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The dissolution of platinum-iron alloys in oxygeneted acid chloride solutions Scott, John Wilfred 1972

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THE DISSOLUTION OF PLATINUM-IRON ALLOYS IN OXYGENATED ACID CHLORIDE SOLUTIONS BY JOHN WILFRED SCOTT B.Sc., Queens U n i v e r s i t y , 1968 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE i n the Department of METALLURGY We accept t h i s t h e s i s as conforming to the req u i r e d standard THE U N I V E R S I T Y OF B R I T I S H COLUMBIA J U L Y 1972 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the Head of my Department or by his representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department of Metallurgy The University of British Columbia Vancouver 8, Canada Date August 10th, 1972 ABSTRACT The d i s s o l u t i o n of platinum from platinum i r o n a l l o y s i n oxygenated hydrochloric acid/sodium chloride solutions has been investigated using an autoclave technique. The d i s s o l u t i o n rate was found to be dependent on a l l o y composition, acid concentration, and oxygen pressure. The d i s s o l u t i o n followed t y p i c a l corrosion k i n e t i c s and analysis of the r e s u l t s indicated that the cathodic reduction of oxygen was the rate c o n t r o l l i n g step i n the d i s s o l u t i o n reaction, at high chloride ion concentrations. An apparent a c t i v a t i o n energy of 16.8 k c a l per mold was found for the d i s s o l u t i o n of PtFe a l l o y s , and 19 k c a l per mole for pure Pt sheet. - i i i -TABLE OF CONTENTS Page INTRODUCTION ....... 1 I. General 1 I I . Mineralogy of Platinum Metal Deposits , 2 I I I . E x t r a c t i v e M e t a l l u r g y of Platinum 6 IV. L i t e r a t u r e Review on Platinum Corrosion ^ V. Theory of Corrosion ^ EXPERIMENTAL r 1 9 I. M a t e r i a l s and Reagents ^ I I . A l l o y P r e p a r a t i o n 19 I I I . Autoclave Design , 20 IV. A n a l y t i c a l Method , 23 V. Experimental Procedure 24 RESULTS 2 5 I. D i s s o l u t i o n of Fe-Pt A l l o y s and Pt Sheet 25 I I . E f f e c t of A c i d Concentration 26 I I I . E f f e c t of Oxygen Presence 26 IV. E f f e c t of C h l o r i d e Ion Concentration 27 DISCUSSION 39 I. L i n e a r D i s s o l u t i o n Curves 39 I I . A c t i v a t i o n Energies and S t i r r i n g E f f e c t s 39 I I I . K i n e t i c A n a l y s i s 41 CONCLUSIONS ...... 49 REFERENCES . . . • 51 TABLES 53 - i v -LIST OF FIGURES Figure Page 1 Phase diagram of the Fe-Pt system 5 2 Eh-pH diagram f o r the systems 0 2 " H 2 0 2 a n d P t _ P t C l 6 _ at 25°C, as a f u n c t i o n of p 0 2 and a[ C l ~ ] 1 0 3 S i m p l i f i e d Pourbaix diagram f o r the system Pt-H^O.... 10 4 Schematic diagram of the P a r r autoclave and the machined Te f l o n sample holder 21 5 A t y p i c a l d i s s o l u t i o n curve f o r Pt-Fe a l l o y at 700 p s i g 0 , 2 M HCl and 150°C 2 8 6 Comparative d i s s o l u t i o n r a t e s f o r PtFe, Pt^Fe, and Pt sheet under s i m i l a r l e a c h i n g c o n d i t i o n s 29 7 The e f f e c t of temperature on the d i s s o l u t i o n r a t e of PtFe a l l o y at 1 M HCl and 700 p s i g C>2 30 8 Arrhenius p l o t f o r the d i s s o l u t i o n of PtFe a l l o y .... 31 9 The e f f e c t of temperature on the d i s s o l u t i o n r a t e of pure Pt sheet at 2 M HCl and 700 p s i g 0 2 3 2 10 Arrhenius p l o t f o r the d i s s o l u t i o n of Pt sheet 33 11 The e f f e c t of HCl concentration on the d i s s o l u t i o n r a t e of PtFe a l l o y at 3 M t o t a l C l co n c e n t r a t i o n , 150°C and 500 p s i g 0 2 3 4 12 ' D i s s o l u t i o n r a t e vs. [H ] f o r PtFe a l l o y J ^ 13 The e f f e c t of oxygen pressure on the d i s s o l u t i o n r a t e of PtFe a l l o y at 150°C and 2 M HCl 35 14 D i s s o l u t i o n r a t e vs. p0 9 f o r PtFe a l l o y d i s s o l u t i o n . . . 35 - v -Figure Page 15 The e f f e c t of [Cl"] on the d i s s o l u t i o n r a t e of PtFe a l l o y at 500 p s i g 0 2 and 2 M HCl 16 D i s s o l u t i o n r a t e vs. t o t a l C l concentration f o r PtFe a l l o y 17 (a) PtFe a l l o y sample showing leached and unleached areas (b) T y p i c a l PtFe a l l o y s t r u c t u r e , etched i n aqua r e g i a (c) E l e c t r o n microprobe photograph of leached PtFe a l l o y 37 38 40 40 40 - v i -LIST OF TABLES Table No. Page S 3 1 A l l o y comparison J J 2 E f f e c t of Temperature on D i s s o l u t i o n Rate 53 3 E f f e c t of [H +] on D i s s o l u t i o n Rate of FePt 54 4 E f f e c t of Oxygen Pressure on D i s s o l u t i o n Rate .... 54 5 E f f e c t of Chloride Ion on D i s s o l u t i o n Rate 55 - v i i -ACKNOWLEDGEMENTS The author wishes to express h i s gratitude to Dr. I.H. Warren for h i s patient and h e l p f u l d i r e c t i o n during the course of th i s work. Thanks are also extended to members of the f a c u l t y and fellow graduate students f o r many valuable discussions. F i n a n c i a l support from the National Research Council of Canada i n the form of a scholarship and research assistantships i s g r a t e f u l l y acknowledged. - 1 -INTRODUCTION I. General The extraction and recovery of platinum and the associated metals of the platinum group from ores and i n d u s t r i a l scrap i s of increasing importance as world demand grows. Present extraction and recovery processes are a l l based on the old and proven aqua regia d i s s o l u t i o n as a f i r s t step, followed by c l a s s i c a l chemical separations of the various metals. As the aqua regia process i s expensive, slow and generally l i m i t e d to a series of batch operations, an i n v e s t i g a t i o n into the p o s s i b i l i t i e s of applying modern high-temperature, high-pressure autoclave techniques to the d i s s o l u t i o n of platinum was thought to be i n t e r e s t i n g . This type of processing would be generally applicable to both ore concentrates and i n d u s t r i a l scrap material, with perhaps a further use i n recovering platinum values from low grade deposits not presently economical to e x p l o i t . A general review of the mineralogy and e x t r a c t i v e metallurgy of platinum shows that the native metal i s most commonly the s t a r t i n g material a v a i l a b l e for extraction. The p r i n c i p l e s of m e t a l l i c corrosion should apply to the chemical d i s s o l u t i o n of the native platinum, as i t i s a metal or metal a l l o y . As part of the introduction to t h i s work then, i t was necessary to look at the mineralogy of platinum occurrence, the chemical -2 -and e l e c t r o c h e m i c a l c o r r o s i o n of platinum and the commonly used recovery processes, as w e l l as reviewing previous attempts at d i s s o l v i n g platinum under pressure. I I . Mineralogy of Platinum Metal Deposits The platinum metals occur mainly as platinum minerals a s s o c i a t e d w i t h n i c k e l - c o p p e r s u l f i d e s or copper s u l f i d e s i n lodes, or as platinum metal a l l o y s disseminated i n u l t r a b a s i c rock and i n p l a c e r deposits derived from these rocks.''" ( i ) Lode Deposits Platinum ores i n lode deposits may be c l a s s i f i e d i n t o three main types, according to the r e l a t i v e c o ncentration and form of the contained platinum metals. In the Sudbury type of d e p o s i t , the platinum metals are produced as a by-product i n n i c k e l and copper production. The platinum metals are contained i n n i c k e l - c o p p e r , copper or copper-cobalt s u l f i d e s that are r e l a t e d to b a s i c or u l t r a b a s i c rocks. In t h i s type of deposit there are no n a t i v e platinum metals or a l l o y s . The platinum minerals are s p e r r y l i t e ( P t A s 2 ) , michenerite [ ( P t , P d ) ( B i , T e ) ] , f r o o d i t e ( P d B i 