Effect of Zinc Phosphate on the Corrosion Behavior of Waterborne Acrylic Coating/Metal Interface

13 Pages·2017·5.62 MB·English

materials
Article
Effect of Zinc Phosphate on the Corrosion Behavior of
Waterborne Acrylic Coating/Metal Interface
HongxiaWan1,DongdongSong1,2,*,XiaogangLi1,3,*,DaweiZhang1,JinGao1andCuiweiDu1
1 InstituteforAdvancedMaterialsandTechnology,UniversityofScienceandTechnologyBeijing,
Beijing100083,China;[email protected](H.W.);[email protected](D.Z.);[email protected](J.G.);
[email protected](C.D.)
2 AerospaceResearchInstituteofMaterialsandProcessingTechnology,Beijing100076,China
3 NingboInstituteofMaterialTechnologyandEngineering,ChineseAcademyofSciences,
Ningbo315201,China
* Correspondence:[email protected](D.S.);[email protected](X.L.);
Tel.:[phone](D.S.);[phone](X.L.)
AcademicEditor:Yong-ChengLin
Received:22April2017;Accepted:30May2017;Published:14June2017
Abstract: Waterbornecoatinghasrecentlybeenpaidmuchattention. However,itcannotbeused
widely due to its performance limitations. Under the specified conditions of the selected resin,
selecting the function pigment is key to improving the anticorrosive properties of the coating.
Zincphosphateisanenvironmentallyprotectiveandefficientanticorrosionpigment. Inthiswork,
zinc phosphate was used in modifying waterborne acrylic coatings. Moreover, the disbonding
resistanceofthecoatingwasstudied. Resultsshowedthataddingzincphosphatecaneffectively
inhibit the anode process of metal corrosion and enhance the wet adhesion of the coating, and
consequently prevent the horizontal diffusion of the corrosive medium into the coating/metal
interfaceandslowdownthedisbondingofthecoating.
Keywords: waterborneacryliccoating;zincphosphate;LEIS
1. Introduction
Seriousenvironmentproblemshavedirectedattentiontowardwater-basedcoatingsbecauseof
theirlowvolatileorganiccompoundsVOCadvantages[1–3]. However,waterbornecoatingshave
performancelimitations; moreover, theyhavebeenextensivelyusedindoors, wherethecorrosion
environmentisbelowtheC3level[4]. Thus,waterbornecoatingsneedtohaveimprovedperformance
forlong-termapplications[5–8]. Acryliccoatingshavegoodbarrierpropertiesandageingresistance,
which has attracted much attention [9,10]. Disbonding is the first sign of coating/metal interface
corrosion [11–14], which is the most common form of failure that limits coatings, especially for
long-termapplicationsofwaterbornecoatings[15].Hence,theabilitytoresistdisbondingcharacterizes
thecoatingservicelife,whichisthemainpurposefordevelopinglong-lastingwater-basedcoatings
suitedforharshenvironments[16]. Traditionalcoatingswouldexhibitsignificantcathodicdisbonding
underdifferentenvironmentsthatwouldaffectthecoatingservicelife. Addinganelectrochemically
activeantirustpigmentisacommonmethodforimprovingtheantirustcapacityofthecoating[17,18].
Therefore,theeffectofanelectrochemicallyactiveantirustpigmentonthecathodicdisbondingprocess
whenaddedintoawaterborneacryliccoatingissignificant.
Muchattentionhasbeengiventophosphateantirustpigmentsfortheirrelativeenvironmental
protection[19–23]. Phosphatepigmentsmainlycontainaluminumtripolyphosphate[24]andzinc
phosphate[25]. Theantirustfunctionandmechanismofzincphosphatehavebeenpreviouslystudied.
Bethencourtetal.[26]studiedthefailureprocessofawaterborneacryliczincphosphatecoatingusing
Materials2017,10,654;doi:10.3390/ma10060654 www.mdpi.com/journal/materials
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electrochemicalimpedancespectroscopy(EIS).Thecorrosivefailureofcompositecoatingsin3.5%
NaCl solution were divided into three stages: the first stage is the water diffusion stage, wherein
Materials 2017, 10, 654 2 of 13
thecoatingimpedancedecreases; thesecondstageinvolvesthezincphosphatecomingincontact
withcwoaattienrg,sw inh 3e.r5e%in NaaCpal ssosilvutaitoino nwreures tdirveisdiesdta inncteo tehffreeec tstraegaecsh: ethsea ficrestr tsataingec oisn tcheen wtraatteiro dnifafuftseiornw ater
absorsptatigoen, ;wanhedrethine tthheir dcosattaingge iismthpeedsatnarcte odfetchreeassheise;l dthine gsedceocnlidn estaafgtee ritnhveolzviensc tphheo zspinhca ptehiosspexhhataeu sted.
YawecioSmhinaMgoa teeirnital sa c2l0o.1n7[,t2 1a70c, ,t62 584w ] isthtu dwiaetder,t hwehceorrerino saio npaisnshiviabtiitoino nruosft zrienscisptahnoces pehfafetcet inreaacnheosi 2l -aobf 1ac3es retdaine poxy
concentration after water absorption; and the third stage is the start of the shielding decline after the
coatingusingEISandelectrochemicalnoise,andshowedthatzincphosphateprovidesarecovery
zinc pcohaotsinpghsa itne 3i.5s% e xNhaaCul sstoeldu.t ioYna wweerie Sdhivaiod edet inatlo. t[h2r7e,e2 8st]a gsetus:d thieed f irtsht es tacgoer rios stihoen w iantehri bdiitfifounsi oonf zinc
effectforcoatingdefects. Currently,researchers[29]foundthatwhenthecorrosivemediumpermeates
stage, wherein the coating impedance decreases; the second stage involves the zinc phosphate
phosphate in an oil-based epoxy coating using EIS and electrochemical noise, and showed that zinc
theoil-basceodmeinpgo xiny ccoonattaicnt gw/imthe twaaltienr,t ewrfhaecreei,nz ian cpapshsiovastpiohna treuspt arretsliystdanisces oelfvfeecst arneadchfoesr ma scmerteatina lsubstrate
phosphate provides a recovery effect for coating defects. Currently, researchers [29] found that when
phosphatecoanncdenstrtaatiboinli azfetesr twhaetepr raobtseorcptitivoen;fi alnmd ,thwe thhiicrdh sctaognet iasi tnhse sγta-Frte ofO the, sαhi-eFldeiOngO dHec,liγne-F aeftOerO thHe ,andiron
the corrosive medium permeates the oil-based epoxy coating/meta2l in3terface, zinc phosphate partly
zinc phosphate is exhausted. Yawei Shao et al. [27,28] studied the corrosion inhibition of zinc
phosphate. Moreover,theprotectivefilmcontainscarboxylandhydroxylgroupsthatenhancethe
dissolpvheoss pahnadt ef ionr mans o mil-beatasel ds uepbosxtrya ctoea ptinhgo supshinagt eE IaSn adn ds tealebcitlriozcehse mthiec apl nrootiseec,t aivned fsihlomw, ewd hthicaht z cinocn tains
combγi-nFaet2iOpoh3n,o αsop-fFhaectOoe OaptrHionv, giγde-aFsn eaOd reOcmoHve,et raaynl .edff Seirocotm nfo erp chrooeasstipenhagar dcteehf.e eMcrtsso. r[Ce3uo0rv]reehnr,ta ltyvh,e ere ppserraoortcpehcoetrsisve [ed2 9fit]l hfmoau tcnozdni tnthacaitn pwsh hcoeanrs bpohxaytle can
formasntdab hltyehdei rrcoooxnrryopls ghivroeos umppehsda ituthemast paeennrhmdaebnalctoeesc tkthheem cooeilmd-bibausimneda tdeipoiofnfx uoysf c icoooanatitniingn/tgmo aetnthadle imnctheetraafnal.cn eSe,o zlmsinaec nrpedhsoessalporhcwahteed rpsoa [wr3tl0ny] choarvreo sion.
