Indian Journal of Chemical TechnologyVol. 4, March 1997, pp. 101-109

Effect of some anions on corrosion behaviour of 1060,1100 and 5052aluminium alloys in orthophosphoric acid

R S Dubey", R Sagar Dubey, Y D Upadhya &. S N UpadhyayDepartment of Chemical Engineering and Technology, Institute of Technology, Banaras Hindu University,

Varanasi 221 005,'India

Received 26 August 1996; accepted 26 November 1996

The effect of chromate, citrate, thiocyanate, tartarate, oxalate and ferrocyanide ions on the corro-sion behaviour of 1060, 1100 and 5052 aluminium alloys in 1% orthophosphoric acid has been studiedby weight loss technique at 30 and 40 ± OSC, whereas corrosion behaviour of 1100 aluminium alloy inthe presence of above anions was studied by potentiostatic polarisation technique. It was observed thatthese anions play a decisive role in determining the ease with which the above alloys undergo dissolu-tion in orthophosphoric acid. Chromates, oxalates and ferrocyanides worked as corrosion inhibitors,whereas citrates, tartarates and thiocyanates accelerated the corrosion reaction at all the concentrations(0.01-0.2%) investigated. The stability of the surface film formed in the presence of these anions wereclosely related to the stability constant of metal-chelate complexes.

The presence of anions in aqueous solutions playsan important role in determining the corrosion be-haviour of aluminium and its alloys. An anion mayact either as a corrosion inhibitor or as an acceler-ator. To study the inhibition effect on corrosion ofaluminium alloys by these aggressive anions, theimportant points are the steps by which these an-ions act on the aluminium. surface'. These stepsinvolve the adsorption of anions on the aluminiumoxide surface, complexing of aluminium cations inoxide lattice with anions present in the electrolyte,thinning of protective oxide film by the dissolutionof soluble corrosion products and direct reactionof aluminium with the electrolyte at sufficientlythinned sites.

In the case of orthophosphoric acid, the anionsmay compete for adsorption sites and retard theformation of soluble aluminium hydrogen phosph-ate or they can compete with POl- to prevent theformation of soluble phosphate species. But in thelatter case, it must be recognised that if an inor-ganic anion forms a stable soluble complex ionwith the aluminium cation, the dissolution willproceed just as it could otherwise proceed withthe formation of aluminium hydrogen phosphate.

In the case of aluminium corrosion, the electro-lyte containing the inhibitor will be in direct con-tact with its oxide film. Therefore, the possible.corrosioin mechanism must include the interactionof the anion (or inhibitor) with aluminium cationin the oxide film. Chelating agents are among one•Author to whom correspondence should be addressed

of the interesting examples of corrosion inhibitors.These are ions or molecules with the two or moreatoms containing an unshared pair of electrons.They donate the lone pair of electrons to the ca-tion to form a stable five or six membered ring, asa result of which metal atom is held in a stableconfiguration. The complexes neither exhibits theproperties of the metal atom nor those of the che-lating agent. These chelating agents can react 'withaluminium cations in oxide film". The stability ofth complex formed depends upon the nature ofthe chelating agents.

The objective of the present work was to inves-tigate the effect of some chelating agents on thedissolution behaviour of aluminium oxide filmformed on the aluminium surface. To fulfil this ob-jective, some organic and inorganic chelatingagents such as potassium sulphate, potassium thi-ocyanate, potassium persulphate, potassiumchromate, potassium citrate, potassium tartarate,potassium oxalate and potassium ferrocyanidewere used as corrosion inhibitors.

Experimental ProcedureOrthophosphoric acid, potassium salts of the

above mentioned anions (AR grade) and tripledistilled water were used throughout the experi-ments. All the weight loss and potentiostatic po-larisation experiments in the present work werecarried out in 1% orthophosphoric acid solution.The potentiostat used in this experiment wasWenking model POS-73. Polarisation experiments

102 INDIAN J. CHEM. TECHNOL., MARCH 1997

Table I-Compositions of aluminium alloys (wt%)

Alloy Si Fe Mn Mg Cu1060 0.12 0.02 0.041100 0.13 0.32 0.07 0.02 0.015052 0.17 0.04 2.30

