Effects of cobalt on high rate dissolution behaviour of nickel-base superalloys in NaNO3 and NaCl solutions

Effects of cobalt on high rate dissolution behaviour of nickel-base superalloys in NaNO3 and NaCI solutions E. Makino*, N. Motoi** and T. Sato*

The dissolution behaviour of Ni-12%Cr-Co and Ni-20%Cr-Co alloys containing from 0% to 40% cobalt has been studied in 3 tool dm-3 NAN03 and 2 mol dm-3 NaCl solutions at 30 ° C. The anodic polarization curves and the dissolution current efficiency were measured in a flow cell. The results show that cobalt facilitates the dissolution of the alloys, dissolving as divalent Co2+ ions under electro-chemical machining conditions. For the Ni-12%CrCo alloys which exhibited secondary passivation and oxygen evolution in NAN03. increasing the cobalt content to 20% or more resulted in secondary transpassive dissolution. A decrease in the degree of the secondary passivity was observed for those alloys which had a high cobalt content, comparable with their nickel content. However. the Ni-20%Cr-Co alloys in NAN03 and all the alloys studied in NaCI dissolved with 100% current efficiency due to the marked effects of chromium; the effects of cobalt had only secondary significance. X-ray photoelectron spectroscopic study indicates that in the surface layers formed in the transpassive and secondary passive states cobalt exists in the +2 oxidation state, such as CoO and/or Co(OH)2. Keywords: electrochemical machining, nickel-containing alloys, current efficiency

Cobalt plays an ,mportant part m the strengthening of mckel-base superalloys, dissolving in matrix "y and prectpitate 3' phases. Cobalt content m commercial mckel-base superalloys ts relatively high, eg about 20% in Nimonic and Udlmet alloys and 29% in Inconel 700. It is important, therefore, to eluctdate how this element influences dissolution behavlour in the electro-chemtcal machining (ecm) of ntckel-base superalloys In the present work, the effects of cobalt were studied tn commonly-used passtvatmg and non-passlvating ecm electrolytes, le in NaNO 3 and NaCI solut,ons. The anodlc polarizat,on and current efficiency measurements of ntckel-chromium-cobalt systems were carried out at 12% and 20% chromium levels taking into account the marked effects of chromium content on dissolution behawourL2 . The curve of current efficiency agamst current dens,ty obtained should be a useful tool for est, mating the machining rate, d,mensional control and surface finish. The metal compounds existmg on the metal surface after d,ssolutlon were studted using X-ray photoelectron spectroscopy (xps).

Experimental

The ingots were machined into cylindrical specimens, 0 5 cm in diameter and about 1 cm long, by edm and turning. Electrical contact was made by soft soldering a copper lead wire to one end of each spec,men. The other end was used as the test surface of the electrode. To prevent undes,rable crevice corrosion in the chloride medium, each specimen was passivated by immersion in 30% NaOH for 3600 s at 60°C and was masked with an epoxy resin adhesive. The specimens were then embedded m epoxy resin so that only the test surface was exposed to the electrolyte. This surface (0.2 cm 2 ) was polished on 1000-grit wet carborundum paper, rinsed in water and degreased w,th acetone. In addition, it was subjected to cathodic preelectrolysis in a working electrolyte at 1.5 V (versus a saturated calomel electrode, sce) for 600 s durmg the polarization measurements.

Table I . Chemical composition and electro-chemical equivalent of Ni-Cr-Co alloys Compositions (mass %) Alloy Ni

Specimen preparation Specimens were prepared from htgh-purlty nickel, chromium, and cobalt. Five-hundred gram charges of each compostt,on were reduction-melted in alumina crucibles m vacuum. Table 1 shows the chem,cal compositions and the theoretical values of electro-chemical equivalent of all the alloys. *Department of Prec,s,on Engineering,Hokka,do Umvers,ty, Sapporo, Japan **Graduate School, Hokka,do Untversity (presently with the Sem,conductor Dw,s,on, Sony Corporation, Atsugi, Japan)

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Electrochemical equivalent (mg/C)

88Ni-12Cr-0Co 78N,-12Cr-10Co 68Ni-12Cr-20Co 58N,-12Cr-30Co 48N,-12Cr-40Co 80N,-20Cr-0Co 60N,-20Cr-20Co 40Ni-20Cr-40Co

