APPLICATION OF THIAZOLE AND TRIAZOLE COMPOUNDS IN CARBON
STEEL CORROSION PROTECTION
ASIAH MOHAMAD
A dissertation submitted in fulfillment of the
requirements for the award of the degree of
Master of Science (Chemistry)
Faculty of Science
Universiti Teknologi Malaysia
OCTOBER 2009
iii
To mak, ayah, family and friends…
iv
ACKNOWLEDGMENT
First and foremost I would like to express my sincere appreciation to Prof.
Dr. Rahmalan Ahamad as my project supervisor for his encouragement, guidance,
critics and friendship. Without his continued support and interest, this dissertation
would not have been the same as presented here. He is always guiding me in doing
my research and writing this dissertation and may Allah bless all his sacrifices and
efforts.
My special appreciation also goes to all friends who have helped and give me
their support whenever I need them. Their help and encouragement is very useful in
finishing my project and report writing. I am also grateful to all my family members
for their morale support and encouragement. Lastly, my thanks also go to everyone
who has supported me all the way.
Thank you.
v
ABSTRACT
Inhibitory effect of 2-mercaptobenzothiazole (MBT) and 1,2,3-benzotriazole
(BTA) on corrosion of carbon steel in 1.0 M HCl and seawater sample respectively
has been studied using weight loss method. All measurements show that inhibition
efficiencies of BTA and MBT increased with increase in inhibitor concentration and
temperature in 1.0 M HCl and seawater sample. The results of the investigation show
that the compound BTA and MBT have fairly good inhibiting properties with
inhibition efficiencies of 98.24% for BTA and 92.98% for MBT in seawater sample
while 87.49% for BTA and 30.15% for MBT in 1.0 M HCl, at 90 °C. Adsorption of
these inhibitors follows the Langmuir adsorption isotherm. Thermodynamic
adsorption parameters (Kads, Gads) of BTA and MBT were calculated using the
Langmuir adsorption isotherm. The adsorptions of BTA and MBT on carbon steel
are spontaneous processes in 1.0 M HCl and seawater sample, indicated by the
negative values of Gads.
vi
ABSTRAK
Kesan hambatan oleh 2-mercaptobenzothiazol (MBT) dan 1,2,3-benzotriazol
(BTA) terhadap pengaratan keluli karbon dalam larutan asid hidroklorik (HCl) 1.0 M
dan sampel air laut telah dikaji menggunakan teknik pengurangan berat. Berdasarkan
analisis yang dilakukan, kecekapan hambatan oleh BTA dan MBT terhadap
pengaratan keluli karbon di dalam HCl 1.0 M dan sampel air laut meningkat dengan
peningkatan kepekatan bahan hambatan dan suhu medium rendaman. Hasil kajian
menunjukkan BTA dan MBT mempunyai ciri-ciri hambatan kakisan yang agak
bagus dengan kecekapan hambatan sebanyak 98.24% untuk BTA dan 92.98% untuk
MBT di dalam sampel air laut manakala 87.49% untuk BTA dan 30.15% untuk MBT
di dalam HCl 1.0 M pada suhu 90 °C. Proses penjerapan kedua-dua bahan hambatan
kakisan tersebut di dalam HCl 1.0 M dan sampel air laut adalah mematuhi isoterma
penjerapan Langmuir. Parameter penjerapan termodinamik iaitu pemalar penjerapan
(Kads) dan tenaga bebas Gibbs (Gads) telah dikira mengikut persamaan isoterma
penjerapan Langmuir. Hasil kajian termodinamik memberikan nilai Gads yang
negatif menunjukkan proses penjerapan bahan hambatan BTA dan MBT ke atas
keluli karbon di dalam HCl 1.0 M dan sampel air laut adalah spontan.
vii
TABLE OF CONTENTS
CHAPTER
TITLE
PAGE
DECLARATION OF POSTGRADUATE PROJECT
PAPER
SUPERVISOR'S DECLARATION
1
2
TITLE PAGE
i
DECLARATION OF ORIGINALITY
ii
DEDICATION
iii
ACKNOWLEDGEMENTS
iv
ABSTRACT
v
ABSTRAK
vi
TABLE OF CONTENTS
vii
LIST OF TABLES
x
LIST OF FIGURES
xii
LIST OF SYMBOLS
xiv
LIST OF ABBREVIATIONS
xv
INTRODUCTION
1.1 Background of Study
1
1.2 Statement of Problem
3
1.3 Research Objectives
3
1.4 Scope of Study
4
LITERATURE REVIEW
2.1 Basic Concept of Corrosion
5
2.2 Types of Corrosion
7
viii
3
2.3 Corrosion Inhibitor
7
2.3.1 Anodic Passivating Inhibitors
9
2.3.2 Cathodic Inhibitors
9
2.3.3 Ohmic Inhibitors
10
2.3.4 Organic Inhibitors
10
2.3.5 Precipitation Inhibitors
11
2.3.6 Vapor Phase Inhibitors
12
2.4 Studies on Carbon Steel Corrosion Control
12
2.5 Commonly Used Corrosion Inhibitor
13
2.6 Thiazole Compounds as Corrosion Inhibitors
18
2.7 Weight Loss Method
19
METHODOLOGY
3.1 Chemicals
20
3.2 Apparatus and Instrumentation
20
3.3 Preparation of Carbon Steel Coupon
21
3.4 Determination of Elemental Composition of Carbon
Steel Coupons
21
3.5 Solutions Preparation
22
3.5.1 1,2,3-Benzotriazole (BTA) 0.5 M
22
3.5.2 2-Mercaptobenzothiazole (MBT) 0.5 M
23
3.5.3 Hydrochloric Acid (HCl) 1.0 M
23
3.5.4 Seawater Sample
23
3.6 Weight Loss Measurements
24
3.6.1 Inhibitor Concentration Effect
25
3.6.2 Immersion Period Effect
26
3.6.3 Temperature Effect
3.7 Microstructure Analysis of Coupons
26
27
ix
4
RESULTS AND DISCUSSION
4.1 Determination of Elemental Composition of Carbon
Steel Coupons
4.2 Weight Loss Measurements
4.2.1 Inhibitor Concentration Effect
4.2.2 Immersion Period Effect
4.2.3 Temperature Effect
4.3 Corrosion Inhibition in Seawater Sample
4.3.1 Inhibitor Concentration Effect
4.3.2 Temperature Effect
4.4 Adsorption Isotherms and Thermodynamics
4.5 Microstructure Analysis of the Carbon Steel Coupons
5
28
29
29
31
33
34
35
36
38
41
CONCLUSION AND FUTURE WORK
5.1 Conclusion
5.2 Future Work
REFERENCES
44
45
46
x
LIST OF TABLES
TABLE NO.
TITLE
PAGE
2.1
Types of Corrosion
8
2.2
Summary on corrosion inhibitor application in various
research areas
15
4.1
Elements composition of carbon steel coupons
28
4.2
Corrosion rate, surface coverage and inhibition efficiency
for various concentration of BTA and MBT for the
corrosion of carbon steel after 24 hours immersion in 1.0 M
HCl obtained from weight loss measurements at 25 °C
30
4.3
Corrosion rate, surface coverage and inhibition efficiency
for carbon steel after 1, 2, 4, 8, and 24 hours immersion in
1.0 M HCl with absence and presence of 10-2 M BTA and
MBT respectively obtained from weight loss measurements
at 25 °C
31
4.4
Corrosion rate, surface coverage and inhibition efficiency
for various immersion temperature of carbon steel after 24
hours immersion in 1.0 M HCl with absence and presence
of 10-2 M BTA and MBT respectively obtained from weight
loss measurements
33
4.5
Corrosion rate, surface coverage and inhibition efficiency
for various concentrations of BTA and MBT for the
corrosion of carbon steel after 24 hours immersion in
seawater sample obtained from weight loss measurements
at 25 °C
35
4.6
Corrosion rate, surface coverage and inhibition efficiency
for various immersion temperature of carbon steel after 24
hours immersion in seawater sample with absence and
presence of 10-2 M BTA and MBT respectively obtained
from weight loss measurements
37
xi
4.7
Thermodynamic parameters obtained from weight loss
measurements for the adsorption of BTA and MBT in 1.0
M HCl on the carbon steel at 25 °C
41
4.8
Thermodynamic parameters obtained from weight loss
measurements for the adsorption of BTA and MBT in
seawater sample on the carbon steel at 25 °C
41
xii
LIST OF FIGURES
FIGURE NO.