Howpervoepro,dstiehsdseo ltevhfeafste aczntinsdc of opfrhzmoisns pmchepathatelo scsuapbnhs tfaroatrteme ap schttoaisbvpleeh afiitrelol neanr psdh osotnaspbtihhliazeteesws t ahatnee dpr rbbooltoerccntkiev mea ecfidrlmyiul, miwc hcdiaicfhtfh ucosoidnotinac iindnst ios b tohen ding
procecshsa,nensγpe-lFesec 2iaOan3ld,l yα s-iFlnoevOwoO ldHvoi, nwγg-nF tehcOoeOrirHno,ts eaionrnfda. icrHeonoc woprherovosepsrih,o antthe.me M eeofcrfheeocavtnsei rso, mtfh zeo ipnfrcco atpetchhtooivsdep ifhcilamatne c doancattnaivioneds icfcialrlpberorosx ycole ns stehse, have
and hydroxyl groups that enhance the combination of coating and metal. Some researchers [30] have
waterborne acrylic cathodic disbonding process, especially involving the interface corrosion
notbeencomprehensivelystudied.
proposed that zinc phosphate can form stable iron phosphates and block medium diffusion into the
mechanism of cathodic and anodic processes, have not been comprehensively studied.
In thicshapnanpeles ra,nwd esloswtu ddoiewdn tchoerrocsoioant.i nHgo/wmeveert,a lthien eteffrefcatsc eof czoinrrc opshioosnphparteo caectsivseo ffillweras toenr bthoer ne acrylic
In this paper, we studied the coating/metal interface corrosion process of waterborne acrylic
coatingswwiathterzbionrcnep hacorsyplich actaethuosdiinc gdiEsbISonadnindg lopcroacleizsse, desEpIeSci(aLllyE ISin)v.oFlvuirntgh ethrem oinrteer,fwacee sctourrdoiseiodn theeffects
coatinmgesc whainthis mzi nocf cpahthoosdpihc aatned u asniondgi cE pISro acnesds elso,c haalvizee ndo Et bISee (nL cEoImS)p. rFeuhretnhseivremlyo srteu,d wieed .s tudied the effects
of active pigments on coating/metal interface corrosion and disbonding processes in waterborne
of active Ipni gthmise nptasp eorn, wceo asttiundgie/md ethtae lc oinattienrgfa/mcee tcaol rinrotesrifoance acnordr odsiiosnb opnrdociensgs opfr owcaetsesrbeso rinne wacarytelircb orne
acrylaiccrcyoliaccto icanotigantsgi.ns gwsi.t h zinc phosphate using EIS and localized EIS (LEIS). Furthermore, we studied the effects
of active pigments on coating/metal interface corrosion and disbonding processes in waterborne
2. Re2s.u Rltessaucrltysli c coatings.
2.1. T2h.1e. CToh2ae. tRCineosgautlAitnsdg hAedsihoensioTne sTtest
FiguFr2ei.g11u. Trshehe 1 oC wsohastoinwtghs eA tdhahede shaiodenhs Tieoessnito n v avlaulueseso off0 0%% aanndd 88%% zzinincc pphhosopshpahtea tceoactoinatgisn. gTsh.eT ahdeheasdiohne sviaolnuevsa lues
werewtheeres tahme seFaiimgnuert ehi n1e st0hh%oew 0as% nth daen a8dd% h8e%sziio nznic nvcpa lhpuehosos ospfp h0h%aat taeen ccdoo 8aa%ttii nznigngscs. p.AhAdosddpidhniagnt egz cinozacitn ipnchgpos.sh Tpohhsea patdeh hadetisedio dnn oivdta lanufeofest cat ftfheec tthe
initiailnciotiaaltw inceorgea mttihneeg cs hammaenec iihcnaa tlnhepic e0a%rl f opanremdrf o8a%rnm czeain.nccT peh.h eoTschpoeha atctieon acgotianftrigna gcfstr.ua Acrtedusdriaensfgt ezarifnttech rp ehthopseup lhpla-uotelfl f-doiedfxf npeoextr paiefmfreiecmtn tethsnet asr earseh own
shownin iitnia Fl icgouartien 2g. mechanical performance. The coating fractures after the pull-off experiments are
inFigure2.
shown in Figure 2.

Figure 1. The adhesion values of 0% and 8% zinc phosphate coatings.
Figure1.Theadhesionvaluesof0%and8%zincphosphatecoatings.
Figure 1. The adhesion values of 0% and 8% zinc phosphate coatings.

Figure 2. The fractures after the pull-off experiments in (a1,a2) 0% and (b1,b2) 8% zinc phosphate
coatings.
Figure 2. The fractures after the pull-off experiments in (a1,a2) 0% and (b1,b2) 8% zinc phosphate
Figure 2. The fractures after the pull-off experiments in (a1,a2) 0% and (b1,b2) 8% zinc
c oatings.
phosphatecoatings.
