extract by Weight Loss Method.

Figure (3.7) shows the inhibition efficiency for mild steel specimens immersed to 1M H2SO4 for 6 hours as a function of inhibitor concentration at 25°C. The result shows that the inhibition efficiency increases as the concentration of plant leaves extract is increased and reaches the maximum value of 90.29 % for 1000 ppm of extract solution. The increase in inhibition efficiency and decrease in corrosion rate may be due to adsorption and desorption phenomena. Such behavior is attributed to availability of significant amount of inhibitor constituents with increased concentration for adsorption that significantly increases the fraction of the surface covered by the adsorbed molecules thereby increasing efficiency of inhibitor solution.

100

80 R² = 0.938

40 Inhibition efficiency Inhibition % 20

0 0 200 400 600 800 1000 1200 Concentration/ppm

Fig. 3.7: Variation of inhibition efficiency as a function of concentration of Calotropis

Gigantea extract in 1 M H2SO4 at 25 °C for 6 hours open in air by weight loss method.

0.0016

0.0014

0.0012

0.001

0.0008

0.0006 Corrosionrate

0.0004

0.0002

0 0 200 400 600 800 1000 1200 Concentration (ppm)

Fig.3.7 (a): Variation of corrosion rate with concentration of Calotropis Gigantea extract in 1 M H2SO4 at 25 °C.

3.8 Variation of Inhibition Efficiency of Calotropis gigantea extract with temperature

The effect of temperature on the corrosion inhibition with and without inhibitors was studied. Figure 3.8 shows the variation of inhibition efficiency with temperature. Also, it can be seen that efficiency increases with increase in temperature. However, the efficiency was increased up to the temperature 55 °C it may be due to the adsorption and desorption of inhibitor molecule at the metal surface. Adsorption and desorption of the inhibitor molecules regularly occur at the metal surface until equilibrium exists between two process at a particular temperature. With raise in temperature, the equilibrium between adsorption and desorption processes was shifted towards desorption until equilibrium was established again at a different values of equilibrium constant. It explains the lower inhibition efficiency at higher temperature. It gives the clue that mechanism of adsorption of the inhibitor contains electrostatic interactions, which disappears at the elevated temperature. However, the efficiency can be seen at higher temperature also, it may be due to strong chemical bonding. It can

45 be seen that after the 55 °C inhibition efficiency is highly decreases, it may be due to loss of inhibitive components in inhibitor. The Calotropis gigantea extract show the maximum efficiency of 86.67% at 55 °C .

100

20 inhibition efficiency% inhibition

0 20 30 40 50 60 70 Temperature/ ºC

[

Figure 3.8: Variation of inhibition efficiency of 1000 ppm Calotropis gigantea extract in

1 M H2SO4 as a function of temperature for 6 hours open in air by weight loss method.

3.9 FTIR Spectroscopic Analysis

FTIR spectrum of the crude extract of Calotropis gigantea along with its liquid form, 1000 ppm extract solution was obtained as in Fig. 3.9 thus subsequently analyzed.

110 100 90 80 70 N-H 60 N-O CO-O-CO

50 C-H Transmittance

% 40 O- H N-H 30 20 4000 3500 3000 2500 2000 1500 1000 500

Wave number(cmˉ1)

Fig 3.9 FTIR spectrum of 1000 ppm extracts solution of Calotropis gigantea.

The spectra display wide range of peaks and bands of stretching and bending suggesting presence of various functional groups, heteroatom‟s and aromatic system resulting in inhibition property of the extract.

FTIR spectrums of crude extract showing broad band at 3300 cm-1 can be assigned to the presence of intermolecular hydrogen bond, stretching mode of an O-H or N-H. The peak obtained at 1650 can be interpreted as the stretching of nitro compound (N-O) or stretching of α,β- unsaturated ketones (C=C). A sharp peak at 2350 cm-1indicates the presence of primary amines (N-H).

The absorption band obtained on spectrum below 1000 cm-1corresponds to alkenes, aliphatic and aromatic C-H group and organic halogen compound (C-X). FTIR spectrum of 1000 ppm extract solution shows strong peak at 1010 cm-1 indicating stretching of anhydride (CO-O-CO). Further, stretching mode of C-N is showed by strong peak at 1380 cm-1 attributed to the stretching of sulfone (S=O). Similarly, the strong peak at 1780 cm-1 indicates the existence of C=O and N-H bonds in primary amides.

The results obtained from the spectrum indicates that extract contains oxygen, nitrogen and sulfur atoms in functional groups (O-H, N-H, C=C, C-

N, C-X, C=O, S=O) and aromatic ring which meets the general consideration of typical corrosion inhibition since corrosion inhibition property of organic compounds having hetero atoms such as N, O and S of high electron density has already been established. Hence it can be showed that the extracted organic compounds are stable in 1M H2SO4.