TITLE
PAGE
1.1
Piece of gas pipeline with external corrosion
2
1.2
Ruptured gas pipeline due to corrosion
2
1.3
Structure of 1,2,3-benzotriazole (BTA)
3
1.4
Structure of 2-mercaptobenzothiazole (MBT)
4
3.1
Carbon steel coupons with dimensions 20mm x 20mm x
2.5mm used in weight loss measurements
21
3.2
Photograph of FESEM-EDX, model JSM-6701F
22
4.1
Variations of the inhibition efficiency calculated from
weight loss measurements at different concentrations of
BTA and MBT after 24 hours immersion in 1.0 M HCl
solution at 25 °C
30
4.2
Variations of the inhibition efficiency of BTA and MBT
calculated from weight loss measurements at different
immersion period in 1.0 M HCl solution at 25 °C
32
4.3
Inhibition efficiency of BTA and MBT in 1.0 M HCl
calculated from weight loss measurements at different
immersion temperature for 10-2 M inhibitor concentration
respectively
34
4.4
Inhibition efficiency calculated from weight loss
measurements at different concentrations of BTA and MBT
after 24 hours immersion in seawater sample at 25 °C
36
4.5
Inhibition efficiency of BTA and MBT in seawater sample
calculated from weight loss measurements at different
immersion temperature for 10-2 M inhibitor concentration
respectively
37
4.6
Langmuir isotherm for adsorption of MBT and BTA on
carbon steel surface in 1.0 M HCl at 25 °C
39
xiii
4.7
Langmuir isotherm for adsorption of MBT and BTA on
carbon steel surface in seawater sample at 25 °C
40
4.8
Microstructure of carbon steel coupons
43
xiv
LIST OF SYMBOLS
Gads
-
Free energy of adsorption
°C
-
Degree Celsius
-
Degree of surface coverage
%
-
percent
C
-
Inhibitor concentration
f
-
Factor of energetic inhomogeneity
g
-
Gram
h
-
Hour
Kads
-
Equilibrium constant of adsorption process
kg
-
Kilogram
M
-
Molar
mg
-
Milligram
mg cm-2 h-1
-
Milligram per centimeter square per hour
mL
-
Milliliter
mm
-
Millimeter
ppm
-
Part per million
R2
-
Correlation coefficient
xv
LIST OF ABBREVIATIONS
APM
-
Ammonium polymolybdate
ASTM
-
American Standard for Testing Materials Society
BTA
-
1,2,3-benzotriazole
EDX
-
Energy dispersive X-ray spectrometer
FESEM
-
Field emission scanning electron microscopy
GA
-
Gum Arabic
IE
-
Inhibition efficiency
MBT
-
2-mercaptobenzothiazole
MDEA
-
Methyldiethanolamine
PAE
-
P. amarus extract
PEG
-
Polyethylene glycol
PVC
-
Polyvinyl chloride
SAMs
-
Self assembled monolayers
VCI
-
Volatile corrosion inhibitor
VPI
-
Vapor phase inhibitor
ZPC
-
Zero point charge
CHAPTER 1
INTRODUCTION
1.1
Background of Study
Corrosion is a naturally occurring phenomenon commonly defined as the
deterioration of a substance (usually a metal) or its properties because of a reaction
with its environment (Delinder et al., 1984). Like other natural hazards such as
earthquakes or severe weather disturbances, corrosion can cause dangerous and
expensive damage to everything from automobiles, home appliances, drinking water
systems, pipelines, bridges, and public buildings (Treseder, 1991).
Corrosion is one of the major problems affecting the performance, safety and
appearance of materials (Rim-rukeh et al., 2006). In many industries, the need to use
constructional materials safely, but cost effectively, is a primary consideration.
Corrosion affects all areas of the economy and it has been estimated that the cost of
corrosion represent 4% of the gross national product. These numbers include direct
losses for replacement of corroded materials and equipment ruined by corrosion,
indirect losses include cost of repair and loss of production, cost of corrosion
protection and cost of corrosion prevention (Landolt, 2006).
2
Frequently, physical requirements can be satisfied easily, but corrosion
effects seriously complicate the selection of suitable materials. Generally, increase
corrosion resistance can only be obtained at increased cost. Despite continuing
advances in corrosion resistant materials, the use of the chemical inhibitors often
remains the most practical and cost effective means of preventing corrosion (AlSarawy et al., 2008).
Figure 1.1 and Figure 1.2 show the examples of corrosion effect in which gas
pipelines have been damaged by external corrosion.
Figure 1.1: Piece of gas pipeline with external corrosion (Thompson, 2001)
Figure 1.2: Ruptured gas pipeline due to corrosion (Thompson, 2001)
3
1.2
Problem Statement
The inhibition efficiency of organic compounds is strongly dependent on the
structure and chemical properties of the layer formed on the metal surface under
particular experimental conditions (El-Rehim et al., 2001). It is also dependent on the
state of the metal surface, type of corrosive medium, composition of the steel and the
chemical structure of the inhibitor (Azhar et al., 2001). Studies report that the
adsorption of the organic inhibitors mainly depends on some physical-chemical
properties of the molecule related to its functional group, as well as on the strength of
the inhibitor–metal bond (Samide et al., 2005). It is also necessary to investigate the
effectiveness of inhibitors under severe conditions, such as acidic and sea water, that
might occurr to a carbon steel. To date, there is no report found in the literature
regarding the use of 2-mercaptobenzothiazole and 1,2,3-benzotriazole for carbon
steel inhibition under acidic and sea water conditions. Therefore, this study on
inhibition of carbon steel corrosion by a thiazole and a triazole compounds was
carried out.
1.3
Research Objectives
The objectives of this research are:
1. to investigate the inhibition efficiency of 1,2,3-benzotriazole (BTA) (Figure
1.3) and 2-mercaptobenzothiazole (MBT) (Figure 1.4) towards carbon steel in
acidic and sea water conditions respectively,
Figure 1.3: Structure of 1,2,3-benzotriazole (BTA)
4
Figure 1.4: Structure of 2-mercaptobenzothiazole (MBT)
2. to study the effect of temperature on the corrosion rate and thermodynamic
parameters related to the corrosion process,
1.4
Scope of the Study
This study was limited to the effects of 2-mercaptobenzothiazole (MBT) and
1,2,3-benzotriazole (BTA) as corrosion inhibitors for carbon steel corrosion
protection under acidic and sea water conditions respectively. The technique applied
in this study was a chemical technique that involved weight loss experiments. The
study also involved elemental analysis of the carbon steel used and the
microstructure of the carbon steel coupons with and without the application of
inhibitors.
5
CHAPTER 2
LITERATURE REVIEW
2.1
Basic Concept of Corrosion
Corrosion may be defined as the deterioration of a substance (usually metal)
caused by chemical or electrochemical reaction with its environment (Bosich, 1970).
Other material than metal, such as ceramic, polymers or concrete may also be subject
to corrosion. However, it is normally referred to metal (Wranglen, 1972). Examples
of corrosion phenomena include transformation of steel into rust, cracking of brass in
the presence of ammonia, oxidation of an electrical contact made of copper,
weakening of high-resistance steel by hydrogen, hot corrosion of a super-alloy in a
gas turbine, swelling of PVC in contact with a solvent, chemical attack of a nylon
tube by an oxidizing acid, alkaline attack on refractory bricks and chemical attack of
mineral glass by an alkaline solution (Landolt, 2006).
In industry, corrosion is a serious problem which can lead to weakening of
metal structures, failure of plant, contamination of product and can affect safety, also
appearances (Tuomi, 1979). For examples, automobiles are painted because rusted
surfaces are not pleasing to the eye. Badly corroded or rusted equipment in a plant
would also leave a poor impression on the observer. In aspect of contamination of
6
product, the market value of a chemical plant product is directly related to its purity
and quality (Fraunhofer, 1974). For examples, in manufacturing of transparent
plastics, food products and drugs, freedom from contamination is a vital factor
(Bosich, 1970). Also, plants are shut down or parts of the process stopped because of
unexpected corrosion failures. This situation is very unpleasant which can cause a
direct loss in revenue, if occur during periods of high demand for the product
(Chilton, 1968). Then, safety is also a critical factor to be considered. Corrosion
failures may cause severe injury or maybe loss of life (Evans, 1981).
The corrosion of metals is due to an irreversible oxidation-reduction (redox)
reaction between the metal and an oxidizing agent present in the environment
(Fraunhofer, 1974). The basic concept of corrosion is the oxidation of the metal is
inseparably coupled to the reduction of the oxidizing agent (Landolt, 2006):
Metal + oxidizing agent
oxidized metal + reducing agent
For example, the corrosion of iron in the presence of hydrochloric acid is due to
reaction (2.1).
