Corrosion of metals and their protection

Reference

• Spotlight P.120-121 • Rusting parts

What is corrosion?

A) The reaction of a metal with oxygen in the air, moisture or other substances in the environment is called corrosion.

B) Gold is so unreactive that it does not corrode at all.

Corrosion of iron: rusting

A) The corrosion of iron is called rusting.

B) Rust is the powdery reddish brown coating

which forms when iron corrodes.

C) Its chemical name is hydrated iron(III) oxide,

Fe2O3•nH2O. Chemical formula

In the first stage of rusting, some iron atoms lose electrons to form Fe2+(aq) ions.

Fe(s) Fe2+(aq) + 2e-

Then a series of reactions follow. The overall reaction can be represented by an equation:

4Fe(s) + 3O2(g) + 2nH2O(l) 2Fe2O3•nH2O(s) 《 rust 》

Rusting is a slow process. It eventually causes structural weakness and disintegration of the metal.

E) For rusting to occur, two things must be present: oxygen and water.

F) During rusting, iron is oxidized by oxygen in the air.

What factors speed up the rusting process?A) Presence of ionic substances

The rusting process speeds up when water containing ionic substances such as sodium chloride.

B) Presence of acidic pollutants

The emission of acidic gases from factories to form acids with moisture in air, thus speeding up the rusting process.

C) Higher temperatureAt a higher temperature, reaction becomes faster. The rusting process also becomes faster.

D) Scratching or bending the surface

Rusting occurs more quickly where the iron surface is scratched or bent.

E) Attachment to a less reactive metal

The rusting process speeds up if iron is in contact with a less reactive metal, such as tin and copper. A food can is usually made of mild steel (an iron alloy) coated with a thin layer of tin. When the tin coating of a tin can is damaged, the steel rusts more quickly than when it is alone.

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Socioeconomic implications of rustingA) Rusting causes structural weakness and disintegration of metal. B) Billions of dollars is lost due to rusting every year.

Which is the most suitable method to protect a particular iron article from rusting? A) Factors to consider when choosing the most suitable method:

1. How long does the article need to last for?2. Where is the article used?3. What is the article used for?4. Is appearance important?5. Is the cost high?

How to prevent rusting?

How to prevent rusting? A) Using protective coatings1. Coating with paint, oil, grease or plastic

Oil or grease can prevent oxygen and water from reaching the iron. It has a lubricating purpose as well.

2. Tin-plating Tin-plate is iron coated with tin. Tin is an unreactive metal. It can protect iron from air and oxygen and hence prevent rusting. Tin-plating is commonly used in making food cans since tin ions are not poisonous. Note: Tin is a less reactive metal than iron.

3. Zinc-plating Iron coated with a layer of zinc is called galvanized iron and this method is called zinc-plating.

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4. Chromium- plating

• Chromium plating also called chrome plating, often referred to simply as chrome, is a technique of electroplating a thin layer of chromium onto a metal or plastic object.

4. Electroplating We can plate a very thin layer of metal on iron by an electrical process called electroplating.

B) Using sacrificial metals5. Sacrificial protection When a lump of a more reactive metal is attached to a piece of iron, the more reactive metal corrodes instead of the piece of iron. The more reactive metal ‘sacrifices’ itself to prevent the piece of iron from rusting.

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C) Cathodic protection6. Cathodic protection Electricity can also prevent rusting. For example, the negative terminal of a car battery is always connected to the car body. This supplies electrons to the iron body, preventing it from losing electron.

Cathodic protection is often used to protect water / fuel pipelines and storage tanks, ships, offshore oil platforms and onshore oil well casings.

**Recall, rusting:

Fe(s) Fe2+(aq) + 2e-

D) Using corrosion resisting alloys7. Using alloys of iron

Class workConsider the following situations. Choose a method to protect the iron or steel from rusting in each case. Give your reason(s) for each choice. 1) To protect a bus stop sign.Answer:

2) To protect the base of an electric iron.Answer:

Painting because it is cheap.

Chromium-plating because both good protection and shiny appearance are important.

3) To protect the steel frame of a swing in a park.Answer:

4) To protect the body of an old car. Answer:

Painting because it is cheap.

Painting because it is cheap.

5) To protect steel scissor blades.Answer:

6) To protect a steel pencil-sharpener.Answer:

Chromium-plating because both good protection and shiny appearance are important.

Zinc-plating or tin plating because both methods give good protection with medium cost.

7) To protect steel railings along the pavement.Answer:

8) To protect a food can.Answer:

Painting because it is cheap.

Tin-plating because tin ion is non-poisonous and the method gives fairly good protection with medium cost.

Iron rusting could be useful

• Handwarmer

Ingredients in hand warmer:

Apparatus:• Small size Plastic zip-zag bag• Beaker X 1 • Spoon X 1Chemicals:• Iron 3 teaspoon• Salt 1 teaspoon• Activated carbon 2 teaspoon• Vermiculite 2 teaspoon • Water 1ml

Suggest methods to improve the hand warmer

Aluminium foil in acid