<text><span class="style10">etals (3 of 4)</span><span class="style7"></span><span class="style10">Tarnishing and corrosion</span><span class="style7">Nearly all metals are prone to surface oxidation, i.e. the surface of the metal reacts with oxygen or other components of the atmosphere. The major exceptions are the coinage metals and those of the platinum group (see Periodic Table), and even these react with sulfur compounds in industrially polluted atmospheres and turn black. All other metals should, in principle, react with moisture and the oxygen in air, yet some corrode badly and others appear to be inert. In fact, they all oxidize, but in many cases a thin layer of oxide adheres firmly to the metal surface and prevents further reaction. This is the case with aluminum and titanium. On the other hand, iron forms porous oxides that readily break away, allowing corrosion to continue. Stainless steels are produced by alloying iron with chromium and sometimes also with nickel, which form a protective oxide on the surface; the thickness of this layer is so small that the surface still appears shiny and metallic.An alternative way of preventing corrosion is essentially electrochemical. Corrosion can be prevented by connecting a metal object to a piece of more reactive metal and completing the circuit through the earth. The more reactive metal be comes the anode and gradually `dissolves' or degrades. This method is sometimes used for metal tanks that have to stand outdoors: a block of magnesium buried and connected by a wire to the tank slowly oxidizes - it acts as a `sacrificial' anode. This is also the reason why galvanization works. Contrary to what would be expected, the layer of zinc on the iron needs to be somewhat porous so that water can bring the two metals into electrochemical contact. It is then the zinc that reacts instead of the iron.RVP</span></text>
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<text><span class="style10">etal corrosion.</span><span class="style7"> A flake of badly corroded car bodywork, magnified and shown in false color: the rust at the bottom is covered by three coats of original paint and one coat of a later respray. Most metals are prone to corrosion, or surface oxidation; the problem is particularly acute in the case of iron, because it forms porous oxides that break away and allow corrosion to continue.</span></text>
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<text>ΓÇó ELECTROMAGNETISMΓÇó ELECTRICITY IN ACTIONΓÇó ATOMS AND SUBATOMIC PARTICLESΓÇó ELEMENTS AND THE PERIODIC TABLEΓÇó CHEMICAL BONDSΓÇó CHEMICAL REACTIONSΓÇó MINING, MINERALS AND METALSΓÇó IRON AND STEELΓÇó HUMAN PREHISTORY</text>
