Corrosion of metalsWhat is corrosion?

ReadYou have already learned about some chemical reactions

Lorna C. Webster 1

West Lothian Council

burning is a reaction that makes heat. The scientific word for burning is combustion.

neutralisation is the reaction of an acid to make water.

Corrosion is a chemical reaction that involves metals. When a metal corrodes it is eaten away chemically and the surface of the metal changes from an element into a compound.

Rusting is a special type of corrosion. Rusting is the name we use for when iron rusts. The word rusting can be used for steel because steel is an alloy made from iron and carbon but the word rusting should not be used for any other metal.

Your teacher may show you a video about rusting (SS18).

Write a heading and try the work below.1. What is the name of the reaction that produces heat?2. What is the name of the reaction where and acid reacts

to make water?3. What happens to metals in the reaction that is called

corrosion?4. Why is rusting a special type of corrosion?5. Why can we use the word rusting for steel but not for

any other metals?

What causes rust?ReadMost people think water causes rusting. Experiments can be set up to test this. Look at the display (2.14) called "what causes rust".

Lorna C. Webster 2A B C

boiled water

oil

waterdrying chemical

NOTE:

Tube A: contains a special chemical that absorbs water to make sure that the oxygen in the tube is dry.

Tube C: Gases like oxygen dissolve slightly in water. This dissolved oxygen can be removed by boiling the water. Oil is put on the surface to stop oxygen dissolving again.

Write a heading and try the work below.1. Draw a diagram of the experiment.2. Why has the water in tube C been boiled?3. Why is there a layer of oil on the surface of the boiled

water in tube C?4. Copy and complete the table below.

5.Copy and complete the sentence below.

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test tube water present?

oxygen present?

rust?

ABC

This experiment shows that for iron to rust both _________ and _________ must be present.

looking for rustReadTo see whether a piece of iron has rusted is easy. All we need to do is to look for a brown substance forming on the iron but this can take a long time. Scientists need quicker ways to find out whether a piece of iron is beginning to rust. To do this they use an indicator.

IndicatorsYou have already found out about pH indicator in chemistry. pH indicator changes colour depending on whether a substance is acid, alkali or neutral. The following experiment is to find out the name of an indicator that will change colour depending on whether there is rust or not.

Collect:a test tube rack

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4 test tubes1 glass rod1 bottle of rust solution1 bottle of each indicator (labelled A, B, C, D)4 droppers (one for each indicator)

Set up the experiment as shown in the diagram below:

Now add 20 drops of

indicator A to the first tube,B to the second tube,C to the third tube,D to the fourth tube

Mix each tube with a clean glass rod each time and watch to see what happens. Write a heading and try the work below in your jotter.1. Why do scientists need an indicator to show them if iron

is rusting?2. From your experiment which indicator (A,B,C or D)

changed colour when it was put into rust?3. What colour was this indicator before you put it into the

rust?4. What colour did it change to when it touched the rust?5. Ask your teacher to tell you the names of the chemicals

that were in the indicator bottles and copy and complete the sentences below.

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rust solution

The name of the indicator that can be used to find rust is called ________ indicator. Rust indicator changes from ____________ to __________ if rust is present.

Use a fresh dropper each time!

Changing the speed that iron rustsRead If you've ever been to Florida or Arizona you may have noticed that there are many very old cars that are still in excellent condition. This is something that is rare in Britain. Most old cars have rusted and have been sent to the scrapyard.

Iron rusts at different speeds depending on the conditions that it is in.

Write a heading then do the following experiment.Collect 4 test tubesa test tube rack4 clean iron nails1 bottle of salty water1 bottle of acid rain (1mol/l hydrochloric acid)1 bottle of rust indicator

1. Set up the test tubes in a rack.2. Half fill each test tube with the solutions as shown in the

following diagram.

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The aim of the following experiment is to find out if iron rusts faster than normal when it is acid and when it is in salt.

3. Add enough rust indicator to each tube and mix each tube with a clean glass rod each time to give a yellow colour.

4. Put a nail into each tube and watch what happens.

Now try the work below in your jotter.1. What was the aim of the experiment?2. Why was the tap water test tube included in the

experiment?3. How could you tell if the iron had rusted or not?4. Draw a diagram of the test tube rack to show the nails

and colour the tubes in blue to show how much the rust indicator changed colour.

