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The reaction of copper-gold alloys in aqueous ammonia under oxygen pressure Fisher, James Irwin 1953

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THE REACTION OF COPPER-GOLD ALLOYS IN AQUEOUS AMMONIA UNDER OXYGEN PRESSURE  by JAMES IRWIN FISHER  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 thesis as conforming to the standard required from candidates f o r the degree of MASTER OF APPLIED SCIENCE  Members of the Department of Mining and Metallurgy THE UNIVERSITY OF BRITISH COLUMBIA September, 1953  V  ABSTRACT  An investigation was conducted of the corrosion of coppergold alloys in ammonia solutions under oxygen pressure. The reaction, which took place i n an autoclave, was followed by sampling and analysis of the solutions.  It was found that only copper was dissolved  from the alloys, the gold being left behind in a film on the surface of the corroding specimen. The reaction of pure copper as well as of four alloys ranging in gold content from 2 to 15 atomic percent were studied. variables examined include the concentrations of NH  3  Other  and NHj i n the  solution, the oxygen pressure and the temperature. It was found that while the rate curves for the dissolution of pure copper were linear, those for the alloys were generally parabolic in shape. The rate of dissolution of copper from the alloys appears to be determined by the transport of reactants and products through the gold rich films.  Some copper oxide may also be precipit-  ated i n the pores of the film or in the region between the film and the underlying metal, further impeding the transport processes and contributing to the lowering of the rate.  ACKNOWLEDGEMENT  The author i s indebted to the National Research Council of Canada f o r the financial aid which enabled this project to be carried out. The author i s grateful to the members of the Department of Metallurgy for  their assistance throughput this work, and i s  especially grateful to Dr. J. Halpern, vbo ably directed this investigation.  TABLE, OF CONTENTS Page: Introduction  1  Previous Work on the Corrosion of S i m i l a r A l l o y Systems;  2  Object and Scope of the Present I n v e s t i g a t i o n  4  Experimental  5  Ao  Preparation of the A l l o y s  5  Bo  Apparatus  7  Co  Chemical Reagents and S o l u t i o n s  7  D«  Measurement of the Rates o f D i s s o l u t i o n  9  E  A n a l y t i c a l Procedures  9  (1)  Copper  9  (2)  Gold  10  (3)  Ammonia  12  0  Results and D i s c u s s i o n  13  Nature of the Reactions and Rate Curves  13  E f f e c t of NH  16  Concentration  3  24  E f f e c t of (NH ) S0j„ 4  a  E f f e c t of Varying NH  3  i n the Presence of Constant NH+.  34  E f f e c t of Oxygen P a r t i a l Pressure  41  E f f e c t of Temperature  46  Structure and Composition of the Films  50  Conclusions  59  Summary of Experimental R e s u l t s  59  I n t e r p r e t a t i o n of the Results  61  TABLE OF CONTENTS (continued)  References  LIST OF FIGURES Figure No.  Page  1.  Diagram of Pressure Vessel  8  2.  C a l i b r a t i o n Curve f o r Copper Carbamate  3.  T y p i c a l Rate Curves f o r Copper and A l l o y 5 i n NH  4o  Rate Curves f o r Copper i n NH  5.  Rate Curves f o r A l l o y 5 i n NH  6.  Parabolic Plots f o r A l l o y 5 i n NK  7.  Comparative Rate Curves f o r the Alloys i n NH  20  8.  E f f e c t of NH  21  9.  E f f e c t of Gold on the Rate of Copper Dissolution i n  3  18  3  19  3  3  3  on the Hate of Copper Dissolution  3  Alone  Rate Curves f o r Pure Copper with NH  22 Constant and  3  NH£ Varying  11.  Rate Curves f o r A l l o y 10 with NH  25  3  Constant and NH£  Varying  12.  Parabolic Plots f o r A l l o y 10 with NH Ml  13.  14  17  3  Solutions containing NH 10.  11  26  3  Constant and  Varying  27  Comparative Rate Curves f o r Alloys with Constant NH , 3  Constant NH£  28  LIST OF FIGURES (continued) Figure No. 14.  E f f e c t of NH£ on the Rate of Copper Dissolution at Constant NH  15.  E f f e c t of Gold on the Rates of Copper Dissolution with NH  16.  3  3  Constant and NHJ Varying  Rate Curves f o r Copper with NHlt Constant and NH  3  Varying 17.  Rate Curves f o r A l l o y 5 with NHj Constant and NH  3  Varying  18.  Rate Curves f o r A l l o y 10 with NHj Constant and NH  3  Varying 19.  Parabolic Plots f o r Alloy 10 with NH£ Constant and NH  3  Varying  20.  E f f e c t of NH  21.  E f f e c t of Gold on the Rates of Copper Dissolution  3  i n Solutions Containing Constant NH£  with NH£ Constant and NH  3  Varying  22.  Rate Curves f o r Copper at Various Oxygen Pressures  23.  Rate Curves f o r Alloy 10 at Various Oxygen Pressures  24.  Rate Curves f o r Copper at Various Solution Temperatures  25.  Rate Curves f o r A l l o y 2 at Various Solution Temperatures  LIST OF FIGURES: (continued)  Figure No.  Page  26.  Surface Structure of Alloy 2  51  27.  Surface Structure of Alloy 2  51  28.  Surface Structure of Alloy 5  52  29.  Surface Structure of Alloy 10  52  30.  Surface Structure of Alloy 10  53  31.  Surface Structure of Alloy 10  53  32.  Surface Structure of Alloy 15  54  33.  Surface Structure of Alloy 15  54  LIST OF TABLES Table No. ,1 II  Data on Experiments with Solutions Containing NH3 Alone Data on Experiments with Solutions i n which NH£ was Varied and NH  3  III  Held Constant  29  Data on Experiments with Solutions i n which NH was Varied 3  and NHj. Held Constant IV V VI  Page 23  40  Data on Experiments i n which. Oxygen Pressure was Varied  45  Data on Experiments i n which Temperature was Varied  49  Analyses of Films  55  THE REACTION OF GOLD-COPPER ALLOYS WITH AQUEOUS AMMONIA UNDER OXYGEN PRESSURE  INTRODUCTION  This study of the kinetics of the d i s s o l u t i o n of coppergold a l l o y s i n aqueous ammonia and ammonium s a l t solutions under oxygen pressure had i t s inception i n a s i m i l a r study on the kinetics of the d i s s o l u t i o n of copper recently conducted i n t h i s laboratory ( l ) .  In  the course of the l a t t e r investigation, i t was shown that the t o t a l rate of d i s s o l u t i o n of copper i n ammonia solutions was the sum of the rates of two independent reactions, f i r s t order i n r e l a t i o n to the concentrations of ammonia and ammonium i o n respectively.  These rates  were found to be independent of the oxygen concentration, provided the oxygen was present i n excess so that i t s transport to the copper surface d i d not l i m i t the rate of the reaction.  It was also found  that the rates were independent of the concentrations of hydrogen ion and dissolved copper.  On the basis of these r e s u l t s , the follow-  ing mechanism f o r the d i s s o l u t i o n of copper was proposed; (i)  Adsorption of dissolved oxygen onto the copper surfaces  Cu + (ii)  1/2  0  2  fast — Cu....O  .,,,...(1)  Reaction of an ammonia molecule or ammonium i o n with the copperoxygen complex on the surface?  2.  Cu» p • © 0 + NH  slow  3  Cu  O • e e  0 + NH*  NH  3  + HOH  fast  + OH"  Cu(NH ) % 20H"  (2)  3  — Cu(NH ) 3  + 20H  «  «  O  s  .(3)  o • •  Following this work on copper, i t seemed of interest to examine the effect on the dissolution reaction of varying the composition of the metal phase by suitable alloying, copper remaining the principal or only dissolving metal. Gold was chosen as the alloying metal because: (a)  i t i s inert to attack by a l l the reagents contemplated.  (b)  i t forms a continuous solid solution with copper, and the structural properties of this system have been extensively investigated.  (c) there appeared to be some possibility of studying the^ effect of ordering on the corrosion properties of the alloy. Previous Work on the Corrosion of Similar Alloy Systems.. The corrosion properties of copper-gold alloys have been studied by Tammann (2) and Muller, Freissler and Plettinger (3).  How-  ever, their work was mainly restricted to establishing parting limits of the alloys in various corroding media. Landau and Oldach (4) have compiled a comprehensive summary of qualitative and quantitative data on the corrosion properties of various solid solution alloys, and have noted several interesting-correlations.  