Anodic dissolution of Zn in aqueous salt solutions

Corrosion Science, 1971, Vol. 11, pp. 153 to 159. Pergamon Press. Printed in Great Britain

ANODIC

DISSOLUTION OF Zn IN AQUEOUS SOLUTIONS*

SALT

J. W. JOHNSON,Y. C. SON and W. J. JAMES Departments of Chemical Engineering and Chemistry, University of Missouri-Rolla, Rolla, Missouri, U.S.A. Abstract--The anodic dissolution of Zn has been studied in aqueous solutions containing Ci-, Br-, I-, Ac-, SO~- and NO~. An apparent deviation from Faraday's law was found in NO~ solutions which was a function of N O ; concentration, c.d. and temperature. The deviations (in terms of an apparent valency) were correlated using an equation derived previously for Cd in which the mode of dissolution was described as the normal (faradaic) anodic process accompanied by local corrosion and surface disintegration. The normal anodic process was also found to be consistent with the mechanism proposed previously for Cd. R6sum6---On a 6tudi6 la dissolution anodique du Zinc en solutions aqueuscs contenant Cl-, Br-, I-, Ac-, SO~-, et NO;. Une d6viation apparente de la loi de Faraday a 6t6 observ~e dans les solutions de NO], cette d6viation est une fonction de la concentration de NO~, de Cd et de la teml~rature. Lcs dfviations sous forme d'une valence apparente ont 6t6 raises en cor61ation au moyen d'une fiquation d6rivfie pr~.alablement pour le Cd dans laquelle le mode de dissolution a ~t6 d6crit en tant que proc6d6 anodique normal (faradique) accompagn6 d'une corrosion locale et d'une d6sint6gration de la surface. On a constat6 que le proc6d6 normal anodique faisait partie du m~canisme propos6 pour Cd. Zusammenfassung--Es wurde die anodische Aufl6sung von Zn in wasserigen L6sungen untersucht, die CI-, Br-, I-, Ac-, SOi-, und NO~ enthalten. Es wurde eine anscheinende Abweichung vom Gesetz Faraday's in NO~ L6sungen gefunden, welche eine Funktion der NO~ Konzentration, c.d. und Temperatur war. Die Abweichungen wurden (ira Sinne einer anscheinenden Wertigkei0 aufeinander abgestimmt, in dem eine Gleichung benutzt wurde, welche zuvor fiir Cd abgeleitet war und in welcher die Art der Zersetzung als der normale, anodische Faraday Prozess beschrieben wurde, welcher yon/Srtlichem Korosions- und Oberfl~chen ZerTall begleitet ist. Man fand aueh, dass der normale anodische Prozess mit dem zuvor fiir Cd vorgeschlagenen Mechanismus vereinbar ist.

INTKODUCTION

THE ANOOICdissolution of metals in aqueous salt solutions has been the subject of many investigations. One interesting facet of these investigations has been concerned with metals for which the weight-loss during electrolysis is greater than that calculated from Faraday's law using the normal valency of the metal cations in solution. The extent of the deviation from Faraday's law is often expressed in terms of an "apparent" valency, i.e. a calculated (average) valency of the ions going into solution based on the weight-loss of the anode. In the absence of unrelated anodic reactions, e.g. 02 evolution, the apparent valency is usually less than the normal valency and is rarely an integer. A summary of metals exhibiting this phenomenon and explanations which have been proposed can be found in a review: Apparent valencies of < 2 for Zn ions in the presence of oxidizing anions have been reported by several investigators, del Boca 2 found that less than 1 F was required *Manuscript received 20 March 1970. 153

