Monitoring Corrosion

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Report on Monitoring and Limiting Corrosion in Marine Environments.
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Part One: Monitoring and Limiting Corrosion in Marine Environments.
Introduction
Corrosion is a steady deterioration of metallic material by a chemical reaction with the
surroundings.
Corrosion oxidizes the metal into a more chemically stable form; the stable chemical
form could be its oxide, chloride, hydroxide, or sulphide. The corrosion of metals involves bond
formation with the nonmetals like oxygen and release of energy in the form of heat, hence
increasing the stability of the formed compounds (Talbort D.E & Talbort J.D., 2018). The
chemical process occurring entails the interaction of chemicals to form new compounds with
different compositions. Another mechanism which causes corrosion is the electrochemical
transfer of electrons between metals with a different potential difference in the presence of an
electrolyte. Corrosion of the less reactive metal will occur as a result of electrolytic reaction
which will develop
Oxidation reaction chemically
4 Fe (s) + 3O2 (g) → 2Fe2O3 (s)
Oxidation of zinc electrochemically
Zn (s) + 2H+ (aq.) → Zn2+ (aq.) + H2 (g)
Corrosion by oxidation interferes with the metal composition hence compromising the
structure of the metal. Metallic decay due to corrosion accounts for a major loss of equipment's,
machinery and structures. The destruction of structure in the marine environment has a
significant impact on the environment, which might be catastrophic (Morcillo et al., 2013).

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Exposure of ships and all kind of sea locomotives to harsh sea conditions at sea, these
conditions are high humidity, relatively high temperature, salinity, winds, abrasion due to
movement in saline water and sea disturbance. All the named factors above have a significant
effect on the rate of corrosion in sea structures.
Factors Affecting Corrosion.
Humidity
Humidity is the amount of water vapour suspended in the air, water vapour is the gaseous
state of water. There are three ways of measuring humidity; relative, absolute, and specific.
Humidity at sea is high due to the continuous vaporization of the seawater by the sun's heat
during the day, and the relative warmth at night resulting from the heat absorbed by the water
masses during the day.
Vaporization process
H2O (l) + Heat energy → H2O (g)
Water vapour condenses on the metallic surface of the ships or metallic structures late at
night due lagging in temperature as the surrounding air rise, thus making them act as condensers.
Condensation occurs when warm rich with water vapor cools down, causing it to lose its capacity
to hold water.
Condensation process
H2O (g) → H2O (l) + Heat energy

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A film of dew resulting from condensation of water vapour on the surface of the ship
coupled with sea salt or acid sulphates and acid chlorides present in the atmosphere provides a
destructive electric conducting substance known as electrolyte, when the substances dissolve in
water they form anions and cations which facilitate electron transfer during corrosion (Leygraf et
al., 2016). Due to air pollution by industrial plants, chlorides are found in atmospheric gases and
suspended salt in the atmosphere. Chlorides have a corrosive effect on metals and are therefore
considered to be a harmful pollutant. The marine steel structure surfaces which slant horizontally
are affected the most than those which slant horizontally due to the retention of moisture and
atmospheric particle for a longer period (Talbot D & Talbot J., 2018).
The water vapor in the skies absorbs pollutants by reacting chemically to form weak acid
rains. Either way, the condensed water on the metallic surfaces of the structures combines with
the pollutants chemically. An example is Sulphur (IV) oxide gas, which dissolves in water or
water vapour to form a weak sulphuric (VI) acid.
Sulphur (VI) oxide gas dissolves in water form sulphuric acid.
SO3 (g) + H2O (l) → H2SO4 (aq.)
The weak acid reacts with the metallic parts of the structures to form their metallic salt,
which is soluble.
An example is an iron which is a component of steel which react with sulphuric (VI) acid
to give out iron (II) sulphate and hydrogen gas as products.
H2SO4 (aq.) + Fe (s) → FeSO4 (s) + H2 (g)
Aluminum reacts with sulphuric (VI) acid to give out aluminum sulphate and hydrogen gas.