2 ) , and minor unnamed minerals. The Merensky type of deposit c o n s i s t s of platinum-bearing copper-n i c k e l s u l f i d e s i n which the platinum values are high enough to c o n s t i t u t e the p r i n c i p a l ore m i n e r a l . The platinum occurs mainly as s p e r r y l i t e and cooperite [ ( P t , N i , P d ) ( S ) ] . i n lenses of p e r i d o t i t e or chromite i n the host u l t r a b a s i c rock. Small amounts of n a t i v e platinum metal a l l o y s are a l s o present. - 3 -The t h i r d important type of deposit i s a concentration of native platinum metal a l l o y s disseminated i n p e r i d o t i t e s and sometimes i n perknites. Most of these deposits are i n dunite which i s commonly alter e d to serpentine. This type of deposit i s the source of the Uralian placers, the Goodnews Bay deposit, and the Tulameen placers. Most of these deposits are e i t h e r too small or too low grade for d i r e c t mining, although some high grade concentrations i n masses of chromite i n dunite have been found. The Transvaal deposits of p l a t i n i f e r o u s i r o n -r i c h dunite are mined for platinum occurring i n t h i s way. There are also a number of minor platinum metal occurrences, for example i n gold ores, i n platinum bearing meteorites and in contact metamorphic copper ores. ( i i ) Placer Deposits Platinum placers are a l l u v i a l deposits that contain economic amounts of native platinum metal a l l o y s . The two types of a l l o y s may be t y p i f i e d as 'platinum' and 'osmiridium' with the two often occurring together. 'Platinum' consists mostly of that metal, but contains a l l the other platinum metals i n varying amounts. "Osmiridium' consists dominantly of i r i d i u m and osmium but also includes ruthenium, rhodium and platinum. Both these a l l o y s can also contain base metals such as i r o n or copper; ei t h e r as minerals or a l l o y s , i n varying amounts. Platinum placers are derived from dunites, s e r p e n t i n i t e , or perknites i n which the native platinum metals are very highly disseminated. As i n gold placers, the platinum placers are assumed to be close to t h e i r bedrock sources i n the absence of extensive g l a c i a t i o n . _ 4 -The major p l a c e r deposit i n B r i t i s h Columbia i s i n the Tulameen area near P r i n c e t o n . This s e r i e s of p o s t - g l a c i a l stream and t e r r a c e p l a c e r s i s derived from a l a r g e i n t r u s i v e mass of pyroxenite and gabbro c o n t a i n i n g two smaller bodies of p e r i d o t i t e . The p e r i d o t i t e i s considered a more important source rock than the p y r o x e n i t e . The platinum occurs i n these p l a c e r s as s m a l l rounded grains of platinum r i c h a l l o y , w i t h small p i t s and some adhering chromite and magnetite. 2 Dana has given the composition of the n a t u r a l l y o c c u r r i n g Fe-Pt a l l o y s , known as polyxene and f e r r i a n ( f e r r o p l a t i n u m ) . Polyxene i s 80-90% platinum and 3-11% i r o n w h i l e f e r r i a n i s about 28% i r o n . An assay of two types of platinum found i n the Tulameen P l a c e r deposits gave the f o l l o w i n g r e s u l t s : Magnetic Non-magnetic % Pt 78.4 68.2 % Fe 7.87 9.8 The weighted average was 72.0% platinum and 8.6% i r o n . I f the Fe-Pt r a t i o i s taken as i n d i c a t i n g an all,oy composition, these assays would put tie Fe-Pt a l l o y from the Tulameen area as being somewhere i n between FePt and FePt^. These two a l l o y s are i n t e r m e t a l l i c compounds, as shown i n the phase diagram Figure 1. These a l l o y s should be more e a s i l y d i s s o l v e d than pure Pt and as they occur commonly i n t h i s p a r t i c u l a r deposit the i n v e s t i g a t i o n of t h e i r d i s s o l u t i o n p r o p e r t i e s should prove of more p r a c t i c a l value than s i m i l a r i n v e s t i g a t i o n s on pure P t . - 5 -F i g . 1 Phase d i a g r a m o f t h e F e - P t sys tem - 6 -I I I . Extractive Metallurgy of Platinum The extractive metallurgy of platinum i s e s s e n t i a l l y the same whether the concentrate i s produced as a by-product i n Cu-Ni production or as a primary metal value from placer or lode deposits. Platinum metal concentrates derived from Cu-Ni s u l f i d e ores are produced from matte anodes as anode slimes. The copper and n i c k e l are dissolved e l e c t r o l y t i c a l l y from the anode i n an acid s u l f a t e bath. The anode slimes are c o l l e c t e d i n anode bags, .then removed and washed. The washed slimes are roasted to remove s u l f u r , ground to - 40 mesh, then leached i n hot s u l f u r i c acid to remove r e s i d u a l copper and n i c k e l . The r e s i d u a l slimes concentrate i s washed and f i l t e r e d and sent to a platinum r e f i n e r y . In placer and lode platinum operations the platinum metals are concentrated by gravity techniques, usually a dredging operation i n placer mines, followed by t a b l i n g . F l o t a t i o n has also been used i n some lode operations. The r e s u l t i n g concentrates from e i t h e r type of deposit are then sent to the platinum r e f i n e r y . Inmost platinum r e f i n e r i e s the steps followed are almost the same; 3 the following d e s c r i p t i o n i s of the Engelhard r e f i n e r y process which i s probably t y p i c a l of the industry. The bulk concentrate i s f i r s t leached i n hot aqua regia. This i s done i n glass l i n e d vessels as follows: Five hundred pounds of concentrate are heated to 80°C i n f i v e hundred gallons of 5:1 concentrated hydrochloric acid. One hundred gallons of n i t r i c acid i s added over a period of eleven hours, then the residue i s f i l t e r e d o f f . The s o l u t i o n , which i s r i c h i n dissolved platinum, palladium and gold, i s evaporated to drive o f f - 7 -n i t r a t e s , then r e d i s s o l v e d i n HCl and sent to an ammonium c h l o r i d e p r e c i p i t a t i o n . This p r e c i p i t a t e s an impure platinum ammonium c h l o r i d e , which i s c a l c i n e d to platinum metal, then r e d i s s o l v e d i n aqua r e g i a and r e p r e c i p i t a t e d as pure platinum ammonium c h l o r i d e . This p r e c i p i t a t e i s then c a l c i n e d to produce commercial platinum sponge. The s o l u t i o n a f t e r the f i r s t p r e c i p i t a t i o n i s t r e a t e d to e x t r a c t the other platinum metals, ( i . e . i r i d i u m , osmium, rhodium and ruthenium). Most recovery operations f o r scrap platinum e i t h e r i n c a t a l y s t s or i n metal form are based on a s i m i l a r procedure. They d i f f e r only i n minor d e t a i l s and are a p p l i c a b l e to platinum recovery from c a t a l y s t s , and other sources of contained platinum metal. IV. L i t e r a t u r e Review on Platinum Corrosion ( i ) Anodic D i s s o l u t i o n of Platinum The extensive use of platinum as an anode i n e l e c t r o l y t i c c e l l s and f o r cathodic p r o t e c t i o n i n sea water as w e l l as i t s use i n pol a r o g r a p h i c analyses has lead to some i n v e s t i g a t i o n s of c o r r o s i o n under anodic c o n d i t i o n s . Even though the platinum has been considered to be " i n e r t " , i t i s slowly attacked and e v e n t u a l l y has to be replaced. L l o p i s and 4 Sancho found that the anodic c o r r o s i o n of platinum r e q u i r e s high over p o t e n t i a l s and can only be s t u d i e d i n the presence of complex-forming i o n s . In HCl and c h l o r i d e s o l u t i o n s , platinum d i s s o l v e s as P t C l g at the anode. The nature of the platinum surface has some e f f e c t on the co r r o s i o n c h a r a c t e r i s t i c s , w i t h p l a t i n i z e d t i t a n i u m anodes being more s u s c e p t i b l