Measurements have been made of the aluminum/metal galvanic couple. A wide range of geometries were investigated varying the areas of anodic and cathodic surfaces and employing specially designed galvanic cells with crevices. In situ ionic current density mapping was used to monitor galvanic corrosion and currents flowing between separated metals was measured

Barbour, J. C.

Isaacs, H. S.

Jeffcoate, C. S.

Missert, N. A.

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Proc. Vol. 1999 Joint Intern. Mtg. of the 196th Mtg. of the‘Electrochemical Society and 1999 Fall Mtg. of the ElectrochemicalSociety of Japan, Honolulu, Oct. 17-22, 1999. BNL-67015?IonicCurrentMapping TechniquesandApplications toAIuminum-Copper CorrosionI H. S. Isaacs”, C. S. 3eff60ate+,N. A. Missert”,J. C. &rbour-“BrookhavenNational Lab,Upto~NYll 973‘Ohio State University, Columbus, OH 43210‘SandiaNationalLaboratory, NM aE”cE!v~~ABSTRACTMeasurements have been made of the alumimudmetal galvanic couple. A OST$wide range of geometries were investigated varying the areas of anodicand cathodic surfaces and employing speciaIly designed galvanic cellswith crevices. In situ ionic current density mapping was used to monitorgalvanic corrosion and currents flowing between separated metals wasmeasured.INTRODUCTIONThe high corrosion rate of copper containing aluminum alloys comparedaluminum is attributed to both the cathodic nature of alloying elementto pureand thesusceptibility of inclusions to deaiioying. For example AA202&T3 has two mainprecipitates found in the matri~ cr-phase A.W@e, Mn) and 6-phase &CuMg. Theprecipitate phases have electrode potentials of-675 and –920mV vs SCE respectfidly (1).Trenching around c-phase precipitates has been observed by AFM studies (2).This present study repcxts an initial series of measurements that are designed to identifithe important fhctors in the initiation and propagation of corrosion of Am&m.Metal/metal galvanic couples of various designs have been studied using very simplestandard electrochemical methods and in dtu current density mapping in solution. Inprinciple the polarization characteristics determine the galvanic corrosion rates.However, the details of the changes in the pol~lon characteristics complicates aquantitative predktion of the cmosion rates. The polarization behavior changes withtime and concentration buifdup in pits and crevices as they are formed, as welf as at thefkely exposed anode and cathode.. The extent of the chemical changes are fbturecomplicated by the close proximity of anodes and cathodes as is the case with mostcomding systems. In order to start to try and take the latter effkcts into account currentdensity mapping has been used as it closely represents many of the corrosion processes.Ionic current density mapping has been used to investigate different types ofcorrosion(3): crevice(3> pitting(4~ and intergramdar corrosion(5). The technique,utilizing a scanning vibrating microprobe, detects cuments flowing in solution. Thevibrations transform a dc potentitd gradkmt due to current flow in a resistive solution.The vibration is used to convert the dc gradient to an ac signal allowing cleaner signalampliktion. W@ galvanic corrosion the cathodic areas are well defined but it isnecewary to spatial resolve anodic sites which can develop adjacent to the cathodes or onthe exposed aluminum surfkce.— .—. -—--.—-. ——-...Z——-————-—— —.— — —.....__ ———EXPERIMENTALrtThe Applicable Electronics (Forest&de MA) current density ma]pping has been in detailin another publication (6). Siply a fine tipped PtAr probe with a 20 ~ diameter of Ptblack is scanned in a plane parailel to and 100-200 above a sample. The probe vibrates inthe X and Z directions at dHerent fkquencies with amplitudes. l?hase sensitive detectionis used to measure the variation in the electric field in each vibration direction. Thecurrent density is proportional to the magnitude of the measured field and can becalibrated using a point source. All samples were masked ofT with 3M type 5 tape,leaving and exposed surface area of approximately 3 x 3 mm. In addition a visual recordis kept of the samples during the experiment to correlate apparance changes with andlccuments and to assist in dktinguishing between pitting and crevice corrosion. In addhiona visual record is kept of the samples chuing the experiment to correlate appearancechanges with anodic wrents and to assist in distinguishing berween pitting and crevicecorrosion.Couples of noble metals imbedded in Al were tested. Cu and Ft wires were placed inaxial holes in rods of pure 99.999% and then swaged to contact the wires and AL Theresulting wire diameter was 0.5 mm. Using a similar approach a copper wire in a Teflonshrink tube was imbedded in Al and electrical contact was made externally. The areaexposed to solution was limited with adhesive tape (3M Type 5) with a 3x3 mm cut outwindow.Samples of Al thin films with copper disks have been described previously (7). Al films150-200 nm thick were deposited by on a Si (100) substrates. The samples were thenexposed to air to form a native oxide. Photoresist was spun olntothe samples and wasdefined