87.2 8al. 6al. Bal. Bal. 79.9 59.5 39.8

Cr

12.0 11.5 11.8 11.7 11.0 19.4 19.0 20.4

Co

9.78 20.5 29.0 42.5 20.9 40.0

N,2+ Cr 3+ NI2+Cr6+

Co2+

Co 2+

0.283 0.282 0.281 0.282 0.283 0.270 0.268 0.266

0.238 0.239 0.237 0.238 0.241 0.209 0 207 0.204

0 1 4 1 - 6 3 5 9 / 8 3 / 0 2 0 0 6 5 - 0 8 S03 0 0 © 1983 Butterworth & Co (Pubhshers) Ltd

65

Makino, Motol and Sato - effect of Co on dissolution of NI-base superalloys m NaNo3 and NaC/ A p p a r a t u s and p r o c e d u r e The experiments were carried out under forced convection conditions using the electro-chem~cal f l o w cell apparatus shown schematically in Fig 1 The electrolyte was contmuously circulated between the reservoir and the f l o w cell by a flex.ble rotor pump. A rotameter was used to measure the electrolyte f l o w rate. The electro-chemical cell consisted of a rectangular f l o w channel 0.5 mm high (the d~stance between the two electrodes) and 5 mm wide, formed by inserting a Teflon spacer between two acrylic plates. The anode and the cathode were installed facing each other and were located 90 mm from the inlet of the cell to prowde fully developed velocity profiles at the electrodes. A c~rcular platinum cathode and a brass cathode (5 mm diameter) were used for the polanzat=on and current efficiency measurements respect=vely. A saturated calomel electrode was connected to the cell through a capillary hole pos,tioned near to and upstream from the anode All potentials were measured and reported with respect to this electrode. The electrolytes used were 3 tool dm -3 NaNO~ and 2 mol dm -3 NaCI prepared from reagent grade chemicals and d=stdled water. The electrolyte f l o w rate was kept at 6 . 7 m s -1 (Reynolds number, R e = 6 7 0 0 ) =nthe f l o w channel. Electrolyte temperature was maintained at 30-+ 2° C for all the experiments Polarization measurements were carried out by a 'stepping' techmque using a potentlostat (N=chla-ke~kl EP-200H) Starting at the steady state corrosion potential, this was then increased by 20 mV increments, anodlc current was recorded after 60 s at each potential Current efficiency for metal dissolution was determined by applying weight-loss measurements. The anode was machined at a constant current controlled by a dc power supply (Takasago-seisakusho GP55-20) for a constant charge of 500 C cm -~ The anode was carefully weighed

c

Fig 1 Schematic o f flow cell apparatus," A electrolyte reservolr, B flexible rotorpump, C rotameter, D flow cell, E anode, F cathode, G Luggin capillary, H saturated KCI bridge, I sce, Jgas collector, K gas outlet to burette, L dc power supply/potentiostat, M recorder, N coulomb meter

66

using an analytical balance before and after each experl ment to determine the amount of metal removal. The current efficiency was then calculated on the basis of NI 2+ Cr 3+ Co 2+ and NI 2+ Cr ~+ Co 2+ formation for the dissolution in the active and transpasslve states respectwely -- using the electro-chemical equwalent data given in Table 1 Reaction products formed during galvanostat~c electrolysis were collected and analysed quantitatively The amounts of mckel, chromium and cobalt dissolved during the electrolysis for 500 C cm -2 were determined by the dlmethylglyoxlme, dlphenylcarbazlde, and nltroso-R salt spectrophotometrlc methods respectively The amount of oxygen in the gaseous products formed during the electrolysls for 2500 C cm -2 was determined by the absorption method using alkahne pyrogallol solution, and the current effic.ency for oxygen evolution was then calculated taking the theoretical electro-chemical equivalent of 5 8 x 10 -8 N m 3 C -1 Because of the difficult of gas collect,on, the electrolyte f l o w rate was decreased to 2 m s-v at low current densities. An xps study was made of the electrode surface after dissolution using a Vacuum Generators ESCA 3. The speclmens were irradiated with AI K~ radiation (1486.6 eV) at a pressure of 2 7 x 10 -~ Pa (2 0 x 10 -9 Torr)

