Page 167 - The City and Guilds Textbook: Plumbing Book 1 for the Level 3 Apprenticeship (9189), Level 2 Technical Certificate (8202) and Level 2 Diploma (6035)

Chapter 3 Scientific principles

Conductivity IMPROVE

Conductivity is the property that enables a metal to carry heat (thermal YOUR MATHS
conductivity) or electricity (electrical conductivity). Thermal conductivity is
measured in watts per metre
● Thermal conductivity: here, heat is transferred from molecule to molecule kelvin (W/mK). Electrical
through the substance. How fast or how well the heat travels will determine conductivity is measured
the material’s thermal conductivity. For example, metals, such as copper, in ohms (Ω).
transfer the heat quickly and are said to be good conductors of heat, whereas
other materials, such as polyurethane, allow the passage of heat only very
slowly and so are poor conductors of heat. The inability of polyurethane to
allow the passage of heat makes it a very good insulator with the ability to keep
heat in. Thermal conductivity is measured in watts per metre kelvin (W/mK).
● Electrical conductivity: this is the ability of a material to allow an electrical
charge or current to pass through it. It is measured in ohms (Ω). Materials
that allow an electrical current to flow freely, such as copper and gold, are
known as good conductors, whereas those that do not allow the passage of an
electrical current, such as wood, ceramics and PVC, are known as insulators.

Oxidation, corrosion and degradation
of solid materials

All solid materials will corrode or degrade over time. The amount that
materials corrode or degrade will depend upon the material’s resistance and the
environment in which the material exists. In this section of the chapter, we will
investigate these three processes and how they affect plumbing materials.

Oxidation of metals

Metals are oxidised by the presence of oxygen in the air. This process is more
commonly called corrosion. Electrons jump from the metal to the oxygen
molecules. The negative oxygen ions that are formed penetrate into the metal,
causing the growth of an oxide on the metal’s surface. As the oxide layer
increases, the rate of electron transfer decreases. Eventually, the corrosion stops
and the metal becomes passive. However, the oxidation process may possibly
continue if the electrons succeed in entering the metal through cracks, pits or
impurities in the metal, or if the oxide layer is dissolved.

Corrosion

Corrosion is the main reason for metals deteriorating. Most metals will corrode
on contact with water (and moisture in the air), acids, salts, oils, and other solid
and liquid chemicals. Metals will also corrode when exposed to some gases,
such as acid vapours, ammonia gas and any gas containing sulphur.
Corrosion specifically refers to any process involving the deterioration or
degradation of metal components. The best-known case is that of the rusting
of steel and iron where the formation of ferrous oxide occurs. The corrosion
process is usually electrochemical.

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