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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 t h e Department of MINING AND METALLURGY  We a c c e p t t h i s t h e s i s as conforming t o t h e s t a n d a r d r e q u i r e d from c a n d i d a t e s f o r the Degree o f Master o f A p p l i e d  Science  THE UNIVERSITY OF BRITISH COLUMBIA October,  I960  In p r e s e n t i n g  t h i s thesis i n p a r t i a l f u l f i l m e n t of  the r e q u i r e m e n t s f o r an advanced degree a t the U n i v e r s i t y o f B r i t i s h Columbia, I agree t h a t the L i b r a r y s h a l l make i t freely  a v a i l a b l e f o r r e f e r e n c e and s t u d y .  agree t h a t p e r m i s s i o n f o r e x t e n s i v e  I further  copying of t h i s  thesis  f o r s c h o l a r l y purposes may be g r a n t e d by t h e Head o f my Department o r by h i s r e p r e s e n t a t i v e s .  I t i s understood  t h a t c o p y i n g or p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l not be a l l o w e d w i t h o u t my w r i t t e n  Department  Of  M i n i n g and M e t a l l u r g y  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  permission.  ABSTRACT  The k i n e t i c s of the d i s s o l u t i o n of zinc i n aqueous s o l u t i o n s of ammonia and ethylenediamine under oxygen pressure were i n v e s t i g a t e d at a temperature of 25°C,  The v a r i a b l e s  investigated  c e n t r a t i o n , oxygen pressure and s t i r r i n g  rate  included  f r e e amine con-  0  Two regions of k i n e t i c c o n t r o l were observed having d i f f e r e n t dependences on oxygen p r e s s u r e d i s s o l u t i o n of zinc i s f i r s t  0  At low oxygen pressures, the rate of  order i n oxygen pressure, dependent on s t i r r i n g  rate and independent of the free amine c o n c e n t r a t i o n  0  At higher oxygen  pressures, an adherent oxide l a y e r i s formed on the zinc surface rate of d i s s o l u t i o n i s f i r s t and  order with respect  to free amine  and the  concentration  independent of oxygen pressure.  These r e s u l t s i n d i c a t e that at low oxygen pressures the rate i s c o n t r o l l e d by the d i f f u s i o n of oxygen to the z i n c surface? oxygen pressures, the rate of r e a c t i o n i s chemically solution interface.  and at high  c o n t r o l l e d at the f i l m -  -i i  ACKNOWLEDGEMENT  The author wishes to g r a t e f u l l y acknowledge the a s s i s t a n c e and encouragement given by members of the Department of Mining and M e t a l l u r g y  0  He i s e s p e c i a l l y g r a t e f u l to Mrs* A, M, Armstrong f o r her capable d i r e c t i o n ^ f o r her fund of ideas, and f o r her c o n s t r u c t i v e c r i t i c i s m o f f e r e d during the w r i t i n g of t h i s t h e s i s . The author i s g r a t e f u l to the N a t i o n a l Research C o u n c i l of Canada f o r f i n a n c i a l a s s i s t a n c e which allowed  f o r the completion of t h i s p r o j e c t .  TABLE OF CONTENTS  INTRODUCTION  o  o  o  o  o  o  o  o  Scope o f t h e P r e s e n t  Investigation, ,  EXPERIMENTAL Preparation  o f the Z i n c  Samples  Preparation  of Solutions  ,  0  0  0  0  1)  Dissolution Solutions  2)  Polarographic Solutions  Apparatus  0  0  Experimental Analytical  0  0  0  0  0  0  0  Procedure  0  Procedure,  0  0  o  0  0  o  0  0  0  0  0  o  0  0  0  0  0  0  0  o  0  0  0  0  o  RESULTS Preliminary  Experiments  Reproducibility Effect  O  O  O  O  of Experimental  O  O  O  O  O  Results  ,  o f S u r f a c e A r e a <, , „ » , » . , , ,  Ammonia  System  Effect  o f Oxygen  Effect  o f Ammonia  Effect  o f Ammonium I o n C o n c e n t r a t i o n  Effect  of Stirring  Ethylenediamine  DISCUSSION CONCLUSIONS  O  O  n  O  o  O  O  Pressure Concentration  Rate  O  O  O  O  O  O  O  O  O  O  O  »O  O  O  O  O  0  0  0  0  0  « , . , , ,  0  System, ^ , ,  O  0  O  D  O  O  O  O  O  O  O  O  O  O  O  O  O  O  RECOMMENDATIONS FOR FURTHER WORK. APPENDIX I BIBLIOGRAPHY.  . . . . . . . . . . . . . . .  LIST  OF FIGURES Page  NOo 0  0  0  0  0  0  0  7  solutions.0  0  0  0  0  0  0  10  c  0  0  0  0  11  f o r 1.0M NH^OHo . . . 0 .  0  •  0  0  0  0  16  o f rate curves  f o r 0.5M NH^OH. . .  0  0  0  0  0  0  17  Family  o f rate curves  f o r 0.3M  0  0  0  0  0  0  18  7.  Family  of rate curves  f o r 0.25M NH^OH . . . 0 0  •  "  0  0  0  19  8 0  Plot  o f rate versus  0  0  0  0  0  0  20  9o  Rate  curves  0  0  0  0  0  0  23  0  0  0  0  0  0  24  1.  Schematic  2o  Calibration  3.  Calibration  4o  Family  of r a t e curves  5o  Family  6 0  diagram  of the autoclave.  curve  for zinc  0  i n NH.OH  .  0  .  0  4 curve  f o r zinc  i n en s o l u t i o n s  NH^OHo  oxygen p r e s s u r e  at constant  of r a t e curves  0  llo  Plot  12  0  Family  o f r a t e curves  f o r 0.5M en .  13  0  Family  of rate curves  f o r 0.3M  14.  Family  of rate curves  f o r 0 0 6 5 M en . .  15.  Family  o f rate curves  f o r 0.03M e n .  