154

J.-W. Jom~so~, Y. C. SuN and W. J. J^r,lLs

to anodically dissolve 1 g equiv, of Zn in liquid NHa q-- Zn(NOa)z electrolyte. Epelboin 3 reported an apparent valency of 1.4 in glacial CHaCOOH or CzHsOH in the presence of CIO~. Aqueous solutions containing oxidizing anions for which apparent valences less than two have been reported are NO~, a-6 CIO~,a and BrO~. 7 In papers by Straumanis and James et al., 5-7 evidence was presented which attributed the low apparent valencies to a combined effect of self-dissolution and disintegration that accompanied anodic dissolution. The anodic dissolution phenomena reported for Zn are similar to those for Cd. In an earlier paper, s a mode of dissolution for Cd was proposed which involved the simultaneous occurrence of anodic dissolution, local corrosion and disintegration. This led to a mathematical relationship which successfully correlated the apparent valency with c.d. and NO~ concentration. This paper reports the extension to Zn of the mode of dissolution proposed for Cd. 2.0

t

c 1.9 o oh.-

~ 1.8

c-

>~ L7

(b) 400C

(a) 25"C 1.6

I

0

" I

004 i,

I

0.08

t

O.04.

0.02

a m p cm " z

J,

arilp cm " 2

2.0

1.9

"~ 1.8 i:::

g 1.7

1.6

(c) 55~ I.~

I

0

I

0.02 i,

0.04 a m p cm - 2

FIG. 1. Apparent valency of zinc dissolving anodically in KNO~-K2SO~ solutions at (a) 25, (b) 40, and (c) 55~ ( e , 0-333 M K~SO~; o , 0.05 KNOa--0.317 K~SO~; A, 0"10 KNO3-0"30 K2SO~, ,t, 0"30 KNOz-0-233 KzSO~; V, 0"50 KNOz-0-167 KzSO~; Y, 0"70 KNOa-0"I0 K_.SOI; t3, 1"0 KNOa, III, 2'0 KNOs).

t:)-.

Anodic dissolution of Zn in aqueous salt solutions EXPERIMENTAL TABLE 1.

AND

155

RESULTS

APPARENT VALENCY OF Z n IONS DURING THE ANODIC DISSOLUTION OF Z n METAL IN VARIOUS, ELECTROLYTES AT UNIT IONIC STRENGTtl

Apparent valence Electrolyte (g mole/l)

c.d. (A/cm -2) x 10 ~

25~

40~

55cC

0"1 ZnSO, + 0-20 K..SO,

1 10 50

2'01 1-97 1-99

1"96 2'02 1.98

2'02 1"96 2-01

0"1 ZnClz + 0.23 K2SO~

1 I0 50

2.00 1"97 2.02

1-96 1"98 1.97

1"98 1-98 1"95

0"1 ZnBr~ + 0"23 K..SO,

1 10 50

1.94 1-95 1"94

1.93 1"96 1"94

1.95 1"93 1"93

0'1 Znla + 0"23 KaSOa

1 10 50

1-96 1-98 1"98

1"97 1"95 1"99

2.00 1-97 1-94

0.1 Zn (At)* + 0"23 K~SO4

1 I0 50

2"01 2"01 1-97

1.96 2"01 2.00

1.98 1"96 1"96

0'1 Z n (NOn)2 + 0"23 KzSO,

1 10 50

1 "96 1"90 1"89

1.96 1"92 1-86

1.94 1-86 1-77

*Ac = acctate.

The Zn anodes were prepared from ASARCO (99.99 + o//o) rod. All solutions were made using analytical grade chemicals and distilled water. The ionic strengths were held constant at unity to insure good conductance. The electrode holders, cells, equipment, analytical procedures, calculations, eto. have been described previously)

Apl)areltt i,alellgy illeasurelllelltS The apparent valencies of Zn ions were measured at temperatures of 25, 40 and 55~ and were reproducible within 1-2%. The values for KNO3-K,.SO4 solutions are shown in Figs. l(a-c) and those for other solutions containing various anions and Zn ~§ in Table 1. It is seen that the NO~-containing solutions are the only ones in which the apparent valencies deviate significantly from the normal value. The dissolution rates of the anodes were negligible for periods oftime equal to those of the experiments when no external current was passing. Currenl-polenlial measttremenls The results of the polarization studies are given in Table 2. The c.d. was varied from 10-4 to 10-1 A/cm -~, and a linear Tafel relationship was usually found over the region 10-4 to 10-"-A]cm -2. The individual curves were reproducible within

