AD-A268 203

ARMY RESEARCH LABORATORY fl

Effect of Molybdenum Ion Implantationon the Pitting Corrosion of Depleted

Uranium - 0.75 Titanium Alloy

K.-S. Lei, F. Chang, and M. Levy

ARL-TR-144 July 1993

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13~. ABSTRACT (Maximumn 200 *or)

Pitting corrosion of molybdenumn-ion-implanted. depleted uranium -0.75 Ti (DU -0.75 Ti) has beenstudied electrochemically in acidic, neutral, and alkaline solutions containing sodium ch~oride, and theresults have been compared to those of the unimplanted DU -0.75 Ti. The data show that Moimplantation shifts the pitting potential of DU -0.75 Ti in the noble direction in acidic and alkalinesolutions. In neutral 50 ppm CF' solution, however, there is no beneficial effect of Mo implantationAuger analysis studies show that before exposure to the solutions, all the molyhdenum is in the oxide,which is approximately 1000 A thick After electrochemical scans in the acidic and alkaline chloridesolutions, most of the Mo disappears from the oxide However, no decrease in Mo concentration isfound after exposure in neutral chloride solution. It is proposed that the implanted molybdenumdissolves in the acidic and alkaline solutions and forms simple or complex motlybdates that inhibitpitting corrosion. The implanted molybdenum does not dissolve in the neutral chloride solution andinhibition does not occur.

14. SU6JECT TERMS 15, NUMBER OF PAGES

Uranium alloys, Uranium titanium alloys, Molybdenum, 1Ion implantation, Corrosion, Pitting, Surface inhabition 1.PIECDanai~l ss_______ _

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Effect of Molybdenum Ion Implantationon the Pitting Corrosion

of Depleted Uranium -0.75 Titanium AlloyK.-S. Lei,* F. Chang," and M. Levy

AbstractPitting corrosion of mofybdenum-ion-implanted, depleted uranium -0.75 Ti (DU -0.75 Ti) has beenstudied electrochemically in acidic, neutral, and alkaline solutions containing sodium chloride, and theresults have been compared to those of the unimplanted DU -0.75 Ti. The data show that Moimplantation shifts the pitting potential of DU -0.75 Ti in the noble direction in acidic and alkalinesolutions. In neutral 50 ppm Cf- solution; however, there is no beneficial effect of Mo implantationAuger analysis studies show that before exposure to the solutions, all the molybdenum is in the oxide,which is approximately 1000 A thick. After electrochemical scans in the acidic and alkaline chloridesolutions, most of the Mo disappears from the oxide. However, no decrease in Mo concentration isfound after exposure in neutral chloride solution. It is proposed that the implanted molybdenumdissolves in the acidic and alkaline solutions and forms simple or complex molybdates that inhibitpitting corrosion. The implanted molybdenum does not dissolve in the neutral chloride solution andinhibition does not occur.

Introduction in a Variant electric-arc furnace under vacuum (10-s Torr) and castDepleted uranium (DU) has good ballistic properties but poor into a cylindrical steel mold to form a rod of 1.0-in (2.54-cm)

ccrrroston resistance in moist air and in salt-laden environments, diameter. The alloy was hot rolled, solution treated, water quenched.Alloying depleted uranium with Ti, Mo, or Nb reduces the corrosion and aged. Samples of 5/8-in. (1.59-cm) diameter and 1/8-inrate of pure DU 1

-3 However, these alloys are still susceptible to thickness were fabricated from the 1 .0-in, rod. As indicated in the

localized corrosion when chloride ions are present Various coatings phase diagram, the equilibrium structure of DU -0 75 Ti consists ofand surface treatments have been explored as protective schemes a major a phase and a minor U 2 Ti phase 8 The alloy has a martensiticfor uranium alloys." Surface-modification techniques such as ion structure and a grain size of about 400 Im.implantation offer several advantages. e.g . Cdmensiona? stabilityand good adhesion, while maintaining the bulk properties. Further- Ion implantationmore. this approach has been shown to increase the corrosion The ion implantation was performed at the Naval Researchresistance of various alloy systems"' Thus. ion implantation may be Laboratory. After cleaning, the samples were implanted with 75 KeVa nrghly desirable method fo1 providing not only corrosion resistance Mo tons to a dosage of 6 x 1016 ions/cm2 , and then with 150 KeVo DU but also maintaining the high density necessary for anta rnor Mo÷ ions to 1.2 x 10'7 ions/cm 2 . This implantation procedure wasappications. This research effort studied the pitting corrosion intended to produce a Mo concentration profile such that theresistance of DU -0.75 Ti after ion implantation of Mo* onto its concentration of molybdenum (35 at%) remains constant at the firstsi'face Electrochemical studies were conducted in acidic, neutral, several hundreds of angstroms under the surface instead of theand alkaline solutions containing sodium chloride Electron disper- normal distribution type of concentration profile usually obtained bys,se spectroscopy (EDS) and Auger electron spectroscopy.(AES) one-step implantation. But the actual composition profile displayedanalyses were used to characterize the surface before and after a normally distributed molybdenum as shown latereyposure to these solutions. Experimental results of DU -0.75 Ti andMo-,mplanted DU -0.75 Ti (Mo/DU -0 75 Ti) are compared and theposs~ble mechanisms describing the effects of molybdenum implan- Electrochemical studiestalton on ptlir,.u are discussed Electrochemical measurements were performed using an EG&G

