: What causes rusting? ‘Chemicals in the air may attack metals causing the surface to get eaten away. We call this corrosion. Rusting is the corrosion of iron and steel. Only iron and steel rust. An experiment to investigate rusting 1 Stand three identical nails in three test-tubes. 2 Now prepare the test-tubes as below, so that: = test-tube 1 contains dry air — test-tube 2 contains water but no air ~ test-tube 3 has both air and water. 3 Leave the test-tubes to one side for several days. 1 Dryair stopper ary air }— iron nail calcium 8& | Wy—22 2 Boiled water 3 Air and water layer of to keep out \—av unboled water Conrosiin 7 metals * ‘Result After several days the mails in test-tubes 1 and 2 show mo signs of rusting But the milin test tube 3 has aston it. This is because: Rusting requires orygen and water. {mn act the iron is oxidise, inthis reaction: 4Fels)+30,(@ +400) + 2Fe,0,28060) fron + orygen + water — byte iro) oxide (rest) BeamscannerHow to prevent rusting Iron is the most widely used metal in the world - for everything from needles to ships. But rusting destroys things. How can you prevent it? There are two approaches.~ 1 Cover the iron The aim is to keep out oxygen and water. You could use: © paint. Steel bridges and railings are usually painted. © grease. Tools and machine parts are coated with grease or oil. © another metal. Iron is coated with zine, by dipping it into molten zihe, for roofing, Sil is electroplated with zine, for car bodies. Coating with zinc Has a special name: galvanising. For food tins, steel is coated with tin by electroplating. 4. Roofs of galvanised iron called corrugated iron {Beamscamner2 Let another metal corrode instead During rusting, iron is oxidised: it loses electrons. Magnesium is more reactive than iron, which means it has a stronger drive to lose electrons. So when a bar of magnesiumis attached to the side of a steel ship, or the leg of an oil rig, it will corrode instead of the iron. Without magnesium: Fe — Fe?* + 2e7 With magnesium: = Mg— Mg?’ + 2e7 The magnesium dissolves. It has been:sacrificed to protect the iron. This is called sacrificial protection. thing faa ‘The magnesium bar must be replaced before it al] dissolves. Faxing bars of zine toa ship’ hull prevents the ship's steel body ‘rom rusting REE Heavy blocks of magnesium attached to underground pipes made of iron protects the pipes fom rusting RRR. These metas attache are more reactive than trom. ‘and wil corrode instead of iron. As long as magnesia be tine |S present, tron will not rust. This is called sacrificial protection because the magnesium or tn is “acrficed to protect the iron orsted. {BeamscamnerTable 14.10 summarises the common methods of rust prevention. Ropeaes Me on de ‘oiling ox greasing | tools and machine parts If the paint on the metal surface is scratched, rusting will take place under the painted surtace. ‘The protective film of ol or grease gathers dust and must be renewed. plastic coating | kitchenware such as draining racks If the plastic layer is torn, the iron starts to rust. pees water buckets, dustbins, ‘zinc’ ine-plating) _| roots, kitchen sinks ‘The metal does not rust even if the zinc layer is damaged. (This is because zinc is more reactive than iron, 0 zinc corrodes instead of iron.) the tin layer i scratched, the iron beneath it rusts. chrome-plating Heh re ae tes This gives a bright shiny finish as welt as rust protection. metal block of zinc or ‘Magnesium and zinc corrode in place of iron because ‘they are more reactive metals stainless steel Saree vent pipes in chemical plants Stainless steel contains chromium and nickel, which do Rot rust. Beamscanner