156

J. ~.V. JOltNSON, Y. C. SUN and W. J. JAMES

q- 10-20 mV; the variation normally associated with a shift of the entire curve rather than a scatter of individual points. Extrapolation of the linear sections back to the rest potentials gave exchange currents of 10-5-10 -6 A/cm -~ No passivation regions were found. TABLE 2.

REST POIENIIALS OF TIlE ZI1 ANODE AND TAFEL SLOPES FOR TIIE ANODIC DISSOLUTION OF Z n IN VARIOUSELECTROLYTESAT UNIT IONIC S'I'RENGIII Rest potential (SHE)

Electrolyte (g mole/l)

0.050 KNOa + 0.317 K2SO~ 0-100 KNO, + 0.300 K~SOt 0.500 KNOa + 0.167 K:SO~ 1'00 KNO3 0.010 ZnSOl + 0.320 K2SO4 0-100 ZnSO~ + 0-200 K~SO, 0-010 ZnCI2 + 0-320 K2SO, 0.100 ZnCl: + 0"230 K:SO, 0"010 ZnBr= + 0-320 K2SO, 0"100 ZnBr: + 0"230 K2SOt 0010 Znlz + 0"320 KzSO, 0.I00 ZnI: + 0"230 K2SO, 0"010 Zn (Ae)z + 0-320 K2SO~ 0"100 Z n (Ac)~ + 0-230 K2SO4 0010 Zn (NO3), + 0"320 K~SO4 0'100 Zn (NO,)a + 0.230 KzSOI

Tafel slope (V)

25~

40~

55~

25"C

40~C

55~

--0-83 -0-79 --0-79 --0-72 -0-82 --0-80 --0-86 --0.83 --0-86 -0-84 --0-86 --0-82 --0-87 --0-84 --0.83 --0-79

--0.81 --0.81 --0.77 --0-73 --0.87 --084 --0.87 --0'84 --0-87 -0"83 --0'86 --0.84 --0"87 --0-81 --0'84 --0.79

--0-85 -0.82 --0-77 -0-76 --0.84 --0"81 --0'87 --0-82 -0.86 -0-84 --0.86 --0-83 --0"87 --0-83 --0'85 --0.85

0.075 0.065 0065 0.080 0.025 0.020 0-020 0-020 0.040 0.020 0.020 0015 0.025 0-020 0'015 0.060

0-065 0.065 0.065 0.085 0020 0.020 0.020 0-020 0.020 0015 0.015 0-015 0-020 0-015 0.045 0.065

0-070 0.065 0.060 0.075 0015 0'015 0'020 0-015 ~: " 0.020 0020 0.020 0015 0.015 0"015 0"050 0.040

DISCUSSION

Of the various ions used in this study, NO~ alone affected the apparent valence of Zn ions entering into solution. A comparison of Figs. l(a-c) with data reported previously for Cd 8 shows that the effect of NO~ concentration and c.d. is similar for the two metals, though the apparent valencies for Zn are generally higher than for Cd. For Cd, it was proposed that NO~ functioned as a depolarizer which allowed local corrosion to accompany anodic dissolution* with a resultant disintegration of the anode surface. Zn is also a relatively high H2 overpotential metal and a production of NO~ nearly equivalent to the metal dissolved outside the faradaic circuit has been reported during anodie dissolution in NO~ solutions.4'9 Because of the similarities of the two metals, it is of interest to see if the expression derived to correlate the data for the anodic dissolution of Cd is also applicable for Zn. The basic considerations in the previously derived equation were that the total dissolution rate during the passage of an anodic current consisted of the sum of three separate rates, (1) the anodic dissolution rate as calculated from Faraday's law using the normal valency, (2) the rate of local corrosion, and (3) the rate of disintegration. The expression resulting from these considerations is: 8 *Cd has a relatively high Hi overpotential and the amount o f local corrosion that accompanies anodic dissolution is observed to be small in the absence of depolarizers.

