STUDIES ON INHIBITION OF METALLIC CORROSION IN ORGANIC ACIDS DISSERTATION SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE AWARD OF THE DEGREE OF JWaster of ^l)ilo«Dpt)p IN j APPLIED CHEMISTRY BY FARHAT AISHA ANSARI DEPARTMENT OF APPLIED CHEMISTRY FACULTY OF ENGINEERING & TECHNOLOGY ALIGARH MUSLIM UNIVERSITY ' - ALIGARH (INDIA) 2002
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STUDIES ON INHIBITION OF METALLIC CORROSION IN ORGANIC ACIDS
DISSERTATION SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS
FOR THE AWARD OF THE DEGREE OF
JWaster of ^l)ilo«Dpt)p IN j
APPLIED CHEMISTRY
BY
FARHAT AISHA ANSARI
DEPARTMENT OF APPLIED CHEMISTRY FACULTY OF ENGINEERING & TECHNOLOGY
ALIGARH MUSLIM UNIVERSITY ' - ALIGARH (INDIA)
2002
( » •
DS3400
Department of Applied Chemistry FACULTY OF ENGINEERING & TECHNOLOGY
Aligarh Muslim University, Aligarh - 202 002 (India)
M.A.Quraishi Reader
Date: ^ V S os
Certificate
Tfiis is to certify tHat tfie wor^em6odie(f in this dissertation entitled "Studies on
Inhibition of MetaCCic Corrosion in Organic ^cids" wfiicfi has Been suSmitted
6y 9/is. TaiHat ^isfia Jinsari contains an originaCpiece of research, carried out under
my supervision and it has not Been suBmitted elsewhere for the award of a degree ordipCgma.
furaishi (Supervisor)
Acknowledgement
Ad^praises Be to^Udfi, the cherisher and sustainer of the ivorCcC, who Bestowed
upon me enough guidance and benevolence to carry out this wor^
It is good fortune and a matter ofpriviCege forme to have the esteemed supervision
of (Dr. M. A- Quraishi, ^ader, (Department of Applied Chemistry, Z. K College of
(Engineering and Technology, JL M.!), JLl^igarh, for his never fading inspiration, schoCarCy
guidance, constant support atufaBove a[[ Benevolent attitude. A large deBt of gratitude is
owed to him.
I would R^ to thankt (Dr. S. V. Kfian, Chairman, (Department of Applied
Chemistry, for providing necessary research facilities during the period of study.
I am highly thankful to (Dr. ^ 0^. Singh, (Department of Chemistry, (Banaras
Tlirufu Vniversity, ^araruisi, for providing instrumentalfaciRty arufhelp.
It is my pleasure to Be thankful to my senior (Dr. Danish Jamal, (Research
Associate (CSI^, for his ^en interest and encouragement throughout the tenure of this
wor^ I sincerely express my than^ to my seniors andlaB colleagues (Drjaya (Rawat, Dr.
(Mrs.) Shahana Anwar, Ms. (Rfltia Sardar, Mr. Jfariom Kjimar Sharma and Mr.
'H' fW (Bhardwajfor various help, discussions aruffriendly interactions.
"With than^ let me place on record the faithfid, unwarranted and unqualified
services of my close friends Ms. IN'ahid (Parveen, Ms. Waseefa (patma, Mrs. (Rpohi
iBad, Mrs. 'Kjiinat MujtaSa, Ms. ^azma andMs. Sarah, who followed me through
thic^andthin aruf leruC their moral aruf intellectual help andassistatwe in the darkest hour
of my strain aruf stress.
I am deeply Beholden to my dear (Papa, Mummy, sister LuBna, Brother (Faisal
aruCaU other well wishers whose love, support and encouragement are the Base of my every
success.
Conclusively, let's pay our erwomiums glowing and rich triBute to late Sir Syed
Ahmad'Kfi.an, who dedicated his Gfefor the estaBlishment of this remrumed University.
^ ( V - ^ - ^
(Farhai Aisha Ansari)
PREFACE
Corrosion is a costly and severe material science problem.
Corrosion manifests itself in multifarious forms in our daily lives.
The seriousness of the problem has made the corrosion scientist aware
and conscious. Corrosion prevention technology has many options at
its disposal for the successful corrosion mitigation of materials. One
of the very important methods of minimizing corrosion today is the
use of inhibitors. Corrosion inhibitors are extensively used in various
applications and many plant operations are dependent on their
successful operation. In the present study, the use of some azathiones
and thiourea derivatives as corrosion inhibitors of mild steel in
organic acid media has been investigated.
Several corrosion inhibition studies on mineral acids has been
done, but are scarce in organic acid. This dissertation highlights the
corrosion behaviour of organic acid and its inhibition. It includes an
introduction, reflecting the economic significance of corrosion
problem. The form and theories have been described to explain its
mechanism. Prevention and corrosion-control has also been described,
with emphasis on mode of action of inhibitors. A brief description of
different techniques employed for investigation of corrosion
inhibitors has also been given. The literature on corrosion inhibition
studies has been incorporated.
The synthesis of the inhibitors and the method employed for the
study of the inhibitive action such as weight loss method,
potentiodynamic polarization, AC impedance technique have been
described in the experimental part.
