STUDIES ON SOME ORGANIC CORROSION INHIBITORS

. . ' • ' J . ' . •

DISSERTATION Submitted in partial fulfilment of the requirement-s

# for the Award of the Degree of

iWaster of ^Ijilosfoplip 4 IN

APPLIED CHEMISTRY

^•f\

Supervisor

DR. M. A. QURAISHI Co-Supervisor

PROF. MOHD. AJMAL

SHAGUFASHAHEENSHERE

DEPARTMENT OF APPLIED CHEMISTRY FACULTY OF ENGINEERING & TECHNOLOGY

ALIGARH MUSLIM UNIVERSITY ALIGARH (INDIA)

1995

f e d ^ COKIPUIW*

.f- Xco No.

• 1 ,

' 'V, ^ • ^ i ' . . L , - . U c K

DS2837

To, DADDY

DEPARTMENT OF APPLIED CHEMISTRY ALIGARH MUSLIM UNIVERSITY, ALIGARH-202 002 (INDIA)

Ref. No. DAC/.

Dated..

iRes. (External) ; Uni\. Tclcx : 564—230 AMU T^

CERTIFICATE

Certified that this dissertation entitled " STUDIES OF SOME ORGANIC CORROSION BVHIBrrORS ", submitted by Ms Shagufe S. Shere is a record of the candidate's own work carried out by her under our supervision and guidance. The results presented in this dissertation have not been submitted else\ iiere and are suitable for the submission to the award of M.Phil degree.

'.:•?'

Supervisor (M.A.Quraishi) Department of Apphed Chemistry Z. H. College of Engg.& Tech. A.M.U., Ahgarh

]^-Supervisor "^ (M.Ajmal)

Department of AppMed Chemistry Z.H. College of Engg. & Tech.

A.M.U., AJigarh

ACKNOWLEDGMENTS

This dissertation is a confluence of opportunity, ideas and effort supported by my supervisor Dr. M.A. Quraishi and co-supervisor Prof. M. Ajmal. I am grateful to Prof. K.M. Shamsuddin, Chairman, Deptt. of Applied Chemistry for providing necessary facilities.

All my lab colleagues specially Dr. M.A.W. Khan deserve a word of praise for being cordial and helpful.

From the oceanic depths of my heart I wish to express my thanks to my husband, Farrukh Kidwai for his support, compassion and encouragement. Arham, my sweet little angel deserves a special thank you for being patient and making every thing worthwhile.

C.S.I.R is gratefully acknowledged for providing financial assistance.

^"i>~i^-jH'''4- t'v

(SHAGUFA S. SHERE)

PREFACE

Large quantities of metals are lost to mankind every year

due to corrosion. This has made world extremely corrosion

conscious. This phenomena is an universal problem leading to

considerable loss of economy and at the same time is also

detrimental to human health. Although various techniques have

been devised for corrosion control but prevention through the

use of inhibitors is the most widely accepted one due to its

simplicity of fairly good results. Use of inhibitors is

specific for different systems and thus it needs to be studied

thoroughly. The present work deals with the investigation of

various aromatic schiff bases as Corrosion inhibitors for mild

steel in acidic environment.

In the present dissertation first and foremost is an

introduction which highlights the economic and technical

significance of corrosion. Further various forms of theories

of corrosion have been described which help in understanding

the mechanism of corrosion inhibitors. Special emphasis has

been given to the mode of action of inhibitors toward

corrosion prevention. Various techniques for investigation

have also been discussed briefly.

The literature on organic corrosion inhibitor has been

surveyed from 1967-1995.

Experimental section deals with the synthesis of

inhibitors, its description and material used. Further various

experimental techniques viz weight loss and polarization have

also been studied.

C O N T E N T S

Page No.

1 . Chapter 1 1 - 4 4 Introduction

2. Chapter 2 45-53 Material & Methods

3. Chapter 3 54-64 Results Sc Discussion

4. Chapter 4 65-73 References

5. Smnmary 74-7 8

CHAPTER 1 INTRODUCTION

Gradual deterioration of material caused by the chemical

and electrochemical reaction with its environment is known as

corrosion. Noble metals such as gold, silver and platinum

remain unaffected for ages while metal such as Iron, Copper,

Aluminum get corroded when exposed to aggressive environment.

Corrosion as a chemical reaction is a characteristic of the

metals. The electrons in the metals, being loosely bound to

their atoms are easily removed in chemical reaction. The

refined metal have a constant tendency to return back to their

natural state or ores. Therefore corrosion can also be defined

as conA;-ersion of metal back into oxide/ sulphites. The

foundations of corrosion science are based on metallurgy and

physical chemistry.

One more term in addition to corrosion is "Erosion" which

means gradual destruction of material by physical causes like

mechanical wear/ abrasion. When corrosion and erosion occur

simultaneously the damage is much greater in comparison to the

damage caused when they occur individually. Erosion-Corrosion,

stress corrosion cracking, corrosion fatigue are some examples

of this type of conjoint action. Rusting of iron and iron

based alloys is a very common example of corrosion. Non-

ferrous metals corrode too. Corrosion process form an

interesting area for scientific studies which are frequently

undertaken by chemists, particularly electrochemists,

metallurgists and chemical engineers.

1.1. LOSSES DUE TO CORROSION:

Corrosion is an economic problem and is associated

closely with the loss of capital assets and business profits.

It is estimated that approximately 1/4 part of the total

population of metal and alloys go waste due to corrosion.

Technological and economic crisis occurring due to wastage of

metal can no longer be ignored any further. A number of

reports have appeared time to time highlighting the enormous

financial losses in India and abroad. According to a survey

done in India in 1958, the annual loss due to corrosion was

estimated to be of the order of 50 crores of rupees which is

indeed is remarkable amount. According to NACE International

bulletin ( 1 ) the annual losses due to corrosion in USA were

estimated to be more than $ 200 billions. In India the annual

losses due to the impact of corrosion has increased to more

than Rs. 1200/- crore ( 2 ).

Therefore to safeguard the economic interest of the

nation it is important for scientist and engineers to adopt

various means by which loss due to corrosion can be reduced.

Use of metals and their alloys has increased remarkably due to

vast technological and industrial developmental and thus any

step in the direction of understanding the nature of

corrosion, its mechanism and the way to control it , would be

of immense help to the nation's economy.

1.2 CAUSES OF CORROSION;

Corrosion as a chemical reaction is a characteristic of

the metals. Except for noble metals all the metal occur in

the earth's crust as certain stable compounds viz oxides,

hydrated oxides or sulphide. Sometimes basic sulphates , basic

chlorides or carbonates etc. Free energy of these compounds is

almost always lower than that of the metal in metallic state

therefore during reduction of 'ore' to metallic state, energy

must be expanded to overcome the affinity between the metal

and non-metal. These refined metals have a great tendency to

return back to their natural state or ores and this process is

termed as corrosion. As in metallic state the metal represents

an energy rich state and therefore when it is exposed to

oxygen, sulphur compounds, water which usually happens during

service they revert to the lower energy state in which they

originally occurred in the earth through the reactions

involving drop in free energy and this reaction is an

spontaneous one. It is not surprising to find that iron heated

in air acquires a scale of oxides while iron exposed to air

and water produces rust (hydrated oxide). Similarly copper

exposed to an atmosphere containing a trace of certain sulphur

compounds develops tarnish films. Since the earth is

surrounded by oxygen and water vapours, it would be sensible

instead of asking the question 'why do metal corrode ?' to

enquire how under these circumstances can metals manage to

escape corrosion.

4

In terms of thermodynamics the amount of free energy

required or stored up for the conversion of ores to metallic

state varies from metal to metal. Noble metals like

gold,platinum and silver require least energy to convert their

ores to metals and are therefore most corrosion resistant,

whereas for metals like magnesium, aluminium, iron this energy

is relatively high. Metals of general use are iron, nickel,

chromium, copper, zinc, aluminium etc. Some metals or alloys

corrode heavily in certain environment in which others remain

unaffected.

Iron when left in an industrial atmosphere forms a

reaction product layer or rust (Fe203. H2O) which is unable to

protect the metal. This deposition of the corrosion product on

the surface of the metal helps to reduce further progress of

corrosion in some cases for example in similar corrosive

atmosphere copper forms an adherent green platina CuSO^

(3Cu(OH)2 which protects the metal by isolating it from the

environment. 17 % Chromium is required in stainless steel to

produce good corrosion resistance whereas, in Fe-base metallic

glasses or amorphous alloys only at 8 % Cr is necessary to

develop the same degree of corrosion resistance. Broadly

speaking both metallurgical structure and environmental

conditions are important factors in the study of corrosion

phenomenon.

1.3.FORMS OF CORROSION!

Corrosion may be classified on the basis of :

i) the mechanism of corrosion process

5

ii) Nature of environment

iii) Type of Corrosion deterioration.

iv) Type of corrosion reactions.

On the basis of above heads corrosion is classified into

8 forms ( 3 ) namely Uniform attack, Crevice corrosion,

Pitting , Intergranular corrosion, Stress corrosion,

Dealloying, Microbial corrosion and Hydrogen damage.

1.4 ELECTROCHEMICAL THEORY OF CORROSION;

Though corrosion problem is as old as man's knowledge

about the uses of metals but its mechanism was not known

uptill the eighteenth century. Wollaston ( 4 ) produced the

first paper in 1801 regarding the mechanism of corrosion.

Though several theories of corrosion have been proposed but

the most widely accepted Electrochemical theory of corrosion

was put forward by Whitney ( 5 ) in 1903. Various other

theories namely acid theory ( 6 , 7 ) direct chemical attack

theory ( 8 ) , colloidal theory ( 9 ) have also been proposed

but they are mainly restricted to specific systems only. The

electrochemical theory of corrosion is universally accepted

and is applicable to most of the corrosion process.