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H2SO4 (aq.) + Al2 (SO4)3 (s) + H2 (g)
This product gets off the metal surface leaving behind a corroded surface which is
subjected to further corrosion, hence, depletion of the metal with time.
Pollutants
Pollutants are substances that pollute the environment by introducing undesirable
outcomes. They can be in solid, liquid, or gaseous form. These are either gases from motor
vehicles, poisonous waste from industrial. Gaseous pollution results majorly from the
combustion of fuels in power plants, factories, and automobiles (Morcillo et al., 2013). The gas
which is majorly released and found in industrialized countries are sulphur (IV) oxide, ozone,
nitrogen (IV) oxide. Others are chlorides (hydrogen chloride and chlorine gas).
These pollutants react with oxygen and water to form acids. An acid is a chemical
substance that gives protons or hydrogen ions and or accepts electrons, an acid dissociates in
water to yield a cation and anion.
An acid dissociation in water
HCl (aq.) + H2O (l) → H3O+ (aq.) + Cl- (aq.)
Oxides of carbon form carbonic acid, oxides of sulphur form sulphuric acid, and the
oxides of nitrogen form nitric acids (Leygraf et al., 2016)
Oxides of carbon reacting with water to form carbonic acid
CO2 (g) + H2O (l) → H2CO3 (aq.)
Cation Anion

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The sulphur (IV) oxide released by the car exhaust and industrial plants get oxidized to
sulphur (VI) oxide first.
SO2 + O2 (g) → SO3 (g)
The formed oxide of sulphur (sulphur (VI) oxide gas) reacts with water to sulphuric (IV)
acid.
SO3 (g) + H2O (l) → H2SO4 (aq.)
Oxide of nitrogen forming nitric acid
3NO2 (g) + H2O (l) → 2HNO3 (aq.) + NO (g)
Oxides of chlorine reacting with water to form acid, chlorine (VII) oxide reacts with
water to form chloric (VII) acid, which is a powerful acid.
Cl2O7 (g) + H2O (l) → 2HClO4 (aq.)
A compound capable of donating free chlorine ions (Cl-) to a water-based aqueous
solution is potential to cause corrosion in steel. Chlorine is highly electronegative. It has a
high tendency to attract electrons from other elements to itself hence resulting in corrosion.
Chlorine ions form a compound with iron which is soluble in water resulting in degradation of
the metal with time. The pollutants in the form of chlorine gas and its oxides dissolve in water
to form acid, which reacts with metals (Speight, 2015).
Fe (s) + 3Cl+ (aq.) → FeCl3 (aq.)
Iron (III) chloride formed is soluble in water.

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The acid formed from various types of pollutant gases combines chemically with the
different parts of the metallic structures to form various salts.
The reaction of nitric acid with; zinc, aluminum, and iron.
Zn (s) + 2HNO3 (aq.) → Zn(NO3)2 (aq.)+ H2 (g)
Fe (s) + 2HNO3 (aq.) → Fe(NO3)2 (aq.)+ H2 (g)
2Al (s) + 6HNO3 (aq.) → 2Al(NO3)3 (aq.)+ 3H2 (g)
The other acid too resulting from pollutant gasses react with the metals to form their
respective oxides.
Metals nature and reactivity.
There are two properties which the elements largely depend on when taking part in
chemical reactions. These are the tendency to gain or lose electrons. The tendency to loss or gain
electrons is mostly dependent on the structure of the element's atom. The two factors that affect
the stability of an element is ionization energy and electron affinity.
Ionization energy
Ionization energy is the energy required to remove one or more electron from a neutral
atom to form a positively charged particle in its gaseous state. Ionization energy is a physical
property that affects the chemical behavior of an atom. There are two types of ionization energy.
First ionization energy and second ionization energy.
The tendency of a metal to undergo corrosion majorly depends on its nature. Some metals
easily get oxidized while others are stable and do not readily get oxidized. The bigger the size of

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the atom, the less tightly the electrons are held by the nucleus; hence, the ionization energy will
be less.
Metals have different properties from each other, which influence the choice of its use.
For a given metal to attain stability, electrons must be lost or given out. This electron loss or gain
takes place during a chemical reaction. Metals with the list ionization energy are the most
unstable. The following factors affect ionization energy; nuclear charge, electrons shell number,
effective nuclear charge, and type of orbital (Sanderson, 2012). Atoms having stable electronic
configurations have a lesser tendency to lose electrons hence have higher ionization energy.
First ionization energy increases from left to right across the rows of a periodic table,
meaning that the elements (metals) on the left side of a periodic table are the most unstable
because they have lesser ionization energy. Low ionization energy makes them vulnerable to
oxidation. The first ionization too decreases as we go down the periodic table, which is the
column, signifying the same, easiness to get oxidized.
Increasing ionization energies in (kJ/mol.)
Least stable elements (metals) more stable elements (metals)
Examples (first ionization energies)
Na (g) → Na + (g) + e- Energy= 495.8 kJ/ mol.
Li (g) → Li+ (g) + e- Energy= 520.3 kJ/ mol. Increasing metal stability.
Fe (g) → Fe+ (g) + e- Energy= 762 kJ/ mol.
Au (g) → Au+ (g) + e- Energy= 890.3 kJ/mol.