Materials2017,10,654 3of13
Figure2showstheadhesivefractureinthesampleswithoutandwith8%zincphosphatecoating.
Materials 2017, 10, 654 3 of 13
Thefracturemodeforthe8%zincphosphatecoatingafterthepull-offexperiment,whichisthemixture
Figure 2 shows the adhesive fracture in the samples without and with 8% zinc phosphate
ofthecoatingbodyandadhesiveinterlayerfractures,showedstrongadhesion. Thisresultisthesame
cMoaatetriinalgs .2 0T1h7,e 1 f0r, a65c4tu re mode for the 8% zinc phosphate coating after the pull-off experiment, wh3ic ohf 1is3
with the sample without zinc phosphate coating, only the ratio of the coating body and adhesive
the mixture of the coating body and adhesive interlayer fractures, showed strong adhesion. This
interlayerfrFaicgtuurree 2s isshodwifsf etrheen ta.dhesive fracture in the samples without and with 8% zinc phosphate
result is the same with the sample without zinc phosphate coating, only the ratio of the coating body
coating. The fracture mode for the 8% zinc phosphate coating after the pull-off experiment, which is
and adhesive interlayer fractures is different.
2.2. ThetCheo amtiinxtguMre oorfp thhoel ocgoyating body and adhesive interlayer fractures, showed strong adhesion. This
2re.2s.u Tlth ies Cthoaet sinagm Me woripthho tlhogey s ample without zinc phosphate coating, only the ratio of the coating body
Figure3showsthescanningelectronmicroscope(SEM)sectionofthe8%zincphosphatecoating.
and adhesive interlayer fractures is different.
Figure3ashFoigwurse th3 eshsoewcos ntdhea rsycaenlneicntgr oenlecmtroonrp mhioclroosgcyopaen d(SEFMig)u sreect3iobn sohfo twhes 8th%e zsicnac ttpehroisnpghaetlee ctronic
coating. Figure 3a shows the secondary electron morphology and Figure 3b shows the scattering
morpho2l.o2g. Ty.heT Choeatzining cMpohrpohsoplohgya tecoatinghasahomogeneousdispersionandcombinedwiththeresin
electronic morphology. The zinc phosphate coating has a homogeneous dispersion and combined
tightly. MorFeiogvuerer ,3a dshdoiwngs zthine cscpahnonsinpgh aelteectdroidn nmoitcrsohscoowped (eSfEeMct)s .section of the 8% zinc phosphate
with the resin tightly. Moreover, adding zinc phosphate did not show defects.
coating. Figure 3a shows the secondary electron morphology and Figure 3b shows the scattering
electronic morphology. The zinc phosphate coating has a homogeneous dispersion and combined
with the resin tightly. Moreover, adding zinc phosphate did not show defects.
Figure 3. The section morphology of the 8% zinc phosphate coating: (a) secondary electron morphology
Figure3.Thesectionmorphologyofthe8%zincphosphatecoating:(a)secondaryelectronmorphology
and (b) back scattering morphology.
and(b)backscatteringmorphology.
FFiigguurree 3 .4 T shheo swecsti otnh em morapchroolomgyo ropf hthoel o8%gy z ionfc tphheo scpohaattien cgo awtiintgh:o (au)t s aecnodn dwariyth e l8e%ctr oznin mc oprphhooslpoghya te in
3.5% aNnda C(bl )s boalcukt isocnat taefrtienrg v maoriropuhso liomgym. ersion days. At the beginning, the 0% and 8% zinc phosphate
Figure4showsthemacromorphologyofthecoatingwithoutandwith8%zincphosphatein3.5%
coating morphologies were the same. However, after two days of immersion, the 0% zinc phosphate
NaClsolutioFniguarfete 4r svhaorwiosu tsheim mmacerrosmioonrpdhoalyosg.yA otf tthhee cboeagtiinngn iwnigth,otuhte a0n%d wanitdh 88%% zziinncc pphhoospshpahtea tien coating
coating generated obvious corrosion. The coating generated serious corrosion after four days of
morpho3l.o5%gi eNsawCle sroeluthtioens aafmteer .vHarioowuse ivmemr,earfstieonr tdwayos.d Aaty tsheo fbeimginmneinrgs,i othne, 0th%e a0n%d 8z%in zcinpch pohsopsphhaattee coating
immersion, and the defective coating bubbled after eight days. In the 8% zinc phosphate coating, rust
generatoceocdcautoirnbrgev dmi oaoufrtepsrhc fooolurorgro idesasi yowsn eo.rfeT i mthhmee scearomsaioetni. nH agnodwg eiennvceerrer,aa astfeetdedr a tsfwteerori odeiaugyhsst c odofa riymrso,m sbieuortsn niooan fb,t utehbrebf 0loi%nug rz widnacas yp oshboossefprivhmeadtme. ersion,
coating generated obvious corrosion. The coating generated serious corrosion after four days of
andthedefectivecoatingbubbledaftereightdays. Inthe8%zincphosphatecoating,rustoccurred
afterfouimrmdaeryssioonf, aimndm theer dsieofenctaivn ed coinatcirnegaa b0su%ebd bZalnef3dt(eP arOftee4)ir2g ehigthdt dayabys s8,.%b Inu Z ttnhn3e(o P8O%b4u )z2bi nbcl pinhgoswphaasteo cbosaetirnvge, dru.st
occurred after four days of immersion and increased after eight days, but no bubbling was observed.
a 0% Zn3(PO4)2 b 8% Zn3(PO4)2
0 day
0 day
1 day
1 day
2 days
2 days
Figure4.Cont.
Materials2017,10,654 4of13
Materials 2017, 10, 654 4 of 13
Materials 2017, 10, 654 4 of 13
4 days
4 days
8 days
8 days
Figure 4. The macromorphology of (a) 0% and (b) 8% zinc phosphate coating scratches with different
Figure4.Themacromorphologyof(a)0%and(b)8%zincphosphatecoatingscratcheswithdifferent
immersion days.
immersiondays.
