MTL TR 92-56 AD-A257 498

ADHESION AND CORROSION BEHAVIOR OFAI-Zn AND TiNITi/TiN COATINGS ON A DTICDU-0.75 WT% ALLOY S ELECTED

CF. C. CHANG, M. LEVY, R. HUIE, M. KANE, and

R BUCKLEY..U.S. ARMY MATERIALS TECHNOLOGY LABORATORYMETALS RESEARCH BRANCH

Z. KATTAMIS A rUNIVERSITY OF CONNETICUT •€C)STORRS, CT

G. R. LAKSHMINARAYANU.S. ARMAMENT RESEARCH, DEVELOPMENT, AND ENGINEERING CENTERPICATINNY ARSENAL, NJ

August 1992

Approved for public release; distribution unlimited.

US ARMYLMORATORY COMMAND U.S. ARMY MATERIALS TECHNOLOGY LABORATORYMiis .MW• LMeuTOWt Watertown, Massachusetts 02172-0001

The findings in this report are not to be construed as an officialOepwrTmnt of the Army position, unless so designated by otherauthorized documents.

Mention of any tade names or mariufactulrs in this reportshall not be construed as advertising nor as an officialindorsoment or approval of such products or companies bythe United States Government.

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I. REPORT NUMBER '. GOVT ACCESSION NO. 3. RECIPIENT'S CATALOG NUMBER

MTL TR 92-564. TITLE (and Subtitle) S. TYPE OF REPORT A PERIOD COVERED

ADHESION AND CORROSION BEHAVIOR OF Al-Zn ANDTiN/Ti/TiN COATINGS ON A DU-O.75wt% Ti ALLOY

6. PERFORMING ORG. REPORT NUMBER

7. AUTmOR(s) 4. CONTRACT OR GRANT NUMBER(s)

F. C. Chang, M. Levy, R. Huie, M. Kane,P. Buckley, T. Z. Kattamis,* andG. R. Lakshminarayant

S. PERFOMMING ORGANIZATION NAME AND AODRESS 10. PROGRAM ELEMENT. PROJECT, TASK

U.S. Army Materials Technology Laboratory AREA A WORK UNIT NUMBERS

Watertown, Massachusetts 02172-0001ATTN: SLCMT-EMM

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U.S. Army Laboratory Command August 19922800 Powder Mill Road 13. NUMBER OF PAUES

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IS. SUPPLEMENTARY NOTES

*Dept. of Metallurgy, Univ. of Connecticut, Storrs, CTWU.S. Armament RD&E Center, Picatinny Arsenal, NJ(SEE REVERSE)

IS. KEY WORDS (Conit•ne an #ever** sede II ne'esuan and identtlfy bWock number)

Uranium alloys Corrosion resistanceUranium-titanium alloys Mechanical propertiesVapor deposition Scanning electron microscopy

20. ABSTRACT (CntW.ui -v ewver@*e side II necieuwp and identlfy b• block islmle'm

(SEE REVERSE SIDE)

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Block No. 18. continued

Presented at Proceedings of the 1991 International Conference on MetallurgicalCoatings and Thin Films. Published in Surface and Coatings Technology, 49(1991) 87-96.

Block No. 20

ABSTRACT.

Al-Zn alloy and multilayer TiN/Ti/TiN thin coatings were deposited onDU-0.75Ti alloy specimens by a cathodic arc plasma physical vapor depositionprocess. The quality, soundness and adhesion of the coatings to the substratewere evaluated by automatic scratch testing, in combination with optical and

-scanning electron microscopy examination of the scratch morphology. The gal-vanic corrosion behavior of DU-O.75Ti alloy coupled to the coated alloys andaluminum alloy 7075-T6 was also investigated by electrochemical tests in a0.5 N NaCl aqueous solution.

