Corrosion The rusting of iron
The rusting of iron • Most metals react with their surroundings to form oxides or hydroxides, e.g. iron rusts to form iron oxide, copper turns green in the atmosphere as copper hydroxide is formed, and silver tarnishes in air-silver oxide is formed. • Gold, stainless steel and platinum are among the few metals which do not corrode. • Corrosion is the changing of the surface of a metal into a compound. • Rusting is the special name given to the corrosion of iron. • Corrosion of metals is very costly, 20% of the annual production of steel is to replace that lost by corrosion.
air + water water/ no air air/ no water dry air oil freshly boiled water (no air present) nail calcium chloride to absorb water water The rusting of iron rusting no rusting no rusting Both air (oxygen) and water are needed for rusting to occur.
Corrosion What happens when iron rusts?
iron iron water containing ferroxyl indicator and sodium chloride water containing ferroxyl indicator What happens when iron rusts?
What happens when iron rusts? • Rusting is a complicated process. However, the first step consists simply of iron atoms losing electrons and forming iron (II) ions: Fe Fe2+ + 2e • Before the iron atoms can lose electron, there has to be a substance present to accept them, such as oxygen. • An electrolyte must also be present. An electrolyte is an ionic compound which conducts electricity when dissolved in water. Electrolytes are often present in rain water. Sodium chloride is an electrolyte, and so rusting occurs faster when salt is present. • To find out if rusting is taking place, we can use ferroxyl indicator. This is a pale yellow-coloured solution which turns blue when it reacts with Fe2+ ions. In general, the more blue colour there is, the more rusting has taken place.
More on corrosion • Reactions which involve a loss of electrons are oxidation reactions. When iron rusts, the initial oxidation reaction is: Fe Fe2+ + 2e • When corrosion occurs metals lose electrons to form compounds. • Rusting is, however, a complicated process which involves more than one step. Further oxidation of the iron (II) ions can occur: Fe2+ Fe3+ + e • The iron (III) ions which are produced can form iron (III) oxide. This is the substance which we call rust.
More on corrosion • When an oxidation reaction occurs there must be a corresponding reduction reaction. In the case of rusting, this involves water and oxygen molecules reacting as follows: 2H2O + O2 + 4e 4OH - • This reduction reaction helps to explain why water and oxygen are needed for rusting to occur. • Water is also required because it usually contains electrolytes. During rusting, electrolytes assist with the movement of electrons from the iron to the water and oxygen molecules. • In most cases, the higher the concentration of electrolytes present the faster the iron will rust.
Corrosion Rusting and redox
mA carbon iron ferroxyl indicator Direction of electron flow during corrosion • A simple chemical cell can be used to show that electrons flow away from iron as it corrodes. • In the iron/carbon cell, the blue colour around the iron shows that it is rusting. • The meter shows that the electrons are flowing from the iron to the carbon. • These electrons are produced by the oxidation of iron. • The pink colour round the carbon is due to the formation of OH- ions produced in the corresponding reduction reaction.
mA magnesium iron ferroxyl indicator Direction of electron flow during corrosion • In this figure there is no blue colour around the iron. • It has not rusted. • This is because electrons have been flowing towards the iron from the magnesium. • Magnesium is higher than iron in the ECS. • The pink colour around the magnesium shows the formation of OH- ions.
mA tin iron ferroxyl indicator Direction of electron flow during corrosion • In the iron/tin cell the rusting off iron is particularly rapid. • Iron is above tin in the ECS. • Electrons flow from the iron to the tin. • This effect is seen with tin-plate which is made from thin sheets of steel plated in tin. • Tin-plate is used to make ‘tin’ cans for food. • If a tin can is badly bashed or scratched the steel may become exposed. • Rusting then occurs rapidly as electrons flow from the iron to the tin.
Corrosion Accelerating corrosion
5 4 1 iron nail + salt water iron nail + water iron nail + acid Accelerating corrosion Extreme Corrosion Extreme Corrosion Corrosion
Factors affecting the rate of corrosion • Salt Cars that are driven during winter or are parked near the sea tend to corrode more quickly than cars that are not subjected to these conditions. This is because the salt dissolves in water to form a solution containing a large number of ions.This provides a much better conducting medium. • Pollutants in the air In industrial areas, especially those containing chemical plants, there are relatively high concentrations of sulphur dioxide, nitrogen dioxide, and carbon dioxide. The gases dissolve in rain water to form dilute acidic solutions.
Corrosion Preventing corrosion by physical methods
Paint Plastic coating Electroplating Galvanising Varnish Oil or grease Tin plating Physical barriers to corrosion A surface barrier to air and water can provide physical protection against corrosion e.g.
Galvanising Iron objects are dipped in molten zinc to give a coating of zinc metal. Galvanising provides physical protection, keeping out air and water. Galvanising can also provide chemical protection because even if the zinc coating is broken electrons can flow from the zinc (higher in the ECS) to protect the iron. • Electroplating Electroplating is a process in which a layer of metal, such as zinc, nickel, silver etc., is deposited on another metal by electrolysis. • Tin-plating Tin provides only physical protection for iron. When the tin coating is broken, the iron rusts rapidly. This is because iron is above tin in the ECS and electrons flow from the iron to the tin.
Corrosion Preventing corrosion by chemical methods
Electrical protection • When a metal corrodes it loses electrons. • If electrons could be forced back into the metal, it would not corrode. • Electrons can be supplied from the negative terminal of a battery. • 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. • Many piers and jetties and even some ocean liners when they are in port also use electrical protection to cut down on corrosion.
Sacrificial protection • When a metal corrodes it loses electrons. • If electrons could be forced back into the metal, it would not corrode. • Electrons can be supplied from a metal higher in the ECS to a metal lower in the ECS. • If iron is in contact with zinc then electrons will flow from the zinc to the iron and this will prevent the iron from losing electrons and corroding. • The zinc loses electrons so it corrodes. • The zinc is ‘sacrificed’ to protect the iron. • Some ships have zinc blocks attached to them. Underground pipes can be protected from corrosion by connecting them to scrap magnesium.