5. Copy the following sentences and circle the correct words.

6. In the winter salt is spread on the roads to help stop them being icy. What will this do to the speed that the car will rust?

7. John lives near the beach at St Andrews. His brother

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tap water

acid rain

salty water

The test tube in the experiment that contains tap water is needed to see how fast the iron rusts when it is not in acid or salt.

Acid rain speeds up/slows down rusting.Salt water speeds up/slows down rusting.

lives in Armadale. Even though both brothers bought their new cars on the same day John noticed that his was rusting faster than his brother's. Can you explain this?

8. You should already remember that acid rain is caused by nitrogen dioxide gas from car exhausts dissolving in cloud water. Explain why cars that are in cities rust faster than cars that are in country areas.

Stopping rustRead When a metal corrodes it becomes weakened. This can cost a lots of money because the weakened parts need to be replaced so it important to try to find ways to stop corrosion.

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You should already know that for iron to rust it has to be in contact with oxygen and water. This means that one way to protect iron from rusting is to put a barrier between the iron and the oxygen and water. There are many types of coatings that we can use to cover the iron. To find out if they work try the experiment below.

Collect:a test tube rack5 test tubes1 glass rod1 bottle of rust indicator1 bottle of salty water1 ordinary nail1 plastic coated nail1 galvanised nail1 greased nail1 painted nail

1. Set up 5 test tubes in the rack and half fill them with salt water.2. Add enough rust indicator and mix it into the salty water with a glass rod to give a yellow colour in each tube.3. Put the different nails into each tube as shown in the diagram below and observe what happens.

Lorna C. Webster 9ordinary nail

plastic coated nail

grease coated nail

paintcoated nail

galvanised nail

Iron

coating/barrier

oxygenwater

An ordinary nail is needed in this experiment to allow you to see how much rusting happens without any coating.

Write a heading and try the work below.1. Copy the diagram of the experiment into your jotter and

add blue colour to show how much rusting has happened.

2. Why was an ordinary nail needed in the experiment?3. Copy and complete the notes below

GalvanisingReadGalvanised iron is iron that has been coated with a metal called zinc. To do this the zinc is heated up until it melts and then the iron is dipped into it. The iron is then lifted out and when the melted zinc cools, it turns into a solid coating. Galvanising is used to coat iron nails, chisels and motorway crash barriers. This is useful because if the zinc is scratched, the iron will still not rust. Paint would be no use for a crash barrier because if the paint is scratched the iron would start to rust.

Write a heading and try the work below.1. When iron is galvanised, what is the name of the metal

that it is coated with?

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Coating iron with a substance stops iron from rusting because the coat stops the ____________ and ____________ in the air touching the iron.

Iron

Zinc

2. If you had an iron nail, a lump of solid zinc and the normal apparatus that is in the lab how you would galvanise the nail?

3. Give two uses of galvanised iron.4. Why is galvanised iron better for crash barriers than

painted iron?5. Copy the following table and complete it by looking at

the "coatings in use" poster.

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coating usestin-plate

crash barriers, chisels and dustbins

greasing

electrical wiring washing-up racks clothes hangers

Electroplating one metal with anotherReadYou should already know that iron is coated with zinc when it is called galvanising but to stop iron rusting, it can also be coated with other of metals.

You have probably heard of silver-plated jewellery and gold-plated jewellery. This is a cheaper way to make jewellery than using solid silver or gold.

The shiny bumpers on cars or bathroom taps are often coated with a metal called chromium. This helps stop rusting and also makes the bumpers and taps look good.

To plate silver, gold or chromium onto other metals we use electricity. This process is called electroplating.

Write a heading and do the work below.1. List some everyday examples of coating one metal with

another metal.2. Why do some shops sell silver-plated and gold-plated

jewellery instead of solid silver or gold?3. What is the process called that uses electricity to coat

one metal with another?