Influence of cobalt on dissolution behaviour in NAN03 A n o d i c p o l a r i z a t i o n b e h a v i o u r in N A N 0 3 F~gure 2 shows the anod=c polarization curves for the N~-12%Cr-Co alloys m NaNO3 All the alloys studied were passive at low potentials, and the transpasslve region began at about 0.8 V. The increase of current in this region corresponds to uniform dissolution with chromate (CrO4 ~-) formation The 88Ni-12Cr-0Co alloy exhblted secondary passivatJon. This behaviour was characterized by two distinct current peaks at 1.3 V and 1.6 V, as previously reported 2 . Cobalt-containing alloys also exhibited secondary passwat.on at potentials above 1 3 V. As the cobalt content was increased to 40%, the tendency for secondary passwation decreased. Figure 3 shows the current efficiency 77t for the transpassive dissolution of 68Nl-12Cr-20Co and 48N1-12Cr-40Co alloys at different potentials for 100 C cm -2 . Both alloys dissolved with 100% current efficiency at potentials.below 1.3 V, but above this value the efficiency decreased due to secondary passivity. The current efficiency for the 68NJ12Cr-20Co alloy dissolution at potent,als above 1 7 V was about 10%, and the current was presumably consumed for oxygen evolution. On the other hand, for the 48Nl-12Cr40Co alloy a current efficiency of about 80% was still maintained at potentials between 1.3 V and 1 6 V. This result suggests that the secondary passivity of this alloy is imperfect and that the predominant process in this potential region is dissolution. The results shown Jn Figs 2 and 3 Jnd=cate that the tendency for secondary passivation of the 12%Cr alloys decreases with increasing cobalt content. However, this behawour does not occur until the cobalt content is increased to more than 40%, w=th an accompanying decrease in the level of nickel - ~tself also important for secondary passwation. Therefore, it is considered that the decrease in the degree of secondary passiwty is caused by the decrease in nickel content, as well as by the increase =n cobalt content

APR 1983 VOL 5 NO 2

Makino, Motoi and Sato - effect of Co on dissolution of Ni-base superalloys in NaNo 3 and NaCI I0

I0

I

--

Io-' -

5

N

'E

I0-'

(J
.= ,£ U

id 2

g ~ ida (,.)

_

i0 "3

_

i() 4 0

o

8 8 N i - 1 2 C r - OCo

•

78Ni-12Cr-

•~

68NI- 12Cr- 20Co

n

58Nl-12Cr-30Co

•

48Nl-12Cr-

80Ni-

lOCi

=o-3 _

OCo

o

6ONi-2OCr-

2OCo

•

40N=-2OCr-

4OCo

40Co

I

I

I

I

I

05

I0

15

2.O

25

10-4

I

0

Potenhal ,V (vs sce )

Fig 2 Anodic polarization curves for Ni- 12%Cr-Co alloys in 3 r a i l dm-3 NAN03 120

05

~t

I

I

I

I

I0

15

20

25

Potenhal , V ( v s sce)

Fig 4 Anodic polarization curves for Ni-20%Cr-Co alloys in 3 r a i l dm -3 NaNO~ Figure 4 shows the anodic polarization curves obtained for the Ni-20%Cr-Co alloys m NaNO3. The transition from the passive to transpassive state occurred at about 0.8 V, similar to the behaviour of the Ni-12%Cr-Co alloys. For the 80Ni-20Cr-0Co alloy, no secondary passivation was observed in the transpassive region. This behaviour is characteristic of high chromium content alloys ] . Since neither cobalt nor chromium exhibit secondary pass~vation - as ment.oned above - the anod=c polarization behaviour of cobalt-containing alloys was similar to that of the 80Ni-20Cr-0Co base alloy.

tOO

80

=;

20Cr-

60

Current e f f i c i e n c y in N A N 0 3

o~ o

4 0 --

20

--

05

•

4 8 N I - I Z C r - 40Co

o

6 8 N = - 1 2 C r - 20Co

I

I

10

15

I 20

Potentmol, V (vs sce }

Fig 3 Current efficiency for potentiostatic dissolution of Ni-12%CF-Co alloys in 3 r a i l dm -3 NAN03

PRECISIQN ENGINEERING

Results of chemical analysis of solutions containing metal dissolved during galvanostatic dissolution of the 68Ni12Cr-20Co and 60Ni-20Cr-20Co alloys are shown in Fig 5. The composition of dissolved metal coincided well with the alloy composition shown .n Table 1, indicating that the alloys exhibit uniform dissolution without preferential attack of selected components not only in the transpass=ve state but also in the secondary passive state. Thus, the current efficiency can be calculated using the nominal alloy composition for the composition of the dissolved metal. Figure 6 shows the current efficiency Tit for the transpassive dissolution of the 12%Cr and 20%Cr alloys in NaNO3 as a function of current density. Although the valency of the cobalt dissolution reaction is assumed to be 2 of 3, it has not been clarified under extremely high

