The relation between corrosion and paint

Abstract

iiccotiiptuiyiug ligurc the three types iire illustriitctl l)y ciirvcs yicldcd by copper, zinc, i i i i d iron ri!spcctivcly ; itiid iti citclt.c:tso :i ~ilicitoiuicrogrii~~li of t Iic corr~~s~iontliiig rcnction prndiict is given for coml)itrison. Tlic firtit ciirvc--~ qtiiiclriitic pit riilJolit-rcl)rcseitts tlic turnishin,n of coppcr in nil ortliniiry irttloor :itniosplicrc. lZerc tlic reaction procliict, frotti tlio eiirlicst stages, tnkcs t,lic forin of II contittuoits citvclopc, t81irougli nliicli tlic nttncking iitniosplic:re is iiblc to tlilfusc a t it rate that is iiivcrscly proiwrtioiiiil t o tlic tliickticss of the film n t iiny given tinic. Siiicc tlic primtiry filtii is in opt.ionl contiict with tlict iii~!titI, its riitc of thickciting itiity IIC followed by ol)scrviitg tlicl cltiiriictcristic S C~~I C I I C C of i~itcrferc~ico colours to wliicli the process gircs rise. l'lint the process is coiitrollctl cnt.ircly by tltc propertics of the itii~iiilIy-foritic(I f i l i n is sltowi by tlic fiict t l i i t t whcn oiice tlic tiictiil sttrfiicc is covered, tlic composition of tho i i t~i t~~p l i c r~ tiiiiy vary \vitltin wide liinits without ~~rodiiciiig iiuy cltringc iti tltc 1)rogress of tlic tittrick ; the quiidriitic piiritlwlii is uiitlcflrctcd from its coursc.A sul)-division of tlio lirst group is ttcccssriry t o includc tlic case of ti tiictnl sucli its iiluniiiiioni, in which the siirfiicc t~ccouics quickly covcrcd with it tihi wliich, under f i i~o~r i~l~l c conditions, in pritcticnlly iiitpcrvious to furt,licr advnncc of t,lic ntmosplierc ; tlic cliiirrtctcristio curve (slioivn in I)rokcn litics iii tlic fgiirc) tlius bcco~nes ptirtillcl to tlic t h e nsis. It is ivcll ktiowii tlt~it tho resisttilit, qtiiilit.icu of ciluinitii~im iirc tluc cntircly to the pro1)crtics of thc osidc film with which it is normnlly covcrcd ; if tlic cottditioiis nrc so clioscn t h t tltc oxide film ciinnot IIC muiiitiiinctl (ns obtnitis for csimplc.wltcn the s i i r f i i~o is ttniiilgnnintcd) tltcn corrosion 1)rocccds at :t riipicl pnco cliriritctcriRtic of tho intrinsic rctictivity of t,hc mctiil. 21 thC JXIrt Of thC I l l C t i l~ to " gCt lJllCk 10 lllItl1N!." ollly