Figure 4. The macromorphology of (a) 0% and (b) 8% zinc phosphate coating scratches with different
imTmheer smioinc rdoasytrsu. ctures after immersion are shown in Figure 5. The coating defects without zinc
phosphate bubbled in 3.5% NaCl solution after eight days, and that with 8% zinc phosphate did not
The microstructures after immersion are shown in Figure 5. The coating defects without
buTbbhlee . mTihcersoes trreuscutlutsr eins daifctaetre dim thmaet rtshieo nc oaartein sgh wowithn zinin Fc ipghuoresp 5h.a Tteh cea cno iamtipnrgo vdee ftehcet sd wisbitohnoduint gz inc
zinc phosphate bubbled in 3.5% NaCl solution after eight days, and that with 8% zinc phosphate
phroesspishtaantec eb. ubbled in 3.5% NaCl solution after eight days, and that with 8% zinc phosphate did not
did not bubble. These results indicated that the coating with zinc phosphate can improve the
bubble. These results indicated that the coating with zinc phosphate can improve the disbonding
disbondingresistance.
resistance.

Figure 5. Microstructures of coating with (a) 0% zinc phosphate and (b) 8% zinc phosphate immersed
for eight days in 3.5% NaCl solution.

2.3. The Electrochemical Behavior of Zinc Phosphate Coating
Figure 5. Microstructures of coating with (a) 0% zinc phosphate and (b) 8% zinc phosphate immersed
Figure5.Microstructuresofcoatingwith(a)0%zincphosphateand(b)8%zincphosphateimmersed
for eight days in 3.5% NaCl solution.
for2e.i3g.1h.t Mdaacyrsoisnco3p.5ic% ElNecatCrolcshoelmutiicoanl .Behavior
2.3. TheF Eigluecrter o6c shhemowicsa lt hBee hEaIvS iroers oufl tZsi onfc cPohaotisnpghsa tiem Cmoeatrisnegd in 3.5% NaCl solution. The drop in the low
2.3. ThefErelqecuternoccyh eimmpiceadlaBneceh avvailoure ooff Zthine c0%Ph zoisnpch pahteosCpohaattien cgoating indicates the fast corrosion along the
2.3c.o1a. tMinagc/rmoestcaolp iinct eErlfeaccter.o Tchheem hiicgahl- fBreehquaveniocry impedance value of 0% zinc phosphate coating was
2.3.1. Mraecdruocsecdo apfitcerE tlwecot rdoacyhs eomf iimcamleBresihoanv, iionrdicating that the corrosive medium in the coating/metal
Figure 6 shows the EIS results of coatings immersed in 3.5% NaCl solution. The drop in the low
interface was diffused and that disbonding of the coating occurred. However, for the coating with
Ffrigequureen6cys himopwesdatnhcee EvIaSluree osuf ltthse o0f%c zoiantci npghsosipmhmatee rcsoeadtining i3n.d5i%catNesa tChel sfaosltu ctoiorrno.siTonh ealdonrogp thien the
8% zinc phosphate, the low-frequency impedance value declined within one day of immersion, but
lowfrceoqathutiene ngv/acmlyueeit marle pmienadtienarenfdacc eue.nv cTahhlauen egheoidgf hta-hfftreeer0q ou%neenz cidyna cyim.p Thpheoeds aplonhwcae-tf reveacqlouueaent iconyfg i0mi%np dezdiicnaanct cepes hvtoahslepuehf aawtseta scc oofiartttreiondsg ii onwn aas long
reduced after two days of immersion, indicating that the corrosive medium in the coating/metal
thecoatFiniggu/rem 7eat. aAlti tnhtee rbfeagcinen.iTnhg,e thhei glohw-f-rfereqquueennccyy iimmppedeadnacne cvealvuaelsu oef othfe0 0%% zainndc 8p%h zoisnpc hpahtoespchoaattei ngwas
reduciendtceoraaffatticneegr swt wwasoe rdedi fafausyssseesdos efadni,m da mnthdea trt hsdiaiots nboo,f nitndhdien igc8 a%ot fi nztihgnec t chpoahatotistnphghe aotcceoc urcorroraetsidinv.g eH romemweaedivnieuerdm, faoifrnt etrht heo enceoc aodtiaanytgi n owgf /ithm etal
8%im zimnecr psihoons. pAhsa teim, tmhee rlsoiown- ftriemqeu einnccyre iamsepde, dtahnec elo vwa-lfureeq dueecnlciny eidm wpeitdhainnc oe nvea dluaey ooff itmhem 8e%rs iozinn,c b ut
interfacewasdiffusedandthatdisbondingofthecoatingoccurred. However,forthecoatingwith
thep hvoaslpuhea rtee mcoaaitninegd w uansc ahna onrgdeedr oaff mtear gonniteu ddea hyi.g Thhere t hloawn t-hfree vqauleunec oyf tihme p0%ed zainncce p hvoaslupeh awtea cso aftitintegd. in
8%zincphosphate,thelow-frequencyimpedancevaluedeclinedwithinonedayofimmersion,but
FigTuor set u7da.y A tht et hzein bc epghionsnpihnagt,e t hcoea ltoinwg-, ftrheeq euqeunicvya liemntp ceidrcaunict ew vaasl fuitetse do fi nt hFeig 0u%re a7nbd. 8% zinc phosphate
the value remained unchanged after one day. The low-frequency impedance value was fitted in
coa tings were assessed, and that of the 8% zinc phosphate coating remained after one day of
Figuriem7ma.erAsitotnh. eAbse giminmneinrsgio,nth etimloew i-nfcrreeqauseedn,c ythiem lpowed-farneqcueevnaclyu eimspoefdthanec0e% vaalnude 8o%f tzhien c8%p hzoinspc hate
coatinpghsowspehraetae scsoeastsinegd ,waansd anth oartdoefr tohf em8a%gnzitiundcep hhiogshperh tahtaenc tohaet ivnagluree omf athine e0d%a zfitnecr pohnoesdpahyatoe fcoimatminger. sion.
AsimTmo esrtusidoyn thtiem zeinicn pchreoasspehda,tet hcoealtoinwg-, ftrheeq euqeunivcyaleimntp ceirdcuaint cweavsa fliutteedo ifnt hFiegu8%re 7zbin. cphosphatecoating
wasa norderofmagnitudehigherthanthevalueofthe0%zincphosphatecoating. Tostudythezinc
phosphatecoating,theequivalentcircuitwasfittedinFigure7b.
