Abstract
The effectiveness of Terminalia catappa (tropical almond) and Tamarindus indica (tamarind) as
corrosion inhibitors for stainless steel in 1M hydrochloric acid at 30°C, 40°C and 50°C was
investigated in this research. The gravimetric method of analysis was employed for 20 days at a
4-day interval using 0.9, 1.1 and 1.3g/15mL of Terminalia catappa leaves extracts and 1.4 and
2.3g/15mL of Tamarindus indica leaves extracts. The results showed that the two extracts acted
as good inhibitors for stainless steel. The corrosion rate, surface coverage, inhibition efficiency,
inhibition mechanism and the effects of temperature were analyzed. The results showed that the
highest inhibition efficiency for tamarind was 97.71% at 30°C using 2.4g/15mL, while the
lowest was 87.95% with 2.3g/mL at 30°C.The highest efficiency for tropical almond was
97.89% with 1.3g/mL at 50°C and the lowest was 88.7% with 0.9g/mL at 50°C. The isotherms
showed that tropical almond acted as a mixed type inhibito
1.1 PROBLEM STATEMENT
Metals are used in many industries and they come in contact with a lot of chemicals. The oil
industry especially makes use of metals in different applications, which include, pipeline
construction, oil tankers, oil well engineering, oil rigs construction, etc. Metal surfaces stand a
risk of being attacked by acids used in cleaning them; this leads to corrosion. Corrosion has a lot
of effects in different areas in our lives; these areas include economic, safety and environmental.
Economically, a lot of cost are put into consideration and accounted for due to corrosion.
Corrosion could wear out a machine and could render it useless when not discovered on time.
The machine will then have to be replaced adding to the costs incurred. Costs are also incurred as
regards to controlling corrosion, by either maintaining the machine or repairing it, or specially
designing the machine to resist or withstand attacks by corrosive media1
.In safety, corrosion
poses a great threat to human life, aquatic life and the life of other animals. For example, the
corrosion of iron hulls in ships and the subsequent effects poses a serious threat to the lives of the
people aboard.
The corrosion of oil pipelines poses a big threat to the people living in that area, as the
subsequent breakdown and explosion of the pipeline could destroy lives. Also, corrosion of drill
pipes could cause a breakage and explosion, leading to loss of lives and injury. Breakdown of
bridges due to corrosion has caused the loss of lives and property1
. Furthermore, airline accidents
due to corrosion have caused the loss of many lives and people have lost their jobs due to loss of
confidence by customers. A popular example is that of the Aloha airlines Boeing 737 air accident
in 1988 where the plane lost a major portion of the upper fuselage in full flight at 24000 ft. above
the ground. Although only one loss was recorded, several injuries were also recorded2
.
2
Corrosion has many effects on the environment with the fact that corrosion related failure of
pipelines and oil tanks can cause serious problem for the environment as regards the pollution of
lands, water and the air. Oil spills due to corrosion have caused loss of vegetation and aquatic
lives. Many have lost their farmlands and means of livelihood due to corrosion related oil spill
and other activities1
.
In this research, a comparative study was carried out with tamarind and tropical almond leaves
extracts on the corrosion inhibition of stainless steel in 1M hydrochloric acid with focus on
maximizing the inhibition efficiency of the inhibitors.
1.2 MOTIVATION
Corrosion affects the integrity of materials, especially metals used in different industries. The oil
industry is one of the most important industries in Nigeria, with oil being its main source of
revenue. It is known that the industry makes use of metallic materials either as pipelines, gas
tanks, oil rigs, or in well engineering. Ships and boats used in transporting materials offshore are
also made using metals.
At the Shell Nigeria Exploration and Production Company (SNEPCo) dedicated base, it
discovered that due to non-availability of space inside their warehouse some of their materials
are being kept outside and get beaten by rain and sunshine. These materials get attacked by the
extreme conditions and often corroded. The company thus invests a lot of time and money into
keeping the materials in shape, by cleaning them and repainting them. Also, the company had to
off-hire vessels because corrosion and other minor damages had made them unfit for sail. If the
maximum efficiency is obtained in this research, the company could have an option of using
cheap, easily available, and long-lasting inhibitors to preserve their metal equipment.