Fe(s) + 2 HCl(aq)
FeCl2(aq) + H2(g)
(2.1)
Under neutral and alkaline conditions, the corrosion of metals is generally
due to a reaction of the metal with oxygen (Landolt, 2006). For example, when
exposed to air and to humidity, iron form rust, FeOOH;
4 Fe + 3 O2 + 2 H2O
4 FeOOH
(2.2)
Metals also react with oxygen in acidic environments, but under these conditions the
concentration of oxygen is usually so much lower than that of protons, so that its
effect can be neglected (Evans, 1981).
7
2.2
Types of Corrosion
Corrosion damage can be grouped into eight forms which consist of uniform
corrosion, galvanic corrosion, crevice corrosion, pitting corrosion, intergranular
corrosion, selective corrosion, erosion corrosion and stress corrosion (Bosich, 1970).
Table 2.1 shows the summary of types of corrosion with its description.
2.3
Corrosion Inhibitor
An inhibitor is a substance which retards or slows down a chemical reaction.
Thus, a corrosion inhibitor is a substance which, when added to an environment,
decreases the rate of attack by the environment on a metal (Delinder et al., 1984).
Corrosion inhibitors are commonly added in small amounts to acids, cooling waters,
steam, and other environments, either continuously or intermittently to prevent
serious corrosion (Dillon, 1982). Some inhibitors retard corrosion by adsorption to
form a thin and invisible film while some form visible bulky precipitates which coat
the metal and protect it from attack. Another common mechanism consists of causing
the metal to corrode in such a way that a combination of adsorption and corrosion
product forms a passive layer (Perez, 2004). Commonly, there are six classes of
inhibitor which are anodic passivating inhibitors, cathodic inhibitors, ohmic
inhibitors, organic inhibitors, precipitation inhibitors and vapor phase inhibitors
(Delinder et al., 1984, Dillon, 1982).
8
Table 2.1: Types of Corrosion
Types of Corrosion
Descriptions
Uniform Corrosion
Loss of material distributed uniformly over the entire
surface exposed to the corrosive environment. Usually
involved metals in contact with strong acids. (Bosich,
1970)
Galvanic Corrosion
Also called bimetallic corrosion. This type of corrosion is
results from the formation of an electrochemical cell
between two metals. (Evans, 1981)
Crevice Corrosion
Caused by a difference of oxygen availability between two
sites on a passive metal that lead to the formation of an
electrochemical cell. A selective attack within cracks and
at other sites of poor oxygen access is frequently observed.
(Bosich, 1970)
Pitting Corrosion
Observed on passive metals in presence of certain anions
(in particular chloride) when the potential exceeds a
critical value. This process typically produces cavities with
diameters in the order of several tens on micrometers.
(Evans, 1981)
Intergranular Corrosion Selective attack of grain boundaries. Often, it is related to
thermal treatments that lead to preferred precipitation of
phases at grain boundaries. (Bosich, 1970)
Selective Corrosion
Also called selective leaching or dealloying. It implies the
selective dissolution of one of the components of an alloy
that forms a solid solution. It leads to the formation of a
porous layer made of the more noble metal. (Bosich, 1970)
Erosion Corrosion
Results of an electrochemical reaction combined with a
material loss by mechanical wear due to impingement of
solids or a fluid. (Landolt, 2006)
Stress Corrosion
Results from the combined action of corrosion and of
mechanical stress. It manifests itself by crack formation at
stress levels well below the ultimate tensile strength of a
material. (Bosich, 1970)
9
2.3.1
Anodic Passivating Inhibitors
Anodic passivating inhibitors are inhibitors that cause a large shift in the
corrosion potential. They are also called dangerous inhibitors because, if used in
insufficient concentrations, they cause pitting and sometimes an increase in corrosion
rate. There are two types of passivating inhibitors which are oxidizing anions such as
chromate, nitrite, and nitrate which can passivate steel in the absence of oxygen and
the nonoxidizing ions such as phosphate, tungstate and molybdate which require the
presence of oxygen to passivate steel. With careful control, passivating inhibitors are
frequently used because they are very effective in sufficient quantities (Delinder et
al., 1984).
The mechanism by which chromate passivates steel appears likely that
protection is afforded by a combination of adsorption and oxide formation on the
steel surface. Adsorption helps to polarize the anode to sufficient potentials to form
very thin hydrated ferric oxides which protect the steel. Since the oxide film is
invisible on steel, articles protected by chromate remain bright in otherwise
aggressive environments. The oxide film is a mixture of ferric and chromic oxides
and is kept in good repair by adsorption and oxidation with very little loss of metal as
long as sufficient chromates remains in solution (Dillon, 1982).
2.3.2
Cathodic Inhibitors
Cathodic inhibitors either slow the cathodic reaction itself, or they selectively
precipitate on cathodic areas to increase circuit resistance and restrict diffusion of
reducible species to the cathodes. The cathodic reaction is often the reduction of
hydrogen ions to form hydrogen gas. Some cathodic inhibitors make the discharge of
hydrogen gas more difficult and they are said to increase the hydrogen overvoltage.
10
Compounds of arsenic and antimony are examples of this type of inhibitor which are
often used in acids or in systems where oxygen is excluded. Another possible
cathodic reaction is the reduction of oxygen. The inhibitors for this cathodic reaction
are different from those mentioned for the more acidic systems (Delinder et al.,
1984).
2.3.3
Ohmic Inhibitors
Ohmic inhibitors are inhibitors which increase the ohmic resistance of the
electrolyte circuit. Since it is usually impractical to increase resistance of the bulk
electrolyte, increased resistance is practically achieved by the formation of a film, a
microinch thick or more, on the metal surface. If the film is deposited selectively on
anodic areas, the corrosion potential shifts to more positive values while if it is
deposited on cathodic areas, the shift is to more negative values and if the film
covers both anodic and cathodic areas, there may be only a slight shift in either
direction (Delinder et al., 1984).
2.3.4
Organic Inhibitors
Organic compounds constitute a broad class of corrosion inhibitors which
cannot be designated specifically as anodic, cathodic or ohmic. Anodic or cathodic
effect alone are sometimes observed in the presence of organic inhibitors, but in
general rule, organic inhibitors affect the entire surface of a corroding metal when
present in sufficient concentration (Delinder et al., 1984). Typically, corrosion
inhibition increase with inhibitor concentration and it is suggested that inhibition is
11
the result of adsorption of inhibitor on the metal surface (El-Rehim et al., 2001,
Fouda et al., 2006).
Organic inhibitors will be absorbed according to the ionic charge of the
inhibitor and the charge on the metal surface (Dillon, 1982, Delinder et al., 1984).
Cationic inhibitors which positively charged, such as amines, or anionic inhibitors
which negatively charged, such as sulfonates, will be absorbed preferentially,
depending on whether the metal is charged negatively or positively which is opposite
sign charges attract. The in-between potential at which neither cationic nor anionic
molecules are preferred is known as the zero point charge or ZPC. Thus, a
combination of cathodic protection and an inhibitor which is adsorbed more strongly
at negative potentials gives greater inhibition than either cathodic protection or an
inhibitor when used alone (Atkinson and VanDroffelaar, 1985).
2.3.5
Precipitation Inhibitors
Precipitate-inducing inhibitors are film forming-compounds which have a
general action over the metal surface and which interfere with both anodes and
cathodes indirectly. The most common inhibitors of this class are silicates and
phosphates (Delinder et al., 1984). In water with a pH near 7.0, a low concentration
of chlorides, silicates and phosphates cause passivation of steel when oxygen is
present, which makes they behave as anodic inhibitors (Clubley, 1988). Another
anodic characteristic is that corrosion is localized in the form of pitting when
insufficient amounts of phosphate or silicate are added to saline water (Atkinson and
VanDroffelaar, 1985). However, both silicates and phosphates form deposits on steel
which increase cathodic polarization. Thus, their action appears to be mixed which
by a combination of both anodic and cathodic effects (Delinder et al., 1984).
12
2.3.6
Vapor Phase Inhibitors
Vapor phase inhibitors (VPI), also called volatile corrosion inhibitors (VCI),
are compounds which are transported in a closed system to the site of corrosion by
volatilization from a source (Delinder et al., 1984). These inhibitors are usually salts
of moderately strong bases and weak volatile acids, which sublime at room
temperature. When inserted inside contained spaces, its saturate the air with the
vapors, and condense on the metal surface of the wrapped object, forming a
protective layer on the metal. This layer prevent the contact of the metallic surface
with the aggressive medium, which then making it less susceptible to corrosion
(Estevao and Nascimento, 2001).