Most of the corrosion process involve two or more partial

reactions i.e. oxidation or reduction. An oxidation process is

loss of electrons by the metals, thereby acquiring a positive

charge.

M >M+ + e' (1)

This reaction constitutes the basis of corrosion of metals.

Similarly a reduction reaction involves gain of electrons. For

6

every oxidation reaction there must be a corresponding

reduction reaction. In aqueous solutions various reduction

reactions are possible depending upon the system . Some

examples of reduction reactions are as follows:

Hydrogen evolution: 2H" + 2^ >• H2 ( in acid system) .... (2)

Oxygen reduction:02 + 4H' + 4e' — > 2H20(acid solution) ... (3)

02 + 2H2O + 4e' > 40H" (in neutral and alkaline soln.)

Metal ion reduction : M +" + e' > M" (n-1) (4)

Metal deposition : M" " +ne' >M (5)

Oxidation reactions are known as anodic reactions while

reduction reactions are cathodic . During corrosion more then

one anodic and cathodic reactions may occur. Oxidation-

reduction (redox) reaction can be understood by the example of

corrosion of mild steel in sulphuric acid contaminated by

ferric ions, Anodic reaction will occur as below.

M • . M"" + ne" 5(a)

All the component elements of mild steel (example Fe, Mn,

Ni, Cr) go into solution as their respective ions. The

electron produced by the anodic (oxidation) reactions are

consumed by the cathodic (reduction) reactions. In this case,

reaction(4)can be be represented as below.

Fe+3 + e" > Fe+2 (6) ,

Deleting one of the available cathodic reactions [eg

reaction (6) by removal of the Fe""*"" ions] will lead to a

reduction in corrosion rate.

Thus the essentials for electrochemical corrosion can be

summarized as follows:

(a) Formation of anodic and Cathodic areas.

(b) Electrical contact between anode and cathode for the

condition of electrons.

(c) Presence of electrolyte (moisture)

When a metal or alloy is immersed in a corrosive

environment (conductive) different potential zones are

developed on the surface of metal or alloy due to the presence

of different metallic phases, grain boundaries, segregates,

crystalline imperfections, impurities etc. This difference in

potential leads to formation of anodic and cathodic areas on

the metallic surface where oxidation and reduction reactions

occur, respectively. These areas result in formation of local

action cell on the metallic surface. Local action cell can

also form where there are variations in the environment or in

temperature.

The electrode potential is calculated from the Nernst

equation:

RT (oxid) E = E G + In (7)

ZF (red)

where,

Eo = Standard electrode potential

R = Gas constant (1.98 cal/gm equivalent)

F = Faraday constant (96, 500 coulombs/gm equivalent)

T = Absolute temperature (gm equivalent)

Z = No. of electrons transferred in the reaction

(OX) = Concentration of oxidized species (mol/1)

(red)= concentration of reduced species (mol/1)

The potential of a reaction is related to its free energy (6G) by:

G = zFE (8)

A negative value for the free energy corresponds to a

spontaneous reaction, whereas a positive value of G indicates

that the reaction has no tendency to proceed. The change in

free energy accompanying an electrochemical or corrosion

reaction can be calculated from a knowledge of the cell

potential of the reaction.

It is the redox potential by which one can predict

whether a metal will corrode in a given environment or not.

Limitations of the EMF series have been largely overcome

by a new series namely "Galvanic Series". In galvanic series

metals and alloys are arranged in accordance with their actual

measured potential in a given environment , when one or more

metal is used in structure, galvanic series helps to find out

the most anodic metal. Such as series of metals and alloys

immersed in sea water is given in Table 1.2. Due to simplicity

and because corrosion potential of a given metal often shows

considerable variation, the actual potential values are

usually eliminated and the table is merely presented as a list

of metal and alloys with positive corrosion potentials usually

referred to as active or anodic at the bottom. In the absence

of actual tests in a given environment, the galvanic series

helps to establish the corrosion resistance of metal and

alloys with the exception of few limitations. For example,

TABLE 1 . 1

S t a n d a r d - O x i d a t i o n - R e d u c t i o n (Redox) p o t e n t i a l s a t 3 9 8 ° K

(E.M.F S e r i e s ) ( 1 0 , 1 1 )

Au+^ + 3e" Au ^ P t ^=:i P f ^ + 2 e 0 ^ + 4 H+ + 4 e " -Pd ^ = ^ Pd"""" + 2 e " Ag ^=:^ Ag"^ + e 2Hg «=:^ Hg2+"'

2 H 2 O

+ 2e

0" + 2H;,0 + 4e" . 5 = ^ 0 H ' Cu Cu+2 + 2e" Sn"+ 2H'' Pb ^ Sn ^ Ni v= Co ^ Cd ^ Fe ^ Cr ^ Zn ^ Al ^ Mg ?: Na ^ K ^=

+ +

2e' 2e"

Sn +2

Pb +2 H

Sn+2 Ni+2

Cd+2 Fe +2

Cr Zn Al Mg

+ 3 + 2 + 3 + 2

2^-2e" 2e" 2e" 2^

2e" 3e" 2e' 3 e " 2 e

e"

1 . 4 2 1 .2 1 . 2 3 0 . 8 3 0 . 7 9 8 0 . 7 7 1

0 . 4 0 1 0 . 3 4 0 . 3 4 0 . 1 5 4 0 . 0 0 0

- 0 . 0 1 2 6 - 0 . 1 4 0 - 0 . 2 3 - 0 . 2 7 - 0 . 4 0 2

• - 0 . 4 4 - 0 . 7 1 - 0 . 7 6 3 - 1 . 6 6 - 2 . 3 8 - 2 . 7 1 - 2 . 9 2

( N o b l e )

10

aluminum is anodic to zinc according to EMF series whereas

zinc is shown anodic to aluminum in galvanic series (Table

1.2). In marine atmosphere it is actually observed that zinc

is anodic to aluminum. Although more useful than EMF series,

the galvanic series cannot be used to predict the corrosion

rates. A galvanic series is more relevant and practical for

predicting galvanic effects than the EMF series.

1.5 PREVENTION OF CORROSION;

Many methods are adopted to minimize the damage caused

due to corrosion. These are essentially (a) selection and

control of the material, (b) appropriate design of the

structure, (c) applications of various types of coating (d)

cathodic and anodic protection (e) alteration of environment.

Details of these various methods can be found in extensive

literature on corrosion control ( 12-14 ).

One of the major methods of corrosion control

particularly in closed systems is by the use of corrosion

suppression reagents called inhibitors. An inhibitor is a

chemical substance which when added in small concentrations to

the environment causes a substantial reduction in the rate of

corrosion of metal either by reducing the probability of its

occurrence (deterrent) or by reducing the rate of attack

(retardant) or by both. Therefore an inhibitor can be regarded

as a retarding catalyst. An inhibitor useful for a particular

corrosion system may be harmful to another under certain

situations.

11

TABLE 1.2

Galvanic Series in sea water ( 3 % NaCl) of some commercial

metals and alloys.

Metals & Alloys Platinum Gold Cathodic (Noble) Graphite Silver 18-8-3 Stainless steel, Type 316(passive) 18-8- Stainless steel, Type 304 (passive) 13 % type 410 chromium stainless steel, (active) Nickel Alloy (Passive) Bronze Copper Brass Nickel (active) Manganese Bronze Muntz metal Tin Lead Stainless steel type 316, 304, 410 (active) Cast Iron Wrought Iron Mild Steel Aluminum 2 024 Cadmium Al Clad Aluminium Galvanized Steel Zinc Magnesium A l l o y s Anodic ( ac t ive ) Magnesium

12

1.6 INHIBITORS;

The term inhibitor has been defined in several ways.

According to NACE, "Inhibitor is a substance which retards

corrosion when added to an environment in small

concentrations( 15 ) while recent ISO defines inhibitor as "

A chemical substance which decreases the corrosion rate when

present in corrosion system at a suitable concentration

without significantly changing the concentration of any other

corrosive agent." ( 16 ).

Inhibitors may also be defined on electrochemical basis

as substances that reduce the rates of either or both of

partial anodic oxidation or cathodic reduction reaction.

From 19th century onwards vegetable wastes, plant

extracts ( 17-19 ) an animal protein were used as inhibitors.

Putilova et al ( 20 ) have reviewed metallic corrosion

inhibitors. Several reviews on organic inhibitors ( 21-23 )

and organic sulphur compounds ( 24 ) have been published.

Several books too have been published on this subject ( 25,

26). Besides this, the university of Ferrara, Italy , conducts

a symposium on corrosion inhibition once in every five years

( 27 ) . All the international seminars on corrosion discuss

the development and application of corrosion inhibitors

( 28, 2 9 ) . Various books on corrosion, review the subject in

a precise manner ( 30,31 ). These show that an extensive

amount of information is available on this topic. It also

shows the immense importance of the topic.

13

Inhibitors may be classified into several groups on the

basis of their mechanism and composition i.e. passivators,

precipitators, Vapour phase inhibitor, anodic inhibitor,

cathodic inhibitors, neutralizers and adsorbents.

The passivating type inhibitors are mainly inorganic

oxidizing chemicals or oxidizers namely chromates, nitrate,

nitrites and ferric salts which can passivate steel in absence

of oxygen and when added to the corrodent, facilitate the

anodic process of oxide film formation by retarding the

cathodic current density or by enhancing the field potential

value to more negative values and subsequently promoting the

anodic process. The other passivating inhibitors are inorganic

non-oxidizing chemicals. (e.g. phosphate, tungstate, molybdate

etc. ) which require the presence of oxygen to passivate

steels. Passivating inhibitors are considered to be most

effective of all inhibitors because they can stop corrosion

completely. Passivating inhibitors are also known as

"dangerous inhibitors" because under certain conditions they

can accelerate corrosion.

