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A kinetic study of the dissolution of zinc in aqueous ammonia and ethylenediamine Dilworth, Louis Rivet 1960

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A KINETIC STUDY OF THE DISSOLUTION OF ZINC IN AQUEOUS AMMONIA AND ETHYLENEDIAMINE by LOUIS RIVET DILWORTH A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE i n the Department of MINING AND METALLURGY We accept t h i s t h e s i s as conforming to the standard required from candidates f o r the Degree of Master of Applied Science THE UNIVERSITY OF BRITISH COLUMBIA October, I960 In presenting t h i s t h e s i s i n p a r t i a l f u l f i l m e n t of the requirements f o r an advanced degree at the U n i v e r s i t y of B r i t i s h Columbia, I agree that the L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r reference and study. I f u r t h e r agree that permission f o r extensive copying of t h i s t h e s i s f o r s c h o l a r l y purposes may be granted by the Head of my Department or by h i s r e p r e s e n t a t i v e s . I t i s understood that copying or p u b l i c a t i o n of t h i s t h e s i s f o r f i n a n c i a l gain s h a l l not be allowed without my w r i t t e n permission. Department Of Mining and Metallurgy The U n i v e r s i t y of B r i t i s h Columbia, Vancouver 8, Canada. Date November k, i960  ABSTRACT The kinetics of the dissolution of zinc i n aqueous solutions of ammonia and ethylenediamine under oxygen pressure were investigated at a temperature of 25°C, The variables investigated included free amine con centration, oxygen pressure and s t i r r i n g r a t e 0 Two regions of kinetic control were observed having different dependences on oxygen pressure 0 At low oxygen pressures, the rate of dissolution of zinc i s f i r s t order in oxygen pressure, dependent on s t i r r i n g rate and independent of the free amine concentration 0 At higher oxygen pressures, an adherent oxide layer i s formed on the zinc surface and the rate of dissolution i s f i r s t order with respect to free amine concentration and independent of oxygen pressure. These results indicate that at low oxygen pressures the rate i s controlled by the diffusion of oxygen to the zinc surface? and at high oxygen pressures, the rate of reaction i s chemically controlled at the f i l m - solution interface. - i i ACKNOWLEDGEMENT The author wishes to gr a t e f u l l y acknowledge the assistance and encouragement given by members of the Department of Mining and Metallurgy 0 He i s especially grateful to Mrs* A, M, Armstrong for her capable direction^ for her fund of ideas, and for her constructive c r i t i c i s m offered during the writing of this thesis. The author is grateful to the National Research Council of Canada for f i n a n c i a l assistance which allowed for the completion of this project. TABLE OF CONTENTS o o o o o o o o INTRODUCTION Scope of the Present I n v e s t i g a t i o n , , EXPERIMENTAL P r e p a r a t i o n of the Zinc Samples P r e p a r a t i o n of S o l u t i o n s 0 , 1) D i s s o l u t i o n S o l u t i o n s 2) P o l a r o g r a p h i c S o l u t i o n s 0 0 0 0 0 0 0 0 0 0 Apparatus Experimental Procedure A n a l y t i c a l Procedure, 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 o o o o o O O O O O O O O O RESULTS P r e l i m i n a r y Experiments R e p r o d u c i b i l i t y of Experimental R e s u l t s , E f f e c t of Surface Area <, , „ » , » . , , , Ammonia System E f f e c t of Oxygen Pressure E f f e c t of Ammonia Concentration E f f e c t of Ammonium Ion Concen t r a t i o n E f f e c t of S t i r r i n g Rate 0 « , . , , , Ethylenediamine System, ^ , , » 0 0 0 0 0 0 O D O O O O O O O O O O O O O O O O O O O O O O DISCUSSION CONCLUSIONS n o O O O O O O O O O O O O O O RECOMMENDATIONS FOR FURTHER WORK. APPENDIX I . . . . . . . . . . . BIBLIOGRAPHY. . . . . LIST OF FIGURES NOo Page 1. Schematic diagram of the aut o c l a v e . 0 . 0 . 0 0 0 0 0 0 0 0 7 2o C a l i b r a t i o n curve f o r z i n c i n NH.OH s o l u t i o n s . 0 4 0 0 0 0 0 0 10 3. C a l i b r a t i o n curve f o r z i n c i n en s o l u t i o n s 0 . 0 c 0 0 0 0 11 4o Family of r a t e curves f o r 1.0M NH^OHo . . . 0 . 0 • 0 0 0 0 16 5o Family of rate curves f o r 0.5M NH^OH. . . 0 . . 0 0 0 0 0 0 17 6 0 Family of rate curves f o r 0.3M NH^OHo 0 0 . 0 0 0 0 0 0 0 0 18 7. Family of r a t e curves f o r 0.25M NH^OH . . . 0 0 • " 0 0 0 19 8 0 P l o t of rate versus oxygen pressure f o r NH^OH 0 0 0 0 0 0 0 20 9o Rate curves at constant ammonium c o n c e n t r a t i o n 0 0 0 0 0 0 23 10o Family of r a t e curves at reduced s t i r r i n g r a t e . 0 0 0 0 0 0 24 l l o P l o t of r a t e versus oxygen pressure at reduced s t i r r i n g r a t e 25 12 0 Family of r a t e curves f o r 0.5M en . 0 0 . 0 . 0 0 0 0 0 0 0 27 13 0 Family of rate curves f o r 0.3M e n o o o . o . o 0 0 0 0 0 0 28 14. Family of rate curves f o r 0 o065M en . . 0 . 0 . ° 0 0 0 0 0 29 15. Family of rate curves f o r 0.03M en. . 0 . 0 0 . 0 0 0 0 0 0 30 16. P l o t of rate versus oxygen pressure f o r en. 0 0 0 • 0 0 0 0 31 17. P l o t of rate versus amine c o n c e n t r a t i o n i n region of oxide f O I°I0.£l t X 0X1 O O O O O O O O O O O O O O O O O O O O O O O O O LIST OF TABLES No. Page 1. Formation Constants for Zn and Cu amine complexes , , , , . 3 2. Determination of Reproducibility. . . . . . . . . . . . . . 13 3. Effect of Surface Area on Rate of Dissolution of Zinc . . . 14 4. I n i t i a l Dissolution Rates of Zinc at the Investigated Ammonia Concentrations. . . 0 . . 0 0 0 0 . 0 . 0 . 0 . . . 21 5. Rate of Dissolution of Zinc at Various Ammonia Cone©nt>x*cit>i.onSo o o o o <> o o o « o o <* o o <> o o o o o o o 22 6. Rate of Dissolution of Zinc at a reduced s t i r r i n g rate. . . 