RUSTING

 

INTRODUCTION

When reactive metals come into contact with oxygen and moisture, oxides of those metals are produced. This process is known as oxidation. More reactive metals show stronger oxidation effects. The oxidation reaction causes them to wear off. One metal which shows stronger oxidation reaction is iron. 

 When iron reacts with oxygen or moisture, in a process known as corrosion, iron (III) oxide (hydrated iron oxide) is formed. This iron (III) oxide is a thin reddish-brown layer that forms on the surface of the iron called rust. 

The oxide is a solid that retains the same general form as the metal from which it is formed but, porous and somewhat bulkier, is relatively weak and brittle. Corrosion and rust Corrosion is the chemical reaction which causes metals to wear away when it reacts with air and moisture. When corrosion persists for a long time, it results in rust.

 Rust is the reddish-brown coating on the surface of iron or steel that forms when the metal is exposed to air and moisture.

Conditions necessary for corrosion or rusting of iron 

 Air or oxygen 

 Water or moisture or vapour 

NB: Both conditions must be available for rusting to occur.

 Experiment to show that air and water are necessary for rusting of iron 

 Collect three test tubes, labelled A, B and C, with stoppers, clean iron nails, freshly boiled water (with the dissolved air in it removed), silica gel and liquid paraffin. 

 Place a nail in each test tube. 

 Fill test tube A with ordinary tap water and stopper it. (Tap water contains dissolved oxygen) 

 Fill test tube B with the freshly boiled water and cover the water with a layer of the oil and stopper it. (The layer of oil prevent air from entering the water) 

 Put a small quantity of the calcium chloride in test tube C and stopper it. (The calcium chloride absorbs moisture and keeps the atmosphere dry). 

 Leave the test tubes for 4 to 6 days. 

Observation 

After four days, it would be observed that the nail in test tube A has begun rusting whiles the nails in test tubes B and C show no sign of rusting. Conclusion Test tube A, which contain both water and air, supports rusting while test tubes B and C, which have water only and air only respectively, do not support rusting. Therefore, both water and air are necessary for rusting.

Reactive metals which do not rust

 Some metals, such as aluminium, although very active chemically, appear not to corrode under normal atmospheric conditions. Actually, aluminium corrodes rapidly, and a thin, continuous, transparent layer of oxide forms on the surface of the metal, protecting it from further rapid corrosion.

Lead and zinc, although less active than aluminium, are protected by similar oxide films. Copper, a comparatively inactive metal, is slowly corroded by air and water in the presence of such weak acids as carbonic acid, producing a green, porous, basic carbonate of copper. Green corrosion products, called verdigris or patina, appear on such copper alloys as brass and bronze, as well as on pure copper. 

Non-reactive metals which do not rust 

Some metals, called noble metals, are so inactive chemically that they do not suffer corrosion from the atmosphere; among them are silver, gold and platinum. A combination of air, water, and hydrogen sulphide will act on silver, but the amount of hydrogen sulphide normally present in the atmosphere is so small that the degree of corrosion is negligible except for the black discoloration, called tarnishing, produced by the formation of silver sulphide. 

PREVENTION OF RUSTING

Rusting and corrosion destroy iron and steel structures. The most effective method of preventing iron or steel from rusting is to keep it away from water and air. 

The following are some methods of keeping iron from water and air. 

Galvanizing 

Galvanizing is the process of coating the iron or steel with a thin layer of zinc. Since zinc does not rust, the zinc layer protects the base iron from rusting. Some examples of commonly galvanized products are garbage cans, corrugated sheets for roofing, iron pipe, and fence wires. 

Enamelling 

Enamelling involves coating the surface of the metal with enamel. Enamel is a special paint which consists of zinc oxide and lithopone in brown linseed oil and high-grade varnish. Products which are normally enamelled are refrigerators, gas cookers, auto mobiles etc. 

 

                                                                       (Enamelled ware) 

Painting 

Oil-based paints when applied to the surfaces of irons harden and become affixed, thereby preventing air and water from having direct contact with the iron. 

Tinplating 

Cans and tins, especially those for commercial products are often tinplated. Tin plating involves coating the surface of the iron or steel with a thin film of tin (which is resistant to rusting). 

 Electroplating 

Cadmium, chromium, copper, gold, nickel, silver and tin are electroplated on irons or steel. This is because they resist rusting. In electroplating, the iron is placed in a solution and is connected to the negative terminal with the coating metal connected to the positive terminal of an electricity source.

Current is then allowed to pass through which causes atoms from the coating metal to move to cover the surface of the base iron. 

 Alloying 

One effective method of preventing corrosive metals from rusting is chemically mixing them with metals which do not rust. Iron, for example, is normally alloyed with carbon and chromium or nickel to give a rust-proof metal known as stainless steel. Objects which are made from stainless steel include cutlery set, surgical instruments etc. 

Cladding 

 Cladding is the process of bonding a non-corroding metal to a corrosive metal. One example of a clad metal is rolled gold, which consists of an iron or steel core with a layer of gold on the outside. Cladded aircraft components may have a thick layer of strong aluminium alloy in the centre and thin outer layers of corrosion-resistant pure aluminium sheets. The various layers or plies of metal are usually heated and rolled together. Other cladding methods include casting, welding, and pouring molten metal around a hardened core. In addition to sheets and strips, clad metals are produced in wires, bars, and tubing. 

Oiling and greasing 

Corrosive metals can be oiled or greased to protect their surfaces from rusting. One disadvantage of oiling or greasing is, it does not last long and the metal will have to be oiled or greased regularly. 

Plastic coating 

Some metals are also coated with a thin film of plastic. Such metals are prevented from contact with fire or heat as that will cause the plastic coating to melt off exposing the underlying corrosive metal.