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0 MTL TR 87-37 EI

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< CORROSION BEHAVIOR OFOHIGH DENSITY TUNGSTEN ALLOYS

MILTON LEVY and FRANK C. CHANGMETALS RESEARCH DIVISION

DTICdflb LECTE

August 1987 UCT 2 9 1987,

ja,

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LABORATORY COMMANDMATERIALS TECHNOLOGY U.S. ARMY MATERIALS TECHNOLOGY LABORATORYLABORATORY Watertown, Massachusetts 02172-0001

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CORROSION BEHAVIOR OF HIGH DENSITY TUNGSTEN Final Report

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Presented at the Second International Conference on Environmental Degradationof Engineering Materials, VPI, 1981.

19. KEY WORDS (Continue on fre.e srd. it n sory end Identify by block number)

Tungsten alloys Liquid immersion tests&Density Electrochemical tests /Corrosion resistance Polarization tests

20. ABSTRACT (Continue on rev.erse aide If necessary end Identsfy by block number)

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ABSTRACT

The corrosion behavior of high density tungsten alloys (W > 90 wjo)',hasbeen examined by electrochemical potentiostatic polarization methods and weightloss measurements in full immersion tests. Electrochemical tests were cajriedout in aqueous solutions, buffered to pH 4, 9, 12 with and without 0.1MVaC.and immersion tests in 5% NaCLsolution. Pure tungsten and all the alloysstudied undergo active-passive transitions and corrosion rates increase withincreasing pH. The chloride ion generally increases corrosion rates slightly, 'V

but in pH12 solution, the chloride ion inhibits the anodic reaction to soreextent. A 97W-2Ni-0.7Fe-0.3Co alloy was found to be the most corrosion resist-ant in solutions containing chloride ions. The data also suggests that copperas an alloying element accelerates corrosion of tungsten alloys in NaCL solu-tion. Both general dissolution and localized attack at grain boundaries wereobserved. ,'

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TAB

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________UNCLASSIFIEDSfCTIIYY CLASSIFIATION Of T.?iS PAG

f ' T 41

INTRODUCTION

Sintered tungsten alloys are candidate materials for certain U.S. Army ammuni-tion components because of their high density (17.0 to 19.0 g/cm 3). The Army has a10 year life requirement for ammunition stored in an uncontrolled environment. Ifthe environment can be controlled, the storage life requirement can be extended to20 years. Until recently, the corrosion of tungsten alloys has not been considereda problem. The disclosure by Andrew et al. that a 90%.W, 7.5%Ni,--,5-o-_ij!_yreadily corrodes when exposed to air saturated water vapor has caused the U.S. ?myto reassess the situation. The work reported herein was aimed at evaluating thecorrosion behavior of various sintered tungsten alloys under development by poten-tiostatic polarization methods in aqueous chloride solutions at varying pH's and byweight loss measurements after total immersion in chloride solution. The effects ofcontrolled temperatures and humidity are also being assessed, but testing is stillin progress.

EXPERIMENTAL PROCEDURE

The sintered tungsten alloys were fabricated using powder metallurgy techniques(isostatic compaction of the mixed metal powders followed by sintering in hydrogen).

The basic tungsten content 2 varied between 90 and 97%. The matrix compositionsalso varied and included elements such as Fe, Ni, Co, and Cu. Figure I is a micro-graph of a typical liquid-phase sintered tungsten alloy microstructure showing ,rounded tungsten particles surrounded by a layer of matrix solid solution. In gen-eral, tungsten particles in these alloys were in contact and the matrix phase wasconfined to discontinuous or continuous interparticle interstices. The observedparticle sizes varied from a few microns up to 8W , while the average mean particlesize ranged from 25 to 40p, depending on the matrix composition and the sinteringconditions. The matrix mean-free path was less than 2P and the tungsten grain con-tiguity was always from 2 to 3 particles. The alloy designations and compositionsof the test materials are listed in Table 1. Pure tungsten was included for baseline data. Some of the compositions were fabricated in-house; the remaining weresupplied by a commercial vendor.

Table I . COMPOWITIrNS OF ALLOYS TESTE-

ht

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I ANI)RI W . J I .BAKI R, M I ind Ill Ko)\, 11 1 vi,,,l Pp-r /,',, 11-1 PI ,. , 'l, : :n Il, ~ ,, l~,;( ,U ', ;. 11,

o I Ic Sccond ( 'hl nl mIjc (I [ I,' I v n, li c , 'i I I ) 'III\ 0, IK r 1 r1 , t \h i 0, Ilh ,,' Q I s ' r h\ibr li Icl I II I 1. , Ind1diam; IC Revjr il (enlctr. \% itcrtu,"t. \I.1,, -l i l

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Figure 1. Typical microstructure

of a high density tungsten alloy -

unetched at 130X.