e than s o l i d platinum anodes. For p l a t i n i z e d t i t a n i u m i n sea water,~" consumption r a t e s of from 6-50 mg/Ampere/year have been - 8 -reported, while under s i m i l a r conditions pure platinum anodes were uniformly consumed at a rate of 6-7 mg/ampere/year over a wide range of current density. In an e l e c t r o l y s i s c e l l f o r the production of sodium hypochlorite f o r sewage treatment platinum consumption rates 6 of 5CK100 mg/ampere/year were reported, and could be c o n t r o l l e d by c o n t r o l l i n g the anode p o t e n t i a l . The platinum loss was postulated to be r e l a t e d to a p o t e n t i a l dependent change i n surface structure, corresponding to the formation of platinum oxide. On p l a t i n i z e d titanium anodes"' i n a c i d i f i e d chloride solutions, t h i s anodically formed oxide dissolved at open c i r c u i t . A recent study of the passivation of platinum i n 1-8 M HCl and 1-5 M NaCl solutions showed rapid corrosion below the passivation p o t e n t i a l . This p o t e n t i a l was [H +] independent, thus r u l i n g out d i r e c t oxide formation as the cause of passivation. It was concluded that passiva-t i o n i s due to the formation of a series of adsorption complexes, with a f i n a l surface of PtO -nHOH. x 4 In HCl solutions p r i o r to passivation, platinum dissolves to form PtClg . The rate of corrosipn increases with temperature, chloride ion concentration and a c i d i t y . The a c t i v a t i o n energy for d i s s o l u t i o n was found to be 25 Kcal/mole corresponding to a highly i r r e v e r s i b l e step. Superimposed a l t e r n a t i n g current increases corrosion by minimizing concentration p o l a r i z a t i o n e f f e c t s . ( i i ) Chemical D i s s o l u t i o n of Platinum Platinum i s a very noble metal, and extreme conditions must be 9 used to dissolve i t chemically. According to the Pourbaix diagram, - 9 -(Fig. 3) platinum i s stable i n aqueous solutions of a l l pH's, i n the absence of complexing agents, except under ce r t a i n very low pH and highly o x i d i z i n g conditions. At room temperature platinum i s unattacked by water, caustic a l k a l i s and acids and attacked slowly by o x i d i z i n g agents, except when complexes are formed. The best known and most commonly used reagent for d i s s o l v i n g platinum i s aqua regia. This acid combines the o x i d i z i n g power of n i t r i c acid with the complexing power of hydrochloric acid and dissolves platinum as c h l o r o p l a t i n i c acid, R^PtCl^.. Pure HCl hardly attacks platinum, but i f i t contains dissolved chlorine or oxygen the combination of o x i d i z i n g and complexing actions w i l l dissolve the metal. Some platinum a l l o y s , e s p e c i a l l y those with i r i d i u m , are insoluble i n aqua regia, and must be dissolved by some other method. Some methods^ which have been used are closed tube fusions with s a l t s , closed tube diss o l u t i o n s with HCl and added o x i d i z i n g agents, and open boat chlorinations both with and without added s a l t . Fusion with base metals followed by acid attack has also been used as a method of d i s s o l v i n g r e s i s t a n t platinum metal a l l o y s and i s the basis of the c l a s s i c a l f i r e assay techniques. 11 Tronev studied the d i s s o l u t i o n of the platinum metals i n hydrochloric acid under high a i r pressures. He found that acid concentra-t i o n , p a r t i c l e s i z e of the platinum, over-pressure of a i r and temperature were the important variables i n increasing the d i s s o l u t i o n rate. He proposed a mechanism of d i s s o l u t i o n i n v o l v i n g a free chlorine molecule produced from the HCl oxygen reaction. - 10 --2 0 2 U 6 8 10 12 U pH F i g . 3. S i m p l i f i e d P o u r b a i x diagram f o r t h e system P t - H ? 0 - 11 -2HC1 + 0 9- HOH + C l 2 Pt + C l + 2HC1 —*- R \ P t C l , -« 2 6 As the e q u i l i b r i u m i n equation 1 goes to the r i g h t as the temperature r i s e s , t h i s mechanism gives an i n c r e a s i n g r a t e x^rith i n c r e a s i n g temperature and hence f i t s h i s r e s u l t s . Tronev made no attempt to f u r t h e r c h a r a c t e r i z e the d i s s o l u t i o n r e a c t i o n , simply s t a t i n g h i s r e s u l t s i n t a b u l a r form. 12 Wichers, Schlecht and Gordon developed a closed, tube method of d i s s o l v i n g r e f r a c t o r y p l a t i n i f e r o u s m a t e r i a l s . They f e l t that temperature could be the c o n t r o l l i n g f a c t o r i n the i n e r t n e s s of i r i d i u m and other platinum a l l o y s to aqua r e g i a i n open v e s s e l s . They assumed the same type of r e a c t i o n as Tronev, i . e . 7HC1 + HC10. 4C1„ + 4H o0 10HC1 + 2HN03 5C1 2 + N 2 + 6H 0 HCl + NaC10 3 5- C l 2 - + H 20 They found that some HCl was necessary f o r attack to occur, using L i C l as a source of c h l o r i d e i o n . They al s o found a maximum att a c k on i r i d o - p l a t i n u m a l l o y s w i t h the f o l l o w i n g mixtures at temperatures from 100°C to 150°C. - 12 -.27 ml HN0 3 + 4.2 g HCl .37 g NaC10 3 + 4.2 g HCl The authors concluded that the pressure developed, which could go as high as 4000 p s i , was unimportant as i t was c o n t r o l l e d by the r e a c t i o n mixtures. I n t e r e s t i n g l y enough, they a l s o mention that Tronev's work on d i s s o l v i n g platinum under a i r pressure i s equivalent to t h e i r added oxidant method of producing c h l o r i n e , w i t h i n c r e a s i n g a i r pressure being regarded as i n c r e a s i n g the c h l o r i n e concentration. ( i i i ) C orrosion Mechanism f o r Platinum i n Oxygenated A c i d C h l o r i d e S o l u t i o n s (a) Anodic Platinum D i s s o l u t i o n L l o p i s ^ has reviewed the l i t e r a t u r e on the anodic d i s s o l u t i o n mechanism of platinum i n c h l o r i d e s o l u t i o n s . As mentioned above, Pt d i s s o l v e s as P t C l , at the anode below the oxide formation p o t e n t i a l , o The mechanism proposed can be expressed i n the form of two consecutive r e a c t i o n s : Pt + x C l ~ — P t C l ~ X x P t C l " X - ne" — ^ P t C l - ( x - n ) x x The f i r s t r e a c t i o n corresponds to s p e c i f i c adsorption of the anions on the metal surface w i t h complex formation, the second to i o n i z a t i o n - 13 -and desorption of the adsorbed complex. The adsorption of h a l i d e ions has been shown to depend on the presence of adsorbed oxygen, and v i c e versa. However, i t appears that over the range of 0.35 to 0.75 v o l t s , Pt d i s p l a y s an e n t i r e l y gas-free s u r f a c e , w i t h adsorption of oxygen lea d i n g t o C K i d e formation at p o t e n t i a l s above 1.5 v o l t s . ^ For h y d r o c h l o r i c a c i d and NaCl s o l u t i o n s up to 3 M HCl, i t appears that a p o t e n t i a l of at l e a s t 1.2 v o l t s i s r e q u i r e d f o r any p a s s i v a t i o n to occur. The c h l o r i d e i o n dependence of the Pt d i s s o l u t i o n r a t e was found to f o l l o w the rate law: - 0 9 Rate = k [ C l ] U ' y which supports a C l one e l e c t r o n t r a n s f e r step at the Pt surface as the rate l i m i t i n g step. The o v e r a l l e l e c t r o d e h a l f - r e a c t i o n f o r Pt d i s s o l u t i o n at 25°C i n c h l o r i d e s o l u t i o n s i s : Pt + 6 C l ~ — v P t C l , = + 4e 6 E: = 0.72 + 0.015 l o g a ( P t C _ 6 = ) - 0.089 l o g a(Cl") and depends only on the a c t i v i t y of P t C l ^ [ a ( P t C l ^ )] and the a c t i v i t y of C l " [ a ( C l ~ ) ] at 25°C. - 14 -(b) Cathodic Reduction Reaction and Eh-pH R e l a t i o n s h i p For Pt c o r r o s i o n by oxygen saturated a c i d s o l u t i o n s , the r e d u c t i o n of molecular oxygen to water w i l l be the cathodic r e a c t i o n . Therefore an o v e r a l l e l e c t r o c h e m i c a l c o r r o s i o n r e a c t i o n f o r the d i s s o l u t i o n would be: 2Pt + 12C1 + 0„ + 4H + — a - 2 P t C l ^ + 4H„0 I b I As both the h a l f r e a c t i o n s are o x i d a t i o n - r e d u c t i o n type of r e a c t i o n s we can c a l c u l a t e Eh-pH or Pourbaix type diagrams to represent the thermodynamic e q u i l i b r i a of the various species. For the anodic process the p o t e n t i a l i s described by: E = 0.72 + .015 l o g a P t C l r - .089 l o g aCl D at 25°C. This p o t e n t i a l i s independent of pH and gives a s t r a i g h t h o r i z o n t a l l i n e on a Pourbaix diagram. For higher temperatures various methods of c a l c u l a t i o n have been proposed f o r the c o n s t r u c t i o n of Eh-pH diagrams. However none have been c a l c u l a t e d f o r the P t - C l - P t C l g system. The Eh-pH diagrams f o r the P t - P t C l , and 0 o-H o0„ re a c t i o n s at o Z 2. I 25°C and f o r various 0^ pressures and C l a c t i v i t i e s are shown below ( F i g . 