In particular, they  The parting limit of an alloy may be defined as the maximum concentration of the less noble element of an alloy at which selective oxidation can occur. Beyond this limit the alloy behaves like the pure noble metal (5).  3.  emphasized the importance of p a s s i v i t y l i m i t s , and noted the e x i s t ence of minima i n the corrosion rate - composition curves of certain a l l o y systems  0  Several theories r e l a t i n g to the corrosion of a l l o y s have been proposed and, although the present work was  not undertaken with  the idea of applying or confirming any p a r t i c u l a r one of these theories, i t may  be u s e f u l to r e f e r b r i e f l y to some of them  0  In 1921  Tammann (2)  proposed a c o r r e l a t i o n between parting l i m i t s and the atomic r a t i o of the metals i n the a l l o y , noting c e r t a i n exceptions i n terms of s p a c e - l a t t i c e isomerides.  Russell (6)  and explaining these noted a somewhat  s i m i l a r c o r r e l a t i o n between a l l o y structure and p a s s i v i t y . Wulff ( 8 ) ,  Uhlig and  extending Russell's ideas, proposed the Electron Configur-  ation Theory i n which passive metals are characterized by u n f i l l e d d energy bands.  These passive metals are able to transmit t h e i r charact-  e r i s t i c s to other less passive metals i n a l l o y s by electron sharing. Landau (9), t r e a t i n g theories based on a l l o y structure with considerable reserve, extended the l o c a l c e l l theory f o r the corrosion of pure metals to a l l o y s .  It should be noted here that most a l l o y corrosion theories t r e a t p a s s i v i t y rather than parting.  Since i n the present system the  a l l o y i n g metal (gold) i s thermodynamically immune to corrosion i n the media used, the concept of parting i s of greater significance than  Uhlig and Mears (7) give the following d e f i n i t i o n of p a s s i v i t y ; "a metal or a l l o y i s passive i f i t s u b s t a n t i a l l y r e s i s t s corrosion i n an environment where thermo-dynamically there i s a large free energy decrease associated with i t s passage from the metallic state to appropriate corrosion p r o d u c t s ,,  0  that of passivity, Ob.ject and Scope of the Present Investigation This study was undertaken with the object of establishing the effect of a l l o y i n g copper with an i n e r t metal on the d i s s o l u t i o n k i n e t i c s of the former.  Specimens of copper-gold a l l o y s were exposed  to ammonia solutions i n an autoclave maintained under controlled temperature and pressure conditions, and the rate of d i s s o l u t i o n of copper determined by periodic sampling and analysis of the s o l u t i o n . variables examined included a l l o y composition, NH  3  ions, oxygen pressure and temperature.  The  The  and NH£ concentrat-  experimental procedures  used and the r e s u l t s obtained are described and discussed below.  5.  EXPERIMENTAL  A«  Preparation of the Alloys: The copper used in this work was supplied as half-inch  rods by Ellett Copper and Brass Company. The gold was supplied as fine gold shot by Birks Limited.  Spectrochemical analyses of these  metals and of the alloys are given in Appendix I. At the outset, i t was thought that three alloys at 5 atomic percent intervals (i.e. corresponding to 5, 10 and 15 atomic percent gold) would be sufficient to establish a pattern of results which would suggest what other alloys should be prepared and studied. The metal constituents for these alloys were carefully weighed, and then degassed and melted by induction heating in a carbon crucible under 150 microns of argon pressure. On cooling, a cylindrical ingot about 1.25 inches in diameter and weighing about 25 grams was obtained. It was subsequently decided to prepare and study another alloy containing 2 atomic percent gold.  Since a carbon crucible was not easily  obtainable at that time, an alundum crucible was used in preparing this alloy. It was also desired to conduct some comparative dissolution experiments on pure copper, and a sample of pure copper was melted and heat treated under the same conditions as the alloys.  "1  '  - — —  _  Kindly carried out by the Department of Mines, Victoria, B.C. Henceforth the alloys w i l l be referred to as alloy 2, alloy 5» etc., where the number corresponds to the gold content in atomic percent.  6.  A l l the a l l o y s were homogenized i n an argon atmosphere at 850°C. f o r periods ranging between 60 and 75 hours. furnace cooled to 500°C. and water quenched.  They were then  This treatment  was  designed to produce disordered structures f o r , i f studies were to be made on the effects of order on the d i s s o l u t i o n rates of the a l l o y s , i t was decided to treat disordered a l l o y s f i r s t .  Although ordering  i s most s i g n i f i c a n t i n a 25 atomic percent gold alloy, a consistent treatment f o r a l l the alloys was desirable.  The c r i t i c a l  temperature  for the order-disorder transformation i n a 25 atomic percent a l l o y i s 391°C  The ingots were turned on a lathe to a uniform diameter which was measured with a micrometer.  They were then sawed i n half  through a plane perpendicular to the c y l i n d r i c a l axis, and mounted i n bakelite with the c i r c u l a r face exposed.  Samples from the saw cut-  tings and from d r i l l i n g s , which were taken at the completion of the work, were assayed f o r gold.  The assays were found to correspond very  c l o s e l y to the intended compositions (see Appendix I I ) . Metallographic t e s t s f a i l e d to show any evidence of coring or inhomogeneity  i n the  ingots, but i n some cases i n t e r d e n d r i t i c shrinkage c a v i t i e s were observed toward the upper end.  It was assumed that t h i s shrinkage d i d  not affect the surface area appreciably.  P r i o r to each experiment the mounted a l l o y samples were refaced i n a lathe to remove any f i l m or surface inconsistencies i n composition which might have resulted from the previous  experiment.  About 3/1000 inches were machined from the exposed surfaces of the a l l o y s , which were then polished using standard techniques and stored  7.  i n a dessicator f o r subsequent experiments. B„  Apparatus; The experiments were conducted i n a stainless s t e e l auto-  clave (see Figure 1) designed f o r working pressures up to 150 psig. The bakelite mount (which also served as e l e c t r i c a l insulation) containing the metal specimen was held by a stainless s t e e l rod, tapped into the l i d of the autoclave, so that the surface of the a l l o y was exposed to the s o l u t i o n about 0.25 inches above the blades of the impeller.  Fresh solution was forced against the face of the a l l o y  by the impeller, which was 1 0 . 9 cm. i n diameter and rotated at 720 RPM.  The temperature of the solutions was regulated by a Wheelco  c o n t r o l l e r coupled to a platinum resistance thermometer, which  Was  placed i n a s t a i n l e s s s t e e l w e l l extending from the l i d of the autoclave into the solution.  The thermometer leads were connected i n the  controller across a double pole, single throw microswitch which a c t i v ated solenoid valves on gas and water l i n e s .  The gas was fed to a  r i n g burner mounted under the autoclave, and the water l i n e was connected to cooling c o i l s running through the corroding solution.  A  c y c l i c temperature v a r i a t i o n resulted which could be held to within ±1°F. at 77°F. (25°G:.) and ±3°F. at 132°F. (50*©.).  Pressure was  controlled by a standard pressure regulator on the oxygen cylinder.  G.  Chemical Reagent s and.. Solutions...  Chemically pure grade ammonium hydroxide and ammonium sulphate, supplied by the Nichols Chemical Company, were used. Oxygen was supplied i n cylinders by the Canadian Liquid A i r Company,  8.  Figure L, Diagram of Stainless Steel Pressure Vessel and Internal Parts. A - Cooling Coil B3 -• Impeller C - Gas Inlet Tube  D - Sampling Tube £ - Thermometer Well F - Metal Specimen i n Bakelite Mount  and was used without further p u r i f i c a t i o n . Solutions were prepared by d i l u t i n g a measured volume of ammonium hydroxide to 3»0 l i t r e s with d i s t i l l e d water.  