Materials2017,10,654 5of13
Materials 2017, 10, 654 5 of 13
Materials 2017, 10, 654 5 of 13
Fbi2gbF-bFBui2i2g-g-orBuBeutoreor6ete)t.e e 68)6T). %. 8h 8TT%%ehzh eei ezzn l ieeiencnlclececptc rc phtpotrhrochooohscocspheshpmpehehmhamiataciteicatceealac cliclo mo ioiamamatptitpipneinendeggdgdassasan..n. n ccceee s sspppeeeccctttrrrooossscccooopppyyy (((EEEIIISSS))) ooofff (((aaa111---NNNqqquuuiisistst,,t a,a2a2--2BB-oBottoee)t) e 00)%%0 % aannaddn (d(bb1(1-b-NN1q-qNuuiqissutt,, i st,

Figure 7. (a) Low-frequency impedance value and (b) equivalent circuit of EIS of the 0% and 8% zinc
Figure 7. (a) Low-frequency impedance value and (b) equivalent circuit of EIS of the 0% and 8% zinc
Figpuhroesp7h.a(ate) cLooawtin-fgres.q uencyimpedancevalueand(b)equivalentcircuitofEISofthe0%and8%zinc
phosphate coatings.
phosphatecoatings.
Figure 7b shows the equivalent circuit to fit the impedance spectrum. In Figure 7b, Rs is the
Figure 7b shows the equivalent circuit to fit the impedance spectrum. In Figure 7b, Rs is the
solFuitgiounr ere7sbistsahnocwe, sQthc eis etqhue icvoaalteinntg cciarcpuaictittaoncfiet, tahnedi mRcp ies dtahne cceoasptiencgt rruemsis.taInnceF.i gAulsroe, 7Qbd,l Ris tihset he
solution resistance, Qc is the coating capacitance, and Rc is the coating resistance. Also, Qdl iss the
capacitance of the double layer between the coating/metal interfaces that can characterize the coating
solcuatpioancitraensciest oafn tchee, dQocuibslet hlaeyceor abteitnwgeceanp thacei ctaonatcien,ga/mndetaRlc inistetrhfaecceos athtiantg carne scihsatarancctee.riAzel stoh,e Qcodaltiinsgth e
capddaiisscbbitooannnddcieinngog f cctoohnnedddiittoiioounnb.. l ReRccltt a iisys etthrheeb cechthwaarreggeeen ttrtrahannesscffeoerra rtreienssigiss/ttaamnncecetea.. lIInnin ggteeenrnfeearrcaaell,s, aat h hhaiigtghhc a RRncctt c rrheeflaflreeacccttsts e lloroiwwze cctoohrrerroocssoiioaontni n g
because the exchange current is directly associated with the electrochemical process of corrosion;
disbbeocnaduisneg thcoe nedxicthioann.gRe cctuisrrtehnet cihs adrigreecttrlayn assfesorcrieastiesdta wnciteh. Itnheg eenleecrtarlo,cahhemigihcaRl cptrroecfleescst solfo cworcroorsrioonsi;o n
therefore, Rct can show the degree of corrosion of the coating. The values of Qdl and Rct are shown in
becthaeurseefotrhee, Recxt cchanan sgheowcu trhree ndtegisrede ioref cctolryroassisoonc oiaft tehde wcoiatthintgh.e Tehlee cvtarloucehs eomf Qicdal lapndro Rcects asreo fshcoowrrno sinio n;
Figure 8. As immersion time increased, the Qdl values of 0% and 8% zinc phosphate coatings increased
theaFrniegdfuo trrheee ,8 RR. Acctt soc ifam nbmosthehro sdwioecntrh teeiamsdeeed ign, rctehreeeaoresfebcdyo, rstrhhoeos wQioidnnl vgoa fsluethreiseo oucfos 0 ac%toinr argno.dsTi o8h%ne bzveiantwlcu peeehsnoo scfpoQhaatditlneag cn/omdateRitnactgl asi nrientecsrrhfeaoacwseesnd. in
FigHaunordwe 8teh.veAe rRs, ctith moefm Qbeodrlt svhiao dlnueecti rmoefae tsheinedc ,8 rt%eha ezsreiendbc,y tp hshheooQswpdhilnavgtae sl uceoreiasotoiunfsg 0c %worarason ssdiiog8nn% ibfiezctiawnnctelepynh l oocwsopaehtrian tthge/amcno etahttaailnt igonsft eitnrhfcear c0ee%ass. ed
anzdHinothcw epeRhveoctrs,po tfhhabet oeQt hdclo dvaaetlicunrege a,o safe ntdhd,e t i8hts%e rR zecibtn yvc asplhuhoeow swpinhaags tess iecgroniaoiftuiicnsagcn owtlryar soh ssiiigoghnneibfri ectathwnatnelye t nlhoecw oQeartd ilt nhvgaa/nlum tehe aottaf ol0fi% ntht eezr i0fna%cc es.
Hozwinecv perh,otshpehQate cvoaaltuinego, fatnhde i8ts% Rzcti nvcalpuhe owspahs astiegnciofiactainntglyw haisghsiegr ntihfiacna nthtley Qlodwl vearluthe aonf t0h%at zoifncth e
dl
0%zincphosphatecoating,anditsR valuewassignificantlyhigherthantheQ valueof0%zinc
ct dl
Materials2017,10,654 6of13
Materials 2017, 10, 654 6 of 13
pphhoosspphhaattee ccooaattiinngg.. TThheerreeffoorree,, tthhee aaddddeedd zziinncc pphhoosspphhaattee iinnttoo tthhee ccooaattiinngg sslloowweedd ddoowwnn tthhee ccoorrrroossiioonn
rreeaaccttiMiooannte raaiattl stt 2hh0e1e7 c,c 1oo0aa, t6ti5inn4 gg ddeeffeecctt.. 6 of 13
phosphate coating. Therefore, the added zinc phosphate into the coating slowed down the corrosion
reaction at the coating defect.

FFiigguurree 88.. QQddll aanndd RRcct tvvaalulueess oof f00%% aanndd 88%% zzinincc pphhoosspphhaattee ccooaattininggss. .
Figure 8. Qdl and Rct values of 0% and 8% zinc phosphate coatings.
2.3.2. Microscopic Electrochemical Behavior
2.3.2. MicroscopicElectrochemicalBehavior
2.3.2. Microscopic Electrochemical Behavior
Figure 9 shows the point measurements of the local alternating current impedance of the (a) 0%
Figure9showsthepointmeasurementsofthelocalalternatingcurrentimpedanceofthe(a)0%
aanndd ((bb)) 88%%Fi zgziuinnrcec p9p hshhooosswpphsh atahtteee cpcoooaianttitin nmgge ddaisisusttraaennmcceeenss tfsfrr ooofmm th ttehh leeo scscacrlr aaatlttccehhr nddaeetfifneegcct tc auatrt 1r1e66n00t0 0i mµµmpme diinan n33c.5e.5 %o%f NtNhaeaC C(al l)s s0oo%lulu titoionn. .