UNCLASSIFIEDSECURITY CLASSI FICATION Of rWuS PAGE (Whom 0000 PR....ej

Sudo c ,and C'ouoet L Iv hn/,ootb . 41) (1991 •57 96 x7

Adhesion and corrosion behavior of Al-Zn and TiN/Ti/TiN coatingson a DU-0.75wt.% Ti alloy

F. C. Chang, M. Levy, R. Huie, M. Kane and P. Buckleyt S .Arm Altics T Iohoatorri. Watertown. VA 021724X)1 1.S...

T. Z. KattamisDIc'parlmin of .Ahtall/,rg,'i. • "lr'rsitr of Connclictt. Snorrt. CT (0269-3 136 (U .S..4.)

G. R. Lakshminarayan8U.S. .4rownteo'nt RC'N4'or(th. I)A re/olhonwii and Eineic'ring Ct'n'Ic'r. I'•'oinoni" .41A"en i,. .NJ 0)7806 il . A.).

Abstract

Al Zn .allov and rnultilaier TiN/Ti/TiN thini coatings %%ere deposited on DUi 0.75Ti ;illo. spcciniens ht a cathodic arc plasmaphysical ,vapx)r dep)sition process. The quality. soundness and adhesion of the coatings to the substrate were esaluated byautomatic scratch testing. in combination with optical and scanning electron microscopy examination of the scratch morphologp-The galvanic corrosion behavior of DU 0.75Ti allo% coupled to the coated alloys and aluminum alloy 71075-T6 %,as alsoinvestigated by electrochernical tests in t (0.5 N NaCI aqueous -olution.

I. Introduction The relative mechanical strength ofcoatings and of thecoating - substrate interfaces may be conveniently evalu-

In a previous study [II of various coatings deposited ated by scratch testing. This procedure consists ofon DU -0.75Ti alloy (where DU means depleted uranium progressively straining the substrate by deforming theand where the composition is in weight per cent) by a coating-substrate interface with a diamond indenter andcathodic arc plasma physical vapor deposition (PVD) evaluating the cohesive load L, . which is the minimumprocess using elemental targets. AI-Zn coatings were load required for crack initiation within the coating. asfound to be anodic (sacrificial) with useful life governed well as the adhesive load L,. which is the minimum loadby their thickness and integrity. Titanium and TiN at which the coating is detached from the substrate [3-7].coatings. on the contrary. were found to be cathodic: The interpretation of the critical loads for coatinghence to be effective they must be defect free. Surface cohesion and adhesion has been analyzed by Steinmannmorphology studies by scanning electron microscopy et al. [8] and applied previously to similar systems [2].(SEM). as well as electrochemical polarization and The aim of the present investigation was to evaluatelong-term immersion tests in aerated 3.5 wt."',, NaCI further the quality. soundness. adherence and the galvanicaqueous solution indicated that AI-Zn alloy is the best corrosion behavior of the two most promising coatings:of four metallic sacrificial coatings tested for improving AI-Zn alloy and multilayer TiN/Ti/TiN on DI 0.75Tithe corrosion resistance of DU -0.75Ti. In a subsequent specimens. The cohesive and adhesive loads were deter-evaluation[ 21 ofthe adhesion. soundness and comparative mined using an automatic scratch testing apparatus.quality of various coatings by automatic scratch testing in combination with microscopic observations of thein combination with optical and SEM observations of scratch and the adjacent coating surface. The galvanicthe scratch and the adjacent coating surface, it was corrosion behavior was evaluated bvelectrochemical testsconcluded that (I) alloyed metallic coatings. AI-Zn and in a, 0.5 N NaCI aqueous solution in combination withAl- Mg.on DU-0.75Ti specimens exhibit bighercohesive microscopic examination of corroded surfaces.and adhesive (critical) loads than do elemental coatings.such as aluminum, zinc, magnesium and titanium (theseanodic coatings adhere well to the substrate and ofler 2. Experimental procedureexcellent protection) and (2) TiN and the dual-layerAl/TiN coatings also exhibit good cohesion and adherence 2. I. Specimen preparation

to the substrate (however, unless such cathodic coatings Disk specimens of DUI -0.75Ti. 25.40 or 15.89 mm inare defect free they will perform rather poorly). diameter, and 6.35 or 3.18 mm thick respectively, were