The results, obtained from potentiodynamic and weight loss
measurements have been discussed in sections in terms of various
corrosion parameters such as inhibition efficiency, corrosion rate,
corrosion current and corrosion potential. The values of activation
energy and free energy of adsorption have also been evaluated. To
reveal some more information about the mechanism of inhibition,
selected compounds have been evaluated by AC impedance technique.
The influence of inhibitor concentration, solution temperature,
immersion time, acid concentration and molecular structure of
compounds on corrosion inhibition has also been discussed. The
mechanism of corrosion inhibition has also been discussed.
PAGE NO.
CHAPTER 1
Introduction 1-37
CHAPTER 2
Experimental 38 - 46
CHAPTER 3
Results and Discussion
SECTION -1
Azathiones as Corrosion Inhibitors 47 - 52
SECTION - 2
Thiourea Derivatives as Corrosion Inhibitors 53 - 59
REFERENCES 60 - 67
SUMMARY 68 - 70
Corrosion can be defined in many ways. The one more preferred in
literature is the degradation of useful properties of material as a result
of chemical or electrochemical reaction with its environment [1].
Degradation due to purely mechanical forces is not called corrosion but
is known as wear, fretting etc. In some cases chemical or
electrochemical attack may be accompanied by physical deterioration
and is described by terms corrosion-erosion, corrosion-wear or fretting
corrosion.
The well-recognized example of metallic corrosion is the rusting
of iron and steel. Beside this there are numerous other familiar
examples of corrosion reactions. Silver articles.tarnish and finally go
black in the atmosphere; the transformation of silver to its black
sulphide does not amount to a serious loss of the costly metal, but
steps have to be taken to restore the surface to its former luster. A
similar loss of appearance is involved in the dulling of brass and
fogging of nickel.
Trends in corrosion research had changed rapidly over the years.
In fifties, polarization studies and their applications had been the topic
of interest [2,3]. In the seventies corrosion research was concentrated
on the mechanistic studies on metal dissolution, localized corrosion
and high temperature corrosion [4-7]. In recent years corrosion
research has been diversified into several newer fields. Optical
techniques have revolutionized the field. Surface analytical techniques
play a major role since; they give more insight into the understanding
of the nature and the influence of surface of surface oxides on the
corrosion of metals and alloy. These techniques are helpful to
characterize the thickness, the structure and the composition of films.
Computers [8] and microprocessors [9] find application in analyzing
the corrosion data.
Corrosion engineering is the science and art to prevent or control
corrosion economically and safely. The ultimate objective of all
electrochemists, metallurgists and chemical engineers is to understand
the mechanism of corrosion and minimize corrosion failures.
1.1 ECONOMIC IMPORTANCE OF CORROSION
Corrosion poses a very serious problem to industries affecting
both to the cost and the productivity. Loses due to corrosion are so
high that it has assumed great economic importance throughout the
world. It is expected that 25% of the total product of the metal and
alloys go waste due to corrosion. The losses due to corrosion, which
were modest when process and material were simple, grew
exponentially, to the astronomic figures of over Rs. 6,40,000 million
per year by 1977, amounting for losses equal to about one percent of
the gross world product [10].
Even in the industrially developed countries like USA and UK,
corrosion is posing very serious problems, which can be appreciated by
the fact that Canada is spending $1 billion annually to control
corrosion, while in UK the total loss due to corrosion is of the order of
£600 million [11]. According to NACE (International) bulletin [12] the
annual losses due to corrosion in USA were estimated to be more than
$300 billion. In India the annual losses due to corrosion has increased
to more than Rs.25,000 crores per year.
Thus from economic point of view, it is necessary for corrosion
specialists to study corrosion mechanism and various ways and means
to minimize corrosion damage.
1.2 CLASSIFICATION OF CORROSION
Corrosion has been classified in many different ways as low
temperature and high temperature corrosion, direct oxidation and
electrochemical corrosion, etc. The preferred classification is (i) Wet
or electrochemical corrosion (ii) Dry or chemical corrosion.
i. Wet or Electrochemical Corrosion which involves an interface.
It can be further separated into:
(a) Separable anode / cathode type: In these cases certain areas of
the metal can be experimentally identified as predominantly anodic or
cathodic. The distances of separation of these areas may be very small,
of the order of fractions of millimeter. There is a macroscopic flow of
charge through the n\etal.
(b) Interfacial anode / cathode type: In this type one entire
interface is cathode and the other is the anode and the charge is
transported through a file of reaction product on the metal surface.
(c) Inseparable anode / cathode type: Here the anodes and the
cathodes cannot be distinguished by experimental methods, though
there presence is postulated by theory, e.g. the uniform dissolution of
the metal in fused salt non-aqueous solutions, acid, alkaline or neutral
solutions.
ii Dry or Chemical Corrosion which involves direct chemical
reaction of a metal with its environment. There is no transport of
electric charge and the metal remain film free. This would include
corrosion in gaseous environments when the reaction product is
volatile, corrosion in liquid metals, fused halides and organic liquids.
