More Protection Techniques

Direct Electrical Protection

When a metal corrodes it loses electrons (oxidation). If electrons could be ‘pushed’ back onto the metal then it would not corrode.

If the metal is connected to the negative terminal of any direct electrical supply like a battery, a supply of electrons can be provided, and so prevents corrosion.

This method of protection is most widely used in cars and trucks where the negative terminal of the battery is connected to the steel bodywork.

Sacrificial Protection

Water +Ferroxyl indicator

Results:

Test tube Observation

1 Lots of blue colour

2No blue colour + pink colour present

3 Little blue colour

Conclusion:Since Mg is higher in the ECS than Fe, it loses electrons and ‘pushes’ them onto iron.

Mg(s) Mg2+ + 2e-

This prevents Fe from corroding, but Mg corrodes Sacrificial protection.

When two metals are joined together in a simple cell, electrons will flow from the metal higher in the electrochemical series to the one lower down in the series.

Uses: Magnesium, which is higher than zinc in the

electrochemical series will give even better protection as it loses electrons more readily.

Underground gas pipes can be protected from corrosion by attaching scraps of magnesium at regular intervals.

Some ships have blocks of zinc attached to their hulls to protect them against the corrosive action of seawater.

Tin-plating is another example of sacrificial protection. Tin-plate is used to make tin cans for food. If a tin can is bashed or scratched, the steel below becomes exposed. Rusting rapidly occurs as the iron quickly loses electrons which flow to the tin. In this way, the iron is ‘sacrificed’ and the tin is protected.

* Anodising – Protecting Aluminium * Some metal corrode to form a flaky

oxide layer or oxide layer which reacts with other chemicals.

Aluminium smooth oxide layer which does not react further.

Aluminium oxide layer protects metal and so we deliberately increase this layer.

Oxide layer:1Resistant to chemical attack2Allows a dye to be applied – improve appearance