Another important type of inhibitor is the pickling

inhibitor. These are usually nitrogen or sulphur containing

organic compounds like thiourea, amines, azoles etc.

Urotropin, { 32 ) is a nitrogen containing inhibitor and has

wide application in hydrochloric acid.

These pickling inhibitors mainly act through adsorption

mechanism.

14

1.6.1 Il^IBITORS CLASSIFICATION:

Inhibitors can be classified for near-neutral pH aqueous

systems under the following heads:

a) Safe or dangerous inhibitors

b) Anodic or cathodic inhibitors

c) Oxidizing or non-oxidizing inhibitors

d) Organic or inorganic inhibitors.

Each inhibitor should be present above a certain minimuin

concentration for it to be effective. At insufficient

concentration safe inhibitor will allow a uniform type of

corrosion where as a dangerous inhibitor will lead to an

enhanced localized attack or pitting.

Anodic or Cathodic inhibitors are classified on the basis

whether the inhibitors cause an increased polarization of

anodic reaction (metal dissolution) or cathodic reaction i.e.

oxygen reduction ( in near neutral solution) or hydrogen

discharge (in acid solution).

The oxidizing or non oxidizing inhibitors are

characterized by their ability to passivate the metal. Non-

oxidizing inhibitors require presence of dissolved oxygen in

liquid phase for the maintenance of passive film while in case

of oxidizing inhibitors presence of dissolved oxygen is not

necessary.

Inhibitors can be set to be organic or inorganic on the

basis of their chemical nature e.g. sodium salts of carboxylic

acid are organic inhibitors.

15

'^corr

corr

'carr ' cor r

CURRENT

( a )

•^ '^corr c o '^corr a

' cor r 'corr

CURRENT

( b )

- E

c 'o a

m corr

^ C O r r

'corr 'corr CURRENT

( c )

F l G i . l M E C H A N l S M OF ACTION OF CORROSION I N H I B I T O R S

BASED ON POLARIZATION E F F E C T S

16

1.6.2. ANODIC INHIBITORS:

Those substance which reduce the anode area by acting on

the anodic sides and polarize the anodic reaction are called

anodic inhibitors. In the presence of anodic inhibitors,

displacement in corrosion potential (E orr occurs in positive

direction, the corrosion current (I orr gets suppressed and

there is a reduction in a corrosion rate. In aqueous acid

media the corrosion of metals occurs at the anodic area

through metal dissolution. The cathodic reaction generally

involve hydrogen evolution or oxygen reduction. These

reactions are represented schematically in the given figure

1.1. The curve E ^^.^^ A represents the anodic reaction while

E QQJ-J. C represents the cathodic reaction and the point B where

both anodic and cathodic reaction intersects corresponds

corrosion potential (E orr ^ " corrosion current! I orr •

The substances which retard the anodic reaction lead to

the enhancement of anodic polarization. In this situation,

anodic curve E ^ Corr A (Fig 1.1a) and the current I ^ corr

corresponding to 0 is less than I ^^^^ (corrosion current in

absence of inhibitors) thus making a reduction in the

corrosion.

Anodic inhibitors which cause a large shift in corrosion

potential are called passivating inhibitors but if the same

are used in insufficient concentrations, they can cause

pitting and some time lead to an increase in corrosion rate.

With careful dosage control, anodic inhibitors can be

17

frequently used because they are remarkably effective. Anodic

inhibitors cause passivity by speeding up the corrosion

reaction to the extent that the anodes are polarized to a

passive potential if corrosion or metal or alloy is controlled

by the anodic reaction (anodic control), it is obvious that

the decrease in overall corrosion should be proportional to

the portion of anodic areas polarized^ On the other hand if

corrosion is controlled by cathodic control there is no effect

of decrease of anodic areas on the corrosion current or the

total amount of corrosion. In this case, the same amount of

corrosion must be distributed over a smaller anodic area,

resulting in intensified localized attack or pitting.

The inhibition mechanism of anodic corrosion inhibitors

has been a matter of dispute for a very long time and there

are two views put forward to explain their action. One theory

supports the formation of a protective insoluble film on metal

in the presence of the inhibitor while according to the other

one inhibitors get adsorbed by specific force of interaction

or through chemisorption on the surface of metal.

a) PROTECTIVE FILM MECHANISM:

It has been observed that most of K and Na salts

containing anions act as anodic inhibitor by the formation of

a sparingly soluble salt with the metal. Hoar and Evans ( 33 )

have shown that chromates reacts with ferrous ions and

precipitates and adherent protectic film of hydrated ferric

and chromic oxides on the anodic areas. It has been shown that

during corrosion processes by sodium hydroxide { 34 ) ,

18

orthophosphate ( 35 ) , nitrite ( 36 ) , chromate ( 37 ) and

other anodic inhibitors like sodium carbonate, acetate,

benzoate, molybdate ( 38 ) in aerated solutions, there occurs

a formation of an invisible protected film by r - Fe203.

b) ADSORPTION MECHANISM:

Eminent scientist Uhlig ( 39 ) suggested that formation

of oxide film is not necessary for inhibition,- According to

him primary inhibition by chromates and other oxidizing

inhibitors is due to physical and activated chemisorption, by

which the valence forces of the surface of metal atoms are

satisfied. These views were confirmed from the measurements of

electrode potential as well as the measure of residual

activity of an iron sample immersed in radioactive chromate

solution and subsequently washed thoroughly with distilled

water ( 40-42 ). Later on Hackerman ( 42 ) too declared that

anion adsorbed at the oxide solution interface were

responsible for inhibition rather than formation of metal

oxide films.

1.6.3 CATHODIC INHIBITORS:

Those substance which reduce the cathode area by acting

on cathodic sides and polarize the cathodic reactions are

termed as cathodic inhibitors. They displace the corrosion

potential in the negative direction (E ^ Corr) and reduce

corrosion current, thereby retarding the cathodic reaction and

finally resulting in a reduction in corrosion rate(Fig.1.1 b) .

In this situation the point of intersection is at 0 and

corresponding current I '^ Corr will be lower than that without

19

inhibitor (I ^Q-^^) •

The cathodic inhibitors, with a few exceptions ( 43 ) do

not lead to intensified or localized attack since cathodic

areas are not attacked during corrosion. When corrosion is

controlled by cathodic reactions, the added cathodic

inhibitors results in a decrease in cathodic area and hence

overall corrosion rate.

Mann et al ( 44-46 ) and other investigators ( 47-50 ) ,

working on numerous organic inhibitors in acid media have

proposed the onium structure of an inhibitor and according to

them these inhibitors get adsorbed on the cathodic areas of

the surface by force of physical adsorption and chemisorption.

Contrary to this Bockris and Convey { 51 ) claim that the

action of cathodic inhibitors is due to an increase of

hydrogen over voltage. They did not agree with the inhibitor

film theory. The cathodic inhibition due to general adsorption

of the inhibitors on the metal surface remains however the

most accepted theory ( 52,53 ).

Cathodic inhibitors too are safe like the anodic ones

when present in solution in insufficient quantities and

involve no additional risk of pitting attack. Cathodic

inhibitors can be further subdivided into three categories.

a:Cathode poisons:

When cathodic reaction is the reduction of hydrogen ions to

hydrogen gas, several steps are involved. The hydrogen ions

are reduced to hydrogen atoms which are adsrobed on the

surface of the metals.

20

2H' + 2e' — > 2H° (ads) (13)

Two hydrogen atoms may combine to form a hydrogen

molecule which is discharged from the surface in form of gas.

2H° (ads) * H2 t (14)

Cathodic poisons are substances which interfere with the

formation of hydrogen atoms or recombination of hydrogen atoms

leading to production of hydrogen gas on the surface of the

corroding metal.

The rate of the cathodic reaction decreases and since

both the anodic and cathodic reactions must proceed at the

same rate the overall corrosion process is slowed. Some

cathodic poisons like sulphide and selenides get adsorbed on

the metal surface. Other compounds of arsenic, bismuth and

antimony are reduced at the cathode to deposit a layer of

respective metals. Sulphide and selenides are not considered

to be useful inhibitors as they are almost insoluble in anodic

solutions and precipitate many metal ions and are toxic too.

A serious disadvantage is of cathodic poisons is the

formation of blisters on the steel and thus increasing the

susceptibility of the steel to hydrogen embrittlement. Since

the recombination of hydrogen atom is increased and a greater

function of hydrogen produced by the corrosion reaction is

adsorbed by the steel ( 54 ).

21

b)Cathode Precipitates:

These are filming type of inhibitors for example

CalHCOj), ZnSO^, compounds with cations having a tendency to

migrate towards the surface and react with cathodically formed

alkali (mild) to produce insoluble protective film or layer

are termed as cathode precipitates. These reduce the corrosion

current by making the cathode reaction more difficult.

Ca'' + 2 H CO^' + OH" >CaC03 + HCO3" + H2O

(From Hard) (cathodic (Cathodic film) ....(15) water) alkali)

Zn+2 . OH" > Zn (OH) 2

From Cathodic Cathodic Cathodic film .... (16)

inhibitor alkali

Sand waters are less corrosive to iron than soft or

distilled water,

(c) Cathode Scavengers:

These function by removing corrosive reagents from the

solution. Common oxygen scavengers are sodium sulphite,

sulphurdioxide and hydrazine. These remove dissolved oxygen

from aqueous solution.

2Na2S03 + O2 > 2Na2S04 (17.1)

2SO2 + O2 + 2H2O > 2H2SO4 .... (17.2)

N2H4 + O2 »N2 + 2H2O (17.3)

These i nh ib i t o r s function very e f f i c i e n t l y in closed

systems where oxygen reduction i s the con t ro l l ing corrosion

reaction but are inef fec t ive in s t rong acid so lu t ions .