A general scheme for the classification of corrosion processes is
presented separately in the form of a Table 1.1. Various important form
of corrosion with definitions and examples are summarized in Table 1.2
1.3 ELECTROCHEMICAL THEORY OF CORROSION
Most of the corrosion reactions, especially those occurring in
aqueous media are electrochemical processes. The overall corrosion
process is the contribution of two reactions, the oxidation of metal
(anodic process) and an equivalent reduction reaction (cathodic
process). An oxidation reaction is indicated by production of electrons
as given below :
Tabic 1.2 Various important forms of corrosion with examples
S. No. Corrosion t\'p€ Dcfinitioa Examples
1.
3.
4.
5.
7.
10.
I I .
Dr}' corrosion
Wet corrosion
Uniform corrosion
Involving chemical reaction with non-electrolytic gas or liquid.
Corrosion in contacts with electrolyte such as aqueous solution of salt, alkali and acid.
Uniform attack of
Pitting corrosion
Crevice corrosion
Galvanic corrosion
Intcrgraaular corrosion
Stress corrosion cracking
High temperature oxidation
Erosion corrosion
Corrosion fatjouc
electrochemical or chemical reaction over the entire surface.
Localized attack in the form of pit.
Corrosion of steel with SO2, CO2, Oj, etc.
Corrosion of steel in sea water, acids and alkalis.
Steel immersed in dilute sulphuric acid.
Stainless steel, aluminium alloys, copper alloys, and nickel alloys immersed in chloride solution.
Intense localized corrosion in The crevices under bold and rivet shallow holes. heads.
Dissimilar metals immersed in a corrosive media and connected electrically
corrosion occurring in the vicinity of grain boundaries.
Cracking caused by simultaneous presence of tension stress and particular corrosion medium.
Oxidation reaction with the products of fijel combustion.
Acceleration of corrosion because of relative movement between corrosive fluid and the metal. Combined action of corrosive medium and variable stresses.
Zinc and iron in salt solution.
Weldments of stainless steel
Season cracking of brass and caustic embrittlement of steel.
Corrosion of steel with combustion products such as C02, SO2, O2, etc.
Corrosion in pumping equipment, corrosion in the area between bearings and shafts.
Heal exchanger tubes of chemical equipments.
M - ^ M^+ e" (1)
This reaction constitutes the basis of corrosion of metals. In a
similar fashion, a reduction reaction is indicated by the consumptipn of
electrons. For 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
2.5 SYNTHESIS OF p-SUBSTITUTED A R Y L T H I O U R E A
(Scheme-2) [86,87]
An appropriate p-substituted aniline (0.1 mol) was dissolved in a
mixture of concentrated HCl (9 ml) and water (25 ml) by warming on
water bath. The solution of amine hydrochloride thus obtained was
cooled and the solid ammonium thiocyanate (0.1 mol) added. After the
addition, the reaction mixture was heated on a water bath for 5 hours.
40
NHjNH-C-NHNHj
S = C = S + NH2-NH2
2.R = C2H5,R1=CH3 3.n=l
RCOR
(CH2)n
R R,
H N ^ ^ N H 1 I
HN. NH
S (1-2)
HN I
HN
O
Y S
(3)
(CH2)„
NH I
NH
SCHEME -1
41
NH,
R
4.R = H 5 R = CH3
NH4SCN/HCI
SCHEME - 2
NHCSNH,
R (4-5)
\ \ /
Ab. alcohol
^ ^ ^ " ^ ^ reflux ' \ ^ N H - C - N H - ^ ^
R R R^
6. R=H 7. R= CH3
(6-7)
SCHEME -3
42
Thereafter, the reaction mixture was cooled and the precipitated
crude product was filtered, washed with water, dried and crystallized
from aqueous ethanol. p-substituted arylthiourea , thus prepared are
phenylthiourea (PTU) and p-tolylthiourea.
2.6 SYNTHESIS OF N, N'-DISUBSTITUTED THIOUREA
(Scheme-3)
An appropriate amount of aniline / toulidine (0.43 mol)
was added to carbon disulphide (0.39 mol) dissolved in 64 ml of
absolute alcohol. The mixture was heated under reflux for 8 hours on a
water bath gently until the content solidifies. After the reaction was
over, carbon-disulphide and alcohol was distilled off and the semi solid
product, thus obtained, was treated with dil. Hydrochloric acid. The
solution was then filtered, washed with water, dried and crystallized
from ethanol. N, N'- diphenyl thiourea and N, N'- ditolyl thiourea.
Compounds No.
4. 5. 6. 7.
Name of the compounds
(Abbreviated) PTU TTU
DPTU DTTU
Yield (%)
71 68 75 64
m.p. ("O
148 180 153 150
2.7 DETERMINATION
PARAMETERS
OF THERMODYNAMIC
2.7.1 DETERMINATION OF ACTIVATION ENERGY
The values of activation energy (Ea) were calculated using the
Arrhenius equation:
In (r2/ri) = (-E. x AT) / (R x Tj x T.) —(32)
Where ri and r2 are corrosion rate at temperature Ti and T2
respectively, AT is the difference in temperature (T2-T1).