16.  Plot  o f rate versus  17.  Plot  o f rate versus O  O  O  O  0  .  0  0  0  ammonium c o n c e n t r a t i o n  Family  f O I°I0.£l t X 0X1  . .  0  f o r NH^OH  10o  o f r a t e versus  .0  0  a t reduced  stirring  rate.  a t reduced s t i r r i n g  oxygen p r e s s u r e  0  0  0  0  0  0  27  0  0  0  0  0  0  28  °  0  0  0  0  0  29  .  0  0  0  0  0  0  30  f o r en. 0 0  0  •  0  0  0  0  31  0 .  0  0 . 0  e n o o o . o . o  o  .  oxygen p r e s s u r e  . 0 .  0  0  .  0  0  amine c o n c e n t r a t i o n i n r e g i o n o f O  O  O  O  O  O  O  O  O  r a t e 25  O  O  O  O  O  O  O  oxide O  O  O  O  O  LIST OF TABLES No.  Page  1.  Formation Constants f o r Zn and Cu amine complexes , , , , .  2.  Determination of R e p r o d u c i b i l i t y . . . . . . . . . . . . . .  13  3.  E f f e c t of Surface Area on Rate of D i s s o l u t i o n of Zinc  14  4.  I n i t i a l D i s s o l u t i o n Rates of Zinc at the Investigated Ammonia Concentrations. .  5.  .  0  .  .  0  0  0  0  .  0  .  0  .  0  . . .  .  .  3  21  .  Rate of D i s s o l u t i o n 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 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 . . .  26  7.  Rate of D i s s o l u t i o n of Zinc at the Investigated Ethylenediamine Concentrations. . a , , , , , ,  32  8.  9. 10,  ,  ,  ,  ,  ,  ,  Comparison of Rate and Formation Constants f o r Zn and Cu i n the Ammonia and Ethylenediamine Systems. . . . . . . . .  36  Data from F i l m No. 1529 . . . . . . . . . . . . . . . . . .  39  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 i s of great industrial Importance* The greatest use of zinc i s 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 i s desired).  As well, much zinc i s used i n rolled products where resistance to  atmospheric corrosion i s of prime importance. The protective effect of zinc i s 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 i s 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 i s reported that on exposure to the  atmosphere, a protective coating of zinc oxide i s f i r s t formed.  This i s  converted i n 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 s i l i c a t e s which form zinc salts of low s o l u b i l i t i e s .  Corrosion rates are low i n the pH range of 6 to 12.5 and domestic waters g e n e r a l l y f a l l w i t h i n t h i s range.  In a c i d and a l k a l i s o l u t i o n s  of c o r r o s i o n i s high i n comparison w i t h other metals.  s  the r a t e  Very low coneentra-  t i o n s of most of the common acids give r a t e s which render zinc u s e l e s s under these c o n d i t i o n s . high.  The a t t a c k of a l k a l i s i s l e s s severe but s t i l l  Aqueous ammonia i s reported  to corrode z i n c rather r e a d i l y with the  5 formation  of a s o l u b l e complex hydrate of zinc and ammoniao Zinc i s anodic i n aqueous s o l u t i o n to most of the common i n d u s t r i a l  5 metals ~~ i r o n , n i c k e l , lead, copper, aluminum and t h e i r various  alloys.  This g a l v a n i c behaviour of zinc has l e d to i t s use as s a c r i f i c i a l p r o t e c t i o n f o r aluminum i n chemical equipment, f o r i r o n and s t e e l i n o i l and gas fields,  f o r brass  on ships h u l l s , and f o r s t e e l p i l i n g s i n sea water  0  Although much e m p i r i c a l data i s a v a i l a b l e on the c o r r o s i o n of z i n c under a v a r i e t y of c o n d i t i o n s , l i t t l e the fundamental mechanisms. instability  attempt has been made to examine  A b a s i c d i f f i c u l t y i s the thermodynamic  of z i n c with respect  to the decomposition of water.  r e a c t i o n i s normally prevented by the high hydrogen overvoltage but  This on z i n c ,  traces of metal i m p u r i t i e s having a lower hydrogen overvoltage  the system can cause the a d d i t i o n of an e l e c t r o c h e m i c a l chemical r e a c t i o n under i n v e s t i g a t i o n .  within  r e a c t i o n to the  I f such i m p u r i t i e s are not excluded  c o r r o s i o n studies i n aqueous media cannot be r e a d i l y i n t e r p r e t e d i n terms of basic mechanisms. Recently  s t u d i e s have been c a r r i e d out w i t h i n the Department  of Mining and Metallurgy  on the d i s s o l u t i o n of copper i n ammoniacal and  organic amine s o l u t i o n s ^ * ' * ' I t was f e l t 9  that a comparative i n v e s t i g a t i o n  9  of  zinc would be i n t e r e s t i n g i n view of the r a t h e r d i f f e r e n t  tendencies of the two metal i o n s  complexing  Formation constants f o r t h e i r complexes  0  with ammonia and ethylenediamine are known and are shown i n Table  1  0  TABLE I Complex  Cu(NH ) 5  Cu(en)  + + 4  v  + + 2  Zn(NH ) 3  Formation Constant  + + 4  Zn(en) "*"  r  13O32  6  19o60  7  9o46  6  12 09  789  o  3  These show the g r e a t e r s t a b i l i t y corresponding complexes of z i n c  Reference  9  9  of the copper amine complexes over