2.4
Studies on Carbon Steel Corrosion Control
An increasing number of wet gas carbon steel pipelines use glycol for hydrate
prevention and the pH-stabilization technique for corrosion control (Dugstad et al.,
2003). The basis of pH stabilisation is addition of alkaline chemicals (e.g. NaOH or
methyldiethanolamine, MDEA) to corrosive media in order to increase pH of the
glycol/water mixture and thus improve the protective properties of the corrosion
films (Kvarekval and Dugstad, 2006).
Numerous studies have applied various types of corrosion inhibitor for
corrosion control of carbon steel. A study has used 2-mercapto-benzothiazole (MBT)
for corrosion inhibition of carbon-steel in 10-3 M ammonia solution, and it shows that
MBT behaves as a corrosion inhibitor for carbon-steel in that medium (Samide et al.,
2004). The other study adopted ammonium polymolybdate (APM) as inhibitor on the
corrosion of carbon steel in 1 M HCl solution (Samide et al., 2008). Also, lithium
13
nitrate (LiNO3) was successfully used in mild steel corrosion in a lithium bromideethylene glycol solution as corrosion inhibitor (Sarmiento et al., 2008).
The other form of corrosion control for carbon steel was studied by using self
assembled monolayers (SAMs) of hydroxamic acids CH3(CH2)nCONHOH with
different alkyl length (Alagta et al., 2008). Hydroxamic acids successfully deposited
and form protective self-assembled layers on carbon steel surfaces. Adsorption of
hydroxamic acid molecules on carbon steel surfaces reduce the corrosion process
with inhibition efficiencies reached 96% for C10 and 99% for C18 (Alagta et al.,
2008).
2.5
Commonly Used Corrosion Inhibitor
Corrosion inhibitor is substance which when apply in suitable concentration
can effectively reduces the corrosion rate of a metal exposed to certain environment
(Harrop, 1988). Corrosion inhibitors act by different mechanisms, by adsorption,
passivation, film formation by precipitation, or by elimination of the oxidizing agent
(Mercer, 1988). In most cases, inhibition is achieved through interaction or reaction
between the corrosion inhibitor and the metal surface. It is then resulting in the
formation of an inhibitive surface film which may occur directly on the metal surface
or the environment interface (Gao et al., 2008). Inhibitors can be classified in
different ways which are by their field of application, effect on the partial
electrochemical reactions or by their reaction mechanism (Landolt, 2006). Inhibitors
are widely used in the corrosion protection of metals in several environments (Rehim
et al., 2008). Table 2.2 shows the summary on corrosion inhibitor application in
various research areas.
14
There are numerous studies on corrosion inhibitors with different types of
mechanisms and field of applications. The use of inorganic inhibitors such as
chromates, nitrates and molibdates to evaluate the corrosion rate of carbon steel has
been studied by Samiento-Bustos et al (2008). The results showed that the inhibitor
efficiency increased with the concentration, except for chromates, where the highest
efficiency was reached with 20 ppm of inhibitor. The highest efficiency was obtained
with 50 ppm of LiNO3 which gave 95% inhibition efficiency. The inhibition
mechanism was by passivation (Samiento-Bustos et al., 2008).
The influence of different types of organic inhibitors has also been
extensively studied (Behpour et al., 2008). For example the [2,5-bis(n-pyridyl)-1,3,4thiadiazoles] was used successfully as an inhibitor of corrosion for mild steel in 0.5
M H2SO4 and 1 M HCl, but better performance in the presence of 1 M HCl. The
inhibition mechanism was by adsorption and they behave as mixed-type inhibitors in
acidic media (Azhar et al., 2001).
The use of polymers and naturally occurring substances as inhibitors also has
drawn considerable attention. Polyethylene glycol (PEG) which is a synthetic
polymer and Gum Arabic (GA) which is a naturally occurring polymer was adopted
as corrosion inhibitors on mild steel in strong acidic solution, with PEG being a
better inhibitor than GA (Umoren et al., 2008). The other study adopted natural
products of plant origin as corrosion inhibitor was using P. amarus extracts (PAE).
PAE was successfully used as inhibitor for mild steel corrosion in HCl and H2SO4
solutions (Okafor et al., 2008).
Mild steel
Mild steel
2-Methylbenzimidazole
Benzimidazole
Phenanthro[9,10-c]-1,2,5-thiadiazole
1,1-dioxide
Bis (benzimidazol-2-yl) disulphide
1M HCl
Mild steel
35
25
6.5 x 10-7
0.25M H2SO4
Copper
120 ppm
25
25
25
250 ppm
250 ppm
250 ppm
1M HCl
1M HCl
1M HCl
Mild steel
2-Mercaptobenzimidazole
25
25 ppm
1M H2SO4
API 5L X52 steel
25
5.0 x 10-3
1M HNO3
Copper
25
5.0 x 10-3
1M HNO3
Copper
25
5.0 x 10-3
1M HNO3
Copper
60
1.5 x 10-4
1M HCl
Mild steel
60
1.5 x 10-4
Mild steel
2,5-Bis(2-thienyl)-1,3,4-thiadiazoles
(2-TTH)
2,5-Bis(3-thienyl)-1,3,4-thiadiazoles
(3-TTH)
1-(Phenylsulfonyl)-1H-benzotriazole
(PSB)
1-(3-Pyridinylsulfonyl)-1Hbenzotriazole (3PSB)
1-(2-Pyridinylsulfonyl)-1Hbenzotriazole (2PSB)
2-Mercaptoimidazole (2MI)
T (°C)
IC (M)
Corrosive
Medium
1M HCl
Metal Sample
Corrosion Inhibitor
Table 2.2: Summary on corrosion inhibitor application in various research areas
98.2
50
88.7
57.1
52.2
98.5
82.4
87.39
92.37
98.26
97.89
IE (%)
15
(Bentiss et al.,
2005)
(Bentiss et al.,
2005)
(Khaled et al.,
2009)
(Khaled et al.,
2009)
(Khaled et al.,
2009)
(ÁlvarezBustamante et al.,
2009)
(Aljourani et al.,
2009)
(Aljourani et al.,
2009)
(Aljourani et al.,
2009)
(Grillo et al.,
2009)
(Ahamad and
Quraishi, 2009)
References
25
25
25
200 ppm
5 ppm
1 x 10-5 M
1 x 10-5 M
15% HCl
15% HCl
0.5M H2SO4
0.1M Na2SO4
Mild steel
Low carbon steel
Copper
Copper
Di-phenyl-sulfoxide
0.1M Na2SO4
200 ppm
25
25
70
70
70
70
25
Mild steel
0.1 M
1 x 10-3 M
1 x 10-2 M
1 x 10-2 M
200 ppm
0.1M HCl
0.1M HCl
0.1M HCl
0.1M HCl
15% HCl
Mild steel
Mild steel
Mild steel
Mild steel
Mild steel
5-Amino-1,2,4-triazole
5-Amino-3-mercapto-1,2,4-triazole
1-Amino-3-methylthio-1,2,4-triazole
5-Amino-3-methylthio-1,2,4-triazole
2-{[(2sulphanylphenyl)imino]methyl}]phenol
2-{[(2)-1-(4methylphenyl)methylidene]
amino}benzenthiol
2-[(2-sulphanylphenyl)ethanimidoyl)]phenol
Quaternized polyethyleneimine
Di-benzyl-sulfoxide
25
1.0 x 10-2
0.5M H2SO4
Mild steel
2-Amino-4-(p-tolyl)thiazole
25
1.0 x 10-2
0.5M H2SO4
Mild steel
2-Methoxy-1,3-thiazole
25
1.0 x 10-2
0.5M H2SO4
Mild steel
Thiazole-4-carboxaldehyde
35
Mild steel
Bis (benzimidazol-2-yl) disulphide
T (°C)
In. Conc.