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1.3 AIM AND OBJECTIVES
The aim of this project was to use the predicted concentrations of Tamarindus indica and
Terminalia catappa leaves extracts to obtain maximum inhibition efficiencies of Tamarindus
indica and Terminalia catappa leaves extracts on corrosion of stainless steel in 1 M HCl. The
objectives of this project were:
1. To extract the Tamarindus indica and Terminalia catappa inhibitors from their leaves.
2. To study corrosion inhibition of stainless steel in 1M HCl using 0.9g/15mL, 1.1g/15mL,
1.3g/15mL, concentrations of the Terminalia catappa extracts at 30?C, 40?C, and 50?C
temperatures.
3. To study corrosion inhibition of stainless steel in 1M HCl using 2.4g/15mL, 2.3g/15mL,
5.4g/15mL concentrations of Tamarindus indica extracts at the same temperatures as Terminalia
catappa.
4. To determine the inhibition efficiency of both leaves extracts on stainless steel in 1M HCl.
5. To determine the effects of temperature on the inhibition efficiency of the extracts and to
calculate the adsorption kinetics of the extracts.
1.4 SCOPE OF PROJECT
This research was focused on the following scopes of study:
CHAPTER 1
This chapter looks into the introduction of the topic, the aim and objectives of the research, the
problem to be solved in the research, the overview of the research, and motivation.
CHAPTER 2
This chapter examines different articles and researches on corrosion, its sources, and various
ways that have been applied in the past and potential inhibition strategies by others. Factors
4
which affect the selection of a particular type of technique and inhibitor, for instance in what
media, which inhibitor works best in, acidic, alkaline, saline or neutral. The inhibition
mechanism is also covered here.
CHAPTER 3
This chapter deals with the materials/reagents used in carrying out the research and the procedure
used in successfully carrying out the experiment. It also includes the different formulas and
methods used in analyzing the results.
CHAPTER 4
This chapter contains the results and discussion on the weight loss measurements for the metals.
These results include values from the calculations of the inhibition efficiency, corrosion rates,
surface coverage, adsorption kinetics, and temperature effects. The graphs that compare these
terms for the two extracts used are also shown.
CHAPTER 5
In this chapter, a summary of the whole research is discussed, the importance of the research is
also emphasized and recommendations for improving this area of research are given as well.
1.5 RESEARCH OVERVIEW
1.5.1 Corrosion
Corrosion is the spontaneous oxidation or deterioration of the surface of a metal through the
effects of its interaction with its environment. Corrosion was obtained from the Latin word
“corrodere”, which means to “chew to pieces” or “eat away”. Mostly corrosion occurs as an
electrochemical mechanism where the metal is oxidized and the oxidizing agent reduced
simultaneously. Corrosive environments could be water, air, carbon dioxide, salt solutions,
5
sulfur; and rarely neutron beams, ultraviolet light, nuclear fission fragments and gamma
radiation3
.
1.5.2 Types of corrosion
Corrosion exists in two types: dry or direct chemical corrosion, and wet or electrochemical
corrosion.
1.5.2.1 Dry or direct chemical corrosion
This type of corrosion occurs when there is a direct chemical action of atmospheric gases such as
oxygen, halogens, hydrogen sulphide, carbon dioxide, oxides of sulphur, nitrogen, and hydrogen,
or molten metals on metal’s surface in the absence of moisture4
.
Corrosion by oxidation: This type of corrosion happens due to attack on metals by oxygen either
at low or high temperatures in the absence of moisture. Alkali and alkaline earth metals easily
get oxidized at low temperatures, while other metals except gold, silver and platinum get
oxidized at high temperatures. The metals form either stable oxides, which prevent further
corrosion, or unstable oxides, or volatile oxides, which volatilize and leave fresh surface for
attack4
.
Corrosion by hydrogen and other gases: Steel when exposed to hydrogen gas at high temperature
helps the formation of hydrogen atom, the hydrogen atom then reacts with carbon in the steel to
form methane and the methane exerts pressure on the steel causing cracking. On the other hand,
when chlorine reacts with some metals, it forms either protective films or volatile films. With
silver, chlorine reaction forms a protective film, while it forms a volatile (non-protective) film
with tin. Also, hydrogen sulphide gas forms porous films with steel4
.
Liquid metal corrosion: This occurs as a result of attack on a solid metal or alloying by a molten
metal at high temperature. The corrosion reaction is either by dissolution of a solid metal by a
6
molten metal or the molten metal penetrates into the solid metal, causing the metals to weaken.
