consider the dissolution of led n hydrochoric acid . the standard electrode potential ( E0) of lead is -0.13 volt . it should therefore , readily , dissolve in hydrochloric acid , evolving hydrogen . zinc (  E0 = -0.76 volt ) shouild do so even more readily . however , in practise , neither lead nor zinc dissloves in hydrochloric acid nor does the evolution of hydrochloric acid take place . this is because of hydrogen overvoltage . the bubble overvoltage of hydrogen over lead is 0.64 volt and that over zinc is 0.70 volt . however , if pure zinc is connected to a piece of copper and both re dipped in dilute acid solution , ( as in simple voltaic cell ) , the dissolution of lead take place vru readily and hydrogen is evolved at the copper electrode .

in this case , while zinc electrode dissolves to produce zn2+ ions , the hydrogen ions are discharged and evolved as hydrgen gas at the copper electrode , the bubble voltage over coper being only 0.2 volt . similar effect is observed if zinc conmtains copper orany other metl having  low hydrogen overvoltage as he impurity . in this case zinc dissolves giving zn2+ ions while hydrogen is evolved at the copper part ( or the impurity part ) of the znc rod itself . this results in corrosion or dissolution of zinc . the same thing can happen to any other base metal ( lead , iron etc. ) if it contains a metal with low hydrogen overvoltage as the impurity .

in the presence of oxygen or air or any other oxidising agent ( depolarizer ) , there may be no evolution of hydrogen gas as it may get soon oxidised as formed . there is thus no evolutin of the gas . therefore , even if a base metal ( lead , iron etc . ) contains a metal with a high hydrogen overvoltage , it will dissolve in acid solution readily in  the presence of air and oxygen . this is the reason why iron " present " in tin plate gets readily corroded in air . but in the absence of oxygen or air it does not dissolve at all because of high hydrogen overvoltage over tin .

it is also a well known laboratory experience that pure lead and pure zinc which , ordonarily , do not dissolve in acidic sol;utin , do so if air is bubbled in the solution .

corrosion may be inhibited by coating the metal surface with an impermeable layer of a suitable metal suc as a paint . this may prevent the excess of air . however , this method fails if  the paint becomes porous because of the atmospheric oxygen then gets acess to the exposed metal , and corrosion occurs beneath the layer of the paint . another technique of preventing corrosion is galvanising i.e. coating a piece of iron with zinc . since the reduction potential of zn2+ , zn couple is more negitive then that of fe2+,fe couple , the corrosion of zinc is thermodynamically favoured  and the iron survives ( the zinc survives because it is covered with a ydrated layer of zinc oxide . )