10.08.4 Preventing Corrosion

Syllabus

Corrosion is the destruction of materials by reactions with substances in the environment.

Rusting is an example of corrosion. Both air and water are necessary for Iron to rust.

Corrosion can be prevented by applying a coating that acts as a barrier, such as greasing, painting or electroplating.

Aluminium has an Oxide coating that protects the metal from further corrosion.

Some coatings are reactive and contain a more reactive metal to provide sacrificial protection, eg Zinc is used to galvanise Iron.

Students should be able to:

1. describe experiments & interpret results to show both air & water are needed for rusting
2. explain sacrificial protection in terms of relative reactivity.

What does this mean?

What causes rusting?

Rusting is an oxidation reaction in which Iron picks up Oxygen.

But the rust formed is not pure Iron Oxide, it also contains water.

Iron + Oxygen + Water --> Hydrated Iron III Oxide (rust)

All three reactants are needed for rusting.

So, rust won't form on Iron in dry air - which is why surplus military equipment and aeroplanes are sometimes kept in dry deserts.

And rust can't form on Iron in water without Oxygen (such as the very deep ocean) which is why so much of the Titanic is still there over 100 years after it sank.

The Rusty Nail Experiment

Chemistry examiners love the rusty nail experiment.

Examiners are strange people.

This means that it's a good idea to learn the results of this experiment and what those results mean

A typical Chemistry Examiner

In the experiment to the right- in A there is plenty of water and Oxygen.

It will rust particularly at the surface of the water (salt or acid would speed up the rusting but are not necessary - they are catalysts).

In B the water was boiled to remove Oxygen and then sealed with oil to stop more dissolving.

It will not rust.

In C there is plenty of Oxygen and no water vapour because a drying agent has been added.

It won't rust

So, the experiment proves that Water and Oxygen are both needed

Preventing rust.

Barriers

The simplest way to stop rusting is to provide a barrier that prevents either Oxygen or Water or both getting at the surface of the Iron.

Paint is a good option for the body-work of a steel car - but if scratched this method stops working.

Moving engine parts, or bike-chains, can't be painted because the paint would flake off. But they can be oiled and greased to keep out water and Oxygen.

Chain-link fence and padlock chain is sometimes coated with plastic to stop the steel wires rusting - this only works until the first hole appears in the coating, however.

Sometimes Iron is plated with less reactive metals such as Chromium or even Gold

These metals don't corrode, so no oxygen or water gets at the Iron below - until the coating is scratched.

Then the rust will spread below the coating & it will all flake off.

Galvanising

Galvanising is coating Iron with a more reactive metal - Zinc

Zinc reacts with Oxygen to form Zinc Oxide, which is a hard substance without cracks or pores.

It stops the Oxygen/water vapour getting at the Iron below and, if scratched a little, will simply generate more Zinc Oxide.

Galvanised corrugated iron roofing can last for decades before rusting.

Sacrificial protection.

It is possible to slow the reaction of water with iron by simply attaching blocks of more reactive metals to the Iron.

Zinc blocks are frequently hung from the sides of ships or attached to buried Iron pipes.

These don't cover the Iron but the sea-water reacts with the blocks in preference to the Iron, stopping the ship corroding so quickly.

Sometimes Zinc is referred to as a Sacrificial coating, rather than Chrome plating which would be a Noble coating.

We could show the result of sacrificial protection in College by putting Iron nails in water to rust and attaching some to more reactive metals, some to less reactive metals and leaving some alone.

Zinc is more reactive than iron: the water corrodes the Zinc and leaves the nail un-rusted.

Silver and Copper are less reactive than iron; the water rusts the nail and leaves the less reactive metals un-corroded.

The Iron nail that is left alone is there purely as a comparison - a control.