0257-5972/91 /3.5( 1991 Flsc'icr .kequoui. Lausanne

88 F C. ( (/I,um oi a/. 41 /.A an i" Ti TiN '\ni,,ni: in DM 0. 75 Ti ,ih, i

prepared and coated by Nuclear Metals. Concord. spectroscopy (EDS) analyses were performed for evi-MA. using the cathodic arc plasma (PVD) process in a dence of corrosion.Multi-Arc Vacuum System (Multi-Arc Vacuum Sys- Anodic and cathodic potentiodynamic polarizationtems. St. Paul. MN). as previously described [2]. scans of the each uncoupled specimen combinationWhereas in the previous study [2] elemental aluminum were made to complement the galvanic coupling data.and zinc cathodes were used. the present study used The intersection of the anodic segment of the alloypre-alloyed AI-45wt."/. Zn targets (supplied by Bethle- behaving as the anode of a couple and the cathodichem Steel Corporation. Bethlehem, PA). Higher zinc segment of the alloy behaving as cathode representedevaporation rates make control of the coating compo- the potential and current density generated by thatsition rather difficult. Coated specimens 25.40 mm in couple. The PAR potentiostat-galvanostat model 273diameter in the as-received condition were used for in conjunction with a PAR Softcorr 342 program wasautomatic scratch testing. used for the potentiodynamic polarization scans. A

The galvanic corrosion behavior of DU-0.75Ti scan rate of 0.3 mV s ' beginning at E,_, was used withcoupled to the following alloys was studied: aluminum a reference saturated calomel electrode (SCE). and twoalloy 7075. DU-0.75Ti coated with an Al -Zn alloy, high density non-permeable graphite rod counterelec-and DU-0.75Ti coated with a multilayered system trodes. a PAR standard flat specimen holder modelconsisting of an inner TiN layer, an intermediate KI05 with a sealing knife edge washed of Teflon ex-titanium layer and an outer TiN layer. These speci- posed I cm2 of specimen area to the test solution.mens were machined into disks with a diameter of Measurements began after immersion for I h to allow15.89mm and thickness of 3.18 mm. The uncoated specimens to stabilize.DU and aluminum alloys were ground and polishedusing 600 grit silicon carbide paper to a surface rough-ness of about 0.25 pm r.m.s. After coating. the DU 3. Results and discussiondisks were tested in the as-received condition. All spec-imens were degreased in acetone. followed by a 3. 1. Qualilo. .su,,tIbw.ss and adhesion of ihe c£,,,ingsmethanol rinse and air dried prior to electrochemical 3.1.1. (.41- Zn)-coated DU- 0. 75Ti specinenscorrosion testing in a 0.5 N NaCl solution at room The coating thickness is fairly uniform with an aver-temperature. age value of 8.33 pm. as measured optically from trans-

verse sections (Fig. I(a)). The surface morphology.2.2. Specimen testing (Figs l(c)-I(e)) consists of an agglomeration of