CONCLUSIONS

In this study, the corrosion inhibition efficiency of the Green corrosion inhibitor Calotropis gigantea extract were performed using the weight loss method and potentiodynamic polarization measurement in 1M H2SO4. Following conclusions are drawn on the basis of the above results and discussion.

Calotropis gigantea acts as a good and efficient inhibitor for corrosion of mild steel in 1M H2SO4 by forming the protective layer in the surface of the mild steel coupon. The OCP measurement showed that the inhibitor acted as mixed type.

Polarization behavior of mild steel in presence and absence of inhibitor showed that corrosion current density significantly decreases with increase in inhibitor concentration and best works when sample is immersed in inhibitor solutions for 1 hr and 24 hours as compared to non-immersed samples.

With the time of immersion of samples in corrosive medium, corrosion loss increases and inhibitor solutions are less effective for prolonged time exposure to such corrosive environment and inhibitor would be effective up to 6 hours of exposure to such aggressive environment.

From result obtained in polarization, the inhibition efficiency increased with increasing the concentration of inhibitor in both the cases but the optimum efficiency of 89.23% was found in the sample immersed for 24 hours in 1000 ppm concentration of inhibitor solution.

The weight loss results shows effectiveness of inhibitor in corrosion loss to be approximately 90.29% as compared to loss in 1M H2SO4.

Inhibition efficiency of inhibitor decreases at elevated temperatures thereby indicating adsorption of inhibitive molecules on the surface of metal is primarily physiorption involving electrostatic interaction between phytoconstituents and sites of adsorption of metal.

FTIR analysis confirms presence of heteroatom in functional groups and aromatic ring which validates the consideration of corrosion inhibition property and further reveals that the extracted organic compounds are stable in 1 M H2SO4 since spectra of both crude extract and extract solution are comparable.

Hence, it was concluded that the Calotropis gigantea can be used as an excellent, eco-friendly, green corrosion inhibitor for mild steed in 1M H2SO4 solution. On the other hand, this dissertation has explored potent properties of Calotropis gigantea plant.

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APPENDIX

Table 1: Inhibition efficiency for the as dipped sample

Undipped Corrosion current Corrosion current Efficiency (%) Concentration (ppm) with inhibitor without inhibitor 200 0.0008 0.0013 37.75

400 0.0005 0.0013 60.84

600 0.0004 0.0013 66.30

800 0.0004 0.0013 70.04

1000 0.0003 0.0013 75.42

Table 2 : Inhibition efficiency for 1 hour dipped sample

1 hour dipped Corrosion current Corrosion current Efficiency(%) Concentration (ppm) with inhibitor without inhibitor 200 0.0005 0.0013 57.41

400 0.0004 0.0013 66.30

600 0.0004 0.0013 69.89

800 0.0003 0.0013 80.49

1000 0.0002 0.0013 83.77

Table 3 : Inhibition efficiency for 24 hour dipped sample

24 hours dipped Corrosion current Corrosion current Efficiency (%) Concentration (ppm) with inhibitor without inhibitor 200 0.0005 0.0013 61.93

400 0.0004 0.0013 70.01

600 0.0003 0.0013 75.42

800 0.0002 0.0013 82.99

1000 0.0001 0.0013 89.23

Table 4: Wet loss, Corrosion rate, surface coverage and inhibition efficiency

Time (hours) Wt. loss Wt. loss Corrosion Surface Efficiency (Acid) (inhibitor) rate coverage (%)

3 0.3053 0.0499 0.0005 0.836 83.65

6 0.6894 0.1018 0.0005 0.852 85.23

9 1.5020 0.2110 0.0007 0.869 86.95

12 1.6236 0.3018 0.0008 0.814 81.41

24 2.6103 0.4403 0.0006 0.830 83.06

Table 5: Variation of concentration with surface coverage and inhibition efficiency

Concentration Surface coverage Efficiency (%)

200 ppm 0.684 68.4

400 ppm 0.783 78.27

600 ppm 0.834 83.44

800 ppm 0.863 86.25

100 ppm 0.903 90.29

Table 6: Variation of temperature with corrosion rate, surface coverage and inhibition efficiency

Temperature (oC) Wt. Wt. Surface Efficiency loss(acid) loss(inhibitor) coverage 25 0.9881 0.1948 0.8144 81.44

35 1.4582 0.2698 0.8245 82.45

45 2.3156 0.3448 0.8536 85.36

55 2.9016 0.7459 0.8667 86.67

65 3.1254 1.0987 0.6323 63.23

Table 7: Functional groups in Calotropis gigantea extract

Wave number(cm-1) Functional group

3300 O-H

1660 N-O or C=C

2350 N-H (primary amine)

1015 CO-O-CO (anhydride)

Below 1000 Alkenes, aliphatic and aromatic C-H group

3230.76 N-H

III