Ammonium  sulphate was weighed and added to the s o l u t i o n i n the autoclave. D.  Measurement of the Rates, of. Dissolutions. After the desired specimen and solution were placed i n  the autoclave, the l a t t e r was sealed and the solution brought to temperature.  When the correct temperature was attained, the oxygen was  introduced and maintained at the desired value.  The f i r s t sample of  the solution was taken within one minute of the pressure  increase.  To follow the rate of the reaction 25 ml. samples were withdrawn through the sample tube shown i n Figure 1 at 30 minute i n t e r v a l s and analysed f o r copper. ' Since the surface area of the a l l o y and the v o l ume of solution were known, the amount of copper dissolved per unit area could be calculated.  E.  Analytical.Procedures.:. (1)  Copper; Copper concentrations were determined  a l l y by the Carbamate method (11).  spectrophotometric-  Sodium diethyldithiocarbamate forms  a stable, colored chelate compound with cupric ions, showing a maximum l i g h t absorption at 437 millimicrons.  A Beckman model DU  spectrophoto-  meter was used to determine the o p t i c a l density of the solutions at t h i s wavelength.  The o p t i c a l density was found to be proportional t o  the copper concentration.  (A c a l i b r a t i o n curve f o r t h i s a n a l y t i c a l  10. procedure i s given i n Figure 2). 7 ml. of a carbamate s o l u t i o n  To prepare the samples f o r analysis,  were added to an aliquot of the sample  taken from the autoclave, and the mixture diluted with d i s t i l l e d water to 50 ml. i n a volumetric f l a s k .  The aliquot used varied from 1 to 5  ml. depending on the copper concentration.  Since the s t a b i l i t y of  the copper carbamate complex i s affected by daylight ( l l ) , solutions were prepared i n an a r t i f i c i a l l y l i t room.  analytical  With c a r e f u l  technique, r e s u l t s obtained by t h i s method could be duplicated within 1 percent.  (2)  Gold: It was also thought advisable t o develop a method f o r  determining  gold concentrations i n the corroding solutions.  A color-  imetric procedure was again.adopted using p-dimethylaminobenzalrhodanine which forms a colored chelate complex with gold ions.  s  A 15 ml.  aliquot of the s o l u t i o n was made 0,1 N i n HCl and 1 ml. of a 20 percent saturated solution of rhodanine i n e t h y l alcohol was added. The r e s u l t i n g s o l u t i o n was d i l u t e d to 50 ml. and i t s absorption measured i n s i l i c a c e l l s at 300 millimicrons. While i t was found that t h i s method did not give q u a n t i t a t i v e l y reproducible r e s u l t s , i t was very sensitive and made i t possible to detect v i s u a l l y less than l y of gold per ml. by the purple color developed.  Copper had no e f f e c t on  the v i s u a l t e s t , although i t could be detected by the :  —  _  _  _  _  spectrophoto-  _  Preparation of the carbamate solutions Solution As 0.4 grams carbamate i n 400 ml. d i s t i l l e d water. Solution Bs 10 grams gum arabic and 1 ml. toluene i n 1000 ml. d i s t i l led water. Solutions A and B were mixed and f i l t e r e d , and stored i n the dark.  11.  20  h  0  6  0  8  0  Cu f y / m l . Figure 2 .  C a l i b r a t i o n curve f o r copper carbamate.  100  12.  meter.  I t was found that the o p t i c a l densities of copper and gold  rhodanine were a d d i t i v e . (3)  Ammonia: Ammonia concentrations were determined by potentiometric  t i t r a t i o n with standard HCI.  13o RESULTS AND DISCUSSION  Nature of the Reactions and Rate Curves.  In a l l the experiments, specimens of copper or of a coppergold a l l o y were exposed to solutions containing NH3 (and, i n some instances, (NH*)aSO*,.) under oxygen pressure. In general, i t was found that the specimens were corroded under these conditions and that copper was dissolved. solutions.  At no time could any gold be detected i n the  The reaction involved thus appears to be represented by  the equation:  C u  (ioo-n) (n) A u  +  7  /  2  °  2 +  W  Cu(ioo-.r)M(r5  +  3  +  y H  * ° ~*  yCu(NH )r • 2y0lT  ..(4)  3  where r > n, n and r being the atomic percent gold i n the a l l o y s before and a f t e r corrosion.  A l l the r e s u l t s described below thus  r e l a t e t o measurements of the rate of d i s s o l u t i o n of copper.  The shapes of the two rate curves, shown i n Figure 3, are t y p i c a l of those obtained f o r pure copper and f o r the copper-gold a l l o y s , respectively, under most conditions.  The rat<e plots f o r the  d i s s o l u t i o n of pure copper were always found to be l i n e a r , the amount of copper dissolved being proportional to the time, i . e .  Cu -  fcjt  (5)  On the other hand, the rate of d i s s o l u t i o n of copper from coppergold a l l o y s generally f e l l o f f with time, corresponding i n most cases  14.  ALLOY  O—  TIME - HOURS F i g u r e 3.  T y p i c a l rate curves f o r the d i s s o l u t i o n of copper f o r pure copper and a l l o y 5, showing r e l a t i v e rates. T <= 25 C; 0 - 6.8 Atm.j NH - 0.5 <a/l. 9  2  3  15. to a parabolic r e l a t i o n , i . e .  £cuj  = a +  kT  00.0.0.(6)  2  where Cu i s the amount of copper dissolved per square centimeter of surface i n time T, and k p kg and a are constants  0  Such a r e l a t i o n usually denotes the buildup of a surface f i l m on the corroding specimen which slows the reaction as i t t h i c k ens by impeding the transfer of reactants and products.  I t was  concluded that t h i s f i l m was due to the gold which was l e f t behind at the surface of the a l l o y when the copper dissolved.  This was  confirmed by subsequent analyses of some of the films which showed them to be much r i c h e r i n gold than the o r i g i n a l a l l o y s . Although i t was found that the rate curves obtained i n most of the experiments on a l l o y s conformed c l o s e l y t o a parabolic r e l a t i o n , there were not infrequent exceptions t o t h i s r u l e .  This,  together with the f a c t that the rate curves f o r pure copper were l i n e a r , made i t impossible  to compare the rates f o r the d i f f e r e n t  specimens i n terms of a uniform rate constant.  For these reasons i t  was decided to adopt the amount of copper dissolved during the f i r s t hour of any experiment as a measure of the rate.  A l l subsequent  correlations of, the e f f e c t s of variables are expressed i n terms of t h i s quantity.  Where possible, such correlations were compared with  those based on parabolic rate constants, and i n general the trends were found t o be s i m i l a r .  In the following sections are described the r e s u l t s o f studies on the d i f f e r e n t v a r i a b l e s .  Unless otherwise stated, the  16. pressure and temperature i n a l l the experiments  were 6.8 atmospheres  of oxygen and 2 5 ° C ,  E f f e c t of NHS  Concentration.  The corrosion of copper and each of the four  copper-gold  a l l o y s was investigated using a s e r i e s of solutions containing d i f ferent concentrations of NH  3  generally ranging from 0 . 1 5 to 2 . 5 m/1.  No NHi£ s a l t s were added to these solutions.  Some t y p i c a l rate curves  obtained during t h i s series of experiments are shown i n Figures 4 to 7, and the r e s u l t s are summarized i n Figures 8 and 9 and i n Table I.  In general i t was found that increasing the concentration of NH  3  resulted i n a higher rate of d i s s o l u t i o n of copper f o r the  a l l o y s as w e l l as f o r pure copper.  This i s shown i n Figures 8 and 9 .  Figures 4 and 5 demonstrate the rate curves obtained f o r copper and for  an a l l o y with increasing NH  3  concentrations.  The parabolic  character of the a l l o y rate curves i s shown i n Figure 6,  vAiere the  square of the amount of copper dissolved i s plotted against time. It should be noted that the extrapolations of the l i n e s i n Figure 6 do not go through the o r i g i n , but intersect the ordinate, i n d i c a t i n g a high i n i t i a l rate of copper d i s s o l u t i o n .  