Specimens for both polarization and immersion tests were 0.250-inch in diameter

and 0.500-inch in length. The specimens were polished with 240 and 600 A siliconcarbide paper, rinsed with distilled water, acetone degreased, air dried, and imme-

diately used.

The solutions used for polarization tests were argon saturated and buffered topH - 4, 9, and 12, with and without 0.IM NaCl. They were prepared from laboratoryreagent chemicals and distilled water. The buffers used to fix the pH's included

citric acid, disodium phosphate, and sodium hydrogen carbonate. Polarizationmeasurements were also made on specimens in distilled water containing only the

buffers to assess the effect of chloride ion. A 5% NaCl solution was used for the

total immersion tests.

Polarization measurements were made utilizing the potential sweep method ofpotentiostatic polarization. The electrode potential was continuously changed at a

constant rate of 4 V/hr, and current was simultaneously recorded. A Wenking poten-tiostat, in conjunction with a motor potentiometer for automatic programming of the

operating potential and an X-Y recorder, was employed to automatically record cur-rent versus voltage. Prior to sweeping the potential, the specimen was allowed toremain in the solution for 90 minutes to let the potential come to a steady statecorrosion potential before applying and varying the potential from the open circuitor corrosion potential to 2.0 V vs SSE. The polarization cell and general proce-

dure have been described previously. In order to determine corrosion rates by

3. LIVY. M (',rrovion v. 23. no. 30. 10 7

2

V-

extrapolation of Tafel slope to the corrosion potential, the anodic polarizationcurve was rerun between the corrosion potential and +0.2 V (more noble) at a slowerscanning rate of 0.1 mV/sec.

One hundred cc of 5.0% NaCI solution was used to completely immerse test speci-'Pmens for weight loss experiments. The volume of solution was appropriate for thesurface area of specimen exposed. Immersion test specimens were weighed before andafter 10 weeks of exposure. After testing and prior to weighing, exposed specimenswere rinsed under running tap water, ultrasonically cleaned in distilled water for10 minutes, rinsed in acetone, and air dried.

RESULTS AND DISCUSSION

Anodic Polarization

Pure Tungsten

Figure 2 shows the effect of pH and chloride ion on the anodic behavior ofpure tungsten in buffered distilled water solutions. Tungsten undergoes an active/passive transition in all the solutions shown. Generally, the corrosion potential I plebecomes more active, and the maximum dissolution current density and passive currentdensity increase as the pH increases from 4 to 9 to 12. The greatest change occursas the pH is increased from 4 to 9. Regardless of pH, the chloride ion produces arelatively slight increase in the magnitude of the maximum dissolution current. A -distinct "knee" in the polarization curves is not observed in solutions of pH4, withor without chloride ions; at the higher pH's, however, particularly in the solutionscontaining chloride ions, there are two knees attributed to the tungsten. None ofthe polarization curves show a transpassive region.

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- 1 41Nd

Figure 2. Polarization curves of

W in various pH solutionswith and without 0.1M NaCl.

7-6 jH ',(X,)I -* I )H I"

A I 1 I I •

(urrent NA 'i,'

%

3 'p -

Alloy W2 (97W, 1.6Ni, 0.7Fe, 0.5Cu, O.lCo)

The effect of pH and chloride ion on the polarization curves for tungsten alloy

W2 in buffered distilled water solutions is shown in Figure 3. The alloy undergoes

an active/passive transition in all solutions. In all cases, the corrosion poten-

tial becomes more active as the pH is increased. In the absence of chloride ions,

the maximum dissolution current density and the passive current density increase as

pH increases from 4 to 12. Transpassivity occurs between 1.2 and 1.4 V vs SCE. The

addition of chloride ions increases the dissolution current density significantly at

potentials more noble than the corrosion potential. Above 0.500 V vs SCE, the

polarization curves become essentially the same regardless of pH, and there is no

transpassive region. At pH 9 and 12, they show three knees. The knee at -0.500 V

vs SCE is due to another phase introduced by the alloying elements. Below this

potential, the two knees are attributed to tungsten. Comparing the anodic behavior

of pure tungsten with that of alloy W2 in non-chloride solutions, the alloy is more

easily passivated and exhibits a smaller passive current density. In the presence

of chloride ions, the situation is reversed, and pure tungsten exhibits lower corro-

sion rates.

