Chapter 13
Corrosion of metals and their protection
13.1 Corrosion of metals
13.2 Corrosion of iron — rusting
13.3 Factors that speed up rusting
13.4 To observe the rusting of iron using rust
indicator
13.5 Methods used to protect iron from rusting
13.6 Socio-economic implications of rusting
13.7 Corrosion resistance of aluminium
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Key terms
Progress check
Summary
Concept map
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13.1 Corrosion of metals
Corrosion: gradual deterioration of a metal,
resulting from the reaction of the metal with air,
water or other substances in the environment
Learning tip
When a metal corrodes, metal atoms at the surface
lose electrons and change to metal ions. As a
result, a solid (which is usually a metal oxide), forms
on the metal surface.
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(a)
(b)
(c)
Figure 13.1 Some corroded metal objects.
(a) Corroded iron water pipes. Some reddish brown solid is found on their
surface.
(b) Corroded copper roofs. Some green solid is found on their surface.
(c) Corroded silver spoons. Due to corrosion, they have become tarnished.
The more reactive a metal, the more rapidly it
corrodes.
13.1 Corrosion of metals
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Figure 13.2 Golden Bauhinia
Square. The surface of bauhinia is
covered by a layer of gold which
does not corrode at all.
Key point
The reaction of a metal with air, water or other
substances in the environment, leading to
gradual deterioration of the metal, is called
corrosion.
13.1 Corrosion of metals
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13.2 Corrosion of iron — rusting
Conditions for rusting
Rusting: corrosion of iron
To find out whether both water and air are
necessary for rusting to occur, prepare the tubes
as shown in Figure 13.3.
The test tubes are allowed to stand for several
days.
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anhydrous calcium
chloride
cotton wool
iron nail
air
oil layer
boiled
iron nail
distilled water
iron nail
tube 1
tube 2
distilled
water
tube 3
Figure 13.3 Experimental set-up used for
investigating the conditions for rusting.
SBA note
Anhydrous calcium chloride is a drying agent. It
removes water (moisture) from the air.
Concept check
Think about
13.2 Corrosion of iron — rusting
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Tube
Has rusting
occurred?
Explanation
1
No
Anhydrous calcium chloride removes moisture
from the air. Hence, tube 1 has air but no water.
2
No
Boiling the water removes dissolved air. The oil
layer on top prevents air from dissolving in water
again. Hence, tube 2 has water but no air.
3
Yes
Distilled water contains dissolved air. Thus, both
water and air are present.
Table 13.1 Results for the experiments shown in Figure 13.3.
Key point
Rusting refers to the corrosion of iron. For
rusting to occur, two things must be present:
water and air (oxygen).
13.2 Corrosion of iron — rusting
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Chemistry of rusting
Rusting is a slow and complex chemical process.
In the initial stage of rusting, some iron atoms lose
electrons to form iron(II) ions, Fe2+(aq).
Fe(s) → Fe2+(aq) + 2e–
The dissolved oxygen and water accept the
electrons to form hydroxide ions, OH– (aq).
O2(aq) + H2O(l) + 2e– → 2OH–(aq)
13.2 Corrosion of iron — rusting
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Learning tip
The process of rusting involves redox reactions, which
will be further discussed in Book 3B, Chapter 30.
Learning tip
The uneven, scratched, bent or sharp area of an iron
piece usually loses electrons more readily.
13.2 Corrosion of iron — rusting
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air
water film
iron
electron flow
Figure 13.4 A diagram
illustrating the initial
stage of rusting.
The Fe2+(aq) and OH–(aq) ions formed then
combine in the water film, forming iron(II) hydroxide
precipitate, Fe(OH)2(s).
Fe2+(aq) + 2OH–(aq) → Fe(OH)2(s)
13.2 Corrosion of iron — rusting
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The precipitate further reacts with dissolved
oxygen and water, forming iron(III) hydroxide,
Fe(OH)3(s).
4Fe(OH)2(s) + O2(aq) + 2H2O(l) → 4Fe(OH)3(s)
Iron(III) hydroxide changes to hydrated iron(III)
oxide
rust
The overall reaction can be represented by the
following equation:
4Fe(s) + 3O2(g) + 2nH2O(l) → 2Fe2O3nH2O(s)
hydrated iron(III) oxide (rust)
(reddish brown solid)
13.2 Corrosion of iron — rusting
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Learning tip
‘Hydrated’ means ‘combined with water’. In
hydrated iron(III) oxide, the number of water
molecules combined with the iron(III) oxide varies.
n is a variable number.
