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