Anodic dissolution of Zn in aqueous salt solutions

157

L4

1.2 113

0.8 0.6 1.3

09 ,

0.7 05

0.9

(el

55"C

O5

o3L~''~ 1.7

I 1,8

., I 19

I 20 -log

I 2.1

I 22

! 2.3

i

FIG. 2. Effect of e.d. on the apparent valency of zinc at (a) 25, (b) 40 and (c) 55~ (O, 0.05 KNO3-0-317 K~SO,; A, 0.10 KNO3-4).30 K2SO~; V, 0-50 KNOa-0"I67 K~SO(; I3, l'0 KNO3.) I~ -----2 - - k i r~ C~NO;

(1)

where V~is the apparent valency, i is the c.d. and k, m, n are constants. Log-log plots o f (2-Vi) vs. c.d. and NO~ concentration are shown in Figs. 2(a-c)and 3(a-c) (values of Vi have been taken from the curves o f Figs. l(a-c). The data are seen to be correlated reasonably well. The best fit value of m, n and k from these data are shown in Table 3 along with the corresponding values for Cd for comparison. It is seen that the current exponents m are almost the same for Zn and Cd. This might be expected from the proposed mode of dissolution in that the current would be responsible for disrupting the surface film which in turn allows local corrosion to occur. The concentration exponents n associated with tile NO~ reduction also are not greatly different and indicate a possible similarity in these reactions at tile local cathodic sites. An Arrhenius plot for the k-values from Table 3 gave a linear relationship allowing k to be expressed as 58.8 exp (1960/T). With these parameters, the resultant equation* for calculating the apparent valency o f Zn ions as a function ofc.d. (0-001 < i <: 0.030 A/cm-~-), N O ; concentration (0.05 ~ CNo~ __< 1"0 M), and temperature (25 < t _< 55~ is Vi -----2 - - 58"8 i ~ r,o-33 9-'No~ exp ( - - 1960/T). (2)* 9This expression cart also be used to calculate the efficiency of Zn anodes under the stated conditions.

158

J.W. JOHNSON,Y. C. SON and W. J. JAMES !. 4

1.2~

o

(a) 25"C

I.CI 08 ! 0.(5: 1.2, (b)

~

400C

~ ~....-.-'~

09

9

0.5 l.I

o

-(c) 550C 0.9~ 0.7 0.5

0.3 ~

0

0.2

I

0.4 0.6 - log

I

0.8

I

1.0

I

1.2

C,~o;

FIG. 3. Effect of NO~ concentration of the apparent valency of zinc at (a) 25, (b) 40, and (c) 55~ ( o , 0.005 A/cm-2; A, 0-010 A/cm-2; V, 0.020 A/cm-~.)

TABLE3. PARAMETERSFORCALCULATINGTHEAPPARENTVALENCY OF Zn AND Cd IONSDURINGANODICDISSOLUTIONOF TIlEMETALS I N "NO~SOLUTIONS

Parameter m n /~25 k~o k~ A* E~,

Zn

CA

0.52 0"33 1"80 2.57 3.39 58.8 3920

0.49 0-41 2.23 2"35 2'47 8.72 900

*Parameters for tile Arrhenius equation,

k = A exp(--E,,/RT).