Princeton Applied Research' (PAR) K0047 Corrosion Cell. a PAR273 Potentiostat/Galvanostat, an Apple' lie personal computer, and

Experimental a PAR 332 Softcorr program Solutions were made of reagent-gradechemicals and distilled water Before testing. the solution was

Sample preparation deaerated for 1 h by purging with Ar All DU -0 75 Ti specimensThe Du -0.75 wt% Ti alloy used in this study was first melted were polished through 600-grit SiC paper and ultrasonically cleaned

in acetone before immersion into the solution Ion-implanted spec-irrns were only ultrasonically cleaned in acetone without polishing.

Cortest Laboratories. Inc., Cypress. TX. Polarization icans at a scan rate of 5 V/hr (1.39 mV/s) commenced"Army Materials Technology Laboratory, Watertown, MA02172-0001. Trade name.

460

at E.., after the samples were equilibrated in the solution for 1 h. All Acidic Chloride Solutions. The pitting corrosion potential (E.) of

potentials were measured against a saturated calomel electrode DU -0.75 Ti in acidic solutions was measured in 1 N H2SO 4 + x

(SCE). ppm Cl. Cyclic polarization started at Eo., and was reversed whenthe CD reached either 1 or 10 mA/cm2 in order to locate the pitting

Surface analyses potential and the repassivation potential (Er,) For DU -0,75 Ti. theE,* and the passivation CD increased slightly when the chlorideScanning electron microscopy (SEM) and EDS were performed cnetain[I]icesdfo o10pmWe C a

on a JEOLI 840 microscope with a Tracorl Northern EDS attach- concentration [Cl1 increased from 0 to 150 porn When [Cl-] was

mont. A PHL8 model 548 ESCATAuger instrument was used for AES higher than 150 ppm, current fluctuations on the reverse scan wereThe sPectramwere t4Akugen atianseletromnvltwagsed ofo 5 K observed, suggesting that the passive film became unstable Whe"

analysis. The spectra were taken at an electron voltage of 5 KeV. (Clf- was equal to 300 ppm, the reverse scan did not intercept IveSputtering was accomplished in an Ar atmosphere with a sputtering forward scan and crevice corrosion was observed after the expe,-

rate equivalent to the removal of 1000 A of Ta 2 0 5 in 13 min. During kpent. Visual examination showed wevidence of pitting when the

sputtering, the peak-to-peaý.height of each element was recorded chloride concentration was 300 ppm and below, but pittig was

and converted to atomic concentration afterwards by taking the orie whent[Cli was 400 and 500 p ur 2ishowssenstivty actrs ito ccont.observed when [.C-I- was 400 and 500 ppm. Figure 2 shows t,,e

sensitivity factors into account. cyclic scans of DU -0.75 Ti in 1 N H2SO4 + 300.400. and 500 cl"n'

CI-. Also shown are duplicate polarizarton curves of implante,' OU

Results -0.75 Ti in 1 N H-.SO 4 + 400 ppm Cr- for comparison with those ofthe unimplanted specimen (only one curve of each specrme' wasshown) The implanted sample in duplicate scans had E_.. a: 70

Electrochemical tests and 97 mV. which were about 550 mV more noble than the -470 mVAcidic Solution. The polentiodynamic polarization curves of DU of the unimplanted specimen The Mo/DU -0 75 T, also hao E_ at

-0.75 Ti and Mo-implanted DU -075 Ti in deaerated 1 N H2 SO4 250 and 290 mV, which were 75 to 130 mV more noble than Ine 160