43
2.7.2 DETERMINATION OF FREE ENERGY OF ADSORPTION
The free energy of adsorption at different temperature was
calculated using the equation given below:
AGads = -RTIn(55.5K) —(33)
and K is given by:
K = e /C( l -e ) —(34)
where 9 is degree of coverage on the metal surface, C is concentration
of inhibitor in mole/lit, T is temperature, R is a constant and K is
equilibrium constant.
2.8 TECHNIQUES EMPLOYED
The experimental work was carried out with the help of the
following techniques:
1. Weight Loss Method
2. Potentiodynamic Polarization Technique
3. AC Impedance Technique
2.8.1 WEIGHT LOSS METHOD
Specimens of size2.0 cm x 2.0 cm x 2.5 cm were cut from
the mild steel and mechanically polished with 1/0 to 4/0 grades of
emery papers. After polishing, the specimen with acetone. The weight
of the specimen was measured before exposing it to corrodent on an
electrical balance. During weight loss experiments, the specimen were
fully immersed in 200 ml test solution using beaker of 250 ml capacity.
After a definite exposure time, the specimen was taken out and washed
with distilled water. Specimens were then dried and loss in weight was
recorded. The thermostatic chamber was used for carrying out the
weight loss experiments at higher temperatures. Thermostat was within
an accuracy of ± 2°C. The percentage inhibition efficiency and surface
coverage (9 ) were calculated using the following equation:
44
Wo - W IE(%) = X 100 —(35)
Wo
W o - W —(36)
Wf
where IE (%) = Percentage Inhibitive Efficiency
0 = Surface Coverage
Wo = Wt. Loss or Corrosion Rate in Uninhibited System
W = Wt. Loss or Corrosion Rate in Inhibited System
2.8.2 POTENTIODYNAMIC POLARIZATION TECHNIQUE
The following instruments were used for carrying out the
polarization studies:
(I) POTENTIOSTAT (EG & G PARC MODEL: 173)
(i) Log current converter (model: 376)
(ii) Universal programmer (model: 175)
(iii) X-Y Recorder (model RE 0089)
For potentiostatic polarization studies, working electrodes 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
then thoroughly washed with distill water and finally with acetone,
unwanted area of the electrode was coated with lacquer to get a well
defined area. The polarization studies were carried out using
Potentiostat (EG 7 G PARC model: 173), Universal programmer
(model: 175), X-Y Recorder (model RE 0089).
All the experiments were carried out at (26 ± 2°C). A platinum
foil of 3 cm X 3 cm was used as the auxiliary electrode and a saturated
calomel electrode was used as reference electrode.
45
All the potentials were measured against a saturated calomel
electrode. The inhibition efficiency were calculated using the
following equation:
1° corr - I corr IE (%) = X 100
1° corr
1° corr = Corrosion Current Density Without Inhibitor
I corr = Corrosion Current Density With Inhibitor
2.8.3 AC IMPEDANCE TECHNIQUE
The compounds CPTAT and DTTU were studied by AC
impedance technique. In this technique a conventional three electrode
single compartment Pyrex glass cell using an electrochemical
impedance system, which is comprised of a lock-in-amplifier (model
5210, PARC, USA), a potentio-galvanostat(model 273A, PARC, USA)
and a PS/2 (model 35 SX) IBM computer. A bright platinum foil of
relatively large surface area (8 cm^) was used as counter electrode and ^
saturated calomel electrode was used as a reference electrode.
Impedance measurements were performed at Ecorr (corrosion
potential) with the a.c. voltage amplitude ± 5 mV in the frequency
range of 5 Hz - 100 Hz. A time interval of a few minutes was given
the open circuit potential (O. C. P) to read steady value. The
potentiostat was set at O. C. P using electrochemical interface.
Plots for real part (Z') and the imaginary part (Z") were made
from the impedance diagram (Nyquist plot). The charge transfer
resistance (Rt) and double layer capacitance (Cdi) were obtained using
the Nyquist and Bode plots respectively. The percent inhibition
efficiency was calculated using equation:
(1/Rto)- ( l /R. ) IE (%) =
(1/ R.o)
46
Rto = Charge Transfer Resistance Without Inhibitor
Rt = Charge Transfer Resistance With Inhibitor
The double layer capacitance can be-determined from the frequency at
which Z" is maximum from the relation:
1 FZ",
2% Cdi X Rt
RESUUSAKDDmjSSKM
SECTION-1
AZATHIONES AS CORROSION INHIBOTORS
47
In this section, influence of azathiones on the corrosion of mild
steel in 20% formic acid and 20% acetic acid has been investigated.
The molecular structure and other details of azathiones used as
corrosion inhibitors are given in table 3.1.1.
3.1.1 WEIGHT LOSS STUDIES
The various corrosion parameters such as percentage weight
inhibition efficiency and corrosion rate of mild steel in 20% formic
acid and 20% acetic acid in the absence and presence of various
azathiones at different concentrations at 30°C are summarized in Table-
3.1.2. It has been observed from the results that inhibition efficiency
for all the compound increase with increase in concentration. The
maximum inhibition efficiency of each compound was achieved at 500
ppm. A further increase in the concentration of the compound does not
causes any change in their performance.