The soundness and quality of 25.40 mm diameter spheroidal or flattened particles of a wide size distribu-coated specimens were evaluated primarily with a tion between about I and 35 pm. Defects such as pitsCSEM-Revetest (Centre Suisse d'Electronique et de and micropores are also observed.Microtechnique. CSEM. CH-2007. Neuchdtel. Switzer- The variation in AE intensity. F, and i,* vs. appliedland) automatic scratch testing apparatus. The original normal load F, between 0 and 80 N is illustrated intip radius of the diamond indenter was 200 pm. The Fig. 2. For this particular scratch a cohesive loadapparatus and testing procedure have been described L, = 38.2 N. and adhesive (critical) load L, = 70.4 Nelsewhere [2. 7]. In all tests the sample table transla- and an average friction coefficient It* =-0.41 were mea-tion speed was 10 mm mrin with a loading rate of sured. Average values of L, = 43.72 N and L, = 68.64100 N min ': hence dL/d.v = 10 N mm '. The acoustic N were determined (Table I). using five scratches onemission (AE) signal intensity, the frictional force F, two specimens 25.4 mm in diameter.and the friction coefficient It* were plotted vs. applied With increasing load the coating deforms plasticallynormal load F,. The scratch track and coating surface and the surface particles in the track gradually merge intomorphology in the vicinity of the scratch were exam- a single mass ( Fig. I(c)). The first microcracks within theined by optical microscopy and SEM. coating are observed at a load of about 37.9 N near the

Galvanic coupling was accomplished by an electrical edges of the track (Fig. 1(d)). These transverse. pre-short circuit between the sample electrodes 5 cm apart. sumably tensile microcracks form at a load of about 40 NA PAR mo('el 273 systen. functio.iing as a zero-resis- and appear to be parallel to the trailing edge of thetance ammeter measured galvanic currents continuously moving stylus. Longitudinal striations are observed allas a function of time. The galvanic corrosion cell was along the track within the coating and toward the endinstrumented so that a positive current density indi- of the scratch within the substrate. Similar observationscated that the DU-0.75Ti alloy was cathodic. con- may be seen in Fig. I(e) where a secondary system of veryversely a negative value indicated anodic behavior. The fine microcracks exists at the edges of the scratch inexposed areas of each anode and cathode pair were the addition to the primary system within the track. Coatingsame. Post-test SEM examination and energy-dispersive debris appears to be smeared on the sides of the track.

4L 4uol

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P (C. (01i1t!0 ,t 1. I t (o lnd I I\ h 1A o\w in W)1 0. 5 Ti alh,

(AI-Zn).¢oated OU-OSTII (TINITIITIN)-coated DU-O.71TI

IiiL AI.wLlA)TIMITIrM

LAViLAJTINIOU

X z

JAM

ILC IN LAI a LAS LAS

F,, NOWL LORA0 F, , Norma Lo#AN

(a) (hI

Fig 2. AE signal inensit%. frictional ICorcc /-, and friction coeflicient I "* r. normal load F/, hIt% Ln O) and XII N for (a) (. Al Zn)-cooahed and

h) multilayer TiNiTiTiN-cotted DU (0.75Ti spLcinicns

TABLE I. A•crage cohesime load. I.,. adhesie Icritical) load I.\ For the scratch in Fig. 2(b) the average /i* was 0.34.Lind (riction coefficicnt I* Average values using six scratches are given in Table 1.

(TiNýTi'TiNI-coated D U (75Ti TiN debris and islands of exposed titanium middle

I., 1, .i 27.15 N layer are illustrated in Fig. 3. In addition to longitudinal

1- I. 32.55 N striations. very fine transverse microcracks, parallel to1- 0 \ 52.17 N the trailing edge of the stylus and presumably tensile, are

L \; " \6,.. ,, 2.73 N observed in the scratch.t* o.34 Comparison of the two coatings (Table I) clearly

(Al Zn(-coated DU 0.7MTi shows that (1) the cohesive load of the Al Zn alloy1., 43M72 N coating is noticeably higher than that of the upper TiN

6., 6.64 N layer in the multilayer TiN/Ti/TiN coating: and (2) the14* 0.41 critical or adhesive load of Al- Zn on DU 0.75Ti is also

slightly higher than that of the multilaver coating. Forthese tests all intrinsic parameters 12. 7. 8] that can affectthe critical load values were kept constant (d0.,dx = I0 N min ' and the stylus tip radius of 200 pm