2). This diagram shows that thermodynamically i t i s p o s s i b l e f o r c o r r o s i o n of platinum to occur w i t h the r e d u c t i o n of oxygen to hydrogen peroxide as the cathodic r e a c t i o n . - 15 -V. Theory of Corrosion Electrochemical Theory The basic premise of the electrochemical theory of corrosion i s that the o v e r a l l chemical reaction i s divided i n t o two l a r g e l y independent processes. 1. Anodic d i s s o l u t i o n of metal - the t r a n s f e r of metal into s o l u t i o n as hydrated or complexed ions, with the loss of a number of electrons to the metal. 2. Cathodic reduction - the gain of electrons from the metal by depolarizers which are atoms, molecu].es or ions i n the s o l u t i o n capable of reduction. Naturally for the d i f f e r e n t processes to occur independently at d i f f e r e n t parts of the surface there must be a heterogeneous surface structure. This i s known as the l o c a l c e l l theory. If the corrosion i s considered to be analogous to a galvanic c e l l , the anodic and cathodic areas on the airface may be due to a number of factors acting together or independently. Surface heterogeneties are c l a s s i f i e d as macroscopic, microscopic, or submicroscopic. Macro-couples are formed by coupling d i f f e r e n t metals, or d i f f e r e n t i a l aeration, and lead to d e f i n i t e areas of l o c a l corrosion. Micro-couples may be formed by such things as s o l i d s o l u t i o n segregation, grain boundaries and c r y s t a l anisotropy leading to microscale corrosion such as intergranular corrosion, s t r u c t u r a l l y s e l e c t i v e corrosion and p i t t i n g corrosion. A good basic example i s metallographic etching. Submicroscopic corrosion couples e x i s t on metal surfaces within groups of atoms and are caused by factors l i k e s u b s t i t u t i o n a l impurity atoms, surface topography, and thermal o s c i l l a t i o n s of atoms i n - 16 -the l a t t i c e . These submicro-couples are very unstable and l e a d to uniform surface d i s s o l u t i o n by changing a c e r t a i n p o i n t from anodic to cathodic r a p i d l y and f r e q u e n t l y . In the case of an u l t r a pure metal w i t h no i m p u r i t i e s or h e t e r o g e n e i t i e s , c o r r o s i o n would be impossible by the l o c a l c e l l theory. The theory f o r c o r r o s i o n of u l t r a pure metals was f i r s t suggested by Wagner and 13 Traud i n 1938. The b a s i s of t h i s theory i s that f o r c o r r o s i o n to occur, s p a t i a l l y separated anodic and cathodic areas are not necessary: The necessary and s u f f i c i e n t c o n d i t i o n f o r c o r r o s i o n i s that the metal d i s s o l u t i o n and some cathodic r e d u c t i o n r e a c t i o n proceed simultaneously on the surface. For t h i s to happen the , p o t e n t i a l d i f f e r e n c e across the i n t e r f a c e must be more p o s i t i v e than the e q u i l i b r i u m p o t e n t i a l of the M-^M11"1" + ne and more negative than the e q u i l i b r i u m p o t e n t i a l of the r e d u c t i o n r e a c t i o n A + ne — D i n v o l v i n g a s o l u t i o n species. 14 Hoar recognizes three general s i t u a t i o n s of c o r r o s i o n attack. 1. Base metal - the metal d i s s o l v e s to an etched s u r f a c e , and cathodic r e d u c t i o n occurs on the same surface over a p e r i o d of time. At any i n s t a n t the number of a c t i v e anode s i t e s ( i . e . K i n k s , edges) i s r e l a t i v e l y s m a l l , and these s i t e s are c o n s t a n t l y changing and g i v i n g a uniform d i s s o l u t i o n . In t h i s case the whole surface acts as anode and cathode so we may take t h e i r r e s p e c t i v e areas as being equal. 2. Metal d i s s o l v i n g i n w e l l defined zones - f o r example at oxygen starved areas or at obvious h e t e r o g e n e i t i e s or i n c l u s i o n s . 3. Metal p i t t i n g - that i s o f t e n due to a p a s s i v a t i n g fUlm and the anode area i s n e g l i g i b l e compared to the cathode area. - 17 -A l l o y Corrosion When a homogeneous binary a l l o y i s dissolved, there are two possible modes of d i s s o l u t i o n . 1. Simultaneous d i s s o l u t i o n of the two metals i n the a l l o y , 2. P r e f e r e n t i a l d i s s o l u t i o n of the le s s noble metal x^ith surface enrichment of the noble metal. In an a l l o y consisting of a noble metal and an active metal, i n s o l i d s o l u t i o n , i t has been found"''"' that there i s a more or less sharp increase i n corrosion resistance at a c e r t a i n r a t i o of a l l o y components. Normally, t h i s increase i n s t a b i l i t y occurs at an atom r a t i o of n/8 where n i s an integer from 1-6. For a Cu-Au system i n hot n i t r i c acid t h i s occurred at n = 4. The simplest explanation of t h i s phenomenon i s surface enrichment, however, i n t e r m e t a l l i c compound formation may play a part as w e l l . If an active phase i s f i n e l y dispersed i n a noble matrix, i t w i l l be dissolved out and surface enrichment w i l l occur. If the active phase i s continuous or i n excess t h i s cannot happen, as a small number of noble atoms w i l l probably be detached along with the large number of active atoms. Surface enrichment should only occur i n d i l u t e heterogeneous a l l o y s , however a homogeneous a l l o y may be regarded as a heterogeneous a l l o y i n the f i n e s t possible state of dispersion and therefore surface enrichment could oontrol the d i s s o l u t i o n . In a binary a l l o y corroding i n a system i n x^hich both components can d i s s o l v e , the s i t u a t i o n i s f a r from cle a r . I t i s apparent that the corrosion rate of the noble component i s raised while that of the less noble component i s lowered. In an i d e a l s i t u a t i o n the corrosion p o t e n t i a l x<rould vary between that of the base and noble components l i n e a r l y with the composition. As was shown above for the Cu-Au al l o y s - 18 -t h i s i s not the case, and d e v i a t i o n s are due to s o l i d s t a t e i n t e r a c t i o n s and non-ideal s o l u t i o n behavior. Uhlig"'"^ made an attempt to e x p l a i n the p a s s i v a t i o n phenomena i n s t a i n l e s s s t e e l s and c o r r o s i o n behavior i n some b i n a r y a l l o y s through the use of e l e c t r o n theory. In p a r t i c u l a r he r e l a t e d the number of unpaired d e l e c t r o n s to the a b i l i t y 17 to adsorb oxygen at the surface. L e i d h e i s e r explained the c o r r o s i o n behavior of c e r t a i n s t a i n l e s s s t e e l a l l o y s on the b a s i s of a nearest neighbour theory and a rat e c o n t r o l l i n