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Methods of preventing rusting.

Answer Text:
Prevention of rusting.
Note:
- Rusting destroys materials; equipment and roofs made of iron;
- Rust is porous and thus allows air and water to reach the iron beneath.
- Thus if not removed iron will continue corroding until it is all eaten up.
Methods of preventing rusting.
1. Galvanizing.
- Is the coating of iron with a small layer of zinc;
- Can be done by either dipping the iron object in molten zinc, spraying with a spray of molten zinc, or by electroplating (electrolytic deposition);
- On exposure to air the zinc acquires an inert layer of zinc oxide that is impervious to both air and water;
- The iron beneath is thus prevented from air and water and thus rusting.
Note:
- The iron is protected evenif the zinc coating is scratched.
Reason:
- Upon scratching both the iron and zinc get into contact with air and water;
- Since zinc is more reactive than iron, air and water reacts with zinc at the expense (instead) of iron;
2. Electroplating:
- Refers to electrolytic coating of metals (iron) with less reactive (less corrosive metals);
- This is done through the process of electrolysis where flow of electric current causes the less reactive metal to coat the metal being protected from rusting;
Example:
- Most tin cans are in fact made yup of steel coated with a thin layer of tin.
- Other than being non-toxic tin is unreactive and rarely reacts with the contents of the can or air;
Note:
- Unlike in galvanizing, when an electroplated material gets scratched, the metal underneath (iron) rusts, and very fast;
Reason:
- Both iron and the electroplating metal (tin) are exposed to air and water;
- Since iron is more reactive than tin (the less reactive electroplating material) it reacts with oxygen and water in preference to tin;
- This explain why galvanization is more durable than electroplating;
- Other less reactive metals that are used to coat iron objects include chromium, silver and gold;
- Some electroplating metals such as gold and silver also increase the aesthetic value of the electroplated object;
3. Sacrificial protection.
- Blocks of a more reactive metal such as zinc or magnesium are attached to the iron structure;
- The more reactive metalwill be corroded in preference to iron;
- To keep the iron structure from rusting, the block of reactive metal has to be replaced regularly;
- This metal is used for the protection of underground water pipes as well as ship hulls;
- The blocks of reactive metal are either attached directly to the iron structure or connected to it by a wire.
4. Painting.
- The paint coats the metal surface and thus prevents contact with air and water hence no rusting;
- However if the paint is scratched, rusting occurs quickly;
- It is used mainly in ionrailings, gates, bridges, roofs, ships ad cars;
5. Alloying.
- Alloys are mixtures of two or more metals;
- Thus to prevent iron from rusting it may be mixed with one o more metals resulting into a substance that does not rust;
Example:
- Stainless steel is an alloy of iron with chromium, nickel and manganese and it resistant to rusting.
7. Oiling and greasing;
- Oil is used in moving engine parts while grease is used I other movable metal joints;
- The oil // grease forms abarrier that prevents water and air from coming into contact with the metal surface and hence preventing rusting;
- Oiling and greasing are unique in the sense that they are the only methods that can be used to prevent rusting in movable car parts;