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H11
Materials and the Basic Electrochemistry
The corrosion of iron is an electrochemical process that is accelerated in marine
conditions. This chapter investigates the conditions that promote the corrosion of iron,
the types of steel alloys used to make ships and the nature of the electrochemical
process.
Oceans
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Sodium chloride content of seawater is 0.47 mol L-1.
Seawater is an electrolyte solution.
The two major sources of salts in seawater are
o Leaching by rain and ground water
o Dissolution of salts in hydrothermal vents
Historical
development

Galvani credited with the first generation of electric
current
Volta explain the source of electricity and produced the
first galvanic cell
Davy applied electrical energy to chemical reactions and
isolated sodium, potassium and other active metals
Faraday related the mass of the substance produced and
the quantity of electricity used.
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Steel
Alloy of iron with no more than 2% carbon and often with
varying amounts of other metals or silicon.
The properties of steels dependent on the presence and
quantity of other metals and carbon.
Cementite
Iron carbide (Fe3C) produced when molten iron is cooled
quickly.
Quenching
Rapid cooling of molten iron, carbon occurs as cementite
Annealing
Slow cooling of molten steel, carbon occurs as graphite
Tempering
Holding the iron at high temperature below the melting point
for some time to make the steel tougher and less brittle while
retaining its hardness.
Working
Rolling or hammering at different temperatures to improve the
tensile strength and other mechanical properties of the steel.
Electrolysis
Endothermic reactions in which electrical energy is converted
to chemical energy. The electrode reactions depend on:
 The nature of the electrolyte
 The concentration of the ions present
 The nature of the electrodes
Electrolytic reactions
At the anode (positive electrode)
 Bromide and iodide ions are always oxidised
 Nitrate, sulfate and fluoride ions are never oxidised
(water oxidised instead)
 Chloride ions are oxidised if relatively concentrated
At the cathode (negative electrode)
 Metals with positive standard electrodes are always
reduced
 Metals with electrode potentials more negative than 1V
are never reduced
 Other metals are reduced under special conditions such as
high concentrations, suitable electrode material or high
pH).
Electrolysis of molten electrolytes
 Metal ion must be reduced
 Anion must be oxidised
Rate of electrolysis
The higher the current the greater the rate of formation of the
product. The current is affected by:
 The voltage used
 The concentration of the ions in the electrolyte solution
 The surface area of the electrodes
 The distance between the electrodes
Faraday’s first law of
electrolysis
The mass of a substance formed at an electrode during
electrolysis is proportional to the quantity of electricity that
passes through the electrolytic cell.
Quantity of electricity
Q = It
Where Q = charge in coulombs
I = current in amperes
t = time in seconds
Faraday constant
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Quantity of electricity carried by one mole of electrons.
96 500 coulombs