I t i s advantageous to use hydrazine instead of sulphite

in boi ler water corrosion control because the former causes an

22

increase in the hardness of water in addition to formation of

boiler scale in the presence of free calcium ions. The

liberation of ammonia by the following reaction.

2N2H4 > N2 + 4NH3 (18)

leads to alkalisation of water to reduce free carbondioxide

concentration. The reaction rate of sulphite, sulphurdioxide

and hydrazine with oxygen at low temperature is slow.

Therefore catalysts like Co, Mn, and Cu are usually added to

increase the corrosion rate.

1.6.4 Mixed Inhibitors:

Number of chemicals which inhibit the metallic corrosion

by interfering with both the anodic and cathodic reactions are

termed mixed inhibitors. These type of inhibitors are

represented by fig (1.1 c) . The anodic and cathodic reactions

are represented by E' Corr and E^ Corr^ respectively and

corrosion current 1^ corr in presence of such type of

inhibitor is considerably less than that in their absence.

Glue, gelatin and other high molecular wt. substances fall in

this category. It is believed that the action of such type of

inhibitors at the metal liquid interface is due to their

concentration or coagulation providing a shield to the metal

surface. Machu ( 55,56 ) claims that their action is mainly

due to formation of a porous layer.

1.7 MECHANISM OF INHIBITION IN ACIDS

It is believed that the inhibitive action of organic

compounds on the metallic surface is due to the interaction

between the inhibitor and metal surface, phenomenon being that

23

of adsorption. In this process ( 57 ) the molecules are held

on to the surface of the adsorbent by valence forces i.e.

variation in the charge of the adsorbed substance and a

transfer of charge from one phase to another. Therefore, the

molecular structure of the inhibitors assumes special

significance! 58 ) . The electron density at atoms of

functional group constituting a reaction center affects the

strength of the adsorption bond ( 57 ) . It is also dependent

on the properties of the metal, as well as on the

polarizability of the functional group { 58-61 ). Inhibition

by adsorption can be explained by LFER correlation ( 62,63 ) .

1.7.1. FACTORS INFLUENCING ADSORPTION MECHANISM;

1.7.1.1 Surface Charge of metal;

The magnitude and sign of the surface charge play a very

significant role in establishing the adsorption bond. The

effects exercised by the organic inhibitors on the electrode

reactions must be connected with the modifications induced in

the structure of the electrochemical double layer because of

their adsorption. In solution the charge on a metal can be

expressed by its potential with respect to the zero charge

potential . This potential, often referred to as the 0

potential is more important than the potential on a hydrogen

scale and sign of these two potentials are different ( 64 ) .

Positive potential favours adsorption of ions and negative

potential favours adsorption of cations.

24

1.7.1.2 Reaction of Adsorbed Inhibitors;

In some cases, the adsorbed corrosion inhibitors may

react to form a product by electrochemical reduction which may

also be inhibitive in nature. Inhibition due to added

substance is termed as primary inhibition and that due to the

reaction product, secondary inhibition ( 65 ) . In such cases,

the inhibitive efficiency may increase or decrease with time

according to whether the secondary inhibition is more or less

effective than the primary inhibition ( 66 ) .

1.7.1.3 Interaction of Adsorbed inhibitor species;

Lateral interactions between adsorbed inhibitor species

becomes significant with increase of surface coverage of the

adsorbed species. This lateral interaction may be either

attractive of repulsive . Attractive interaction occurs

between molecules containing large hydrocarbon components.

Repulsive interactions occur between ion or molecules

containing dipoles and lead to weaker adsorption at high

coverage ( 67 ) .

1.7.1.4 Interaction of the inhibitor with water molecule.

The surfaces of the metals in aqueous solutions are

covered with adsorbed water molecules. Adsorption of

inhibitors occurs by the displacement of adsorbed water

molecules from the surface which involve free energy for

adsorption. It is found to increase with the energy of

solution of adsorbing species ( 68 ).

25

1.7.1.5 Structure of Inhibitors and their adsorption;

Inhibitors bond the metal surface by electron transfer to

the metal leading to formation of adsorption bond. Generally

the inhibitors are electron donors (Lewis base) and metal is

the electron acceptor (Lewis acid). The strength of this bond

depends on the characteristics of both the adsorbate and

adsorbent. Electron transfer from the adsorbed species is

formed by the presence of relatively loosely bound electrons,

such as those which may be found in anions and neutral organic

molecules containing lone pair of electron or ^ electrons

systems associated with multiple especially triple bonds or

aromatic rings.

Most organic compounds have at least one polar atom, i.e.

nitrogen, sulphur, oxygen and in some case selenium and

phosphorus. In general, the polar atom is regarded as the

reaction center for the establishment of chemisorption ( 57 )

process. In such cases, the adsorption bond strength is

determined by the electron density of the atom acting as the

reaction center and also by the polarizability of the polar

atoms. The effectiveness of the polar atoms with respect to

the adsorption process varies in the following sequence( 59 ) .

Selenium > Sulphur > Nitrogen > Oxygen. It was

Donahue ( 62 ) who evaluated the importance of electron

density in chemisorption of organic substances in relation to

inhibition phenomenon. The idea of electron density acquires

remarkable importance in aromatic and heterocyclic inhibitors

26

whose structures may be affected by the introduction of

substituents in different position of the rings ( 61 ) . The

availability of electron pairs can thus be altered by regular

and systematic variations of the molecular structure.

1.7.2 EFFECT OF INHIBITOR ON CORROSION REACTION;

We know that an inhibitor is a substance which when added

in small concentrations to the environment may lead to a

decrease in either the anodic, cathodic or both the processes,

hence reducing the corrosion rate. The change in corrosion

potential on addition of the inhibitor is the indication of a

retarded process ( 69 ) . Shift of corrosion potential in the

positive direction indicates mainly the retardation of the

anodic process (anodic control). Whereas shift in the negative

direction indicates the retardation of cathodic process

(Cathodic control). Little change in the corrosion potential

suggests that both anodic and cathodic process are retarded.

In the presence of a inhibitor, a shift of polarization

center without a change in the Tafel slope reveals that the

adsorbed inhibitors act by blocking active sites, thus

inhibiting the reaction instead of affecting the mechanism of

the reaction ( 70 ) . A change in the Tafel slope depicts that

the reaction mechanism is affected.

Inhibitors in acid solution affect the corrosion

reactions of metals in the following ways:

1.7.2.1 Formation of Diffusion barrier;

The adsorbed inhibitor which forms a surface film on

metal surface, can act as physical barrier to restrict the

27

diffusion of ions or molecules to or from the metal surface

consequently retarding the corrosion reaction. This type of

behaviour occurs in inhibitors containing large molecules

( 71 ) .

1.7.2.2 Blocking the reaction sites

The inhibitor may adsorb on the metal surface to prevent

the surface metal atoms from participating in either the

anodic or cathodic reactions of corrosion. This blocking

process reduces the surface metal atoms at which these

reactions can occur and hence the fates of these reactions.

The mechanism of the reactions are not affected and the Tafel

slopes of the polarization curves remain unchanged. Adsorption

of inhibitors at low surface coverage tends to occur at anodic

sites, causing retardation of the anodic reaction. At high

surface coverage adsorption occurs on both anodic and cathodic

sites, and both reactions are inhibited.

1.7.2.3 Participation in the Electrode Reaction:

The electrode reactions involve the formation of adsorbed

intermediate species with surface metal atoms. The presence of

adsorbed inhibitors will interfere with the adsorbed

intermediate but the electrode process may then proceed by

alternative paths through intermediates containing the

inhibitor. In these processes, though the inhibitor affect the

reaction it remains unchanged with change in the Tafel Slope

( 72 ). Inhibitors may retard the rate of hydrogen evolution

on metals by affecting the mechanism of reaction with the

increase in Tafel Slopes of cathodic polarization curves. This

28

effect has been observed on iron in the presence of inhibitors

such as phenyl thioureas ( 73 ) .

1.7.2.4., Alteration of the Electrical Doiible Layer;

The adsorption of ions or species which can form ions on

the metal surfaces will change the electrical double layer at

the metal solution interface and this in turn will affect the

rates of the electrochemical reactions.

1.7.2.5 Adsorption Isotherms;

An adsorption isotherm gives the relationship between the

coverage of an interface with an adsorbed species (the amount

absorbed) and the concentration of the species in the solution

{ 74 ) . Various adsorption isotherms have been formulated.

Table 1.3 gives the list of isotherms and their corresponding

equations ( 75 ) .

Interpretation of the inhibition characteristics of

organic molecules can be made by fitting the data to one of

the adsorption isotherms.

29

Table 1.3

ADSORPTION ISOTHERMS ( 75 )

S.No

1.

2.

3.

4.

Isotherm

Freundlisch

Langmuir

Frumkin

Temkin

gc

gc

SC

i?n

Equations

= e

+ 9/ l-e

= 9 / 1 - 9 exp(-2ae)

exp (a 9) -1

5.

6.

Blomgren- Bockris

Parsons

1-exp. [-a(l-9)]

fiC = 9/1-exp. (pe /2 .q Q:

9 2-9 EC = exp exp (-2a(

9 (1-9) 2'

7. Bockris Devanathon and muller

LogC+Log9/l-9 = (+S9-''

where:

E _ g-G ads/RT _ adsorption constant

G = Free energy adsorption

8 = Surface Coverage

C = Concentration

a = interaction parameter

a>0 = > attraction and a< 0 = > repulsion

p and q = Constants expressed in terms of dipole moments

30

1.8 METHODS OF STUDYING CORROSION;

The analysis of corrosion inhibitors covers a spectrum of

activities ranging from studies for protection mechanisms

through the search for new inhibitors and the appraisal of

competitive commercial producers to the monitoring industrial

system in which the inhibitors are being used. Recently, the

measurement of corrosion rates in the presence of corrosion

inhibitors by weight loss and electrochemical methods have

been reviewed by Mercer ( 76 ) . Various important techniques

employed for the investigation of corrosion inhibitors have

been listed as follows:

i) Weight loss method,

ii) Potentiostatic method,

iii) A.C. Impedance technique,

iv) Hydrogen Permeation technique

v) Scanning Electron microscopy (SEM)

vi) Anger Electron Spectroscopy.

