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OPTION 2
7
WORKSHEET
CORROSION PROTECTION
Syllabus reference 9.6.4
1
Classify each of the following statements as true or false. For those statements that are false,
rewrite them so they are true.
a
Paint protects an object from rusting because it stops air and water contacting the metal.
TRUE
b
A tin coating on cans stops corrosion of the can because the tin corrodes instead.
FALSE
A tin coating on cans stops corrosion because the TIN STOPS OXYGEN AND
WATER REACHING THE IRON/STEEL.
c
Covering iron with vitreous enamel is more effective than paint because it is tougher and binds
to the metal better than paint.
TRUE
d
Galvanised iron is iron coated with less reactive zinc which protects the iron.
FALSE
Galvanised iron is iron coated with MORE reactive zinc, which protects the iron BY
FORMING AN IMPERVIOUS ZINC OXIDE LAYER.
e
Scratched tin cans rust less rapidly than iron alone.
FALSE
f
Scratched tin cans rust MORE rapidly than iron alone.
Cathodic protection works by attaching iron to an object or covering it with another more
reactive metal that is the cathode.
FALSE
Cathodic protection works by attaching iron to an object or covering it with another
more reactive metal that is the ANODE.
Copyright © 2007 McGraw-Hill Australia
CONQUERINGCHEMISTRY HSC
OPTION 2 WS 7
g
A block of zinc or magnesium is often attached to the hull of a ship because it corrodes in
preference to the iron.
TRUE
h
An applied voltage between an object to be protected and an inert anode prevents rusting.
TRUE
Use the following standard electrode potentials for some half-cell reactions to answer the questions
which follow.
Al3  3e → Al
1.66
2

Zn  2e → Zn
0.76
Fe2  2e → Fe
0.44
2

Sn  2e → Sn
0.14
Cu2  2e → Cu
0.34
3

2
Fe  3e → Fe
0.77
Ag  e → Ag
0.80
2

Hg  2e → Hg
0.85
2
Pieces of two different metals were partially immersed in an electrolyte solution without touching
each other. The non-immersed ends were joined together. Assuming no reaction with the
electrolyte, predict which metal would corrode (i.e. go into solution as ions).
a Fe and Zn
Zn
b
Al and Cu
Al
c
Sn and Ag
Sn
3
A student, while absentmindedly chewing on the end of a pencil, experienced a sharp pain in a
tooth. On further inspection the student noticed the end of the pencil had a steel metal casing
holding what remained of an eraser. Careful experiments showed that touching the metal against
an amalgam filling in the tooth caused pain like an electric shock, but touching the tooth itself did
not.
Use the table of electrode potentials above to explain what is happening.
Amalgam and the steel metal casing formed an electrochemical cell causing an electrical current
which affected the nerve.
4
Some gardeners had mixed up a dilute copper sulfate solution to use as a fungicide on garden paving
stones. They had some solution left over and poured it into a steel can for storage. A few days later
they noticed the can was leaking at the base. Suggest what may have caused the can to leak.
Cu2 ions reacted with the Fe can causing the Fe to rust (producing Fe2 ions) and the can seam to
disintegrate.
5
Describe what is likely to happen if steel bolts are used to fix copper plates together.
Steel will corrode in preference to copper and the plates will come apart.
Copyright © 2007 McGraw-Hill Australia
CONQUERINGCHEMISTRY HSC
OPTION 2 WS 7
6
Why does a dented tin-coated iron can rust more quickly than a dented galvanised can?
Tin is less active than iron so the iron will act as a sacrificial anode to the tin.
7
Why does a scratched painted iron wheelbarrow rust more rapidly than a similarly scratched
galvanised one?
Once the paint is scratched oxygen and water attack the iron causing rusting, but a galvanised
wheelbarrow has zinc acting as a sacrificial anode protecting the iron.
8
A chemistry student was helping assemble an aviary (a structure to house birds). The instructions
advised joining the metal sheets with galvanised screws. With only a couple of joins to go there
were only iron screws left. The student advised against using these. Give an explanation of why the
student gave this advice.
The iron screws are likely to corrode. If the aviary metal sheets are galvanised or made of less
active metal they will rust more rapidly.
9
Hot water systems are often protected from corrosion with a sacrificial anode of a block of
magnesium metal.
a Explain what is meant by a sacrificial anode.
A sacrificial anode is where a block of more active metal is attached to a structure, protecting it
because the more active metal corrodes preferentially.
b
Draw a diagram, giving half reactions and showing electron flow, to explain how this works.
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10 One form of cathodic protection is to use an inert anode and apply a voltage between it and the
object to be protected.
a Give one example of where this is used.
Protecting the hull of a ship or underground pipeline
Copyright © 2007 McGraw-Hill Australia
CONQUERINGCHEMISTRY HSC
OPTION 2 WS 7
b
Draw a diagram showing direction of electron flow and give half reactions.
11 The use of materials for shipbuilding has changed over time. Complete the following table showing
how the use of materials for shipbuilding has improved.
TIME PERIOD
CHOICE OF MATERIALS
PROBLEMS/DIFFICULTIES
Pre World War II
Steel
Steel brittle and subject to
stress cracking
Post World War II
Mild steel for hulls,
Steel propellers corrode
more quickly
Structural steel for girders
Corrosion
Mild steel
Corrosion
Aluminium and non-ferrous
alloys in smaller vessels
not as strong as steel
Current
Copyright © 2007 McGraw-Hill Australia
CONQUERINGCHEMISTRY HSC
OPTION 2 WS 7