Typical AE. F, and p* curves vs. F,, between () and with not much tip wear during testing of this batch of80 N are illustrated in Fig. 2(b). For this particular specimens). For the extrinsic parameters of substratescratch the shape characteristics of AE and to it lesser hardness and roughness. prior to coating there %%ere noextent F, indicate that crack initiation within the upper differences since all the substrate disks were sectionedTiN layer occurs at L( = 27.5 N. The crack reaches the from the same rod using the same procedure. Also. theinterface between the upper TiN and titanium layers. coating thickness was roughly the same. However. thecausing delamination at a load of' LI = 34.5 N. It coating roughnesses. and thus the frictional forces andsubsequently propagates U-rough the titanium layer. the friction coefficients. were not the same. It wouldreaching the Ti-lower TiN layer interface and causing therefore be speculative to generalize that Al- Zn coat-delamination at a load L,, = 56.7 N. Finally. the crack ings adhere better to the substrate than do the multilayerreaches the interface between the lower TiN layer and TiN/Ti/TiN coatings. None the less, in the latter thethe substrate and causes delamination at the critical three layers delaminate from each other at substantiallyload. L,, = 64.9 N. Metallographic observations using lower loads. Thus it appears that. for the set of processthe golden color of TiN and silvery color of titanium variable values used. the Al Zn coating is mechanicallyconfirmed this interpretation of these measurements. superior to the multilayer TiN/Ti/TiN coating.

V. C . ChIumgi, et al. 4/I n wid riN 7?ý T,.\ oawnn ' i DU 0. '5 T1 all'.

'SUDING DIRECTIONL

Fig. 3. 1a). (h) Optical micrographs of it scratch corresponding to normal loads of' (it 42 N and Ih) 79 2 N foir rnultila~cr TiN Ti TiN-coated DUI 0.75Ti specimens. (c) SEM micrograph of' the same scratch correspo~nding to it normal load of 55.6 N IMaenitication%(al. Ib 20) x . c) 754) x.1

37.2. Galvanic corrosion bheluriar the oxide film was removed from the At -Zn anode.3.1.1. (DU -0. 75Ti) c:c. (At - Z,,)-coiafd DU' - 0. 75Ti) Several cycles in current density followed before aThe current flow-time curve for this couple (Fig. steady state value of 10 pA cm (Table 2) wats reached

4(a)) ralls in a relatively low current density range. The after immersion for 70 h (2.5 x I Ws) in the 0. 5 N NaClinitial current density drop indicates oxide film forma- solution. The positive current density (Table 2) indi-tion on the anode: this is followed by at gradual rise ats cated that Al-Zn wits anodic and DUi 0.75Ti was

12 7141m! ( .1~t 401 1/ / A/ '0 1, ri II II I m , ,u t!• m• I) 1 ,- I//( I

galvgak CompleDU.3/4 T1i vs. AIZa (NMI)

In & N NaC

w.24WVC1M2

(Interpolated)

-i alvanic Current i

-I Il

-a , iiU/i2

(a)DepIcied Ur~niumn vs. AIZn (NMI) Coating

Polazatin .10.5 N NaG'Scan rae0 Nm~.\o Purge

E. 61L-Vanu-kof" 10pA/ca2 -

ly ,eut '- n 9. 3!ec nd I

EDS ol'Al~n af'teru test~ IMI o

SE Zn~ atris

J . o1n o cas-n0J. .a

(e)Optical microgra~ph ot DLi .itler test at 5()1 (aigainst Au~nt

Fig. 4. Gialvanic. corrosion o1 (Al Znp-coailed ivv unicoated DU 0 .75Ti. f it gaiiinic couple DU U. 75Ti ik. AI Zn in 0.5 NATl fb DU. 1,Al Zn in 0.5 N Na1 I scan rate. (0.3 mV,' no purge). (Ic) IDS of Al /n after test: Id) SFI m icrogra ph olS rt 0 nler test: me) opticalmicrographs of Di alter test I against Al Zn). (Magnification: e) ,i)