67

Making, Motoi and Sato - effect of Co on dissolution of Ni-base superalloys m NaNo~ and NaCI potential and, hence, current density condmtlons such as occur In ecru Here, ~t was determined f r o m the current etflcJency data of the 40NI-2OCr-20Co alloy. Since this alloy dissolved in the transpassmve state, mckel and c h r o m i u m dissolve as NI 2+ and Or 6+ ions respectwely ] Moreover, since no secondary passlvatlon and no oxygen evolution were observed, this alloy should dissolve w i t h 100% current efflcaency Figure 6 indicates that the experimental values of the current efficiency based on Ni 2+ Cr 6+ Co 2+ f o r m a t i o n o f this alloy were 100%, whereas they were 112% based on Ni 2+ Cr 6+ Co 3+ f o r m a t i o n Current efficiency values exceedmg 100% may be caused by the shedding of minute grams due to severe intergranular attack, or shedding of noncorrodible phases - such as carbide and/or 3/ phase - f r o m the electrode surface However, no intergranular attack was observed in the experiments, and thDs alloy does not contain the components which form non-corrodible phases. The current efficiency values exceeding 100% are therefore incorrect, the assumption of f o r m a t i o n of Co 3+ Ions IS demed, and It IS shown t~at cobalt dissolves as divalent Co ~+ lOBS even in the transpass~ve state. For similar reasons, this result IS also true for the secondary transpasslve d~ssolutlon of the N~-12%Cr-Co alloys containing 20% or more cobalt (discussed later) Current efficiency for the dissolution of alloys contaming 30% or less cobalt was m a r k e d l y low due to secondary passivity ( F i g 6 ) Figure 7 shows the current efflclency f ° r d~ssolutlon and for oxygen evolution w i t h the 68Nl-12Cr2 0 C o a l l o y The total current efflclency for b o t h a n o d l C reactions was about 100%, indicating that the p r e d o m i n a n t reaction in the secondary passwe state ~s oxygen evolution. The 68Nl-12Cr-20Co and 58Ni-12Cr-30Co alloys exhibited d~ssolut~on ~n the secondary transpasslve state at current densities above 50 A cm -~, and thus the current efficiency increased. Although the current efhmency for dissolution of the 48Nb12Cr-40Co alloy was 7 0 - 8 0 % at current densities below 10 A cm -2, because of ~mperfect secondary passivity, It became 100% w i t h increasing current density The 20%Cr alloys dissolved w i t h 100% efficiency, mdependent of cobalt content and current density. These results show that increasing cobalt content facdltates dissolution m the transpasslve and secondary transpasswe states in NaNO~. However, since slmdar and

,2°t

©

8 0 N I - 2OCr-OCo

•

48Nl-12Cr

•

6ONI-20Cr-20Co

[]

58NI-[2Cr-3OCo

4OC0

4ONI - 2OCr- 40Co O

I00 ~80-u-~

60 3

40

-

o

88NI- IZCr-OCo

•

7 8 N I - I Z C r - IOC0

.j

z~ 68NI - 1 2 C r - 2 0 C o 20

/

J

~

I

/

-

I

I

I

[

J

I

5

IO

50

I00

0

Current densffy, Acm ~2

Fig 6 Current efficiency for dissolution of Ni-Cr-Co alloys in 3 mol dm-3 NAN03 120 I

2 0 m s'l " i ' ~

IO0

G

I ~

6 7 m S-I

Q

0

0

0

o

80

.4 60 -= E ~,

Dissolution and oxygen evoluhon

Oxygenevoluhon Dissolution

40 --

D

I

o

I____

IO

50

Current density. A c m

I I00

-2

Fig 7 Current efficiency for dissolution and oxygen evolution of 68Ni- 12Cr-20Co alloy m NAN03 (,.7tool dm -3 )

I00 68Nl-12Cr-

.4

~3 ~

6 O N i - 2 O C r - 20Co

2OCo

8o

Ni

6o

.1.

