Although corrosion is a natural process, it can be controlled by using effective methods and strategies. There are mainly five primary ways to control corrosion. Thus, the following methods may be adopted for preventing or reducing corrosion:
(a) Materials selection
(b) Design
(c) Coating
(d) Cathodic and anodic
protection
(e) Inhibitors
(a) Material Selection
The most common and
important method of controlling corrosion is the selection of the right and
proper materials for particularly corrosive environments. The corrosion behavior
of each metal and alloy is unique and inherent and corrosion of metal and alloy
has a strong relationship with the environment to which it is exposed. A
general relation between the rate of corrosion, corrosivity of the environment,
and corrosion resistance of materials can be elucidated as follows:
Rate of corrosive attack = Corrosion resistance of metal / Corrosivity of environment ..(1)
The rate of corrosion
directly depends upon the corrosivity of the environment and is inversely
proportional to the corrosion resistance of the metal. Hence, the knowledge of
the nature of the environment to which the material is exposed is essential. Moreover, the corrosion resistance of each metal can be different in
different exposure conditions. Therefore, the right choice of materials in the
given environment (metal corrosive environment combination) is very essential
for the service life of equipment and structures made of these materials. It is
possible to reduce the corrosion rate by altering the corrosive medium. The
alteration of the corrosive environment can be brought about by lowering the temperature,
decreasing velocity, removing oxygen or oxidizers, and changing concentration.
Consideration of corrosion resistance based on the corrosion behavior of the
material and the environment in which it is exposed is an essential step in all
industries. The alternative materials for some of the corrosive materials are
listed in Table.1.
(a) Pure materials have
fewer tendencies towards pitting, but they are expensive and soft. Therefore,
only aluminum can be used in pure form.
(b) Improved corrosion
resistance can be obtained by adding corrosion-resistant elements. For example,
intergranular corrosion occurs in stainless steel. This tendency can be reduced
by the addition of a small amount of titanium.
(c) Nickel, copper, and
their alloys are used in the non-oxidizing environment, whereas chromium-containing
alloys are used in an oxidizing environment.
(d) Materials that are
close in the electrochemical series should be used for fabrication.
(e) Corrosive materials
are taken with suitable construction material:
Table.1: Corrosive
Materials and Their Alternative Metals
|
Corrosive
material |
Suitable
material |
|
Nitric
acid Hydrofluoric
acid Distilled
Water Dilute
sulphuric acid Caustic |
Stainless
steel Monel
metal Tin
Lead
Nickel |
(b) Proper Design of Equipment
The structural design of
equipment is equally important as that of the choice of construction materials.
It greatly reduces the time and cost associated with corrosion maintenance and
repair. The proper design of equipment or tools made-up of metals and alloys
must consider mechanical and strength requirements along with corrosion
resistance. Prior knowledge about the corrosion resistance of the probable
material and the environment in which it is supposed to function is very
essential for the proper design of any equipment. The most common rule for
design is avoiding heterogeneity. As corrosion frequently happens in dead
spaces or crevices thus it is recommended to eliminate or minimize such areas
while designing. All the components of structures should be designed by
considering their expected service life. If not, the premature collapse of the
component or structure is inevitable and a large sum of money should be spent on
its repair or replacement or can go waste. The everchanging environment during
the different stages of manufacture, transit, and storage as well as the daily
and seasonal variations in the environment in which the components are exposed
should be taken into consideration for its maximum service life. It is highly
important to avoid bimetallic corrosion cells in components by coupling
dissimilar metals. The metals involved in a coupling should be widely separated
in the galvanic series to have a maximum service life of components. Dissimilar
corrosion rate can also be minimized by keeping the anodes as large as possible
in the particular component or location to reduce the current density.
Corrosion using proper design can be minimized in the following conditions:
(a) Design for complete
drainage of liquids.
(b) Design for ease of
cleaning.
(c) Design for ease of
inspection and maintenance.
(d) Direct contact
between two metals should be avoided.
(e) They may be insulated
from one another.
(c) Coating or Lining
The corrosion-resistant
coating may be applied on metal surfaces to improve corrosion resistance. It
also separates the metal from a corrosive environment. Protective coatings are
the most generally used method for preventing corrosion. The function of a
protective coating is to provide a satisfactory barrier between the metal and
its environment. Coatings can be broadly classified as metallic coatings,
inorganic coatings, and organic coatings. Usually, an anticorrosive coating
system is multifunctional with multiple layers with different properties. A
typical multifunctional coating can provide an aesthetic appearance, corrosion
control, good adhesion, and abrasion resistance. The functioning of any
protective coatings is based upon barrier protection, chemical inhibition, and
galvanic (sacrificial) protection mechanisms. The metals and alloys can be
completely isolated from their environment to achieve barrier protection.