This type of corrosion occurs in nuclear powers plants4
.
1.5.2.2 Electrochemical or wet corrosion
For electrochemical corrosion to occur there must be contact between a metal and a conducting
liquid, or when alloys/metals with different potentials, which have been joined together are
dipped in a solution. Anodic and cathodic areas are created due to this; there is a transfer of
electron-current between the anodic and cathodic sides. The anodic area is oxidized (corroded),
while the cathodic area is protected. A corrosion product is then formed at an area between the
anodic and cathodic sides. Electrochemical corrosion could either be in form of
galvanic/bimetallic corrosion or differential cell corrosion4
.
Galvanic corrosion: This occurs when two dissimilar soldered metals come in contact with an
electrolyte. The metal with a lower potential/higher in the electrochemical series acts as the
anode and corrodes, while the one lower in the electrochemical series acts as the cathode and is
protected. Example, when copper and zinc are joined and exposed to a conducting liquid; zinc
being higher in the electrochemical series acts as the anode and corrodes, while copper acts as
the cathode4
.
Differential cell/aeration corrosion:
This type of corrosion occurs when the surface of a metal is attacked when exposed to varying
concentrations of an electrolyte. Also, when the metal surfaces are exposed to different air
concentrations difference in potential between these areas occur. For example, when a metal is
not completely immersed in an electrolyte, the part above the electrolyte is more exposed to
oxygen (aerated) and becomes cathodic. On the other hand, the part in the solution becomes the
7
anode, since it is less aerated, therefore suffering corrosion. Examples of cases under this form of
corrosion include pitting corrosion, water line corrosion and crevice corrosion5
.
Pitting corrosion is the formation of pin-holes on the metal surface as a result of drops of water,
sand, dirt and dust. These impurities create a concentration cell of large cathodic area and small
anodic area on the surface of the metal. The area covered by the dirt and water drops, are the
anodic areas, while the clean sides are the cathodic areas5
.
In water-line corrosion, the differential cell is set at the areas above the water line and below the
water line. The area above the water line has more access to oxygen and thus acts as the cathode,
while the area just below the water line has insufficient oxygen and acts as the anode. This part
suffers corrosion5
.
Crevice corrosion is another type of differential corrosion in which metallic areas under space,
like screws trap dirt or electrolyte in them and are corroded. The trapped area acts as the anodic
side due to insufficient oxygen, while the exposed part act as the cathode5
.
1.5.3 Corrosion control
Different methods have been used to control corrosion depending on the varied conditions and
types of corrosion. These methods include properly designing the metal so that two dissimilar
metals are not used in a corrosive environment, using metals close to each other in the
electrochemical series, use of highly pure metals, the use of organic and inorganic coatings and
paints and electroplating. Cathodic protection, a process in which the metal is forced to act as a
cathode in an electrolytic cell eliminating the anode could be used, anodic protection, a process
where the metal is forced to act as the anode could also be used. The most recent form of
corrosion control, although it has been in existence for decades is the use of inhibitors6
.
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Corrosion inhibitors are organic or inorganic chemical substances which are effective in small
concentrations when added to an aggressive environment to reduce the rate of corrosion7
.
1.5.3.1 Inorganic inhibitors
Inorganic inhibitors are anodic or cathodic in nature. Anodic inhibitors inhibit corrosion by
forming a non-soluble compound with produced metal cations, which then adsorbs on the surface
of the metal to form a layer of passive film on the corroding surface of the metal. They are often
used to repair cracks of oxide films or porous oxide films on metal surfaces, and pitting
corrosion. Examples of inorganic inhibitors are Chromates, phosphates, tungstate, nitrates, and
molybdates etc7
.
Cathodic inhibitors on the other hand are classified based on the nature of the cathodic reaction,
because the cathodic reactions differ for different media. In an acidic medium, the main reaction
is the liberation of hydrogen gas. This reaction is controlled by slowing down the rate at which
the liberated gas diffuses through the cathode. Examples of inhibitors for this type of reaction
include amines, mercaptans, thiourea, etc. meanwhile, in a neutral medium; the cathodic reaction
is the formation of hydroxyl ions. Corrosion here is controlled by either eliminating oxygen from
the medium, or slowing down its diffusion rate to the cathode. Oxygen can be eliminated by
addition of reducing agents like sodium trioxosulphate (IV), Na2SO3 or hydrazine, N2H4. Also,
the rate of diffusion of oxygen is controlled by using magnesium, zinc or nickel salts4
.