26 7. Rate of Dissolution of Zinc at the Investigated Ethylenediamine Concentrations. . a , , , , , , , , , , , , 32 8 . Comparison of Rate and Formation Constants for Zn and Cu in the Ammonia and Ethylenediamine Systems. . . . . . . . . 36 9. Data from Film No. 1529 . . . . . . . . . . . . . . . . . . 39 10, Comparison of ASTM Standard Zn and ZnO with Sample Layer. , 40 A KINETIC STUDY OF THE DISSOLUTION OF ZINC IN AQUEOUS AMMONIA AND ETHYLENEDIAMINE INTRODUCTION The corrosion resistance of zinc is of great industrial Importance* The greatest use of zinc is in the galvanizing industry where zinc coatings are used to protect iron and steel on exposure to the atmosphere, to natural waters and to sea water, (or wherever corrosion resistance is desir ed). As well, much zinc is used in rolled products where resistance to atmospheric corrosion is of prime importance. The protective effect of zinc is twofold; i t affords s a c r i f i c i a l protection to many other metals and a film of corrosion products is formed which limits the eventual rate of the corrosion of zinc. A great deal of work has been done to determine corrosion rates of zinc under exposure to various atmospheric conditions and much empirical 12 3 data has been tabulated. ' ' It is reported that on exposure to the atmosphere, a protective coating of zinc oxide is f i r s t formed. This is converted in time to basic zinc carbonate in indoor atmospheres, and to basic zinc carbonate, sulphate or both in outdoor atmospheres. Both these coatings are very adherent and have no tendency to flake off. The corrosion rates of zinc in aqueous media have also been studied extensively. It has been found that domestic waters are less corrosive 4 to zinc than d i s t i l l e d waters because of the presence of anions such as carbonates and silicates which form zinc salts of low solubilities. Corrosion rates are low i n the pH range of 6 to 12.5 and domestic waters generally f a l l within this range. In acid and a l k a l i solutions s the rate of corrosion i s high in comparison with other metals. Very low coneentra- tions of most of the common acids give rates which render zinc useless under these conditions. The attack of a l k a l i s i s less severe but s t i l l high. Aqueous ammonia i s reported to corrode zinc rather readily with the 5 formation of a soluble complex hydrate of zinc and ammoniao Zinc i s anodic i n aqueous solution to most of the common in d u s t r i a l 5 metals ~~ iron, n i c k e l , lead, copper, aluminum and their various a l l o y s . This galvanic behaviour of zinc has led to i t s use as s a c r i f i c i a l protection for aluminum in chemical equipment, for iron and st e e l in o i l and gas f i e l d s , for brass on ships hu l l s , and for s t e e l p i l i n g s in sea water 0 Although much empirical data i s available on the corrosion of zinc under a variety of conditions, l i t t l e attempt has been made to examine the fundamental mechanisms. A basic d i f f i c u l t y i s the thermodynamic i n s t a b i l i t y of zinc with respect to the decomposition of water. This reaction i s normally prevented by the high hydrogen overvoltage on zinc, but traces of metal impurities having a lower hydrogen overvoltage within the system can cause the addition of an electrochemical reaction to the chemical reaction under investigation. I f such impurities are not excluded 9 corrosion studies i n aqueous media cannot be readily interpreted in terms of basic mechanisms. Recently studies have been carried out within the Department of Mining and Metallurgy on the dissolution of copper in ammoniacal and organic amine solutions^ 9*'*' It was f e l t that a comparative investigation of zinc would be interesting in view of the rather different complexing tendencies of the two metal ions 0 Formation constants for their complexes with ammonia and ethylenediamine are known and are shown in Table 1 0 TABLE I Complex Formation Constant Reference Cu(NH 5) 4 + + 13O32 6 C u ( e n ) 2 + + v 19o60 7 Zn(NH 3) 4 + + 9o46 6 Zn(en)3"*"r 12o09 7 98 99 These show the greater s t a b i l i t y of the copper amine complexes over the corresponding complexes of z i n c 0 Moreover9 since ZnsQ complexes are pre~ ferred over CusO complexes, i t would be expected that the oxidation of zinc i n aqueous ammonia and amine solutions would indicate a greater tendency for formation of oxide or hydroxide films than was observed i n the analogous copper study Q In addition^, there i s considerable interest at the present time in the behaviour of zinc electrodes in the Zn-AggO wet-storage battery c This useful battery i s greatly handicapped by the short l i f e of the batjbery due to corrosion of the zinc electrodes in the a l k a l i battery e l e c t r o l y t e 0 It was thought that a kinetic study of zinc in amine solutions would be preliminary to an extensive kinetic investigation of zinc under the conditions experienced with the Zn-AggO battery Q Scope of the Present Investigation It was the purpose c f this investigation to determine the kinetics of the dissolution of zinc in aqueous ammonia and ethylenediamine 9 a comparable organic amine0 The behaviour of z i n c i n the above s o l u t i o n s has been s t u d i e d i n respect to the e f f e c t of oxygen pressure, c o n c e n t r a t i o n of the amine s t i r r i n g r a t e , presence of ammonium i o n , and the area of the z i n c sample, EXPERIMENTAL Pr e p a r a t i o n of the Zinc Samples The z i n c used i n a l l the recorded experiments was s u p p l i e d by the C o n s o l i d a t e d Mining and Smelting Company of Canada Li m i t e d , They report a t o t a l impurity content of 4 p a r t s per m i l l i o n (2 ppm Cu, 1 ppm of Cd and Pb), I t was s u p p l i e d i n the form of round bar stock, approximately 4 i n c h i n diameter and 8 inches l o n