Rusting continues until the iron piece corrodes
completely.
Key point
Rust is in fact hydrated iron(III) oxide
(Fe2O3nH2O), where n is a variable
number. It is a reddish brown solid.
13.2 Corrosion of iron — rusting
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13.3 Factors that speed up rusting
Presence of acidic pollutants
Acids speed up the rusting of iron.
Learning tip
Figure 13.5 Rusting
occurs at a higher rate in
industrial areas.
Acids speed up the rusting of iron because they
promote the formation of Fe2+(aq).
Factors that speed up rusting
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Presence of soluble ionic compounds
Soluble ionic compounds, such as sodium
chloride, also speed up the rusting of iron.
Figure 13.6 Iron-made objects near
the seashore rust more quickly.
Think about
13.3 Factors that speed up rusting
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High temperature
An increase in temperature always increases
the rate of chemical reactions, including rusting.
car exhaust pipe
Figure 13.7 The car exhaust
pipe corrodes easily.
13.3 Factors that speed up rusting
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Attachment of less reactive metals to iron
When iron is attached to a metal lower in the
reactivity series (such as tin or copper), rusting
becomes faster.
Learning tip
Tin is less reactive than iron but more reactive than
lead.
13.3 Factors that speed up rusting
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Scratched, bent or sharp area of an iron-made object
Rusting also becomes faster where the iron
surface is scratched or bent.
Rusting occurs faster at the sharp area of an ironmade object.
Class practice 13.1
13.3 Factors that speed up rusting
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13.4 To observe the rusting of iron using rust
indicator
Place an iron nail in a warm gel containing a rust
indicator.
Rust indicator contains
potassium hexacyanoferrate(III) K3[Fe(CN)6],
phenolphthalein, and
sodium chloride.
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When iron rusts, iron(II) ions and hydroxide ions
form.
Iron(II) ions turn potassium
hexacyanoferrate(III) blue.
Hydroxide ions turn phenolphthalein pink.
Appearance of blue and pink colours near the iron
nail indicates that rusting has occurred.
13.4 To observe the rusting of iron using rust indicator
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iron nail
Petri dish
warm gel containing
rust indicator
(a)
(b)
Figure 13.8 Investigating rusting of an iron nail using a rust indicator.
(a) An iron nail (before rusting) is placed in a Petri dish with a rust
indicator added.
(b) The iron nail rusts. The appearance of blue colour near the iron nail
indicates the presence of iron(II) ions, while the appearance of pink
colour indicates the presence of hydroxide ions.
Experiment 13.1
Experiment 13.1
13.4 To observe the rusting of iron using rust indicator
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13.5 Methods used to protect iron from
rusting
Several methods can be used to protect iron from
rusting or to slow down the rusting process.
Learning tip
Steel is an alloy of iron. It is produced by mixing
0.15% to 1.5% carbon with iron. It can also undergo
rusting as it contains iron.
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Applying a protective layer
Coating with paint, plastic, oil or grease
A layer of paint prevents the iron from contacting
air and water
cheap way
This method can be applied to bridges, ships, car
bodies, fences and other large iron-made objects.
Figure 13.9
(a) The fence and
(b) the car body
are painted to
prevent rusting.
(a)
(b)
13.5 Methods used to protect iron from rusting
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Small iron-made objects like coat hangers and
paper clips are often protected by coating them
with a layer of plastic.
Objects coated with plastics can look better and
last longer but is more expensive than painting.
Figure 13.10
Plastic is coated on
(a) coat hangers
and (b) paper clips.
(a)
(b)
13.5 Methods used to protect iron from rusting
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Moving parts of machines and woodworking tools
are not painted or coated with plastic.
∵ the paint or plastic would be scratched off
easily.
They are protected from rusting by oiling or
greasing.
Oil and grease can serve as a lubricant.
Figure 13.11
(a) Bicycle gear and
chains and
(b) woodworking tools
are oiled or greased to
prevent rusting.
(a)
(b)
13.5 Methods used to protect iron from rusting
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Coating with another metal
Galvanizing (zinc-plating)
Galvanizing: to coat the surface of iron with a
thin layer of zinc
Galvanized iron
The layer of zinc prevents iron from contacting
air and water.
Figure 13.12 A bucket made
of galvanized iron.
13.5 Methods used to protect iron from rusting
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Tin-plating
Tin-plating: to coat the surface of iron with a
thin layer of tin
It protects iron from rusting by preventing it
from contacting air and water.