AIRemainderRemainderRemainder

were carried out in a 250 mL pyrex glass cell con-taining an auxiliary platinum electrode and a satu-rated calomel reference electrode. A flag shapedpiece of aluminium with the stem coated with lac-quer to have a working area of 2 ern? was used asthe working electrode. The reference electrodewas connected with the help of a Luggin capillarywhich was placed close to the working electrode.The distance between the capillary tip and theworking electrode was kept constant throughoutthe experiment. Open circuit potential (OCP) wasrecorded after 30 min of exposure of aluminiumalloy samples to the test solution. During poten-tiostatic polarisation experiments, the potentialwas changed in an instalment of 50 mV after re-cording the steady state current density. The com-positions of the aluminium alloys used in thisstudy are given in Table 1. All the samples werepolished with emery paper of grades 1/0 to 4/0.The specimens were then cleaned and degreasedin acetone. Experiments were carried out in elec-tronically regulated air thermostat maintained at30 ± OSC and 40 ± OSc.

Results and DiscussionWeight loss studies-Corrosion data obtained

from immersion tests for 1060, 1100, and 5052 al-uminium alloys at 30°C are given in Tables 2-4.Influence of these anions on above aluminium al-loys have been determined at various concentr-ations of the inhibitors ranging from 0.01 to 0.2%.It is clear from these tables that potassium chrom-ate, potassium oxalate and potassium ferrocyanideworked as corrosion inhibitors, whereas potassiumthiocyanate, potassium citrate and potassium tar-tarate accelerated the corrosion reaction at all theconcentrations investigated.

The. corrosion behaviour of 1060, 1100 and5052 aluminium alloys in orthophosphoric acid atdifferent concentrations and the mechanism of dis-solution of passive film on aluminium alloy surfacehave been reported by Dubey? in recent studies.The corrosion resistance of UOO aluminium alloywas found to be highest followed by 1060 and5052 aluminium alloys. Therefore, 1100 alumini-um was selected to investigating the effect of var-ious aggressive anions in the presence of ortho-phosphoric acid by electrochemical polarisationtechnique. The mechanism of inhibition and accel-

eration of corrosion by the above mentioned an-ions can be explained in view of their interactionwith aluminium oxide film. The oxide film formedadjacent to the aluminium surface is a thin com-pact barrier film"!', whereas a thicker morepermeable layer is also present in the bulk of thefilm 1• In the presence of chromates, oxalates andferrocyanides the corrosion resistance may be at-tributed to the incorporation of these anions tothe continuous oxide layer next to the metal. Themagnitude of the corrosion rate is expected to de-pend on the solubility and composition of the bulkfilm".

Effect of potassium chromate-The inhibition ef-ficiency of potassium chromate towards 1060,1100 and 5052 aluminium alloys is given in Tables2-4. It is clear from the results incorporated inthese tables that potassium chromate acts as agood corrosion inhibitor for these aluminium al-loys in 1% orthophosphoric acid solution. In thepresence of of chromate ions the open curcuitpotential (OCP), shifted in noble direction.

Anodic and cathodic polarization curves for1100 aluminium alloy in 1% orthophosphoric acidat various concentrations of chromate are shownin Fig. 1, whereas different electrochemical par-ameters are listed in Table 5. It is obvious fromthis figure that anodic polarization curves shiftedtowards the lower current density region with in-crease in concentration of potassium chromate.However, the nature of the curves remains almostthe same. It is also c1ear that anodic polarisationcurves are slightly polarised in the presence of po-tassium chromate. Both anodic and cathodic partsof the polarisation curves shift in the same direc-tion with increase in the concentration of chrom-ate ions.

It has been reported by Kravchenko et al.13,that chromate is more effective as an inhibitorthan molybdate, permanganate, ferrocyanide andnitrate. The mechanism of the inhibition of alumi-nium corrosion by chromate has been studied bymany workers'vP, It has been found that chrom-ate acts as an oxidizer in the inhibition of alumini-um corrosion. The Cr203 gets incorporated intothe passive nlm of aluminium. This is responsiblefor a good inhibition of aluminium by chromates.

