TABLE OF CONTENTS
1.1 Scope of
and Benefits of the Study
That Influence Corrosion
factors related to metal:
factors related to environment:
1.4 Types of
1.4.1 Causes of
for corrosion prevention can be categorized into six basic
2.2 Uses of
Of Corrosion Inhibitors
considered in applying inhibitors
3.1 Aims and
4.0 RESULTS AND DISCUSSION
1.0 Background of Study
Corrosion is an irreversible interfacial reaction of
a material (metal, ceramic, and polymer) with its environment which results in
consumption of the material or in dissolution into the material of a component
of the environment. Corrosion could also be defined as the degradation
of materials’ properties due to interactions with their environments, and
corrosion of most metals (and many materials for that matter) is inevitable of
the 105 known chemical elements, approximately eighty are metals , and
about half of these can be alloyed with other metals, giving rise to more than
40,000 different alloys. Each of the alloys will have different physical,
chemical, and mechanical properties, but all of them can corrode to some
extent, and in different ways .Corrosion eventually causes failure of components and systems both in the processing andmanufacturing
industries and in the service life of many components. Corrosion control of
metals and alloys is an expensive process and industries spend huge amounts to
control this problem.We have all seen corrosion and know that the process
produces a new and less desirable material from the original metal and can
result in a loss of function of the component or system. The corrosion product
we see most commonly is the rust which forms on the surface of steel.
Figure 1: STEEL RUST
Corrosion is a natural
phenomenon. When newly made steel is first exposed to the air, its originally
shiny surface will be covered with rust in a few hours. The tendency of metals
to corrode is related to the low stability of the metallic state. Metals occur
either in the pure metallic state, the zero oxidation state, or in the
form of compounds with other elements (they acquire positive states of
oxidation. The chemical reactions that take place in corrosion processes are reduction-oxidation (redox)
reactions. Such reactions require a species of material that is oxidized (the
metal), and another that is reduced (the oxidizing agent). Thus the complete
reaction can be divided into two partial reactions: one, oxidation; the other,
reduction. In oxidation, the metal loses electrons. The zone in which this
happens is known as the anode. In the reduction reaction, the oxidizing agent
gains the electrons that have been shed by the metal, and the zone in which
this happens is the cathode.[3-5]
Corrosion processes not only
influence the chemical properties of a metal but also generate changes in its
physical properties and its mechanical behavior. This is why the effects of
corrosion are manifested in a variety of forms. The most common form is uniform
corrosion, whereby there is a generalized, overall "attack" of the
entire exposed surface of the metal, leading to a more or less uniform
reduction in the thickness of the affected metal.
processes affect many areas of human activity in which metal products are used.
In general, as levels of economic development increase, so do costs incurred as
a result of corrosion. It is estimated that the costs attributable to the
corrosion of metallic materials amount to 4 percent of the gross domestic
product of the developed countries. And this cost, representing a loss of
resources, would be even higher if methods of protection against corrosion were
not so widely applied. It is estimated that because of this protection, populations
are able to reduce these potential losses by a factor of about 30 percent.
1.1 Scope of Study
the outcome of the project can be applied in nearly all sectors, in the oil and
gas industry which have corrosion problem, the scope of this project as have
been limited to Corrosion/Corrosion Inhibitors.
Thus, corrosion detection and control or reduction techniques have been
discussed in detail. Also, within the scope of the project is the overview of
the different types of corrosion and corrosion inhibition and the terrain they
Significance and Benefits of the Study
failures can result to personal injuries, fatalities and cost billions of
dollars, through spontaneous shutdown and environmental contamination. Hence,
the critical need for better methods to monitor the actual deterioration of a
component once it is placed in service in a corrosive environment, analyse that
information, and, based on decision-making reasoning, provide a reasonable
forecast of the time remaining before maintenance or replacement becomes
necessary. Through corrosion inspection
and monitoring, the condition of the metals works can always be ascertained and
proper corrosion control and maintenance strategies put in place.