g . The bars were machined i n a l a t h e to remove outer l a y e r i r r e g u l a r i t i e s and cut i n t o c y l i n d e r s i n c h i n diameter and g i n c h i n l e n g t h . Three such c y l i n d e r s were mounted i n l u c i t e i n a m e t a l l o g r a p h i c specimen mounting pre s s . Four such specimens were prepared and used throughout the whole s e r i e s of experiments. In each, 2 the area of z i n c metal exposed to r e a c t i o n was 2,80 cm, , T h i s surface c o n s i s t e d of long d e n d r i t i c c r y s t a l s r a d i a t i n g out from the c e n t r e . No m o d i f i c a t i o n of t h i s c r y s t a l s t r u c t u r e was attempted i n order to assure no impurity contamination of the h i g h - p u r i t y z i n c . Before each d i s s o l u t i o n experiment, the z i n c specimen was p o l i s h e d ^ a 2/0 p o l i s h i n g paper being the f i n a l paper usedo Thi s was f o l l o w e d by 12 e t c h i n g with a 9 to 1 ethanol and n i t r i c a c i d etchant. E t c h i n g times of 3 minutes were r e q u i r e d to remove the outer s t r e s s e d l a y e r s of z i n c Preparation of Solutions 1 0 Dissolution Solutions Solutions of known ammonia concentration were prepared by diluting measured amounts of Nichols C P , Ammonium Hydroxide up to the required 2 l i t r e s , by the addition of d is t i l led water. Sodium perchlorate added to the dissolution solution was supplied by the G, Frederick Smith Chemical Coo This was added in the form of a 100 ml, aliquot of a 2 M G sodium perchlorate solution, Ethylenediamine solutions were prepared similarly using Carbide and Carbon Chemical supplied ethylenediamine (98,7$ en, remainder water). The base concentration of each solution was determined by t i t r a  tion against standard 0,5 N HC1 using methyl red as an indicator with the ammonia solutions and methyl orange with the en solutions. Potential titrations with a glass electrode were made in each case to determine the pH of the end point, which would act as a guide to the choice of indicator, 2, Polarographic Solutions The required ammonium hydroxide and ammonium chloride solutions were prepared from Nichols C P , Ammonium Hydroxide and Nichols C P , Hydro chloric Acid, Gelatin and sodium sulphite solutions^ prepared daily, u t i l i z Baker and Adamson chemicals. In a l l cases d i s t i l l e d water was used to dilute the solutions to the required concentration. Apparatus The series of experiments necessary for this investigation was conducted in an autoclave with a maximum working pressure of 8 atmospheres. - 6 - The autoclave was fabricated from 316 stainless steel. A l l parts that actually entered the solution were made of titanium (see Figure l ) . However, because zinc is strongly anodic to both commercial grade titanium and steel, i t was necessary to coat a l l interior parts of the autoclave with a chemically inert material. An air-drying heresite primer supplied by Industrial Coatings, Vancouver, B.C., was used. This coating had to be renewed approx imately every 4 days to assure a complete and unbroken cover. A polyethelene liner was used to contain the solution. The lucite mounted zinc specimen was held by means of a stainless steel rod screwed into the autoclave cover plate. Agitation of the solution was provided by two four-bladed impellers mounted on a single shaft sweeping out a cylinder 3 inches in diameter. The rotational speed of the impeller throughout the series of experiments was 860 rpm. A second speed of 575 rpm was used for a few experiments as w i l l be indicated later within the thesis. The zinc specimen was mounted such that the corrosion face was tangential to the swept-out cylinder and was spaced l/8 inch from the circumference of this cylinder. Oxygen and nitrogen, supplied by the Canadian Liquid Air Company, were delivered from standard 2000 p'sig bottles through standard two stage pressure regulators. As a further check, a 0-100 psi pressure gauge was set in the delivery line. Temperature control was maintained by means of a heating c o i l immersed in the autoclave solution and connected with a thermostatically controlled water bath. Water at the required temperature was forced through the heating coil by means of a centrifugal pump. A mercury control thermo- regulator set in a well of the autoclave cover was used to maintain control of the temperature of the water bath. The temperature in a l l experiments - 7 H W>>i>)>>>>m>»i>M 6 A. S h a f t B„ Thermoregulator w e l l C o H e a t i n g c o i l D o Sampling Tube E„ Z i n c sample F o I m p e l l e r G 0 A u t o c l a v e L i n e r H o R e l a y I 0 Pump J o Heater F i g u r e lip Schematic diagram of the a u t o c l a v e a n d temperature c o n t r o l s y s t e m 0 was maintained at 25- 0,1 C o Experimental Procedure In a l l experiments an autoclave solution volume of 2 l i t res was used. Measured amounts of ammonium hydroxide or ethylenediamine and sodium perchlorate solution were diluted to this volume and charged into the auto- clave 0 The solution was brought up to temperature9 the zinc specimen was affixed in place and the autoclave cover bolted down. The impellejr was then rotated at its fixed speed, the autoclave was flushed three times with oxygen and the desired oxygen pressure applied. An i n i t i a l 50 ml 0 aliquot was removed from the solution to be used for base concentration and pH determinations, A further 40 ml, sample was withdrawn for zinc determination. Withdrawal of the second sample constituted zero time0 Further 40 ml, samples were withdrawn at suitable time increments during the course of the run, which generally lasted for 90 minutes. Analytical Procedure All analyses for zinc content during the course of dissolution were made using a Sargent Polarograph Model XXI fit ted with