the  0  Moreover  9  s i n c e ZnsQ complexes are pre~  f e r r e d over CusO complexes, i t would be expected that the o x i d a t i o n of z i n c i n aqueous ammonia and amine s o l u t i o n s would i n d i c a t e a g r e a t e r tendency f o r formation of oxide or hydroxide f i l m s than was copper s t u d y  Q  observed i n the analogous  In addition^, there i s c o n s i d e r a b l e i n t e r e s t at the present  time i n the behaviour of z i n c e l e c t r o d e s i n the Zn-AggO wet-storage This u s e f u l b a t t e r y i s g r e a t l y handicapped  thought  c  by the short l i f e of the batjbery  due to c o r r o s i o n of the zinc e l e c t r o d e s i n the a l k a l i battery I t was  battery  electrolyte  0  that a k i n e t i c study of zinc i n amine s o l u t i o n s would be  p r e l i m i n a r y to an extensive k i n e t i c i n v e s t i g a t i o n of zinc under the c o n d i t i o n s experienced w i t h the Zn-AggO b a t t e r y  Q  Scope of the Present I n v e s t i g a t i o n  It was of  the purpose c f t h i s i n v e s t i g a t i o n to determine  the k i n e t i c s  the d i s s o l u t i o n of zinc i n aqueous ammonia and ethylenediamine  comparable organic amine  0  9  a  The i n respect stirring  b e h a v i o u r o f z i n c i n the  to the  effect  above s o l u t i o n s has  o f oxygen p r e s s u r e ,  r a t e , p r e s e n c e o f ammonium i o n , and  been  concentration the  area  of  o f the  studied  the  amine  zinc  sample,  EXPERIMENTAL  Preparation  of  The the a  and  recorded  Smelting  8 inches  used  million  The and  bars  cut  supplied  Cu,  stock,  by  They  1 ppm  of  report  Cd  approximately  were machined i n a  into cylinders  lathe  inch in  T h r e e s u c h c y l i n d e r s were mounted  specimen mounting p r e s s .  throughout  (2 ppm  form o f round b a r long.  g inch i n length.  e x p e r i m e n t s was  Company o f Canada L i m i t e d ,  of 4 p a r t s per  layer irregularities  in a metallographic prepared  and  s u p p l i e d i n the  4 i n c h i n d i a m e t e r and t o remove o u t e r  i n a l l the  content  I t was  d i a m e t e r and  Samples  Mining  impurity  Pb),  Zinc  z i n c used  Consolidated  total  and  the  in  lucite  F o u r s u c h s p e c i m e n s were  the whole s e r i e s o f e x p e r i m e n t s .  In  each,  2 the  area  consisted  of z i n c m e t a l exposed of long  modification no  impurity  of  a  2/0  dendritic crystals  this  of  the  2,80  r a d i a t i n g out  c r y s t a l s t r u c t u r e was  contamination  Before  t o r e a c t i o n was  high-purity  the  final  ,  This  f r o m the  attempted  surface  centre.  i n order  to  No  assure  zinc.  e a c h d i s s o l u t i o n e x p e r i m e n t , the  p o l i s h i n g paper being  cm,  p a p e r usedo  z i n c s p e c i m e n was T h i s was  polished^  followed  by  12 e t c h i n g w i t h a 9 to 1 e t h a n o l  and  nitric  o f 3 m i n u t e s were r e q u i r e d t o remove the  a c i d etchant. outer  Etching  times  s t r e s s e d l a y e r s of  zinc  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 litres,  by the addition of d i s t i l l e d 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 The base concentration  (98,7$ en, remainder water).  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  t i t r a t i o n s 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 , Hydrochloric Acid,  Gelatin and sodium sulphite solutions^ prepared d a i l y , 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.  - 6The 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 i s 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 approximately 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 i n 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 A i r 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 i n the delivery l i n e . Temperature control was maintained by means of a heating c o i l immersed i n the autoclave solution and connected with a thermostatically controlled water bath. Water at the required temperature was forced through the heating c o i l by means of a centrifugal pump. A mercury control thermoregulator set in a well of the autoclave cover was used to maintain control of the temperature of the water bath. The temperature i n a l l experiments  -  H  6  W>>i>)>>>>m>»i>M A. B„ Co Do E„  Fo G Ho I Jo  Shaft Thermoregulator w e l l Heating c o i l S a m p l i n g Tube Z i n c sample  F i g u r e lip  0  0  Impeller Autoclave Liner Relay Pump Heater  Schematic diagram o f the a u t o c l a v e a n d c o n t r o l system 0  temperature  7  was maintained at 25- 0,1  Co  Experimental Procedure In a l l experiments an autoclave solution volume of 2 l i t r e s was used.  Measured amounts of ammonium hydroxide or ethylenediamine and sodium  perchlorate solution were diluted to this volume and charged into the autoclave  0  The solution was brought up to temperature  9  the zinc specimen  was affixed in place and the autoclave cover bolted down.  