This effect was p a r t i c u l a r -  l y pronounced with a l l o y 1 0 ( s e e Figure 7)  }  creasing NH  3  concentrations.  and increased with i n -  However, a f a l l i n g o f f of the rate was  always observed following t h i s i n i t i a l period, i n d i c a t i n g that ultima t e l y the f i l m became rate c o n t r o l l i n g i n every  case.  The e f f e c t on the rate of increasing the gold content of the a l l o y i s shown i n Figures 8 and 9 o  P a r t i c u l a r reference may be  17.  160 \ -  120  o  I S o  CO CO  o0  TIME - HOURS  Figure 4.  Rate curves for pure copper in solutions containing NH alone. T - 25°C; 0 = 6.8 Atm.  3  2  18  s o  1 o  CO CO  M Q  o  TIME - HOURS  Figure 5. Rate curves for the dissolution of copper from alloy 5 i n NH solutions, T * 25°C; 0 =6.8 Atm. 3  2  Figure 6„  Parabolic plots f o r the d i s s o l u t i o n of copper from a l l o y 5 i n NH solutions. T = 25°Cj 0 <= 6„8 Atm. 3  2  20.  TIME - HOURS Figure 7. Comparative rate curves for the various alloys under similar corroding conditions i n NH solutions. NH = 0.5 m/1; T = 25°C; 0 - 6.8 Atm. 3  2  3  ALLOY • - Cu  NH Figure 8.  E f f e c t of NH T = 25°C; 0  3  2  3  - M/L  on the rate of copper d i s s o l u t i o n . = 6.8 Atm.  22.  Figure 9.  E f f e c t of gold on the rate of copper d i s s o l u t i o n i n solutions containing NH alone. T = 2 5 ° C ; 0 = 6.8 Atm. 3  2  23. TABLE  I.  Data on Experiments w i t h S o l u t i o n s C o n t a i n i n g NH^ A l o n e .  ± Run No.  NH  3  Alloy  m/1  C-6 C-34 C-3 G-4 C-5  0.5 0.5 0.5 0.5 0.5  C-35  0.75  NH Titrated 3  Cu 2  L  15  0.47 0.48 0.46 0.48 0.47  P P P  -  5  0.75  P  -  Cu 2  0.99 0.96 1.00  L P P P  5  10  C-24  1.0 1.0 1.0  1.0  5 10  G-23  1.0  15  C-99 C-39  1.5 1.5  5  1.57  15  1.60  C-103  2,0  2  2.00  C-60  2.0  Cu  0=42 C-41  ±kk Linear Rate  Parab. Rate  m/1  C-36 C-21 G-22  c-27 c-25  Rate Type  1.05 1.04  I  -  -  -  —  _  P P  2.0 2.0 2,0  5  2.00  10 15  1.99 2.15  2.3 2.3  5 10  2.28 2.30  P I  (0.019) 0.02 0.013 0.013  =  =  5.25 4.0 10.4  0.19 0.15 0.08 (0.11)  62.0 12.6 18.3 16.6 10.0  0.39 0.60  27.1 18.0 36.0 82.2  82.2  -  20.5 2.89 4.41  0.60  66.6  I  L P I P  1.89  21.0  • Cu Dissolved i n 1 hour mg  0.88 (0.34) 0.81  42.5 39.8 22.1  1.03  45.6  —  a. 5  ± Rate t y p e s s  ±k  L = Linear|  P = Parabolic;  Rates i n p a r e n t h e s e s are d o u b t f u l . [mgCu/ cm2j 2 / h o u r .  I = Intermediate;  Units of parabolic rates are  ±M U n i t s o f l i n e a r rates are mgCu/cm /hour. 2  24. made t o t h e pronounced decrease i n r a t e which o c c u r s when as as 2 atomic p e r c e n t g o l d i s added t o the c o p p e r . o f t h e s e experiments  (i.e.  Under t h e  little conditions  when s o l u t i o n s c o n t a i n e d NH3 but no NH4,  salts)  further  a d d i t i o n o f g o l d had r e l a t i v e l y  rate.  F o l l o w i n g a minimum at about 2 atomic p e r c e n t g o l d , t h e  appeared t o i n c r e a s e s l i g h t l y w i t h f u r t h e r  little  effect  on t h e rates  gold a d d i t i o n , reach a  maximum i n t h e r e g i o n o f 5 t o 1 0 atomic p e r c e n t g o l d , and t h e n d e c r e a s e a g a i n a s t h e g o l d was i n c r e a s e d t o 1 5 atomic p e r c e n t . the  Since  shapes of t h e r a t e curves f o r t h e v a r i o u s a l l o y s i n NH3 are  complex and d i f f e r  s i g n i f i c a n t l y f o r the d i f f e r e n t  o f the amount o f copper d i s s o l v e d i n the f i r s t t h e r a t e i s p o s s i b l y not j u s t i f i e d therefore  hour as a measure of  in t h i s case.  The r e s u l t s s h o u l d  be i n t e r p r e t e d w i t h c a u t i o n , s i n c e the d i f f e r e n c e s  are r e l a t i v e l y  E f f e c t of  a l l o y s , the use  involved  small.  (NH/.) S0 , 9  It w i t h no f r e e  A  was found t h a t s o l u t i o n s c o n t a i n i n g NH* s a l t s o n l y , N H 3 , d i d not r e a c t w i t h c o p p e r .  A p p a r e n t l y some f r e e  NH3 i s n e c e s s a r y t o f o r m t h e s o l u b l e cuprammine i o n . t h e NH4 i o n was t h e r e f o r e which t h e f r e e  of  i n v e s t i g a t e d u s i n g a s e r i e s of s o l u t i o n s  NH3 c o n c e n t r a t i o n was kept  c o n c e n t r a t i o n of  The e f f e c t  in  constant at 0 . 5 m/1 and the  (NH*) S04 v a r i e d between 0 . 0 1 and 0.06 m/1 ( c o r r e 2  sponding t o NH£ c o n c e n t r a t i o n s r a n g i n g from 0,02  to 0 . 1 2 m / l ) .  This  s e r i e s o f s o l u t i o n s was used t o s t u d y t h e c o r r o s i o n o f copper and o f each of t h e a l l o y s .  Some of t h e t y p i c a l r a t e curves o b t a i n e d i n these  experiments are shown i n F i g u r e s 10 t o 1 3 , and a l l t h e r e s u l t s summ a r i z e d i n T a b l e II,  + The r a t e s are p l o t t e d as f u n c t i o n s o f t h e KH/,.  TIME - HOURS Figure 10. Rate curves for pure copper in solutions with constant NH and varying SJH+. NH = 0.5 m/1; T = 25°Cj 0 » 6.8 Atm. 3  2  3  Figure 11. Rate curves for alloy 10 i n solutions with constant NH varying NH*.. NH - 0.5 m/lj * = 25°Cj 0 - 6.8 Atm.  3  3  2  and  27  Figure 12.  Parabolic plots for alloy 10 i n solutions with constant NH and varying NH?. NH - 0,5J T = 25 C; 0 - 6.8 Atm. P  3  3  2  T J M E - HOURS  Figure 13. Comparative rate curves for the alloys i n solutions with constant NH , constant NHJ. Nh*£ • 0.04 m/1; NH - 0.5 m/1: T m 25°Cj  3  0 - » 6.8 Atm. 2  3  29. TABLE I I .  Data on Experiments with Solutions i n which + NHfe was Varied and Nfo Held Constant.  ft  Run No, NH*  NH  3  NH Rate Linear Alloy Titrated Type Rate 3  m/1  m/1  C-73 C-80 C-18 C-37 C-19 C-20 C-17  0.006 0.012 0.02 0.02 0.02 0.02 0.02  0.5 0.5 0.5 0.5 0.5 0.5 0.5  Cu Cu Cu  C-10 C-43 C-8 C-9 C-ll  0.04  Cu  0.04 0.04  0.04  0.5 0.5 0.5 0.5 0.5  C-31 C-45 C-14 C-16  0.06 0,06 0.06 0.06  0.5 0.5 0.5 0.5  Cu  C-13 C-46 C-26 C-15 C-12  0.08 0.08 0.08 0.08 0.08  0.5 0.5 0.5 0.5 0.5  Cu  C-56 C-38  0,10 0.10  0.5 0.5  C-44 G-59 C-29 C-30  0.12 0.12 0.12 0.12  0.5 0.5 0.5 0.5  ±  M  0.04  L = Linear;  m/1  2 5  10 15 2 5  10 15 2 5  10 2 5  10 15  mg  L L L I I P  33.3 52.5  76.8 60.0  0,50 0.51  L L P P  -  0.53 0.50 0,50 0.51  L I P P  80.8 68.3  0.52 0.52  L L P P  83.3 67,8  -  5.64 2.36  mm mm  6,86  61.0 44.0  mm mm  85.5 68.0 59.0 44.9  0.49  0,51 0.49  0.50 0.50 0.48 0.49 0.49 0.53 0.53  0.49 0.51 0.51 0.51  P P  Cu  0.51 0.51 0.45 0,49  L I P P  10  P = Parabolic?  -0 -  -  0  mm  -  . 0,52  2 5  -  62.2  -  10  5  Parab. Cu Rate Dissolved in 1 hour  0  89.6 (59.0)  -  —  33.3 52.5  —  (1.51) (1.41) 0.53 0  __,  _•  3.60 1.14 0  „  5.47 1.72  -  0  2.74  7.04 2.67  62.2 31.5  28.8 17.2 0  76.8 60.0  45.0  25.5 0  80.8 68.3 55.0  32.8 83.3 67.8 54.8 37.8 0  I Intermediate! 3  Rates i n parentheses are doubtful. Units of parabolic rates are {mgCu/cm2/ /hour. 2  Units of linear rates are mgCu/cm/hour. 2  30. i o n concentration and of the a l l o y composition i n Figures 14 and  15,  respectively. An examination of the r e s u l t s reveals the f o l l o w i n g effects? (1)  A l l the rate curves f o r pure copper were again found t o be l i n e a r . Most of those f o r the a l l o y s (but not a l l ) were parabolic (compare Figures 10, 11 and 12).  (2)  Increasing the gold content of the a l l o y r e s u l t e d i n a lowering of the r a t e of d i s s o l u t i o n (see Figures 13 and 15).  The rate  decreased continuously w i t h i n c r e a s i n g gold concentration u n t i l the r e a c t i o n ceased at about 15 atomic percent gold.  In no case,  when the s o l u t i o n s contained NH]£ ions as w e l l as f r e e NH , 3  any r e a c t i o n be detected w i t h a l l o y 15.  could  These r e s u l t s are i n  marked contrast t o those observed with s o l u t i o n s containing NH  3  only and discussed e a r l i e r , where i t was found that the i n i t i a l a d d i t i o n of a s m a l l amount of gold t o the copper r e s u l t e d i n a large decrease i n the r a t e , while f u r t h e r gold a d d i t i o n had r e l a t i v e l y l i t t l e e f f e c t (compare Figures 9 and 15). (3)  Increasing the gold content of the a l l o y a l s o tended t o make the r a t e curves more p a r a b o l i c as i s shown i n Figure 13. u t i o n containing 0.08 m/1  of NH^,  In a s o l -  i t was found that the r a t e  curve f o r a l l o y 2 remained l i n e a r u n t i l a considerable amount of copper had d i s s o l v e d . The rate curves f o r a l l o y s 5 and  10,  on the other hand, were parabolic from the s t a r t . (4)  The rates of d i s s o l u t i o n f o r pure copper and f o r a l l o y s 2, 5  32.  -  O-  M/L  0.02  • - 0.04 O -  o.o6  V-  0.08  \ \  \\ \  \ \  \  • \  \  v  \  \  \  \ \ \ N\\\  10  15  ATOMIC % GOLD Figure 15.  E f f e c t of gold on the rates of copper d i s s o l u t i o n i n solutions with constant NH and varying NH£. NH = 0.5 m/l> T = 25°C; 0 2 =6.8 Atm. 3  3  33. and 10 increased with increasing NH4 concentration u n t i l the l a t t e r reached about 0.06  m/1.  Further addition of NH£ d i d not  increase the d i s s o l u t i o n rates (see Figures 14 and 15)«  The  l i m i t i n g value of the rate decreased with increasing gold concentration i n the a l l o y s , and became zero f o r the 15 atomic percent a l l o y . attainment  For pure copper, there i s evidence  ( l ) that the  of a l i m i t i n g value of the rate i s due to control by  oxygen transport through the s o l u t i o n to the surface of the metal. With the a l l o y s , i t i s apparent that other considerations are involved.  (5)  With a l l o y 2 i t was noted that the rate curves tended toward l i n e a r i t y when the NH^ the rate curve was  ion concentration was increased.  parabolic below 0.02  m/1  Thus  of NH^ ion and l i n e a r  at higher concentration, but with a tendency to become parabolic i n l a t e r stages of the reaction. Without going into d e t a i l e d considerations of the mechanism at t h i s point, some general conclusions r e l a t i n g to the effect of NH^ may  be ,noted.  The presence of NH£ appears to have a large effect on  a l l o y s of low gold content.  This effect i s to increase the rates  very markedly and to make the rate curves tend toward l i n e a r i t y , or i n other words, to o f f s e t the effects which were observed when only NH3 was present i n the solutions and which were attributed to the presence of a f i l m .  Since i t i s not l i k e l y that NHj£ could have any  e f f e c t on a f i l m consisting only of gold or a gold-rich m e t a l l i c phase, there i s some suggestion here that an oxide, such as copper oxide whose s t a b i l i t y i s very sensitive to NH^,  i s constituting part of the  34. f i l m structure.  It should be emphasized that t h i s applies only to +  the low gold a l l o y s .  For the a l l o y s higher i n gold, NH* had a much  smaller effect and i n the case of a l l o y 15 a c t u a l l y i n h i b i t e d the reaction.  The E f f e c t of Varying NH^  i n the Presence of Constant  NH*  In view of the differences noted above between solutions containing NH3 only and those containing both NH  3  and NH*,  i t was  decided to undertake a series of experiments on a l l the alloys i n which the NH* NH  3  concentration was  held constant at 0 . 0 2 m/1  concentration varied between 0,25 and 1,0 m/1.  and the  Some t y p i c a l  r e s u l t s of these experiments are represented by Figures 16 to 20 and summarized i n Table I I I .  These r e s u l t s present a very s t r i k i n g feature, namely that increasing the concentration of NH and NH*  3  i n solutions containing both  NH  3  has a very s i m i l a r e f f e c t to that obtained by increasing the  concentrations of NH* i n such solutions and discussed e a r l i e r .  In  p a r t i c u l a r , the following points of s i m i l a r i t y may be noted5  (1)  Increasing the NH  3  proach l i n e a r i t y . It was  concentration caused the rate curves to apThis was  noted with both a l l o y 2 and a l l o y  shown e a r l i e r that increasing the NH*  5.  concentration had  a similar effect.  (2)  The rate increased with increasing NH  3  ed a l i m i t i n g value at 0.75 t r a t i o n s a tendency was but the e f f e c t was  m/1  of NH . 3  concentration, and reachAt higher NH  3  concen-  observed f o r the rate to f a l l o f f again,  small and i t s v a l i d i t y i s open to question i n  35.  TIME - HOURS Figure. 16  Rate curves f o r pure copper i n solutions with constant NH+. and varying NH . NH* = 0.04 m/l> T = 25°Cj 0 «• 6.8 Atm. : 3  2  36.  TIME - HOURS Figure 17. Rate curves for alloy 5 i n solutions with constant NH£ and varying NH . KH* = 0.04 m/lj T = 25°Cj 0 - 6.S Atm. 3  2  37  Figure IB. Rate curves for alloy 10 i n solutions with constant NH* and varying NH . NH* - 0.04. m/1; T = 25°C; 0 - 6.8 Atm. 3  2  Figure 19.  Parabolic plots f o r a l l o y 10 i n solutions with constant m% and varying NH . NH* = 0,04 m/1; T - 25°C; 0 = 6.8 Atm. 3  2  39  ALLOY  Figure 20.  E f f e c t of NH i n solutions containing constant NH* NH* = 0.04 m/1; T = 25°C; 0 = 6,8 Atm. 3  2  f  3  40.  TABLE I I I .  Data on Experiments with Solutions i n which NH^ was Varied and NHt Held Constant.  Run No.  •k  NH  NH£  NH Rate A l l o y T i t r a t e d Type 3  3  ±±k  Linear Parab. Rate Rate  m/1  m/l  G-63  0.04  0.15  Cu  0.14  L  48.2  G-51 C-62 C-47 C-50  0.04 0.04 0.04 0.04  0.25 0.25 0.25 0.25  Cu  2 5 10  0.26  68.4  0.25  L I P P  C-97  0.04  0.35  Cu  0.35  L  75  0.04 . 0.5 0.04 0.5 0.04 0.5 0.04 0.5 GJ-ll 0.04 0.5  Cu  0.49 0.53 0.53 0.50  76.8 60.0  0.51  L L P P  -  .C-52 C-68 C-49 C-53  0.04 0.04 0.04 0.04  0.75 0.75 0.75 0.75  Cu  0.74 0.71 0.72  L I P P  86.6 67.2  -  8.29 2.36  C-58 C-70 C-54 C-55 C-48  0.04 0.04 0.04 0.04 0.04  1.0 1.0 1.0 1.0 1.0  Cu  2 5 10 15  1.02, 0.92 1.02 1.00 0.99  L I I P  80.9  -  0.12 0.20 0.25 0.5 1.0  10  0.12  C-10 C-43 C-8 C~9  C=56 0.12 C-61 0.12 C-62 0.12 C-30 0.12 C-64 0.12  L - Linear;  m/1  2 5 10 15 2 5 10  10 10 10 10  0.24 0.26  0.80  I  -  1.21 0.42  68.4 22.8 26.1 14.3  75  0  -  ~  (54oO)  0  „  =.  3.60 1.14 0 =.  .  =.  ,„  3.72 0 0.50  P P I P I  0.93  P = Parabolic!  _.  "  0.21 0.24 0.49  48.2  f~  ea  Cu Dissolved i n 1 hour  —  0.91 —  —  —  76.8 60.0 45.0 25.5 0 82.8  67.2 60.5 38.5  78.3 61.0 54.0 45.0 0 17.5  24.5  26.0  44.5 47.0  Intermediate;  =  Rates i n Parentheses are d o u b t f u l . [mgCu/ cm^ /hour.  Units of parabolic rates are  2  Units of l i n e a r rates are mgCu/cm /hour. 2  view of the e r r o r s involved i n both the measurements and method used to estimate the r a t e .  the  More s i g n i f i c a n t i s the f a c t  that the l i m i t i n g rates obtained on i n c r e a s i n g the NH  3  concen-  t r a t i o n were very close t o those which r e s u l t e d o n i n c r e a s i n g the NH]£ concentration.  The value of t h i s l i m i t i n g rate thus appears  to be c h a r a c t e r i s t i c only of the composition of the a l l o y . might a l s o be noted t h a t the e f f e c t of NH , 3  s i m i l a r t o that of NH%. t r a t i o n s of NH  3  Figures 14 and (3)  At very low NH  was  It  while q u a l i t a t i v e l y  smaller, considerably higher concen-  being required to achieve the l i m i t i n g rate (see 20).  3  concentrations the pronounced i n i t i a l decrease  i n rate on a d d i t i o n of a s m a l l amount of gold to the copper was again observed, w i t h an apparent minimum i n the rate o c c u r r i n g at about 2 atomic percent gold (see Figure 21).  A similar effect  has already been noted f o r s o l u t i o n s very low i n  NH^.  E f f e c t of Oxygen P a r t i a l Pressure. The e f f e c t s of varying the p a r t i a l pressure of oxygen from 1,7  t o 6.8  the a l l o y s .  atmospheres were i n v e s t i g a t e d f o r copper and f o r each of In t h i s s e r i e s of experiments the compositions of the  s o l u t i o n s were held constant at 0.5  m/1  NH  3  and 0.02  m/1  NH£.  The  rates were thus w e l l below the l i m i t i n g values attained at higher NH  3  or NH4 concentrations.  Some t y p i c a l rate curves d e p i c t i n g the  e f f e c t s of v a r y i n g the oxygen pressure on pure copper and on one  of  the a l l o y s (the e f f e c t s f o r the other a l l o y s were s i m i l a r ) , are shown i n Figures 22 and 23.  