H11Materials 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 /
  • 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
  • Leaching by rain and ground water
  • 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.

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 /
  • Quantity of electricity carried by one mole of electrons.
  • 96 500 coulombs