Topic 12: Corrosion

Topic 12: Corrosion

Introduction

This topic is concerned with the corrosion of metals and in particular the rusting of iron. The effect of salt, eg from road “gritting”, on the rate of corrosion of iron is given prominence as an everyday example. Prevention of corrosion should be dealt with through real, everyday examples to which pupils can relate.

At General Level, the Learning Outcomes required:

·  some understanding of the macroscopic aspects of corrosion, and its prevention

·  knowledge that rusting is a change from iron atoms to iron ions, and that an indicator exists

·  which can detect this change

·  comprehension of the ways of preventing this oxidation.

At Credit Level, in addition, the Learning Outcomes require an awareness of the operation of a cell in rusting, and knowledge of the redox reactions involved. The scratched tinplate effect on the rate of rusting is dealt with at this Level.

Topic 12: Corrosion Learning Outcomes

GENERAL LEVEL (Grades 4, 3) CREDIT LEVEL (Grades 2, 1)

Pupils should be able to: and in addition to:

1 State that corrosion is a chemical reaction

which involves the surface of a metal

changing from an element to a compound

2 Give examples to show that different

metals corrode at different rates

3 Explain that corrosion is an example

of oxidation

4 Apply the term rusting to the corrosion of

iron

5 State that both water and oxygen (from State that water, oxygen and dissolved

the air) are required for rusting carbon dioxide, or another electrolyte,

are required for rusting

6 State that when iron rusts, initially the iron State that the iron (II) ions can be

atoms lose two electrons to form Fe2+ ions further oxidised to give iron (III) ions

7 State the that electrons lost by the

iron during rusting are accepted by the water and oxygen to form hydroxide ions

GENERAL LEVEL (Grades 4, 3) CREDIT LEVEL (Grades 2, 1)

8 State that ferroxyl indicator for Fe2+(aq)

can be used to show the extent of the

rusting process

9 State that salt spread on roads increases

the rate of corrosion on car bodywork

10 Explain that salt acts as an electrolyte

11 State that iron does not rust when attached

to the negative terminal of a battery

12 State that electrons flowing to the iron

prevents rusting

13 Explain the reaction at the iron in

an iron/carbon cell using the

colour change of Fe2+ indicator and direction of electron flow

14 State that with metals higher in the Explain the reactions in a cell using

electrochemical series, electrons flow to the colour change Fe2+ indicator and

the iron, and with metals lower in the the direction of electron flow

series, electrons flow from the iron

15 Explain what is meant by the following:

·  electroplating

·  galvanising

·  tin-plating

16 Explain that a surface barrier to air and

water can provide physical protection

against corrosion, eg

·  painting

·  greasing

·  electroplating

·  galvanising

·  tin-plating

·  coating with plastic

17 Explain the effect of sacrificial protection Explain the effect of scratching tinplate

in terms of electron flow (eg galvanising, in increasing the rate of rusting of iron

scrap magnesium)

18 give everyday examples of anti-corrosion

methods in 15, 16 and 17 above