THE ANODIC BEHAVIOUR OF F.ERROMANGANESE

BY ALAN NEWTON CAMPBELL, M.Sc., Ph.D., F.I.C.

Received 25thJune, 1926.

Introductory.

The anodic behaviour of pure manganese and of an indifferent anode in a manganese electrolyte having previously been examined by the author and o t h e r ~ , l - ~ it was thought that the anodic behaviour of ferromanganese, an alloy containing roughly 80 per cent. of manganese, should be of con- siderable interest, in view of the fact that iron is a metal which exhibits the phenomenon of passivity very markedly, whilst manganese, at least under the conditions of the author’s observations, is largely free from it. From the analogy between chemical and electrochemical passivity and the facts that ferromanganese is comparatively stable, even in damp air, whilst pure manganese is not, a certain amount at least of passivity was expected. This passivity was actually found and the various methods of inhibiting or reducing passivity were applied. The current efficiency, the nature of the slimes and of the components entering solution have also been investigated.

The author would like to draw particular attention to the final graph, which shows very clearly the effect of the ratio of superposed alternating current to direct current in reducing passivity : the method of expressing the three quantities on one graph appears to be new.

Experimental.

The sample of ferromanganese used was kindly supplied by Sir Robert Hadfield. I t had the following composition :-

C Si P Mn 6.9 per cent. 0.66 per cent. 0.21 per cent. 77.9 per cent.

Unfortunately, apparatus was lacking wherewith to cast the ferro- manganese into regular rods. The apodes were therefore formed of irregular cubes of ferromanganese, to which copper wires were soldered, the soldered junction being kept well above the surface of the electrolyte. The apparatus consisted of a beaker, containing the electrolyte in use : the cathode was a cylindrical sheet of platinum foil, in the axis of which the anode was suspended. The potential of the anode was measured by means of a N/I calomel electrode, Lugin capillary pressed against the surface of the ferromanganese and potentiometer arrangements as usual. The E.M.E of the calomel was taken as 0-560 volt, i.e., the potentials are in so- called absolute volts.

226

Publ

ishe

d on

01

Janu

ary

1926

. Dow

nloa

ded

by N

orth

east

ern

Uni

vers

ity o

n 27

/10/

2014

22:

42:3

6.

View Article Online / Journal Homepage / Table of Contents for this issue

A. N. CAMPBELL 2 2 7

Pot en t io metr i c Res u Its. T h e current-potential curves were obtained for a number of electrolytes

and conditions. Each curve commences with a static potential which, under the circumstances, has probably no very definite meaning. The tint series of experiments, all carried out a t room temperature, is sum- marked in Fig. I. The area of immersed surface of the anode was in each case about 10 cm2, so that the current density per dm.2 is obtained by multiplying the current figure by 10.

I t will be observed that passivity attains a maximum in neutral molar manganese sulphate solution. U p to about 5 milliamps. (0.050 amp./dm.2) the curve is a normal solution curve. At 5 niilliamps. passivity suddenly sets in and the potential rises to that of visible oxygen discharge. The

2'0

1'5

I '0

0.5

0'0

- 0.3

I. 11. Oxygen

111.

IV.

V.

evolved.

1

25 50 75 I00 Current in milliamps.

FIG. 1.-Curve I. Neutral molar manganese Fulphate. , I 11. ,, ferrous sulphate. ,, 111. ,, ,, manganese nitrate. > I w . 1 9 t f ,, chloride.

V. Molar ferrous chloride (slightly acid).

curve is then an ordinary oxygen discharge curve. The potential of visible oxygen discharge from a neutral electrolyte, at room temperature, on a ferromanganese surface is about + 2-15 volts (abs.), if the current density be not too great. This is in the neighbourhood of the potential of permanganate formation in a manganese sulphate electrolyte and such formation was actually observed.

The curve for neutral molar ferrous sulphate shows passivity setting in more slowly, but at a current density of 0 - 7 5 amp./dm.2 the potential of visible oxygen discharge is again reached. There was here no formation of permanganate, for this is an effect requiring a high concentration of manganous ion in the electrolyte, apart from the fact that the ferrous sulphate would immediately reduce any permanganate formed.