The impellejr  was then rotated at i t s 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, determination.  A further 40 ml, sample was withdrawn for zinc  Withdrawal of the second sample constituted zero time  0  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 f i t t e d with a dropping 14 mercury electrode c e l l . constant at 3,5  Volume of the c e l l was 20 ml, and drop time was  seconds.  The polarographic sample solution for determination of zinc in ammonia solutions consisted ofs . ,  . i ) 25 mis,  IN NH C1  i i ) 10 mis.  IN NH^OH  i i i ) 10 mis,  4  unknown Z n  ++  s o l ' n drawn from 40 ml, sample  iv)  2 mis,  0,2$ gelatin (by weight)  v)  4 mis,  2M Na S0 9  - 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 i g h t l y .  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 i n 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 r e r .  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 curve for determination of zinc in ammonia solutions.  11 -  Polarographic Wave Height FigurOo  (mm ) 0  C a l i b r a t i o n curve f o r determination of zinc i n ethylenediamine solutions,,  -  were v e r y the It  slow b u t v a r i e d w i d e l y  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  would a p p e a r t h a t g a l v a n i c  and  as a r e s u l t  lection  f r o m one r u n t o t h e nexto  of melting  cells  The  next  to the bulk  e x p e r i m e n t s were c a r r i e d  autoclave  using  a titanium l i n e r  and  stainless  steel  p a r t s o f the autoclave  avoid  ( 9 9 , 9 9 9 6 ^  that  any m e l t i n g  cause l o c a l  o u t u n d e r oxygen p r e s s u r e i n  and d i r e c t  i m m e r s i o n o f the t i t a n i u m  i n the r e a c t i o n s o l u t i o n .  process  i n the a s - r e c e i v e d  concentration  of impurities already  varied widely  h i g h p u r i t y C M , and S  c o n d i t i o n s i n c e i t was f e l t  would add t o t h e i m p u r i t y  out i n 0 , 5 M and 1 , 0 M NH^OH a n d a g a i n  carried  that  Zn) was u s e d  content  present.  or at least Studies  r a t e s were o b s e r v e d  i t was c o n s i d e r e d of stainless  therefore  taking  full  advisable  to  factory,  the p o s s i b i l i t y  parts  inert  a l l metal parts  layer,  and was f o u n d  o f s u c h by  from the s o l u t i o n ,  A polyethylene  i n contact  of zinc.  with the  A h e r e s i t e p r i m e r was p a i n t e d on  t o be c o m p l e t e l y  the s o l u t i o n , a g l a s s autoclave  be t o o f r a g i l e ,  of galvanic  advantage o f the h i g h hydrogen o v e r v o l t a g e  s o l u t i o n with a chemically the autoclave  t o minimize  s t e e l and t i t a n i u m s u r f a c e s  T h i s was done by c o v e r i n g  contain  showed  common.  exclusion  all  which  disappeared but  Although c o r r o s i o n p i t s a r e not p o s i t i v e evidence cells,  were  f r o m r u n t o r u n . An e x a m i n a t i o n o f t h e z i n c s u r f a c e  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 a l m o s t  e t c h p i t s were  through the c o l -  o f the g r a i n s .  the p r e f e r e n t i a l g r a i n b o u n d a r y d i s s o l u t i o n ,  zinc  boundaries,,  were a c t i v e i n t h e d i s s o l u t i o n p r o c e s s ,  the  To  -  The s u r f a c e o f  corrosion at grain  and c a s t i n g , g r a i n b o u n d a r i e s ,  o f i m p u r i t i e s , were a n o d i c  12  l i n e r was f i r s t  4 litre  satisfactory. tried  To  but proved  b e a k e r was f o u n d t o be s a t i s -  showing no c o r r o s i o n when u s e d w i t h  t h e ammoniacal s o l u t i o n s o r  0  - 13 physical damage on repeated insertion and removal from the autoclave  0  Under these conditions;, p i t t i n g and grain-boundary corrosion of the zinc surface were no longer observed and kinetic studies now gave reproducible r e s u l t s . It should be noted, however, that inspection of the heresite layer was necessary after each run. presence  Pin-hole flaws did develop but their  could be detected easily at the end of an experimental run by  the appearance of the zinc sample which would show many small corrosion pits.  When breaks in the coating developed, the autoclave parts were repainted  and allowed to a i r - d r y for 12 hours.  Drying in a warm-air blast was found  to leave the coating in a b r i t t l e 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 A series of experiments was carried out at constant ammonia concentration, oxygen pressure, s t i r r i n g rate, and temperature in order to determine the reproducibility of the experimental technique.,  The results  of these tests are shown in Table II, TABLE II Determination of Reproducibility Rate of Dissolution (mgZn/cm, / h r , )  Run No,  45ol 47.0 46,1 45.8 46.0  1 2 3  4 Conditionss  Average  Deviation  -1,95 +2.17 +0.22 =0.44  Temperature 25°C s t i r r i n g rate 860 rpm., oxygen pressure 5,1 atm.o NELOH l 0M NaClO. 