/

/

1 '4 -N, I

Figure 3. Polarization curves of

SI 97W-1.6Ni-0.7Fe-0.5Cu-0.1Co

PH 4 1 in various pH solutions with andp9without M NaCI.

o l pH 4

PH d

0 14 'E

S I 1 I 1 I),)t)1 [ ) j(l)3' 104 1() Il

( l 1

'

I. r r L , NA C

Alloy 31 (97W, 2Ni, 0.7Fe, 0.3Co)

Figure 4 shows the effect of pH and chloride ion on the polarization behavior

of the alloy in buffered distilled water solutions. Active/passive transitions are

observed for the alloy in all solutions shown. The corrosion potential becomes more

active as pH is increased. The maximum dissolution current and passive current in-

crease with increasing pH. A transpassive region is observed in all solutions

(-1.4 V vs SCE). The chloride ions increased the dissolution rate of pure tungsten

and the W2 alloy, but reduced the maximum dissolution rate and the passive current

density of Alloy 31 in solutions of pll 9 and 12. The 1nhiht Io,) may be due to the

- t!1.*

/ /-- (I /

lO.. . f ( I Io t 4(;I

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Figure 4. Polarization curves ofS "97W.2NI-0.7F°-0,3Co In various

" pH • pH solutions with and without

'ii' .1M Naed,

o/

i 1 1 0 1 I I ,

Current, NAlcm 2

formation of an insoluble protective tnsten-basic chloride film, or the chlorideItself may stabilize the oxide film.3'

In the absence of chloride ions, the corrosion resistance of both alloys

appears to be slightly superior to pure tungsten in a solution of pH4 . There islittle, if any, difference in the pH 9-12 range. When chloride ions are present(0.1M NaCi), alloy 31 exhibits better corrosion resistance (more noble corrosionpotential, smaller maximum dissolution and passive currents) than the pure tungsten,but the W2 alloy appears to be less corrosion resistant (greater maximum dissolutioncurrent and passive current) than both pure tungsten and alloy 31. The major dif-

ference between the W2 and 31 compositions is the 0.5% Cu content of the W2 alloy.Thus, Cu may have a deleterious effect on corrosion resistance.

Other Alloys

Because of similarities in the anodic polarization behavior of the remaining

alloya, additional polarization curves are not shown. The electrochemical para-meters obtained for these alloys are contained In Tables 2-4. Generally, alloyscontaining copper are more 8usceptible to chloride ton attack.

Corrosion Rates by Tafel Slope Extrapolation

Tafel slopes were determined from slow scan polarization nasurements and the

corrosion current estimated by extrapolation to the corrosion potential. Corrosion

rates in mpy calculated from the Faraday Equation are contained In Table 5. Pure

4, KO(piI R, JW ( -r itl'l/ 1J h1'1 1'11... T, N /all 5 /rii anl/ I1/, C , w'N fiu ll l / IwI Iq/.a p', /1111,h,' .N0,1,1 ' ' alni f I.6063, Unionh ("whh k i 1 ,j'i Y-1 2 Nhow. lk I ',0AIwai V,'r 1'14

Table 2. POLARIZATION CURVE PARAMETERS FOR ALLOYS IN pH4 SOLUTIONS

No NoCl 0,1)M N,,el

Matorialp s#m I I ot oc #m i

Pure W -11,155 0,15 .0,200 0,474 0,2 0,2

D7W-2,SNi.0,5Po -0,250 0.03 -0,275 0,162 0,00 0,07 1,400

07W-2N.0.71Po .0ie 0,31 0.07 0,03 1,400 -0,250 0,175 0,05 0,03 1,2000,3Co

6W-2Ni-1I'o-Co -0,250 0,350 0,07 0,02 1,310 -0,337 -0,063 0,30 0,07 1,300

95W-2,5Ni-l'251fei 0. ,231 0,357 0,06 0,04 1,320 -0,350 -0,100 0,20 0,08 1,3251 .2SCo

DOW-5NI-2,So- -0,252 0,262 0,10 0,06 1,025 -0,337 -0,025 10,0 (1 1,4002,50~

97W-I .6N1.1.li'*- .0,150 .0,263 0,00 0,08 N,I), -0,200 0.0 0,0230,2W.u

07W.,6Ni-Fo- -0,037 *0,137 0.00 0,02 1,400 -0.137 -0.052 1,0 0,03 1,400

97-0,SN -. 0 -0,050 +0,050 0,03 0.02 1,400 -0,138 .0,320 10,0 8,0O, 7 F*-O.5Cu-0. 1 (o