(M)
120 ppm
Corrosive
Medium
0.5M H2SO4
Metal Sample
Corrosion Inhibitor
Table 2.2 continued
25
92
78
65.9
99.4
79
90
94
95
99
98.1
93.4
92.3
99.1
IE (%)
16
(Behpour et al.,
2008)
(Gao et al., 2008)
(Telegdi et al.,
2000)
(Telegdi et al.,
2000)
(Ahamad and
Quraishi, 2009)
(Khaled and
Amin, 2009)
(Khaled and
Amin, 2009)
(Khaled and
Amin, 2009)
(Hassan, 2007)
(Hassan, 2007)
(Hassan, 2007)
(Hassan, 2007)
(Behpour et al.,
2008)
(Behpour et al.,
2008)
References
30
30
30
30
1 x 10-6 M
1 x 10-6 M
1 x 10-6 M
1 x 10-6 M
2M HCl
2M HCl
2M HCl
2M HCl
Carbon steel
Carbon steel
Carbon steel
Carbon steel
36.3
29.6
25.3
18.1
75.63
73.9
82
88
66
IE (%)
17
(Al-Sarawy et al.,
2008)
(Al-Sarawy et al.,
2008)
(Al-Sarawy et al.,
2008)
(Telegdi et al.,
2000)
(Telegdi et al.,
2000)
(Telegdi et al.,
2000)
(Abdallah, 2004)
(Samide et al.,
2005)
(Al-Sarawy et al.,
2008)
References
IE (%): Percentage Inhibition Efficiency
25
25
250 ppm
150 ppm
1M H2SO4
10-3 M NH3
Carbon steel
Carbon steel
T: Temperature
25
1 x 10-5 M
0.1M Na2SO4
Copper
25
1 x 10-5 M
0.1M Na2SO4
Copper
5-(4'isopropylbenzylidene)-2,4dioxotetrahydro-1,3-thiazole
5-benzylidene-2,4-dioxotetrahydro-1,3thiazole
Guar gum
N-ciclohexil-benzothiazolesulphenamida
2-(acetyl-ethoxy carbonyl-methyleno)3-phenyl-4-(phenylhydrazono)-1,3thiazolidin-5-one
2-(acetyl-ethoxy carbonyl-methyleno)3-phenyl-4-(3methoxyphenylhydrazono)-1,3thiazolidin-5-one
2-(acetyl-ethoxy carbonyl-methyleno)3-phenyl-4-(2methoxyphenylhydrazono)-1,3thiazolidin-5-one
2-(acetyl-ethoxy carbonyl-methyleno)3-phenyl-4-(4methoxyphenylhydrazono)-1,3thiazolidin-5-one
* IC: Inhibitor concentration
25
1 x 10-5 M
Copper
Di-p-tolyl-sulfoxide
T (°C)
IC (M)
Corrosive
Medium
0.1M Na2SO4
Metal Sample
Corrosion Inhibitor
Table 2.2 continued
18
2.6
Thiazole Compounds as Corrosion Inhibitors
The effectiveness of an organic substance as an inhibitor depends on its
structure (Azhar et al., 2001). The variation in inhibitive efficiency mainly depends
on the type and the nature of substituents present in the inhibitor molecule (Samide et
al., 2005). Corrosion inhibitors are necessary to reduce corrosion rates of metallic
materials in corrosive media such as chloride solutions. Many organic molecules are
used to inhibit corrosion (Chen et al., 2004). Organic molecules such as pyrazole,
pyrimidine, thiadiazole and benzimidazole have been shown to have a high inhibiting
efficiency (Scendo and Hepel, 2007).
Since the S atom has strong adsorption on copper, many heterocyclic
compounds containing a mercapto group have been developed as copper corrosion
inhibitors for different industrial applications. These compounds include 2,4dimercaptopyrimidine
(Walter,
1996),
2-amino-5-mercapto-thiadiazole,
2-
mercaptothiazoline (Trachli et al., 2002) and potassium ethyl xanthate (KEtX)
(Scendo, 2005b, Scendo, 2005a). It has been suggested that the interaction of the S
atom with the metal surface results in the formation of an insoluble protective
complex (Scendo and Hepel, 2007). Also, researchers suggest that the corrosion
inhibitor is chemisorbed on the Cu surface through the S atom (Scendo, 2005b,
Scendo, 2005a). Most investigations on the application of thiazole inhibitors were
focused on copper surface. Information on the application of these inhibitors on
carbon steel, which commonly used in long distance gas supply line is still lacking.
Therefore there is a need for an investigation on the action of thiazole compounds in
inhibiting corrosion of carbon steel. To date, there are no reports found in the
literature regarding the use of 2-mercaptobenzothiazole and 1,2,3-benzotriazole for
carbon steel inhibition under acidic and sea water conditions.
19
2.7
Weight Loss Method
Weight loss is the conventional method for evaluating the corrosion of steel
by measuring the loss of mass of a metal coupon after a period of exposure to a
corrosive environment (ASTM, Designation G31-72). Weight loss measurement
does not require any definite size or shape, but a large area-to-volume ratio is used
for better sensitivity. Usually, a flat square or rectangular is used to simplify
measurement of surface area. The specimen is kept relatively small to permit simple
and accurate weight measurements (Delinder et al., 1984). There are numerous
studies applied the weight loss measurements for evaluating the corrosion of metals
(Behpour et al., 2008, Khaled and Al-Qahtani, 2009, Fouda et al., 2006, Benabdellah
et al., 2006).
20
CHAPTER 3
METHODOLOGY
3.1
Chemicals
All chemical used in this study are from analytical reagent grade. The
chemicals used was 1,2,3-benzotriazole (BTA); (C6H5N), 2-mercaptobenzothiazole,
(MBT); (C7H5NS2), hydrochloric acid (HCl), nitric acid (HNO3), acetone (C3H6O),
and ethanol (C2H6O).
3.2
Apparatus and Instrumentation
All the glass wares and other reusable items such as beaker, volumetric flask,
pipette and dropper were soaked overnight in 10% nitric acid solution and rinsed
with deionised water to ensure all the apparatus used in analysis was not
contaminated with interfering ions that may affect the result of analysis. The
analytical balance used in weight loss measurements was XT 220 A model by Atama
Tech Sdn Bhd, which gave the readings at four decimal places.
21
3.3
Preparation of Carbon Steel Coupon
The carbon steel coupons were modified according to ASTM corrosion
testing standard to make rectangular (20mm x 20mm x 2.5mm) coupons (Figure 3.1),
prior to use in weight loss measurements. Before each measurement the carbon steel
coupons were polished with a sequence of grit SiC papers of different grades (240,
320, 600, 1000), degreased in acetone, rinsed with double distilled deionised water,
dried between two filter papers and stored in desiccator until used (ASTM,
Designation G1-03).
Figure 3.1: Carbon steel coupons with dimensions 20mm x 20mm x 2.5mm used in
weight loss measurements
3.4
Determination of Elemental Composition of Carbon Steel Coupons
Elemental compositions of the carbon steel coupons were determined by
energy dispersive X-ray spectrometer (EDX) which attached to JSM-6701F Field
Emission Scanning Electron Microscopy (FESEM), which currently available at the
Ibnu Sina Institute of Fundamental Research, Faculty of Science, UTM.
22
Figure 3.2: Photograph of FESEM-EDX, model JSM-6701F
3.5
Solutions Preparation
All the solutions used were prepared from analytical reagent grade with
double distilled deionised water. These solutions include 1,2,3-benzotriazole (BTA),
2-mercaptobenzothiazole (MBT), hydrochloric acid and nitric acid.
3.5.1
1,2,3-Benzotriazole (BTA) 0.5 M
A 0.5 M 1,2,3-benzotriazole (BTA) solution was prepared by dissolving
2.978 g BTA into 40 mL ethanol. The solution then transferred into 50 mL
volumetric flask. The volumetric flask was then filled up to the mark with ethanol.
23
3.5.2
2-Mercaptobenzothiazole (MBT) 0.5 M
A 0.5 M 2-Mercaptobenzothiazole (MBT) solution was prepared by
dissolving 4.175 g MBT into 40 mL acetone. The solution then transferred into 50
mL volumetric flask. The volumetric flask then filled up to the mark with acetone.
3.5.3
Hydrochloric Acid (HCl) 1.0 M
A 1.0 M hydrochloric acid solution was prepared using 82.81 mL of
concentrated hydrochloric acid (12.076 M) which diluted into 1000 mL double
distilled deionised water.
3.5.4
Seawater Sample
Seawater (5 liters) sample was taken by using a plastic bottle from Pantai
Batu Buruk, Kuala Terengganu on 5th September 2008. It was filtered using filter
paper prior to use.
24
3.6
Weight Loss Measurements
The weight loss measurements were carried out in the absence and presence
of different concentrations of inhibitors. A graduated glass beaker with a 6 cm inner
diameter and a total volume of 250 ml was used as reaction basin. For each
experiment, 100 ml of the test solution was pour into the reaction basin. The test
solution was seawater sample and 1.0 M HCl, which was made from the commercial
reagent and double-distilled deionised water. The concentrations of the inhibitors
used were in the range of 10-5 to 10-2 M.