Effect of potassium thiocyanate-Potassium thi-ocyanate is usually regarded as an activator ofcorrosion of iron" by activating the dissolution ofpassive film. Thiocyanate ion is known to be agood complexing agent for iron ion'". '{he anodicpolarization curves of 1100 aluminium alloy 1%.orthophosphoric acid and at various concentr-ations of thiocyanate ions are given in Fig. 2. Shift

DUBEY et al: CORROSION BEHAVIOUR OF SOME AWMINIUM AUDYS 103

Table 2-Percentage inhibition efJiciency of 1060 aluminum Table 3-Percentage inhibition of 1100 aluminium alloy in thealloy in the presence of different concentrations of anions at presence of different concentrations of anions at 30 and 40

30 and 40 ± OSC ±OSCExposure period = 48 h Exposure period - 48 h

Inhibitor and 30·C 4O·C Inhibitor and 30·C 40·Cpercentage percentage

concentration Wt.loss Percentage Wt.loss Percentage concentration Wt.loss Percentage Wt.loss Percentagemg efficiency mg efficiency

mg efficiency mg efficiencyPlain solution 125 249 E E

Potassium Plain solution 147 6 292chromate Potassiumom 100 21 186 25 chromate

0.02 79 38 144 42 om 121 18 216 260.05 57 55 107 57 0.02 105 28 193 340.10 48 62 82 67 0.05 101 31 180 380.15 41 68 70 72 0.10 80 45 129 560.20 31 76 47 81 '0.15 64 56 105 64

Potassium 0.20 51 65 85 71Thiocyanate Potassium

0.01 134 -5 266 -7 thiocyanate0.02 138 -9 278 -12 om 160 -9 327 -120.05 142 -12 286 -15 0.02 169 -15 344 -180.10 ISO -18 301 -21 0.05 173 -18 353 -210.15 153 -21 317 -25 0.10 185 -26 385 -320.20 160 -26 328 -32 0.15 194 -32 400 -37

Potassium 0.20 202 -38 420 -44citrate Potassiumom 135 -7 269 -8 citrate0.02 144 -13 291 -17 0.01 164 -12 338 -16

0.05 ISO . -18 296 -19 0.02 173 -18 362 - 240.10 154 -21 311 -25 0.05 185 -22 376 -29

0.15 166 -31 336 -35 0.10 188 -28 402 -38

0.20 171 -35 343.6 -38 0.15 199 -36 411 -41

Potassium 0.20 208 -42 429 -47

tartarate Potassiumtartarate

0.01 132 -4 261 -5 0.01 154 -5 315 -80.02 135 -7 274 -9

0.02 187 -7 321 -100.05 138 -~ 281 -13 0.05 158 -8 327 -120.10 142 -12 296 -19 0.10 166 -13 338 -160.15 147 -16 301 -19 0.15 173 -80 356 -220.20 149 -18 311 -25 0.20 177 -21 374 -28

PotassiumPotassium

oxalate oxalate0.01 104 18 189 24 0.01 169 15 231 210.02 95 25 172 31 0.02 177 21 213 270.05 79 38 137 45 0.05 202 38 177 410.10 62 51 102 59 0.10 217 48 140 520.15 46 63 82 67 0.15 229 56 123 51!0.20 41 68 69 72 0.20 242 65 98 71Potassium Potassium

ferrocyarude ferrocyanideom 121 5 231 7 om 154 5 269 80.02 116 8 221 l\ 0.02 163 II 248 150.05 104 18 196 21 0.05 167 14 242 170.10 Hhl 21 184 26 0.10 17.1 18 .~22 240.15 ':I!. 27 171 11 O.I~ 1M2 24 :'10 28u ,!.{'; -.J"~ 35 157 J' 02( }'H 32 17I 41

.. -'---"-'-' -.- --"'--'-'-~-.'..- _ ....-.-- ..-. _._--_._----_ ..-._---------_.-.-

104 INDIAN J. CHEM. TECHNOL., MARCH 1997

Table 4-Percentage inhibition efficiency of 5052 aluminiumalloy in the presence of different concentrations of anions at