catastrophic incidences resulting from corrosion failure had been historically
recorded. Corrosion costs the oil and gas industry tens of billions of dollars
in lost income and treatment costs every year. The total annual cost of
corrosion in the oil and gas production industry is estimated to be $1.372
billion, broken down into $589 million in surface pipeline and facility costs, $320
million in capital expenditures related to corrosion. Corrosion costs US
industries alone an estimated $170 billion a year in which the oil and gas
industry takes more than half of these costs
1.3 Factors That Influence Corrosion
nature and extent of corrosion depend on the metal and the environment. The
important factors which may influence the corrosion process are:
1.3.1 Primary factors related to metal:
Nature of the metal:
The tendency of a metal to undergo corrosion is dependent on the nature
of the metal. Metals with lower reduction potential undergo corrosion easily whereas
metals with higher reduction potential do not undergo corrosion easily. The
reactive metals like Na, K, Mg, Zn are more susceptible for corrosion. The
noble metals like Ag, Au, Pt, Pd are less susceptible for corrosion.
Surface state of the metal:
surface phenomenon, larger the surface area or finer the grain size of the
metal, more will be the corrosion. Smooth surface resist corrosion than the
rough surface. Due to ups and downs on the rough surface, there will be
formation of large number of air concentration cells with anodic and cathodic
regions. Hence the metal suffers corrosion.
Nature of the corrosion product:
It largely decides the rate of corrosion. If the corrosion product is
insoluble, stable, uniform and nonporous, it acts as a protective film
preventing the further corrosion. If the corrosion product formed is soluble, unstable,
porous and non-uniform, the corrosion continues.
Hydrogen over voltage:
hydrogen over voltage of metal is low, it is more susceptible for corrosion.
When the cathodic reaction is of hydrogen evolution type with lower hydrogen
over voltage, hydrogen gas is evolved easily and thus cathodic reaction is
faster and corrosion of metal becomes fast. In metals with higher hydrogen over
voltage, cathodic reaction is slow and corrosion of metal becomes slow.
1.3.2 Secondary factors related to environment:
pH of the medium:
lower the pH higher is the rate of corrosion. If the pH is greater than 10,
corrosion of iron is very less due to the formation of protective coating of
hydrous oxides of iron. If pH is between 10 and 3, then presence of oxygen is
essential for corrosion of iron. If the pH is 3 or lower than 3 severe
corrosion occurs in the absence of air due to the continuous evolution of H2
at cathode. However metals like Al, Zn etc. undergo fast corrosion in highly
temperature increases, rate of corrosion also increases. This is because
increase in temperature increases the conductance of the aqueous medium .As a
result rate of diffusion also increases.
Presence of oxidizing agents:
The presence of oxidizing agents increases the corrosion rate of the
metal. Even noble metals undergo corrosion in the presence of oxidizing agents.
Most of the metals corrode faster in a humid atmosphere than in dry air.
There is a particular value of humidity called critical humidity above which
corrosion rate steeply increases. Humidity (moisture) provides conducting
medium which helps in
Formation of electrochemical cell on the
Dissolution of gases like O2, CO2,
SO2etc.That help in corrosion.
Presence of impurities in the
Presence of impurities like SO2, HCl in the environment
increases the rate of corrosion due to acidic conditions created by their
dissolution. For example, when SO2 is present as impurity in the
atmosphere, it combines with moisture or rain water forming sulphuric acid. In
the presence of an acid metals like iron are more susceptible for corrosion.
Conductance of the medium:
Presence of conducting species in the atmosphere increases the rate of
corrosion. This is because, higher the conductivity of the medium, faster the
ions can migrate between anodic and cathodic regions of the corrosion cell, in
turn faster will be the exchange of electrons at the electrode surfaces.
Therefore, corrosion problem is more in the sea water than in fresh water.
Smaller the anodic area, larger the cathodic area, faster will be the
rate of corrosion and conversely, larger the anodic area, smaller the cathodic
area, slower will be the rate of corrosion. This is because electrons liberated
at anode (smaller the anodic area) are consumed quickly by the large cathodic
area and hence, the rate of corrosion will be more.
8. Polarization at anodic and cathodic
of cathode or anode decreases the rate of corrosion. If anodic polarization
takes place due to some reaction, then tendency of metal to undergo oxidation
decreases hence dissolution of metals as metal ion decreases. This is usually
due to increase in concentration of ions of the dissolved metals in the
vicinity of electrode or also due to the anodic passivity.
polarization decreases the cathodic reaction hence hindering the combination of
cathode reactant and electron. For the corrosion to continue both anodic and
cathodic reaction should take place simultaneously if any one reaction is
slower than the rate of corrosion is slower. Use of depolarizes reduces the
polarization effect hence the rate of corrosion reaction increases.