a dropping 14 mercury electrode c e l l . Volume of the c e l l was 20 ml, and drop time was constant at 3,5 seconds. The polarographic sample solution for determination of zinc in ammonia solutions consisted ofs . , . i ) 25 mis, IN NH4C1 i i ) 10 mis. IN NH^ OH i i i ) 10 mis, unknown Z n + + sol 'n drawn from 40 ml, sample iv) 2 mis, 0,2$ gelatin (by weight) v) 4 mis, 2M Na9S0 - 9 - Experimentation proved that this was the most suitable polarographic solution. This concentration of gelatin completely removed the very large polarographic 15 wave maxima normally observed with zinc. The use of NagSO^ was found to remove oxygen from the polarographic solution more completely and more rapidly than did the use of nitrogen bubbled through the solution. The resulting solution .was allowed to stand for 15 minutes to allow for complete removal of dissolved oxygen. The supporting electrolyte for determination of zinc in en solutions had to be modified s l ight ly . Use of the above solution gave rise to a three-step polarographic wave for zinc, indicating in a l l probability the step-wise reduction of the triamine complex. After lengthy experimenting, i t was found that the most suitable solution for polarographic use was obtained i f 2N NH.C1 and 2N NH.OH were used in the same proportions as above. 4 4 Separate plots of polarographic wave height versus zinc contained within a 10ml. sample are shown in Figures 2 and 3. RESULTS Preliminary Experiments In order to determine the possible extent of electrochemical decomposition of water, the f i r s t experiments in this investigation were carried out at atmospheric pressure in a glass beaker with agitation by a Teflon covered magnetic s t i r rer . The zinc used was 99.9^ Zh and had been melted under a reducing flame and cast into cylinders in a graphite mold. The resulting cylinders, f inch in diameter, were mounted in bakelite and immersed in the ammoniacal solution (l.ON NH.OH). Rates of dissolution Figure 2 Calibration solutions. curve for determination of zinc in ammonia 11 - Polarographic Wave Height (mm0) F i g u r O o Calibration curve for determination of zinc i n ethylenediamine solutions,, - 12 - were very slow but v a r i e d widely from one run to the nexto The su r f a c e of the z i n c showed e t c h p i t s and p r e f e r e n t i a l c o r r o s i o n at g r a i n boundaries,, It would appear that g a l v a n i c c e l l s were a c t i v e i n the d i s s o l u t i o n process, and as a r e s u l t of melting and c a s t i n g , g r a i n boundaries, through the c o l  l e c t i o n of i m p u r i t i e s , were anodic to the bulk of the g r a i n s . The next experiments were c a r r i e d out under oxygen pressure i n the autoclave u s i n g a t i t a n i u m l i n e r and d i r e c t immersion of the t i t a n i u m and s t a i n l e s s s t e e l p a r t s of the autoclave i n the r e a c t i o n s o l u t i o n . To a v o i d the p r e f e r e n t i a l g r a i n boundary d i s s o l u t i o n , high p u r i t y CM, and S 0 z i n c ( 9 9 , 9 9 9 6 ^ Zn) was used i n the a s - r e c e i v e d c o n d i t i o n since i t was f e l t that any melting process would add to the impurity content or at l e a s t cause l o c a l c o n c e n t r a t i o n of i m p u r i t i e s already present. Studies were c a r r i e d out i n 0 , 5 M and 1 , 0 M NH^OH and again r a t e s were observed which v a r i e d widely from run to run. An examination of the z i n c surface showed that p r e f e r e n t i a l g r a i n boundary d i s s o l u t i o n had almost disappeared but etc h p i t s were common. Although c o r r o s i o n p i t s are not p o s i t i v e evidence of g a l v a n i c c e l l s , i t was considered a d v i s a b l e to minimize the p o s s i b i l i t y of such by e x c l u s i o n of s t a i n l e s s s t e e l and t i t a n i u m s u r f a c e s from the s o l u t i o n , t h e r e f o r e taking f u l l advantage of the high hydrogen overvoltage of z i n c . T h i s was done by covering a l l metal parts i n contact w i t h the s o l u t i o n with a c h e m i c a l l y i n e r t l a y e r , A h e r e s i t e primer was p a i n t e d on a l l the autoclave p a r t s and was found to be completely s a t i s f a c t o r y . To co n t a i n the s o l u t i o n , a g l a s s autoclave l i n e r was f i r s t t r i e d but proved to be too f r a g i l e , A polyethylene 4 l i t r e beaker was found to be s a t i s  f a c t o r y , showing no c o r r o s i o n when used with the ammoniacal s o l u t i o n s or - 13 - physical damage on repeated insertion and removal from the autoclave0 Under these conditions;, pitting and grain-boundary corrosion of the zinc surface were no longer observed and kinetic studies now gave reproducible results. layer was necessary after each run. Pin-hole flaws did develop but their presence could be detected easily at the end of an experimental run by the appearance of the zinc sample which would show many small corrosion pi t s . When breaks in the coating developed, the autoclave parts were repainted and allowed to air-dry for 12 hours. Drying in a warm-air blast was found to leave the coating in a brit t le condition which resulted in breakage at the corners of the impeller. Drying at room temperature was found to be more satisfactory. Reproducibility of Experimental Results concentration, oxygen pressure, s t i rr ing rate, and temperature in order to determine the reproducibility of the experimental technique., The results of these tests are shown in Table II, It should be noted, however, that inspection of the heresite A series of experiments was carried out at constant ammonia TABLE II Determination of Reproducibility Run No, Rate of Dissolution (mgZn/cm, /hr , ) Deviation 1 2 3 4 45ol 47.0 46,1 45.8 46.0 -1,95 +2.17 +0.22 =0.44 Average Conditionss Temperature 25°C 9 s t i r r ing rate 860 rpm., oxygen pressure 5,1 atm.o NELOH lo0M9 NaClO. 