For experimental, the carbon steel plate has been cut into three pieces with
dimension 20 mm × 20 mm × 2.5 mm. This gave a constant surface area of 200 mm2
to contact with the test solution. The three pieces carbon steel samples then
mechanically polished, degreased in acetone, rinsed with double distilled deionised
water, dried between two filter papers and weighed (m1). The samples then were
suspended by nylon thread at the edge of the basin, and under the surface of the test
solution by about 1 cm. After specify periods of time, the samples were taken out
from the test solution, rinsed with double distilled deionised water, dried as before
and reweighed (m2). The average weight loss for certain immersion period for each
set of three samples was recorded, (m= m1– m2). These weight loss measurements
were carried out according to the ASTM standard procedure (ASTM, Designation
G31-72).
Corrosion rate and inhibition efficiencies were calculated from the following
equations (Khaled, 2008):
C.R = m / AT
(3.1)
IE% = [(C.Ro – C.R) / C.Ro ] x 100
(3.2)
25
Where m
= weight loss in milligrams
A
= total surface area in cm2
T
= time of exposure in hours
C.Ro
= corrosion rates in (mgcm-2 hour-1) without inhibitors
C.R
= corrosion rates in (mgcm-2 hour-1) with different
concentrations of the inhibitors
The inhibition efficiency depends on the degree of coverage of the carbon
steel surface by molecules of the inhibitor and can be expressed as in the following
equation:
= [(C.Ro – C.R) / C.Ro ]
(3.3)
In order to get a comparative view, the variation of the percentage inhibition
(IE %) of the two inhibitors with their molar concentrations will be calculated
according to equation (3.2), which stated previously. So, from the calculation we can
determine the inhibition efficiencies for all two investigated inhibitors. (Fouda et al.,
2006).
3.6.1
Inhibitor Concentration Effect
Four concentration values of 1,2,3-benzotriazole (BTA) used which were in
the range of 10-5 to 10-2 M. Two to two thousand μL of 1,2,3-benzotriazole (0.5 M)
solution pipetted into reaction beaker contained 100 mL test solution to give 1 x 10-5
M, 1 x 10-4 M, 1 x 10-3 M and 1 x 10-2 M concentration value of BTA in reaction
beaker accordingly. The immersion period of carbon steel coupons and the
26
temperature of the reaction medium were set to be constant. Each analysis was done
in three replicates. Those steps were then repeated for 2-Mercaptobenzothiazole
(MBT) concentration effect analysis.
3.6.2
Immersion Period Effect
The study of immersion time effect of BTA and MBT inhibition efficiencies
on carbon steel coupons were carried out in the range of 1-24 hours. The carbon steel
coupons were immersed in the reaction beaker for 1, 2, 4, 8 and 24 hours. The
inhibitor concentration and the temperature of the reaction medium were set to be
constant. Each analysis was done in three replicates.
3.6.3
Temperature Effect
The study of temperature effect of BTA and MBT inhibition efficiencies on
carbon steel coupons were carried out in the range of 30-90 °C. The reaction beaker
were covered with aluminium foil and placed in the oven for 24 hours. The
temperature of the oven was set to 30, 50, 70 and 90 °C. The concentration of the
inhibitor used in the reaction beaker was 1.0 x 10-2 M. Each analysis was done in
three replicates.
27
3.7
Microstructure Analysis of Coupons
The microstructure of the studied carbon steel coupons have been analysed by
Nikon Image Analyzer, which currently available at the Material Laboratory, Faculty
of Mechanical, UTM.
28
CHAPTER 4
RESULTS AND DISCUSSION
4.1
Determination of Elemental Composition of Carbon Steel Coupons
Carbon steel coupons were analysed for their specific elemental compositions
using the energy dispersive X-ray spectrometer (EDX) which attached to JSM-6701F
Field Emission Scanning Electron Microscopy (FESEM). The results obtained were
as presented in Table 4.1.
Table 4.1: Elements composition of carbon steel coupons
Fe
C
Mn
Elements Composition (%w/w)
No. of Observation Mean Value Standard deviation
3
96.23
1.02
3
1.34
0.13
3
0.68
0.39
Results of the analysis show that the composition of the elements in carbon
steel coupons is similar to those reported for the standard properties of carbon steel
(ASTM A29). Carbon was found to be the major alloying element in the carbon steel
coupons.
29
4.2
Weight Loss Measurements
Gravimetric corrosion measurements were carried out according to the
ASTM standard procedure (ASTM, Designation G31-72). This experiment would
enable determination of the physical changes and the corrosion rates of carbon steel
coupons in corrosive medium exposed. In brief, carbon steel coupons in triplicate
were immersed in corrosive medium which was the test solution. Two corrosive
medium has been exploited in this study, which are acidic and seawater conditions
respectively with absence and presence of inhibitors studied. In the present study, for
the specified experimental conditions, relative differences between replicate
experiments were found to be smaller than 5%, indicating good reproducibility. For
further data processing, the average of the three replicate values was used. The
calculations of corrosion rate, degree of surface coverage and inhibition efficiency
were as discussed in Section 3.7.
4.2.1
Inhibitor Concentration Effect
The inhibitor concentration effect was determined with a range of inhibitor
concentration from 10-5 M to 10-2 M. The values of percentage inhibition efficiency
IE%, corrosion rate and surface coverage obtained from gravimetric measurements
with the addition of different concentrations of inhibitors after 24 hours immersion in
1.0 M HCl solutions at 25 °C are summarized in Table 4.2. The inhibitors studied
were 1,2,3-benzotriazole (BTA) and 2-mercaptobenzothiazole (MBT).
The inhibition efficiency as a function of concentration is shown in Fig. 4.1.
Inspection of the data in Table 4.2 reveals that both tested compounds appeared to
act as inhibitors over the studied concentration range. The corrosion rate values in
30
the presence of various inhibitors indicated that the BTA has the greatest inhibition
effect on the dissolution of carbon steel in 1.0 M HCl.
Table 4.2: Corrosion rate, surface coverage and inhibition efficiency for various
concentration of BTA and MBT for the corrosion of carbon steel after 24 hours
immersion in 1.0 M HCl obtained from weight loss measurements at 25 °C
Concentration
(M)
0
Corrosion rate
(mg cm-2 hour-1)
0.90
BTA
10-5
10-4
10-3
10-2
0.68
0.53
0.37
0.22
0.25
0.41
0.59
0.76
25.00
41.01
59.00
76.01
MBT
10-5
10-4
10-3
10-2
0.88
0.84
0.83
0.72
0.03
0.07
0.08
0.20
3.01
7.01
7.98
20.00
Inhibitor
Blank
Surface
Inhibition
Coverage () Efficiency (IE%)
0.00
–
Figure 4.1: Variations of the inhibition efficiency calculated from weight loss
measurements at different concentrations of BTA and MBT after 24 hours immersion
in 1.0 M HCl solution at 25 °C
The reduction in the dissolution of carbon steel in the presence of these tested
compounds was attributed to the amino group and the heterocyclic rings. These
31
groups (heterocyclic rings and amino groups) are electroactive and interact with the
metal’s surface to a greater extend (Khaled and Al-Qahtani, 2009). It has been
observed that the inhibition efficiency increased with increase in inhibitors’
concentration and reached a maximum value of 76% at a concentration of 10-2 M of
BTA after 24 hours immersion in 1.0 M HCl solution at 25 °C.
4.2.2
Immersion Period Effect
The effect of immersion period on carbon steel corrosion inhibition by BTA
and MBT has been studied for 1, 2, 4, 8 and 24 hours immersion in 1.0 M HCl at 25
°C. The values of percentage inhibition efficiency IE%, corrosion rate and surface
coverage obtained from gravimetric measurements with the addition of 10-2 M BTA
and MBT respectively after 1, 2, 4, 8, and 24 hours immersion in 1.0 M HCl at 25 °C
are summarized in Table 4.3.