(pH 1) are shown in Figure 1 The corrosion potential of the and 175 mV of the unimplanted specimen The unimpla"ted Du

implanted specimen was about 300 mV more noble than that of the -0,75 Ti did not repassivate even after a reverse scan to E.... wn,,e

unimplanted DU. Passive regions without marked active-passive the implanted material repassivated at about + 130 mV Whe,'

transitions were observed for both materials where the passive pitting occurred, a large amount of black powder, probabry UO,Current densities (CDs) were in the range of 1 to 5 I.A/cm2 . There formed at the pits of both samples. The surface of MoDU was darkwas no color change of the solution and little gas evolution was blue after the conclusion of the experiment The unimpianted DUobserved at the electrodes The polarization curve for MoIDU -0.75 -0.75 Ti appeared to have larger pits, which might resjd•: fron MneTi shows a peak at 370 mV. which probably results from the longer pit growth period during the reverse scan The imp;anted

dissolution of Mo into the solution. The absence of another dissolu- alloy repassivated quickly so there was little time to, pits to grOv.

tion peak on successive scans (not shown in Figure 1) indicates thatall Mo was probably dissolved during the initial scan

600

-- - DU-'0.5 Ti in IN H2SO- + 400 pD cr2

MIMO/DU-.0.75 1 in IN HZS04 + 400 300 C-

10- 10 10 o ,0.: 1 r,

S-30

- O , * .. ..... ,,,,,,I~ p , , , , , ,,,,6, - -,.T.i.. ,I 1250.. I f1.CA/cr2)

I .ICr.•i FIGURE 2 -- Polarization curves of ianimptanted andMe÷- Implanted DU -0.75 Ti in deaeIiHied 1 N HPS0 4 +

FIGURE 1 -- Anodc polarization curves of DU -0.75 TI 300, 400, and 500 ppm C-.and Mo/DU -0.75 Ti In dowUafed V N H2SO+ at room

temperature.Neutral Chloride Solutions. Unimpranted Dl -0 75 pined

readily in the neutral solution with a chloride concentlal,on as 10* as

50 ppm and did not repassivate In 50 ppm Cr- sovufon a irnied

Trade name. passive region was observed, while in 0 005 M NaCI soluti•n. th

460

current increased steadily from E.., and the anodic current was 120generally one magnitude greater Gas bubbles were observed on OU-ji.if I .ii M N.OR +0.5 N NaO

the surface during scanning and pits were found after the experi- MoDU-0.75 TI In 0.SN NaWH +0.5 lN NaC3 (Run 11)

ment Two pitting scans in 50 ppm CI- were performed on each - M/DU-0.75 _i in 0.51N WOHN .5N faO (Run #Z2

specimen: the results are shown in Figure 3. The Eco, of MoJDU r --075 Ti was at least 50 rnV more noble than the Ec,, of DU -0.75 10"Ti, however, the CD of the implanted specimen was higher than thatof the unimplanted specimen. The pitting potential of the implantedDU was 95 mV on one run and was difficult to define on the other runbecause no clear passive region could be found. The polarization 400-'curves of the unimplanted DU -0.75 Ti displayed two breakdownpotentials, one at a low current level (1 to 2 pAcm

2) and one at ahigher current level (10 to 30 piAcm2 ). The low breakdown potential Ziwas 50 mV for both runs, while the higher breakdown potentials were "1

130 and 260 mV for both runs, respectively. The low breakdownpotential may represent the first pit event: the higher breakdown 0,.,..ppotential may be related to secondary pitting. Repassivation was not

observed on either specimen. Nonadherent black powder wasagain found on both pitted specimens, but pitting was more severein the unimplanted alloy. This may have resulted in part from ,Wterminating the reverse scan early for the implanted alloy but running -O

the gamut to E, for the unimplanted alloy.

01.1.O-0.75 Ti in 50 ppm CO" (Run III)1- 10 0I 02 l I0 O l0

(Rn0110-1 IGO 101 102 103 10' 105 t0-....... DU-0.75 ri in 50 ppm CI (Run 92) i 4Jcii2)

-Mo/DU-0.75 1i in 50 ppm a- (Run 11)100 --- M/DU-0. 75T i In 5ppm C" (Run 12) FIGURE 4 - Pitting scans of DU -0.75 TI and Mo/DU

-0.75 TI In 0.5 N NaOH + 0.5 N NaCI solution.

