GCSE OCR Gateway Chemistry C6.1p Mitigation of Iron Corrosion

C6.1q To be able to explain how mitigation of corrosion is achieved by creating a physical barrier to oxygen and water and by sacrificial protection

How to stop iron rusting

Protective methods

One obvious way of protecting iron and steel from rusting is to coat the metal with a material that water and air cannot penetrate.

Traditionally paint such as Hammerite has been used fairly successfully to protect iron from rusting.  

Similarly, oiling the iron gives a more flexible coating that water cannot penetrate. 

These traditional methods work but are not suitable for all conditions particularly when iron is exposed to sea water a much more vigorous and corrosive environment. 

So how can iron and steel be protected from corrosion especially since most boats and ships are steel hulled?

The answer is in what’s called sacrificial protection.

Sacrificial methods

Because corrosion is an oxidative process connecting iron to a more reactive metal will protect it from rusting.

What happens is that the more reactive metal will oxidise instead of the iron it protects

Typically, magnesium and zinc are used to protect iron .  Its called sacrificial since the other more reactive metal is eventually consumed and sacrificed to protect the iron,

You can see blocks of magnesium bolted to ships hull for this very purpose

You can also set up an interesting and colourful experiment to show sacrificial protection happening. 

You need to make up a hot agar solution containing traces of both potassium hexacyano ferrate(III) (K₃Fe(CN)₆)  and the acid base indicator phenolphthalein.

Pour this solution into test tubes that contain iron nails, one on its own, another wrapped in copper wire, another wrapped in zinc plate and a fourth wrapped in magnesium ribbon. 

Here is the set up:

And in photo:

The blue colour is due to the formation of Prussian Blue a distinctive blue colour that shows the presence of iron (III) (Fe³⁺) ions in rust. 

The magenta colour shows how the magnesium or other reactive metal is protecting iron from corrosion.  The magnesium has reacted to form an alkaline solution hence the phenolphthalein has turned magenta. 

The next photo shows how a boat hull is being protected from corrosion using blocks of magnesium or zinc bolted to the hull. 

The other photo shows a corroded zinc block on a ships hull.

GCSE OCR Gateway Chemistry C6.1p Corrosion of iron and other metals

GCSE OCR Gateway Chemistry C6.1p Corrosion

C6.1p To be able to describe the process of corrosion and the conditions that cause corrosion in iron and other metals.

Corrosion in iron and other metals

Causes and conditions of the corrosion of iron

Iron easily corrodes to form brown rust a flaky substance that peels off the surface of the exposed iron.

It is possible to show that rust is produced from the action of both water and oxygen on the surface of the iron.

Chemically the reaction is an oxidation type reaction in which hydrated iron(III) oxide is formed as in the typical equation below:

4Fe + xH₂O + 3O₂ ⟶ 2Fe₂O₃.xH₂O

This reaction can be confirmed using the following experiment in which simple test tube reactions are set up with either air or water removed.  In both cases, there is a reduction or a complete absence of the formation of rust. 

The first test tube is the control experiment in which there is an iron nail immersed in water and in contact with air.  The iron nail rusts in this tube after a couple of days.

The second tube contains an identical iron nail in water which has been boiled for a significant length of time in order to remove all dissolved oxygen.  This set up is then sealed with a layer of oil to prevent air re-dissolving in the water.  No corrosion is observed after several days.

The third tube contains an identical iron in absolutely dry air.  Calcium chloride (CaCl₂) in the tube removes all water from the air in the tube.  After several days the iron nail has not corroded.

The fourth tube contains the same nail this time immersed in salt water and in contact with air.

If salt water is used instead of water in the control experiment then we observe an acceleration of rust formation.  We see this in the results of the experiment in the photo below:

These results to this simple experiment suggest that water and air are required for iron to corrode.

Corrosion in some other metals

Other metals do corrode but not as severely or as rapidly as iron

Copper

Copper we know and see turns green on exposure to air and rainwater.  Copper looks very attractive when first constructed as we see below:

The green colour is due to the formation of verdigris or basic copper carbonate on exposure to air water and carbon dioxide as we can see here on the Minneapolis City Hall building below:

Aluminium

If aluminium corrodes then its tough adhesive surface layer of one molecule thick aluminium oxide (Al₂O₃) has been broken through.  Usually this occurs in the presence of strong alkali like sodium hydroxide (NaOH) or strong acid such as sulphuric acid (H₂SO₄). 

GCSE OCR Gateway Chemistry C6.1o Alloys

GCSE OCR Gateway Chemistry C6.1o Alloys

C6.1o  To be able to describe the composition of some important alloys in relation to their properties and uses: Alloys to include steel, brass, bronze, solder, duralumin

Alloys

Composition and uses

Name of alloy	Composition	Example of use
Steel	Iron, carbon (0.2%-2% by weight) and other metals commonly from manganese, chromium, nickel, silicon, molybdenum, tungsten, vanadium even uranium.	Tools, sheet metal, pipes, construction materials of all kinds, military shells and armour, cutlery,
Brass	Typically 65% copper 35% zinc but there are a range of specialist types and applications which you can find here	Pipes, plumbing, cartridge cases, coinage, ships bells and propellers, musical instruments etc….
Bronze	Typically 88% copper and 12% tin but there is a range of different types according to their use.  You can find out more here	Coinage, sculpture, architectural features, military weapons etc…
Solder	Typical soft solder is 60% tin with 40% lead though new lead free solders are coming into more common uses such as tin–silver–copper solders (18/64/18%) There is much more to be found here	Electronics, avionics, electrical applications of many types, plumbing,
Duralumin	Typically duralumin contains Aluminium with copper 4%, manganese 0.5-1% magnesium 0.5-1.5% and possibly some silicon.	Aircraft construction, electrical cabling

In this table what I’ve done is to take a specific example of each of the alloys and link it to its use based on that specific property.

Alloy	Property	Use
High carbon steel iron with 2% carbon sometimes called cast iron	Hard, strong but brittle	Pots e.g. cast iron ware such as Le Creuset cookware
Cartridge brass 70% copper 30% zinc	Good cold working properties	Ammunition cases, plumbing and hardware
Bell bronze 20% tin 80% copper	Highly resonant and sonorous	Bells
Soft lead solder 63% tin with 37% lead	Specific low melting point of 183oC the lowest of all lead solders and a true melting point not a melting range.	Electrical and electronic work
2024 aluminium alloy or duralumin that is aluminium with 4.5% copper 0.5% manganese and 1.5% magnesium	It has a low density of 2.78g/cm3	Used to form aircraft parts