$c " Corrosion potential, volts

fm 9 Maximum dissolution current potontiul. volt%

Im a Maximum dissolution current density, mA/cm-

IF a Passive ci-rrent density, mA/cm2

t z 'rranspassive potential; N,D w Not I)otormJned,

All Potentials refer to SCiE,

Table 3, POLARIZATION CURVE PARAMETERS FOR ALLOYS IN pH9 SOLUTIONS

No NaCi O,1M NoCI

Materials ce o Im I t 4c m In 1

Pure W -0,400 0.562 1.05 0.0 .0,487 0,075 2.0 1.0

07W.2,bNi-0,5'o -0,487 0,275 1,00 0,7 1,250 -0,500 0,.300 1.0 0,1) 1,300

07IV-2Ni-,T7Po-. -0,487 0,237 1,00 0.8 1,400 .01.375 0,300 1,05 0,15 J,1000,3Co

06W-2N-IHe-lCo .0,475 0.30 1,00 (OA 1,150 .0,475 0,300 1,0 0.6 1,100

0SW-2,NI-1,25Fo- .0,400 0,300 1,00 0,8 1,10(0 .0,500 0,300 1,0 0,8 1,2001,25("o

0VW.SNI-2.SPo- -0,512 0,250 1,0)1 0,8 1,200 -0,500 (1,287 1,0 0,11 1.3502,5(,'o

07W-1,6Ni-l1o- -0.400 0,425 1,00 0.A NA), -0,40(0 0,074 f,2 1.0 0,0000,2Cu

07W-1,6NI-lPo- -0,512 0,262 1,00 0,8 1.700 -0,364 0,050 3,0 2.0 1.6000,4Cu-0, ICo

C7W-l,5Nl-0,o- .0,462 0,.300 1,00 (1.8 1.250 -0,462 0,450 0,0 HAO,5cu-O, Ice

te " Corrosion potential, volts

om " Maximum dissolution current potential, volts

Im " Maximum dissolution current density, mA/cm2ip 'ussive currernt density, nA/cm 2

0 t 'rransposive potential N)., g Not Determined,

AlI potontialo refer to SCH,

6

Table 4. POLARIZATInN CIURVr PARAMETERS FdR ALLOYS IN P1 SOLUTIONSNo Noi(I 0,IM Nael

Moeil C o mI 4t #C c m Im I..... .,,, r,,,, *°c m rn ,. . 2...11ire W -0,525 0,300 1,0 0,8 -0,612 0,340 1.05 1,0

17W-2,5NI.-1,5110 -0,575 0,324 1.0 0,A 1.400 .0,640 1,00 1,200

,7-2J -0,71.o -, 0 5 1.1500 -0,500I (0, 1, 400WI:2NI-110-I Co -0,522 0-324 1,8 0,7 1,100 -0,550 1,00 NP.

95SW- 2,5N i-!, 2511c.. 1.550 0 1 Y 1 -00 01562 02,00 N9I .251CO

UOW.-NI,512 0,250 t,7 0, 1.150 -0.575 1.00 1,.300

07-,N-,1c. -1, 562 0,9 NX, -0,550 -0,375 0,2 0,1.5 0,300, 2Co

,7W-I6Ni-IIo-52 0.8 1,200 -0,431 0,520 4,00 1.0 1,400O,4Cu-0.ICo7W-I,5Ni-0,7Pc. -0,500 0,275 0,8 0,7 1,200 -0,580 0,450 0.00 8,0

0,5Cu-0, iCo

$c f Corrosion potential, voltsH Magimum di eolution curront potontial, volto

In I Maxlmuim ditsolution current density, A/cm2

I u Passlvo current density, mA/cm 2

*t a 'run~palve potential; N,D, - Not Determined,AJI potonthilo refer to 50.,

Table 5. CORROSION RATES OF ALLOYS IN AQUEOUS ENVIRONMENTS, MPY

No NaCi O.IM NnCI

materinix 1)114 pli p1112 4 p119 1,111

Pure IW 0,02 0,04 1.2 0,02 0.00 0.1727 0.23 0,47 2.5 0.02 0.04 0.5831 0.03 0,03 1,2 0.01 0,10 0.0235 0,02 0,07 1.8 0,01 0,01S 0.0942 0,04 0,07 IA.4 0 0108 O. |

45 0.27 0.11 0.72 0,17 0,17 0,2146 0.02 0.02 1,1 0,02 0,05 0,02

tungsten and all the alloys tested exhibited acceptable initial corrosion rates(below 5 mpy). Pure tungsten appears'to be more corrosion resistant than most ofthe alloys in aqueous solutions. Generally, corrosion rates increase with increas-ing pH, except in solutions of pHl2 containing chloride ions that show an inhibitingeffect. The copper containing alloy exhibited the highest corrosion rate in most ofthe solutions.