0,1M 9  o  9  = 14 =  Reproducibility for duplicate experiments was considered to be of the order of 4%  0  I f , 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 2 80 cm TABLE III 0  0  Effect of Surface Area on Rate of Dissolution of Zinc Surface Area of Zinc (cm.2)  Rate of Dissolution (mgZn/cm / h r ) 0  2.80  2<,80 2 01 2.01  6.9  =>lo4  7.1 7.0 6,9, 7,0  0  Conditions?  Deviation %  0  Average  +lo4 0 -1„4  Temperature 25°C, s t i r r i n g rate 860 rpm , oxygen pressure 6o4 atm, NH^OH 0.3M, NaC10 0.1M. 0  4  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 i n 1 OM NH^OH. 0  pressures, 1.7 to 7.1 atm.  9  Over a wide range of Og  l i n e a r i t y of the family of rate curves was  good (Figure 4) and when rate was p l o t t e d versus oxygen p r e s s u r e curve showed a f i r s t  order dependence o f rate on 0^ pressure  9  the r e s u l t i n g  (Figure 8 )  0  Experiments were next c a r r i e d out i n 0,5M NH^OH over the same range o f 0^ pressureso  The same r a t e s of d i s s o l u t i o n were observed  Og pressures 2 4 to 6 4 atm, 0  be concluded  C  over the range of  (Figure 5)o Within t h i s range  that rate o f d i s s o l u t i o n of z i n c was f i r s t  9  i t could then  order i n oxygen  pressure and zero order i n ammonia concentration,,  At higher Og pressures, i n 0,5M NH^OH, the rate showed a sudden decrease, reaching a constant value o f 11 mg Zn/cm ^/hr 0  of  such a rate t r a n s i t i o n had been noted  64  atm  0  0  The f i r s t  0  indications  i n the experimental run a t  0^ pressure, i n which the rate changed a b r u p t l y a f t e r 40 minutes but  s t i l l maintained  linearity.  On removal from the autoclave, the surface of  the zinc sample showed i n t e r f e r e n c e colours i n d i c a t i n g the presence o f a t h i n coherent atm,  9  film.  The surface of the z i n c  showed the presence  9  a f t e r runs at 6 8 and 7,1  of an opaque white f i l m ,  U  Again  9  as with l 0 M o  NH^OH s o l u t i o n s , a l l curves were l i n e a r ,  A 0,3M NH^OH s o l u t i o n was next i n v e s t i g a t e d over an 0^ pressure range of 2,4 to 6,4 atm, first  (Figure 6) P a r a l l e l r e s u l t s were obtained?  order dependence on 0^ pressure from 2,4 to 4,4 atm,  decrease  i n r a t e at higher pressures.  and a sudden  The value of the l i n e a r r a t e at high  pressures, 7,0 mg Zn/cm, / h r , i n d i c a t e d that r a t e s i n t h i s region had become first  order i n NH^OH concentration and zero order i n oxygen p r e s s u r e .  The same break occurred i n the curve f o r an oxygen pressure of 4,4 atm, which corresponds the other.  to the t r a n s i t i o n from one r a t e - c o n t r o l l e d r e g i o n to  120  O 7.1  100  CM  a o  CD >  r-i  atm.  • 3.7 atm.  • 6.4  "  • 3.0  "  A  5.8  "  • 2.4  "  7 5.1  «  T  1.7  "  "  80  60  O  CO CO  40  20 _  0 10  20  30  40  50 Time  Conditions!  60  70  80  90  (min.)  Temperature 25°C, s t i r r i n g rate 860 rpm., NH^OH 1.0M,  NaClO^  0.1M. ON  Figure 4  Rate curves f o r the d i s s o l u t i o n of Zn i n 1.0M oxygen pressures.  ammonia s o l u t i o n at various  c  1  T  O 6o4 atm "  D  3d  A  4o4 "  0  1  1  1  1  i  1  r  v 3»7 atm. O 3o0 "  Time (min.) Conditionss Figure 6  Temperature 25°C, s t i r r i n g rate 860 rpm„, NH^OH 0 3M o  9  NaClO^ 0.1M  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 various oxygen pressures. o  Figure 7  Rate curves f o r 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 f o r oxygen pressures.  various  0  i 1.0  i 2.0  I 3.0  l 4.0  Pressure of Oxygen Conditions: Figure 8  I 5.0  I 7.0  (atm.)  Temperature 25°C, s t i r r i n g rate 860 rpm., NaClO^  E f f e c t of oxygen pressure concent r a t i ons.  I 6.0  0.1M  on the rate of d i s s o l u t i o n of Zn at various ammonia  =  In a subsequent at  experiment  ~  t h e z i n c sample was o x i d i s e d f o r 3 h o u r s  and an O g p r e s s u r e  t h i s anmonia c o n c e n t r a t i o n  f i l m was i d e n t i f i e d  9  21  by X - r a y d i f f r a c t i o n  o f 6 . 4 atm  Zn0  procedures as  The r e s u l t a n t  0  o  (Appendix  l)  TABLE I ? Initial  D i s s o l u t i o n R a t e s o f Z i n c a t the. I n v e s t i g a t e d Ammonia  ConcentrationSo  (mgZn/cmo / h r ) 0  Oxygen P r e s s u r e (atm )  0  o  2 5 M  0o30M  0o50M  l  0 M  o  0  ll 0 lloO  7ol  67o5  o  608 6 4  7o0  0  5o8 5ol  3o0  2o4  6 0  5 4 o 0  46o5  4 6  o  0  39o0  4 1  o  0  4 o 7  3 2  o  0  30o0  3 2 o 0  4 o 7  2 2  o  0  17o8  24o5  19o0  I80O  1 4 o 0  Conditionss  Temperature  Further  to  0o25  o f 2 . 4 atm  NH^OHo  5oOM  5 0  o  2  and  and  49oO  show a f i r s t  0  0  0 . 