30 and 40 ±OSCExposure period ~ 48 h

Inhibitorpercentage

concentration

30°C

Wt. loss Percentagemg efficiencyE

160Plain solutionPotassiumchromate

om 137.60.02 126.40.05 116.80.10 92.80.15 78.40.20 67.2

Potassiumthiocyanate

om 177.60.02 188.80.05 198.40.10 209.60.15 2160.20 227

Potassium citrateom 1840.02 193.60.05 206.40.10 216.00.15 225.60.20 236.8

Potassium tartarate0.01 171.20.Q2 179.20.05 142.40.10 148.00.15 190.40.20 198.4

Potassium oxalate0.01 140.80.Q2 118.4

0.05 105.60.10 89.60.15 76.80.20 67.2

Potassiumferrocyanide

0.01 153.60.Q2 146.60.05 1400.10 1200.20 115.2

42127425158

-11-18-24-31-35-42

-15-21-29-35-41-48

-7-12-14-15-19-24

40°C

Wt.loss Percentagemg efficiency

370

292.3266.4151.6196.1162.8155.4

432466496537562596

432.9462.5492.1514.3540.2569.8

403.3425.5436.6447.7463.0488.0

122634445258

311252

218.3196155

129.5

212832475665

-17-26-34-45-52-61

-17-25-33-39-46-54

-9-15-18-21-25-32

163241475865

712173238

Table 5-Electrochemical parameters of corrosion of 1100aluminium alloy in 1% H3P04 with different anions

Inhibitor Ecorr Icorr2 IE(0.2%), w/v mV/SCE mAlcm2 %1% orthophosphoric acid - 720 0.7Pot. chromate - 710 0.13Pot. thiocyanate -740 0.58Pot. citrate -750 0.95Pot. tartarate -755 0.92Pot. oxalate - 720 0.25Pot. ferrocyanide -710 0.20

28.57-21.43-35.7-31.4

64.328.6

of the anodic polarisation curves towards highercurrent density region with increase in concentra-tion of potassium. thiocyanate suggest the activa-tion of corrosion reaction in the presence of thioc-yanate ions The activating behaviour of thiocyan-ate ions may be attributed to the strong ligandfield around the metal ions. Complexing of thiocy-anate ion with AP + resulted in the extraction ofaluminium ion from the Al203 film. The activedissolution of aluminium from the surface oxidefilm in presence of thiocyanate ion is clear fromthe weight loss data incorporated in Tables 2-4.The complex compounds formed during the reac-tion with aluminium are very stable in solution.

Effect of potassium citrate and potassium tartar-ate-Potassium citrate and potassium tartarate aresalts of hydroxy carboxylate acids. The inhibitionefficiencies of these inhibitors at various concentr-ations are given in Tables 2-4. Anodic polarizationcurves of 1100 aluminium alloy in 1% orthophos-phoric acid with different concentrations of pota-ssium citrate and pot. tartarate are plotted inFigs 3 and 4, respectively. Both potassium citrateand potassium tartarate are chelating agents. It isclear from the anodic polarization curves thatthese ions accelerate the corrosion of aluminiumat all the concentrations investigated. Althoughthere is only a slight shift in OCP towards nega-tive direction, the loss in weight ,pbserved in theirpresence is of considerable extent. Bryan" carriedout a series of investigations on the corrosion ofaluminium alloys in citric and tartaric acids. Hefound that the solubility of AI(OH)3 in these acidswas more than double of that needed to combinewith the hydrogen of the - COOH groups. It wasfurther observed that the rate of corrosion of alu-minium increased by decreasing the concentrationof citric acid. This was unaffected by the additionof sodium citrate, but the presence of NaCI in-creased the rate of corrosion and evolution of hy-drogen with increasing acid concentration.

Katoh 16 reported that pure aluminium did notcorrode in 1M citric acid solution because of the

49122528

344325307

251.6229.4

DUBEY- et al: CORROSION BEHAVIOUR OF SOME AWMINIUM AlLOYS

2~1.-~------------------------~ __ ~~-n--~~~------ _

1500

t! 1000

E

500!!C•.~0-

0•.'C

~u•.iij-soo

-1000

V 1v. H)P04 ptaino 0'01'/, Pot.chromate + 1·0'l.H3~A 0'02'1,D 0-05'1.• 0'10'1.• 0'15'/.• 0·20'/.

-IS00,L

O,.----'--------'--'--'---'-..LJ.. ..•..~.L,o2.----....L..-----''--'--'---'-..LJ...L~.L,CJ3.----.;a...--~~-'-·..u,04

Current density, ]!A /,,,,2

Fig. I-Anodic and cathodic polarization curves for 1100 alu-minium alloy in 1.0% H)PO. + potassium chromate

2000r-----------------------------------------~~~~~~

c•.~0.~~u.!