1.8.1 Non-Electrochemical Methods:

The weight loss studies and gasometric techniques are

categorized under the non-electrochemical methods:

The effectiveness of an inhibitor is assessed in terms of

Inhibition efficiency (I.E) which is expressed by the

following formula.

WQ-W, IE % = X 100

Wo Where W^ = Weight loss of specimen in the given method in absence of inhibitor,

Vl-^ = Weight loss in the presence of Inhibitor.

31

Similarly the volume of hydrogen gas liberated in the

presence and absence of inhibitor can be measured using

gasometric technique and the inhibition efficiency is

calculated by the following formula.

V - V IE % = — X 100

Vo and V are the volumes of the hydrogen gas liberated in

absence and presence of inhibitor respectively. This method

may give inaccurate results when:

(i) the inhibitors react with hydrogen and

(ii) there is considerable hydrogen penetration.

1.8.2 Electrochemical Method:

These methods are much more widely employed for the study

of inhibitors.

1.8.2.1 Polarization Method;

In this procedure the inhibitors behavior is studied by

means of a Tafel plot (Fig 1.2) . both in the absence and

presence of inhibitor.

The percentage inhibition is derived from the formula

o corr ~ corr

o corr IQ = Corrosion current density (Corrosion rate) in

absence of inhibitor.

I - Corrosion current density (Corrosion rate) in presence of inhibitor.

The corrosion rate is determined from the polarization

data in two ways:

(a) Tafel extrapolation method

(b) Linear polarization method.

32

Noble t

^ V H .

'o^ *^/*^2 / ' ^

E ( V )

— Theorelicol curves ^— Experimeniai curves

"corr (M )

Active ( - )

Log I -

FIG^.2 POLARIZATION CURVES FOR A CORRODING ELECTRODE

Eco r r = Corrosion potent ia l

' co r r = Corrosion cur ren t

33

In the Tafel extrapolation method the linear portion of

the Tafel curve is extrapolated. The point of intersection is

referred to as Icorr

Linear polarization method, provides the volume of

absloute corrosion rate from the following expression.

Sa. Sc Icorr = X 1/ Rp

2.3 (Sa + Ec) Where:

Ea and £c are Tafel constants l/Rp = (i / |AjE = polarization resistance.

1.8.2.2 A.C Impedance;

The inhibition efficiency of the inhibitor can be

determined by means of AC Impedance techniques ( 77-78 )

employing the following formula.

1/Rto - 1/Rt IE % = X 100

1/Rto Rt and Rto are charge transfer resistance with and

without inhibitor.

For the determination of Rt and very small potential is

applied as a function of frequency (usually 10 KHZ- Im Hz).

The impedance of the corroding system for various

frequencies can be measured using lock in amplifier. A plot of

Z' (real) /vs Z" (imaginary) for various frequencies gives a

semicircle ( Nyquist plot ) which cuts the real axis at higher

and lower frequencies. At higher frequency it corresponds to

Rg and at low frequency it corresponds to ( Rgt R ) . The

difference between the two values gives R.. From the value of

R the corrosion current can be calculated using Stearn -

Geary equation.

34

Ea . Ec corr ^ / t

2.3 (Ea + Ec)

The double layer capacitance can be determined for the

frequency at which Z" is maximum from the relation.

FZ" max = 1/2 Y Cdl. R .

1.8.3 Other Methods;

Various other methods such as radio tracer techniques

( 79 ) , spectroscopic methods ( 80-82) , X-ray photo electron

spectroscopy ( 83 ) , Auger Electron spectroscopy ( 84 ) ,

Ellipsoraetry ( 85 ) , Hydrogen permeation ( 86 ) have also been

employed for studying the inhibition phenomenon.

1.9 SYNERGISM IN INHIBITION

"Synergism" is the term applied to the marked

reinforcement of the inhibitinhg action of one inhibitor by

the addition of small amounts of second inhibitor, even though

the second inhibitor is less effective when used alone.

It was shown by Foley ( 87 ) that tetraisoamyl ammonium

sulphate has little influence on the dissolution of iron in 4N

sulphuric acid. However where 0.005 KI was added, the organic

cation is adsorbed reducing the double layer capacity and the

dissolution of iron is very much decreased.

The inhibition efficiency of acetylenic compounds has

been greatly improved when combined with amines or thio

compounds ( 88 ).

35

1.10 CORROSION INHIBITION OF STEELS IN ACIDS;

Inhibitors play an important role in controlling the

corrosion of steel ( 89,90 ) . The major use of inhibitors in

acidic solutions is in pickling processes ( 90,91 ) for the

removal of rust, scale and corrosion products. Inhibitors can

protect metallic materials, especially ferrous metals and

alloys in various acids such as hydrochloric sulphuric,

nitric, hydrofluoric , phosphoric and various organic acids

( 32 ) .

Various properties required for inhibitors in acid

pickling are as follows:

(i) Should effectively inhibit the metal dissolution,

(ii) Should be effective at low concentration and high

temperature.

(iii) Should be thermally stable and chemically inert,

(iv) should not cause overpickling in the presence of

higher iron salt contents.

(v) Should inhibit hydrogen uptake by the metal,

(vi) Should possess good surfactant and good foaming

characteristics.

The pickling inhibitors should also not lead to the

formation of surface films with electrically insulating

properties that might interfere with subsequent electroplating

or other surface treatments. Pickling inhibitors require a

favourable polar group or groups by which the molecule can

attach itself to the metal surface. These include organic

36

nitrogen, amine, sulphur and hydroxy groups. The size,

orientation and shape of the molecule also plays an important

part in the effectiveness of inhibition ( 92 ). The surface

charge of the metal and its constituents effect the relative

strength of adsorbed bond on corrosion inhibition (93 ).

Granese and Resales ( 94 ) elucidated the mechanism of

corrosion inhibition of iron and steel in HCl media.

Inhibitors effective in corrosion prevention of Iron and steel

on treatment with HCl mainly belong to the group of nitrogen

containing compounds such as alkyl and aryl amines, saturated

and unsaturated N-ring compound, condensation product of

amines with aldehydes and ketones and carboxylates amines,

nitriles aldoximes, keto-oximes. Granese and Resales ( 94 )

observed reduced corrosion by N-containing organic compounds

like acridine , Hexamethylene, quarternary ammonium sulphate

etc. at 85° C. Urotropin a nitrogen containing inhibitor has

a wide application in HCl.

According to Hanna et al ( 95 ) the anion of the pickling

acid may also take part in the adsorbed film accounting for

the difference in inhibition efficiencies for the same

compound in Hydrochloric acid as compared to sulphuric acid.

Hanna et al used ethoxylated unsaturated fatty acid.

Pickling inhibitors may act as good inhibitor for iron

but not for other metals vice versa due to specific electronic

interactions of polar groups with the metal. Sometimes

temperature plays a significant role in effecting the

inhibition efficiency ( 96,97 ) eg. 0-tolyl thiourea in 5 %

37

H2SO4 acts as a good inhibitor at elevated temperature than at

room temperature due to increased adsorption.

In USA various acid inhibitors are prepared by utilizing

the industrial by-products. Katapin A which is alkyl benzyl

pryridine chloride ( 98 ) and its analogues prove to be

efficient in preventing the corrosion of high C steel.

Compounds containing N or S have shown vast application

as corrosion inhibitors ( 99,100 ) . Machu ( 100 ) has shown

the use of sulphur containing compounds for H2SO4 and nitrogen

containing compounds for HCl solutions. Hackermann ( 101 ) too

gave the idea that inhibitive action is due to the presence of

a high percentage of orbitals containing free electrons. N

containing compounds used as acid inhibitors include

heterocyclic bases such as pyridine, quinoline and various

amines ( 101,102 ). S containing compounds like thiourea and

its derivatives, mercaptans and sulphides in concentrations

.003- 0.01 % give 90 % protection ( 103-105 ). According to

Every and Riggs ( 106 ) , a mixture of N and S compound was

better than either type alone. Highly substituted N atoms

increase the inhibition efficiency due to increase of electron

density. Alkyl substitution on N atom or p-position of

aromatic nucleus improved inhibition efficiency in contrast to

meta derivatives. Effect of anions such as I" and SH' in

promoting a remarkable inhibiting action by organic cations in

acid solutions are well known ( 107-109 ) .

38

1.11 VARIOUS ORGANIC AND HETROCYCLIC CORROSION INHIBITOR

The literature reports various organic compounds as

potential corrosion inhibitors. The inhibitive action of

pyrrole and its derivative was investigated in 5N HCl and 5N

H2SO4 at 20°C by weight loss and polarization techniques. It

was found that the inhibition efficiency is dependent on the

dipole moment and pK values { 110 ).

F. Zucchi and G. Trabanelli investigated a series of

heterocyclic compound and showed that 2-Mercaptopyrimidine

had the highest efficiency in the range of 80-90 % ( 111 ).

2-Mercapto-benzi-imidazoles was studied by Balezin et al

and was found to be effective for mild steel in H2SO4 upto

70°C.( 112 ). Lee ( 113 ) evaluated the corrosion inhibiting

effect of 7-nitroso-8-hydroxy quinoline against mild steel in

HCl solution. The efficiency was more than 90 %.

S.L Granese and co-workers ( 114 ) studied various

heterocyclic N-Compounds such as n-hexadecyl derivative of

pyridine, quinoline, acridine in HCl by electrochemical and

surface analysis and concluded that acridine had the strongest

interaction with iron and steel surface while pyridine had the

least. According to them the efficiency of these compounds

increases with number of aromatic systems and electrons

availability in the molecule.