2 5 M  The r e s u l t s  o f these  o  l M  solution  NH^OH  T h e s e showed t h e same  I n t h e low 0^ p a r t i a l p r e s s u r e  in  0 M  out i n a  r p m , NaClO^  abrupt  experiments  I V , t h e range o f ammonia i n v e s t i g a t e d was  o f amine c o n c e n t r a t i o n  o  Q  860  Concentration  independent 2  rate  ( F i g u r e 7)0  0  i n d i c a t e d i n Table  loOM  stirring  i n Table I V o  o f Ammonia  As  9  e x p e r i m e n t s were c a r r i e d  t r a n s i t i o n a t an O g pressure summarized  C  25  o v e r a range o f 0 ^ p r e s s u r e s  Effect  0  7o0  1»7  are  o  34o5  4o4 3o7  5 7 . 0 50o0  0  NH^OH s o l u t i o n s a t  mgZn/cm order  0  /hr  0  region,  To f u r t h e r c h e c k t h i s ,  5,5 a t m  I n the high  c  0^ p r e s s u r e  oxygen p r e s s u r e  dependence on amine c o n c e n t r a t i o n .  the r a t e s a r e r u n s were made  and gave r a t e s o f region^  the rates  A s i n g l e r u n was made 2  in  0 . 2 7 5 M  NH.OH a t  3.7 atm. 0  o  pressure  and gave a r a t e o f  5.5 mgZn/cm / h r 0  0  =  22  ~  Effect of Ammonium Ion Concentration The  NH^/NH^OH  ratio was controlled by the addition of perchloric  acid to the ammonia solution.  The effect of added  the whole range of ammonia concentrations  NH  4  +  was checked over  (0.5M)  and gave rate curves  (Figure 9)  whose i n i t i a l slopes are shown in Table V  Q  TABLE V Rate of Dissolution of Zinc at Various^Ammonia Concentrations in the presence of 0.5M N H ^ (mgZn/cm / h r « ) +  0  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  38.4  33.2  4 . 0  Conditions?  Temperature 2 5 ° C , s t i r r i n g rate 8 6 0 rpm., N H ^  The results  the  0  2  of the  +  0 5Mj, NaClO^ 0 . 1 M o  indicate that the added N H . does not a l t e r rates within 4 pressure dependent region ( i e . O045, 0 o 4 2 j , 0 . 9 2 M NH^). In the case 0 . 2 5 M  NH, 3  +  solution, at  oxide formation was noted  u  5.1  atm.  0^ £  without the presence of  The addition of  0  o  5 M  N H . 4  +  has at this  tration prevented the formation of an oxide layer and thus the  N H . 4  NH_, ?  +  ions  9  concen-  transition  to slow oxide dissolution rates. Effect of S t i r r i n g Rate To investigate  the p o s s i b i l i t y of transport  control, a reduced  s t i r r i n g rate was used over a range of oxygen pressures  in  0o3M  NH^OH  solutions.  Rates were obtained which showed a p a r a l l e l l i n e a r i t y but reduced value in the oxygen dependent region compared with rates obtained at the same ammonia concentration but at the usual s t i r r i n g rate of 8 6 0 rpm  0  (Figures 1 0 and l l )  0  1  :  1  1  I  f"  i  1  1  r  Time (min.) Conditionss  Figure 9  Temperature 25°C, s t i r r i n g rate 860 rpm., oxygen pressure NH. 0.5M.  Rate curves  f o r the d i s s o l u t i o n  5.1 atm.?  of Zn at a constant ammonium c o n c e n t r a t i o n .  6  t  3  s 60  50  .  .  \  a  1  r  1  1  1  r  O 5.1 a t m , •  4c4  "  A  3„7  »  V 2.4  "  o  Time Conditions; Figure  10  Temperature  25°C  0  stirring  (min.)  r a t e 575 r p m .  ?  NH OH 0 . 3 M , NaCIO  0.1M  Rate c u r v e s f o r the d i s s o l u t i o n o f Zn i n 0.3M ammonia s o l u t i o n a t v a r i o u s oxygen p r e s s u r e s .  .0  O Stirring rate 860 rpm 60 _  o  "  "  575 "  50  40 -  30 _  20  10  0  X  X  0  l o O  2  o  0  3o0  4o0  5 c 0  6 c 0  7o0  Pressure of Oxygen (atm ) 0  Conditions?  Temperature  25  C, N^OH 0 o 3 M  ?  NaClO^  0  £  1M  Figure 11 Effect of s t i r r i n g rate on the rate of dissolution of Zn at a constant ammonia concentration*.  - 26 The  rate  i n the  region of oxide  f i l m s r e m a i n e d unchanged,,  TABLE  VI  Rate o f D i s s o l u t i o n o f Z i n c a t a r e d u c e d s t i r r i n g Oxygen p r e s s u r e atm  0.3M NH 575 rpm,  0  4.4 3.7 2.4  Ethylenediamine  i n v e s t i g a t i o n o f the  Initial  resulting  rates  showed the  to  ( F i g u r e s 12 and  en  fact,  ( F i g u r e 15)  remain constant  These r e s u l t s  The  are  individual  en  the  to v e r i f y i n the  summarized  ( F i g u r e 16)  and  used  and  0,1M  order  en and  0,3M  o f amine  and  the  observed.  analogous en.  The  concentration pressure,  sharp  transition  (Figure 14)  A  u  i n v e s t i g a t e d o v e r a wide range o f  t h a t the  i n Table  r a t e o f d i s s o l u t i o n o f Zn region  (3.4  to 7,8  was  excellent. low  The  did, in  atm,).  pressures  plot  o f 0^  rate of d i s s o l u t i o n i s f i r s t o f amine c o n c e n t r a t i o n .  In  of r a t e  pressure.  versus  ( f o r each order  the  becoming p r o p o r t i o n a l t o the  i n oxygen  0^  VII,  shows t h a t at the  i n 0.5M  oxygen p a r t i a l  was  "high" pressure  independent  zero  out  (0,065M en)  pressure  dependence i s r e v e r s e d ,  concentration  NaClO^  dependence on  of a l l curves  concentration)  oxygen p r e s s u r e region,  7.0 34.5 32,0 16,0  Z n - e t h y l e n e d i a m i n e s y s t e m gave  o f 0,03M was  linearity  oxygen p r e s s u r e  order  i n c r e a s i n g 0^  concentration  pressures  0o3M NH OH 860 rpm  13) were i n d e p e n d e n t  c o n c e n t r a t i o n was  lower r a t e s w i t h  final  /nr.)  Q  e x p e r i m e n t s were c a r r i e d  same f i r s t  A much l o w e r en  2 0  System  results.  