I* to 200 mV lower, than the unimolanted alloy. Both specimens did not

S....appear to repassivate, and the reverse scans were terminated at-'" .,,,,, about 100 mV after reversal to avoid extensive uranium corrosion

... 4 , Black powder was present around the pits

:, Surface analyses100 (I I Electron Dispersive Spectroscopy EDS was used to determine

j:) I :the composition of the material in the pits and on the surface, before-" " 1 and after tests However, because Ti was only 0 75 wt% in the bulk

alloy. and the amount of Mo in the whole excitation vnlume (1 cu I.m'.. was very small, EDS did not provide conclusive results In most

cases, EDS showed that U is above 99%. while Ti and Mo are less., than 0.5%, which was very close to the resolution limits of EDS EDS

mapping was attempted in order to identity the reaction products

S'. formed in the pits Even distributions of Ti and Mo on the surfacearound the pits and in the pits were found by EDS mappings, whichsuggests that either no compouhds of Ti or Mo were present in the

- - pits, or that the concentrations of these elements were less than the"detection limit of this technique (0 5 to 1%) Sodium chloride wasidentified on both DU -0,75 Ti and Mo/DU -0.75 Ti surfaces after

"-950 I IuIi,.Il , 3.l3.,! 33 ii liln l lii u g inn? ill h ittleexperiments in the 05 N NaOH + 05 N NaCl solution

ir1 1o0 101 102 103 1o4 1o5 1o6 Auger Electron Spectroscopy AES analysis combined with Arsputtering was used to generate the composition depth profiles ofspecimens before and after tests Since the disk specimens were

FIGURE 3 - Pitting wcant of DU -0.75 TI and covered at the edges during polarization experiments, the outside

Mo'-Implanted OU -0.75 TI In deaeratod 50 ppm CI- protected region was analyzed as representative of the before-test

solution, condition The central area exposed to the solution and subjected toelectrochemical tests was considered the after-test condition

(1) Acidic solution. The AES profile of the Mo'-implantedAlkaline Chloride Solutions The DU -0.75 Ti was much more sample before test is shown in Figure 5 Mo is normally distributed

resistant to pitting attack in NaOH than in H2SO and neutral under the surface instead of the anticipated Mo distribution (dotted

solutions. It did not pit in 1 N NaOH with up to 0.2 N NaCI added. line in Figure 5) The oxygen concentration decreases while the DUCrevice corrosion was observed in 1 N NaOH + 0.5 N NaCl and 1 concentration increases continuously from the surface to the bulkN NaOH + 1 N NaCI solutions. Pitting of DU -0. 75 Ti in 0.5 N NaOH Both Mo and O peaks diminish after 10 to 15 min of sputlering. which+ 05 N NaCI occurred at 300 mV, as stx)wn in Figure 4. The suggests that almost all of the implanted Mo exists in the oxide, andMo-implanted specimen was much more resistant to pitting in this very little Mo is in the bulk The oxide thickness is approximatelyenvironment with a pitting potential at about 800 mV (Figure 4) On i0oo A. estimated from sputtering of Ta.o0 (1000 A in 13 mm ofthe other hand, the implanted sample had a lower E",,. about 150 sputtering). The Mo concentration is about 5 at% (7 wt%) at the

461

after test. and in the pit The profile before test was the same as

previous ones After test, however, the oxide became very thick

o t(Figure 7). such Mat after sputtering for 55 mi . the oxygenconcentration remained at 25 at%. This implies that the oxide film

o0o pgrew considerably dunng test (>3000 A) The profile on the pit wasConcentrteionre similar to the one taken from the after test surface. i e.. low Mo

11 MoIDU-0.5 Ti before test concentration and thick oxide. The AES profile taken in the pitSshowed very low chloride concentration s(i at%) throughout the

do0 oxide film. The fact that the Mo concentration was very low in bothafter test and in the pit profiles suggests that Mo disappeared during

0 U the test, and Mo compounds were not formed in the pits.

a 0A Mo Discussiono 1t The effect of ion implantation on corrosion has been studied by

10 ~Several reseairChers. 7 '12 1 and different mechanisms have beenproposed Ashworth and coworkers? suggested that ion implanta-

tion. per se. has very little or no effect on the corrosion prqperties of10 base matenals; the implantation effect is similar to alloying in the

base- metal solid solution. AI-Saffer. et al .' explained the effect of4 Mol implantation on At as einer the incorporation of Mo into the Al

passive film or dissolution and reprecipitation as some Mo-con-0 0 20 30 taming species on the film. Natishan. McCafferty. and Hubler2 0

-"

Sputtering Tint (Minute) proposed a theory based on the concept of pH of zero charge to

FIGURE 5 - Depth profile of AES on Mo*-Implanted DU explain their results on the pitting resistance of implantat:on of Mo

