Corrosion

pp. 412-415

What is Corrosion?

Corrosion is the break-down of metals as a result of OXIDATION.

Disadvantages of Corrosion

Advantages of Corrosion

Rusting of Iron

• Redox reaction• Involves Fe (s), O2 (g) and H2O (l).

The water comes from the moisture in the air.

• Corrosion is in the form of rust, which is a mix of oxides and hydroxides of iron. It’s red and flakes off easily allows for further corrosion.

Rusting of Iron: Redox Equation

Factors that Affect the Rate of Corrosion

1. Moisture (presence of water)– The presence of water speeds up the

corrosion rate– Corrosion is nearly impossible without water– Metals in desert climates last longer than

those in temperate climates (where humidity is higher)

2. Electrolytes– Salts (eg. NaCl) are electrolytes that do NOT

cause corrosion, BUT help speed up the process– When mixed with water (eg. snow, rain), the

negative Cl- ion pulls away the positive Fe2+ ions from the surface of the metal. This removes iron.

– Road salt is bad for cars – Cars within reach of the ocean

also suffer from the salt-water

spray in the air

3. Contact with Less Reactive Metals– Corrosion can happen when 2 different

metals come into contact (eg. Copper and steel, copper and iron)

– Metals differ in their reactivity (pg. 386), some are more reactive than others

– To prevent this, use the same metal (copper pipes and copper nails)

4. Mechanical Stress– When metal is bent, shaped or cut – the

point where it is bent, shaped or cut is the weakest point

– Weakest point - This is where corrosion occurs

Blue colour shows where iron nail is being oxidized

Preventing Corrosion

• Corrosion can be costly, especially within the trade industries (eg. metals, automobiles, etc).

• There are three methods to prevent the onset of corrosion, some more effective than others

1. Protective Coatings

• Simplest method

• Cover the metal with:– Rust-preventative paint– Tape– Plastic

• Problem: If chips or scratches occur, then corrosion may start to set in

2. Corrosion-Resistant Metals

• Use metals that are NOT susceptible to corrosion– Stainless steel (mix of iron, chromium, tin)– Aluminum

• Recent models of cars feature these metals, as well as essential items (eg. Sinks, artificial hips, etc)

• Problem: Can be costly.

3. Cathodic Protection

• Using a layer of metal to protect the layer of a similar or different metal underneath– To cover iron or steel, you must use a metal

more reactive than iron (such as zinc) becomes galvanized.

• Problem: May not last as long. Once the metal layer is corroded, you must replace it.