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CORROSIONS

Corrosion of iron

Mechanism of rusting The formation of rust can occur at some distance away from the actual pitting or erosion of iron. This is possible because the electrons produced via the initial oxidation of iron can be conducted through the metal and the iron ions can diffuse through the water layer to another point on the metal surface where oxygen is available. This process results in an electrochemical cell in which iron serves as the anode, oxygen gas as the cathode and the aqueous solution of ions serves as a "salt bridge".

Corrosion occurs in the presence of moisture. For example when iron is exposed to moist air, it reacts with oxygen to form rust, Fe₂O₃. XH₂O.

The amount of water complexed with the iron (III) oxide (ferric oxide) varies as indicated by the letter "X". The amount of water present also determines the colour of rust, which may vary from black to yellow to orange−brown. The formation of rust is a very complex process, which begins with the oxidation of iron to ferrous (iron +2) ions.

Fe Fe⁺² + 2e^–

Both water and oxygen are required for the next sequence of reactions. The iron (Fe⁺²) ions are further oxidized to form ferric ions (Fe⁺³) ions.

Fe⁺² Fe⁺³ + 1e^–

The electrons provided from both oxidation steps are used to reduce oxygen as shown:

O₂(g) + 2H₂O + 4e^– 4OH^–

The ferrous ions then combine with oxygen to form ferric oxide [iron (III) oxide], which is then hydrated with varying amounts of water. The overall equation for the rust formation may be written as:

Equation for the rust formation

Corrosion of aluminium Aluminium is very much prone to corrosion. However, aluminium corrosion is aluminium oxide, a very hard material that protects aluminium from further corrosion. Aluminium oxide corrosion also looks a lot more like aluminium (dull grey to powdery white), hence it is not easy to notice as rusted iron. Aluminium doesn't rust but it is encrusted with dust, calcium, and grease making it difficult to wash.

Corrosion of aluminium

It is a common observation that aluminium vessels lose their shine and become dull very soon after use. This is due to the corrosion of aluminium metal when exposed to moist air. These variables include environment, temperature, alloy in question, flow velocities, impurities present in the environment and chemical conditions to which it is exposed.

An additional factor that affects the corrosion of aluminium is pre–treatment. Many of the factors are controlled by design and construction, such as alloy type, temperature, and flow velocities.

Experiments have shown that pre−filming limits corrosion on aluminium–clad fuel assemblies. In the tests conducted, the ratios of oxide film thickness to non–pre−filmed and pre−filmed elements were on the order of 2 to 3 and in some cases even greater. Impurities are major contributors to the corrosion of aluminium.

Corroded copper Signs of high levels of copper in drinking water include a metallic taste or blue to blue–green stains around sinks and plumbing fixtures. The solubility of these by–products ultimately determines the level of copper at our taps.

Corrosion of Copper

The copper objects lose their shine after some time due to the formation of a layer of copper oxide on them. When a copper object remains in damp air for a considerable amount of time, then copper reacts slowly with the carbon dioxide and water vapour in the air to form a green coating of basic copper carbonate on the surface of the object.

Copper exhibits good resistance to corrosion in urban, marine, and industrial atmospheres. The major factors that control the initial rate of attack on copper are moisture, temperature, and the level of pollution. Soon after exposure of copper to the atmosphere, copper oxidizes, and the bright copper surface takes on a dull tan tarnish. After a few years, this tarnish gradually changes to dark brown or black.

At a later stage the corrosion products of copper turn green. This green coating of basic copper carbonate is a mixture of copper carbonate and copper hydroxide, CuCO₃. Cu(OH)₂. Since copper metal is low in the reactivity series, therefore, the corrosion of copper metal is extremely slow. The corroded copper vessels can be cleaned with a dilute acid solution. The acid solution dissolves green−coloured basic copper carbonate present on the corroded copper vessels and makes them look shiny,red–brown again.

Corrosion of silver The two silver coins on the left side in this picture are freshly made so they are bright and shiny. On the other hand, the two silver coins on the right side are very old so they have been tarnished (or corroded) by the action of air and hence turned black.

Corrosion of Silver

When a shining metal object loses its shine and becomes dull, we say that it has been tarnished. When silver objects are kept in the air, they get tarnished and gradually turn black. This can be explained as follows:

Corrosion of silver by Sulphur compounds:

Silver is a highly unreactive metal so it does not react with the oxygen of air easily. But air usually contains a few sulphur compounds such as hydrogen sulphide gas (H₂S). So, the silver objects combine slowly with the hydrogen sulphide gas present in the air to form a black coating of silver sulphide (Ag₂S).

The shining silver objects become tarnished due to the formation of silver sulphide coating on their surface. Thus, silver ornaments(and other silver articles) gradually turn black due to the formation of a thin silver sulphide layer on their surface by the action of hydrogen sulphide gas present in the air.

Silver is a bright, shiny metal which is chemically quite unreactive. Silver metal is used to make silver coins, jewellery and silverware (such as silver utensils and decorative articles) because of its bright shiny surface and resistance to corrosion.