-0.75 TI aspelmen before ts. Dotted line repreeni the Si, Nb, Zr. and Al on aluminum. Rubio and coworkers" thoug-ot the

antlcipsted Mo c•ncentratuon profile. implantation effect comprises a mechanical effect, resulting from thestructure change produced by implantation, and a chemical effect.resulting from the chemical nature of the implanted spec~es

surface and reaches 27 at% (18 wt%) halfway into the oxide. After Implanted species have also been found by Walker and Chance' 9

test, most of the Mo in the oxide disappeared (Figure 6); the highest to thicken the passive film on steel and hence improve the fitrMo concentration was only about 10 at% and was distributed more stability.evenly throughout the oxide (5 at%). The oxide thickness, however, Improvement of the pitting corrosion resistance of steels byremained the same. molybdenum alloying has been studied extensively 22 Kolotyrkir

and Knyaheva thought that Mo improves the stability of tne passive

100 film on stainless steels (SSs).2 3 Molybdenum was shown by Hash,-moro and coworkers to form compounds on active surface sites of

Sferntic SS and to reduce their activity. 2' Galvele. et al proposed Matthe dissolution rate of a CrCI3 salt layer on the surface of 18% Cr SS

Co ncenTrato r of is reduced by Mo." ' Sugimoto and Sawada suggested that Moa" isinMIDU-N45 T after Ctr present in the chromium oxyhydroxide on CrNiMo SS and imp'oves

70 - its pitting tesistance.2 Ambrose found that a protective salt film

containing Mo formed in the crevice of the FeMo alloys and resu red601 in repassivation of the crevice.27 2' Ogawa and Sugimoto believed

that the molybdenum in SSs forms molybdate in the solution anr50 a U acts as an inhibitor for pining corrosion.' 2

a 0 We propose a mechanism similar to that proposed by Ogawaa-0 •0and Sugimoto for molybdenum in SS,29 30 namely that the impla'ted

A Mo Mo dissolved into the solution and formed a molybdate that aced as

0o T an inhibitor to prevent or retard pitting initiation This mechanism isexplained in detdil for the different environments. since our study

a showed that the effect of Mo-ion implantation on the pitting corrosion

of OU -0.75 Ti depends very much on the pH of the solution10

0 In alkaline solution

0 10 a Molybdenum-ion implantation on DU -0 75 Ti markediV Shifts

Sputtering Time Minute) the pitting potential in deaerated NaOH + NaCI solution (pH 14) in

the noble direction. The implanted molybdenum dissolved into theFIGURE 6 -- AES depth profile of Mo-mplanted U solution as indicated by the decrease in Mo concentration in the-0.75 IG aftrE tes Ide p N 4SOf4 400 W pp C- -Ilt*o . oxide film after testing (Figure 7). From the potential-pH d:ag'am 3'

molybdenum forms molybdate in aqueous sc lion when pH > 6

Molybdate has long been used as a corror- n inhibitor for both(2) Neutral aetuflee. The AES profile of the before-test ferrous and nonferrous metals"l Generd , oxygen must be

specimen is very similar to the previous one. which indicates that the present in solution for the molybdate to be elf, v.ye. 3 but B,lm andion implantation process produces consistent results. The AES Gabe found that aeration is not necessary ft, moiybldate to nn,bilprofile after test was also the same as that before test, i ethe one the corrosion of zinc.-' Furthermore. it has been recently found thatshown in Figure 5. All Mo was still in the oxide and the Oxide sodium molybdate. inhibits uniform corrosion and reduces the

thickness did not change. passive CD of DU -0.75 Ti and DU -2Mo in deaeraled NaF and

(3) Alkaline solutiorn For the Mo/U -0:75 1i tested in 0.5 N NH4HFa solutions."NaOH + 0.5 N NaCI solution, three profiles were taken: before test. To confirm our proposed mechanism. polarization exper-rents

4U2

OMMMUMlrplft dO "U-0.751 It dw id il 8I JN OMla * UN ItalC

DU-0.75 I1 In 0.51N N&.01 , O.U NR- - DU-0.73 TI In 0.3 1011 *.05N NaCI

at• 14D -oS.IM 1N 2MO04

31.

FIGURE 7 - AES depth profile of Mo-4mplanted DU -400-0.7511 after pittling can In 0.5 N NeON + 0.5 N NeCIsolution.