Protection of metals through chemical inhibition is achieved by adding
inhibitor molecules into the coating system. An active metal is coated on the
surface of the metal to achieve sacrificial or galvanic protection.
(d) Cathodic and Anodic Protection
Cathodic protection is an
electrochemical way of controlling corrosion. The object to be protected is the
cathode. Cathodic protection is achieved by suppressing the corrosion current
in a corrosion cell and by supplying electrons to the metal to be protected.
The principle of cathodic protection can be explained with the help of a
typical corrosion reaction of a metal ‘M’ in an acid (H+) medium.
For example, consider an
electrochemical reaction in which metal dissolution and hydrogen evolution are
taking place;
M → Mn+ + ne …
(2)
2H+ + 2e → H2
…(3)
Equations (2) and (3)
show that the addition of electrons to the structure would reduce the metal
dissolution and increase the rate of evolution of hydrogen.
Cathodic protection of a
structure can be achieved by an external power supply and appropriate galvanic
coupling. Cathodic coupling by galvanic coupling is realized by using active
metal anodes, for example, zinc or magnesium, which are connected to the
structure to provide the cathodic protection current. In this case, the anode
is called a sacrificial anode, since it is consumed during the protection of
the steel structure. In contrast to cathodic protection, anodic protection is
one of the more recently developed electrochemical methods for controlling
corrosion. Anodic protection is based on the principles of passivity and it is
generally used to protect structures used for the storage of sulphuric acid.
The difference between anodic protection from cathodic protection is how the
metal to be protected is polarized. The component that is to be protected is
made as an anode in anodic protection. Since anodic protection is based on the
phenomenon of passivity, metals and alloy systems, which exhibit active-passive
behavior when subjected to anodic polarization, can be protected by anodic
polarization. The corrosion rate of an active-passive metal can be
significantly reduced by shifting the potential to the passive range. Anodic
protection is used to make a protective passive film on the metal or alloy
surface and thereby controlling the corrosion.
(e) Inhibitors
A corrosion inhibitor is
a substance that retards corrosion when added to an environment in small
concentrations. An inhibitor can be considered a retarding catalyst that
reduces the rate of corrosion. The mechanism of inhibition being complex is not
yet well understood. It is established that inhibitors function in the following
way:
(a) Adsorption of a thin
film on the corroding surface of a metal;
(b) Forming a thick
corrosion product, or
(c) Changing the
properties of the environment and thereby slowing down the corrosion rate.
Corrosion inhibitors can
be broadly classified as passivation, organic inhibitors, and vapor phase
inhibitors. The inhibitors can also be classified based on their mechanism of
inhibition and composition. A large number of inhibitors fall under the
category of adsorption-type inhibitors. These are generally organic compounds
and function by adsorbing on anodic and cathodic sites and reducing the
corrosion current. Another class of inhibitors is hydrogen evolution poisons. Arsenic
and antimony are generally used as hydrogen evolution poisons and they
specifically retard the hydrogen evolution reactions.
This type of inhibition
is very effective only in those environments where hydrogen evolution is the
main cathodic reaction and hence these inhibitors are very effective in acid
solutions.
The inhibitive
substances, which act by removing the corrosive reagents from the solution are
known as scavengers. Sodium sulfite and hydrazine are these types of
inhibitors, which remove dissolved oxygen from aqueous solutions. These
inhibitors function very effectively in those solutions where oxygen reduction
is the main cathodic reaction. Oxidizers are also a kind of inhibitor.
Substances such as chromate, nitrate, and ferric salts act as corrosion
inhibitors in certain systems. Generally, they inhibit the corrosion of metals
and alloys that exhibits active-passive transitions.
Inorganic oxidizing
materials such as chromates, nitrites, and molybdates are generally used to
passivate the metal surface and shift the corrosion potential to the noble
direction. Paint primers containing chromate pigments are widely used to
protect aluminum alloys and steel. Inhibitors that are very similar to organic
adsorption types with very high vapor pressure are known as vapor phase
inhibitors. They are also known as volatile corrosion inhibitors (VCI). VCI’s
are secondary-electrolyte layer inhibitors that possess appreciable saturated
vapor pressure under atmospheric conditions and thus allow vapor-phase transport
of the inhibitive substance. These inhibitors are generally placed very near to
the metal surface to be protected and they are transferred by sublimation and
condensation to the metal surface. Hence, these inhibitors can be used to
protect metals from atmospheric corrosion without being placed in direct
contact with the metal surface.
Vapour phase inhibitors
are very successful if they are used in closed packages or the interior of
equipments.
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