0,1M = 14 = Reproducibility for duplicate experiments was considered to be of the order of 4%0 If , during a series, deviations from the experimental curve were greater than the heresite coating on the autoclave was replaced and the experiment repeatedo Effect of Surface Area Another series of tests was carried out in the i n i t i a l stages of this investigation to check on the effect of the surface area of zinc exposed to the reaction solution 0 The results are shown in Table III and demonstrate that surface area has no effect on the rate. In a l l further 2 experiments, the zinc sample had a surface area of 2080 cm0 TABLE III Effect of Surface Area on Rate of Dissolution of Zinc Surface Area Rate of Dissolution Deviation of Zinc (cm.2) (mgZn/cm0 / h r 0 ) % 2.80 6.9 =>lo4 2<,80 7.1 +lo4 2001 7.0 0 2.01 6,9, -1„4 Average 7,0 Conditions? Temperature 25°C, st irring rate 860 rpm0, oxygen pressure 6o4 atm, NH^ OH 0.3M, NaC104 0.1M. This series of tests also served as a further check on the reproducibility of rates. Effect of Oxygen Pressure The effect of oxygen pressure on the rate of dissolution of zinc was investigated i n i t i a l l y in 10OM NH^OH. Over a wide range of Og pressures, 1.7 to 7.1 atm. 9 l inearity of the family of rate curves was good (Figure 4) and when rate was plotted versus oxygen pressure 9 the resulting curve showed a f i r s t order dependence of rate on 0^ pressure (Figure 8 ) 0 Experiments were next carried out in 0,5M NH^ OH over the same range of 0^ pressureso The same rates of dissolution were observed over the range of Og pressures 2 04 to 6 C4 atm, (Figure 5)o Within this range 9 i t could then be concluded that rate of dissolution of zinc was f i r s t order i n oxygen pressure and zero order i n ammonia concentration,, At higher Og pressures, in 0,5M NH^OH, the rate showed a sudden decrease, reaching a constant value of 11 mg Zn/cm0^/hr0 The f i r s t indications of such a rate transition had been noted i n the experimental run at 6 04 atm0 0^ pressure, in which the rate changed abruptly after 40 minutes but s t i l l maintained l i n e a r i t y . On removal from the autoclave, the surface of the zinc sample showed interference colours indicating the presence of a thin coherent f i l m . The surface of the z i n c 9 after runs at 6 U8 and 7,1 atm, 9 showed the presence of an opaque white f i l m , Again 9 as with l o0M NH^ OH solutions, a l l curves were linear, A 0,3M NH^OH solution was next investigated over an 0^ pressure range of 2,4 to 6,4 atm, (Figure 6) P a r a l l e l results were obtained? f i r s t order dependence on 0^ pressure from 2,4 to 4,4 atm, and a sudden decrease in rate at higher pressures. The value of the linear rate at high pressures, 7,0 mg Zn/cm, /hr, indicated that rates in this region had become f i r s t order i n NH^ OH concentration and zero order i n oxygen pressure. The same break occurred i n the curve for an oxygen pressure of 4,4 atm, which corresponds to the transition from one rate-controlled region to the other. CM a o CD > r-i O CO CO 120 100 80 60 40 20 _ 0 O 7.1 atm. • 6.4 " A 5.8 " 7 5.1 « " 10 20 • 3.7 atm. • 3.0 " • 2.4 " T 1.7 " 30 70 80 90 40 50 60 Time (min.) Conditions! Temperature 25°C, s t i r r i n g rate 860 rpm., NH^ OH 1.0M, NaClO^ 0.1M. Figure 4 Rate curves for the dissolution of Zn in 1.0M ammonia solution at various oxygen pressures. O N c T 1 1 1 1 1 i 1 r O 6o4 atm0 v 3»7 atm. D 3 d " O 3o0 " A 4o4 " Time (min.) Conditionss Temperature 25°C, s t i r r i n g rate 860 rpm„, NH^ OH 0o3M9 NaClO^ 0.1M Figure 6 Rate curves for the dissolution of Zn in 0o3M ammonia solution at various oxygen pressures. Figure 7 Rate curves for the d i s s o l u t i o n of Zn i n 0.25M ammonia s o l u t i o n for various oxygen pressures. i i I l I I I 0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 Pressure of Oxygen (atm.) Conditions: Temperature 25°C, s t i r r i n g rate 860 rpm., NaClO^ 0.1M Figure 8 Effect of oxygen pressure on the rate of dissolution of Zn at various ammonia concent rat i ons. = 2 1 ~ In a subsequent experiment 9 the z i n c sample was o x i d i s e d f o r 3 hours at t h i s anmonia c o n c e n t r a t i o n and an O g pressure of 6 . 4 atm 0 The r e s u l t a n t f i l m was i d e n t i f i e d by X-ray d i f f r a c t i o n procedures as Zn0o (Appendix l ) TABLE I ? I n i t i a l D i s s o l u t i o n Rates of Zinc at the. I n v e s t i g a t e d Ammonia ConcentrationSo (mgZn/cmo / h r 0 ) Oxygen Pressure 0 o 2 5 M 0 o 3 0 M 0 o 5 0 M l o 0 M (atm 0) 7 o l l l o 0 6 7 o 5 608 l l o O 6 0 4 7 o 0 5 7 . 0 6 0 o 0 5 o 8 5 0 o 0 5 4 o 0 5 o l 7 o 0 4 6 o 5 4 6 o 0 4 o 4 3 4 o 5 3 9 o 0 4 1 o 0 3 o 7 4 o 7 3 2 o 0 3 0 o 0 3 2 o 0 3 o 0 4 o 7 2 2 o 0 2 4 o 5 2 o 4 1 7 o 8 I80O 1 9 o 0 1 » 7 1 4 o 0 Conditionss Temperature 2 5 C 9 s t i r r i n g r a t e 8 6 0 rpm 0, NaClO^ 0 o l M F u r t h e r experiments were c a r r i e d out i n a 0 . 2 5 M NH^OH s o l u t i o n over a range of 0 ^ p r e s s u r e s 0 (Figure 7)0 These showed the same abrupt t r a n s i t i o n at an O g pressure of 2 . 4 atm Q The r e s u l t s of these experiments are summarized i n Table I V o E f f e c t of Ammonia Concentration As i n d i c a t e d i n Table IV, the range of ammonia i n v e s t i g a t e d was 0 o 2 5 to l o O M NH^ O H o In the low 0^ p a r t i a l pressure r e g i o n , the r a t e s are independent of amine c o n c e n t r a t i o n 0 To f u r t h e r check t h i s , runs were made i n 2 o 0 M and 5 o O M NH^OH s o l u t i o n s at 5,5 atm c 0^ pressure and gave r a t e s of 5 0 o 2 and 4 9 o O mgZn/cm0 / h r 0 In the high oxygen pressure region^ the r a t e s show a f i r s t order dependence on amine c o n c e n t r a t i o n . A s i n g l e run was made 2 i n 0 . 