g n u c l e a t i o n step. He proposed that a noble metal (slow corroding atom) i s unaffected i n i t s r a t e of a c t i v e anode s i t e n u c l e a t i o n , w h i l e the a b i l i t y of a base metal atom to nucleate an a c t i v e anode s i t e i s reduced to a n e g l i g i b l e value by haying a noble nearest neighbour (or i s unaffected i f i t has a l l the same nearest neighbours), thereby reducing the r a t e of d i s s o l u t i o n to one c o n t r o l l e d by the noble metal. This theory assumes no ord e r i n g and a s t a t i s t i c a l d i s t r i b u t i o n of d i f f e r e n t atoms, a l l o w i n g the c a l c u l a t i o n of most l i k e l y nearest neighbours from a p r o b a b i l i t y f u n c t i o n . This type of a n a l y s i s could apply to i r o n platinum a l l o y s but only i n the d i l u t e platinum r e g i o n . - 19 -EXPERIMENTAL I. Materials and Reagents (i) Materials The materials used i n t h i s i n v e s t i g a t i o n were pure platinum r o l l e d sheet as supplied by Engelhard, p u r i f i e d Armco i r o n powder and iro n platinum all o y s made from these two materials. ( i i ) Reagents A l l reagents used were reagent grade. Deionized water was used for a l l solutions. The oxygen was cylinder grade, supplied by Canadian L i q u i d A i r . I I . A l l o y Preparation The Armco i r o n powder was reduced under cracked ammonia at 700°C for four hours to remove any surface oxide, then cooled under the reducing gas. The platinum sheet was cleaned i n aqua regia, washed, degreased with acetone, and dried i n an a i r b l a s t . Stoichiometric proportions of the two metals, platinum and fr e s h l y reduced i r o n , were then weighed in t o a 10 cc r e c r y s t a l l i z e d alumina c r u c i b l e . This small c r u c i b l e was then placed i n a tight f i t t i n g graphite susceptor c r u c i b l e , and placed i n alumina powder i n a larger c r u c i b l e . The crucible assembly was then placed inside a Vycor tube and the metal. - 20 -melted under a helium atmosphere i n an i n d u c t i o n furnace. A f t e r m e l t i n g the metal was w e l l s t i r r e d by magnetic i n d u c t i o n and then allowed to c o o l s l o w l y under helium. The r e s u l t i n g a l l o y button was then annealed under cracked ammonia at 700°C f o r twelve hours to ensure homogenization. A f t e r annealing the samples were cleaned and p o l i s h e d by standard m e t a l l o g r a p h i c techniques through 3-0 carborundum papers and on the 6 y and 1 p diamond dust wheels. I I I . Autoclave Design A t i t a n i u m autoclave ( F i g . 4) of 2000 ml c a p a c i t y , manufactured by the Parr Instrument company was used f o r the l e a c h i n g experiments. This s e r i e s 4500 autoclave as s u p p l i e d had the head, c y l i n d e r and inner wetted parts of t i t a n i u m w i t h the e x t e r n a l valves and f i t t i n g s of s t a i n l e s s s t e e l . As the leach i n g experiments were to be done i n hot a c i d c h l o r i d e s o l u t i o n s the s t a i n l e s s s t e e l sampling tube and valve had to be replaced. An a l l t i t a n i u m sampling tube and a T e f l o n -t i t a n i u m sampling valve were f a b r i c a t e d and f i t t e d . In order to pro t e c t the t i t a n i u m c y l i n d e r a t i g h t f i t t i n g g l a s s l i n e r was used, and a p r o t e c t i v e glass tube was f i t t e d over the thermowell. The rea c t o r contents were s t i r r e d by a l a r g e magnetic s t i r r e r bar on the bottom of the g l a s s l i n e r , which was pro t e c t e d w i t h a t h i n Teflon sheet. This was found to be the only p r a c t i c a l method of s t i r r i n g as the top s t i r r i n g arrangement proved to be s u s c e p t i b l e to c o r r o s i o n and leakage. The autoclave was heated by a 1500 watt e l e c t r i c heater b u i l t i n t o an i n s u l a t e d s t e e l s h e l l . The autoclave s l i d e s i n t o the heater and r e s t s on the magnetic s t i r r e r at the bottom. Automatic temperature - 2 1 -TITANIUM BODY SLASS LINER THERMOWELL GAS INLEF SAMPLE TUBE * SAMPLE HOLDER MAGNETIC STIRRER TEFLON SHEET i g . 4. Schematic diagram o f t h e P a r r a u t o c l a v e and the machined T e f l o n sample h o l d e r - 22 -control was achieved with a Thermistemp temperature c o n t r o l l e r Model 71 (Yellow Springs Instrument Co., Inc., Yellow Springs, Ohio). The thermistor probe was of i r o n constantan and s l i d into the thermistor well i n the autoclave. A second thermocouple was used to follow and record the temperature v a r i a t i o n . The temperature was controlled to within 1°C. The pressure gauge used had a 4 1/2" d i a l graduated from 0-1000 p s i 2 i n 5 l b / i n . subdivisions. This was connected on the pressure side of the gas e x i t tube valve. The oxygen cyl i n d e r was connected to the autoclave with high pressure woven wire tubing. A check valve was i n s t a l l e d before the i n l e t valve on the head of the autoclave to prevent a c c i d e n t a l backflow of corrosive gas or l i q u i d . The sample tube ins i d e the autoclave was made of Teflon with a f r i t t e d glass f i l t e r on the end. The i n t e r n a l pressure forces the sample through this tube and out the sampling valve. A'small cold water cooler was used on the sampling tube outside the autoclave to prevent f l a s h i n g of the high temperature l i q u i d . The sample holder (Fig. 4) was constructed of Teflon i n such a manner as to maintain a constant surface (area exposed to the s o l u t i o n . A Teflon cylinder was machined on one end to provide a r e s t r a i n i n g l i p to hold the sample. The sample was pressed against the l i p by a threaded rod, the edges being sealed by using a r i n g of soft Teflon valve packing on the r e s t r a i n i n g l i p . The sample holder was supported on the sample tube i n s i d e the autoclave with the exposed face held at an angle to the flow of the s o l u t i o n . This arrangement of sample posi t i o n i n g worked very w e l l and gave no evidence of uneven leaching - 23 -or stagnant pockets of gas on the sample surface. IV. A n a l y t i c a l Method The d i s s o l u t i o n r a t e of the a l l o y s was followed by determining the concentration of Pt (IV) i n s o l u t i o n . A c o l o r i m e t r i c method u t i l i z i n g the colored S n ^ ^ - P t chloro-complex was used. The method i s described i n d e t a i l by Beamish."^ The optimum concentration range i s from 3-25 ppm platinum, however the system conforms to Beer's law f a i r l y w e l l up to about 60 ppm. The procedure used was as f o l l o w s . A s o l u t i o n 1.0 M i n HCl was prepared. A 15 ml sample a l i q u o t was put i n a 25 cc v o l u m e t r i c f l a s k , and 2.5 ml of concentrated HCl was added. Then 5 ml of the S n C ^ s o l u t i o n was added and the volume d i l u t e d to the mark w i t h de i o n i z e d water. The f l a s k was shaken and the colour allowed to develop f o r f i f t e e n minutes. The transmittance of the sample was measured at 403 m i l l i m i c r o n s i n a Beckman DU spectrophotometer using g l a s s c e l l s . A blank sample was prepared from the SnC^ s o l u t i o n and d e i o n i z e d water and used as a reference. A standard sample co n t a i n i n g 10 ppm platinum was prepared from H^PtClg s o l u t i o n and measured against the reference to c a l i b r a t e the transmittance curves.. The platinum content was c a l c u l a t e d from the curves using Beer's law. The t o t a l amount of d i s s o l v e d platinum was c a l c u l a t e d from the known volume of s o l u t i o n and the concentration determined s p e c t r o p h o t o m e t r i c a l l y . The i r o n concentration was checked 18 by using an Ortho-phenanthroline method as described by Bath. - 24 -V. Experimental Procedure The a l l o y buttons were p o l i s h e d and mounted i n the sample holder. The excess Teflon packing was c a r e f u l l y trimmed away to give a c i r c u l a r surface area and the sample holder was washed and mounted on the autoclave sample tube. The l e a c h i n g s o l u t i o n was made up by adding the proper amounts of reagent grade NaCl and concentrated HCl to deionized water and making the t o t a l volume up to 1500 ml. The autoclave was then assembled and placed i n s i d e the heating j a c k e t . The temperature c o n t r o l l e r was set and the autoclave f l u s h e d through w i t h oxygen. When the autoclave reached the d e s i r e d temperature the s t i r r e r was s t a r t e d and the oxygen pressure r a i s e d to the desired pressure. The oxygen pressure was c o n s t a n t l y maintained at the de s i r e d pressure. Samples were taken at r e g u l a r i n t e r v a l s , using 10 cc to f l u s h the sampling l i n e and keeping a 25 cc sample f o r c o l o r i m e t r i c a n a l y s i s . At the end of each run, the a l l o y button was removed from the sample h o l d e r , washed, d r i e d and examined m i c r o s c o p i c a l l y . Photographs were taken a f t e r some runs and e l e c t r o n probe photographs were a l s o taken to f u r t h e r r e v e a l surface s t r u c t u r e . - 25 -RESULTS The d i s s o l u t i o n data obtained are presented i n g r a p h i c a l form; the numerical r e s u l t s are tabulated i n the appendix. The r a t e curves are c a l c u l a t e d from a n a l y t i c a l r e s u l t s and are corrected f o r i n i t i a l surface area and r e a c t i o n volume changes due to sample removal. I. D i s s o l u t i o n of Fe-Pt A l l o y s The d i s s o l u t i o n curves are a l l based on the t o t a l amount of d i s s o l v e d platinum a f t e r c e r t a i n time i n t e r v a l s . These values were then used to c a l c u l a t e the amount of d i s s o l u t i o n per u n i t i n i t i a l surface area. A l l the d i s s o l u t i o n curves found were l i n e a r a f t e r a short i n i t i a l n o n - l i n e a r p o r t i o n . ( i ) A t y p i c a l d i s s o l u t i o n curve f o r an Fe-Pt a l l o y i s given i n F i g . 5. As noted above the curve i s l i n e a r , apart from a s l i g h t i n i t i a l curvature. This curvature may be due to p o l i s h i n g e f f e c t s on the surface or to a d e f i c i e n c y i n the a n a l y t i c a l technique at low platinum concentrations. ( i i ) The curves i n F i g . 6 compare the le a c h i n g rates f o r Pt sheet, Fe-Pt, and Fe^Pt a l l o y s . The curves f o r the Pt sheet, Fe-Pt and Pt^Fe are again l i n e a r , however, no platinum d i s s o l v e d from a Fe^Pt a l l o y , and the r e s u l t s were not plotted.' - 26 -( i i i ) The e f f e c t of temperature on the l e a c h i n g r a t e of FePt i s given i n F i g . 7. The ra t e increased w i t h temperature as expected. An Arrhenius type p l o t of t h i s data i s presented i n F i g . 8 from which an apparent a c t i v a t i o n energy f o r d i s s o l u t i o n of 16 kcal/mole i s found. This high an apparent a c t i v a t i o n energy would i n d i c a t e 19 chemical c o n t r o l r a t h e r than d i f f u s i o n c o n t r o l . ( i v ) A s e r i e s of curves showing the temperature e f f e c t on the leach i n g r a t e of pure platinum sheet i s given i n F i g . 9. As f o r the Fe-Pt a l l o y an Arrhenius p l o t of t h i s data i s given i n F i g . 10. An apparent a c t i v a t i o n energy of 20 kcal/mole i s found, again i n d i c a t i n g chemical c o n t r o l . I I . E f f e c t of A c i d Concentration + The e f f e c t of v a r y i n g the H concentration at a constant C l - i o n concentration (3 M) on the d i s s o l u t i o n of Pt from Fe-Pt a l l o y i s given i n F i g . 11. The a c i d c oncentration was l i m i t e d to 3 M because of the danger of autoclave c o r r o s i o n . A p l o t of the r a t e of d i s s o l u t i o n versus a c i d concentration i s given i n F i g . 12. This i s a curve, however + 2 a p l o t of r a t e of d i s s o l u t i o n versus [H ] ( F i g . 12) i s l i n e a r . I I I . E f f e c t of Oxygen Pressure The e f f e c t of oxygen pressure on the r a t e of d i s s o l u t i o n of Fe-Pt a l l o y i n 2 M HCl at 150°C i s given i n F i g . 13. A l l the curves are again l i n e a r , and the ra t e increases w i t h i n c r e a s i n g 0^ p a r t i a l pressure. A p l o t of 0^ p a r t i a l pressure versus d i s s o l u t i o n rate ( F i g . 14) i s a l s o l i n e a r . As the s o l u b i l i t y of 0 i n aqueous s o l u t i o n s i s p r o p o r t i o n a l to pressure, t h i s p l o t t h e r e f o r e gives the v a r i a t i o n i n r e a c t i o n rate ( d i s s o l u t i o n rate) as a f u n c t i o n of 0^ concentration as w e l l as pressure. IV. E f f e c t of C h l o r i d e Ion Concentration The e f f e c t of i n c r e a s i n g the c h l o r i d e i o n concentration through the a d d i t i o n of NaCl to 2 M HCl s o l u t i o n i s shown i n F i g . 15. Increased C l i o n leads to an increase i n r a t e , but a s a t u r a t i o n type of curve i s obtained when a r a t e versus ( C l ) p l o t ( F i g . 16) i s made. Estimate of Err o r s i n Results The c a l c u l a t e d e r r o r i n the raw data used to ob t a i n d i s s o l u t i o n rates i s ± 5%. This i s composed of a t I I accuracy i n a n a l y t i c a l r e s u l t s and a ± 4% e r r o r i n the c a l c u l a t e d surface area of the sample. The a c t u a l e r r o r s i n the derived r a t e values are than ± 5% plus an e r r o r i n c u r r e d i n drawing a s t r a i g h t l i n e through the s c a t t e r e d experimental p o i n t s . Repeated experiments gave i d e n t i c a l r a t e s of d i s s o l u t i o n , using the l i n e drawing technique. - 28 -1 2 3 • k 5 6 TIME (hrs.) F i g . 5. A t y p i c a l d i s s o l u t i o n c u r v e f o r P t - F e a l l o y a t 700 p s i g 09, 2 M HCl and 150°C - 29 -F i g . 6 C o m p a r a t i v e D i s s o l u t i o n r a t e s f o r P t F e , Pt3Fe and Pt s h e e t under s i m i l a r l e a c h i n g c o n d i t i o n s . - 30 -TIME (hrs.) F i g . 7. The e f f e c t o f t e m p e r a t u r e on the d i s s o l u t i o n r a t e o f P t F e a l l o y a t 1 M HCl and 700 p s i g 0 ? - 31 -F i g . 8. A r r h e n i u s p l o t f o r the d i s s o l u t i o n o f P t F e a l l o y - 32 -2 k 6 8 TIME (hrs.) F i g . 9. The e f f e c t o f t e m p e r a t u r e on the d i s s o l u t i o n r a t e o f pure Pt s h e e t a t 2 M HCl and 700 p s i g 09 - 33 -F i g . 10 A r r h e n i u s p l o t f o r the d i s s o l u t i o n o f Pt s h e e t - 34 -[ H + ] (M.) F i g . 11. The e f f e c t o f HCl c o n c e n t r a t i o n on the d i s s o l u t i o n r a t e o f P t F e a l l o y a t 3 M t o t a l C l c o n c e n t r a t i o n , 150°C and 500 p s i g 0g  2 M 6 8 10 [ H + ] 2 (M2) + 2 F i g . 12. D i s s o l u t i o n r a t e v s . [H ] f o r PtFe a l l o y - 35 -TIME (hrs.) F i g . 13. The e f f e c t o f oxygen p r e s s u r e on the d i s s o l u t i o n r a t e o f P t F e a l l o y a t 150°C and 2 M HCl OXYGEN PRESSURE (p.ai.) F i g . 14. D i s s o l u t i o n r a t e v s . p0 9 f o r P t F e a l l o y d i s s o l u t i o n - 37 -r. i i i i i , i 1 2 3 L 5 TIME (hrs.) F i g . 15. The e f f e c t o f [ C l - ] on the d i s s o l u t i o n r a t e o f P t F e a l l o y a t 500 p s i g 0 9 - a n d 2 M HCl - 38 -I I I— I J 1 1 1 1 L. 2.0 2.5 3.0 TOTAL Cf (M.) F i g . 16. D i s s o l u t i o n r a t e v s . t o t a l C l " c o n c e n t r a t i o n f o r P t F e a l l o y - 39 -DISCUSSION I. L i n e a r D i s s o l u t i o n Curves In a l l instances i n t h i s study, the d i s s o l u t i o n r a t e of the platinum from both pure Pt and Fe-Pt a l l o y s was l i n e a r w i t h time. Mic r o s c o p i c examination of the surface showed general uniform c o r r o s i o n ( F i g . 