I 7

Publ

ishe

d on

01

Janu

ary

1926

. Dow

nloa

ded

by N

orth

east

ern

Uni

vers

ity o

n 27

/10/

2014

22:

42:3

6.

View Article Online

2 2 8 THE ANODIC BEHAVIOUR OF FERROMANGANESE

% 0.5 v

m

> - . .'

cd

Y - .9 0'0

.- w g a" - 0.5

The molecular normal ferrous chloride electrolyte was prepared by dissolving iron in hydrochloric acid and contained a little free acid. I t is possible that the complete absence of passivity observed in this case is to be attributed, partially at least, to the free acid.

Neutral molar manganese chloride showed considerably more passivity, although not to the same extent as the sulphate electrolytes. Under the experimental conditions, the discharge potential of chlorine was not reached.

" 11.

I.

25 50 75 I00 125 Current in milliamps.

FIG. 2.-Molar manganese sulphate at 75" C.

The curve given by manganese nitrate solution is peculiar in that (I) this electrolyte does not give the maximum passivity, as is usually observed, and ( 2 ) the passivity is very unstable. At certain periods of time and of potential it appears suddenly to break down, so that the curve given cannot be considered as other than illustrative of the oscillatory nature of the

the passivity in various electrolytes at room showing the maximum passivity, viz., neutral

passivity. I After thus investigating

temperature, the electrolyte

FIG. curve I. Molar manganese sulphate + N H,SO, at 15" C. Y , 11. 9 , 9 9 ,, + N NaCl at 16" C.

molar manganese sulphate, was investigated for the variation of passivity under various conditions, with various additions and with various treat- ments of the anode.

Temperature was the most obvious condition to vary and Fig. 2 is the current-potential curve for neutral molar manganese sulphate at 7 5 O C.

I t will be observed that the passivity is completely inhibited. Similar observations at 50" C. and even at 25" C. gave almost identical curves, indicating that the passivity is of a very unstable nature.

Fig. 3 contains the current-potential curves for (I) an electrolyte con- sisting of molar manganese sulphate and N H,SO,, at room temperature ; and, ( 2 ) , molar manganese sulphate + NNaCl.

Publ

ishe

d on

01

Janu

ary

1926

. Dow

nloa

ded

by N

orth

east

ern

Uni

vers

ity o

n 27

/10/

2014

22:

42:3

6.

View Article Online

A. N. CAMPBELL 229

In the case of (I) , this strength of acid attacks ferromanganese which accounts for the irregular form of the curve. The curve of case ( 2 ) shows that the presence of the sodium chloride reduces passivity, whilst not com- pletely inhibiting it.

The methods adopted were ( I ) charging the anode with hydrogen, by making it the cathode in an electrolyte of very dilute sulphuric acid and (2) etching the anode with fairly strong sulphuric acid. The curves obtained are given in Fig. 4 (AA and BB).

The action in the second case may be the same as in the first, siz. charging with hydrogen.

I n view of the observation of Grube and Metzger,6 that pure manganese dissolves anodically in hot caustic soda to form manganate, similar experi-

Experiments were now tried on the activation of the anode.

Both methods of treatment almost entirely inhibited passivity.

I.

1'5

11. 111.

B. I\ A.

25 50 75 I00 125 150 I75 Current in Milliamps.

I. Molar manganese sulphate at 15" C. + superposed A.C. FIG. +-Curve ,, ,, ,,

11. N KOH at 15" C . 111. N KOH at 75" C. IV. Molar manganese sulphate a t 15" C. (A = anode charged wit

H, ; B = anode etched with H,SO,).

ments were carried out with a ferromanganese anode. The electrolyte chosen was N KOH.

Both curves show a fairly gradual rise of potential until the potential of visible oxygen discharge is reached, which, of course, is lower at 75" than at IS". There was no formation of manganate and nothing entered solution. The form of the curve before the discharge of cjxygen is remark- able, for a fir ly considerable current passed without any apparent anodic process. There was, however, a heavy formation of slimes, exceeding the loss in weight of the anode. I t is suggested that possibly the manganese or iron enters into solution, to be immediately precipitated. Some such explanation is necessary to explain the low current efficiencies encountered elsewhere.

