corrosion prevention

Chemistry only

Corrosion prevention basically falls into 3 categories:

Barrier methods

plastic coated fence Barrier methods of preventing corrosion rely on some sort of barrier to block air and/or water; which are both necessary for corrosion. For example cars and bikes are both painted which blocks both air and water from the surface of the metal. Fences are often plastic coated which again puts up a barrier to air and water. The problem with some barrier methods are once the barrier is broken then the metal will corrode. Painting and plastic coating are also not suitable for moving metal parts e.g. motorcycle and bike chains, engines and tools such as spanners, hammers etc. Here the moving parts maybe covered with oil or grease to help prevent corrosion.

Electroplated objects are metal objects which are given a thin layer of metal as an additional coating. Objects are usually electroplated to make them more attractive to look at or to help them resist corrosion. Car, motorcycle and bike parts for example are often electroplated with chromium metal to make them look shiny and also to help them resist corrosion. Engine parts, exhausts, mirrors and bumpers on cars and motorcycles are commonly electroplated. Inexpensive jewellery or costume jewellery is often made of a cheaper less expensive metal which is then silver or gold plated. Rings, braclets, watches are often electroplated. Cutlery maybe electroplated with silver, an unreactive metal which will resist corrosion from acids found in certain foods. The image below shows common everyday metal objects which are electroplated:

electrolated objects

Sacrifical protection

tin cans Tin cans which are used to hold foodstuffs are actually made of the alloy steel and not the metal tin. Although the steel can has a layer of the less reactive metal tin coating the inside of it. Tin being an unreactive metal resists corrosion and attack by any acids in the foodstuffs better than the steel can.

However if the can is dropped or damaged such that the protective tin coating is broken then the steel can will start to rapidly corrode and spoil the food inside the can. A more reactive metal when connected to a less reactive metal will "sacrifice" itself to prevent the corrosion of the less reactive metal . We can take advanatge of this to help slow down the corrosion of metals e.g.

Galvanising metals

galvanised items

Zinc is a more reactive metal than iron. When a metal item, usually steel is galvanized it is dipped into a bath of molten zinc or is electroplatd using a zinc solution. Either way this coats the metal item in a layer of zinc. This layer of zinc prevents the iron/steel from corroding. However if the zinc layer is scratched or damaged and the iron exposed to air and oxygen it would simply corrode. However the more reactive zinc will now sacrifice itself and slow the corrosion of the iron.

When iron rusts it forms Fe3+ ions by losing 3 electrons, the equation is given below:

FeFe3+ + 3e
However the formation of rust, hydrated iron(III) oxide occurs in a number of steps. One of the steps involves the formation of Fe2+ ions, as shown below.
Fe Fe2+ + 2e
However since the zinc is more reactive than the iron it will immediately reduce the iron ions (Fe2+) back into iron atoms:
Fe2+ + 2e Fe
and the zinc atoms will be oxidised (lose electrons) to form zinc ions (Zn2+):
Zn → Zn2+ + 2e
As long as the zinc and iron are in contact the iron will not corrode. The zinc will sacrifice itself to protect the iron. Unfortunately this method cannot be used in food cans as zinc can be toxic if the dose is sufficiently high. Cars are zinc coated or galvanised to prevent corrosion, this means that even if the paint is chipped and the steel is exposed to air and water no corrosion will occur. Boats and oil rigs have a similar method of corrosion protection. Boats have large pieces of zinc plate bolted to the underside of their hulls and oil rigs have long zinc strips attached to their legs. These zinc strips help prevent corrosion by sacrificing themselves and corroding to protect the steel which makes up the boats and oil rigs. Periodically these zinc blocks will have to be replaced with fresh blocks as they slowly corrode away.

It is not necessary for the object to be completely covered in a more reactive metal in order for it to be protected from corrosion, as is the case with galvanising. As long as the two metals are in contact then corrosion can be prevented. As an example consider underground steel pipes which can be protected from the effects of corrosion by being connected to pieces of scrap magnesium ( a more reactive metal) by a length of wire. The magnesium will corrode or be oxidised and send electrons down the wire which will prevent the steel pipes from corroding. This is simply another example of sacrificial protection. It is sometimes referred to as cathodic protection.

protecting underground pipes from corroding

Alloying

Alloys are mixtures of metals and occasionally non-metals. Mild steel is an alloy made by mixing 99.5% iron with 0.5% carbon. The small amount of carbon makes the iron harder and stronger, mild steel's strength makes it a valuable material for use in bridge building, construction and in making motor car bodies. However mild steel is liable to undergo corrosion. If iron is mixed with chromium (20%) and nickel (10%) a new alloy called stainless steel is made. Stainless steel is harder and stronger than mild steel and it does not corrode, but it is very expensive to produce. Brass (70% copper, 30% zinc) and bronze (90% copper, 10% tin) are another two alloys which are corrosion resistant. They are used to make items such as statues, monuments and musical instruments.

Key points

Practice questions

Check your understanding - Questions on corrosion prevention

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