V C. C *hIng ,et a/. 41 /i and Il.V/Ti;TiN ains in DM 0.7Ti Ft ,lliui

TABLE 2. P.-_ and current density from polarization scns. and current density Ifrom gals anic couple measurements

I Polarization Galvanic(mV (S(CE)) t mV (SCEO) current density current density

(pA cm -() (MAcm 21

DU -735DU rs. TiN-coated DU - 297' -720 -5 -10

DU vs. Al alloy 7075 -809h -796 12 10DU r.s. (Al Zn)-coated DU - 1138' -1120 24 10

Positive current densities indicate that the DU is cathodic while negative values indicate that the DLI is anodic Ior each couple."*'E,,,r for TiN-coated DU.'E_, for aluminum alloy 7075.

"IE,,, for (Al Zn)-coated DII.

cathodic. Figure 4(b) shows the results of polarization 3.2.2 (DU-O.75Ti) ts. (Multilayer (TiN/Ti/TiN)-measurements of AI-Zn as anode and DU-0.75Ti as coated DU-O. 75Ti0cathode. The extrapolated intersection of the anodic Figure 5(a) represents the current flow characteristicsand cathodic potentials represents the potential and the of this couple. The current density remained steady atcurrent density of the short-circuited galvanic couple. - 10 1A cm 'for 1S h (5.4 x 10' s) before a series of fallsTable 2 compares the values of the corrosion potentials and rises was observed until a pseudo-steady state valueE,,.., and the galvanic couple potential EC,,,,)j derived of - 15 pA cm - was reached after exposure for aboutfrom these polarization scans. These data show that 110 h (3.9 x 10' s) to the chloride solution. The negativeDU-0.75Ti is polarized significantly in the cathodic values shown in Table 2 indicated that the DU-0.75Tidirection and behaves as cathode. On the contrary, alloy behaved as the anodic member of this couple.E.,,up, for Al-Zn is slightly anodic to E,,,,,. suggesting Figure 5(b) contains polarization scans of DU -0.75Tithat Al-Zn could support the anodic reaction. The alloy as the anode and the coated alloy as cathode. Aanodic curve for AI-Zn intersects the extrapolated comparison of Ec,,n•,u with E,.... of the uncoupled alloysDU-0.75Ti cathodic curve along the oxygen reduction (Table 2) revealed that DU-0.75Ti was polarized in theregion where concentration polarization becomes im- anodic direction and therefore behaved as anode. Theportant as the reduction rate approaches the limiting (TiN/Ti/TiN)-coated alloy was significantly polarized indiffusion current density. A comparison of the mea- the cathodic direction indicative of cathodic behavior.sured galvanic current density and the current density The intersection of the anodic curve for DU -0.75Ti andextrapolated from anodic and cathodic polarization the cathodic curve for the coated alloy occurs along thescans shows reasonable agreement (Table 2). Figure region of the limiting current densit% of oxygen reduc-4(d) is a scanning electron micrograph of the (AI-Zn)- tion where concentration polarization becomes impor-coated DU-0.75Ti specimen after galvanic corrosion tant. The good agreement between the measuredtesting for 90 h in a 0.5 N NaCI solution. Also shown in galvanic current density and the current density extrap-Fig. 4(c) is the EDS scan of the corroded surfaces. The olated from the anodic and cathodic polarization scansglobular and mud-cracked corrosion products are is shown in Table 2.mainly aluminum or zinc chloride compounds. There Figures 5(c) and 5(d) contain micrographs of bothwas no evidence of exposure of the underlying DU- the DU-0.75Ti alloy and the multilayered coated0.75Ti alloy. The EDS concentrations of the unexposed alloy surfaces (includes an EDS scan (Fig. 5(e)) afterAl-Zn alloy coating were 47 at.% Al and 53 at.% Zn galvanic corrosion testing for 110 days in the chloridewhich is in reasonable agreement with the original solution. The DU-0.75Ti alloy is completely coveredcomposition of the AI-Zn alloy target used in cathodic with corrosion products indicative of relatively severearc plasma PVD processing. Higher zinc evaporation corrosion of this alloy. On the contrary the coatedrates makes the slight zinc enrichment of the coating an alloy is relatively corrosion free except for minorexpected effect. Figure 4(e) shows the DU-0.75Ti amounts of white corrosion products which appear tomember of the couple after the same exposure in chlo- be chlorides. The silvery coating which is mainly tita-ride solution. There is very little evidence of corrosion nium (see the EDS scan) appears to have some poros-indicative of the galvanic protection from the AI-Zn ity and in these areas the EDS scan shows the presencecoating. of some DU.

