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Corrosion of metals and their protection

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Presentation on theme: "Corrosion of metals and their protection"— Presentation transcript:

1 Corrosion of metals and their protection

2 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.

3 Ingredients in hand warmer:
Chemicals: Iron 3 teaspoon Salt 1 teaspoon Activated carbon 2 teaspoon Water 1ml Vermiculite 2 teaspoon

4 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

5 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》

6 Rusting is a slow process
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.

7 What factors speed up the rusting process?
Presence of ionic substances  The rusting process speeds up when water containing ionic substances such as sodium chloride.

8 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.

9 C) Higher temperature At 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.

10 K Na Ca Mg Al Zn Fe Sn Pb Cu Hg Ag Au
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.

11 How to prevent rusting? A) Using protective coatings 1. 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.

12 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.

13 3. Zinc-plating  Iron coated with a layer of zinc is called galvanized iron and this method is called zinc-plating. K Na Ca Mg Al Zn Fe Sn Pb Cu Hg Ag Au How to galvanize metal

14 4. Electroplating  We can plate a very thin layer of metal on iron by an electrical process called electroplating. In the video, it can be iron instead of copper

15 B) Using sacrificial metals 5. 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. K Na Ca Mg Al Zn Fe Sn Pb Cu Hg Ag Au

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