Types of Corrosion
are different types of corrosion which depend on the environment surrounding
the material, type of material, chemical reaction etc. Some general types of
corrosion are described below.
is also called General corrosion. It is
a very common method of corrosion. It deteriorates the whole surface of the
metal and makes the surface thin. The damage is done at a constant rate on the
entire surface. It can be easily detected by its appearance. It can be
controlled but if it is not, it then destroys the whole metal.
type of corrosion occurs with an electrolyte like seawater. Metals have
different values of electrical potentials. When they become electrically
connected and put in an electrolyte, the more active metal which has a high
negative potential becomes the anode. Due to its high negative potential, it
corrodes fast. But the less active metal becomes the cathode.
flow of electric current continues till the potentials are equal between both
electrodes. So at the joint where the two non-similar metals meet, the galvanic
corrosion appears. The Galvanic Series shows the list of metals from the most
active to the least active (most noble). Thus galvanic corrosion can be
controlled by selecting the two metals which are close in series. As platinum
is the least active, it is also less active for corrosion.
occurs because of random attacks on particular parts of the metal's surface.
This makes holes which are large in depth. These holes are called
"pits". The pit acts as the anode while the undamaged part of the
metal is the cathode. It begins with a chemical breakdown in the form of a
scratch or spot. The pitting process makes the metal thinner and increases
fatigue. For example, it can be very harmful in gas lines.
occurs in the presence of a corrosive environment like saltwater. It is a
combination of cyclic stress and corrosion. Corrosion fatigue is produced when
a metal breaks at a stress level which is lower than its tensile strength. It
is strongly affected by the environment in which the metal resides which
affects the initiation and growth rate of the cracks. These cracks are too fine
to detect easily. So the stress coupons (metal sample) are used to detect the
can be produced by the influence of various types of stress like stresses
applied, thermal expansion, thermal contraction, welding, soldering, cleaning,
heating treatment, construction process, casting etc. To prevent corrosion
fatigue, the designing and construction process of the materials should be done
properly, by eliminating any stress and environmental factors and by
the granular composition of metals and alloys, grains (small crystals) are
present and their surfaces join with each other. This forms the grain
boundaries. Thus the grains are separated by grain boundaries. Intergranular
corrosion is also known as inter crystalline corrosion. The Intergranular
corrosion is developed on or near the grain boundaries of a metal. This can be
due to welding, stress, heat treating or improper service etc. The metal can lose
its strength due to the Intergranular corrosion.
is also known as concentration cell corrosion. This is due to the trapping of
liquid corrosive between the gaps of the metal. As the electrolyte has
aggressive ions like chlorides, the corrosion reaction is started after
settling of liquid in gaps. Oxygen is consumed during the reaction.
Thus an anodic area is developed near the oxygen-depleted zone while the
external part of the material acts as a cathode. Crevice corrosion is similar
to pitting corrosion. It’s very difficult to detect crevice corrosion. It can
be initiated by materials like gaskets, fasteners, surface deposits, washers,
threads, clamp etc.
is a type of concentration cell corrosion. This develops on coated metallic
surfaces with a thin organic film. The corrosion generates the defect on the
protective coating of metallic surface. The filaments of corrosion product are
the cause of degradation of the coating. The filaments look like thin threads.
They exist as long branching paths.
actively growing filaments do not intersect the inactive filaments. The reflection
process takes place when filaments collide with each other. Filiform corrosion
is a very specific process because it only affects the surface’s appearance,
not the metallic material.
is also called flow-assisted corrosion. This is due to the movement of
corrosive liquids on metal surface which damages the material. It can be seen
in ship propellers which are constantly exposed to sea water or in soft alloys.
The damage can be seen as waves or rounded holes etc. It shows the flow of the
corrosive liquid. It can be controlled by the use of hard alloys, managing the
velocity and flow pattern of the fluid.
is a form of erosion-corrosion. It shows as the combined effect of corrosion
and fretting of metal. Due to this corrosion, the material surface starts to
disappear. Fretting corrosion exists in the form of dislocations of the surface
and deep pits. Oxidation is the main cause of fretting corrosion. It can be
controlled by using lubricates, controlling movement etc.[8-9]
Causes of Corrosion
below are some of the factors that cause corrosion.
