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L2 Plumbing Unit 4 Apply scientific principleswithin mechanical services engineering

Pearson Education Ltd 2011. Printing and photocopying permitted.

Learning outcome 2

Know the properties of materials

used in the mechanical services

industry

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Density of solids

Solid materials with the same size and shape can frequently have a

completely different mass.

The density of an object or material is a measure of its mass (grams)compared to its volume (cm3) and can be worked out using the following

formula:

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Density of liquids

Like solids, liquids and gases also have differing densities depending on

the number of molecules that are present within a particular volume of the

substance.

The density of water changes with the waters temperature. Water is less

dense when it is heated:

1 m3 of water at 4 C has a mass of 1,000 kg

1 m3 of water at 82 C has a mass of 967 kg.

This is because heat energy excites the molecules so that they move

further apart and the water becomes less dense.

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Relative density (specific gravity)

Relative density is an effective way of measuring the density of a

substance or object by comparing its weight per volume to an equal

volume of water.

Water has a relative density (specific gravity) of 1.0. For example:

1 m3 of water has a mass of 1,000 kg

1 m3 of mild steel has a mass of 7,700 kg.

The mild steel is 7.7 times heavier than water and therefore has a relativedensity of 7.7.

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Density of gases

Just as water is said to have a relative density of 1.0 to enable

comparisons of relative density for solids and liquids, air has a relative

density of 1.0 to enable comparisons between gases, depending on

whether they are lighter or heavier than air.

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Properties of materials

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Properties of metals

Property Description

Elasticity The ability of a material to resume its normal shape after being

stretched or squeezed

Plasticity The exact opposite of elasticity a material that is not able to return to

its original shape when deformedDuctility The ability of a material to withstand distortion without fracture, such as

metal that can be drawn out into a fine wire (plastic deformation under

tension)

Durability A materials ability to resist wear and tear

Fusibility The melting point of a material, i.e. when a solid changes to a liquid

Malleability The ability of a metal to be worked without fracture

Temper The degree of hardness in a metal

Tenacity A materials ability to resist being pulled apart

Thermal expansion The amount a material expands when heated

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Main causes of corrosion

The effects of air

The effects of water

The direct effects of acids, alkalis and chemicals, e.g. from

environmental sources Electrolytic action

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Atmospheric corrosion (1)

Pure air and pure water have little corrosive effect, but together in the form

of moist air (oxygen + water vapour) they can very quickly attack ferrous

metals such as steel and iron to form iron oxide or rust.

The corrosive effects of rusting can completely destroy metal. This process

is called oxidisation.

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Atmospheric corrosion (2)

Coastal areas also suffer from increased atmospheric corrosion due to the

amount of sodium chloride (salt) from the sea, which becomes dissolved

into the local atmosphere.

Non-ferrous metals, such as copper, aluminium and lead, have significant

protection against atmospheric corrosion. Protective barriers (usually

sulfates) form on these metals to prevent further corrosion.

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Corrosion by water

Ferrous metals are vulnerable to the effects of corrosion caused by water.

These are seen in central heating systems as black ferrous oxide and red

rust build-up in radiators. A by-product of this process is hydrogen gas,

which accumulates in the radiator, leading to the need for the radiator to be

bled.

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Electrolytic action and corrosion

Electrolytic action describes a flow of electrically-charged ions from an

anode to a cathode through a medium known as the electrolyte (usually

water).

Electrolytic corrosion takes place when the process of electrolysis leads to

the destruction of the anode. The length of time it takes for the anode to be

destroyed will depend on:

the properties of the water that acts as the electrolyte if the water is

hot or acidic the rate of corrosion will be increased

the position of the metals that make up the anode and the cathode in

the electromotive series.

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The electromotive series

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Example of possible electrolytic corrosion in pipework systems

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Properties of water

Water is a chemical compound of two gases: hydrogen and oxygen (H2O).

It is formed when hydrogen gas is burned.

One of the most important properties of water is its solvent power. It can

dissolve numerous gases and solids to form solutions.

The purest natural water is rainwater collected in the open countryside. It

contains dissolved gases such as nitrogen, oxygen and carbon dioxide, but

this does not affect its potability.

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Behaviour of water at different temperatures

Matter, such as water, can exist in what is known as three different states:

Solid (ice) Liquid Gas (steam)

L2 Pl bi U i 4 A l i ifi i i l

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Capillary action: forces of attraction

Surface tension describes the way in which water molecules cling

together to form what is effectively a very thin skin.

Surface tension and cohesion Adhesion

L2 Pl bi U it 4 A l i tifi i i l

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Examples of capillary action

L2 Pl bi U it 4 A l i tifi i i l

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pH value and corrosion (1)

pH value refers to the level of acidity or alkalinity of a substance.

Both acids and alkalis can cause corrosion and thereby damage plumbing

materials. Metals are at particular risk.

L2 Plumbing Unit 4 Apply scientific principles

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pH value and corrosion (2)

All water has a pH value.

Rainwater is naturally slightly acidic, due to small amounts of carbon

dioxide and sulfur dioxide in the atmosphere being dissolved into it,

forming very weak carbonic and sulfuric acids.

The pH value of groundwater is affected by the different rock types it

passes though. For instance, water with dissolved carbonate from chalk or

limestone is alkaline.


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Hard water

Water is classified as hard if it is difficult to obtain a lather with soap.

Hard water is created when it falls on ground containing calcium

carbonates or sulfates (chalk, limestone and gypsum), which it has

dissolved and taken into solution.


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Water hardness

Permanent hardness the result of ions of nitrates and sulfates. It

makes it difficult to form a lather and cannot be removed by boiling.

Temporary hardness the result of the amount of carbonate ions in the

water. The hardness can be removed from the water by boiling, which

results in the carbonate being precipitated out as limescale.


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Soft water

Water is said to be soft when it is easy to produce a lather with soap. This

is because of the absence of dissolved salts such as calcium carbonates

and calcium sulfates.

Soft water can cause corrosion in plumbing components because it is

relatively acidic.

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