Tin-plating is commonly used in making food
cans since tin and tin ions are not
poisonous.
Figure 13.13 ‘Tin cans’ are
made from iron coated with
a thin layer of tin.
13.5 Methods used to protect iron from rusting
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Electroplating
Electroplating: an electrical process in which
a thin layer of metal is plated on an object
Common metal to be electroplated: Chromium
Iron plated with chromium has a beautiful
shiny appearance but this method is quite
expensive.
13.5 Methods used to protect iron from rusting
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(a)
(b)
Figure 13.14 The corrosion resistant chromium protects
the iron underneath from rusting.
(a) A chromium-plated water tap.
(b) A motor cycle with chromium plated parts.
13.5 Methods used to protect iron from rusting
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Cathodic protection
Iron can be protected from rusting by cathodic
protection.
An iron-made object is connected to the
negative terminal of a d.c. power supply.
A conductor (such as graphite or platinum
alloy) is connected to the positive terminal.
The battery supplies electrons to the ironmade object
prevents iron from losing
electrons
13.5 Methods used to protect iron from rusting
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Cathodic protection has many uses:
To protect car body, underground water
pipelines and storage tanks, and the steel pier
legs
d.c. power supply
electron
flow
iron-made
object (as
cathode)
graphite or platinum
alloy (a conductor)
electrolyte
Figure 13.15 An experimental set-up illustrating the principle of
cathodic protection.
P. 31 / 64
Learning tip
The electrode connected to the negative terminal of
the d.c. power supply is called the cathode, which will
be further discussed in Book 3B, Chapter 31.
Sacrificial protection
During rusting, iron loses electrons to form iron(II)
ions.
If iron is attached or connected to a more reactive
metal, that metal will lose electrons more readily
than iron
prevent iron from forming iron(II) ions
Sacrificial protection
Concept check
13.5 Methods used to protect iron from rusting
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1. Galvanized iron
When the zinc coating of galvanized iron is
undamaged, the iron is protected from rusting.
In case the coating is partly scratched, the
exposed iron is still protected
∵ zinc is more reactive than iron
Zinc will corrode instead of iron
‘sacrificed’ to ‘save’ iron
13.5 Methods used to protect iron from rusting
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zinc is
zinc coating
broken surface
zinc coating
iron
oxygen and water
cannot reach iron,
so no rusting occurs
iron
oxygen reacts with
zinc instead of iron
— no rusting
Figure 13.16 Sacrificial protection of iron by zinc.
Galvanized iron is not used in making food cans
because zinc ions are poisonous.
13.5 Methods used to protect iron from rusting
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2. Attaching zinc blocks to ship hulls
Most ships are made of steel.
To protect steel from corrosion, zinc blocks are
attached to the ship hull.
Zinc will corrode instead of iron.
Zinc blocks need to be replaced before they have
completely corroded.
13.5 Methods used to protect iron from rusting
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zinc blocks
(a)
(b)
Figure 13.17 The zinc blocks need to be replaced regularly,
but this is certainly cheaper than replacing the ship.
(a)The ship is protected from corrosion by zinc blocks.
(b)Zinc blocks for attachment to the ship hull.
13.5 Methods used to protect iron from rusting
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3. Connecting magnesium blocks to underground
pipelines
Sacrificial protection is also used to protect
underground iron pipelines from rusting.
Magnesium blocks are connected to the
underground pipelines.
Magnesium corrodes instead of iron.
Magnesium blocks should therefore be replaced
from time to time.
13.5 Methods used to protect iron from rusting
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ground
damp soil
connecting wire
iron
pipeline
electron
flow
bag containing a
magnesium block
Figure 13.18 Protecting underground iron pipelines from
rusting by sacrificial protection. It is much easier and cheaper
to replace magnesium blocks than iron pipelines.
Example 13.1
13.5 Methods used to protect iron from rusting
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Using alloys of iron
Stainless steel is an alloy of iron.
It is produced by mixing the right amounts of
carbon (0.15–1.5%) and other metals (such as
chromium, nickel and manganese) with iron.
Figure 13.19 Stainless
steel cookware.
13.5 Methods used to protect iron from rusting
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Stainless steel is corrosion resistant.
∵ A layer of chromium(III) oxide is formed on the
surface.
The oxide layer is very tough and can protect
the iron underneath from contacting air and water.
Alloying is the most expensive rust prevention
method.