~

•

' (:

NI o

"6 g

4O

~

zo

Co

Co

•

-" O

Cr

[ 5

I I0

Current density, A cm -2

%

] 50

] IOO

%

;o

~=-

~J =

Cr

I

I

I

J

5

IO

50

I00

Current denslty,A cm -z

Fig 5 Chemical composition of anodically dissolved N/-Cr-Co alloys in 3 tool dm -3 NAN03

68

APR 1983 V O L 5 NO 2

Makino, Motoi and Sato - effect of Co on dissolution of Ni-base superalloys in NaNoa and NaCl I0

I

--

i0-I

_

10- 2

_

more marked effects were observed for chromium 2, the effect of cobalt appears to be dependent on the chromuum level, the h=gher the chromium level, the less significant are the effects of cobalt. Alloys containing 20% or more chromium exhibit complete transpassw~ty, nndependent of cobalt content. For lower chromium-content alloys, the effect of cobalt on the decrease m the degree of secondary passivity ~s s~gnuflcant only when cobalt content is suffncientlv h~gh (eg more than 40% cobalt at a 12% chromnum level). The most pronounced influence of cobalt on d=ssolutnon behav~our in NaNO3 =s to facilitate the secondary transpasswe d~ssolut~on of low chromium content alloys At the 12% chrommm level, the addition of 20% or more cobalt allowed secondary transpass~v~ty to be achieved. Increasing the cobalt content decreased the current denmty at whuch secondary transpassnve dissolution occurred

#

o

88Ni-

12Cr-OCo

z~ 6 8 N i - 1 2 C r - 20Co

(.3

48Ni- 12Cr- 40C0

Influence

•

80Ni-

in N a C I

•

60N=- 20Cr- 20Co

A n o d i c p o l a r i z a t i o n b e h a v i o u r in N a C I

•

40N~- 20Cr- 40Co

Anoduc polanzat=on curves of N=-12%Cr-Co and Nn-20%Cr-Co alloys nn NaCI are given In Fig 8. Fngure 9 ~s a set of photomncrographs of the 68N=-12Cr-20Co and 60Nn-20Cr-20Co alloy surfaces after potentiostatic dLssolut~on at d~fferent potentials for 50 C cm -2. For all the alloys studied, the rap=d nncrease in current as the potent,al rose above about 0 V was due to pnttnng attack (Figs 9a and 9d). For the 12%Cr alloys, a large number of mncro-plts were randomly distributed on the metal surface with increasung potential (Figs 9b and 9c), resulting un more general dissolution ~n the active state. For the 20%Cr alloys, p~ttmg attack currents were saturated at about 0.8 V (Fig 9e), above whuch the currents increased again due to the tranmt~on from the passive to transpassuve state. Thus, the pitting attack (actnve dissolution) and general transpassive dissolution occurred simultaneously on the surface of these alloys (F~g 9f)

[3

10-3

o

-01

2 O C r - OCo

I

I

I

05

I0

15

Potentual ,V (vs sce)

Fig 8 Anodic polarization curves for Ni-Cr-Co alloys in 2 mol dm -2 NaCl Fig 9 Photomicrographs of alloy surfaces after potentiostatic dissolution in 2 tool dm -3 NaCh 68Ni-12Cr-20Co alloy (a) 0.3 V, (b) 0.7 V, (c) 1.5 V (versus sce);6ONi-2OCr-20Co a l l o y - (d) 0.3 V, (e) 0.7 V, (f) 1.7 V (versus sce)

+'-"+.++-+.,++'.". +. •. ~ i ~ + ~ . . . . +~'+"a~+-~/~+,

++;.+.-++++,.+.++++,,+'++-+ - "~ ++~_J'~t~

++;,~/" .++~++-" -+--.+ ,

- - ~

~.+=:+]il,.'~ . ~.

+ ++ , , + ;+~+++++++++++++++++++~+E"_+-+5~+ +: + -%+ + - ~ ~ : , , , . + + + + . + +++

++

++.+~v '

-+++++ +. .

'`" EP..+.~:+,+111~+ L "

. . .+ + +,z~, + + + J + , _ + +

.+

+

.

a

,,

PRECISION ENGINEERING

,+

.

~

~

i

l

+

+,.+

+

+"+,.;

+, '"

r.,.~ +

,.+++ "

. ~. +

+,,,4+`

.

,

',

.

.

.

~,h 0 e

.-++ w .

,,j~ +

"+

b

I IOOFm

d

behaviour

+/

+,:.~+,+r,k+ + , , , , , i , + < - . u + ~ . . . ~

Z.,~'~-f.,Jm~.<.-.~

~ .