Galvanic Couple .

Dt-3;4 Ti v-. TiN.Ti-TiN, 5.5 *4 NaCJ

! ., Steady State

Galvanic Current " \ (\l

-o Value of -O1 pA/cm2

t , -,

(a) ...

() Defleted Urnumnt wv.TN-T-TI ComalinPolarailon scam In 0.5 N NaCl

Scan rale 0.3 mV/l. No Purge

(C)(dOpticaiI micrograph of DLI aflter test Mt 50 Optical microgralph ol"TiN-Ti-TiN

coa tin e after test al if)

9UU

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-1 -- 8982

EI)S of Tj-Ti-TiN aftecr test (showing traces (if Uranium)

Fig. 5. Gal van ic corrosion of Ti N Ti ITi N-cmod ldI-%. uncoia ld DU t. 75Ti: (a) givanIalic couple DUI 0.7 STi I.%. Ti NTiTi Ni in 01.5 N. N A( I

I h) DU 1 .ý. Ti N/Ti'Ti NO in 0.5 N NA] I san ralI. 0.3 miV no purge) (I:) optical micrograph III' DU afftr test: (d) l)ptical iiicr rgrafphT

of Ti NT Ti N after test: e) tDS ofTi N /TiiTi4 N;offer test ( showing traces I uranium). IMagnificatlion: l' .r• d l 11

F. C, ( 'Ihai c/I, .4/ An and TiN/Ti]TiN aaning's in DU 0.75 Ti ailh '-i

Galvanic CoupleDU-/4 TI vs.MAl7075 o

in 0.5 N NaCI

- - - - -I °1/:0 40•'•

"(Interpolated)-S0 Stlte -'- - Mm

GavncCuirentj4 .Va.,e o-.f 1 /Cm• 2

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Polarization scans in 0.5 N NaCIScan rate 0.3 mV/s. No Purge

(c)

Optical micrograph of Al after test at 60 x Optical micrograph of DU after test at 10

1Z -S .,-Lg 9 O 11 N:3 2 12 -S w - i g 1C :S :z l "Z-50 -

nution Strt * 6 second *Z£eutin tO 0 K'i.o t329 co..ts Duo-. I E l Vert. lg1s 0 *.t * t 'E7 Ve¢.

C,

0 10

Z.

+, 1O, •, 3 ;• 4- I .III ln, I •12 ••e I l

M EDS of Al 7075 after test EDS after test

Fig. 6. Galvanic corrosion of aluminum alloy 7075 ry. uncoated DU 0.75Ti: (a) galvanic couple DU 0.75Ti vs. aluminum alloy 7075 in 0.5 NNaCl: (bh DU r,. aluminum alloy 7075 in 0.5 N NaCI (scan rate. 0.3 mV s ': no purge). (c) optical micrograph of aluminum alloy 7075 aftertest; (d) optical micrograph of DU after test. (e) EDS of aluminum alloy 7075 after test: (0t EDS of DU! after test. (Magnilication. (c) 60 x;

(d) lOx.)