STSE connections 13.1
Class practice 13.2
13.5 Methods used to protect iron from rusting
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Method of rust Simple
Advantage(s) Disadvantage(s) Example(s)
prevention
chemistry
(a) Painting
(b) Coating
with plastic
(c) Oiling or
greasing
the added • cheap
layer
prevents
the iron
object from • lasts long
contact
• looks good
with air
and water
• does not fall
off like paint
or plastic
• has
lubricating
effect
• scratched off
easily
bridges,
ships,
fences, car
bodies
• more
coat
expensive than hangers,
painting
paper clips
• not ‘once and
for all’
• dirt would stick
to oil or grease
Table 13.2 Different methods of rust prevention.
13.5 Methods used to protect iron from rusting
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moving
parts of
machines,
woodworking
tools
Method of rust
prevention
Simple
chemistry
(d) Galvanizing
the added
• in case the
• zinc ions are
layer
zinc coating
poisonous
prevents
is damaged,
the iron
the iron is still
object from
protected
contact
• tin is
• when the tin
with air and
corrosion
coating is
water
resistant
damaged,
• tin and tin
rusting will
ions are not
occur more
poisonous
quickly than
iron alone
(e) Tin-plating
(f) Electroplating
(e.g.
chromiumplating)
Advantage(s)
• has a
beautiful
shiny
appearance
Disadvantage(s) Example(s)
• quite
expensive
Table 13.2 Different methods of rust prevention.
13.5 Methods used to protect iron from rusting
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galvanized
iron plate
used in
construction,
buckets
‘tin cans’ for
storing food
water taps,
car
bumpers
Method of rust Simple
Advantage(s) Disadvantage(s) Example(s)
prevention
chemistry
(g) Cathodic
protection
the
• convenient
negative
terminal of
an electric
source is
connected
to the iron
object,
supplying
electrons
to prevent
it from
rusting
• not applicable
to many
objects
Table 13.2 Different methods of rust prevention.
13.5 Methods used to protect iron from rusting
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car bodies,
underground
water
pipelines,
storage
tanks, steel
pier legs
Method of rust Simple
Advantage(s) Disadvantage(s) Example(s)
prevention
chemistry
(h) Sacrificial
protection
a more
• an effective
reactive
way of
metal (e.g.
protection
magnesiu
m, zinc) in
contact is
‘sacrificed’
to form
ions; this
would
prevent
iron from
forming
iron(II)
ions
• the ‘sacrificed’
metal needs
replacement
from time to
time
Table 13.2 Different methods of rust prevention.
13.5 Methods used to protect iron from rusting
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galvanized
iron, zinc
blocks
attached to
the ship
hull,
magnesium
blocks
connected
to the
underground
pipelines
Method of rust
prevention
Simple
chemistry
Advantage(s) Disadvantage(s) Example(s)
(i) Using alloys
of iron
iron is
• has a
• most
alloyed with beautiful
expensive
carbon,
appearance
chromium, • a very
nickel and
effective
manganese way of
which are
protection
corrosion
resistant
cookware,
cutlery
Table 13.2 Different methods of rust prevention.
Experiment 13.2
Experiment 13.2
13.5 Methods used to protect iron from rusting
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Activity 13.1
13.6 Socio-economic implications of rusting
Huge sums of money are spent every year to
prevent rusting and replace rusted objects.
Rusting also causes damages to buildings and
even loss of human lives.
Figure 13.20 The Interstate
35W bridge over the Mississippi
River in Minnesota in the United
States collapsed in 2007. It was
found that there was corrosion
in the steel parts of the bridge.
Reading to learn
P. 46 / 64
13.7 Corrosion resistance of aluminium
Protective oxide layer on aluminium
When aluminium is exposed to air, a thin but tough
layer of aluminium oxide forms on its surface.
This layer is impermeable to air and water and
hence can protect the aluminium underneath
from further corrosion.
Aluminium appears to be less reactive than it
really is.
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surface attacked by
oxygen in the air
aluminium
thin protective layer
of aluminium oxide
aluminium
Figure 13.21 Explaining the corrosion resistance of aluminium.
The protective oxide layer on aluminium is very thin.
It can be thickened by a process called
anodization.
13.7 Corrosion resistance of aluminium
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Thickening the protective oxide layer on aluminium
An aluminium sheet is rolled into cylindrical shape
and made the negative electrode (cathode).
The aluminium object to be anodized is made
the positive electrode (anode).
Dilute sulphuric acid is the electrolyte.