Table 4.3: Corrosion rate, surface coverage and inhibition efficiency for carbon steel
after 1, 2, 4, 8, and 24 hours immersion in 1.0 M HCl with absence and presence of
10-2 M BTA and MBT respectively obtained from weight loss measurements at 25°C
Immersion
Inhibitor
time (h)
Blank
1
2
4
8
24
BTA
1
2
4
8
24
Corrosion rate
(mg cm-2 hour-1)
4.24
2.92
2.20
1.61
0.90
Surface
Coverage ()
0.00
0.00
0.00
0.00
0.00
Inhibition
Efficiency (IE%)
–
–
–
–
–
3.27
1.77
1.14
0.77
0.22
0.23
0.39
0.48
0.52
0.76
23.02
39.36
47.90
52.02
76.01
32
Table 4.3 continued
Inhibitor
MBT
Immersion
time (h)
1
2
4
8
24
Corrosion rate
(mg cm-2 hour-1)
3.84
2.51
1.85
1.36
0.72
Surface
Coverage ()
0.09
0.14
0.16
0.15
0.20
Inhibition
Efficiency (IE%)
9.45
13.97
15.97
15.01
20.00
The inhibition efficiency as a function of immersion period is shown in
Figure 4.2. From the figure, it can be seen that the immersion period of the tested
coupons would also affect the values of percentage inhibition efficiency for both
inhibitors studied. It has been observed that the inhibition efficiency increased with
the increase immersion period and gave the highest values after 24 hours immersion
period, which gave 76% for BTA and 20% for MBT for the immersion in 1.0 M HCl
solution at 25 °C.
Inhibition Efficiency (IE% )
80
70
60
50
40
30
20
10
0
0
4
8
12
16
20
24
28
Immersion period (hour)
BTA
MBT
Figure 4.2: Variations of the inhibition efficiency of BTA and MBT calculated from
weight loss measurements at different immersion period in 1.0 M HCl solution at
25°C
33
4.2.3
Temperature Effect
The temperature effect was determined at 30, 50, 70 and 90 °C. The values of
percentage inhibition efficiency IE%, corrosion rate and surface coverage obtained
from gravimetric measurements with the addition of absence and presence of 10-2 M
BTA and MBT respectively at various immersion temperatures are summarized in
Table 4.4.
Table 4.4: Corrosion rate, surface coverage and inhibition efficiency for various
immersion temperature of carbon steel after 24 hours immersion in 1.0 M HCl with
absence and presence of 10-2 M BTA and MBT respectively obtained from weight
loss measurements
Surface
Inhibition
Temperature Corrosion rate
-2
-1
Inhibitor
(°C)
(mg cm hour ) Coverage () Efficiency (IE%)
Blank
30
2.08
0.00
–
50
6.70
0.00
–
70
11.53
0.00
–
90
16.19
0.00
–
BTA
30
50
70
90
0.48
1.27
1.80
2.03
0.77
0.81
0.84
0.87
77.16
81.07
84.39
87.49
MBT
30
50
70
90
1.65
5.17
8.44
11.31
0.21
0.23
0.27
0.30
20.71
22.85
26.77
30.15
The inhibition efficiency obtained with various immersion temperature
obtained are simplified in Figure 4.3. It shows that both investigated inhibitors have
inhibiting properties at all the studied temperatures and the inhibition efficiency
increased with the increases temperature. Thus, the studied inhibitors efficiencies are
temperature dependent. The immersion temperature of 90 °C gave the highest values
of inhibition efficiency for both inhibitors, which gave 87.49% for BTA and 30.15%
for MBT.
Inhibition Effuciency
(IE%)
34
100
90
80
70
60
50
40
30
20
10
0
20
30
40
50
60
70
80
90
100
o
Temperature ( C)
BTA
MBT
Figure 4.3: Inhibition efficiency of BTA and MBT in 1.0 M HCl calculated from
weight loss measurements at different immersion temperature for 10-2 M inhibitor
concentration respectively.
4.3
Corrosion Inhibition in Seawater Sample
Carbon steels are commonly used for petroleum gas piping from the
abstraction sources to various distribution centres. Along the piping lines the pipes
are exposed to various conditions including those severe ones such as sea water.
Thefore, carbon steel corrosion inhibition by BTA and MBT has also been studied in
seawater condition. The effect of inhibitor concentration and immersion temperature
has been investigated. As in the previous weight loss experiments in hydrochloric
acid, the gravimetric measurements for seawater sample were carried out according
to the ASTM standard procedure (ASTM, Designation G31-72). This experiment
would enable determination of the physical changes and the corrosion rates of carbon
steel coupons in seawater medium. The triplicates of carbon steel coupons were
immersed in seawater sample in the absence and the presence of BTA and MBT,
respectively. For the specified experimental conditions, relative differences between
replicate experiments were found to be smaller than 5%, indicating good
reproducibility. For further data processing, the average values of the three replicate
measurements were used.
35
4.3.1
Inhibitor Concentration Effect
The inhibitor concentration effect was determined with a range of inhibitor
concentration from 10-5 M to 10-2 M. The values of percentage inhibition efficiency
IE%, corrosion rate and surface coverage obtained from gravimetric measurements
with the addition of different concentrations of inhibitors after 24 hours immersion in
seawater samples at 25 °C are summarized in Table 4.5. The inhibitors studied were
1,2,3-benzotriazole (BTA) and 2-mercaptobenzothiazole (MBT).
Table 4.5: Corrosion rate, surface coverage and inhibition efficiency for various
concentrations of BTA and MBT for the corrosion of carbon steel after 24 hours
immersion in seawater sample obtained from weight loss measurements at 25 °C
Concentration
(M)
0
Corrosion rate
(mg cm-2 hour-1)
1.15
Surface
Coverage ()
0.00
Inhibition
Efficiency (IE%)
–
BTA
10-5
10-4
10-3
10-2
0.27
0.17
0.12
0.09
0.76
0.85
0.90
0.92
76.18
85.01
89.60
92.40
MBT
10-5
10-4
10-3
10-2
0.41
0.32
0.29
0.28
0.64
0.73
0.75
0.76
64.38
72.53
74.71
75.99
Inhibitor
Blank
The inhibition efficiency of carbon steel in seawater as a function of
concentration is shown in Figure 4.4. Inspection of the data in Table 4.5 reveals that
both tested compounds appeared to act as corrosion inhibitors over the studied
concentration range in seawater sample. The corrosion rate values in the presence of
various inhibitors indicated that the BTA has the greatest inhibition effect on the
dissolution of carbon steel in seawater sample, as compared to MBT.
36
Figure 4.4: Inhibition efficiency calculated from weight loss measurements at
different concentrations of BTA and MBT after 24 hours immersion in seawater
sample at 25 °C
The reduction in the dissolution of carbon steel in the presence of the
inhibitor compounds was attributed to the amino group and the heterocyclic rings.
These groups (heterocyclic rings and amino groups) are electroactive and interact
with the metal’s surface to a greater extend (Khaled and Al-Qahtani, 2009). It has
been observed that the inhibition efficiency increased with increases inhibitors
concentration and reached a maximum value of 92.4% with 10-2 M of BTA and 76%
with 10-2 M of MBT after 24 hours immersion in seawater sample at 25 °C.
4.3.2
Temperature Effect
The temperature effect was determined at 30, 50, 70 and 90 °C. The values of
percentage inhibition efficiency IE%, corrosion rate and surface coverage obtained
from weight loss measurements with the addition of absence and presence of 10-2 M
BTA and MBT respectively at various immersion temperatures are summarized in
Table 4.6.
37
Table 4.6: Corrosion rate, surface coverage and inhibition efficiency for various
immersion temperature of carbon steel after 24 hours immersion in seawater sample
with absence and presence of 10-2 M BTA and MBT respectively obtained from
weight loss measurements
Temperature
Inhibitor
(°C)
Blank
30
50
70
90
Corrosion rate
(mg cm-2 hour-1)
2.32
7.10
11.77
13.21
Surface
Inhibition
Coverage () Efficiency (IE%)
0.00
–
0.00
–
0.00
–
0.00
–
30
50
70
90
0.17
0.38
0.46
0.23
0.93
0.95
0.96
0.98
92.85
94.64
96.12
98.24
MBT
30
50
70
90
0.51
1.20
1.35
0.93
0.78
0.83
0.89
0.93
77.85
83.16
88.56
92.98
Inhibition Efficiency (IE%))
BTA
100
80
60
40
20
0
20
30
40
50
60
70
80
90
100
o
Temperature ( C)
BTA
MBT
Figure 4.5: Inhibition efficiency of BTA and MBT in seawater sample calculated
from weight loss measurements at different immersion temperature for 10-2 M
inhibitor concentration respectively.
The inhibition efficiency obtained with various immersion temperature
obtained are simplified in Figure 4.5. It shows that both investigated inhibitors, BTA
and MBT have inhibiting properties at all the studied temperatures and the inhibition
38
efficiency increased with the increases temperature. Thus, the studied inhibitors
efficiencies are temperature dependent. The immersion temperature of 90 °C gave
the highest values of inhibition efficiency for both inhibitors, which gave 98.24% for
BTA and 92.98% for MBT.