Singh and his co-workers ( 115 ) reported about 97%

efficiency of 2-mercaptobenzothiazole in IN H2SO4 at 40°C at

39

6 X 4 X 10"^ concentration.

Acetylenic alcohols were found to be effective class of

inhibitors of carbon-steel in IN HCl with the help of weight.

loss measurements and electrochemical data( 116 ).

1,10-phenanthroline was examined as corrosion inhibitor

for mild steel by gasometric and gravimetric methods. The

maximum efficiency was found to be 92.1 % and 92.5 % in IN

H2SO4 and IN HCl respectively at 3 X lO'^M concentration.

A macrocyclic compound ( 118 ) was evaluated as acid

corrosion inhibitors for steel by potentiostatic and impedance

methods. It gave 82% efficiency at 25°C in acid chloride

environment.

Use of thiomorpholine, phenothiazine derivatives and

vinyl pyridine polymers as potential pickling inhibitors for

ferrous metals has been reported in US patents (119-121).

Inhibitive action of pyridine, pyrrole, furan and

thiophene was investigated by galvanostatic measurement for

Fe-lN-HjSO^ system. Thiophene exhibited maximum efficiency as

corrosion inhibitor among the heterocyclics examined ( 122 )

The inhibition efficiency of various 4-amino-5 mercapto-

1,2,4-trizolines in 2.5 N H2SO4 was studied( 123 ). The

inhibition efficiency of these triazolines increased on the

introduction of electron donor substituents at 3 position. P-

methoxyphenyl substituted triazoline showed maximum efficiency

amongst the inhibitors studied. 4-amino-3-hydrazine - 5 thio-

1,2,4- triazole and some of its derivatives were studied as

corrosion inhibitors by Abd-el-Nabey et al ( 124 ) . They also

40

found that inhibitor efficiency increases on increasing the

electron density at active centers of the inhibitors.

Anderson et al ( 125 ) examined the effectiveness of 4,

7-diphenyl-l,10-phenanthroline in acid medium. They

established that the inhibitive action is due to the presence

of -ji" electrons in the ring.

The inhibition efficiencies of 1,1'-alkylene-

bispyridinium compounds have been studied for the corrosion of

mild steel in IN H2SO4. The inhibitors namely 1,1'-ethylene 3,

3'-dimethyl-tris pyridinium iodide and 1,1-ethylene bis

pyridinium iodide showed 87.8% and 86.7% efficiencies in

concentration ranges of 250 and 1500 ppm respectively at

3 0°C. The authors have found that substitution in the pyridine

ring has a pronounced effect on the inhibition efficiency

(126 ) .

The influence of some of heterocyclic compounds

containing more than one nitrogen atoms in their molecules on

corrosion of carbon steel in IN HCl was investigated by

Trabanelli and co-workers ( 127 ). With a view to establish

co-relation between molecular structure and the inhibition

efficiency of the various compounds among the examined

substances 2, 2'-Biquinoline, guinoxaline, quinozoline and 2-

mercaptopyrimidine show good inhibiting efficiencies (80 % -

90 %) at temperature 25°-60°C.The influence of some of the

substances on hydrogen penetration into the steel was also

studied by these authors.

Sethumadhavan and his coworkers ( 128 ) studied 1,10-

41

phenanthroline as corrosion inhibitor of mild steel in pure

sulphuric acid and found 87 % efficiency at room temperature

in 1 X 10"^ M concentration.

The inhibitive effect of some substituted phenyl-N-phenyl

carbamates on corrosion of iron in 2N HCl was studied by Fonda

et al ( 129 ) . The authors have found that inhibition of

corrosion occurs by way of adsorbtion through oxygen atom of

phenoxy group and nitrogen atom of - NH group. The inhibitory

character of the compound depends upon the concentration of

the inhibitor as well as its chemical composition.

Agarwal et al studied the corrosion behaviour of HLSA

steel in 1 NH2SO4 solution and suggested that piperidine is an

efficient inhibitor for corrosion and hydrogen evolution

reaction. Inhibition efficiency upto 90 % was observed at 10.1

mM concentration of piperidine ( 130 ).

Jha studied o.m and p-anisidines by weight loss and

electrochemical techniques and established that the inhibition

efficiency increases with increase in inhibitor concentration

and decreases with increase in temperature. However the order

of inhibition was found to be 0-anisidine > m-anisidine > p-

anisidine. o-anisidine showed a maximum efficiency of 82 % at

10"2 M concentration at 30° C ( 131 ).

Stupnisek et al ( 132, 133) investigated the inhibiting

action of various substituted N-aryl pyrroles on corrosion of

steel in strong acid solution ( 5 mol .dm"- HCl) using

electrochemical methods with a view to study the relationship

existing between the molecular structure and inhibition

42

efficiency. They have found that inhibition efficiencies of

pyrroles are significantly influenced by the type and position

of the functional groups. Thus N-aryl-pyrrole bearing flourine

at ortho position gave better performance than other pyrrole

derivatives.

Raicheva and co-workers ( 134 ) investigated several

diazoles such as imidazole, benzimidazole and their

derivatives as acid corrosion inhibitors of iron and steel.

They found a very good relationship between the structure of

diazoles and inhibition efficiency and concluded that the

annelation of the benzene nucleus to diazole ring increases

the protective action of the diazole significantly. Thus the

inhibition efficiency of benzimidazole (IE 29 %) increased to

96 % in 1,2- dibenzylimidazole.

Ajmal et al ( 135 ) studied the inhibitive action of 2-

hydrazino 6-methyl 1-benzothiazole on corrosion of mild steel

in acidic solutions. They found that this compound

acts as a mixed inhibitor in IN H2SO4 and behaves

predominantly as cathodic inhibitor in IN Hcl. They have also

found that the inhibitor effectively inhibits permeation of

hydrogen through mild steel.

Organic surfactants namely 2-hexadecyl imidazoline and 2-

hexadecyl imidazole were evaluated as corrosion inhibitors for

carbon steel in acid media containing H2S. These surfactants

showed 96 % IE. which was remarkably high , the concentration

being > 10"^ M for 2-hexadecyl imidazoline and > 5 X 10"^ M for

hexadecyl imidazole ( 136 ) .

43

A new class of acid corrosion inhibitors viz Azathiones

were synthesized by Quraishi et al ( 137 ) . The inhibitive

action of these compounds was studied on corrosion of mild

steel and in IN HCl and IN H2SO4 by both weight loss and

electrochemical techniques. The authors have found that the

inhibiting action of the azathiones depends on the molecular

size. Among the studied azathiones Cyclohexyltchazothione gave

90 % inhibition efficiency at 500 ppm in IN HCl ( 137 ).

The inhibitive action of 4-amino-5-mercapto-3 methyl-

1,2,4-trizole on corrosion of mild steel in IN H2SO4 and IN

HCl was investigated by potentiodynamic polorization, AC

impedence and hydrogen permeation methods ( 13 8 ) . The results

of the investigations indicated the improved performance of

this compound in H2SO4. The inhibitor was found to be very

effective in bringing down the permeation current considerably

in both the acid solutions.

These authors ( 139 ) have further synthesized a few

anils by condensing 3-alkyl 4-amino-5-mercapto-l, 2, 4,

triazoles with salicylaldehyde with a view to investigate the

inhibitive action of these compounds on the corrosion of mild

steel in acidic medium. They have found that all these

compounds show better performance than the corresponding

amines.

1.12 AIM OF THE PRESENT WORK;

A study of organic corrosion inhibitors is an attractive

field of research due to its usefulness in various industries.

A perusal of recent literature ( 140, 141 ) reveals that most

44

of the commercial inhibitor formulations include aldehydes and

amines as essential ingredients. Turbina et el ( 142 )

observed that condensation products of carbonyls and amines

which are known as anils or schiff bases give higher

inhibition efficiency that of constituents carbonyl and

amines. Desai and co-workers ( 143 ) have studied a few schiff

bases (derived from aromatic aldehyde and amines) as corrosion

inhibitors for mild steel in HCl acid. They found that the

inhibition efficiency of the investigated schiff bases is much

greater than that of the corresponding aldehyde and amines.

Being prompted by the interesting results of these authors it

was thought worthwile to undertake the synthesis of some

aromatic schiff bases with a view to evaluate their

inhibitive properties on corrosion of mild steel in acidic

solutions.

The selection of the inhibitors is based on two

considerations. Firstly they are conveniently synthesised from

relatively cheap raw materials such as aromatic aldehydes and

p-anisidi. Secondly the presence of non-bonding electron pair

on nitrogen and -^ electrons of the aromatic rings are likely

to facilitate greater adsroption of these compounds on the

metal surface, leading to higher inhibition efficiency.

45

CHAPTER 2 MATERIAL & METHODS

46

2.1 MATERIAL

2.1.1 Specimens:

Cold rolled mild steel strips of size 5X2X0.025 cm with

the following composition was used for investigation:

c 0 . 1 4

Mn

0 . 3 5

S i

0 . 1 7

S

0 . 0 2 5

P

0 . 0 3

Fe

r e s t

2.1.2 Test Solution

A. R. grade sulphuric acid and hydrochloric acids were

used . Double distilled water was used to prepare all the

solutions required for the experiments.

2.1.3 Inhibitors

Name Appearance M.P

Cinnamylidene p-anisidine

Vanillidene p-anisidine

Salicylidene P-anisidine

Yellowish green shiny crystals

Deep brown crystals

Silvery green flakes

102° C

108° C

75° C

2.1.3.1 Synthesis of the schiff bases (Scheme-1)

Schiff's bases were prepared by known methods as

described in the literature ( 144, 145 ).

