Bulletin of Electrochemistry 15 (1 J) November 1999, pp 466-47 J 0256-1654/99/$ -50 © 1999 CECRI

RECENT INVESTIGATIONS ON THE CURRENT STATUS OF UNDERSTANDING ON

THE ANODIC BEHAVIOUR OF NICKEL IN FLUORIDE MEDIA

MI HAELNoEL

Central Electrochemical Research Institute, Karaikudi 630 006. INDIA

In fluoride media, a passive layer of nickel fluoride is easily formed on nickel surfaces during anodic

polarisation. This film remains insoluble in liquid HF, non-aqueous fluoride media, low temperature melts

and even in HF media containing upto 20% water. In aprotic media, the fluoride film formed is generally uniform. Nucleative processes become quite significant when the acidity is low. Acidity appears to playa

significant role in dissolving th oxide layer and enhancing uniform dissolution and precipitation of nickel

fluoride. Around 3.0 V, the anodic fluoride fIlm exhibits finite resistivity and allows charge tran fer. The

resistivity of the film itself exhibits some potential dependence. The electron transfer probably leads to

the formation of high valent NiF2 such as NiF3 or nickel fluoride embedded with free radical fluorine. The voltammetric re ponses in liquid HF, aprotic fluoride media as well as low temperature fluoride melts

are being investigated at present.

Keywords: Passivation, anodic behaviour, nickel electrode

INTRODUCTIO techiques have also been employed. These are described in

detail in the respective references cited.

Nickel is generally one of the very stable metals in fluoride RESULTS AND DISCUSSION

media. Nickel and nickel-alloys generally are the materials

of choice for handling fluorine and HF especially at high Anodic behaviour of nickel in aqueous media

temperatures. In liquid HF, nickel is the only anode material

of choice for electrochemical perfluorination processes. Even In dilute aqueous HF solutions, nickel undergoes facile in molten electrolyte media, nickel anodes are used wherever dissolution. However, a thin salt layer of NiF2 film is quite very high purity of fluorinating gases like NF) free from easily formed on the surface of nickel electrode. The CF~ are required. Despite such a wide range of voltammetric responses in 2 M aqueous HF solutions, at electrochemical applications for nickel, there are very few different sweep rates presented in Fia. 1 correspond to this investigations on the anodic behaviour of nickel in different film dissolution precipitation model. The experimental fluoride media response (points) also corresponds we!J with theoretically

expected voltammograms (continuous line). In the In n.:cenl limes this laboratory has undertaken a series of

voltammetric time scale, the NiF2 film gets dissolved investigations on the anodic behaviour of nickel in aqueous

completely within a few minutes under open circuit condition and non-aqueous fluoride media. In what follows, a

[1,2]. comprehensive overview of recent findings in this area

specifically originating from this laboratory is presented. In H solution of higher concentration, two distinct

Cyclic voltammetry was the predominant technique dissolution precipitation processes are observed during lhe

employed. In addition, however, other experimental forward as well as reverse sweep. Typical experimental

466

MICHAEL - Recent investigations on the current status of understanding on the anodic behaviour of nickel in fluoride media

cyclic voltammograms along with theoretical responses are

presented in Fig. 2. The theoretical model assumes two

distinct anodic dissoJ ution processes with two different film

resistivity. In aqueous media of low HF concentration, the

anodic dissolution always proceeds through the oxide layer

present on the nickel surface. With increasing HF content,

part of the oxide layer is chemically dissolved by the acidic

tluoride media. This results in two dissolution possibilities

namely direct dissolution of oxide free nickel surface and

through film dissolution of nickel surface covered with the

oxide layer These films also exhibit time dependent

dissolution behaviour The effect of sweep rate presented in

Fig. 3 exhibits this lime dependent dissolution behaviour

rI.2J.

Effect of monovalent cations

In general, the influence of pH and the attack of anodic

species on the anodic dissolution of metals are well known

However, the likely influence of cationic' species on the

/100

o 0

o 0

o 0

o

o

·t.. g-Q. E~oo

~ c 'U;G.O-O c: "0 "

dissolution behaviour is not sufficiently recognised In

corrosion science. Some investigations carried out in 1 M

HCI04 acid media clearly indicate the influence of

monovalent cationic species on the anodic dissolution of Ni

in fluoride media (Fig. 4). Typical cyclic voltammograms of

100 mM HF and 20 mM KF in 1 M HCl04 under otherwise

identical experimental conditions are pres nted in Fig. 4.

Despite lower fluoride ions concentration. the anodic

dissolution in KF is significantly higher and the dissolutlon

processes also occur over a wider potential range. The

dissolution of nickel especially in the more anodic pntential

region (beyond 0.8 V) is presumably due to the cation

assisted dissolution of NiF2 film as KNiF, and K2NiF4 which

are highly soluble in aqueous medium [3J.