+_....+.,,41~~... •

on dissolution

'I~. - +'t"+.:,+ ;r~,,;,~L'.++~,.-+':'.- b + ""Jt:.~

+ ~ ' + + . - o

+,,+'+', " +,++*

+-++'+. +.+

-

of cobalt

c

,,

@

f

69

Makino, Motoi and Sato - effect of Co on dissolution of NPbase superalloys in NaNo3 and NaC/ C u r r e n t e f f i c i e n c y in NaCI Figure 10 shows the current efficiency r/a for the actwe d~ssolution of the 12%Cr and 20%Cr alloys as a function of current denstty. The 12%Cr alloys d~ssolved m the active state wtth 100% current efftc~ency, independent of cobalt content and current density. On the other hand, the current efficiency data for the 20%Cr alloys fell between 80% and 100%. Thts behav,our appears to be attributable to simultaneous dissolution in the active and transpasswe states, tn accordance wtth the polartzation behaviour mentioned above Figure 11 shows the current efficiency, the composition of d~ssolved metal, and the ratto of dissolved Cr ~+ to total Cr (Cr6+/Cr) for the galvanostat~c dissolution of the 60NJ-2OCr-20Co alloy for 500 C cm -~ The current efflmency values calculated on the basis of N~~+ Cr 3+ Co =+ and N~~+ Cr 6+ Co ~+ formations were about 90% and 120% respectwely The compos~tton of d~ssolved metal in solutton agreed well w~th the alloy composition shown m Table 1, indicating that the departure of the current effictency from 100% was not attributable to the preferential d~ssolution of some components. If dissolution m the active and transpassive states occurs simultaneously on one electrode, then the current efficiency must be calculated using the modified electrochemical equwalent gwen by XI

E a t = IF % - -

{n~ (1 - t ) + n ~

t} ] - '

(1)

M~ where F is the Faraday constant, MI ~s the atomic wmght, x~ ts the composition of dissolved metal m solution, n a and n t are the valences for d~ssolutton m the active and transI passwe states respectively, and t ~s the ratio of the amount of metal removed in the transpasswe state to the total dissolved m the active and transpassive states. Subscrtpt 't' refers to the component q' of dissolved metal. Since mckel and cobalt dissolve with the same valency m either state, Z at can be calculated using the experimental value of the ratio, Cr6+/Cr, as the value of t The current efficlenmes ?7at calculated using E at w e r e about 100%, as shown m F~g 11. Thts result indicates that dtssolution in both acttve and transpass~ve states occurs simultaneously on the N~-20%Cr-Co alloy electrodes m NaCI. It should be noted that these results are independent of cobalt content. The d~ssolution behaviour of the 12%Cr and 20%Cr alloys ~s dominated by the effects of chromium,

120 --

I00 ---~

80 g

#.

60 -

O

o

o

88NI-12Cr-OCo

•

6 8 N I - 12Cr - 20Co

•

6ON=- 2 0 C r - 2OCo

48N=

•

40Ni-2OCr-40Co

12Cr

40Co

8ONi-2OCr-OCo

40 O

I

I

I

I

I

I

5

IO

50

IOO

T

Current density, A cm -z

Fig 10 Current efficiency o f dissolution for Ni-Cr-Co alloys in 2 mol dm -3 NaCI

70

140

[] Cr6+/Cr

•

.r/I

i

[

120

~

+

0

-~

o

o

o®

80

60

NI 0

o

(3

95E

E#~ 40

[] Co

20

ul

Cr

I

]

I

5

I0

50

Ioo

Current density, A c m - z

Fig 11 Corrected current efficiency for dissolution of 60Ni-2OCr-20Co alloy in 2 mol dm-3 NaCI: ~a, current efficiency for active dissolution," ~?t, current efficiency for transpassive dissolution; ~?ar,corrected current efficiency for dissolution. Open squares represent the ratio of dissolved Cr6÷ to Cr, Cr6+/Cr, and the proportions of Ni, Cr and Co in solution are also given as prevtously reported ~ . It ~s concluded therefore, that cobalt has no marked effect on the dissolution behavtour in NaCI

Xps study of the surface layers after dissolution X-ray photoelectron spectroscopy (xps) was applied to charactertze the species which exist on the NPCr-Co alloy surfaces after galvanostatlc d~ssolut~on for 500 C cm -2 under each of the three sets of conditions shown in Table 2 Figure 12 (a--d) shows the xps spectra for oxygen ls, Ni 2p, Cr 2p, and Co 2p electrons, respectively. Asstgnment of the photoelectron peaks in each spectrum was made by comparison with the results obtained by previous workers 3-9 The numbers m parentheses m the Figures refer to the dissolution treatments listed in Table 2 For the surface layers formed on the alloys m the transpassive and secondary passive states, the oxygen ls spectra (Fig 12(a) lines (1) and (2)) can be resolved into two peaks, at bmdmg energies of 530.0 eV and 531.8 eV due to oxide (02- ) and hydroxide ions (OH-) respectively. The surface layers formed under these condttions therefore consisted of metaI-O and metal-OH bonds. The intensity of the OH-peak was htgher than that of O2-peak for the surface layer formed in the transpassive state The ntckel 2p3/2 spectrum shown m Fig 12(b), line (1), can be resolved mto two peaks at 854.5 eV and 856.3 eV These peaks are attributable to nickel oxide (NiO) and nickel hydroxide (Ni(OH)2) respectively3"4 • In the spectrum for the secondary passive film (F~g 12(b), line (2)), the metallic nickel (Nl(O)) peak was observed at 852.9 eV. The maximum which occurs at about 856 eV in this spectrum may be the result of a combination of the peaks for NiO and NI(OH)2 A I t h o u g h l t l s p o s s l b l e t h a t a N i ( [ l [ ) o x J d e species (Ni 2 O3 or NIOOH) ts formed m the secondary