Aftera nthde ( bi)m 8%m zeirnsci opnh,o stphhea LteE cIoSa tvinagl udeis toafn ctehse f r0o%m tzhien scc rpathcohs dpehfeactte a tc 1o6a0t0in µgm d ine c3r.5e%as Neda Ciln s o3l.u5t%io nN. aCl,
Aftertheimmersion,theLEISvalueofthe0%zincphosphatecoatingdecreasedin3.5%NaCl,whereas
After the immersion, the LEIS value of the 0% zinc phosphate coating decreased in 3.5% NaCl,
whereas that of the 8% zinc phosphate coating remained relatively stable after immersion for 6 h. The
thatofthe8%zincphosphatecoatingremainedrelativelystableafterimmersionfor6h. Thesolution
whereas that of the 8% zinc phosphate coating remained relatively stable after immersion for 6 h. The
solution penetrated the coating without zinc phosphate and corrosion occurred under the coating.
penetratedthecoatingwithoutzincphosphateandcorrosionoccurredunderthecoating. However,
solution penetrated the coating without zinc phosphate and corrosion occurred under the coating.
However, the coating with 8% zinc phosphate resisted the medium permeability of the coating.
thecHoaotwinegvewr,i tthhe8 c%oaztiinngc wpihtho s8p%h zaitnecr pehsiosstpehdatthe eremsisetdediu tmhe pmeerdmiuemab pileitrymoeafbtihlietyc oofa tthineg c.oating.
FiguFreigu9.re T9h. eThpeo pinotinmt emaesausruermemeennttss ooff llooccaalliizzeedd EEISIS (L(LEIESI)S )ofo 0f%0 %(a (1a-N1q-uNisqtu, ias2t-,Boat2e-)B oanted) 8a%n d 8%
(b1-Nquist, b2-Bote) zinc phosphate coatings immersed in 3.5% NaCl solution.
(b1-Nquist,b2-Bote)zincphosphatecoatingsimmersedin3.5%NaClsolution.
Figure 9. The point measurements of localized EIS (LEIS) of 0% (a 1-Nquist, a2-Bote) and 8%

(b1-Nquist, b2-Bote) zinc phosphate coatings immersed in 3.5% NaCl solution.
Materials2017,10,654 7of13
Materials 2017, 10, 654 7 of 13
FFiigguurree 1100 sshhoowwss tthhee mmaapp mmeeaassuurreemmeenntt ooff LLEEIISS ooff tthhee 00%% zziinncc pphhoosspphhaattee ccooaattiinngg iimmmmeerrsseedd iinn
33..55%% NNaaCCll ssoolluuttiioonn.. AAtt tthhee iinniittiiaall ssttaaggee ooff iimmmmeerrssiioonn,, tthhee iimmppeeddaannccee vvaalluuee ooff tthhee ccooaattiinngg ddeeffeecctt iinn
ccoonnttaacctt wwitihtht hteheel eecltercotlryotleytseo lusotilountiowna swsaigsn sifiigcnainftilcyanbtellyo wbetlhoewim thpee diamnpceedvaanlucee ivnatlhuee ininta ctthceo iantitnagct.
Acosaitminmg.e rAssio inmtmimeersiinocnr etiamseed i,ntchreeawshedo,l ethaere wahoofliem apreeda aonfc iemwpaesdraendcue cweda.s Trhedeuimcepde. dTahnec eimmpoeddualnucse
vmaloudeuolufst hvealcuoea otifn tghed ecofeacttinagn ddeafreocut annddd arroopupnedd drraoppidpleyda rfatperidimly mafetersr iiomnmfoerrs6iohn. Tfohre 6c ho.a Ttihneg cdoeafteincgt
wdeafseccot rwroadse cdosrreovdereedl yseavnedrethlye aanreda tahreo uanreda tahreocuonadti nthged ceofaetcitnwg adseefexcfot lwiaatesd e.xTfohleiastoeldu.t iTohnem soelduituiomn
pmenedetiruamte dpetnheetbraottetdom thoef btohtetocmoa toifn tgh,ef ocloloawtinegd, bfoyllcoowrreods iboyn .corrosion.
(a) (b)
(c) (d)
Figure 10. The map measurements of LEIS of the 0% zinc phosphate coating immersed in 3.5% NaCl
Figure10.ThemapmeasurementsofLEISofthe0%zincphosphatecoatingimmersedin3.5%NaCl
solution: (a) 3D of 0 h; (b) 2D of 0 h; (c) 2D of 3 h; (d) 2D of 6 h.
solution:(a)3Dof0h;(b)2Dof0h;(c)2Dof3h;(d)2Dof6h.
Figure 11 shows the map measurements of LEIS of the 8% zinc phosphate coating immersed in
Figure11showsthemapmeasurementsofLEISofthe8%zincphosphatecoatingimmersedin
3.5% NaCl solution. At the initial stage of immersion, the impedance value of the coating defect was
3.5%NaClsolution. Attheinitialstageofimmersion,theimpedancevalueofthecoatingdefectwas
significantly below the impedance value in the intact coating, which was the same as the 0% zinc
significantly below the impedance value in the intact coating, which was the same as the 0% zinc
phosphate coating. As immersion time increased, the impedance of the coating defect did not change
phosphatecoating. Asimmersiontimeincreased,theimpedanceofthecoatingdefectdidnotchange
and the coating defect around it slightly increased, which was completely different from the 0% zinc
andthecoatingdefectarounditslightlyincreased,whichwascompletelydifferentfromthe0%zinc
phosphate coating. The added zinc phosphate coating prevented the diffusion of corrosive medium
phosphatecoating. Theaddedzincphosphatecoatingpreventedthediffusionofcorrosivemediumin
in the lateral of coating/metal interfaces and slowed down the corrosion.
thelateralofcoating/metalinterfacesandsloweddownthecorrosion.