2 7 5 M NH.OH at 3.7 atm. 0o pressure and gave a rate of 5.5 mgZn/cm0 / h r 0 = 2 2 ~ Effect of Ammonium Ion Concentration The N H ^ / N H ^ O H ratio was controlled by the addition of perchloric acid to the ammonia solution. The effect of added N H 4 + ( 0 . 5 M ) was checked over the whole range of ammonia concentrations and gave rate curves (Figure 9) whose i n i t i a l slopes are shown in Table VQ TABLE V Rate of Dissolution of Zinc +at Various^Ammonia Concentrations in the presence of 0.5M N H ^ (mgZn/cm0 / h r « ) Oxygen Pressure 0 . 2 5 M 0 o 4 0 M 0 . 4 3 M 0 o 9 2 M (atmo) 5.1 4 4 . 8 4 4 . 8 4 . 4 3 8 . 4 4 . 0 3 3 . 2 Conditions? Temperature 2 5 ° C , s t irr ing rate 8 6 0 rpm., N H ^ + 0o5Mj, NaClO^ 0 . 1 M The results indicate that the added N H . + does not alter rates within 4 the 0 2 pressure dependent region (ie. O045, 0 o 4 2 j , 0 . 9 2 M NH^). In the case of the 0 . 2 5 M N H , solution, at 5 . 1 atm. 0 ^ without the presence of N H . + ions 9 3 £ 4 oxide formation was notedu The addition of 0 o 5 M N H . + has at this NH_, concen- 4 ? tration prevented the formation of an oxide layer and thus the transition to slow oxide dissolution rates. Effect of Stirring Rate To investigate the possibili ty of transport control, a reduced s t i rr ing rate was used over a range of oxygen pressures in 0 o 3 M NH^OH solutions. Rates were obtained which showed a parallel l inearity but reduced value in the oxygen dependent region compared with rates obtained at the same ammonia concentration but at the usual s t irr ing rate of 8 6 0 rpm0 (Figures 1 0 and l l ) 0 1 : 1 1 f" I i 1 1 r Time (min.) Conditionss Temperature 25°C, s t i r r i n g rate 860 rpm., oxygen pressure 5.1 atm.? 6 NH. 0.5M. Figure 9 Rate curves for the dissolution of Zn at a constant ammonium concentration. t 3 s 1 r 1 1 1 r 60 . O 5.1 atm, • 4c4 " 50 . A 3„7 » \ V 2.4 " a o Time (min.) C o n d i t i o n s ; Temperature 25°C 0 s t i r r i n g r a t e 575 rpm. ? NH OH 0 .3M, NaCIO 0.1M .0 F i g u r e 10 Rate curves f o r the d i s s o l u t i o n of Zn i n 0.3M ammonia s o l u t i o n at v a r i o u s oxygen pressures. 60 _ 50 40 - 30 _ 20 10 0 O Stirring rate 860 rpm o " " 575 " X X 0 l o O 2 o 0 3 o 0 4 o 0 5 c 0 Pressure of Oxygen (atm0) Conditions? Temperature 2 5 C, N^OH 0 o 3 M ? NaClO^ 0 £ 1M 6 c 0 7 o 0 Figure 11 Effect of st irr ing rate on the rate of dissolution of Zn at a constant ammonia concentration*. - 26 - The r a t e i n the region of oxide f i l m s remained unchanged,, TABLE VI 2 Rate of D i s s o l u t i o n of Zinc at a reduced s t i r r i n g r a t e , (mgZn/cm0 /nr.) Oxygen pressure 0.3M NH OH 0o3M NH OH atm 0 575 rpm, 860 rpm Q 5.1 6.7 7.0 4.4 6.7 34.5 3.7 23.5 32,0 2.4 10.0 16,0 Conditionss Temperature 25°C.', NH^OH 0.3M, NaClO^ 0,1M Ethylenediamine System An i n v e s t i g a t i o n of the Zn-ethylenediamine system gave analogous r e s u l t s . I n i t i a l experiments were c a r r i e d out i n 0.5M en and 0,3M en. The r e s u l t i n g r a t e s (Figures 12 and 13) were independent of amine c o n c e n t r a t i o n and showed the same f i r s t order dependence on oxygen p a r t i a l pressure, A much lower en c o n c e n t r a t i o n was used (0,065M en) and the sharp t r a n s i t i o n to lower r a t e s with i n c r e a s i n g 0^ pressure was observed. (Figure 1 4 ) u A f i n a l en c o n c e n t r a t i o n of 0,03M was i n v e s t i g a t e d over a wide range of 0^ pressures (Figure 15) to v e r i f y that the r a t e of d i s s o l u t i o n of Zn d i d , i n f a c t , remain constant i n the "high" pressure region (3.4 to 7,8 atm,). These r e s u l t s are summarized i n Table VII, The l i n e a r i t y of a l l curves was e x c e l l e n t . The p l o t of r a t e versus oxygen pressure (Figure 16) shows that at low pressures of 0^ ( f o r each i n d i v i d u a l en c o n c e n t r a t i o n ) the rate of d i s s o l u t i o n i s f i r s t order i n oxygen pressure and independent of amine c o n c e n t r a t i o n . In the high pressure r e g i o n , the dependence i s reversed, becoming p r o p o r t i o n a l to the amine c o n c e n t r a t i o n and zero order i n oxygen p r e s s u r e . Time (min.) Conditions: Temperature 25°C, s t i r r i n g rate 860 rpm., en 0.50M,, NaClO^ 0.1M Figure 12 Rate curves for the dissolution of Zn in 0.5M ethylenediamine solution at various oxygen pressures. Time (min.) . Conditions: Temperature 25°C, s t i r r i n g rate 860 rpm., en 0.30M, NaClO^ 0.1M Figure 13 Rate curves for the dissolution of Zn in 0.30M ethylenediamine solution at vari oxygen pressures. Time (min.) Conditions: Temperature 25°C, s t i r r i n g rate 860 rpm., en 0.065M NaClO^ 0.1M Figure 14 Rate curves for the dissolution of Zn i n 0.065M ethylenediamine solution at various oxygen pressures. 1 1 1 1 1 1 1 I I Time (min.) Conditions: Temperature 25°C, s t i r r i n g rate 860 rpm., en 0.03M NaClO^ 0.1M Figure 15 Rate curves for the dissolution of Zn in 0.03M ethylenediamine solution at various oxygen pressures. T 1 O 0.50H Bthylenediamine • 0.30K n A 0.055M " V 0.03M " 60 - 0 I c O 2 o O O \ v-A- - 9 - 6 c 0 7 o 0 . 