were conducted for unimplanted DU -0.75 Ti in deaerated 0.5 N -j,,NaOH + 0.5 N NaCI + 01 M and 02 M Na 2MoO4 solutions and 101. IGO 101 W0 W) *J 4 WS 10compared to those for the Mo-implanted alloy in the same environ- 1 i@At' iment but without molybdate. The pitting potentials of DU -0.75 Ti inthe molybdate-containing solutions are 1350 and 1420 mV. respec- FIGURE S - Polariatlon scans of Mo/OU -0.7511 and DUtively, as shown in Figure 8. They are at least 110 mV more noble -0.7511 In 0.5 N NeON + 0.5 N NaCI and 04.1 -0.75 11 Inthan the E. in the same solution without molybdate (Figure 8). Figure 0.5 N NaOH + 0.5 N NeCI + 0.1 M NaM ,.8 also shows that the polarization curves of Moe- implanted DU-0.75 Ti in 0.5 N NaOH + 0.5 N NaCI and DU -0.75 Ti in 0.5 NNaOH + 0.5 N NaCI + 0. 1 M Na 2MoO. are very similar. The passive of DU -0.75 Ti. Thus. it was initially believed that mrolybdate wouldCDs are around 10 1&A/cm2 The peaks on the polarization curve of not have an inhibiting effect in neutral chloride solution But addingMo/DU -0.75 Ti at about -400 and 100 mV may be related to the (0.2 M or 0.1 M) molybdate to 50 ppm 0- solution does shift thedissolution of molybdenum. The total amount of Mo implantation is pitting potential of unimplanted OU -0.75 1i m the more noblevery small, about 10"I ions/cm2; the dissolved Mo is even less. direction (Figure 9) In 50 ppm Cr- + 0.1 M NaMoO,, solution. DUHence. it is very difficult to detect the dissolved Mo in the solution. -0.75 Ti has an E, of + 250 mV. which is 200 mV more noble thanCalculations Show that if the dissolved Mo is evenly distributed in the the alloy in the same solution without motybdate. It repassivates atsolution, the molybdate concentration is approximately 100 ppb. In an Ev of - 10 mV in 50 ppm Cl- + 0.1 M Na 2MoO,, compared to noan unstirred solution, the molybdate concentration would be higher repassivation in 50 ppm CI- without Na2MoO, This indicates thatthan 10 ppb near the specimen surface but would not reach a molybdate should inhibit pitting of DU -0.75 Ti in neutral chlorideconcentration of 0. 1 or 0.2 M This explains why the Mo/DU -0.75 Ti solution. But in the case of the Mo-implanted alloy. Auger analysishas a lower pitting potential than the DU -0 75 -r in the same showed that t:-.e k*:; concentration i the passive film did notsolution with 0 1 or 0.2 M sodium molybdale decrease after test. Therefore. Mo did not go into solution, MoO4

2 -Kodama and Ambrose detected Mo in the pits on iron after was not formed, and inhibition was not observed An explanation for

exposure in Cr- and Na 2Moe, solution and concluded that molyb- the absence of Mo dissolution (within the limits of the AES data) isdenum forms an insoluble film and retarded pd growth.28 But this that the pitting potential of DU -0.75 Ti in this environment Is somechanism is not operative in the case of the DU -0.75 Ti alloy close to the Eco,, that pining occurs before Mo dissolvesbecause there is little or no Mo compound formed in the pits. In our MoO2 can be formed in neutral and slightly acid environmentscase. the implanted Mo dictsolves and forms MoO4

2 - in the solution, according to the potential-pH diagram for molybdenum.3 ' If MoO2then the molybdate is preferentially absorbed on the surface in does form during test. the oxygen concentration in the after test AEScompetition with Cr-. thereby reducing the surface Cr- concentra- profile will increase and molybdenum concentration will decrease.tion. and pit initiation is retarded. But thiaý behavior was not observed, and we concluded that no MoO2

The extensive growth of passive oxide film during the polarize- was formed.tion experiments with the Mo-implanted alloy is thought to be a resultof the m., lybdate inhibition that retarded pitting and extended thepassive r.gion The implanted DU -075 has a corrosion potential In acidic solution15010 to?. mV more active than the unimplanted alloy But adding The role of molybdate in the acid environment is very complex.0 1 M at • 10 2 M Na2 MoO, to the solution has very little effect on the as reported previously by other researchers."' -34 At pH below 3.corrouWo. potential of the unimplanted DU -0.75 Ti. The more three species MoeO 2 '-. MoO3 H20. or MoO2 2*, according to

electronegative potential of the implanted alloy cannot be ac- VukasOrich and Farr,32or Mo 6021 s- Mo12041 to- or Mo 240. 12-counted for by the formation of molybdate but probably results from according to Pourbaix, 3 could form depending on the pH Thethe effect of alloying the surface with molybdenum.' condensed isopolymolybdates or their hydrolysis products. hetero-

polymolybdates, do have an inhibiting effect on steels in acidsolutions.3 """it4 In acidic solution, sodium molybdophosphate is a

In neutral. solution passivating agent that does not significantly change the anodicMO/DU -0.75 Ti has a higher passive CD and a lower E. than characteristics of AISI 4340 steel, according to Lizlovs 0 But

the breakdown potential of the unirnplanted DU -0 75 Ti. In other Stranick reported that molybdate is less effective on mild steel inwords, the Mo implantation does no, improve the pitting resistance acidic than in neutral and basic Solutions.3'

463

alkaline chloride solutions but not in neutral Chloride SOlution A

ou-0.7. Ti in 50 ppm Cl" mechanism based on the dissolution of implanted molybdenum and- - OU-0.75 Ti In 50 Ipm C1- * 0.I•05M l•A04 the formation of molybdates is pioposed to account for the corrosion.. OU-0.75 Ti In 50 ppm CI* + 0.02M N12Mo04 inhibition.