The effect of inhibitor concentration, solution temperature,
immersion time and acid concentration on inhibition efficiency of
azathiones has been shown in Figure 3.1.1. and 3.1.2. The following
observations have been noted.
i. the inhibition efficiency of all the tested azathiones increases
with increase in concentration of inhibitors in both the acid
maximum inhibition efficiency was found at SOOppm.
ii. the inhibition efficiency of all the azathiones, does not show any
significant change with increase in solution temperature from 30°
to 50 C in 20% formic acid but increases with increase in
solution temperature from 30° to 50°C in 20% acetic acid,
iii. the inhibition efficiency of all the tested azathiones decreases
with increase in immersion time from 24 hours to 96 hours
iv. the influence of acid concentration on the inhibition efficiency
of azathiones at 24 hour exposure time at 500 ppm shows that
with increase in acid concentration of formic acid the inhibition
efficiency initially increases and then decreases on further.
Table 3.1.1 Name and structure of the azathiones used as inhibitors.
S. No.
1.
2.
3.
Structure
CHsv^CHa
HN NH
T s
CHsv^CaHs
HN NH 1 1
HN,,^^NH T S
Q H N ' T ^ H
HN^JJH
S
Name and Abbreviation
Dimethyl-tetra-hydro-azathione (DMTAT)
Ethyl-methyl-tetra-hydro-azathione (EMTAT)
Cyclopentyl-tetra-hydro-azathione (CPTAT)
48
Table 3.1.2 Corrosion parameter for mild steel in 20% formic and 20% acetic acid in absence and presence of different concentrations of various inhibitors from weight loss measurements at room temperature.
Cone, (ppm)
Blank
DMTAT
100 200 300 400 500
EMTAT
100 200 300 400 500
CPTAT
100 200 300 400 500
20% Formic acid Weight
loss (gm)
308.1
120.35 111.98 99.32 75.25 68.41
95.26 79.55 64.18 52.88 48.12
62.36 59.37 51.43 47.37 42.29
IE (%)
60.93 63.65 67.79 75.57 77.79
69.08 74.18 79.16 82.83 84.38
79.75 80.73 83.30 84.62 86.27
CR (mmpy)
14.31
5.58 5.19 4.61 3.49 3.17
4.42 3.69 2.97 2.45 2.23
2.89 2.75 2.38 2.19 1.96
20% Acetic acid Weight
loss (gm)
150.28
59.01 51.21 42.92 33.65 29.75
42.43 37.55 33.65 29.75 26.82
41.45 36.09 29.96 24.38 21.94
IE (%)
60.73 65.92 71.43 77.60 80.20
71.76 75.00 77.60 80.20 82.15
72.41 75.98 80.06 83.77 85.40
CR (mmpy)
6.97
2.73 2.37 1.99 1.56 1.38
1.97 1.74 1.56 1.38 1.24
1.92 1.67 1.39 1.13 1.01
100
90
£ 80 UJ
70
A
\
k
1
L
(b ) 1
100 200 300 AOO 500
Inhib i torConcentrat ion (ppm)
30 40 50
Temp. ( °C)
100
24 48 72 96 Time (hours)
10 20 30 AcidConcC/o)
Figure 3.1.1. Variations of inhibition efficiency with (a) inhibitor concentration (b) solution temperature (c) immersion time (d) acid concentration in 20% formic acid. (1. DMTAT; 2. EMTAT; 3. CPTAT).
100 300 500 InhlbUor Concentration (ppm)
30 AO 50
Temp.(°C)
2A i!;8 72 96
Time (hours) 10 20 30 Acid Conc.C/o)
Figure 3.1.2. Variations of inhibition efficiency with (a) inhibitor concentration (b) solution temperature (c) immersion time (d) acid concentration in 20% acetic acid. (1. DMT AT; 2. EMTAT; 3. CPTAT)
49
increase in the acid concentration. In acetic acid the IE
decreases with increase in acid concentration.
In the present study, azathiones have been found as good
corrosion inhibitors. The inhibition efficiency value for azathiones
follows the order:
CPTAT > EMTAT > DMTAT
The difference in the order of inhibition efficiency may be
attributed on the basis of their molecular area. Trabanelli [88] reported
that inhibition efficiency can be increased by increasing the molecular
area of the compound. Since CPTAT has larger molecular area than
other azathiones, it gives maximum inhibition efficiency among the
studied azathiones. Increased area causes large coverage of metal
surface, which leads to higher inhibition efficiency [89].
The values of activation energy [Eg) and free energy of
adsorption (AGads) at different temperature are given in Table 3.1.3. It
is found that Eg values for inhibited system are lower than that of
uninhibited system, indicating that these typesaz of inhibitors are
effective at higher temperature [90]. The low and negative values of
A Gads indicate the spontaneous adsorption of inhibitors on the surface
of mild steel. It is found that the A Gads value in less than -9.56 Kcal /
mol indicating that inhibitors are physically adsorbed on the surface of
mild steel [91].