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In the next experiment you are going to try to coat a 10p piece with another metal.Collect:a power pack and plug2 wires2 crocodile clipsa copper stripa 10p piecea petri disha bottle of copper chloride solution

Set up the experiment as shown in the diagram below but do not switch the power on until you read the next instruction.

Place the 10p piece into the copper chloride solution and hold the plastic part of the wire. DO NOT put the crocodile clips into the solution and DO NOT touch the 10p and the copper strip together.

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power pack 4V DC - +

_-

copper chloride solution

copper strip 10p

petri dish

You need to connect the leads to the red and black connections. Red = +Black = -

power pack 4V DC - +

_-

copper chloride solution

copper strip 10p

petri dish

Switch the power on and observe what happens to the 10p piece.4. Draw a diagram of the experiment (copy the second

diagram on page 4).5. What did you see happen to the 10p piece?6. Why do you think this happened?7. A pupil wanted to set up an electroplating experiment to

coat a 10p piece with tin. Copy and complete the labels in the diagram below to show how he would do this.

Protecting aluminium from corrosionReadAluminium is a metal that has a great many uses. It is light so it is used to build aeroplanes. It is very

important that the aluminium does not corrode quickly or the aircraft would soon become unsafe and this would be a danger to the crew and passengers.

Scientists have found that if they leave a piece of aluminium outside, the surface of the aluminium reacts slowly with the oxygen of the air. This turns the surface of the aluminium into a compound called aluminium oxide.

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power pack 4V DC - +

_-

______chloride solution

______ strip _____

petri dish

thin aluminium oxide layer after several days

aluminium

aluminium before it is put outside

Scientists discovered that the aluminium oxide layer protected the aluminium underneath from corrosion. They worked out a way to make this aluminium oxide layer thicker. This process is called anodising.

The main point of anodising is that it protects the aluminium from corroding but another advantage is that the layer can be painted to make the aluminium look more attractive.

Write a heading and try the work below.1. Collect a data book and look up the data book contents

page. What page would you look at to find out about the uses of metals?

2. Use the data book to make a list of the uses of aluminium.

3. Why is it important that aluminium does not corrode in aeroplanes?

4. What did scientists discover happened to aluminium if it is left outside?

5. Copy the diagram to show what happens to a piece of aluminium if it is left outside.

6. Copy and complete the sentences below into your jotter.

7. Apart from stopping the corrosion of aluminium, what other advantage is there to anodising?

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To stop aluminium corroding the thin alumimium oxide layer on the outside is made t__________. This process is called a______________.

Attaching other metals to stop rustingReadUp to this point you have learned that we can stop corrosion by completely covering one metal with another to act as a barrier to the oxygen and water in the air. This is not the only way to stop rusting. We can change the speed of rusting by attaching iron to other metals. With some metals we can stop the rusting even if most of the iron is still touching water and oxygen.

Write a heading and look at display 2.21.1. Copy and complete the table on the next page to show

what you saw in the display and colour in each diagram in the table to show how much blue colour there is.

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diagram of the experimentmetal attached to the ironrust (yes or no)

No metal attached

magnesium

zinc

tin

copper

2. Why do you think a nail on its own was used in this experiment?

3. Which metals speeded up the rusting of iron?4. Which metals stopped the iron from rusting?5. Copy and complete the sentence below by choosing the correct answer. (you should use the data book page 6 to help you if necessary).

6. Collect and read the information sheet called "Stopping corrosion - everyday examples" and try to answer the following questions.

a)What is the main problem with using metals?b)Explain how barrier protection works.c) List some examples of where painting is used to

stop corrosion.d)What type of coating is used to stop bicycle chains

rusting?e)Why is paint not good for using on car engines?f) What are the names of the two scientific ways to

coat one metal with another?g)List some examples where plastic coating is used to

stop rusting.h)Ships are often painted to help stop corrosion but

why do they rust very quickly if the paint is scratched?

i) What is done to help stop ships rusting even if the paint is scratched?

j) Why is paint no good for protecting underground pipelines?

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Iron does not rust if it is attached to a metal that is more/less reactive. This works even if the iron is not completely covered.

k) What is done to stop the pipelines rusting?l) What is the process called that is used to stop

aluminium corroding?

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