and  OH  T e m p e r a t u r e 25°C.', NH^OH 0.3M,  An  (mgZn/cm  6.7 6.7 23.5 10.0  5.1  Conditionss  rate,  high  in pressure  amine  Time (min.) Conditions: F i g u r e 12  Temperature 25°C, s t i r r i n g rate 860 rpm., en 0.50M,, NaClO^ Rate curves f o r the d i s s o l u t i o n of Zn i n 0.5M various oxygen pressures.  0.1M  ethylenediamine s o l u t i o n at  Time Conditions: Figure 13  (min.) .  Temperature 25°C, s t i r r i n g rate 860 rpm., en 0.30M, NaClO^ 0.1M Rate curves f o r the d i s s o l u t i o n of Zn i n 0.30M ethylenediamine s o l u t i o n at v a r i oxygen pressures.  Time (min.) Conditions: F i g u r e 14  Temperature 25°C, s t i r r i n g rate 860 rpm., en 0.065M  NaClO^  0.1M  Rate curves f o r the d i s s o l u t i o n of Zn i n 0.065M ethylenediamine s o l u t i o n at various oxygen pressures.  1  1  1  1  1  Time Conditions: Figure 15  1  1  I  I  (min.)  Temperature 25°C, s t i r r i n g rate 860 rpm., en 0.03M NaClO^ 0.1M Rate curves f o r the d i s s o l u t i o n of Zn i n 0.03M ethylenediamine s o l u t i o n at various oxygen pressures.  T  1  O 0.50H Bthylenediamine  60 -  • 0.30K  n  A 0.055M  "  V 0.03M  " O  \  v-A-  -9-  0  IcO  3o0  2oO  4o0  Pressure o f Oxygen Conditions? Figure 1 6  Temperature 2 5 ° C  9  5o0  6 c 0  7o0.  (atm ) 0  s t i r r i n g rate 8 6 0 rpm»  9  NaClO^  OdM  E f f e c t of oxygen pressure on the rate of d i s s o l u t i o n o f Zn a t various ethyienediamine  concentrationSo  - 32 TABLE VII Rate of D i s s o l u t i o n o f Zinc at the I n v e s t i g a t e d Ethylenediamine Concentrations (mgZn/car. / h r . ) 0.065M  0.3M  0.5M  (atm.) 7.8 7.1 6.4 5.1 3.7 3.4 3.0 2.7 2.4  19.0 38.0 52.5 43.5 30.0  8.4 8.4 8.4 8.4 8.4 19.5 19.5 17.0  Conditions:  62.0  63.0  52.0 40.0  47.0 37.0  17.0  Temperature 25°C, s t i r r i n g rate 860 rpm., NaClO^  0.1M.  DISCUSSION Zinc, due to i t s e l e c t r o p o s i t i v e character, d i s s o l v e s r a p i d l y i n ammonia and ethylenediamine s o l u t i o n s under o x i d i z i n g conditions  according  to equations 1 and 2. Zn + £ 0  2  + 4NH OH — » - Z n ( N H ) 4  5  Zn + £0„ + 3en + H . O Z n ( e n ) _ The experimental of rate c o n t r o l .  + +  + 20H" + 3H 0 2  + 20H~  (l) (2)  r e s u l t s show that there are two d e f i n i t e  The low oxygen pressure  dence on oxygen pressure agent.  + + 4  r e g i o n shows a f i r s t  and zero dependence on concentration  regions  order depeno f complexing  This i n d i c a t e s c o n t r o l of the r a t e of d i s s o l u t i o n of zinc e i t h e r  by transport adsorption  of oxygen w i t h i n the s o l u t i o n to the metal surface or by  of oxygen to the s u r f a c e .  That transport of oxygen to the  i n t e r f a c e i s the c o r r e c t a l t e r n a t i v e i s i n d i c a t e d by decreased rates when slower speeds of a g i t a t i o n are used.  Such a region has been shown to  e x i s t f o r copper and i s known to occur with i r o n i n a l k a l i n e s o l u t i o n s .  18  - 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 l e v e l l i n g off of the fate curves in 0*25M and 0*3M NH^OH (Figures 6 and 7) and the slight deviation from l i n e a r i t y for the curves of 0.03M and 0*065M en (Figures 14 and 15)*  Such a change  in rate, approaching a pardbdlic rate relation* i s indicative of film formation.  In this case i t could indicate p a r t i a l coverage of the zinc  surface by a thin adherent  film.  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.) 5 4  il  ++  + 20H" + 3H 0 ^ = d  ZhO + 4NH OH 4  ^F=^  ZnO * 3en + HgO  fi  Zn(eh)^ + 20H^ ++  (3) .(4)  As Well* visual evidence is available as to i t s presence and positive i d e n t i f i cation of this film as ZnO has been made by X-fay d i f f r a c t i o n 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 i s 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 ethylehediamirie*  The fate of dissolution of zinc at this stage is f i r s t order in  amine concentration and independent of s t i r r i n g rate.  Hence the rate i s  now chemically controlled by dissolution of the ZnO, probably at the f i l m solution interface,  A steady-state condition would be reached in which the  ZnO film enters solution at the same rate at which i t i s 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 i n 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 i s 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**> Wtl  K  3 T  = 294  e x p  en  =21,6  mg.Zn/cm. /nro/M/l 2  mg„Zn/cm, /hr./M/l 2  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 •  O  —  1  ethylenediamine  ammonia  _  /  1 O o l  0  1 0,2  Concentration Conditionss Figure  17  Temperature  1 0o3  1  1  0,4  0o5  o f amine ( m o l e s / l i t r e ) 25°C o s t i r r i n g  r a t e 8 6 0 rpm  P l o t o f r a t e o f Zn d i s s o l u t i o n v e r s u s w i t h i n r e g i o n of oxide formation.  