3.1.1.1 ADSORPTION ISOTHERMS STUDIES
The values of surface coverage (0) obtained from weight loss
data were plotted against log C for different concentration of all the
compounds. The straight lines thus obtain indicates that all the
adsorption of these compounds, from both 20% formic acid and 20%
acetic acid on the mild steel surface follows Temkin's adsorption
isotherms (Figure 3.1.3).
50
Table 3.1.3 Activation energy (Eg) and free energy of adsorption (A Gads) for mild steel in 20% formic acid and 20% acetic acid in the absence and presence of various inhibitors.
Concentration (ppm)
20% Formic acid
EMTAT DMTAT CPTAT
20% Acetic acid
DMTAT EMTAT CPTAT
Ea (K. Cal/mol)
12.27
11.82 12.17 11.01
6.41
5.72 5.80 5.60
A Gads (K. Cal/mol)
30°C
—
3.59 3.85 3.93
—
3.67 3.88 4.15
40''C
--
3.77 3.56 3.63
—
3.68 3.89 4.17
50^C
--
3.87 4.11 4.29
--
3.72 3.93 4.21
0 en o t_
> O
u 0; u O i_ 3
If)
1.00 -
2.3 2.6 log C ( p P m )
2.2 2.A 2.6 2.8
Figure 3,1.3. Temkin's adsorption isotherm plots for adsorption of various inhibitors in (a) 20% formic acid and (b) 20% acetic acid on the surface of mild steel. (1. DMTAT; 2. EMTAT; 3. CPTAT).
51
3.1.2 POTENTIODYNAMIC POLARIZATION STUDIES
The cathodic and anodic polarization curves of mild steel in 20%
formic acid and 20% acetic acid in the presence and absence of
different azathiones at 500 ppm concentrations at 26±2°C are shown in
Figure 3.1.4 and various corrosion parameters such as Ecorr, ba, be and
%IE obtained from these curves are given in Table 3.1.4. It is observed
that Icorr values decreases significantly in the presence of inhibitors.
Maximum decrease in the Icorr was observed in case of CPTAT among
azathiones.
Ecorr values do not show any significant change in presence of
these inhibitors, suggesting that these compounds are mixed type
inhibitors.
Table 3.1.4 Electrochemical polarization parameters for the corrosion of mild steel in 20% formic acid and 20% acetic acid containing optimum concentration of various inhibitors at room temperature
Concentration (ppm)
20% Formic acid
DMTAT EMTAT CPTAT
1
20% Acetic acid
DMTAT EMTAT CPTAT
t'corr
(mV vs SCE)
-416
-404 -426 -402
-404
-400 -417 -428
*corr
(mA. cm'^)
0.350
0.120 0.100 0.060
0.24
0.080 0.068 0.045
ba (mV dec"')
68
64 66 64
60
62 56 1 50
be (mVdec")
104
100 110 106
100
104 100 94
IE (%)
66 71 83
__
66.66 i 71.66 81 25
3.1.3 IMPEDANCE STUDIES
Nyquist plots obtained for the frequency range of 5Hz-100kHz at
the open circuit potential (OCP) for mild steel in 20% formic acid in
UJ (J in
> >
-300 -
-«00 -
- 5 0 0 -
-600 -
0.0001 0.001 0.01
c a> o CL
3 00 -
AOO -
5 0 0 -
600 -
.01 .1 1
C u r r e n t d e n s i t y ( m A c m - 2 )
Figure 3.1.4. Potentiodynamic polarization curves for mild steel in (a) 20% formic acid (b) 20% acetic acid in the absence and presence of various inhibitors at optimum temperature. (1. Blank; 2. DMTAT; 3. EMTAT; 4. CPTAT).
52
the presence and absence of 100 and 500 ppm of CPTAT, understudy,
are shown in Figure 3.1.5. Impedance parameters such as Rt and %IE
derived from Nyquist plots and Cdi derived from Bode plots are given
in Table 3.1.5. It is observed from the results that Rt values increases
and Cdi values decreases with increase in concentration of the
inhibitors. These observations indicate that the corrosion of mild steel
in 20% formic acid is mainly controlled by charge transfer process and
the inhibition of corrosion occurs by adsorption mechanism [92].
Table 3.1.5 Electrochemical impedance parameters for mild steel in 20% formic acid containing different concentration of CPTAT at room temperature.
Concentration (ppm)
20% Formic Acid
CPTAT 100 500
E, (ncm^)
75.00
545.19 120.19
Cdi (Hfcm )
1862.09
33.88 30.19
IE (%)
86.24 89.38
3.1.4. CONCLUSION
The main conclusions obtained from this study are as follows,
i) The azathiones showed good performance as corrosion inhibitors
in both formic as well as acetic acid media,
ii) They inhibited the corrosion of mild steel in acid solution by
adsorption mechanism and the adsorption of these compounds on
the metal surface followed Temkin's adsorption isotherm,
iii) All these azathiones acted as mixed inhibitors in both the acids,
iv) Addition of increasing concentration of CPTAT decreases Cdi
thiourea (DPTU) and Ditolyl thiourea (DDTU) have been synthesized.