47

H3C-O- / V N H ;

R

+ • * H j C O '

( C H r C H ),-CHO

N=CH- (CH = CH),

R,

R-

R

1. n = l , R,= R^r R3= H

2. n = 0 , R,=OH , F(,= R,= H

3. n=0 , R, = H , R2=OCH3, R3 = 0 H

5CHEME-1

48

T A B L E - 1 NAME STRUCTURE

CINNAMYLIDENC-P-ANISIDINE (CPA) VANILLIDENE-P-AMIS I DINE ( V P A ) H 3 C 0 - ^ ^ _ V

S/VLICYLIDENE- _

P-ANISIDINE{SPA) H 3 C 0 - < f ~ V - N = C H - ( ^ J >

49

(a) Cinnamylidene p-anisidine (CPA)

Cinnamaldehyde ( 0.01 mol , 1.3 ml) was added dropwise

to methanolic solution of p-anisidine ( 0.01 ml, 1.3 gm ) . The

resulting mixture was stirred on magnetic stirrer for 15

minutes.The green compound thus obtained was filtered and

crystallized from ethanol. M.P 102° C , yield 87 %, IR( KBr )

1630 ( S, >C=N str) cm '^, 1600 (S, C=C, str) cm.

b) Vanillidene p-anisidine (VPA)

An equimolar mixture of vanillin ( 0.01 mol, 1.5 gm and

p-anisidine (0.01 mol, 1.3 gm was taken in ethanol and stirred

on magnetic stirrer for 5 hrs. and the reaction mixture was

left overnight. The dark brown solid thus obtained was

filtered washed and crystallised from ethanol. M.P 108°C,

yield 70 %, IR ( KBr ) 3350 ( broad 0-H str), 1635 ( S, C=N

str) .

c) Salicylidene p-anisidine (SPA)

Salicylaldehyde (0.01 mol, 1.5 ml) was added dropwise to

an ethanolic solution of p-anisidine (0.01 ml, 1.3 gm) . The

reaction was both heated and stirred simultaneously for 4 hrs.

Green flakes thus obtained were filtered and crystallised from

ethonol. MP 75=C, yield 80 %, IR ( KBr ) 3370 ( broad 0-H str)

1630( S, C= N str ) cm"^.

2.2 TECHNIQUES EMPLOYED;

The inhibitive action of the above mentioned compounds on

mild steel was studied with the help of :

(i) Weight loss or gravimetric technique.

50

(ii) Potentiodyanamic polarization technique.

2.3 WEIGHT LOSS METHOD

Specimen size 5X2X0.025 cm were cut from the polished

mild steel sheets. These specimens were further washed with

distilled water and acetone. The specimens were stored over

silica gel in a desiccator. The weight of the specimen was

measured before exposing it to corrodent on a single pan

electrical balance. During weight loss experiments, the

specimen were fully immersed in 100 ml test solution using a

beaker of 250 ml capacity. After a definite exposure time, the

specimen was taken out and washed with distilled water. If

there is any corrosion product on the mild steel surface it

was removed from the surface by mechanical rubbing with a

rubber cork. Specimens were dried and then loss in weight was

recorded. The thermostatic water bath was used for carrying

out the weight loss experiment at higher temperatures. The

thermostat wass maintained within accuracy of ± 2°C. The

percentage inhibition efficiency and surface coverage ( 9 )

were calculated using the following equations:

Unihibited corros. rate-inhibited corros.rate IE(%)) - ^ ^ ^ X 100

IE(%) Wo -

w Wo -

W

W

U n i h i b t e d C o r r o s .

Or

X 100

r a t e

Wo

Where IE-% = Percentage Inhibitive Efficiency G = Surface Coverage Wo = Wt. loss or corrosion rate in uninhibited

51

system. W = Wt. loss or corrosion rate in inhibited

system

2.2.2 POTENTIODYNAMIC POLARIZATION TECHNIQUE:

The following instruments were used for carrying out the

polarization studies.

(i) Potentiostat ( E.G & G PARC model : 173)

(ii) Log current converter ( model : 376)

(iii) Universal programme ( model : 175)

(iv) X-Y Recorder ( Model RE 0089)

(v) Electric Single pan balance (Model VB, MDD)

(vi) Metallographic microscope (model : Metallux II)

All the experiments were carried out a (35 ± 2°C) . A

platinum foil of 3 X 3 Cm was used as the aiixiliary electrode

and a saturated calorimeter electrode was used as reference

electrode for potentiodyanamic polarization studies.

For the study, working electrode 1 cm X 1 cm with a tag

of 4 cm were cut from the mild steel sheet and polished with

0/0 to 4/0 grade of emery papers. The specimens were

thoroughly washed with distilled and finally with acetone.

Unwanted area of the electrode was coated with a lacquer to

get a well defined working area. The polarization was carried

out using a potentiostat ( EG & G PARC 173)., Universal

Programmer(model: 175) and X-recorder (RE 0089).

All the potentials were measured against a saturated

calomel electrode. The inhibition efficiency were calculated

5 2

POTENTIOSTAT Wo Ao Ro

Copper road

Lecquer

M i l d Steel e l e c t r o d ^ S c " ^

Stop cock

A /

Platinum / f o i l ^ (Courter

electrode)

Satu ra ted calomel e lec t roae

__^-_."___J l___-_^ Saturated

s o l u t i o n

F I G . 2 . 1 5 C H E M A T I C DIAGRAM OF THE ELECTROCHEMICAL

CELL

53

using the following equations:

oCorr- -^Corr IE % = X 100

oCorr

•'•oCorr ~ Corrosion Current density without inhibitor

IQQJ-J. = Corrosion Current density with inhibitor

2.3 DETERMINATION OF CORROSION RATE:

Corrosion rates were measured in iranpy using following

expression:

W X 87.6 C.R = (in rninpy) DAT

where:

W - the weight loss in grams.

A = the area of the specimen in square inches

D = the density of the specimen in gm/ cm^ i.e. 7.86

T = the time in hours

54

CHAPTER 3 RESULTS & DISCUSSION

55

This chapter describes the behaviour of some aromatic

Schiffs Bases as corrosion inhibitors on mild steel in acidic

environment. The following schiffs bases are selected for the

present investigation.

1. Cinnamylidene - p - anisidine ( CPA)

2. Vanillidene - p - anisidine ( VPA )

3. Salicylidene - p - anisidine ( SPA)

The inhibitive action of the above mentioned compounds

was studied employing the weight loss and polarization

techniques.

The weight loss measurements were conducted in both IN

HCl and IN H2SO4 at 40° C with varying concentrations of the

inhibitor ranging from 100 - 500 ppm. Polarization studies

with different concentration of inhibitor were carried out at

room temperature to understand the mechanism of inhibitor.

3.1 WEIGHT LOSS STUDY

Various corrosion parameters such as percentage

inhibition efficiency , corrosion rate obtained by weight loss

method at different concentrations of schiff bases in IN HCl

and IN H2SO4 at 40°C are given in tables 3.1 and 3.2. It is

seen from the results that all compounds inhibit the corrosion

of mild steel in both acids at all concentrations used for the

study. Inhibition efficiency for all these compounds is found

to increase with increase in concentration. SPA is found to

give maximum efficiency at 500 ppm whereas other compounds viz

VPA and CPA give maximum inhibition at a concentration of 400

56

and 3 00 ppm respectively,

3.2 DISCUSSION OF THE RESULTS ON BASIS OF MOLECULAR STRUCTURE

OF SCHIFF BASES

The inhibition efficiency values of examined aromatic

schiff bases follow the order:

CPA > VPA > SPA

The effectiveness of a given compound as a corrosion

inhibitors depends on the structure of the organic compounds

( 148 ) . The variation of inhibitive efficiency mainly depends

on the type and nature of the substituents present in the

inhibitor molecule ( 149 ) . Thus on the basis of the above

mentioned factors, the difference in protective action of the

schiff bases can be explained.

In the present study VPA is found to give better

performance as an inhibitor than SPA. This can be explained on

the basis of the presence of an electron releasing - OCH3

group, which facilitates its greater adsorption on the metal

surface leading to more inhibition than SPA. Among the

compounds used for the study CPA gives the best performance as

an inhibitor. This can be explained on the basis of the

presence of an additional -C=C-bond between the carbon atoms

in conjugation with azomethine group. These extensively

delocalized Y electrons favour its greater adsroption on the

metal suface thereby giving rise to high value of inhibition

efficiency ( 94 % ) at a concentration as low as 300 ppm.

The plots of surface coverage values vs log C gives

57

almost straight line in both the acid solutions indicating

that the adsorption of Schiffs bases on mild steel surface

obeys Scmitin's adsorption isotherm.

3.3 POTENTIODYNAMIC POLARIZATION STUDY :

The potentiodynamic curves of mild steel in absence and

presence of optimum concentrations of schiff basis in IN HCl

and IN H2SO4 at 35 ± 2°C are given in figure 3c and 3d.

Various corrosion parameters such as Ecorr, Icorr, IE % are

summarized in Table 3.3. It is evident from the results that

these compounds bring down I orr values significantly but do

not cause any appreciable change in E Q -J. values. These

observcitions reveal the fact that these compounds are mixed

type inhibitors ie they suppress both the anodic and cathodic

reaction. A similar observation have been found by Agarwal and

co-workers ( 150 ).

The plausible mechanism of corrosion inhibition of mild

steel in acid solutions can be explained on the basis of

adsorption. The adsorption of schiffs bases on the metal

surface can occur through aromatic rings and -^ electrons of

the azomethene group. The adsorption of inhibitor molecules on

the metal surface finds support by the fact that all schiff

base follow Temkin adsorption isotherm (Fig 3a and 3b) .

The salient feature of the investigation is that all

the inhibitors give better performance in IN HCl than in IN

H2SO4. This has been explained on the basis of synergism (

146,147 ), according which chloride ions and schiff base

molecules are jointly adsorbed on the metal surface giving

58

higher inhibtion efficiency.