The effect of monovalent cations may also be clearly seen

in cyclic voltammetric responses of ni kel in l.0 M HCI04•

media containing 0.1 M concentration of NH4F, NaF and Li

as shown in Fig. 5. Hydrated Li ions also assist the anodic

.."" o

o

/'C'l ~o f)

1 / ~ \;//0

E u

:Ii 0.20 .... 1.00

0­E <{

>­

~ ., ')...x 'Q

C a~

" :;C~ u.x."

U ~

U ~

,oc, "'0 ..'" a.o' 1.00

Polen!,al (vall)

Potential (Vo~)

Fig. 1: Experimental (000) and theoretical (---) CV of Fig. 2: Experimental (000) and theoretical (---J CVof

Ni in 2 M HF Influence of sweep rate (mVs-1

) Ni in HF of concentration (a) 10 M and (b) 20 M at

la) v = 10, Ib) v = 80 and Ic) v = 160 same v = 10 (m Vs-1)

467

MI HAE - Recent in stigalions on the current status of understanding on the anodic behaviour of nickel in fluoride m~dia

/1 b

/ IlLMA/CM2 ,:.~

"/"1

i b'~

0 ~ 0-100 iii

z 0 0 UJg 0 ....

0 z:l ..ooo-l~--__..l--.....--~---.,-""-"" UJ

! a .... 0.71 .... l.'l('l Q:.." Q:

30

0

a

...... oOL.-:-----=::::::o::;:s:..------s==~=~-!D."" O

POTENTIAL (V VS

O.ICt.l Fig. 5: V of Ni ill 1 M HClO.; COIl!aUZ;l7g 100 IllM Ij (a) NH4F (b) NaF alld (c) LiF al same v = 10 (mVs )

cPotential (voll)

Fig. 3. !.I!J,·ril1lcn/ll/ (000) lind theoretical (---) CV of /\'i ill /() ,If lit fa) \' = U (mVs-

1) and (b) v = 200 (mVs-

1)

u b131. mAlcm 1 E d­

e::. 0. E $ 2:­;;>­ C.... III 0c:n z C

~ Ot----==~=--------------====-___1 ~ ..... 0 z w a cr cr :::> u

Of. 08 12 POlent<al (va· )

Fig. 6: 'xperilllelltal (000) alld theoretical f---) CV of

Fig. 4: CV of Ni ill / Iv! HCl04 colltaining _I Ni ill CHi CN-H20 ill 2 Iv! HF 11711l1enee of COIICPIl/mtlOll u!1

(al !()() 111M HF (llId (b) 20 III/vI KF at sallie v = 10 (mVs ) CH3CN (a) 20%. (b) 40% and (c) 80% at same v = /0 llllVS )

POTENTIAL ( V V5 Pd/H1)

468

;VII JI EL - Recent investigations on the current status of understanding on the anodic behaviour 0[' nich:el in tluoride media

c!Jssolutioll process hy enhancIng the local acidity near the

Illckel surlau.: Hence, the anodic dissolution In LIP is

c<lll;il.lci·'lhly higher anc! also occurs on a much wider

ptllenlial range [31.

Efl'ect of solvents

The passive NiP2 layer is highly soluhle in aqueous media.

[n olher solvents and solvent mixtures, the anodic dissolution

hch,lviour IS signlficant[y different. 'fypical cyclic

I(Jltal1lnwgrams ot anodic dissolution of nickel in 2 M HF

cOlltainlng different ratios of CH3CN and water under

otherwise idclltical conditions are presented in Fig. 6. With

ilKreasing concentration of H,C ,the passivation potential

corresponding to complete iF2 film formation shifts to more

~1J1odic n:gions. The dissolution of NiF: during the ['everse

sweep also decreases significantly. The resistivity of Nil'

layer as shown by the slopc of the current-rotential curve in

thl' reverse sweep also varies significantly with CH,CN

ctlntenl In thc solvent media r2Al

Fig. 7: Experimenta! (000) and lheorerical 1---) CV of Nl COllliHnill,R 2 M HF in la) 80% melhallo! (b) 80% DMF ([Ild

Ie) 80% dioxane al sa//le v = ! 0 (Ill Vs -I)

469

The nature of the solvent also Innuenc(;s the vo[tammetrlc

responses. Typical cyclic vo!tammograms of 2 M HF 111

aqueous mixtures containing 80% methanol. DMf and

dioxane under otherwi,c identical expel'imental conditions

are presented in Pig. 7 In aqueous methanolic mixlUres, the

iF1 layer is quite insoluble, hut the passivation process is completed bclow 1.6 V. Considerably higher passl V,ll ion

pOlentials and lowCl' film resistivity arc noticed in DMF and

dioxane mixtures [2,51.