3 o 0 4 o 0 5 o 0 Pressure of Oxygen (atm 0) Conditions? Temperature 2 5°C 9 s t i r r i n g rate 8 6 0 rpm» 9 NaClO^ OdM Figure 1 6 Effect of oxygen pressure on the rate of d i s s o l u t i o n of Zn at various ethyienediamine concentrationSo - 32 - TABLE VII Rate of Dissolution of Zinc at the Investigated Ethylenediamine Concentrations (mgZn/car. /hr.) 0.065M 0.3M 0.5M (atm.) 7.8 8.4 19.0 62.0 63.0 7.1 38.0 6.4 8.4 52.5 52.0 47.0 5.1 8.4 43.5 40.0 37.0 3.7 8.4 30.0 3.4 8.4 3.0 19.5 2.7 19.5 2.4 17.0 17.0 Conditions: Temperature 25°C, s t i r r i n g rate 860 rpm., NaClO^ 0.1M. DISCUSSION Zinc, due to i t s electropositive character, dissolves rapidly in ammonia and ethylenediamine solutions under oxidizing conditions according to equations 1 and 2. Zn + £ 0 2 + 4NH4OH —»- Zn(NH 5) 4 + + + 20H" + 3H20 ( l ) Zn + £0„ + 3en + H . O Z n ( e n ) _ + + + 20H~ (2) The experimental results show that there are two definite regions of rate control. The low oxygen pressure region shows a f i r s t order depen dence on oxygen pressure and zero dependence on concentration of complexing agent. This indicates control of the rate of dissolution of zinc either by transport of oxygen within the solution to the metal surface or by adsorption of oxygen to the surface. That transport of oxygen to the interface i s the correct alternative i s indicated by decreased rates when slower speeds of agitation are used. Such a region has been shown to 18 exist for copper and i s known to occur with iron in alkaline solutions. - 33 - There appears to be a competing reaction operative at oxygen pressures immediately before the transition from one region to the other. This is clearly shown by the levelling off of the fate curves in 0*25M and 0*3M NH^ OH (Figures 6 and 7) and the slight deviation from linearity for the curves of 0.03M and 0*065M en (Figures 14 and 15)* Such a change in rate, approaching a pardbdlic rate relation* is indicative of film formation. In this case i t could indicate partial coverage of the zinc surface by a thin adherent f i lm. At high oxygen pressures, the decreased rates indicate the presence of a passivation film* according to the reactions as given by equations 3 and 4 Zri(NH.) i l + + + 20H" + 3Hfi0 ^ = ZhO + 4NH OH (3) 5 4 d 4 Zn(eh)^+++ 20H^ ^F=^ ZnO * 3en + HgO . ( 4 ) As Well* visual evidence is available as to its presence and positive i d e n t i f i  cation of this film as ZnO has been made by X-fay diffraction analysis of powdered surface scrapings* Formation of the fiim ort the attainment of a c r i t i c a l surface concentration of 0H~ is supported by the results when dissolution was carried olit in ammonia solutions containing ammonium (NH^+) ion. The effect of this acid radical would be to neutralize the 0H~ released as the zinc is oxidized* This was shown by the prevention of film formation in a 0.25M NH^ solution at 5*1 atm. 0^  pressure by the addition of 0*5M NH^ + . Without the presence of NH^+. ions* film formation had been observed for that concen tration of NH^ at the particular Og pressure* The oxide film passivates but does not completely protect the zinc metal beneath since the oxide dissolves slowly in ammonia and ethylehe- diamirie* The fate of dissolution of zinc at this stage is f i rs t order in amine concentration and independent of stirring rate. Hence the rate is now chemically controlled by dissolution of the ZnO, probably at the film- solution interface, A steady-state condition would be reached in which the ZnO film enters solution at the same rate at which i t is being restored. An attempt was made to determine the rate of dissolution of the oxide film under nitrogen pressure, A film was grown, the sample was removed from the autoclave and charged into a fresh ammonia solution under nitrogen pressure, . After removal of the f i r s t zinc sample no further change in zinc concentration was observed, indicating that the oxide had dissolved during the time required for removing and replacing the autoclave cover at which time the solution was not under oxygen pressure, A comparison of rates of oxide dissolution from the kinetic experiments and the time elapsed (approximately 14 minutes) shows that the film could have completely dissolved'during this period of time. Rates of dissolution of zinc in ammonia and ethylenediamine solutions are similar within the region of oxygen transport control, as would be expected,, but much higher for the en solutions in the chemically controlled .region where the concentration of amine is rate controlling. Assuming linear dependence (necessary because of the scarcity of experimental points) rate constants for the two systems have been obtained from Figure 17 and have the values? K l**> =21,6 mg.Zn/cm.2/nro/M/l Wtl3 T K e x p = 294 mg„Zn/cm,2/hr./M/l en T A comparison of formation constants for Zn(NH 3)^ + + and Zn(en)^+ (Table l ) would indicate that the experimentally obtained rate constants are in accord with the thermodynamic equilibrium constants. / 1 1 1 1 1 • ethylenediamine O ammonia _ — / 1 1 1 1 1 0 O o l 0 , 2 0 o 3 0 , 4 0 o 5 Concentration of amine ( m o l e s / l i t r e ) Conditionss Temperature 2 5°C o s t i r r i n g r a t e 8 6 0 rpm 0 Figure 17 P l o t of r a t e of Zn d i s s o l u t i o n versus amine c o n c e n t r a t i o n w i t h i n r e g i o n of oxide formation. - 36 - A comparison with the results obtained in the copper-ammonia and copper-ethylenediamine systems as reported by Milants*^, but for a region that showed no oxide-film formation, demonstrates considerable similarity in the order of magnitude. Rate constants and formation constants are shown for both the Zn and Cu systems in Table VIII. TABLE VIII Comparison of Rate*'and Formation Constants for Zn and Gu in the ammonia ahd ethylenediamine systems. Formation Constant Rate Constant* (Log K) (mg. Metal/cm. /hr./M/l) 21.6 294 61.G 245 * The rate constant in the zinc-amine systems is for dissolution of oxide present as a visible film, while in the copper-amine systems the rate constants are postulated as those for attack of a surface Cu**0 complex. This agreement is most surprising since in neither case are solution transport processes rate controlling. It might be inferred that the rate determining step involved the breaking up of an oxide lattice by the complexing agent. Even under these conditions i t would be anticipated that the rates for copper should be the greater because of .the lower stability of the oxide and the higher formation constant