An improvement of inhibition efficiency by organic

inhibitors in presence of chloride ions have been supported by

several investigators ( 151-153 ).

TABLE 3.1

Percentage inhibition efficiency (IE) , loss in weight (mg) and

corrosion rate(mmpy) in presence of different concentrations

of the inhibitors at 40° C, immersion period 3 hrs.

Cone of the Inhibitor(ppm)

Weight loss (mg)

I.E Corrosion rate mmpy

Blank (IN HCl)

CPA

300 200 100

196.

12.2 21.9 53.56

40.49

93.8 88.8 72.7

2.52 4.51 11.05

VPA

400 300 200 100

SPA

500 400 300 200

27.86 37.67 58.86 65.13

64.15 80.24 103.5 132.0

85.8 80.8 70.8 66.8

5.75 7. 77 12 .14 13.4

67.30 59.10 48.70 32.70

14. 18. 21. 25

.98

.6

.44

.50

59

TABLE 3.2

Percentcige Inhibition efficiency (IE) loss in wt (mg) and

corrosion rate (rampy) in presence of different concentrations

of the inhibitors at 40°C,Immersion Period = 90 mins.

Cone.of inhibitors (ppm)

Blank (IN H2SO4)

CPA

300 200 100

wt. loss (mg.)

165.5

16.5 33.1 68.0

I.E (%)

90.00 80.00 58.90

Corrosion rate mmpy

68.31

6.8 13.6 28.06

VPA

400 300 200 100

51.3 67.8 74.4 94.3

69.00 59.00 55.00 43.00

21.17 27.9 30.7 38.9

SPA

500 400 300 200

61.2 76.1 87.7 104.3

63.00 54.00 47.00 37.00

25.3 31.4 36.2 43.05

60

TABLE 3.3

Electrochemical parameters for the corrosion of mild steel in IN HCl and IN H2S0^ in presence and absence of inhibitor

Inhibitor and ^corr " corr •? - ^ concentration (mv) (mAcm" (%) (ppm)

IN HCl -587 0.350

CPA

300 -587 0.020 94.20

200 -571 0.040 88.57

100 -573 0.100 71.40

VPA

400 -576 0.060 82.80

300 -580 0.150 57.14

576

580

578

576

0.060

0.150

0.180

0.200

200 -578 0.180 48.57

100 -576 0.200 42.85

SPA

500 -575 0.130 62.80 575

575

575

0.130

0.150

0.200

400 -575 0.150 57.10

300 -575 0.200 42.85

IN H2SO4

CPA -582 0.320

300 -579 0.065 79.68

VPA

400 -580 0.130 59.37

SPA

500 -580 0.160 50.00

61

1,2

1.1 -

1.0

0.9

0.8

0.7

, 0.6

0.5

O.A

0.3

0.2

0.1

0.0

CINAMALDEHYDE P-ANISIDINE

VANILLIN P-ANISIDINE

SALICYLALDEHYDE P-ANISIDINE

Log C

FIG. 3a. TEMKIN ADSORPTION ISOTHERM FOR THE SCHIFF BASES IN IN HCl.

62

e

1.2

1.1

1.0

0.9

0.8

0.7

0.6

0.5

O.A

0.3

0.2

0.1

0.0

• CINALMALOGHVD£ p-ANISlD(hJE

A VANILLIN P- ANISIOINE

O SALICYLALOEHYDE P-ANIS IOINE

1

Log C

FIG. 3b. TEMKIN ADSORPTION ISOTHERM FOR THE SCHIFF

BASES IN IN HoSO.. z 4

63

<

+ 'd" " * •v ^

o o o o w w w w r j (M CM (N

X S X S

2 Z Z 2

O CO

CM

E u

< 5,

>-i n 2 l iJ O

1— z UJ

ct 3 u

2 H

H5 W W EH W

D f j H

S

o w > a; D U

2 O H EH

< W H « < ^ o O J

u H 2 < 2 >H D O M tH 2 W EH O

w w < QQ b b H S u

fo o w 2 o H E-i < « EH 2 w U 2 O U

s S H EH (14 O

O 2 H 2 H < EH 2 O

& u

o

H fa

o o

o o

o o U3

3 DS SA A u j - 1 V l i N 3 i O d

64

o

f ^

'E u

<

>-1— if) Z UJ Q

1— O 2 - U)

a (X 3 u

o

M

Ed W

U3

a t j M

fa o

>

'«i u M

cn H Di <; o 04

u H

s ri: 2 >H Q O M H 2 W El O

•

•

a H fa

t

w

w < PQ

fa fa H w u w fa o

2 U H FH < EH 2 W n 2 O U

s

H H Ci< O

C5 2 H 2 H

< EH 2 O U

s,\ AUJ' l V l i N 3 i O d

65

CHAPTER 4 REFERENCES

66

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74

SUMMARY

75

STJHICARY

Corrosion is a major problem in several industries. It

causes enormous wastage of metallic materials, which leads to

heavy economic losses all over the world. In India the

monetary losses due to corrosion have been estimated as high

as Rs. 1,200 crores per year.

Among the available methods, of preventing corrosion, the

use of inhibitors is one of the most promising methods

particularly for closed systems. Due to the ease of

application and cost-effectiveness it has attracted a great

deal of attention of corrosion scientists and engineers all

over the world.

Mild steel is one of the most important engineering

metal, which due to it's low cost and excellent mechanical

properties, is widely used as a construction material. The

mild steel is severely attacked in acid solutions, as it

usually comes in contact with HCl and H2SO4 in various

industries, during pickling, cleaning of industrial

equipments and acidization of oil wells etc. Inhibited acid

solutions are commonly used to reduce the corrosive attack of

acids on metals. Use of inhibitors is specific for different

systems and thus, it needs to be studied thoroughly.

The work described in the present dissertation deals with

the study of some schiff bases viz. cinnamylidene-p-anisidine

(CPA), salicylidene-p-anisidine (SPA) and vanillidene-p-

76

anisidine ( VPA) on corrosion of mild steel in hydrochloric

acid and sulphuric acid. The structure of schiff bases

selected for the investigation are given below.

TABLE- 1

NAME.

ClNHAMYLIDKNn:-

P-ANISIDINE ( C P A l ^ i VANILLIDENU:-

P-ANISIDINE (VPA)H s^L lCYL lDL:NL: - _

P-ANISIDINE(SPA) ^

STRUCTURE

co-^^^--

3C0-^Q>-N:

3C0-<Q^N

CH-

rCH -

= CH

CH

< f— i

HO'

OCH:> 1<

> —.

]>

-OH

The dissertation begins with an introduction highlighting

the economic and technological significance of corrosion.

Further various forms and theories of corrosion have been

described which help in understanding the mechanism of

corrosion. Special emphasis has been given to the mode of

action of inhibitors toward corrosion prevention. The account

of various techniques used for investigation of corrosion of

inhibitors have also been discussed briefly. Literature on

organics as corrosion inhibitors has been incorporated. The

aims and objectives of the present investigation have also

been stated.

The apparatus, material used, the description of

synthesis of inhibitors and the experimental techniques

adopted such as weight loss and potentiodynamic polarization

methods have been described in the experimental section.

77

The results obtained from potentiodynamic and weight loss

methods have been discussed in terms of various parameters

such as inhibition efficiency, corrosion rate. Corrosion

current density dcorr^ ^ ' corrosion potential (E QJ. .) . The

influence of concentration of schiff bases on corrosion of

mild steel in both acids has been studied and the optimum

concentration of each inhibitor has been evaluated. The

results show that all the three schiff bases inhibit

corrosion of mild steel in both acids. The inhibition

efficiency values of the schiff bases follows the order :

CPA> VPA > SPA

The corrosion inhibiting properties of schiff bases have

been explained in terms of their molecular structure. CPA is

found to give the highest inhibition efficiency of 94% among

the studied compounds. The excellent performance of CPA as

corrosion inhibitor is attributed to the presence of an

additional (-[]-) bond between carbon atoms -C=C-, through which

its molecules are adsorbed strongly on the metal surface

leading to its high efficiency.

The results of potentiodynamic polarization studies on

mild steel in absence and presence of optimum concentration of

schiff bases, in both the acids clearly bring out the fact

that all compounds under study bring down corrosion current

without causing appreciable change in values of corrosion

potentials, suggesting that all schiff bases are mixed

inhibitors. -^H^^'' '••'••,; X

,f*/ - " '"•- ~-*~t\

" ^ - . w . . . . . . ^ ^ • • - ' . ,

78

All the compounds are found to follow Temkin's absorption

isotherm, which suggest that the inhibition of mild steel

corrosion in both the acids occurs through adsorption of

schiff bases as mild steel surface.

An interesting features of the investigation is that all

the schiff bases give improved performance as corrosion

inhibitors in hydrochloric acid.

CONCLUSIONS

* All the three schiff bases namely Cinnamylidene-p-

anisidine, Vanillidene-p-anisidine(VPA), Salicylidene-p-

anisidine (SPA) are found to perform well as corrosion

inhibitors in sulphuric acid and hydrochloric acid

solutions but better performance is noticed in case of

hydrochloric acid.

* It is observed that CPA gives the highest inhibition

efficiency of nearly 94% even at a low concentration of

300ppm, VPA gives in inhibition efficiency of 85.3% at

400 ppm and SPA gives 67.3% at concentration of 500 ppm.

The inhibition efficiency values of examined aromatic

schiff bases follows the order :

CPA > VPA > SPA

* The schiff bases are found to act as mixed inhibitors.

* They are found to inhibit the corrosion of mild steel by

getting absorbed on the steel surface by pi (-[]-) electrons

of the aromatic rings and azomethine group.

* All inhibitors follow the Temkin's adsorption isotherm.