W-500~-~~

-'OOO~

1500

~ '000~>E

.2 SOO

o

~ '·O'l.H)POt, Plaino 0'01'1. Pot· thiocyanate + '·O'l.H3PO,I::i. 0-02-'. "D 0-05'1.• 0'10'1,• 0·1S'I.• 0·20'1,

Fig. 2-Anodic and cathodic polarization curves for 1100 alu-minium alloy in 1.0% H)PO. + potassium thiocyanate

whereo OH 0II I II

L= -C-C-CH2-C-CH2-C-0-ICOO

o 0- 0II I II

HiL= -O-C-CH2-C-CH2-C-O-ICOO-

protective oxide film. Addition of fluoride ions,however, increased the corrosion rate. The stabil-ity constants of citrate ions formed due to com-plexation are given below";

Reaction Temp. Ionic Log Stab.DC Strength Constant

Al3+ + H)L= AIL + 3H+ 33 (0.25 NaCIO.) 4.7Al(HiLt + H" = AlL 33 (0.25 NaCI04) 3.5

Al(OHXH - IL)- - + H"=AI(HiL) 33 (0.15 NaCI04) 6.8

105

106 INDIAN J. CHEM. TECHNOL., MARCH 1997

2~r----------------------------------------9~o.~~-,

'1.• 0-10'1.• 01!>'1.• 0-20'1.

•••;;:1000>eE 500c•~0.

•..,eu.!•••-500

o

Fig. 3-Anodic and cathodic polarization curves for 1100 alu-minium alloy in l.()'}'oHjPO. + potassium citrate

2oo0r-----------------------------------------~~_C~hr __

1500

E

a 500 -

C.& 0.'0eu

;;,-500

-1~

Fig. 4-Anodic and cathodic polarization curves for 1100 alu-minium alloy in 1.0% H)PO. + potassium tartarate

It is obvious from the above data that the solublechelates accelerate the dissolution of aluminium.Some of the important equations suggested by Ka-toh 16 in his discussion on corrosion of aluminiumin citrate solutions areAI- AP+ + 3e-

AP + +Cit" - - AICit-

0.059 3+E= -1.66+-3-log(AI ) ... (1)

AlCit+ +OH- -(OH) AlCitl-,

K _[(OH) AICit2-1_ 6_62 - [AICit IOHl -10

... (2)

... (3)

DUBEY et al: CORROSION BEHAVIOUR OF SOME AWMINIUM AlLOYS 107

2~r-------------------------------~~~--~~-------'1500 .

108 INDIAN J. CHEM. TECHNOL., MARCH 1997

AI

~

..... I..... : I:... I. .... I

...::::. IOxi ••• I.::..... I.. ;:"-:: :

F.1d o~ni"!lltra~ I. ;:' ..:; I ,#,S

-: .." :.: ::.:~:. Solution

AI

AI

:::".:.::':'.:.: Pllalj)/lorlc acid

.... :: ........:.:

(e ) Ib'

Fig. 7-Schematic representation of the AI-solution interfacewithgalvanic potential distribution in H3P04 (a) at the open

circuit potential, (b) during the anodization

current decreases considerably in the presence ofthese anions. The same result is also observedfrom weight loss data given in Tables 2-4. Thusoxalate ions behave as a corrosion inhibitor forthis metal-electrolyte combination. It has been re-ported in literature-P-" that oxalic acid and tartaricacid are corrosive to aluminium and the rate ofcorrosion is a function of pH. But in the presenceof potassium oxalate in H3P04 solutions, a thinfilm is formed which is visible even to naked eye.Oxalate ions shift the OCP to the noble direction.Consequently, a decrease in corrosion current isobserved at various concentrations of the pota-ssium oxalate. The protective nature of potassiumoxalate may be due to a precipitation type of inhi-bition. The possible reaction of AP + with oxalateare:

Reaction Log. Stability Constant(Ionic Strength 1M)

AP+ + C20~- -+ AIC20: 6.06AP+ + 2C20~- -+ AI(C204)i 11.09

15.12

From the above results it is clear that anionsplay a decisive role in determining the ease withwhich aluminium undergoes dissolution upon ap-plication of some anodic current density.