During anodization, a layer of aluminium oxide
forms on the surface of the aluminium object.
the oxide layer on the object is thickened.
Concept check
13.7 Corrosion resistance of aluminium
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Overall chemical equation of aluminium anodization:
2Al(s) + 3H2O(l) → Al2O3(s) + 3H2(g)
aluminium sheet
(as cathode)
dilute sulphuric acid
(as electrolyte)
aluminium object to be
anodized (as anode)
Figure 13.22 Experimental set-up for anodization.
13.7 Corrosion resistance of aluminium
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Learning tip
Beside an aluminium sheet, a graphite rod can
be used as the negative electrode of the set-up
for anodization.
After anodization, the aluminium object becomes
more corrosion resistant.
Anodized aluminium can be dyed easily to give
attractive colours.
13.7 Corrosion resistance of aluminium
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Figure 13.23 (left) The milk caps and the packaging of chocolate
are made of anodized aluminium.
Figure 13.24 (right) Window frames made of anodized aluminium.
Class practice 13.3
13.7 Corrosion resistance of aluminium
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Key terms
1.
2.
3.
4.
5.
6.
7.
8.
9.
anodization 陽極電鍍
cathodic protection 陰極保護
corrosion 腐蝕作用
electroplating 電鍍
galvanized iron 鍍鋅鐵
galvanizing 鍍鋅
rust 鐵銹
rust indicator 鐵銹指示劑
rusting 銹蝕
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10.
11.
12.
sacrificial protection 犧牲性保護
stainless steel 不銹鋼
tin-plating 鍍錫
Key terms
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Progress check
1.
2.
3.
4.
5.
What is the meaning of corrosion?
What is the meaning of rusting?
What are the essential conditions for rusting?
What are the factors that speed up rusting?
What can we observe when rusting is shown by a
rust indicator?
6. What methods can we employ to prevent rusting?
How does each of these methods work?
7. What are the socio-economic impacts of rusting?
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8. Why is aluminium more corrosion resistant than
expected?
9. How can we enhance the corrosion resistance of
aluminium?
Progress check
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Summary
13.1 Corrosion of metals
1.
2.
Corrosion is the gradual deterioration of a metal
due to reaction with air, water or other
substances in the surroundings.
In general, a metal higher in the metal reactivity
series corrodes faster.
13.2 Corrosion of iron — rusting
3.
Rusting is the corrosion of iron. Rusting requires
the exposure of iron to both water and air. Rust
is in fact hydrated iron(III) oxide, Fe2O3nH2O.
P. 57 / 64
13.3 Factors that speed up rusting
4.
Factors that speed up rusting include:
Presence of acidic pollutants or soluble ionic
compounds
High temperature
Attachment of less reactive metals to iron
Scratched, bent or sharp area of an ironmade object
Summary
P. 58 / 64
13.4 To observe the rusting of iron using rust
indicator
5.
We can observe rusting conveniently using a
rust indicator. It shows blue and pink colours
where rusting occurs.
13.5 Methods used to protect iron from rusting
6.
To prevent rusting, we can make use of a
suitable method. Refer to Table 13.2 on p.17 for
different methods of rust prevention.
Summary
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13.6 Socio-economic implications of rusting
7.
Rusting causes socio-economic problems and
rust prevention methods have to be developed.
13.7 Corrosion resistance of aluminium
8.
9.
Aluminium is resistant to corrosion because it
has a protective oxide layer on its surface.
Anodization is a process used to thicken the
aluminium oxide layer on aluminium. Anodized
aluminium becomes more corrosion resistant,
and can be easily dyed to give attractive colours.
Summary
P. 60 / 64
Concept map
Acidic
pollutants
Gradual deterioration
of metals
presence
of
CORROSION
Soluble ionic
compounds
speeded
up by
High
temperature
Scratched, bent
or sharp area
Less reactive
metal
is called
of iron is called
Rusting
contact
with a
P. 61 / 64
Gradual deterioration
of metals
is called
CORROSION
Aluminium
of iron is called
Rusting
conditions
required
can be
protected
by
Air (oxygen)
Water
Concept map
P. 62 / 64
Anodization
Rusting
can be prevented/
slowed down by
coating with
Protective
layer
Paint
Plastics
Oil/
grease
Other
metals
example
Tin
Concept map
P. 63 / 64
Rusting
can be prevented/
slowed down by
Using alloys
of iron
example
Stainless
steel
Sacrificial
protection
Cathodic
protection
example
Galvanizing
Concept map
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