is often rcgiir(lctl iis ii tlisciisc tltiit. iifllicts : I nlctnl more or 1css fortiiitoiisly ; i i n d it. is coitimoitly I,cIicrctI thiit if tliv ~iictiil I)cltared its ii ~ioriiiiil nictiil sIioit](l, it. \vonltl retiiin iiiilcfiititclg tlic pistitie IJ(!illlty with ~IiicIi it left tlic iiiiinuf;ictii~.~r. Tlic ~irc~tallic stntc is nntitriil :tiid stal)lc only iu tlic rnsc of tlie *' tioliln mctnls * * : in otlicr cnscs i t rcprcscnts n Iiictitstnblc condition from which thcrc mist 1)c n coiwtiint cllort to csciipc i n order . t,hnt stnbi1it.y 11liiY bc rciiclicd. Corrosioii, thcrcforc, iiiiir Iic rcgnrdcti ~iiost simply :IS tlie rcsrtlt of this clfort 011 t,lic .pi\rt, of tplil\cs nletiil t o '. get biick to n:ititrc.'* Recent ivork on tltc corrosion of copper h i i ~ provided i i i i cspcrinicntnl dctiiaiistriition of tlic intiiniitc rchtioiisltiji Iwtmcctt corro.iioti iiiid niiticriil formiit.io~i.~~~ 011 proIoiigc11 rsiio.itire to tlic ojieri air, v o p p r I~~COIIICS covcred wit11 ii c.1tiiritctcristic grccii coiitiiig or *' piititin " wliiclt, itltliorigli it Iii~ppcns to possess ii ilcfitiitc ic.stlict.ic vnliw, is o1)viously t l i v result of :I corrosioti prurcss.
'I'ypical rsiiniplcs of tltc p t i n i i iis it, ouciirs in this country IIII.C I)crti fou~itl to consist itiiiitily of Litsic roppcr sulplintc, cscept nciir the scii-lio:ird, i i t wliicli c:isc bnsic copprr cltloritli~ ~itiiy ~ire~loitiitiiitc ; coittriiry !O the g c w ~n l belief, liiisic CliJiIJCr ciirliottittc is ii itiiitor i i n d uncssentinl rotistitucnt. l'lic p i n t t o wliicli iittcntioit is dircctcd i i t the I I ~O I I I C I ~~ is tliiii tltc coiiiposit ion of the protluct, cliaiigcs progri4v.cly with tiiiic (act\i:iIlg becomiiig i i i ~~~i i ~i ~i ~l ~ hisic) uttril ii1ti1ii:itcIy itlent i ty wit It tlte corrcsl iond I tig miueriil is cst iilJlisltc(1. 1 Itits, in tlic h i t , liiisic copl)cr siilpltiitct coincides in conipo~ition with tlrc initieriil br~ic+li;itititc, CuS0,.3Ci1(011),, iintl Iwic copper cltloritlc with tltc: mineriil iitncitniitc, CuC'1,.3Ci1(0U),. For tltcsc. cl~iingcs to rccicli cotiiyilctioit, ii period of somctliing like 70 yciirs is norninlly rcqitirctl ; ~v l t c i i ottctl tlie cotiipusitioii of the ~nincriil hiis I)ec!ii rciiclicd, Ito\vcvcr, iio fiirthcr 17roiii tlic point of view of clt:ingcs i t t tlic iiiutcriiil :is :I f hole, corrosiwi pltcIi(i~iictiii ~tiiiy IK cotisidcrctl its fiilling into three groups, iis follows. (i) Tlic cliiingcs arc confincd cnt,irc.ly to tlic csposcd surfucc. l k r c tlic iiictnllic residue bcco~~ics progrcssivcly siiiiillcr in tIimciisioiis ns corrosion proceeds, but is otltcrwise ideiiticnl in properties with tltc original nict-al. !l'his inny I)c regiirdcd iis t.hc noritiiil cusc, to which itttcntion will Iic ~nitinly ' tlircctcd iti tlic coiirsi! of tlic lecture. (ii) Tlic cltntigcs iirc Inrgcly coitfiiicd to tltc surfiicr, but it certiiiii :inmitit I J ~ intcrcrystiilliiic penetration takes pliicc ; this leiitls to brittlencss in tltc uitilcrl~i~ig nictnl, iiltltoiigli tltc l~i11k of it 1ii;iy stiNcr 110 cliiittgc. Certain itloininiuni ullogs iire p i r t iculiirlg protit: to this (iii) Tlic cliiiiigcs iirc tlcliiiitcly not confincd to the wfticc, 1 J i l t estciid tliroiigliout tlic niass of tlic iiiatrriiil. 111 cstrciiic ciiscs, whilst the cxtcrnnl for111 liiity not litivc IJCCll iilTcctctl itt, 1111, tlic undcrlying mittcriiil 111iiy Itiivc uttilcrgoiic iL coittplctc cltniigc in conipositioii. By iiniilogy with tlic geological proccss to wliicli thr twin is usiiitlIy itppliccl, sitclt ~~I ' i ~~~~r . I i w l Ic*fi.tc* i l : ~ I!iiritol k ' w t i v i i i t i X ~I V . i , . 1f1:iI. ,> cllitngc takes plncc. of iittiick.
plienonicnn inriy cotivcnieutIy bc clcscribcd as " nictnsoniiitic " in chnritctcr.'g I~s:implcs arc providcd by tbc " gritpltitisntion " of cast iron (in sca wiitcr, soils, or dilute iicids) nnil by tlic so-cnllcd " tlczincificntion ''