This is well known to have a depressing effect on overvoltage and

Two curves for I 5" and 75 " are given.

Finally the effect of superposed alternating current was tried. 87 9

Publ

ishe

d on

01

Janu

ary

1926

. Dow

nloa

ded

by N

orth

east

ern

Uni

vers

ity o

n 27

/10/

2014

22:

42:3

6.

View Article Online

230 THE ANODIC BEHAVIOUR OF FERROMANGANESE

The alternating current was obtained from a motor generator, delivering A.C. at a frequency of 60 cycles and a potential of 160 volts. The voltage was further transformed down by means of a coil transformer to about 8 volts. Lamp resistances and a rheostat reduced the current to any desired amount. The alternating current was measured by means of a vacuo-thermocouple and delicate ammeter. The thermocouple was cali- brated with direct current, so that the values for A.C. are really its equival- ence in terms of D.C. The subsidiary electrode was a carbon rod enclosed in a porous pot. The anode had previously been rendered passive by use in the potassium hydroxide electrolyte. The curve is given in Fig. 4. As the direct current was increased the alternating current was increased at intervals. This has been indicated on the graph by joining the plotted points to their corresponding alternating current value on the current axis. On this method of repre- sentation, the ratio A.C./D.C. is roughly represented by the inclination of the radial lines to the current axis. When the line is perpendicular, A.C. = D.C. The curve shows very clearly the depressing effect of alternating current, when compared with the corresponding curve (Fig. I ) for manganese sulphate electrolyte without A.C. I t also shows the importance of the ratio A.C./D.C., for as this value approaches unity the potential approaches more steeply towards what it would be in the absence of A.C.

Anodic Current Efficiencies.

I t was originally intended to estimate current efficiency from the loss in weight of the anode, but it was found that superficial oxidation and absorp- tion of water occurred. I t was early found that both components of the ferromanganese entered solution and the expedient, admittedly inaccurate, of estimating the constituent not originally present in the electrolyte and of assuming that both constituents entered solution in the proportion in which they were present in the anode, was adopted. The following table summarises the current efficiences obtained :-

The experiments were carried out at 15' C.

TABLE I.

Nature of Electrolyte.

Molar MnSO, . . .

,',' F;~O, . . . ,, FeCl, . . . :: M k l , . . .

3 , 9 , ' * * * . . . 17 9' -

* . . . . . :: M;(NOJ, . .

" Mn&, + N/I'NaCi N;;KOH. . . . N / I K O H . . . .

Y , i * . .

Current in Milliamps.

2-85 4 '5

90'0 5'8

200'0

4 '3 8'0 3-85

3'2 6 '0

150 '

140 150 150

120'0

Temp. OC.

Current Efficiency. Per Cent.

55'99 44'75 10.77 100.7 41'6

100'0 100'0 100'0 42'9 35.1 15-44 6.7

25-8 0'0 0'0

* To this must be added a further 3.93 per cent., corresponding to the amount of permanganate formed.

Mn++ + 4H,O + 5 0 = MnO', + 8H+.

Publ

ishe

d on

01

Janu

ary

1926

. Dow

nloa

ded

by N

orth

east

ern

Uni

vers

ity o

n 27

/10/

2014

22:

42:3

6.

View Article Online

A. N. CAMPBELL 231

In some cases, where the efficiency is below IOO per cent., it is difficult A to see what is the anodic process responsible for the missing energy.

tentative explanation of this has been offered above.

Amount and Nature of Slimes.

I n some cases very heavy slimes were formed, notably when gas was discharged at the surface of the anodes. In these circumstances the anodes disintegrated rapidly and the slimes were largely unchanged ferromanganese. The slimes were collected by surrounding the anode with a porous pot and were subsequently washed, dried, weighed and, in some cases, analysed. In Table II., the quantities of slimes are expressed as percentages of the weight dissolved.

TABLE 11.

Electrolyte.