96 F. C. (han. eat id. 1.41 Zn and Ti,\Ti/TiiN oaatnk.% it Dl I 75 Ti alloi

3.2.3. (DU-O. 75.Ti)O s. .41 alloyv 7075-T6 alloy exhibited severe pitting. Corrosion productsThe galvanic current decreased initially to a very present are in the main chlorides and oxides. The

low (close to zero) value and then gradually increased DU-0.75Ti exhibited only slight corrosion in the formto 15 ,A cm-2 (Fig. 6). later decreasing back to a of oxides with trace amounts or chlorides.steady state value of 10 hAcm- 2 after exposure forabout 100 h (3.6 x 10 5s). The fluctuations in currentobserved after exposure for 40 h (1.4 x l0' s) were 4. Conclusionsprobably due to pitting of the aluminum alloy 7075.The positive current density shown in Table 2 indi- AI-Zn alloy coatings on DU-0.75Ti alloy specimenscated that aluminum alloy 7075 was anodic to DU- provide galvanic protection to the substrate and exhibit0.75Ti. Polarization scans for aluminum alloy 7075 as better mechanical strength than do multilayer TiN/Ti/anode and DU-0.75Ti as cathode are displayed in TiN coatings which can be used only if they are defect-Fig. 6(b). A comparison of E,,,r, and Ec,,upte derived free. Aluminum alloy 7075-T6 can support bothfrom these scans (Table 2) shows that the DU-0.75Ti cathodic and anodic reactions, and therefore its abilitywas not significantly polarized and the aluminum alloy to provide galvanic protection to DU-0.75Ti alloy is7075 was only slightly anodic to E,,, which suggestS limited.that this alloy would support both cathodic and an-odic reactions. The anodic curve for aluminum alloy7075 intersects the DU-0.75Ti curve along the region Referencesof the limiting current density of oxygen reduction.Table 2 shows good agreement between the measured I. F. Chang. M. Levy. B. Jackman and W. B. Nowak. Surf. Coat.

galvanic current density and the current density ex- Technol.. 39-40 11989) 721 -731.

trapolated from anodic and cathodic polarization T. Z. Kattamis. F. Chang and M. Levy. Surf. Coat. Te'hnol.. 43 44(1990) 390 401.

scans. Mclntyre et al (91 have also studied the (DU- 3. J. Ahn. K. L. Mittal and R. H. MacQueen. in K. L. Mittal ied.(.0.75Ti)-Al alloy 7075-T6 couple for a 1:1 area ratio. Adiesion Measurement of Thin Filns. Thick Films and BuIk CoAnntm.They reported that the DU-0.75Ti behaved as the .4STY Spe(.. Publ. 640. 1978, p. 134 (ASTM Philadelphia. PA).anode during the initial immersion for 72 h but, after 4. A. J. Perry. P. Laeng and H. E. Hintermann. Proc. ,1th Int. Cont.

on Chemical IVapor Deposition. Electrochemical Society. Pennington.72 h. the current reversed and the DU-0.75Ti became NJ. 1981. p. 475.

the cathode. "li.rrefore it is not surprising to see DU- 5. A. J. Perry. Thin Solid Films, 107 (1983) 167.

0.75Ti behaving as either cathode or anode when cou- 6. P. A. Steinmann. P. Laeng and H. E. Hintermann. Mater. Tehnol..pled to aluminum alloy 7075-T6 depending upon the 13 (1985) 85.specific experimental conditions. 7. K. J. Bhansali and T. Z. Kattamis. Wear 14 ! 1990) 59.

8. P. A. Steinmann. Y. Tardy and H. E. Hintermann, Thin Solid Filns.Figures 6(c)-6(f) contain micrographs and EDS 154(1987) 333.

scans of both members of the couple after exposure 9. J. F. Mcintyre. E. P. Lefeave and K. A. Musselman, Corrosion. 44

for 110 h to the chloride solutions. The aluminum (8) (1988)502.

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