The r e s u l t s f o r t h i s s e r i e s of exper-  iments are summarized i n Table 17.  NH^ - M/L  2  5  10  15  ATOMIC PERCENT GOLD Figure 2 1 , E f f e c t of gold on the rates of copper d i s s o l u t i o n i n solutions with constant NH* and varying NH . NH* = 0 , 0 4 m/1; T = 25°C; 0 - 6 . 8 Atm. 3  2  PRESSURE  O• -  1.7  ATM.  3.4  1  2  TIME - HOURS Figure 22,  Rate curves f o r pure copper at various oxygen pressures.  NH  3  = 0.5 m/1;  NH* - 0.02 m/1;  T = 25°C  44.  TIME - HOURS Figure 2 3 .  Rate curves f o r a l l o y 10 at various oxygen pressures. NH = 0 . 5 m/1; NH* = 0 . 0 2 m/1; T = 25°C. 3  45. TABLE  IV o  Data on Experiments i n which Oxygen Pressure was Varied.  M  ft  Run No.  Rate Linear Parab. Cu Diss. mt Titrated Type Rate Rate in 1 hr. NH  Alloy Pressure Temp. NH  3  3  Atm.  "C^ m/l  Cu 2 10  1.7 1.7 1.7  25 25 25  0.5 0.02 0.5 0.02 0.5 0.02  0.49 0.51 0.50  L  I I  -  C-92 Cu .J3=89 2 C-71 10  3.4 3.4 3.4  25 25 25  0.5 0.02 0.5 0.02 0.5 0.02  0.49 0.50 0.49  L  42.0  C-96 Cu C-94 2 C-74 10  5.1 5.1 5.1  25 25 25  0.5 0.02 0.5 0.02 0.5 0.02  0.47 0.49 0.50  L  C-18 C-37 C-19 C-20  Cu 2 5 10  6.8 6.8 6.8 6.8  25 25 25 25  0.5 0.5 0.5 0.5  0.02 0.02 0.02 0.02  0.49 0.50 0.50 0.48  L  I I P  -  C-101 Cu  8.5  25  0.5 0.02  0.50  L  61.1  C-79 10 C-85 10 C=76 10  1.7 3.4 6.8  15 15 15  0.5 0.02 0.5 0.02 0.5 0.02  0.52 0.51 0.50  I P P  10 10 10 10  1.7 3.4 5.1 6.8  35 35 35 35  0.5 0.5 0.5 0.5  0.02 0.02 0.02 0.02  0.48 0.48 0.49 0.49  I P I I  C-88 C-83 C-69  C-87 C-91 C=93 C-78  me.  m/1  m/i,  I P P P  20.0  -  •-  53.9  -  62.2  20.0 25.5 12.2  -  .....  .....  0.69 mm  2.0 0.78 _  (1.51) (1.41) 0.53  CO  -  0.64 0.54  -  0.98  -  —  42.0 31.4 18.7 53.9 39.0 19.6 62.2 31.5 28.8 17.2 61.1 11.1 16.8 17.7 15.4 24.7 21.7 22.4  ± Rate typess  L = Linear;  P = Parabolic;  Rates in parentheses are doubtful. [mgCu/cm J /hour. g  Units of parabolic rates are  2  t±k  Units of l i n e a r rates are mgCu/cm /hour. 2  I = Intermediate;  46. For pure copper I t was found that the rate increased with the oxygen pressure and l e v e l l e d o f f at a constant value when the pressure was increased beyond 6 . 8 atmospheres. t h i s has been discussed elsewhere ( l ) .  The s i g n i f i c a n c e of  At low oxygen pressures the  rate i s determined by the transport of oxygen t o the metal surface and i s therefore proportional to the oxygen pressure.  At higher  oxygen pressures, the chemical reaction at the metal surface becomes rate c o n t r o l l i n g and the rate becomes independent of the oxygen pressure.  With the alloys,however  9  the effect of varying the oxygen  pressure was found t o be somewhat d i f f e r e n t .  The rate increased  with the oxygen pressure up t o a maximum value, generally at about 5 . 1 atmospheres, and then f e l l again as the oxygen pressure was f u r t h e r increased (see Figure 2 3 ) .  This was the case f o r a l l the  alloys.  E f f e c t of Temperature. The effect of temperature on copper and on a l l o y s 2 and 10 was investigated using solutions of the same composition as those used i n the i n v e s t i g a t i o n on pressure ( i . e . 0 . 5 m/1 NH NH*).  3  and 0 . 0 2 m/1  The e f f e c t of temperature on a l l o y 5-was investigated using  solutions containing 0 5 m/1 NH , but without NH*. o  3  The temperature  was varied between 15 and 50°C with the pressure held constant at 6 . 8 atmospheres.  T y p i c a l rate curves f o r copper and f o r an a l l o y  are shown i n Figures 2 4 and 25 and the r e s u l t s are summarized i n Table V. While increasing temperature was found to increase the rate  48.  TEMPERATURE: I  O - 15°C • - 25°C  TIME  j  - HOURS  Figure 25. Rate curves for alloy 2 at various solution temperatures. NH - 0.5 m/1; NH* = -.-2 m/1; 0 - 6.8 Atm. 3  2  49. TABLE V»  Data on Experiments i n which Temperature was Varied  Run No.  t ±± NH Rate Linear Parab. Cu Diss. ml T i t r a t e d Type Rate Rate i n 1 hr. m/1 m/1 3  Alloy Temp. Pressure NH °C. Atm. m/1  C-77 C-75  3  Cu 2  C-76  10  C=18  Cu 2  C=37 C-20 6=84  10  C-81  Cu 2  C-78  10  C-98 G=95  Cu 2  15  6.8 6.8 6.8  0.5 0.5 0.5  0.02 0.02 0.02  0.50 0.52 0.50  P  25 25 25  6.8 6.8 6.8  0.5 0.5 0.5  0.02 0.02 0.02  0.49 0.50 0.48  L I P  62.2  35 35 35  6.8  6.8 6.8  0.5 0.5 0.5  0.02 0.02 0.02  0.51 0.51  L P I  61,6  0.5 0.5 0.5  0,02 0,02 0.02  L P P  82.2  0.02 0.02 0.02  15 15  50 50 50  6.8  1.7 3.4  0.5 0.5 0.5  6.8 6.8  1.5 1.5 1.5  C-82  10  C-79 C-85 C-86  10 10 10  15 15  C-99  5 5 5  25 35 50  0=100 0=102  15  6.8  6.8  5.1  6.8  —  0.49 0.49 0.48 0.51 0.52  0.51 0.49 1.57 1.51  * 1.51  L  I  I P I I I I  56.2  =  =  CO  =  -  =  0.54 OB  31,5 17.2  0,75  61,6 32.5  CO  0.33  -  ...  0.48  -  — —  62.2  0,43  (1.51)  0.19  -  56.2 29,8 17.7  —  22.4 82.2  22.7 15.4 11.1 16.8 14.2  27,1 20.5 13.3  t Rate typess L = Linear! P Parabolic; I = Intermediate; ±& Rates i n parentheses are doubtful. Units of parabolic rates are [mgCu/ cm ? /hour. =  2  2  Units of l i n e a r rates are mgCu/cm /hour. 2  50. f o r pure copper, the effect was s l i g h t (corresponding energy of the order of 3 2 0 cal/mole).  t o an a c t i v a t i o n  This indicates some measure of  rate control by transport of oxygen under these conditions.  For the a l l o y s , the effect of temperature was s t r i k i n g l y different.  Here i t was found that, with the possible exception of a  short i n i t i a l period, the rate generally decreased with increasing temperature.  These r e s u l t s suggest that as the temperature i s increased the influence of the f i l m s i n retarding the reaction becomes more pronounced.  This i s supported by the r e s u l t s of p h y s i c a l examinations  of the f i l m s , which indicate that the films formed at higher temperatures are denser and would therefore be expected to be l e s s permeable to reactants or products.  Structure and Composition of the Films.  Following most of the experiments, the surfaces of the a l l o y specimens were examined microscopically and i n some cases the films which had formed during the reaction were removed f o r a n a l y s i s . A s o f t lead spatula was used t o remove the f i l m s , which were very f r a g i l e and, when scraped from the underlying metal, broke up into a f i n e apparently m e t a l l i c powder. Photomicrographs of t y p i c a l f i l m s are shown i n Figures 26 to 33, and the r e s u l t s of chemical analyses of some of the films are l i s t e d i n Table VI. Several attempts were made to determine the c r y s t a l structure of the f i l m p a r t i c l e s using X-ray d i f f r a c t i o n methods, but due either t o very f i n e p a r t i c l e s i z e or imperfect  crystal  Figure 26.  Surface structure of a l l o y 2. Experimental conditions:  NH =0.5 m/1; Nh*J = 0.06 m/1; 3  T = 25°C; 0 = 6.8 Atm. Magnification X75. 2  Figure 27.  Surface structure of a l l o y 2. Experimental conditions: NH - 0,5 m/1; NH£ = 0.02 m/1; T = 50 C; 0 = 6.8 Atm. Magnification X600. 3  e  2  Figure 28. Surface structure of alloy 5. Experimental conditions: NH - 0.5 m/1; T - 25°C; 0 - 6.8 Atm. Magnification X1000. 3  2  Figure 29. Surface Structure of alloy 10. Experimental conditions: NH - 0.5 m/1; T - 25°C; 0 - 6.8 Atm. Magnification X1000. 3  2  Figure  30. Surface structure of alloy 10. Experimental conditions: NH - 0,25 m/1; NHj - 0.04 m/1; T « 25°Cj 0 - 6.8 Atm. Magnification X?5. 3  2  Figure  31. Surface structure of alloy 10. Experimental conditions: NH - 0.5 m/1; NH& - 0.02 m/1; T - 3^'C; 0 - 6.8 Atm. Magnification X500. 3  2  54.  Figure 32. Surface structure of alloy 1 5 . Experimental conditions: NH = 1.5 m/1; T = 25°C; 0 = 6.8 Atm. Magnification X85o 3  2  TABLE  VI.  Analyses of Films.  Run  No.  C-8 C-9  C-3  C-14 C-16 C-15 C-20 C-56 C-49 C-54 C-55 C-48 C-46 C-57 C-61 C-81 C-83  Alloy  Sample wt.-mg  wt. % Copper  5 10 5 5 10 10 10 5 5 5 10  11.05 10.45 10.95 9.50 9.98 8.82 10.72 8.32 11.50 12.91 9.12  2  23.88 35.02 23.78 28.74 40.90 50.56 19.78 30.40 24.62 43.40 60.44 21.52  10  25.86  1 *  18.46  8.50 9.60* 37.0  This was a sample of the unusual f i l m mentioned i n connection with Figure 30.  