The influence of these thiourea derivatives on the corrosion of mild
steel in 20% formic acid and 20% acetic acid has been investigated by
weight loss determination, electrochemical methods. The molecular
structure and other details of the inhibitors studied for the present
investigations are given in Table 3.2.1
3.2.1 WEIGHT LOSS STUDIES
The values of percentage inhibition efficiency and corrosion rate
obtained by weight loss method at various concentrations are given in
Table 3.2.2. It has been found that inhibition efficiency of all these
compounds increases with increase in concentration.
The results of the effect of inhibitor concentration solution
temperatures, immersion times and acid concentration on the inhibition
efficiency of the thioureas have shown in Figure 3.2.1. and 3.2.2. It is
observed from these that:
(i) the inhibition of all the thioureas increases with the increase in
concentration of inhibitors in both 20% formic acid and 20%
acetic acid. Maximum inhibition efficiency was achieved at a
concentration of 300 ppm.
(ii) the inhibition efficiency of all the investigated thioureas except
for DTTU in 20% acetic acid does not show any significant
change with increase in solution temperature from 30°C to 50°C.
The inhibition efficiency of DTTU in 20% acetic acid increases
with rise in solution temperature,
(iii) the inhibition efficiency of all the thioureas decrease with
increase in immersion time from 24 hours - 96 hours.
Table 3.2.1 Names and structures of the thiourea derivatives used as inhibitors.
S. No.
1.
2.
3.
4.
Structure
L ^ ^ N H - C - N H 2
CH3
L ^ ^ N H - C - N H 2
^CH3 HjC^
^ ~ ^ N H - C - N H - ^
Name and Abbreviation
Phenyl thiourea (PTU]
Tolyl thiourea (TTU)
Diphenyl thiourea (DPTU)
Ditolyl thiourea (DTTU)
54
Table 3.2.2 Corrosion parameter for mild steel in 20% formic and 20% acetic acid in absence and presence of different concentrations of various inhibitors from weight loss measurements at room temperature.
Cone, (ppm)
Blank
PTU 50 100 200 300
TTU
50 100 200 300
DPTU
50 100 200 300
DTTU 50 100 200 300
20% Formic acid Weight
loss (gm) 308.1
31.2 30.7 23.6 21.3
28.7 17.9 12.2 9.6
9.4 6.0 5.6 3.8
4.9 4.5 3.4 2.4
IE (%)
--
49.86 90.07 92.38 93.05
90.68 92.66 94.33 96.85
97.76 98.05 98.18 98.74
98.46 98.60 98.81 99.02
CR (mmpy)
14.31
1.45 1.42 1.09 0.99
1.33 1.05 0.81 0.45
0.32 0.28 0.26 0.18
0.22 0.20 0.16 0.14
20% Acetic acid Weight
loss (gm)
150.28
27.31 26.33 25.36 19.06
26.08 23.99 20.28 16.58
24.89 22.89 16.08 13.65
84.09 87.17 92.97 93.56
IE (%)
—
81.82 82.47 83.12 87.34
82.64 84.03 86.50 88.96
83.43 84.76 89.29 90.91
84.09 87.17 92.97 93.56
CR (mmpy)
6.97
1.26 1.22 1.17 0.88
1.35 1.31 1.08 0.76
1.24 1.15 0.90 0.63
1.10 1.08 0.88 0.44
200 300 InhibitorConcentralion (ppm)
30 40 50 Temp. ( °C)
0 2k 46 72 96 Time (hours)
100 r
96
_ 96 «
^ 94
92
90 -1 \^
10
( d )
20 30
AcidConc. (•/.)
Figure 3.2.1. Variations of inhibition efficiency witli (a) inhibitor concentration (b) solution temperature (c) immersion time (d) acid concentration in 20% formic acid.(l. PTU; 2.TTU; 3. DPTU; 4. DTTU).
ot//—I 100 200 300
Inhibitor ConcentrationCppm
97
95
93
£ 91
89
67
Ox-
r A-•-
dvA-i- ±
k t,
-A 2
- • 1
( b )
30 ^0 50 Temp. (°C)
OJ
2A A8 72 96
Time( hours) 10 20 30
Acid ConcC/. )
Figure 3.2.2. Variations of inhiibition efficiency with (a) inhibitor concentration (b) solution temperature (c) immersion time (d) acid concentration in 20% acetic acid. (1. PTU; 2.TTU; 3. DPTU; 4. DTTU).
> "\*i>* ^''"'^ ^'•'v:'^ ^
AS-3-ft70\^.
(iv) the influence of acid conceirtraticms on the inhibition efficiency
of thiourea derivatives shows that with increase in acid
concentration of formic acid the inhibition efficiency initially in
creases and then decreases on further increase in the acid
concentration upto 30%.
The values of Ea and A Gads are given in Table 3.4.3. It is found
that, Ea values for inhibited systems are lower than or equal than that
of uninhibited system. The negative values of A Gads also suggest the
strong interaction of the inhibitor molecules on to the mild steel [95].
3.2.1. ADSORPTION ISOTHERMS STUDIES
The Surface coverage (9) values obtained from weight loss data
were plotted against logC (ppm) thereby straight lines were obtained
(Figure 3.2.3.) suggesting that the adsorption of thioureas on to the
metal surface from both the acid follows Temkin's adsorption isotherm.