4.4
Adsorption Isotherms and Thermodynamics
The adsorption isotherm experiments were performed to have more insights
into the mechanism of corrosion inhibition, since it describes the molecular
interaction of the inhibitor molecule with the actives sites on the carbon steel surface
(Emeregul and Hayvali, 2006). The degree of surface coverage () was evaluated
from the weight loss measurements, Eq (3.3) (Benabdellah et al., 2007). It is
necessary to determine empirically which adsorption isotherm fits best to the surface
coverage data in order to use the corrosion rate measurements to calculate the
thermodynamic parameters pertaining to inhibitor adsorption. The models considered
were (Bouklah et al., 2006).
Temkin isotherm
exp(f .) = KadsC
(4.1)
Langmuir isotherm
/ (1-) = KadsC
(4.2)
Frumkin isotherm
/ (1-) . exp(f .) = KadsC
(4.3)
And Freundlish isotherm
= KadsC
(4.4)
Where Kads is the equilibrium constant of the adsorption process, C is the inhibitor
concentration and f is the factor of energetic inhomogeneity. The correlation
coefficient (R2) was used to choose the isotherm that best fit experimental data
obtained (Table 4.2).
39
The Langmuir isotherm, Eq. (4.2), can be rearranged to obtain the following
expression:
C/ = (1/Kads) + C
(4.5)
where Kads is the adsorption constant and this constant is related to the standard free
energy of adsorption (Gads) by the equation
ln Kads = ln (1 / 55.5) – (Goads / RT)
(4.6)
the value of 55.5 is the molar concentration of water in the solution expressed in
molarity units (M).
0.06000
0.05000
y = 4.9395x + 0.0006
R2 = 0.9999
0.04000
C/
BTA
MBT
0.03000
y = 1.3001x + 0.0002
R2 = 0.9994
0.02000
0.01000
0.00000
0.00000
0.00200
0.00400
0.00600
0.00800
0.01000
0.01200
C (M)
Figure 4.6: Langmuir isotherm for adsorption of MBT and BTA on carbon steel
surface in 1.0 M HCl at 25 °C.
40
0.01400
0.01200
y = 1.3148x + 1E-05
R2 = 1
C/
0.01000
0.00800
y = 1.0806x + 2E-05
R2 = 1
0.00600
BTA
0.00400
MBT
0.00200
0.00000
0.00000
0.00200
0.00400
0.00600
0.00800
0.01000
0.01200
C (M)
Figure 4.7: Langmuir isotherm for adsorption of MBT and BTA on carbon steel
surface in seawater sample at 25 °C.
Figure 4.6 and 4.7 represents the adsorption plots of BTA and MBT on
carbon steel in 1.0 M HCl and seawater sample respectively, which were obtained by
weight loss measurements. Figure 4.6 and 4.7 shows that the adsorption process
obeys Langmuir adsorption isotherm. This isotherm postulates that there is no
interaction between the adsorbed molecules and the energy of adsorption is
independent on the surface coverage (). Langmuir isotherm assumes that the solid
surface contains a fixed number of adsorption sites and each site holds one adsorbed
species (Ali et al., 2003).
The thermodynamic parameters derived from Langmuir adsorption isotherms
for the studied compounds obtained from weight loss measurements are given in
Table 4.7 and 4.8 respectively for 1M HCl and seawater sample. As it can be seen
from Table 4.7 and 4.8, the addition of inhibitors causes negative values of Gads,
which indicate that the adsorption of studied inhibitors (BTA and MBT) is
spontaneous process (Scendo, 2007, Tang et al., 2006).
41
Table 4.7: Thermodynamic parameters obtained from weight loss measurements for
the adsorption of BTA and MBT in 1.0 M HCl on the carbon steel at 25 °C
Inhibitor
BTA
MBT
K (M-1)
5.0 x 103
1.7 x 103
R2
0.9994
0.9999
G°ads (kJ mol-1)
-31.07
-28.40
Table 4.8: Thermodynamic parameters obtained from weight loss measurements for
the adsorption of BTA and MBT in seawater sample on the carbon steel at 25 °C
Inhibitor
BTA
MBT
K (M-1)
5.0 x 104
1.0 x 105
R2
1.0000
1.0000
G°ads (kJ mol-1)
-36.78
-38.50
Generally, according to Ali et al.,(2008), values of Gads up to -20 kJ mol-1
are consistent with the electrostatic interaction between the charged molecules and
the charged metal, by physisorption while those between -80 and -400 kJ mol-1 are
associated with chemisorption as a result of sharing or transfer of electrons from the
inhibitor molecules to the metal surface to form a coordinate type of bond. The
calculated Gads values in the range of 20-40 kJ mol-1 indicate that the adsorption
mechanism of BTA and MBT were electrostatic adsorption which is by
physisorption (Ali et al., 2008).
4.5
Microstructure Analysis of the Carbon Steel Coupons
The polished carbon steel coupons were immersed in 1M HCl for 24 hours at
25 °C with absence and presence of 10-2 M BTA and MBT respectively. The optical
microstructures of the control carbon steel coupon and the sample coupons after 24
hours immersion in 1.0 M HCl are presented in Figure 4.8. Figure 4.8 (a) shows the
microstructure of the control carbon steel coupon, which is clean and not affected by
corrosion. Figure 4.8 (b) shows the microstructure of carbon steel coupon which was
42
immersed in 1.0 M HCl for 24 hours at 25 °C with absence of inhibitor. It can be
seen that the carbon steel coupon is severely corroded. Figure 4.8 (c) shows the
microstructure of carbon steel coupon which was immersed in 1.0 M HCl for 24
hours at 25 °C with presence of 10-2 M MBT. It can be observed that the coupon
surface is less corroded than in Figure 4.8 (b), which indicates that MBT effectively
inhibited the corrosion of carbon steel in 1.0 M HCl. Figure 4.8 (d) shows the
microstructure of carbon steel coupon which was immersed in 1.0 M HCl for 24
hours at 25 °C in the presence of 10-2 M BTA. It is clearly showed that the coupon
surface was less corroded than those with and without MBT. So, it can be concluded
that both inhibitors have inhibition effect on carbon steel in 1.0 M HCl at 25 °C, but
BTA gave greater inhibition effect than MBT.
43
Figure 4.8: Microstructure of carbon steel coupons (a) control coupon, (b) after 24
hours immersion in 1.0 M HCl, (c) after 24 hours immersion in 1M HCl with 10-2 M
MBT, (d) after 24 hours immersion in 1.0 M HCl with 10-2 M BTA
44
CHAPTER 5
CONCLUSION AND FUTURE WORK
5.1
Conclusion
The main objective of this study to investigate the corrosion inhibition of
carbon steel by MBT and BTA has been achieved. Both inhibitors studied; 1,2,3benzotriazole (BTA) and 2-mercaptobenzothiazole (MBT) have corrosion inhibition
effect on carbon steel in 1.0 M HCl and seawater conditions. The inhibition
efficiency of BTA and MBT increased with increasing inhibitor concentration and
immersion temperature for both conditions applied: 1.0 M HCl and seawater sample
respectively. In 1.0 M HCl and seawater sample, BTA gave better efficiency as
compared to MBT. For the highest inhibitor concentration studied (10-2 M) at room
temperature (25 °C), BTA gave 76.01% while MBT gave 20% efficiency in 1.0 M
HCl and BTA gave 92.40% while MBT gave 76% efficiency in seawater sample.
The increase in immersion temperature has increased the inhibition efficiency of both
tested inhibitors, which gave 87.49% and 30.15 for BTA and MBT respectively in
1.0 M HCl, while for the seawater sample; it gave greater efficiency, 98.24% for
BTA and 92.98% for MBT. For the adsorption isotherm analysis, adsorption of BTA
and MBT on carbon steel coupons obeys Langmuir isotherm in both 1.0 M HCl and
seawater condition. This isotherm postulates that there is no interaction between the
adsorbed molecules and the energy of adsorption is independent on the surface
45
coverage (). Langmuir isotherm assumes that the solid surface contains a fixed
number of adsorption sites and each site holds one adsorbed species. The addition of
inhibitors in 1.0 M HCl and seawater sample causes negative values of Gads, which
indicated that the adsorption of BTA and MBT are spontaneous processes in both
conditions applied. The microstructure analysis of tested coupons had confirmed that
BTA and MBT gave inhibition effect on carbon steel.
5.2
Future Work
Due to time limitation, the current study has been limited to weight loss
method. It is recommended that a further study to be carried out by using
electrochemical methods involving Tafel plots to further support the findings by the
weight loss method. It is also recommended that a further investigation into the
microstructure changes of the corrosion surface of the carbon steel due to the BTA
and MBT corrosion inhibition actions.
46
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