After the immersion test in 3.5% NaCl solution, Raman spectroscopy of rust and corrosion
Aftertheimmersiontestin3.5%NaClsolution,Ramanspectroscopyofrustandcorrosionproduct
product in the defect of the 8% zinc phosphate coating were measured. As shown in Figure 12, the
inthedefectofthe8%zincphosphatecoatingweremeasured. AsshowninFigure12,thecorrosion
corrosion product was made up by the composition of γ-Fe2O3, α-FeOOH, γ-FeOOH, and iron
productwasmadeupbythecompositionofγ-Fe O ,α-FeOOH,γ-FeOOH,andironphosphate.
phosphate. 2 3
Materials 2017, 10, 654 8 of 13
Materials2017,10,654 8of13
Materials 2017, 10, 654 8 of 13
(a) (b)
(a) (b)
(c) (d)
(c) (d)
Figure 11. The map measurements of LEIS of the 8% zinc phosphate coating immersed in 3.5% NaCl
Figure11.ThemapmeasurementsofLEISofthe8%zincphosphatecoatingimmersedin3.5%NaCl
Figure 11. The map measurements of LEIS of the 8% zinc phosphate coating immersed in 3.5% NaCl
solution: (a) 3D of 0 h; (b) 2D of 0 h; (c) 2D of 3 h; (d) 2D of 6 h.
solution:(a)3Dof0h;(b)2Dof0h;(c)2Dof3h;(d)2Dof6h.
solution: (a) 3D of 0 h; (b) 2D of 0 h; (c) 2D of 3 h; (d) 2D of 6 h.

Figure 12. Raman analyses of rust and corrosion product in the defect of the 8% zinc phosphate coating.

3. Discussion
FFiigguurree 1122.. RRaammaann aannaallyysseess ooff rruusstt aanndd ccoorrrroossiioonn pprroodduucctt iinn tthhee ddeeffeecctt ooff tthhee 88%% zziinncc pphhoosspphhaattee ccooaattiinngg..
In comparing the EIS results of the 0% and 8% zinc phosphate coatings (Figure 6) at an 85 ± 5 µm
33.. DDiissdcceuufsesscsitisoo nonn the coatings, the metals were shown to corrode at the defect areas. At the beginning of
immersion, the low-frequency impedance value decreased in both, but as immersion time increased,
IIntnh ecc oolommwpp-faarrreiiqnnugge ntthhcyee iEEmIISpS errdeeassnuucllettss v ooafflu ttehh eeo f00 t%%he aa8nn%dd z 88in%%c pzzihinnoccs ppphhhaootess ppcohhaaatttieneg cc ooreaamttiinaniggnsse d((FF siitggauburlreee a 66t) )1 a0at5t O aanhnm 885·5c m±± 25,5 µµmm
ddeeffeecctwtss hooincnh t twhheae scc hooiaagtthiinenrgg stsh,, attnhh eteh mmat eeottfaa tllhss eww 0e%err eez issnhhco opwwhonns pttooh acctooe rr(rrFooigddueer eaa tt7 atth)h. eeM ddoeereffeeoccvtte raa,rr teehaaess R.. cAAt vtt attlhhueee obbfee tgghiienn n8n%iinn gg ooff
iimmmmeezrrissniioco npn,h, tothhspee hlloaotwwe c--fofrraeetqiqnuugee wnncacysy siiimmgnppifeeicddaaannntlccyee h vvigaahllueuree tdhdaeenccr rteheaaasts eoedfd tihinne b0b%oott hzhi,n, bbcu uptht aoasss piimhmamtme ecerorsasitioionnng t t(iFimmigeeu irinenc 8craree)a.a sseedd,,
tthhee lloowTwh--effr raeedqqduueeednn czcyyin iicmm pppheoeddspaahnnaccteee vvinaa lltuuheee ocooff attthhineeg 88 s%%lo zzwiinnedcc pdphohwoossnpp thhhaaett eceo ccrooroaasttiiinonngg rrreeeammctaaioiinnne eaddt stshttaeab bclloeea ataittn 1g10 0d55 eOOfehhcmmts.·· ccmm22, ,
wwhhiicchhT hwweaa Qss dhhl iivggahhleuerer ttohhfa atnnh ett hh8aa%tt oozfifn ttchh epe h00o%%sp zzhiianntcec ppcohhaootsisnppghh awattaees ( (FsFiiigggnuuifrrieec a77naat)l)y.. MMlooworreeeroo vtvheearrn,, ttthhheae tRR ocft vtvhaaellu u0e%e o ofzf itnthhce e 88%%
ct
phosphate coating (Figure 8b), showing that the added zinc phosphate slowed down the coating
zinc phosphate coating was significantly higher than that of the 0% zinc phosphate coating (Figure 8a).
zincphosphatecoatingwassignificantlyhigherthanthatofthe0%zincphosphatecoating(Figure8a).
disbonding. The reason for this may be that, when the corrosive medium permeated the coating/metal
The added zinc phosphate in the coating slowed down the corrosion reaction at the coating defects.
Theaddedzincphosphateinthecoatingsloweddownthecorrosionreactionatthecoatingdefects.
TThhee QQ dl vvaalluuee ooff tthhee 88%% zziinncc pphhoosspphhaattee ccooaattiinngg wwaass ssiiggnniiffiiccaannttllyy lloowweerr tthhaann tthhaatt ooff tthhee 00%% zziinncc
dl
phosphate coating (Figure 8b), showing that the added zinc phosphate slowed down the coating
phosphate coating (Figure 8b), showing that the added zinc phosphate slowed down the coating
disbonding. The reason for this may be that, when the corrosive medium permeated the coating/metal
Materials 2017, 10, 654 9 of 13
Materials2017,10,654 9of13
interfaces, the zinc phosphate was partly dissolved and produced metal substrate phosphate. Zinc
dpihsobsopnhdainteg w. Tithhe trheea scoonmfpoorstihtiisonm oafy γb-eFeth2Oat3,, wα-hFeenOtOheHc,o γr-rFoesOivOemH,e adniudm irponer pmheoastpedhatthee (cFoigatuinreg 1/2m) ectaanl
serve as the protective film. The protective film contained carboxyl and hydroxyl groups, which can
interfaces,thezincphosphatewaspartlydissolvedandproducedmetalsubstratephosphate. Zinc
strengthen the combination of the coating and substrate [29].
phosphatewiththecompositionofγ-Fe O ,α-FeOOH,γ-FeOOH,andironphosphate(Figure12)can
2 3
serveastheprotectivefilm. TheprotectiveFefilmcFoen2taine2deca rboxylandhydroxylgroups,which(c1a)n
strengthenthecombinationofthecoatingandsubstrate[29].