1.5.3.2 Organic inhibitors
Organic inhibitors possess heteroatoms, such as O (oxygen), N (nitrogen), P (phosphorus) and S
(sulphur). They have high basicity and electron density, therefore making them viable as
corrosion inhibitor. They are the active centers for the process of adsorption on the metal surface.
The order of their inhibition efficiency follows the sequence O < N < S < P. Availability of non-
9
bonded (lone pair) and pi-electrons in inhibitor molecules enables electron transfer from the
inhibitor to the metal; therefore, forming coordinates covalent bond involving transfer of
electrons from inhibitor to the metal’s surface. The strength of the chemisorption bond depends
on the electron density on the donor atom of the functional group and the polarizability of the
group. When an H atom is attached to a C atom in the ring, it is replaced by a substituent group
(–NH2, –NO2, –CHO, or –COOH) which then improves inhibition. Chain length, size of the
molecule, bonding, aromatic/conjugate, strength of bonding to the substrate, cross-linking
ability, and solubility in the environment, are some factors that contribute to the efficiency of
inhibitors. Examples are the green organic inhibitors; they are natural inhibitors from plants, they
are biodegradable and contain no toxic compounds7
.
Tamarind (Tamarindus indica): Tamarind belongs to the family of legumes known as Fabaceae.
It is mostly found in tropical regions such as Africa, some part of Asia like India, Pakistan and
Bangladesh. It is mostly used in traditional medicine for treating different diseases like asthma,
ulcer, tuberculosis, wounds, diabetes and stomach upsets. Reports have shown that the seeds of
tamarind possess anti-ulcer, anti-asthmatic, anti-diabetic, and anti-oxidant activities. This is due
to their richness in phenolic compounds, polymeric tannins, fatty acids, flavonoids, saponins,
alkaloids and glycosides8
.
Tropical almond (Terminalia catappa): Tropical almond is a tropical plant, as the name implies,
and is mostly found in Southeast Asia, Sub-Saharan region of Africa and other subtropical areas.
The tree is grown for ornamental purposes and its fruits and nut kernel are edible. A research
states that juice extracted from its fresh leaves is used to prepare medical lotion for scabies and
leprosy. The seeds, bark and leaves have been known to contain carbohydrates, proteins and
vitamins in large amounts. Its leaves, seeds and barks have also been investigated for antioxidant
activities9,10
.
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1.5.4 Stainless steel
Stainless steel is a family of corrosion resistant steels. Although steel is an alloy of iron and
carbon, stainless steel has very low carbon content (less than 0.03%). Compared to other types of
steel, like carbon steel or mild steel, stainless steel possesses several properties that make it
superior. It has a higher corrosion resistance, due to the presence of chromium which gives it a
self-healing passive oxide layer. It has lower maintenance and is very attractive. Some grades of
stainless steel are very strong at either really high or low temperatures. The self-healing property
of stainless steel remains intact even when it is damaged or cut. The different grades of stainless
steel are the austenitic, ferritic, duplex and martensitic. Austenitic stainless steel, used in this
research is made up of 16-20% Cr, 6-12% Ni, 0.03%C, 2%Mn, 0.75%Si, 0.045%P, 0.03%S, 2-
3%Mo, 0.1%N. Stainless steel is used in the oil industry for constructing chemical tanks, heat
exchangers, oil pipelines, etc. Austenitic stainless steel easily gets corroded in the presence of
strong oxidizing species like chlorides, (HCl and NaCl)11
.
1.5.5 Hydrochloric acid
This is a chemical solution comprising of hydrogen chloride and water. It is also found in the
gastric (stomach) acid. It has a pH of 3.01 and dissociates completely in water to give chloride
ion, Cl?
and hydroxonium ion, H3O
+
, reason why it is being called a very strong acid. It is used in
various industries for food processing, industrial waste water treatment, and in cleaning rust on
metals before further processing. In the oil industry, it is mainly used in oil-well acidizing and
fracking. Chloride ion being a strong oxidizing agent attacks the surface of metals readily,
destroying them12