Full Immersion Tests in 5% NaCJ Solution

It should be noted that a constant area of each specimen was in contact withthe glass beaker containing the solution. Within a week of exposure, alloys notcontaining copper (alloys 31, 42, and 45) formed a rust-colortsd reaction product() in the area of contact, indicative of crevice-type corrosion. As exposurecontinued (up to 10 weeks), the reaction product spread over the entire surface ofthe specimen. This type of reaction product was not observed on pure tungsten andalloys containing copper (alloys W2 and 27), but the alloys containing copper formeda yellow reaction product, identified as W03. The pure tungsten appeared to be

7

Tahlo 6 r,, 11 MITii o ilii. I fI4[.10 lou r.,1%

Ol ,vl-Va I t'I ll

', l )r.I I i t l r A ,* i , Ii',I v(le 11 i i 1 I, r, Hil vu ;folllt loll G !fg u( ico huhfv II 0,11

NijutliQ .....!12....,.. Jy) Apin'ui rtincy' OoIr fmrv Alft o'I I to

I'i rv! 112 1, (004 440 1 m

1 2 -' 4 1,1241) , ,3 rN Iky N . IIvXt ,(' 1 1 ,4- .124!1 4..2 Yi I Ilow

27-1 It018 3,47 MJ Iky I )t-, Yv I #)wRe'd - It rowri

31.1 00112 2.12 CI ear IxtC1,; I Ve itt Iv:3 -2 O.0 0 1.93 lcd lhrown Vellow, I4-d. l7Iw,

4Z- I 0,(052 1 ,MO4412-2 00049 .94

45.1 0. 0039 04.7745-3 0,.0029 0.57

* uSptJmei Surface Aren: O, 5 1 l;v

unaffected. The weight loss data contained in Table 6 show that pure tungsten isthe most corrosion resistant, followed by alloys containing no Cu and alloys con-taining Cu in order of decreasing merit. The better performance of the non-Cucontaining alloys is attributed to the formation of the more protective W02. Thedetrimental effect of Cu was shown in both electrochemical and immersion tests.

SEM examination of the corroded surface of the W2 alloy was made after 10 weeksof exposure. The micrographs in Figures 5a and 5b show that the matrix phasecorrodes more readily than the primary tungsten grain phase and that tungsten grainpl -kout can occur due to this intergranular attack. Thus, both general dissolutionand localized attack at the grain boundaries are operative.

CONCLUSIUNS

AnodLc polarization curves show that pure tungsten and all the alloys studiedundergo an active-passive transition; also, as pit increases, the corrosion potentialbecomes more active and the anodic current increases. The chloride ion increasesthe anodic dissolution of tungsten and most of the alloys in p11 4 and 9 solutions,but, in solutions of p1112, there to little or no effect.

Based on the magnitude of the maximum dissolution and passive currents, the97W-2Ni-0.7Fe-O.3Co alloy is the most corrosion resistant material studied ingol-ution of p114 . The chloride ion inhibits the anodic reaction of this alloy Insolutiono of p119 and 12.

In the absence of chloride ion, the alloys appear to be more corrosion resist-ant than pure tungsten in solutions of p114. Alloys containing copper appear to bethe least corrouton resistant of the materials studied in 0. I1 NaCl solution.

In general, corrosion rate data obtained by the Tafel slope extrapolationmethod support the aforementioned conclusions. Datn obtained from total immeratontests in 5% NaCl solution Indicate that pure tungsten has the lowest corrosion

P.-.

,-

1W1

%

(a) (b)

Figure 5. SEM micrographs showing (a) rapid grain boundary corrosion (matrix), and (b) tungsten grain pickout ifter10 weeks exposure of 97W-I.6Ni-O.7Fe-0.5Cu-O.ICo alloy in 5% NaCI solution (350X).

rate and that alloys containing copper have the highest corrosion rates. Regardless

of the method of test, it appears that copper increases the corrosion rate of tung-sten alloys in chloride solutions.

Based on corrosion rates (penetration in mpy) calculated from extrapolatedTafel slopes and weight loss measurements, all the matprials studied exhibit sat-isfactory behavior in aqueous solutions (less than 10 mpy); but the SEM micrographsof the W2 alloy, after 10 weeks of immersion in 5% NaCI solution, showed that thematrix phase is attacked more readily than the tungsten grain, and penetration inthose areas can exceed the calculated penetration rates. Further, this type ofgrain boundary attack can result in loss of strength and ductility.

-.

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P.".-...

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