APR 1983 VOL 5 NO 2

Makino, Motoi and Sato - effect of Co on dissolution of Ni-base superalloys in NaNo3 and NaCI Table 2. Dissolution

passive state, clear identification of this species was difficult because of the complex=ty of the spectrum. The chromium 2p3/2 spectra shown in lines (1) and (2) in Fig 12(c) consist mainly of a peak at 576.4 eV due t o c h r o m t u m o x L d e ( C r 2 0 3 ) s'6 Moreover, in the spectrum obtained for the surface layer formed in the transpassive state (line (1)), a peak at 579.0 eV due to Cr(V[) was also observed s . A Cr(V[) species would appear to be present either as dissolved chromium, CrO42. or as CrO3 formed by oxidation of Cr2 O3 The presence of the CrlV[) species ~n the surface layer suggests that the transpassive dissolution of alloys ts facilitated by the transpassivity of chromium. In the cobalt 2p spectra shown in Fig 12(d) lines (1) and (2), satelhtes of Co 2p3/2 peaks are visible. The difference in bmdmg energy between the principal peak at 780.7 eV and the satellite peak of the Co 2p3/2 level at 786.6 eV - the satellite sphttmg - was 5.9 eV Moreover, the difference in binding energy between the prmmpal peak of the Co 2 p l / 2 level (796.4 eV) and the Co 2p3/2 level -- the spinorbit sphttmg - was 15.7 eV. These results suggest that the surface layers formed in the transpass=ve and secondary passive condition contain cobalt in the +2 oxidation state (Co([[)), such as CoO and/or Co(OH)27-9 The spectra for the secondary passive films, hne (2) in Figs 12 (a)-(d) 7-9, show peaks due to metalhc chromium (Cr(O), 574.1 eV) and metallic cobalt (Co(O), 778.2 eV), as well as Nt(O). The presence of these metalhc hnes indicates

pretreatments

for xps study

Electrolyte

Current density, A cm -2

(1) 60N,-20Cr-20Co (2) 68Ni-12Cr-20Co

NaNO3 NaNO3

50 10

(3) 68Nb-12Cr-20Co

NaCI

50

Alloy

State

Transpass,ve Secondary passive Active

that the thickness of the secondary passive film is less than the escape depth of the photoelectrons, iea few nanometers. The oxygen ls spe(;trum shown in Fig 12(a) hne (3) revealed that the surface species formed in the active state was a metal hydrox=de. The intensities of the peaks due to nickel hydroxide (Fig 12(b) hne (3)) and cobalt hydroxide (Fig 12(d) line (3)) were markedly low, indicating the low content of these species in the surface layer. The absence of cobalt species is consistent with the result that the dissolution behaviour m NaCI is independent of cobalt. On the other hand, in the Cr 2p3/2 spectrum (Fig 12(c), line (3)), a peak at 577.6 eV due to Cr(OH)3 was visible. This species may be formed by precipitation of the Cr 3+ ions dissolved m NaCI z° . A metallic line was also visible in the spectrum of each component. The presence of metallic lines and high dissolution efficiency in the active state md=cate that the surface layer =s as thin as a few nanometres or alternatively IS porous

2p3/2 a

Is

Oxygen

Cr(OH}3

C Chromium

OH" 0z-

Cr(Vl) CrtO3 Cr(O) 2pi/2

f /

\ L

/\

I

534

L

i

i

I

I

526

530

2p3/2

b

Ni(OH) 2

N~ckel

NiO

I

I

I

590 NI(O)

I

i

1

I

I

1

580

d

2pl/2

Cobolt

Co(I])