S-- OU .t 516 In 50 PPm CI +0. 1M IZhWo04 (1) In alkaline Chloride solution, molybdate formed after Mo

dissolved from the surface oxide The molybdate then preferentiallyabsorbed on the surface to reduce the surface chloride ionconcentration with concomitant reduction of susceptibility to pitting

(2) In neutral solution, there was no evidence of Mo dissolutionm and molybdate formation. Since inhibition was not observed. it is

suggested that implanted Mo will not inhibit pitting unless it is-- -dissolved The reason that Mo did not dissolve is probably because

the corrosion potential and the pitting potential ot DU -0 75 Ti areso close that the specimen pits before Mo dissolves

400- (3) In acidic solution, the implanted Mo dissolved and formeda blue complex of polymolybdates that deposited on the surface

SThis deposition would slow down the chloride attack As a conse-quence. the pitting potential shifts in the noble direction

0 •Acknowledgment/ The authors express their gratitude to B D Sartwell of the Na ,a

Research Laboratory for the ion implantation work and to S -S LrPh D. of the Army Materials Technology Laboratory (AMTL) fo, t"e

Auger analyses

- References- 1 M Levy. C V Zabielski, Physical MetallurgV of Uran:u.rn Ailos

J J Burke. D A Coiling A E Gorum. J Greenspan Eds r,-00 ,,897. 1974

1 100 1 IV 103 104 1105 10 2 J T Waber. Proc of 2nd U S International Confere-ce c•IlsAkm2 ) Peaceful Use of Atomic Energy. Vol 6 p 204 1958

3 J M Macki. R L Kochen. Nucl Sci Abst. Vol 25 Nc 14FIGURE 9 - Pitting scans of DU -0.75 TI In SO ppmC C- 326 1971+ 0 M, 0.005 U, 0.02 M, and 0.1 M NsMaO4 solutions. 4 J W Din. H R Johnson. Metal Finishing Vo; 45 No 8 19-E

5 L J Weirick. Physical Metallurgy of Uranium Anions J J

The Mo' implantation markedly shifts the corrosion potential of Burke. D A Colling. A.E Gorum. J Greenspan Eds P

DU -J.75 Ti in the noble direction and also increases (more noble) 1974

the pitting potential in 1 N H2SO4 + 400 ppm C -solution (Figure 2) 6 L J Weirick. D L Douglass Corrosion Vol 32 No 6 l 209

The AES depth profile shows that the concentration of implanted Mo 1976decreased after test. probably a result of dissolution of Mo The 7 V Ashworth. R P M Procter W A Gran? Teai se

Materials Science and Technoiogy J K H.,•o~e" Ecsurface of Mo/DU -0.75 Ti after exposure in the acid solution Aademice Vol 18. p 1 1980

became blue. The blue film on the surface could be the deposition

of iso- or heteropolymolybdates that formed after the implanted Mo 8 A M Ammons Physical Metallurgy of Ura-im A. J

dissolved into the solution This deposit on the surface would Burke D A Colling A E Gorur J Greenspa- Eas c 5'1974

increase the corrosion potenial as well as the pitting potential of DU 9AA

-0.75 Ti. A polarization experiment was attempted on DU -0.75 in 9, C .-

W A Grant R P Procter. Corr Sc Voi 20 r 'deaerated 1 N H2SO6 + 400 porn Ci- -t 0 1 V Na, MoO, solutionto simulate the effect of molybdate But a! the open-c:fcuit potential,

the specimen started to pit in less than 10 mm. The area around the National Association of Corrosion Enginee's T>

pits became dark blue and expanded as pits grew When the 1985

specimen was taken out of the solution. te dark blue regions that 11 C R Clayton. Y F Wang. G K Hubler. Pass, iv or f'.as a-specimenSemiconducters. M Froment. Ed Eisevie' Sc~ence A _',e.-

appeared as fine particles covered the whole specimen surface

The specimen was examined by EDS and Mo was found on the dam. The Netherlands. p 115. 1983

surface and was more concentrated on the corrosion products 12 R VaSior. C Popgost'ev. G K .ubEle Pass h o' ?re3Further investigation is needed to understand this phenomenonSGlass4'3 reported that a Mo salt film replaces TiO 2 on Ti in deaerated dam. The Netherlands. p. 152. 1983