The inhibition efficiency values of examined thioureas at a common
concentration of 300 ppm follows the order:
DTTU> DDTU> TTU> PTU
The difference in inhibition efficiency of the studied a inhibitors
can be explained on the basis of their molecular structure [96].
Among the compound investigated in the present study. TTU are
better performance as an inhibitor than PTU. This difference in
inhibition efficiency can be explained on the basis of hyperconjugation
effect of -CH3 electron releasing group. DPTU gave better inhibition
efficiency than TTU due to the presence of an additional benzen ring
which possess highly delocalized n -electron which favours greater
adsorption on the mild steel surfaces. DTTU shows the best
performance in both the acids, because of both, the presence of an
extra benzene ring and an electron releasing -CH3 group on benzene
ring in the same molecule.
56
Table 3.2.3 Activation energy (Ea) and free energy of adsorption (A Gads) for mild steel in 20% formic acid and 20% acetic acid in the absence and presence of various inhibitors.
Concentration (ppm)
20% Formic Acid
PTU TTU DDTU DTTU
20% Acetic Acid
PTU TTU DDTU DTTU
Ea (K.Cal/mol)
12.27
12.23 12.30 11.80 11.80
6.41
6.30 6.47 6.75 6.47
30°C
-
4.39 4.92 5.49 5.61
-
4.34 4.40 4.53 4.79
-A Gads (K. Cal/mol)
40°C 50°C
-
4.66 4.69 5.09 4.80 5.75 5.86 5.92 6.02
-
4.46 4.58 4.53 4.67 4.67 4.79 4.89 5.16
0/ > o u
u a \-tn
2 2.3 l o g C ( p p m )
Figure 3.2.3. Temkin's adsorption isotlierm plots for adsorption of various inliibitors in (a) 20% formic acid and (b) 20% acetic acid on tlie surface of mild steel. (1. PTU; 2.TTU; 3. DPTU; 4. DTTU).
57
3.2.2 POTENTIODYNAMIC POLARIZATION STUDIES.
The potentiodynamic polarization curves of mild steel in the
absence and presence of 300 ppm concentration of thioureas in 20%
formic acid and 20% acetic acid are shown in Figure 3.2.4. various
electrochemical parameters such as Ecovr, Icon, ba, be and %IE obtained
from cathodic and anodic curves are given in Table 3.4.4. The results
clearly brings out the fact that compounds under study bring down the
corrosion current without causing any appreciable change in values of
corrosion potentials suggesting that all the thioureas are mixed
inhibitors. Maximum decrease in Icorr values was observed in case of
DTTU indicating that DTTU is the most effective corrosion inhibitor
among the studied organic inhibitors.
Table 3.2.4 Electrochemical polarization parameters for the corrosion of mild steel in 20% formic acid and 20% acetic acid containing optimum concentration of various inhibitors at room temperature
Concentration (ppm)
20% Formic Acid
PTU TTU
DPTU DTTU
20% Acetic acid
PTU TTU
DPTU DTTU
t'corr
(mV vs SCE)
-416
-386 -402 -404 -412
-404
-395 -373 -424 -437
^corr
(mA. cm'^)
0.35
0.035 0.034 0.022 0.018
0.24
0.059 0.035 0.033 0.026
b. (mV dec ' )
68
66 64 68 64
60
50 56 54 52
be (mV dec ' )
104
112 108 100 106
100
102 100 90 94
IE (%)
-
90.00 90.23 93.71 94.86
—
75.42 85.42 86.25 89.16
-300 -
- 400 =»
UJ a in
> > E
-500
-600 -
0.001 0.01
c
O
a.
- 3 00 -
-AOO -
- 5 0 0 _
- 6 0 0 -
.001 .01 .1 1 C u r r e n t - d e n s i t y ( m A c m"^ )
Figure 3.2.4. Potentiodynamic polarization curves for mild steel in (a) 20% formic acid (b) 20% acetic acid in the absence and presence of various inhibitors at optimum temperature. (1. Blank; 2. PTU; 3.TTU; 4. DPTU; 5. DTTU).
58
3.2.3 IMPEDANCE STUDIES
Nyquist plots obtained for the frequency of 5 Hz-100 KHz at the
open circuit potential (OCP) for mild steel in 20% formic acid in the
presence and absence of DTTU (ditolyl thiourea) at 28 ± l" C are
shown in Figure 3.2.5.
It is seen that Nyquist plots obtained are not perfect semicircles
and this difference may attributed to frequency dispersion [97]. The
change transfer resistance (Rt) has been calculated using Nyquist plots
form the difference in the impedance at low and high frequencies as
suggested by Haruyama and Tsuru [98]. Double layer capacitance (Cai)
values were calculated using Bode plots [99]. It is seen that the
addition of increasing concentrations of DTTU increases Ri values and
decreases Cdi values in 20% formic acid. These observations indicates
that the corrosion of mild steel in 20% formic acid is mainly controlled
by charge transfer process and the inhibition of corrosion occurs by
adsorption mechanism.
Table 3.2.5 Electrochemical impedance parameters for mild steel in 20% formic acid containing different concentration of CPTAT at room temperature.