17) w i t h some p r e f e r e n t i a l g r a i n boundary c o r r o s i o n . However, e l e c t r o n microprobe a n a l y s i s of the leached surface showed no p r e f e r e n t i a l d i s s o l u t i o n of Fe from the Fe-Pt a l l o y s at greater than 50 at % Pt. Therefore i n the f o l l o w i n g d i s c u s s i o n i t i s assumed that the a l l o y d i s s o l v e d uniformly over the exposed p o r t i o n of the sample, w i t h Fe and Pt d i s s o l v i n g at the same r a t e . The l i n e a r 21 rat e s of c o r r o s i o n i n d i c a t e that the surface area of the specimen remained e s s e n t i a l l y constant over the duration of the leachi n g e x p e r i -ments and aLso that the reactant concentrations remained unchanged. The s m a l l amounts of Pt d i s s o l v e d i n a l l cases tend to support these assumptions, as not enough would d i s s o l v e to appreciably change the surface. I I . A c t i v a t i o n Energies and S t i r r i n g E f f e c t s The apparent a c t i v a t i o n energies f o r the d i s s o l u t i o n of Pt from both Fe-Pt a l l o y s (15.6 kcal/mole) and pure Pt sheet (19.8 kcal/mole) - 41 -19 are high enough that i t appears the r e a c t i o n i s chemically c o n t r o l l e d . A two f o l d i n c r e a s e i n the s t i r r i n g rate had no e f f e c t on the rate of Pt d i s s o l u t i o n , t h e r e f o r e chemical r e a c t i o n c o n t r o l i s d e f i n i t e l y i n d i c a t e d . The chemical r e a c t i o n which i s rate c o n t r o l l i n g must occur at the surface, as the rate i s dependent on i n i t i a l surface area. The a c t i v a t i o n energies of both the cathodic r e d u c t i o n r e a c t i o n and the anodic d i s s o l u t i o n of Pt i n c h l o r i d e s o l u t i o n are close to the values found f o r d i s s o l u t i o n of Pt i n the autoclave. The a c t i v a t i o n energy f o r oxygen re d u c t i o n at a platinum surface i s i i the neighborhood of 20-25 kcal/mole. This value i s found by using -10 2 the magnitude of the exchange current d e n s i t y ( 10 amps/cm ) and c a l c u l a t i n g the a c t i v a t i o n energy by a m o d i f i c a t i o n of the T a f e l equation. The a c t i v a t i o n energy f o r the anodic d i s s o l u t i o n o f Pt i n a l l s o l u t i o n s was found to be 20 kcal/mole.^ The decrease i n a c t i v a t i o n energy f o r the d i s s o l u t i o n of the Fe-Pt a l l o y i s probably an e f f e c t of lowering the corrosion p o t e n t i a l and/or p r o v i d i n g more e a s i l y corrodable s i t e s at the surface. I I I . K i n e t i c A n a l v s i s . 23 The k i n e t i c s of c o r r o s i o n of metals has been presented by Habashi i n the f o l l o w i n g f a s h i o n . For the cathodic h a l f - r e a c t i o n , when f i r s t - o r d e r k i n e t i c s are fol l o w e d , the rate law i s : V = k A [D] c c c - 42 -where V = the reaction v e l o c i t y c k = the cathodic v e l o c i t y constant c A = the surface area of the cathodic zone c [D] = concentration of the depolarizer. S i m i l a r l y , the rate law for the anodic h a l f - r e a c t i o n i s given by: V = k A [C] a a a where V = the reaction v e l o c i t y a J k = the anodic v e l o c i t y constant a . A = the surface area of the anodic zone a [C] = concentration of complexing agent. At the steady state, the rate of the cathodic reaction i s equal to the rate of the anodic reaction; and t h i s rate i s equal to the corrosion rate, i'*e. V = k A [D] = k A [C] c c a a however since A = A + A where A i s the t o t a l surface areaavailable c a for corrosion, the rate equation can be written: k k A[D][C] V = C 3 k [D] + k [C] c a At a r e l a t i v e l y low concentration of D and a high C concentration the rate equation s i m p l i f i e s to - 43 -V = k A[D] c i n d i c a t i n g a c a t h o d i c a l l y c o n t r o l l e d c o r r o s i o n r e a c t i o n . S i m i l a r l y at a high concentration of D and a low concentration of C, the rate equation becomes: V = k A[C] a and the c o r r o s i o n i s a n o d i c a l l y c o n t r o l l e d . For metal c o r r o s i o n i n a c i d oxygenated s o l u t i o n s the cathodic r e d u c t i o n of oxygen i s the most l i k e l y ' d e p o l a r i z a t i o n ' r e a c t i o n . 24 Hoare has reviewed the l i t e r a t u r e on the oxygen r e d u c t i o n r e a c t i o n at platinum e l e c t r o d e s . This r e a c t i o n i s g e n e r a l l y b e l i e v e d to occur i n two steps both i n v o l v i n g a 2-el e c t r o n t r a n s f e r . The f i r s t step i s : 0 2 + 2H + + 2e — H 2 0 2 followed by a second c a t a l y t i c step on the surface: H 20 2 + 2H + + 2e »- 2H 20 The r e a c t i o n mechanism f o r the f i r s t step i s b e l i e v e d to be as f o l l o w s , w i t h the f i r s t one-electron t r a n s f e r r e a c t i o n i n v o l v i n g adsorbed molecular oxygen being r a t e determining: - 44 -° 2 ( s o l n ) ° 2(ads) +e slow 2(ads) ( 0 2 ) ~ a d s ( 0 2 ) ads + H + — ( H 0 2 ) ads ( H 0 2 ) ads + e — ^ - ( H 0 2 > ads ( H 0 o ) ~ ads + H + — * ~ ( H o 0 o ) ads a f t e r which the adsorbed peroxide molecule e i t h e r desorbs or rea c t s f u r t h e r . The second r e a c t i o n step i s b e l i e v e d to be c a t a l y z e d somehow by the surface and a d e t a i l e d mechanism has not yet been proposed. However, the r a t e of t h i s second r e a c t i o n has been found to be an order of magnitude l a r g e r than the f i r s t r e a c t i o n r a t e . Hoare a l s o noted that on a l l o y s of P t , the same o v e r a l l r e a c t i o n mechanism i s observed, however the rates are g e n e r a l l y enhanced, due to a b e t t e r e l e c t r o n t r a n s f e r , an improvement i n c a t a l y t i c a b i l i t y , or a decrease i n i n h i b i t i n g anion adsorption. In t h i s case, tak i n g the f i r s t r e d u c t i o n r e a c t i o n to H 2 0 2 as being the r a t e c o n t r o l l i n g step, we get: 0 2 + 2 H + + 2 e — 5 - H 0 2 Under the co n d i t i o n s f o r determining the oxygen dependence of the d i s s o l u t i o n r e a c t i o n , the H 2 ° 2 c o n c ? e n t r a t : ' - o n ^ s regarded as being - 45 -s m a l l as i t i s a q u i c k l y reduced intermediate. The e f f e c t of c h l o r i d e i o n concentration on the d i s s o l u t i o n r a t e of the PtFe a l l o y i s given i n F i g s . 15 and 16. As shown i n F i g . 16 i t i s apparent that there i s no increase i n d i s s o l u t i o n r a t e w i t h i n c r e a s i n g c h l o r i d e i o n concentration above 2 M. This leads to the c o n c l u s i o n that the d i s s o l u t i o n r e a c t i o n i s i n f a c t c a t h o d i c a l l y c o n t r o l l e d , and therefore only the oxygen pressure and h y d r o c h l o r i c a c i d concentration w i l l have an e f f e c t on the d i s s o l u t i o n r a t e . As the s i t u a t i o n i s one where the concentration of complexing agent ( C l ) i s high w i t h respect to the d e p o l a r i z e r (O^JH" 1") ; according to the k i n e t i c d e r i v a t i o n given above, the o v e r a l l d i s s o l u t i o n r a t e should correspond to a r a t e law of the form: V = k A[D] c where V i s the d i s s o l u t i o n r a t e , k i s a cathodic r a t e constant, A i s the surface area and [D] i s the concentration of d e p o l a r i z e r . The e f f e c t of oxygen pressure on the d i s s o l u t i o n r a t e , determined at a constant h y d r o c h l o r i c a c i d concentration of 2 M i s shown i n F i g s . 13 and 14. The p l o t of d i s s o l u t i o n r a t e vs. pO^ ( F i g . 