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.