using photoiithography. Copper was then electron-beam evaporated through theopenings in the photo resist. The photo resist layer was removed with acetone.Couples of 1 cm2 abraded 99.99 VO Al foii and a 3 mm diameter Cu rod were also tested.The immersed end of the Cu rod was coated except for 3mm. A 3 mm plastic rod wassIso used schematically shown in Fig. 1 When only the Cu rod was used its cross sectionwas pressed against the Al to fbnn a crevice. When the plastic rod was pressed againstthe Al foil the Cu rod was immersed in the solution and electrical connected externally tothe Al through a 1000 Clresistor, which allowed for measurements of the currentReagents used were of analytical or hboratcxy grade. All solutions used were made with18Mf2 de-ionized water.RESULTS AND DISCUSSIONCurrent Densitv Mao&Pt-Al Couples.-. —— — ———.—.———c——— .— .—— ——— .— -.DisclaimerThis report was prepared as an account of work sponsoredby an agency of the United States Government. Neitherthe United States Government nor any agency thereof, norany of their employees, make any warranty, express orimplied, or assumes any legal liability or responsibility forthe accuracy, completeness, or usefulness of anyinformation, apparatus, product, or process disclosed, orrepresents that its use would not infringe privately ownedrights. Reference herein to any specific commercialproduct, process, or service by trade name, trademark,manufacturer, or otherwise does not necessarily constituteor imply its endorsement, recommendation, or favoring bythe United States Government or any agency thereof. Theviews and opinions of authors expressed herein do notnecessarily state or reflect those of the United StatesGovernment or any agency thereof.DISCLAIMERPortions of this document may be illegiblein electronic image products. Images areproduced from the best available originaldocument.tA 3 x 3 mm sr~ of pure Al and galvanically coupled embedded Pt wire, was mapped in50 mM NaC1.Pt, as seen in Fig,2, acts as a very effective cathode and strong currentswere detected cmimmersio~ which grew to a maximum after 20 minutes immersion andthen decreased. Several anodic sites formed quickly. Most were located on the exposedsurface. Pits on the freely exposed surface were evident as seen from the buildup ofcorrosion products in video images afier 30 n@ which continued to darken with time.The anodic site%as determined fimmthe current maps were transitory, but the sites on thefreely exposed surface all showed buildup of corrosion products indicating that they thatremained active with varying intensity throughout the measurement. The site towards thebottomrightof the current density maps in Fig,2 was firmed at the comer of the tape andwas probably stdilized by a crevice. A second “stable” site directly above the Pt seentier 40 mi~ also remained active but showed very low currents sfler 100 @ was alsovery close to the tape. A third’site that showed a high degree of staMity especially after100 minut~ had deposited products away from the tape and did not appear as a crevicecorrosion site.In a number of experiments over prolonged exposure time of 60 mkq a cathodic tail wasobserved extending fm the Pt disk. The extended cathdlc region appeared over thedtion of the abrasive lines. The cause was associated with the surface preparation byabrasion. Pt particles from the wire were carried and deposited onto the Al where thebehaved as catholic sites. It was interesting that the tail was only observed a&r a ratherlong period of time as they must have been active throughoutthemeasurement.Couples r#DepositedCu-Al FilmsOn immersion in 50 mM NaCl the initial cummt density map, Fig. 3., showed highcurrents. The five cathodic locations over the deposited Cu disks were clearly seen withone very active anodic site in the lower right hand corner. The location of this anodic sitewas at the edge of the tape used to mask off the sample. The site either initiated in acrevice or rapidly turned into crevice umrosion. The anodic site remained activethroughout the experiment and the path of the dissolution under the tape appearedrandom. As time progressed the cathodic sites changed in character and the currents fromthe anodic site concomitantly decreased. After 20 minutes of immersion there was asignificant decrease in the cathodic currents over the five Cu sites. No anodic currentsweremeasuredaround the C%discs and once again the intense cathodic currents maskedany weaker adjacent anodic current however as a visible gap formed around the edge ofthe Cu disk Undermining of the Cu was observed opticaily as the currents fkom the Cudemeased. me observations confirm other studies of similar sampks that demonstratedAl dissolution under the Cu (’7).After 100 minutes only 4 Cu sites remained active. Thecurrentflow fi-omthe Cu disk closest to the anodic site was the first to disappear and after160 minutes only one Cu site remained active. Each Cu disk probably became inactiveonce the physical connection with the Al matrix had been lost (’7).As the Cu becameisolated from the M thin fil~ the Cu was then able to dissolve. At the end of theexpiment all the Cu discs had dissolved fkom the sample. EDX analysis was unable tolocate any Cu in the sampie,._. ———. . . ...