14) shows a l i n e a r dependence of the r a t e on the oxygen pressure. As oxygen f o l l o w s Henry's law i n t h i s temperature and pressure range, t h i s dependence i s equivalent to a l i n e a r dependence on the 0^ concentration 2 i n s o l u t i o n . As the d i s s o l u t i o n r a t e i s given i n mg dissolved/cm /hour we can w r i t e a s p e c i f i c r a t e expression. V = k c ' P 0 2 (1) - 46 -where V i s the d i s s o l u t i o n r a t e per u n i t area and k ' i s a rate c constant d e s c r i b i n g the l i n e a r dependence of the d i s s o l u t i o n r a t e on oxygen pressure. Therefore, f o r the s p e c i f i c c o n d i t i o n s of high (> 2 M) c h l o r i d e i o n concentration and a constant a c i d c o n c e n t r a t i o n , the d i s s o l u t i o n r a t e f o l l o w s standard c o r r o s i o n k i n e t i c s . The numerical value of fie late constant i s : 2 k • = 0.00517 mg/cm hr p s i and the rate law i s : V = 0.00517 p 0 2 mg/cm2 hr The e f f e c t of the a c i d concentration on the d i s s o l u t i o n r a t e i s more complex. The a c i d dependence of the ra t e was determined at high (3 M) t o t a l c h l o r i d e i o n concentration and at constant oxygen pressure. As shown i n F i g . 11, the rate dependence on the a c i d concentration i s 2 no n - l i n e a r . However, a p l o t of ( a c i d concentration) vs. d i s s o l u t i o n rate ( F i g . 12) i s l i n e a r over a region from 1.0 M HCl to 3.0 M HCl Below 1 M HCl the rate drops o f f sharply and should become zero at zero a c i d concentration. Therefore over the region from 1' M HCl to 3 M HCl a ra t e law f o r the d i s s o l u t i o n may be w r i t t e n : V = k c" ( [ H + ] 2 + C) (2) - 47 -where V = d i s s o l u t i o n r a t e k " = r a t e constant c [H +] = a c i d concentration C = i n t e g r a t i o n constant. + 2 Numerically, k^" i s the slope of the Rate vs. [H ] curve and ^c"C i s the zero i n t e r c e p t of the s t r a i g h t l i n e e x t r a p o l a t e d to zero. At + constant [H ] the r a t e law was found to be: V = V P°2 there f o r e f o r a s i t u a t i o n where both the a c i d concentration and pC^ are changing, the r a t e law w i l l be: V = k '"(k " [ H + ] 2 + k "C)p0 o (3) c c c z which s i m p l i f i e s to V = k c [ H + ] 2 p 0 2 + k cCp0 2 (4) where k = k '"k " c c c Therefore k i s a r a t e constant f o r the o v e r a l l cathodic r e a c t i o n i n c the r a t e law (4) which i s v a l i d i n the region 1 M < [H +] < 3 M. The anodic h a l f r e a c t i o n of'the d i s s o l u t i o n was not rat e c o n t r o l l i n g under the experimental conditions used i n the autoclave, t h e r e f o r e , no experimental data are a v a i l a b l e f o r c a l c u l a t i n g a rat e constant f o r - 4 8 -the anodic h a l f reaction or the o v e r a l l d i s s o l u t i o n reaction. 8 — Chemodanov et a l . have shown that at low C l concentrations, from about 0.01 to 0.5 M, the rate dependence of the anodic Pt d i s s o l u t i o n on chloride ion concentration i n a c i d s o l u t i o n i s of the form - 0 9 V = k A[C1 ] , y a where k i s an anodic rate constant, A i s the surface area and [Cl ] a i s the c h l o r i d ion concentration. If this dependence on chloride ion i s v a l i d for the d i s s o l u t i o n from PtFe a l l o y s , a rate law describing the o v e r a l l d i s s o l u t i o n rate would have the form: [k [H +] 2p0. + k CpO.]k [ C l " ] 0 ' 9 •y C C Z 3 (k [ H + ] 2 P o 9 + k c Po 9) + k r c i " ] 0 - 9 C _- C Z cl i n a region where both the anodic and cathodic reactions are rate c o n t r o l l i n g to some extent. In the d i s s o l u t i o n experiments done i n the autoclave this region was not studied, therefore, no-numerical values for the rate constants or experimental evidence for the v a l i d i t y of the proposed rate law i s a v a i l a b l e . - 49 -CONCLUSIONS 1. Platinum-iron a l l o y s at platinum concentrations of 50 atom percent or greater dissolve homogeneously i n oxygenated hydrochloric acid solutions. 2. The rate of platinum d i s s o l u t i o n from platinum-iron a l l o y s follows corrosion k i n e t i c s at chloride ion concentrations greater than 2 M, with the rate c o n t r o l l i n g step being cathodic oxygen reduction. The rate i s dependent on acid concentration, temperature, oxygen pressure and a l l o y composition. SUGGESTIONS FOR FURTHER WORK An i n v e s t i g a t i o n of the d i s s o l u t i o n rate under anodic corrosion control would add to the d e t a i l of the d i s s o l u t i o n mechanism. This work would have to be done using an electrochemical technique, as the corrosion problems i n the autoclave are serious. A series of experiments on other PtFe a l l o y s at concentrations both below 50 atom percent and i n a s o l i d s o l u t i o n regime rather than at i n t e r m e t a l l i c compound compositions could lead to better understanding of both a l l o y corrosion and the de-alloying type of phenomena. - 51 -REFERENCES 1. J.B. M e r t i e , U.S. Geol. Surv. P r o f . Pap. No. 630, 1969. 2. Dana's System of Minerology, 7th Ed., J. Wiley and Sons, New York, 1944). 3. Englehard Technical B u l l e t i n . 4. J. L l o p i s and A. Sancho, J. Electrochem. Soc. 108, 720 (1961). 5. J.A. B i t t l e s and E.L. L i t t a u e r , Corr. S c i . JLO, 29 6. C. M a r s h a l l and J.P. M i l l i n g t o n , J. Appl. Chem. 19, 298 (1969). 7. J. L l o p i s , C a t a l y s i s Reviews 2_, 161 (1969). 8. A.N. Chemodanov, et a l . , E l e k t r o k h i m i y a , 4^ , 1466 (1968). 9. M. Pourbaix, A t l a s of E l e c t r o c h e m i c a l E q u i l i b r i a i n Aqueous S o l u t i o n s , Pergamon P r e s s , 1966. 10. F.E. Beamish, The A n a l y t i c a l Chemistry of the Noble Metals, New York, Pergamon P r e s s , 1966. 11. V.G. Tronev, Comptes. Rend, de l'Acad. des S c i . de l'URSS 1_5, 555 (1937). 12. Wichers, Schlecht, and Gordon, J. Res., Nat. Bur. of Stds. 33, 363 (1944). 13. C. Wagner and W. Traud, Z. Elektrochem. 44, 391 (1938). 14. T.P. Hoar, J. Appl. Chem. 11 (1961). 15. H.W. P i c k e r i n g and C. Wagner, J. Electrochem. Soc. 114, 698 (1967). 16. H.H. U h l i g , Trans. Electrochem. Soc. 85, 307 (1944). 17. H. L e i d h e i s e r , 9th Ann. A.E.C. Conf. Boston, May 11, 1960. 18. M.D. Bath, M.A.Sc. Thesis, Dept. of M e t a l l u r g y , U.B.C., 1968. 19. A.R. B u r k i n , The Chemistry of H y d r o m e t a l l u r g i c a l Processes, E. and F.N. Spon L t d . , London 1966. - 52 -20. L.M. Zoss, S.N. Suciu and W.L. S i b b i t t , Trans. A.S.M.E. Jan. 1954. 21. J . Halpern, J . Electrochem. S o c , 100, 421 (1953). 22. J.M. West, E l e c t r a d e p o s i t i o n and Corrosion Processes, D. Van Nostrand Co. L t d . , 1965. 23. F. Habashi, J . Chem. Educ. 42, 318 (1965). 24. J.P. Hoare, The E l e c t r o c h e m i s t r y of Oxygen, I n t e r s c i e n c e P u b l i s h e r s 1968. - 53 -TABLE 1. A l l o y Comparison A l l o y p 0 2 [HC1]/M D i s s o l u t i o n r a t e 2 (mg/cm /hr) Pt 700 p s i g 2 1.4 P t 3 F e 600 p s i g 3 1.9 PtFe 700 p s i g 2 2.9 TABLE 2. E f f e c t of Temperature on D i s s o l u t i o n Rate A l l o y Temperature (°C) D i s s o l u t i o n r a t e Pt-Fe 130 141 150 170 0.40 0.675 1.10 2.20 (1 M HCl 700 psig) Pt 140 150 170 0.90 1.40 4.60 (2 M HCl 700 psig) - 54 -TABLE 3. E f f e c t of [H ] on D i s s o l u t i o n Rate of Fe-Pt [H +] (M) D i s s o l u t i o n r a t e 0.5 1.85 1.0 2.185 1.6 2.33 2.5 2.70 3 2.97 TABLE 4. E f f e c t of Oxygen T = 150°C + Pressure on D i s s o l u t i o n Rate [H ] = 2 M, p 0 2 ( P s i ) D i s s o l u t i o n r a t e 2 (mg/cm /hr) 170 0.75 270 1.80 470 2.40 670 3.25 870 4.45 - 55 -TABLE 5.' E f f e c t of Ch l o r i d e Ion on D i s s o l u t i o n Rate. [H +] = 2 M, p0 2 = 500 p s i g T o t a l [ C l ~ ] (M) D i s s o l u t i o n rate 2 (mg/cm /hr) 2.0 2.2 2.4 2.485 2.8 3.0 3.1 3.9 3.9 3.9 3.9 4.2 

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