Preventing Corrosion

Corrosion can be prevented in a number of ways: Corrosion can be prevented in a number of ways: Physical protection Physical protection Electroplating Electroplating Direct electrical protection Direct electrical protection Sacrificial protection Sacrificial protection

1. Physical Protection For a metal to corrode it must be exposed to water and air. For a metal to corrode it must be exposed to water and air. To prevent corrosion we need a barrier around the metal to keep air and water out. To prevent corrosion we need a barrier around the metal to keep air and water out.

Paint – Girders of the Forth Railway Bridge are painted to keep air and water away. Oil/grease – Used to cover metal surfaces. Oil/grease – Used to cover metal surfaces. Plastic coating – small metal parts e.g. bolts and axles, dipped in molten plastic. The plastic solidifies to form a thin, protective layer over the metal. Plastic coating – small metal parts e.g. bolts and axles, dipped in molten plastic. The plastic solidifies to form a thin, protective layer over the metal.

2. Electroplating Coating one metal with another metal using electrolytes (Ag, Au, Cr, Zn and Cu). Coating one metal with another metal using electrolytes (Ag, Au, Cr, Zn and Cu). Object to be coated is the negative terminal. It is immersed in a solution that contains ions of the metal it is to be electroplated with. Object to be coated is the negative terminal. It is immersed in a solution that contains ions of the metal it is to be electroplated with.

The positive metal ions are attracted to the negative electrode (object). Once the metal ions reach the negative electrode they GAIN electrons and form a thin layer of metal. The positive metal ions are attracted to the negative electrode (object). Once the metal ions reach the negative electrode they GAIN electrons and form a thin layer of metal.

e.g. silver-plated cutlery e.g. silver-plated cutlery Type of electroplatingUse Gold-platingJewellery Silver-platingJewellery, cutlery Chromium plating Parts used in engines of military helicopters Tin-platingUsed to make material for tin cans

Q. During electroplating, what happens to the mass of each electrode?] A. Mass of positive electrode decreases. Mass of negative electrode increases.

Q. Which metal plating will offer an iron object continued protection when scratch and why? a)Zinc b) Tin A. Zinc because it is higher in the Electrochemical series than iron. Electrons will flow from zinc iron. Tin is lower in the ECS than iron and so electrons will flow from iron tin increasing corrosion of iron.

3. Direct Electrical Metal is attached to the negative terminal of a battery. Metal is attached to the negative terminal of a battery. Electrons are forced back to the metal so that corrosion does not occur. Electrons are forced back to the metal so that corrosion does not occur. This method slows down corrosion, but does not stop it immediately. This method slows down corrosion, but does not stop it immediately. Used for cars, lorries etc. Used for cars, lorries etc.

3. Sacrificial Protection a)Galvanising (Zinc) Object to be protected is dipped into molten zinc. The steel is covered in a tough layer of zinc, which prevents air and water getting to the steel. E.g. car bodies, motorway crash barriers, Forth road bridge. Object to be protected is dipped into molten zinc. The steel is covered in a tough layer of zinc, which prevents air and water getting to the steel. E.g. car bodies, motorway crash barriers, Forth road bridge.

Zinc (physical barrier) Iron Zinc (physical barrier) scratch

b) Attaching Magnesium to iron Underground pipes have bags of scrap magnesium attached to them. Underground pipes have bags of scrap magnesium attached to them. As the magnesium corrodes, it gives its electrons to the iron and so prevents corrosion. As the magnesium corrodes, it gives its electrons to the iron and so prevents corrosion.