Anodic behaviour in anhydrous HF/CH3CN

In the abscnee of H20, iF1 film becomes highly qahle ,Ind

hence practically no dissolution of i is ohserved during lh

second and subsequent sweeps. The vo]tammctric r('spons 's

however, still depend on the concentration of HI'" Typic<ll

cyclic voJtammograms ohtained during anodic polaris:llioll

of i in 2 M, 4 M and 6 M AHF in H, N media arc hown

in Fig. 8 In the second and suhsequent SIVCCPS the 'ulrnl

potential cmve practically rctr:1ces, the curve, during lhe: fir. t

"'l

b

/~\,/o

,-,,' ~ "u,.

0.0';

/"-1 //I?---­

~cr.. I

Fig. 8: E\'jJerililenla! (000) (l/U! lheorelica! (---) ell of Ni ill CH lCN coilloillillg ANi'" of cuncelllmlioll

laJ 2, . Ih) "' and (e) 6, v = 12Q (//I VI'

-I)

MICHAEL - Recent investigations on the cun-ent status of understanding on the anodic behaviour of nickel in rIuoriLie meLiia

rcvcrsc sweep. The conductivity of the NiF2 film also

Licpends on HF concentration [2,6].

Addition of small quantity of water influences the

voltammetry response quite significantly. This effect is

shown in Fig. 9. The presence of small quantity of water

improves the conductivity of NiF2 film and also brings down

the critical potential for the conductivity [2,6].

Effect of acidity in CH3CN media

The acidity of HF dissolved in CH:1CN may be conveniently

varied by thc addition of Et}N. This was also found to have

signilicant effect on the anodic behaviour of Ni. Typical

cyclic voltammograms obtained III 0.5 M

(Et) . 6HF/CH}CN) at different sweep rates are shown in

Fig. 10 Around () V, an anodic dissolution peak

corresponding to the formation of monolayer NiF2 film is

noticed. Further oxidation was noticed below 3 V. Scanning

electron microscopic studies (SEM) and i-t transients

measurements indicated that NiF2 film growth occurs

unlrormly on the electrode surfaces, when the solution is

sufficiently acidic as in this ease [7]. When the Et}N

0.000

.,.... ,0:>

/0 o

~.~ ~ l MO)-l-=-----..---,-------,------,---,

1.00 2.00 3.00 '.00 '.00

0.010

0

0.""

0.011

00.012

/ 0.... //V

/

/ /

3.000 ...,).1(1 1.80 0.00"'0

Polt:nlial t \011 )

Fig. 9: Experimenwl (000) and theoretical (---) CV of Ni containing 2 M AHFICHlCN in (a) absence and

. -I (b) presence of 2 M H20 at v = 120 (mVs )

470

concentration is enhanced as in the case of 4.5 M (Et,N 0.66

HF) the anodic behaviour changes substantially. Thc

voltammetric responses obtained under this condition at

different sweep ratcs are shown in Fig. II. The Nif=2 filnl

growth over the monolayer surface occurs through ,I

nueleative film growth processes hcyond 4.5 V Othcr

experimental evidences also supported the view, namely rilm

growth mechanism itself depends on the acidity 01' the

fluoride media [7].

CONCLUSION

The anodic dissolution, formation or a pas~ive NiF2 layer.

conductivity of this passive layer, the stability and solubilily

of NiF2 film and further growth and other electron transrer

processes are found to depend significanlly on the lype of

electrolyte, solvent, the concentration of fluoride ion and

other ionic species present In the media. Hence, it is quitc

important to consider all these factors while interpreting the

1--­~-----------,

~ ill Z W a

Z W a: a: :::> u

18':'lI\A.l'CI"

,

2lS'2m ... "rn

) ) 6 7

POTENTiAL Iv vS ~/H,)

Fig. 10: CV of Ni in 0.5 M E'JN 6HFICI-f;CN at I

differellt v (IIlVS- ) (a) 60, (b) 120 and (e) 240

MICHAEL - Recenl in eSligalions on lhe eun-ent status of understanding on the anodic behaviour of nickel in fluoride media

operating rarameters on the anodic behaviour of Ni 1D

different fluroride media. til

L

REFERE C

I. M Noel and S hidambararn, } I~leuroallal Chelil. 369;i ) (1994) 25

2. M oel. S Ponnusamy and V RaJcndran, Unpublished result,

3. M Noel, R Santhanam and S Chidarnharam. Unpubli,hcd

results 4. M Noel and S Chidambaram, } Fluorilll' CholZ. 68 (19'14)

i21 5. M oel et ai, Unpublished results

6. S Krishnamoorthy. Ph D Thesis, Alagappa University,

Karaikudi (1994)

7. Noel, V Suryanarayanan and S Krishnamurthy, } Fluorille

Chell!. 74 (1995) 241

PO:ENT ALlv ,~ ?C:/h1 )

Fig. 11' CV of Ni i~/5 M £rJN 066 HFICHJCN af

differ!'//[ v (I1IVS ) (a) 6U. (bJ 120 alld le) 240

471