0  amine  concentration  - 36 A comparison with the results obtained i n the copper-ammonia and copper-ethylenediamine systems as reported by Milants*^, but for a region that showed no oxide-film formation, demonstrates considerable similarity i n 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 (Log K) Zn(NH )> Zh(en) 3  5  Cu(NH )^ Cu(en)  ++  5  +  2  Rate Constant* (mg. Metal/cm. /hr./M/l)  9.46 12.09  21.6 294  13.32 19.60  61.G 245  * The rate constant i n the zinc-amine systems i s for dissolution of oxide present as a visible film, while i n the copper-amine systems the rate constants are postulated as those for attack of a surface Cu**0 complex. This agreement i s 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 s t a b i l i t y of the oxide and the higher formation constant for the soluble complex ion.  CONCLUSIONS 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  v e r i f i e d 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 l m .  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, however  n  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 i n a more detailed fashion, may give information about the formation and nature of the oxide f i l m , 3)  The addition of OH to the solutions, in the form of NaOH, would  corroborate  the s u g g e s t e d  be  by a s t u d y  preceded  hydroxyl  ion effecto  o f the Zn-NaOh s y s t e m .  This,  however,  should  - 39 APPENDIX I TABLE IX Data from F i l m No. 1529 Line No. 1 2 3 4  5 6 7 8 9 10 11 12 13 14  dX(obs'd) 5 30 20 50 5 20 100 10  5 5 5  10 10 10  dS(obs'd)  Line No,  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  1.383 1.341 1.330 1.281 1.207 1.159  10 10 10  5 5 5 5  0.9451 0.9068 0.8582 0.8312 0.8224  10  5 5  F i l m No. 1529 i s an X-ray d i f f r a c t i o n powder p i c t u r e of the surface l a y e r formed on a z i n c specimen when reacted with a 0.3N NH^OH s o l u t i o n under an oxygen pressure of 6.4 atmospheres.  The t h i n adherent  l a y e r was scraped  o f f mechanically and as a r e s u l t , some z i n c metal was removed with the l a y e r .  Determination of the composition of t h i s l a y e r 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 f o r zinc  (No. 4-0831) and ZnO (No. 5-0664').  -  40  TABLE X  Comparison Zinc S t ' d . d2  2o473 2 . 3 0 8  i / l  Sample Line Noo  d&\  ZnO S t ' d . i / l  d£  X  Sample  i / l 1  d£  l / l _ 1  2o464  50  2 . 8 1 6  71  2  2 . 8 0 8  60  6  2o302  20  2 . 6 0 2  56  3  2o602  40  2-o 0 8 5  2 . 4 7 6  7  28  12  1 . 3 4 2  25  16  1 . 3 3 2  21  17  l o 2 3 7  100  100  4  2o464  100  1.911  29  9  1 . 9 0 4  10  1 . 6 2 6  40.  13  1 . 6 2 7  20  1 . 4 7 7  35  14  1 . 4 7 9  10  2  1 . 4 0 7  6  23  1 . 3 7 9  28  15  1 . 3 8 3  10  1 . 3 5 9  14  25  0 . 9 0 6 8  10  lo691 1.341 1 . 3 3 0  10  5 5  5  l o l 2 3 6  17  1 . 3 0 1  3.  l o 0 9 0 1  3  1 . 2 2 5  5  1 . 0 4 5 6  5  Oo9454  8  o9093  6  o9064  11  8722'  Line No.  Layer  4  1 . 6 8 7  0  layer  40 100  1 . 1 5 3 8  Layer  Zn a n d ZnO w i t h sample  53  2o091  l o l 7 2 9  o f ASTM S t a n d a r d  1.1812  3  l o 0 9 2 9  10  24  0 . 9 4 5 1  5  1 . 0 6 3 9  3  25  . 9 0 6 8  5  1 . 0 4 2 2  10  1 . 0 1 5 8  5  26  . 8 5 8 2  10  0 . 9 8 4 8  4  5  o8589  9  o8437  2  = 9764  o8245  1  .9555  1  o8225  9  . 9 3 8 2  4  . 9 0 6 9  12  . 8 8 2 6  6  7  . 8 6 7 5  1  o836 9  6  41  -  BIBLIOGRAPHY  1  0  2  0  Ro  and D  Div ,  c  Comincof  0  Schikorr,  G o ,  "The Z i n c  and S c h i k o r r ,  I  |  c  Industry"  Z D A 0  3o  A n d e r s o n , E.A« a n d F u l l e r ,  4o  Burns,  R„Mo  5°  McKay,  R  6  B j e r r u m , J o ; " M e t a l Ammine F o r m a t i o n Son, Copenhagen ( l 9 4 l )  0  7c  o  J  0  Nyman,  C  0  0  10o  Milants,  llo  Sircar,  (1945)  13o  (1939)  f o r Metals"  i n Aqueous S o l u t i o n " P  and Verhoek  0  0  0  0  G B 0  0  ;  <J  0  Am, Chem S o c  0  Danske V i d e n s k a b available)  0  W.M„;  0  Soc  0  77  Selskeb"  0  S „ C | M„So  0  0  4194  , and A n d e r s o n , P | " K g l Medd 22 7 ''1945) (not  0  0  Haase a n d  0  J . Am» Chem  F H 5  9  (1939)  o f M e t a l s and  H o T | Mo S c T h e s i s , Dept„ o f M i n i n g and M e t a l l u r g y of B r i t i s h Columbia ( 1 9 5 8 )  Latimer,  0  10  1334  0  of 12  (1944)  " P r o t e c t i v e Coatings  0  Murbach, E W , a n d M i l l a r d ,  J o ,  Bjerrum, J  152  M e t a l s and A l l o y s  0  McReynolds, J , P .  ( 1 9 5 5 )  9°  0  and W o r t h i n g t o n , R o | " C o r r o s i o n R e s i s t a n c e Alloys" (1936)  0  67  80  0  and Schuh, A E ;  G.A ,  Carlson,  M L j  0  (1948)  Thesis,  British  o f M i n i n g and M e t a l l u r g y ,  D e p t o  Columbia  "Oxidation  0  University  9  University  (1959)  Potentials", Prentice  M e t a l s Handbook Committees  Mat-fys  Hall  Inc  " M e t a l s Handbook", The Am  c  D  (1952)  Soc. f o r  Metals  (1948) 1 4 °  I0M0,  Kolthoff,  and L i n g a n e ,  Inc.  1 5 °  Meites,  16  Evans, W R ?  0  0  17o  Halpern  18o  Halpern,  ?  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