for the soluble complex ion. CONCLUSIONS Zn(NH3)> 9.46 Zh(en)5 12.09 Cu(NH5)^++ 13.32 Cu(en)2 + 19.60 A kinetic study of the zinc-ammonia and zinc-ethylenediamine systems has shown an analogous behaviour for the two systems. At low oxygen pressures, - 37 = transport of 0^ through the solution is rate-limiting and hence, the corrosion rate increases with 0^ pressure. Transport control of the rate has been verified by the results obtained at different speeds of agitation of the reaction solution. Accumulation of hydroxyl ion in the surface region results in the formation of a passivating f i lm. This film has been positively identified as ZnO, Rates of dissolution are decreased in the region of film formation, reaching a constant value which is dependent only on amine concentration. There exists, then, a certain c r i t i c a l oxygen pressure, for each concentra tion of amine, beyond which the reaction is limited by the rate of dissolution of the oxide, RECOMMENDATIONS FOR FURTHER WORK An investigation of these systems has indicated some regions where further amplification is desirable, 1) The effect of ammonium ion concentration should be studied. Those additions reported in the thesis always increased the rate to the region of control by oxygen transport. It is possible, howevern that by use of small increments, some further details of the effect of NH^+ may be obtained, 2) The position and shape of the curve at the transition from trans port control to chemical control, i f studied in a more detailed fashion, may give information about the formation and nature of the oxide f i lm, 3) The addition of OH to the solutions, in the form of NaOH, would corroborate the suggested hydroxyl ion e f f e c t o T h i s , however, should be preceded by a study of the Zn-NaOh system. - 39 - APPENDIX I TABLE IX Line No. Data from Film No. 1529 dX(obs'd) Line No, dS(obs'd) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 100 5 30 20 50 5 20 10 5 5 5 10 10 10 3.334 2.808 2.602 2.464 2.372 2.302 2.085 1.987 1.904 1.814 1.750 1.691 1.627 1.479 15 16 17 18 19 20 21 22 23 24 25 26 27 28 5 5 5 5 10 10 10 5 5 5 5 10 5 5 1.383 1.341 1.330 1.281 1.207 1.159 0.9451 0.9068 0.8582 0.8312 0.8224 Film No. 1529 i s an X-ray d i f f r a c t i o n powder picture of the surface layer formed on a zinc specimen when reacted with a 0.3N NH^ OH solution under an oxygen pressure of 6.4 atmospheres. The thin adherent layer was scraped off mechanically and as a result, some zinc metal was removed with the layer. Determination of the composition of this layer has been made by comparison of the above data with the A.S.T.M. standard X-ray d i f f r a c t i o n data cards for zinc (No. 4-0831) and ZnO (No. 5-0664'). - 4 0 TABLE X Comparison of ASTM Standard Zn and ZnO with sample l a y e r Zinc S t ' d . Sample Layer ZnO St ' d . Sample Layer d2 i / l Line d&\ i / l d£ i / l Line d£ l / l _ Noo X 1 No. 1 2 o 4 7 3 53 4 2 o 4 6 4 5 0 2 . 3 0 8 4 0 6 2 o 3 0 2 2 0 2 o 0 9 1 1 0 0 7 2-o 0 8 5 1 0 0 1 . 6 8 7 2 8 1 2 lo691 1 0 1 . 3 4 2 2 5 1 6 1.341 5 1 . 3 3 2 2 1 1 7 1 . 3 3 0 5 l o 2 3 7 2 l o l 7 2 9 2 3 1 . 1 5 3 8 5 l o l 2 3 6 1 7 l o 0 9 0 1 3 1 . 0 4 5 6 5 O o 9 4 5 4 8 2 4 0 . 9 4 5 1 5 o 9 0 9 3 6 o 9 0 6 4 1 1 2 5 . 9 0 6 8 5 0 8 7 2 2 ' 5 o 8 5 8 9 9 2 6 . 8 5 8 2 1 0 o 8 4 3 7 2 o 8 2 4 5 1 o 8 2 2 5 9 2 . 8 1 6 7 1 2 2 . 8 0 8 6 0 2 . 6 0 2 5 6 3 2 o 6 0 2 4 0 2 . 4 7 6 1 0 0 4 2 o 4 6 4 1 0 0 1.911 2 9 9 1 . 9 0 4 1 0 1 . 6 2 6 4 0 . 1 3 1 . 6 2 7 2 0 1 . 4 7 7 3 5 1 4 1 . 4 7 9 1 0 1 . 4 0 7 6 1 . 3 7 9 2 8 1 5 1 . 3 8 3 1 0 1 . 3 5 9 1 4 1 . 3 0 1 3. 1 . 2 2 5 5 1 . 1 8 1 2 3 l o 0 9 2 9 1 0 1 . 0 6 3 9 3 1 . 0 4 2 2 1 0 1 . 0 1 5 8 5 0 . 9 8 4 8 4 = 9 7 6 4 7 . 9 5 5 5 1 . 9 3 8 2 4 . 9 0 6 9 1 2 2 5 0 . 9 0 6 8 1 0 . 8 8 2 6 6 . 8 6 7 5 1 o 8 3 6 9 6 41 - BIBLIOGRAPHY 1 0 R o and D c D i v 0 , Comincof "The Zinc Industry" ( 1 9 4 8 ) 2 0 S c h i k o r r , G o , and S c h i k o r r , I c | Z 0D 0A 0 1 5 2 ( 1 9 4 4 ) 3o Anderson, E.A« and F u l l e r , M 0 L 0 j Metals and A l l o y s 1 0 ( 1 9 3 9 ) 4 o Burns, R „ M o and Schuh, A 0 E 0 ; " P r o t e c t i v e Coatings f o r Metals" ( 1 9 3 9 ) 5° McKay, R o J 0 and Worthington, R o | " C o r r o s i o n Resistance of Metals and A l l o y s " ( 1 9 3 6 ) 6 0 Bjerrum, J o ; "Metal Ammine Formation i n Aqueous S o l u t i o n " P 0 Haase and Son, Copenhagen ( l 9 4 l ) 7 c C a r l s o n , G . A 0 , McReynolds, J , P . and Verhoek 9 F 0 H 0 5 J . Am» Chem0 Soc 0 6 7 ( 1 9 4 5 ) 1 3 3 4 80 Nyman, C 0 J o , Murbach, E 0W 0 , and M i l l a r d , G 0 B 0 ; <J0 Am, Chem Soc 0 7 7 ( 1 9 5 5 ) 4 1 9 4 9 ° Bjerrum, J 0 , and Anderson, P 0| " K g l 0 Danske Videnskab Selskeb" Mat-fys 0 Medd 2 2 7 ' ' 1 9 4 5 ) (not a v a i l a b l e ) 1 0 o M i l a n t s , HoT 0| Mo S c 0 T h e s i s , Dept„ of Mining and M e t a l l u r g y 9 U n i v e r s i t y of B r i t i s h Columbia ( 1 9 5 8 ) l l o S i r c a r , S „ C 0 | M„So 0 T h e s i s , D e p t o of Mining and Meta l l u r g y , U n i v e r s i t y of B r i t i s h Columbia ( 1 9 5 9 ) 1 2 0 Latimer, W.M„; " O x i d a t i o n P o t e n t i a l s " , P r e n t i c e H a l l I n c D ( 1 9 5 2 ) 1 3 o Metals Handbook Committees "Metals Handbook", The Amc Soc. f o r Metals ( 1 9 4 8 ) 1 4 ° K o l t h o f f , I0M0, and Lingane, J < , J „ | "Polarography" I n t e r s c i e n c e P u b l i s h e r s ^ Inc. ( 1 9 4 6 ) 1 5 ° Meites, L.| "Polarographic Techniques" I n t e r s c i e n c e P u b l i s h e r s , I n C o ( 1 9 5 5 ) 16 0 Evans, W 0R 0? " M e t a l l i c C o r r o s i o n , P a s s i v i t y and P r o t e c t i o n " Edward Ar n o l d and Co. ( 1 9 4 6 ) 1 7 o Halpern ? J 0 ; J . E l e c . Chem0 Soo« 1 0 0 ( 1 9 5 3 ) 4 2 1 1 8 o Halpern, J o and Fisher,, J „ I 0 ; J o E l e c 0 Chem. Soc 0 1 0 3 ( 1 9 5 6 ) 2 8 2 

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