,,

,I,,,,

I

i

57O 2p3/2 Solelhte

Co(n) Co(O)

i 2 860

Bindmgenercjy,eV

8.%0

800

,J, 790 Bmdmcjenergy,eV

,I,

'2 780

Fig 12 xps spectra of Ni-Cr-Co alloy surfaces after dissolution: (a) oxygen ls level, (b) nickel 2p level, (c) chromium 2p level, (d) cobalt 2p level

PRECISION ENGINEERING

71

M a k i n o , M o t o / and Sato - e f f e c t o f Co on dissolution o f N/-base superalloys m NaNo3 and NaCI The o x y g e n spectra shown m Fig 12(a) mdtcate that the surface layers f o r m e d In the actwe and transpass~ve states consist m a i n l y of metal h y d r o x i d e , resulting presumably f r o m a d i s s o l u t i o n - p r e c i p i t a t i o n process ]° However, since dlssolutton occurs w i t h h~gh c u r r e n t efftclency m these states, thts spectes appears to have I~ttle protective character On the o t h e r hand, the increase in the i n t e n s i t y of O 2 - p e a k f o r the secondary passwe f i l m suggests t h a t the metal surface ts p r o t e c t e d against d~ssolutton p r e d o m i n a n t l y by metal oxtdes

Conclusions F r o m the results of this w o r k , tt may be concluded t h a t c o b a l t facd~tates the dissolution of mckel-base super-

alloys, d~ssolvmg as divalent Co 2+ ions under ecm condJ tions For I o w c h r o m ~ u m c o n t e n t a l l o y s w h ~ c h e x h J b b t secondary passivatron m NaNO~, increasing the cobalt c o n t e n t results m a decrease m the degree of secondary pass=vlty and produces secondary transpasswatlon For the alloys which dissolve at 100% current efficiency witho u t passivity or secondary passwlty in NaNO3 and m NaCI, the a d d l h o n of cobalt has o n l y a secondary effect on the dissolution b e h a w o u r The metal surface d~ssolwng at high c u r r e n t efficiency ~s covered w~th n o n - p r o t e c t i v e metal h y d r o x i d e s , whereas in the secondary passive state the metal surface ls protected by metalox~des CobaltexJsts m the surface layers m the +2 o x i d a t i o n state, but on the metal surface which was machined =n NaCI no cobalt species were detected

References 1

2

Makmo E. and Sato T. Electrochemical Machmmg of NtckelBased Superalloys -- Anodtc Dissolut=on Behav=our of N=-Cr Alloys J. Jap. Soc. Precision Engng, December 1980, 46(12), 1518-1524

6

Makino E. and Sato T, Electrochemical Machining of NtckelBased SuperaJloys - Secondary Passtvation Behawour =n

7

Acid Baths J. Electron Spectrosc. and Related Phenomena, 1977, 11, 185-196

Kirn K,S, and Winograd N, X-ray Photoelectron Spectroscopic Studtes of Nickel-Oxygen Surfaces Using Oxygen and Argon

8

Dmkmson T., Povey A.F. and Sherwood P,M.A. Dissolution and Passivation of Nickel -- An X-ray Photoelectron Spectroscoptc Study J. Chem. Soc. Faraday Trans. I, 1977, 73, 327--343

5.

Asam=K. and Hashimoto K. The X-ray Photoelectron Spectra of Several Oxides of Iron and Chromium Corrosion 3cl. 1977, 17,559--570

72

Harber J. and Ungler L. On Chemical Shifts of ESCA and

Auger Lines in Cobalt Oxides 1bid, 1977, 1 2 , 3 0 5 - 3 1 2

Ion-Bombardment Surf. SCL 1974, 43, 625--643 4

Haber J., Stoch J. and Ungier L. X-ray Photoelectron Spectra

of Oxygen in Oxtdes of Co, NI, Fe and Zn /bid, 1976, 9, 459--467

NaNO~ Electrolytes ibid,, March 1981, 47(3), 320--325 3

Bouyssoux G. and Romand M. XPS and AES Studies of Anodic Passive Ftlrns Grown on Chromium Electrodes m Sulphunc

9

Okamoto Y., Nakano H.. Imanaka T. and Teranlshi S. X-ray

Photoelectron Spectroscopic Studies of Catalysts -- Supported Cobalt Catalysts Bull Chern Soc. Jap. 1975, 48(4), 1163-1168 10_ Chin D.T. Anodtc Mechamsrn of Electrochemical Machining, Study of Current Transient on a Rotatmg Electrode J. Electrochem. Soc., 1971, 118(I), 174-179

APR 1983 VOL 5 NO 2