Molar MnSO, . . . . , , . . . . . . . . FegO, . , . . Fib, . , . . . . . .

9 9 . . . . . . . . Mkl, . . . . Mn(NO,), . . . MngO, N / I N ~ C ~ ' :

Current in Milliamps.

2-85 4'5

95 5'8

60.0 4 '3 8.0

6.4

I55 I 60

150 140

Temp. OC *

Slimes Per Cent.

Negligible 53'9

312.7 8.0

73'7 I '09 1.9 2'1 2'2

124.8

13'4 1730'0

Whenever the potential rose to that of visible oxygen evolution, the disruption of the anode was considerable. In addition to the cases above, the caustic potash electrolyte also produced heavy slimes at the higher current densities.

I n certain cases the slimes were analysed and, in one case, the amount of phosphorus entering the electrolyte was determined.

I. Analysis of slimes from FeS04 electrolyte. h'ln . , 52.84 per cent. Fe * 21.53 9 9

Combined Water . * 13-40 9 ,

Free Carbon . ' 3-61 9 ,

Si * 0'32 9 ,

Combined 0,, C, etc. (by diff.) . 8-30 ,,

= o.ooog gm. (per 2.6485 gm. ferromanganese entering solution).

Mn . . 53-51 per cent.

Si . 2-18 ,, C. etc. (by diff.) . . 1-81 ,,

Fe . 88 per cent. * 11.5 ? >

Si Mn . . Absent.

Weight of phosphorus entering solution

2. Analysis of slimes from FeC1, electrolyte-

Fe 41-50 $ 9

3. Analysis of slimes from MnCl2 electrolyte-

Publ

ishe

d on

01

Janu

ary

1926

. Dow

nloa

ded

by N

orth

east

ern

Uni

vers

ity o

n 27

/10/

2014

22:

42:3

6.

View Article Online

232 THE ANODIC BEHAVIOUR OF FERROMANGANESE

4. Analysis of slimes from Mn(No,)g electrolyte- Mn . . 62.25 per cent. Fe . 17-16 ,, C * 7'88 9 ,

H,O . 5-83 ,¶

Combined 0, (by diff.) . - 5-60 9 9

Si . 1-28 ,,

Previous reference has been made to the fact that in a manganese sulphate electrolyte, when the potential has risen above 2 volts, formation of permanganate takes place. The permanganate formed did not enter solution, but crystallised in black flakes on the anode at the liquid junction.

Conel usions.

The anodic behaviour of ferromanganese has been investigated in various electrolytes and under various conditions. Both constituents dissolve and there is never complete passivity, but very large passivity in certain electro- lytes. I n hot potassium hydroxide there is no formation of manganate, in contra-distinction to the behaviour of pure mangane~e.~ The standard methods of reducing passivity are shown to be valid here. The slimes are in most cases heavy and, under a gas discharge, the 'anode disrupts com- pletely. Hence ferromanganese is useless as an insoluble anode.

Finally, if any parallel can be drawn between electrochemical passivity and chemical behaviour, it would seem that the stability of ferromanganese under ordinary conditions is not a very deep-seated phenomenon, since the passivity can be so readily destroyed, notably by rise of temperature.

REFERENCES.

Muller, 2. Elektrochem., 1904, 10, 519. Kuessner, 2. Elektrochem., 1910, 16, 754.

3 Grube and Metzger, 2. Elektr. chem., 1923, 3, 17. 4 Campbell, Y.C.S., 1923, 123, 2327. 5 Campbell, Trans. Faraduy SOC., 1926, 63, 46. 6 2. Elektrochem., 1923, 3, 17. 7 Stager, Helv. Chim. Acta, 1920, 3, 584. 8 Kohlschutter and Schodl, ibid., 1922, 5, 490, 593. 9 Allmand and Campbell, Trans. Furaday SOC., 1924, 59, 379.

Department of Chmistry, University of Aberdeen.

Publ

ishe

d on

01

Janu

ary

1926

. Dow

nloa

ded

by N

orth

east

ern

Uni

vers

ity o

n 27

/10/

2014

22:

42:3

6.

View Article Online