- Reactivity of metal
- Presence of impurities
- Presence of air, moisture, gases like
SO2 and CO2
- Presence of electrolytes.
CHEMISTRY OF CORROSION
Common structural metals are obtained from their ores
or naturally-occurring compounds by the expenditure of large amounts of energy.
These metals can therefore be regarded as being in a metastable state and will
tend to lose their energy by reverting to compounds more or less similar to
their original states. Since most metallic compounds, and especially corrosion
products, have little mechanical strength a severely corroded piece of metal is
quite useless for its original purpose. Virtually all corrosion reactions are
electrochemical in nature, at anodic sites on the surface the iron goes into
solution as ferrous ions, this constituting the anodic reaction. As iron atoms
undergo oxidation to ions they release electrons whose negative charge would
quickly build up in the metal and prevent further anodic reaction, or
corrosion. Thus this dissolution will only continue if the electrons released
can pass to a site on the metal surface where a cathodic reaction is possible.
At a cathodic site the electrons react with some reducible component of the electrolyte
and are themselves removed from the metal. The rates of the anodic and cathodic
reactions must be equivalent according to Faraday’s Laws, being determined by the total flow of electrons from
anodes to cathodes which is called the “corrosion current” Since the corrosion current
must also flow through the electrolyte by ionic conduction the conductivity of
the electrolyte will influence the way in which corrosion cells operate. The
corroding piece of metal is described as a “mixed electrode” since simultaneous
anodic and cathodic reactions are proceeding on its surface. The mixed
electrode is a complete electrochemical cell on one metal surface.
2. Overview of Corrosion process
Rust (Hydrated ferric oxide)
process is composed of three elements: an anode, a cathode, and an electrolyte.
The anode is the site of the corroding metal, the electrolyte is the corrosive
medium that enables the transfer of electrons from the anode to the cathode,
and the cathode forms the electrical conductor in the cell that is not consumed
in the corrosion process. The most
common form of corrosion in the oil and gas industry occurs when steel comes in
contact with an aqueous environment and rusts. When metal is exposed to a
corrosive solution (the electrolyte), the metal atoms at the anode site lose
electrons, and these electrons are then absorbed by other metal atoms at the
cathode site. The cathode, in contact with the anode via the electrolyte,
conducts this exchange in an attempt to balance their positive and negative
charges. Positively charged ions are released into the electrolyte capable of
bonding with other groups of atoms that are negatively charged.[11-13]
anodic reaction for iron and steel is
dioxide (CO2), hydrogen sulphide (H2S), and free water
which are highly corrosive mostly present in oil and gas in the reservoir and are thus corrosive media in oil and gas
wells and pipelines.
Figure 3: Corrosion
A corrosion prevention method is
a technique used to minimize corrosion such as the application of
anti-corrosion coating, cathodic protection or other methods that make metal
resistant to corrosion. Metal corrosion in almost all situations can be slowed,
managed or even put to a stop utilizing the right techniques. These preventive
methods are available in many types, and are deployed depending on the
instances involved in a corroding metal.
Corrosion protection methods are
also known as corrosion control. Corrosion can be prevented by having a good
understanding of the conditions that contribute to corrosion. Making the right
decision in the kind of metal to use could also lead to a significant reduction
in terms of corrosion. Proper observation and eradication of defenseless
surface conditions can be crucial in protecting metals from the deteriorating
effects of corrosion.
1.5.2 Techniques for corrosion prevention
can be categorized into six basic groups:
Surface condition and metal selection
Protective coatings can also be
used to give protection to metals from certain factors like the damaging effect
of gases in the environment. Such coatings can be powder, water soluble or
In cases of galvanic corrosion,
cathodic protection systems are widely employed. This is the method used most
often when two dissimilar metals are in a corrosive environment or electrolyte
such as seawater. This type of corrosion protection converts undesirable anodic
sites found on the surface of the metal to cathodic sites. Plating or metallic
coating can also be applied not only to preserve aesthetic finishes, but to
inhibit corrosion as well. Coating can be applied by:
In other cases, the use of corrosion inhibitors is necessary.
These are chemicals that produce a reaction on the surface of the metal or the
environment, which stops the chemical action leading to corrosion.
- Mechanical plating
- Hot dipping
- Electroless plating
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