The mode of anodic dissolution of Zn suggested by these and other studies is as follows: when a Zn anode is placed in aqueous solution, the oxide or hydroxide film (protective only under limited conditions) will dissolve, a~ This exposes unprotected metal to the electrolyte which makes possible corrosion by local cell action. Due to the high H2 overpotential, the corrosion rate is relatively slow except in the presence of NO~, BrOw, etc., which can participate directly in the local cathodic reaction or act

Anodic dissolution of Zn in aqueous salt solutions

159

indirectly as depolarizers.* In the absence of NO~, the normal or uncomplicated anodie dissolution of Zn occurs and a valence of + 2 is observed; in the presence o f NO~, local corrosion occurs simultaneously with anodic dissolution and the combined action of these two tends to undermine and detach cathodic areas, causing surface disintegration.The Zn that is removed from the anode by local corrosion and disintegration is outside the faradaic circuit and leads to an apparent valency < 2. From Table 2, it is seen that the Tafel slopes for the Zn dissolution tend to fall into two groups; 60-85 mV for solutions containing NO~ and 15--40 mV for those which do not contain NO~. These correspond closest to the theoretical values of 2.3 R T / F and 2.3 RT/2F, respectively. They are also consistent with the observations made on Cd s and the latter values (2.3 R T / 2 F ) with slopes summarized by Gymtryk and Sadzimir u as reported by other investigators. Hence, the type of mechanism suggested for the Cd dissolution appears applicable for Zn, also in agreement with the suggestion by G m y t r y k and Sadzimir that the corrosion current of Zn in sulphate solutions (pH 6-8) is related to the reaction of Zn with non-dissociated molecules o f water. Accordingly, the anodic dissolution reaction is proposed as Zn(s) -I- H.,O(aq) = ZnOH(s) -k H§

-b e

(3)

ZnOH(s) = ZnO(s) + H+(aq) + e

(4)

ZnO(s) + H~O(aq) ~ Zn+-~(aq) + 20H-(aq).

(5)

The corresponding cathodic reaction associated with local cell action would be NO~ + HoO + 2e --~ NO~" + 2 O H - .

(6)

These indicate that the NO~ concentration and c.d. enter into eq. (2) as a consequence o f activation control of the local cathodic reaction and undermining and detachment of local cathodic areas, respectively. *Gmytryk and Sedzimirit report a Tafel slope of --2 (2.3RT[F) for the evolution of H2 on Zn which suggests that H + discharge may be rate determining. Thus, NO-~probably participates directly in the local cathodic reaction rather than reacting with H + at cathodic sites. Acknowledgement--This is contribution No. 50 from the Graduate Center for Materials Research.

It is based on a dissertation presented by one of us (Y.C.S.) in partial ft,lfilment of the degree, Doctor of Philosophy, at the University of Missouri-Rolla. REFERENCES 1. W. J. JA~IES, M. E. S't'RAUMANISand J. W. JOIINSON,Corrosion 23, 15 (1967). 2. M. C. DELBOCA,Helv. chim. Acta 16, 565 (1933). 3. I. EPELBOIN,Helv. chim. Acta 59, 689 (1955). 4. D. T. SORENSEN,A. W. DAVIDSONand J. KLEINI3ERt3,J. hwrg. nttcl. Chem. 13, 64 (1960). 5. W. J. JAslrs and G. E. Sro,'~R, J. Am. eltem. Soc. 85, 1354 (1963). 6. M. E. S'rRAUMANIS,J. L. RrED and W. J. JAMES,J. electrochem. Soc. 114, 424 (1967). 7. M. E. STRAtrMANISand Y. WANO, Corrosion 22, 132 (1966). 8. J. W. JomqsoN, E. DEN6, S. C. LAI and W. J. JAMES,d. electrochem. Soc. 114, 424 (1967). 9. Y. C. Stm, M.S. Thesis, University of Missouri-Rolla (1964). 10. M. POURBAIX,Atlas of Electrochemical Equilibria in Aqueous Sohttions, pp. 406--418. Pergamon (1966). 11. M. GMYTV,'tKand J. SEOZIMm,Corros. ScL 7, 683 (1967).