56. structure, no interpretable r e s u l t s were obtained,  <  It i s not possible at present to explain i n any d e t a i l the structures that are represented i n Figures 26 t o 33$ and the reasons f o r the v a r i a t i o n s observed, but a few features of interest may be noted.  In Figure 26 i s shown the surface of a l l o y 2 a f t e r i t had  been exposed to a corroding s o l u t i o n at 25°C„  V  For t h i s a l l o y the sur-  face structure was the same whether the solutions contained NH^ or not. This i s perhaps s u r p r i s i n g i n view of the very large effect of NH]£ on the rate f o r t h i s a l l o y .  The surface appears to be porous and coarse.  Figure 27 shows the surface of the same a l l o y following reaction at 50°C.  This f i l m was observed to be much thinner and denser than  those formed at lower temperatures and can be seen to have a very d i f ferent surface structure.  This could account f o r the inverse temper-  ature e f f e c t on the rate of d i s s o l u t i o n of copper from the a l l o y s which was noted e a r l i e r . The surface structures of a l l o y s 5 and 10 a f t e r reaction i n solutions containing only NH  3  at 25°C,, are shown i n Figures 28  and 2 9 o These are seen to be very s i m i l a r to each other, but markedl y d i f f e r e n t from the t y p i c a l structures f o r e i t h e r a l l o y 2 (Figure 26) which appears t o be much more porous or a l l o y 15 (Figure 32) which r  seems much more dense.  In contrast to the observations f o r a l l o y 2 , i t was found that the presence of NH^ i n the solution had a considerable effect on the appearance of the surfaces of the higher gold a l l o y s .  This can  be seen i n Figures 30 and 3 1 , which show surface structures of a l l o y 10 a f t e r exposure to solutions containing both NH  3  and NH£.  57. Figure 30 i l l u s t r a t e s an i n t e r e s t i n g effect which was often observed with the higher gold a l l o y s , p a r t i c u l a r l y a f t e r they had been exposed to reaction conditions which gave a low rate (i.e. concentrations, oxygen pressures or temperatures).  low  NH  3  In these instances,  a f t e r the specimens were removed from the reaction v e s s e l , t h e i r surfaces appeared to be uncorroded except f o r a very t h i n tarnished f i l m . However, a f t e r a short exposure to the atmosphere, cracks developed i n t h i s f i l m and fragments of a surface layer, about 1/1000 inches t h i c k , and m e t a l l i c i n appearance, separated from the underlying material.  An example of such a specimen i s shown i n Figure 30.  The  removal of t h i s surface f i l m , which analysis showed to be very high i n gold, exposed another very t h i n layer of a black material which could r e a d i l y be removed as a very f i n e powder to disclose the unattacked metal.  It was  not found possible to c o l l e c t a s u f f i c i e n t  amount of t h i s powder f o r analysis, but i t s appearance suggested that i t could be an oxide rather than a metal.  Figure 32 shows a photomicrograph of the surface of an a l l o y 15 specimen which had been corroded i n a s o l u t i o n containing NH  3  alone.  After the reaction, the surface appeared to the eye to  be only s l i g h t l y tarnished.  Under the microscope the surface f i l m  appeared to be very compact, showing fewer cracks than the f i l m s of a l l o y s lower i n gold content.  When the tarnished layer was removed  by polishing, the underlying surface, shown i n Figure 33, was ed to be m e t a l l i c and obviously very r i c h i n gold. t h i s gold-rich layer was  observ-  Further, when  removed by machining, the shavings were  found to contain a f i n e black powder.  58, No evidence of any surface changes,  such as tarnishing or  f i l m formation, could be observed either v i s u a l l y or under the microscope, on specimens of a l l o y 15 which had been exposed t o utions containing both NH  3  and  sol-  NH*.  Table VI l i s t s the results of the copper analyses which were carried out on some of the surface f i l m s formed during corrosion experiments.  Most of the f i l m s are seen to contain about 10 percent  by weight of copper, corresponding to about 25 atomic percent- copper i f the f i l m material i s assumed t o consist of a copper-goId a l l o y . Qualitative tests confirmed that a l l the films were very r i c h i n gold. The apparent constancy of the compositions of the films i s perhaps s u r p r i s i n g i n view of the wide differences  i n t h e i r physical appear-  ance and i n the compositions of the alloys from which they  w e r e  formed  It i s clear from the above discussion that while the formation of surface f i l m s i s an e s s e n t i a l feature of the corrosion of the copper-gold a l l o y s , the f i l m s which are formed are very complex i n nature, often consisting of more than one layer or component.  Despite  an apparent constancy of the composition (as r e f l e c t e d i n the copper content) of these f i l m s , t h e i r detailed chemical and physical s t r u c t ures have not been determined. t h e i r observable properties  Nor has i t been possible t o r e l a t e  i n any simple way to the compositions of  the a l l o y s , the conditions of the reactions or the reaction  rates.  59.  CONCLUSIONS  Summary of Experimental Results The general pattern of the experimental results which were obtained i n the course of t h i s investigation may be summarized as follows;. (1)  A l l the rate curves f o r the d i s s o l u t i o n of pure copper were l i n e a r . In  general the r e s u l t s obtained on pure copper were i n good agree-  ment with those found i n e a r l i e r studies ( l ) . (2)  Most of the rate curves f o r the alloys showed a decrease i n the rate with time.  In many cases the shapes of the rate curves con-  formed to a parabolic r e l a t i o n , but sometimes they were more complex, showing a greater or smaller tendency f o r the rate to f a l l o f f with time, (3)  In general, increasing the gold content of the alloys resulted i n lower rates of d i s s o l u t i o n of copper and i n a greater tendency for  the rates t o decrease with time.  parent i n solutions high i n NH NH  3  3  This was p a r t i c u l a r l y ap-  and NH£.  In solutions low i n  and NR\, the rates f o r a l l the alloys were s t i l l much smaller  than f o r pure copper, but the rate appeared to be less sensitive to the gold content of the a l l o y ,  (k)  Increasing the concentration of NH  3  i n the solution ( i n the  presence of constant NH£) resulted i n higher dissolution rates for  a l l the a l l o y s .  The rates increased up to a maximum value  beyond which further NH  3  addition had l i t t l e e f f e c t .  The  60.  1 l i m i t i n g rates were inversely dependent on the gold content of the a l l o y s .  Increasing the NH  concentration also tended to make  3  the rate curves, p a r t i c u l a r l y f o r the lower  gold alloys,  more  nearly l i n e a r i n shape.  Increasing the concentration of NH* i n the solution ( i n the presence of constant NH ) had a 3  very s i m i l a r e f f e c t .  The rate  increased with the concentration of NH*, approaching a l i m i t i n g value which was the same i n  the case of each a l l o y , as that reach-  ed by increasing the NH3 concentration. NH* than of NH  3  Much smaller amounts of  were required to produce a comparable e f f e c t .  The rates f o r the low gold alloys ( i . e . a l l o y 2) were p a r t i c u l a r l y sensitive to the NH* concentration.  Those f o r the higher gold  a l l o y s showed a smaller dependence, and f o r a l l o y 15 the dependence was a c t u a l l y reversed, no reaction taking place i n the presence of NH£ s a l t s .  The rates f o r a l l the a l l o y s were found to increase with the oxygen p a r t i a l pressure up to a maximum at about 5 atmospheres. At higher oxygen pressures the  rates f e l l off again.  While the rates f o r pure copper increased with temperature,  those  for a l l the alloys showed an inverse dependence, falling o f f as the temperature was r a i s e d . It was found f o r a l l the a l l o y s that by  the reaction was accompanied  the formation of f i l m s , generally very rich in gold, on the  surface of the corroded specimens.  