—___ .—.CU-AI Couples with a CreviceUnder many conditions the Cu and Al are in direct contact and the Al corrodes in thecreviw between them. Using an apparatus similar to that in Fig. 1, except the Cu rodreplaced the plastic rod show it was found that crevice corrosion did take place betweenthe Cu and the Al. Whh the apparatus shown in Fig. 1 and the plastic rod pressed againstthe ~ crevice corrosion again took place where the plastic was in contact with the Al ina 0.1 M NaCl solution. When no connection was made to the Cu no crevice cxmosionwas observed under the plt@ic after similsr times of exposure. The current and potentialvariations were measured during the exposure and are shown in Fig, 4 and 5 plottedrespectively as a fbnction of time and against each other. A&r contacting the metals thecurrent decreased with time. Smaller potential changes were observed as the potentialfirst decreased and then showed a small increase. A plot of the current and potential inFig. 4 shows the locus decreasing to a minimum and then increasing. The observedchanges were dominated by a decrease in current due to polarization of the Cu. This isillustrated by the schematic polarization cmves superimposed on the current-voltagecurve in Fig. 5. The arrows indicate the polariAon changes fbr anode and cathode. Agreater degree of polarization is noted for the cathode indicating changes at the copperelectrode dominated the decrease in the current whereas the potential of the Alaccommodated the current changes with little effect on its potential.CONCLUSIONSThe galvanic corrosion of Al in contact with Cu appears to be dcmninatedby crevicecomosion probably initiated by pitting. For Pt with its greater cwTent intensity, pitsapptar to be more stable and continue to decorate the Al surface even though the currentswere below detection with the vibrating probeCrevice corrosion was not dependent on the proximity of the Cu cathode as simihrcrevice corrosion processes took place under Cu and plastic. The cathode is polarized to agreater extent than the anode.ACKNOWLEDGEMENTSThis work was conducted at Brookhaven National Laboratory umler the auspices ofDepartment of Energy, Division of Materials Scien~ office of W~enceunder contractNo. DE-ACO2-98CH1O886.C.S-J. was suppofied by the Air Force Office of ScientificResearch contract number F49620-96-1-0479. The authors thank ApplicableElectronics, Inc. for the used of the current density mapping equipmen~ Robert Sabatinifor the EDX analysis and Ken Sutter for producing the samples._—. .——.-—.— . . ..- ——.-....-—------- ———. -.—.- .—— ——-— —— ....—. ——..REFERENCES-1P. Schmutz and G.S. Frankel, J. Ekztrochem. Sot., Vol 145, No. 7,2285 (1998)2P. Schrnutz and G.S. Franke~ J. Electrochem. Sot., Vol 145, No, 7,2295 (1998)3 H. S. Isaacs and B. Vyas. h Electroc heroical Corrosion Testin~ F. Mansfield and U.Bertocci, EditOm, ASTM STP 727, p. 3, 19814 H. S. Isaacs and Y. Ishikawa. J. Electrochem. Sot. ~ 1233-93 (1985).5 B. Vyas and H S. Isaacs. In Intermanu lar Corrosion of StainIess steels ASTM STP656, RF. Steigerwal& Edhor pp. 133-45,1979.6A. 3. Adlykiewic~ Jr., H. S. Isaacs, and A. J. Davenport, J. Electrochem. Sot. ~~10),3342-50 (1995)7 F.D. Wall, K.-A sow N.A, Misse& J.C.Barbour, M.A Martinez KR. Zrwidii, J.P.Sulliv~ RG. Copelan~ W.R Ciesl~ RG. Buchheit, H.S. Isaacs in CriticalFactors in bczz?kedConrosion, ~ G, Kelly, G. S. Frankel, P. M. Natisd@ R C.Ne- Edts, Proceeding Volume 98-17, p. 701,1998.potential@9●e■●m■m— 8CuA ●Figure 1 Schematic of apparatus for studying the effect of a crevice during crevicecmosion-—-——-——- ..--—.-——.. . ——— ———.——.Initial 20 minutes60 minutes 80 minutes40 minutes100 minutes120 minutes 140 minutes 160 minutesFigure 2 Pt wire inside a pure M sample. High cathodic currents are measured centrallylocated over the Pt and several active anodic sites are detected around the edge of theexposed Al sample..... ..-—- --——. ——..— ———.L,#60 minutes120 minutesFigure 320 minutes afler immersion80 minutes. 4 b.140 minutes100 minutes160 minutesCurrent density maps over 5 Cu discs evaporated onto a thin film of A1.Asample area of 3 x 3 mm was exposed to 50 mhdNaC1.—— --- ———.—. .——. .—. . ..C -—. ......#El>g-0.7-0.754! . . . . . . . . . . . . . . . . . . . . . . . . . . ..JFigure 4-0.664).67-0.661600 2ciuolime (s)3dO0Variations of the potential of Al and the current flowingbetween an Al/Cu couple with a crevice formed between the Al and a plastic rod.b“”Figure 5 A plot of the current and potential shown in Fig. 4 for an A1/Cucouple.Superimposed on the cuwe is an Evans type diagram showing anodic andcathodic polarization lines that demonstrate a greater increase in cathodicpolarization leading to the observed changes rather than a decrease inanode polarimtion..—— —. —..—....__.Z -.—.——-— .

Ionic Current Mapping Techniques and Applications to Aluminum-Copper Corrosion

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Ionic Current Mapping Techniques and Applications to Aluminum-Copper Corrosion

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