Corrosion is an electrochemical process where metal converts from a metallic to an ionic state, requiring electricity flow between anodes and cathodes in the presence of an electrolyte. The process involves oxidation of metal, such as iron, and the evolution of hydrogen gas, as well as interactions that can lead to rust formation through compounds like ferrous and ferric hydroxide. Polarization affects corrosion rates, where cathodic polarization reduces and anodic polarization can increase corrosion rates depending on the metal and environment.

2. Electrochemical Principles of Corrosion
• Corrosion is essentially an electrochemical process resulting in part or
all of the metal being transformed from the metallic to ionic state.
• Corrosion requires a flow of electricity between certain areas of a
metal surface through an electrolyte. An electrolyte is any solution that
contain ions. Ions are electrically charged atoms or group of atoms.
• Pure water , for example, contains positively charged hydrogen ions
and negatively charged hydroxyl ions in equal amounts. Electrolyte
may be plain water, salt water, or acidic or alkaline solutions of any
concentration. To complete the electric circuit, there must be two
electrodes, an anode and a cathode, and they must be connected.
Electrodes may be two different kinds of metals or they may be
different areas on the same piece of metal.
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3. Electrochemical Principles of Corrosion
• The connection between the anode and the cathode may be a metallic
bridge, but in corrosion it is usually achieved simply by contact. In
order for electricity to flow there must be a potential difference
between the electrodes.
• If a piece of ordinary iron is placed in a solution of HCl, vigorous
bubbling of hydrogen gas is observed. On the surface of the metal
there are numerous tiny anodes and cathode areas caused by the
inclusions in the metal, surface imperfections, localized stresses,
orientation of the grains, or perhaps variations in the environment (See
figure).
• At the anode, positive charged iron atoms detach themselves from the
solid surface and enter the solution as positive ions, while the negative
charges, in the form of electrons, are left behind in the metal.
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4. Electrochemical Principles of Corrosion
• At the cathode the electrons meet and neutralize some positively
charged hydrogen ions which have arrived at the surface through the
electrolyte. In losing their charge the positive ions become neutral
atoms again and combine to form hydrogen gas.
• So, as this process continues, oxidation and corrosion of iron occurs at
the anodes, and plating out of hydrogen occurs at the cathode. The
amount of metal which dissolves is proportional to the number of
electrons flowing, which in turn is dependent on the potential and the
resistance of the metal.
• In order for corrosion to continue it is necessary to remove the
corrosion products from the anode and cathode. In some cases, the
evolution of the hydrogen gas at the cathode is very slow, and the
accumulation of a layer of hydrogen on the metal slows down the
reaction. This is known as cathodic polarization. (See figure)
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5. Electrochemical Principles of Corrosion
• However, oxygen dissolved in the electrolyte can react with
accumulated hydrogen to form water, thus allowing corrosion to
proceed. For iron and water, the rate of film removal depends on the
effective concentration of dissolved oxygen in water adjacent to the
cathode.
• The products of anode and cathode processes frequently meet and
enter into further reactions that yield many of our common visible
corrosion products. For example, with iron in water, the hydroxyl ions
from the cathodic reaction in their migration through the electrolyte
toward the anode encounter ferrous ions moving in the opposite
direction. These ions combine to form ferrous hydroxide.
• This soon becomes oxidized by oxygen in solution to form ferric
hydroxide, which precipitates as a form of iron rust. Depending on the
alkalinity, oxygen content and extent of agitation, this rust may form
either way from the iron surface or right next to it, where it can further
influence the corrosion process.
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7. Book explanation

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Book figure
slide figure

9. Polarization and Its Effect on Corrosion
• Polarization is the change of potential from a stabilized state, e.g. from the open-
circuit electrode potential as the result of the passage of current. It also refers to the
change in the potential of an electrode during electrolysis, such that the potential of
an anode becomes more noble, and that of a cathode more active, than their
respective reversible potentials. Often accomplished by formation of a film on the
electrode surface.
• In the context of corrosion, polarization refers to the potential shift away from the
open circuit potential (free corroding potential) of a corroding system.
• For all metals and alloys in any aqueous environment, cathodic polarization always
reduce the corrosion rate. Cathodic protection is essentially the application of a
cathodic polarization to a corroding system.
• For a non-passive system (e.g. steel in seawater), anodic polarization always increases
the corrosion rate. For systems showing active-to-passive transition, anodic
polarization will increase the corrosion rate initially and then cause a drastic reduction
in the corrosion rate. Anodic protection is essentially the application of anodic
polarization to a corroding system.

10. • The cathode is polarized by the hydrogen atoms producing a
film covering the cathode surface. The film affects the process
kinetic: it slows down the reaction between the electrons and
hydrogen ions dissolved in the electrolyte.
• At the equilibrium (non-polarized) state the rates of oxidation
and reduction reactions proceeding at any electrode are
equal. For example the ions of Cu are receiving electrons on
the electrode surface and transfer from the electrolyte to the
copper deposit. In parallel the same number of copper atoms
give up their electrons and dissolve in the electrolyte.
Activation Polarization

11. • Concentration polarization of an electrode is a result of formation of a
Diffusion layer adjacent to the electrode surface where there is a gradient
of the ion concentration.
Diffusion of the ions through the layers controls the electrochemical
reaction (corrosion, Electroplating).
Concentration Polarization

13. Important observations