It was not possible to estab-  l i s h the detailed structure of these f i l m s .  61  Interpretation of the Results From the nature of these r e s u l t s i t i s apparent that the system involved i s of such complexity  and the number of variables so  great, that a f a r more extensive study than the present one would be required to elucidate f u l l y the reactions which take place and t h e i r mechanisms.  The i n v e s t i g a t i o n which has been carried out can there-  fore be considered to be only of an exploratory nature, intended to reveal the general features of the system. studied i n greater d e t a i l .  Many aspects remain to be  Nevertheless, the r e s u l t s which have been  obtained form a s u f f i c i e n t l y complete and consistent pattern that i t i s possible to account f o r some of the observed e f f e c t s and to draw some conclusions, of a general nature at l e a s t , concerning the k i n e t i c s and mechanisms of the reactions involved i n the attack of ammonia solutions on copper-goId a l l o y s .  (1)  With pure copper the reaction has been shown to proceed by chemical attack of NH  3  and NH^ on the metal surface, following  the rapid adsorption of oxygen, and r e s u l t i n g i n the d i s s o l u t i o n of copper as the cuprammine ion.  The surface of the corroding  copper remains e s s e n t i a l l y unchanged, with no i n d i c a t i o n of any f i l m formation.  Consequently, the rate curves f o r copper are  linear.  (2)  With the a l l o y s there i s some i n d i c a t i o n that the i n i t i a l attack and d i s s o l u t i o n of copper proceeds i n a somewhat similar fashion, except that some electrochemical reaction due to the presence of regions of d i f f e r e n t composition surface may  also be involved.  ( i . e . gold content) on the metal  This might account f o r the fact  6 2 ,  that under certain conditions, slightly higher i n i t i a l rates were observed with increasing gold content of the alloy. As the corrosion of the alloy proceeds, a film i s soon formed on its surface.  This film appears to be due to the accumulation of  gold, as the copper in the surface region dissolves, resulting in the formation of a gold rich deposit which i s i t s e l f relatively inert to attack and which retards the reaction by presenting a barrier to the transfer of reactants and products between the -underlying metal and the solution. This film thickens as the reaction proceeds, accounting for the parabolic shape of the rate curves ( i . e . for the fact that the rate for the alloys f a l l s off with time). There i s some indication, both from examination of the specimen surfaces after corrosion and from the nature of the kinetic results, that a deposit of oxide (either Cu 0 or CuO) may also 2  form on the surface of the corroding alloys and that this deposit contributes i n the same way as the gold film i t s e l f to the slowing down of the reaction.  Such a deposit might be expected  to accompany or to result from the formation of the i n i t i a l gold rich film, since such a film would give rise to stagnant regions (i.e. i n the pores of the film or between the film and the underlying metal) in which the solution would be deficient in NH and 3  NH* and in which the reaction products (Cu accumulate.  and OH") would  These conditions would favour the precipitation of  CuO and Cu 0. 2  ++  This picture i s supported by the results of exper-  iments made at low NH3 and NH* concentrations, in which i t was  63. found that the rates for the low gold alloys were very low (comparable to those for the higher gold alloys) despite the fact that the gold film was apparently very porous. The fact that the rate passes through a maximum with increased oxygen pressure and then f a l l s off again might also find an explanation in the above picture.  At low oxygen pressures, the  transport of oxygen through the film may control i n part the rate of the reaction. However, as the oxygen pressure i s i n creased, the rate i s increased to a point where there i s insufficient NH3 and NH£ to prevent the accumulation of C u OH™.  ++  and  The resulting precipitation of copper oxides thus causes  the rate to drop again.  This i s comparable to the well known  system involving the corrosion of iron in contact with an aqueous solution, where increasing the concentration of oxygen may f i r s t accelerate corrosion, but at high enough oxygen concentrations a passivating film of iron oxides i s formed and corrosion inhibited o The inverse effect of temperature on the rates of reaction of the alloys may be similarly explained.  At higher temperatures  the formation of denser film structures would be favoured by mechanical failure, as would certainly the precipitation of copper oxides. The increase of the rate with NH and NH4 concentrations may be 3  due to a higher rate of transport of these reactants through the films to the underlying metal which i s dissolving.  At the  same time higher NH and NH£ concentrations would reduce the 3  tendency f o r copper oxides to precipitate, and t h i s would also cause the rates to increase.  Ultimately the rate must become  controlled by the transport of oxygen through the gold f i l m which i s always present, and whose thickness and permeability depend primarily on the composition  of the a l l o y .  This would correspond  to the l i m i t i n g rate region, i n which further increases i n the NH  3  or NH*  concentrations are without effect„  It might be  ex-  pected that i n t h i s region the rate would increase with the oxygen pressure.  No explanation can be offered at t h i s point f o r the f a c t that the a l l o y containing 15 atomic percent gold did not corrode i n solutions containing both NH  3  and NH*,  action i n solution containing NH  3  while i t did undergo re-  only,,  65.  APPENDIX  I.  S.pectro chemical Analyses of Metals and Alloys Used.  Pure Copper Cu Au  Major component  -  Alloy 10  Pure Gold  Alloy 5  0.001-0.005  Major component  Major component  Major component  Major component  Major component  Major component  , Major component  A l l o y 15  Si  0.001-0.01  0.001-0.01  0.001-0.01  0.001-0.01  0.001-0.01  Mg  0.001-0.01  0.001-0.01  0.001-0.01  0.001-0.01  0.001-0.01  Ca  0.001-0.01  0.001-0.01  0.001-0.01  Ag  0.CO1-0.01  0.001-0.01  0.005-0.01  0.005-0.01  0.001-0.01  Fe  0.0001-0.001  -  0.001-0.01  0.001-0.01  0.005-0.01  Cr  0.0001-0.001  -  0.0001-0.001  0.001-0.01  0.0001-0.01  V Ti Mo  -  -  0.001-0.01  -  0.001-0.005  -  0.001-0.005 0.0001-0.001  -  The above figures are percent. Due to the wide variations i n the compositions of the samples, even the percentage ranges quoted f o r the impurities  can only be  regarded as giving the approximate order of magnitude of the concentrations present.  APPENDIX I I  Assays for Gold on Copper-Gold Alloys.  Alloy No,  Cuttings Wt. #Au  Drillings Wt. % Au 6.00  2  Average  Atomic $ Au  Wt. % Au 6.00  2.02  5  13.99  14.12  14.05  5.01  10  25.73  25.64  25.69  10.00  15  35.56  35.56  35.56  15.11  67  REFERENCES  1,  Jo H a l p e r n ,  J . Electrochem. S o c , i n press.  2  Go Tammann,  Z. anorg. allgem,  0  3.  W.JoMuller,  H. F r e i s s l e r , 366-71  4P  Chem., 1 4 2 , 61=72  and E:. P l e t t i n g e r ,  (1925)o 42.  Z*. E l e k t r o c h e m . ,  (1936).  R O Landau and G . S o O l d a c h ,  T r a n s . E l e c t r o c h e m . S o c , 8 1 . 521-558  (1942).  5o  U.R.Evans,  'Metallic  C o r r o s i o n , P a s s i v i t y and  Protection",  A r n o l d , p. 2 2 9 . 6.  A.S.Russell,  Nature 1 1 5 , 455-6 ( 1 9 2 5 ) ; 1 1 7 , 47-8 ( 1 9 2 8 ) , J . Chem. Soc,  7.  H.H.Uhlig,  8.  H.H.Uhlig  9.  R.  1872-81  (1926).  " C o r r o s i o n Handbook", John W i l e y and S o n s , p. 2 1 . and J . W u l f f ,  Landau,  T r a n s . , A . I . M . E . . . 1 3 5 . 494-521 ( 1 9 3 9 ) .  T r a n s . E l e c t r o c h e m . S o c . , 8 1 , 559-571 ( 1 9 4 2 ) .  10.  C.Sykes and F . W . J o n e s ,  11.  E.B.Sandell,  P r o c , Roy. S o c L o n . , A 1 5 7 . 213-233  "Golorimetric Interscience,  (1936).  D e t e r m i n a t i o n of T r a c e s o f Metals", (1950).  

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