1 N 1H2SO,. But in our study. no comparable replacement of Mo for 13 E. McCafferty. G K Hubler. J K Hirvone- 19i6 T, -Se'-

U0 2 can be confirmed. Conference on Corrosior p 435 1979

In summary, Mo* implantation is only effective against pitting 14 J. D Rubio. R R Hat. R B Griffin Corros - Vi 42 N- 9

corrosion of DU -0.75 Ti when the molybdenum is dissolved from p 557. 198615 F A Smidy. NAL Memorandum Report 5 - p 159 19-Et

the surface into solution. After dissolution, it forms a molybdate and 16 M V Zeller, W T Ehihara. U S Army Arn elf R&: Ce-e"behaves like a pitting inhibitor. Without Mo dissolution. Mo* Contractor Report ARSCD-CR-83017implantation does not improve the pitting resistance of DU -0 75 Ti 17 R Wang. J L Brimhall. Ion Implantat;,- a'.d Io" Be3"

Processing of Materials MRS Symposum P-oceei -3s vo

ConclusIon 27. G K Hubler. 0 W Holl.. C R Clayton C W Wr le EcsThe unimplanted DU -0,75 Ti is more resistant to pitting p 729. 1985

corrosion in 0.5 N NaOH than in 1 N H1S10, solutions. It is most 18 J M Williams. G M Beardsley. R A Buchanan R K Bacon

susceptible to pitting in neutral chloride solution The beneficial Ion Implantation. Ion Beam Processing of Mater-a's MRS

effect of Mo" implantation against pitting was found in acidic and Symposium Proceedings. Vol 27. G K Hubler 0 W Holland

464

C. R. Clayton. C W White, Eds, p 735. 1985 29 H Ogawa, H Omata, I Koh. H. Okada, Corrosion Vol 34 No19 M S. Walker. R L Chance, Corosion. 40, No 6. p 307. 1984 2. p 53, 197820 P M Natishan. E McCaftery. G K Hubler, J Electrochem 30 K Sugimoto, Y. Sawada, Corrosion, Vol 32. No 9. p 347.

Soc., 133, No 15. p 1061, 1986 197621. E McCafferty, Corrosion Prevention and Control, Proceedings 31 M Pourbaix, Atlas of Electrochemical Equilibria in Aqueous

of tthe 33rd Sagamore Conference. M Levy, S Isserow, Ed., p Solutions. p 272. NACE. Houston, TX, 1974344, 1986 32 M S. Vukasovich. J. P. G Farr, Materials Performance, Vol 25.

22. Z Szklarska-Smialowska, Painig Cc-"ion of Metals. NACE. No. 5, p 9, 1986.Houston. TX, p 143. 1986. 33 D. R. Robitaille, Chem. Eng., No 20, p 139, 1982

23 Ya. M Koiotyrkn , W M Knya.Lieva, Passivity of Metals, R 34 M A Stranick, Corrosion, Vol 40, No 6. p 296, 1984Fra,•kethal. J. Kruger, Eds, The Electrochemical Society Inc.. 35 E A Lizlovs, Corrosion, Vol. 32, No. 7. p 263 1976Pennington, NJ, p 678. 1978 36. D Bilimi , D. R Gabe, Br Corr J. Vol 18. p 138. 1983

24 K Hashimoto, K Asami, K Teramoto. Corr Sci,, Vol 19, p 3, 37. C Zabielski, J. Scanlon. unpublished data1979 38 E A Lizlovs. J Electrochem Soc., Vol 114, p 1015. 1967

25 J.R. CO-vele. J B Lumsden. R W Staehle. J Electrochem 39 K Ogura. T Malima. Electroche"m Acta. Vol 24. p 325. 1979Soc., Vol 125, p 1204, 1978 40 J N Wanklyn. Corr Sci, Vol 21. No 3. p 211. 1981

26 K Sugimoto , Y. Sawada. Corr Sci, Vol 17, p 425. 1977. 41 K Ogura , T Ohama. Corrosion, Vol 40. No 2, p 47. 198427 J R Ambrose Corrosion, Vol 34, No 1. p 27. 1978 42 G H Cartledge. R F Sympson. J Phys Chem. Vo! 61 p28 T Kodrama J R Ambrose. Corrosion Vol 33. No 5. p 155. 973, 1957

1977 43 R S Glass. Corrosion, Vol 41. No 2, p 89 1985

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