O 2H O4e 4OH (2)
2 2
Fe→Fe2++2e− (1)
Zn PO  2H O4OH 3ZnOH 2HPO 2 (3)
3 4 2 O +2 2H O+4e− →4OH− 2 4 (2)
2 2
Zn (PO )Fe+22HHOP+O442OH−F→eP3OZn4(OHH) +e2H PO 2− (4(3) )
3 4 2 2 2 4
In comparing the LEIS of thFee 20+%+ anHdP 8O%2 z−in→c pFheoPsOpha+teH c+oa+tineg−s, the low-frequency impedan(c4e)
4 4
value of the 8% zinc phosphate coating defect declined slowly and the 0% zinc phosphate coating
IncomparingtheLEISofthe0%and8%zincphosphatecoatings,thelow-frequencyimpedance
defect declined visibly (Figure 9). These results show that the water absorption of the 8% zinc
valueofthe8%zincphosphatecoatingdefectdeclinedslowlyandthe0%zincphosphatecoatingdefect
phosphate coating was decreased significantly. When the corrosion medium penetrated the
declinedvisibly(Figure9). Theseresultsshowthatthewaterabsorptionofthe8%zincphosphate
coating/metal interfaces, part of the zinc phosphate dissolved and reacted with Fe to form stable iron
coating was decreased significantly. When the corrosion medium penetrated the coating/metal
phosphates that can block the medium diffusion [30].
interfaces,partofthezincphosphatedissolvedandreactedwithFetoformstableironphosphates
The impedance values of the 8% zinc phosphate coating increased as the immersion time
thatcanblockthemediumdiffusion[30].
increased (Figure 11), because the chemical activation of zinc phosphate enhanced the wet coating
Theimpedancevaluesofthe8%zincphosphatecoatingincreasedastheimmersiontimeincreased
adhesion. The spread of the corrosive medium in the coating/metal interface was prevented and the
(Figure11),becausethechemicalactivationofzincphosphateenhancedthewetcoatingadhesion.
coating defect corrosion reaction and disbonding were slowed.
Thespreadofthecorrosivemediuminthecoating/metalinterfacewaspreventedandthecoating
The failure mechanisms of the 0% and 8% zinc phosphate coatings are shown in Figure 13. When
defectcorrosionreactionanddisbondingwereslowed.
the 0% zinc phosphate coating with the defect (Figure 13a) was exposed to corrosive media, the
The failure mechanisms of the 0% and 8% zinc phosphate coatings are shown in Figure 13.
corrosion reaction occurred rapidly on the coating/metal interface and grew suddenly. Furthermore,
Whenthe0%zincphosphatecoatingwiththedefect(Figure13a)wasexposedtocorrosivemedia,the
the coating disbanded from the defect. However, the coating defect with zinc phosphate has two
corrosionreactionoccurredrapidlyonthecoating/metalinterfaceandgrewsuddenly. Furthermore,
aspects of protection (Figure 13b). First, when the corrosion medium permeates the coating/metal
the coating disbanded from the defect. However, the coating defect with zinc phosphate has two
interfaces, the zinc phosphate and Fe generates stable iron phosphates and inhibits the anodic process
aspectsofprotection(Figure13b). First, whenthecorrosionmediumpermeatesthecoating/metal
of corrosion. Second, zinc phosphate can combine with the carboxyl and hydroxyl groups of the
interfaces,thezincphosphateandFegeneratesstableironphosphatesandinhibitstheanodicprocess
coating, through which a stable protective film was formed by the zinc phosphate, coating and metal
ofcorrosion.Second,zincphosphatecancombinewiththecarboxylandhydroxylgroupsofthecoating,
substrate. All of these reactions can inhibit the cathode reaction from damaging the coating, prevent
throughwhichastableprotectivefilmwasformedbythezincphosphate,coatingandmetalsubstrate.
the corrosion medium from a horizontal diffusion, and slow down the coating defect corrosion
Allofthesereactionscaninhibitthecathodereactionfromdamagingthecoating,preventthecorrosion
and disbonding.
mediumfromahorizontaldiffusion,andslowdownthecoatingdefectcorrosionanddisbonding.

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Materials2017,10,654 10of13
Materials 2017, 10, 654 10 of 13
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ddeeffeeccttss ffoorr tthhee EEIISS tteesstt wweerree 1100 mmmm aanndd 112200 µµmm,, rreessppeeccttiivveellyy,, aanndd tthhoossee ffoorr tthhee LLEEIISS wweerree 11 mmmm aanndd
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AAccccoorrddiinngg totoI SIOSO4642642s4t asntadnadrdar[3d1 ][,3P1o],s iPToesstidTreaswtdirnagwwinags uwseads tuosdeedt etrom dineetethrmeaindeh etshieo nasdthreensgiothn
ostfrtehnegtohr goaf nthice cooragtainngic. cTohaetin20g-. mThme d20ia-mmmete dridamoleliteesr wdoelrleieds ewgererea sdeedguresainsegda ucesitnogn eacaentdonteh eanndg ltuheedn
tgolutehde stou rftahcee soufrtfhacee cooaf tethdep claotaetsedu spinlagteas twusoin-cgo ma ptownoe-nctomeppooxnye-nbta seepdocxyya-bnaos-eadcr yclyaatenoa-dachreysliavtee.
Aadftheersaivdeh. eAsifvteerc audrihnegs,ivaet ecsutriinngg,a pa ptaersatitnugs wapapsaartattaucsh ewdasto atthtaechloeadd itnog thfiex tluoraedainngd fiwxatusrset raaninde dwaast
5stmramin/edm aint 5u nmtmilt/hmeinco uantitnilg thmea ctoeraitainlhga mdadteertaiaclh headdf rdoemtatchheedsu fbrostmra tthe.e substrate.
TThhee mmiiccrroommoorrpphhoollooggyy ooff tthhee ccooaatteedd sseeccttiioonn ssaammpplleedd wwaass ppeerrffoorrmmeedd uussiinngg SSEEMM ((QQUUAANNTTAA 225500,,
FFEEII,, HHiillllssbboorroo,, OORR,, UUSSAA)).. TThhee mmaaccrroommoorrpphhoollooggyy aanndd mmiiccrroommoorrpphhoollooggyy ooff tthhee ccooaattiinngg ddeeffeeccttss wweerree
oobbttaaiinneedd uussiinngg aa ddiiggiittaall ccaammeerraa aanndd sstteerreeoommiiccrroossccooppee..