For such a system, a model based on anionadsorption can be proposed which explains theactivating role of anions. In the absence of anyadsorption, the cross section of the interface canbe schematically represented as shown in fig. 7.At OCP (Fig. 7), the surface of the metal is nega-tively charged and hence an electric field is

operating across the oxide film opposing transportof either the aluminium ions towards the electro-lyte or oxygen containing anions towards the me-tal. The field stimulates only the transport of theelectrons causing reduction of H+ ions and evolu-tion of hydrogen at the oxide surface. A reversalof the direction of the fields takes place byimposing a suitable voltage from an externalsource to enable the ionization of aluminium andfurther build the oxide layer (Fig. 7a) .

An entirely different situation may be createdby adsorption of negatively charged species at theoxide/electrolyte interface as shown in Fig, 7a. Ifthe concentration of negative charge becomeshigher on the side of the oxide layer than on themetal side, the direction of the field is reversedand it assists dissolution, i.e., transport of the alu-minium ion, towards the electrolyte and the oxy-gen ions in the reverse direction. The mechanismalso holds true for a lower valency state of AI(e.g., AI +) which could pass through the film andreduce water at the oxide/ electrolyte interface.This constitutes an additional mechanism for hy-drogen evolution.

The proposed model also allows for differencesin the effects of various anions. It is sufficient toassume different adsorbability (energies of adsorp-tion) between the anions and oxides. That wouldimply different potentials at which significant ad-sorption takes place and hence sudden activationof anodic dissolution.

ConclusionChromate, oxalate and ferrocyanides were

found to inhibit the corrosion of 1060, 1100 and5052 aluminium alloys, whereas citrate, tartarateand thiocyanate accelerated the corrosion reaction.Inhibition effeciency increases with the inhibitorconcentration and temperature of the test solution.1060 aluminium alloy possess better corrosion re-sistance than 1100 and 5052 aluminium in thepresence of these anions.

AcknowledgementThe authors thank to M/ s HINDALCO,

Renukoot, Mirzapur for providing the samples ofaluminium alloys. One of the author (RSD) isthankful to Banaras Hindu University for financialsupport.

References1 Nguyen T H & FoleyR T, J Electrochem Soc, 126 (1979)

1855.2 Takahashi H, Fujimoto K & Nagagama M J, J Electrochem

Soc, 135 (1988) 1343.3 Dubey RS, Bull Electrochem; 12 (1996) 467.

DUBEY et al.: CORROSION BEHAVIOUR OF SOME ALUMINIUM AlLOYS 109

4 Chiu R L & Chang P H, J Electrochem Soc, 143 (3)(1996) 984.

5 Kobayashi K & Shimizu K, in Aluminium Surface Treat-ment Technology, edited by Alwitt R S & Thompson G E,

Electrochem Soc Proc Ser, (Pennington, NJ), PV 86-111986.

6 ChiuRL,ChangP H& TungCH,ibid, 42(1995) 525.7 Sato S, Ichinose & Masuko N, J Electrochem SQC, 138

(1995) 3705.8 Lobnig R E, Siconolfi & Sinclair J D, J Electrochem Soc.

143(4) (1996) 1175.9 Heine M A & Pryor M J, J Electrochem Soc, 114 (1964)

1001.10 Heine M A, Keir D S & Pryor M J, J Electrochem Soc,

112(1965)24.11 Vedder W 8r Vermilyea D A, Trans Faraday Soc, 65

(1969) 561.12 Kobayashi M & Niioka Y, Corros Sci; 31 (1990) 237.

13 Kravchenko T A, Shatalov A Ya & Yanchuk E K, IzvVyssh Uchebn Zaved Khim Khim Tekhhol; 7 (1964) 56-60.

14 OguraK & Sato K, Corros Sci; 19 (1976) 734-741.15 Wood G C, Skeldon P, Thompson G E & Simizu K, J

Electrochem Soc; 143 1 (1996) 74.16 Katoh M, Corros Sci, 8 (1968) 423-431.17 Sillen L G, Martel A E, Hogfeldt E & Smith R M, Stabil-

ity Constants SuppSp, Pub 25, The Chemical Society,London, 1971.

18 Bryan J M, Dept Sci Ind Research Rept Food Invest Board,1935 (1936) 170-80.

19 Lorking K F, Australian Defence Scientific Service, Aeron-autical Research Laboratory, Metallurgy Note, 61 (Feb1969).

20 Lichienberg H, Aluminium, 19 (1937) 504.21 Mrars B B & Eldriodge G G, Trans Electrochem Soc, 83

(1943) 15, ChemAbstr,37(1943)4342.