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The electropolishing of aluminum
Electrodeposition and Surface Treatment, 3 (1975) 159 - 168
159
© Elsevier Sequoia S.A., Lausanne -- Printed in Switzerland
T H E E L E C T R O P O L I S H I N G OF ALUMINIUM P. NEUFELD and D. M. SOUTHALL Division of Metal Science, Polytechnic of the South Bank, Borough Road, London, S E 1 0 A A (Gt. Britain)
(Received January 22, 1975)
Summary An electron microscopical study of electropolished aluminium films f o r m e d in a variety of different electrolytes is reported. Polishing in various electrolytes produces films which have different morphologies d e p e n d e n t u p o n the electrolyte used. The films have no c o m m o n structure for the range of electrolytes used. It appears that the electrolyte needs only to be able to induce adequate ionic conductivity in the surface film for the p r o d u c t i o n of a s m o o t h and bright surface.
Introduction The c o n c e p t t ha t a solid surface film may be present during anodic brightening was confirmed in 1952 [ 1 ] . Recent work [2, 3] has indicated t h a t a thin solid film m a y be i m p o r t a n t in m any practical instances of electropolishing. Evidence of the general occurrence of these films during electropolishing has been provided by the thickening of the film on zinc to interference tint dimensions, and there is considerable evidence for the presence of films on c oppe r and iron [4, 5]. Electropolishing on a " m i c r o " scale (microsmoothing, as opposed to levelling) depends on the uni f or m anodic dissolution of the metal. During etching the attack is directed to the high energy sites on the metal surface, and so is d e p e n d e n t upon crystal orientation. The distinction between etching and polishing in the majority of practical cases may well be d e p e n d e n t u p o n the presence of a solid film. The film into which the oxidized metal ions pass reduces the energy differentials between various parts o f the surface and thus p r o m o t e s uniform dissolution. Aluminium provides well d o c u m e n t e d evidence of a substantial solid film. Neufeld and Ali [6] observed a transition from anodizing to polishing film in citrate solutions, due to an increase in pore wall dissolution, brought a b o u t by increasing the solvent pow e r of the solution, and n o t because o f any f u n dam e nt a l alteration in film m orphol ogy. The self structure observed on aluminium [7] has a close resemblance to the "porebase" p atter n which is left after dissolving o f f the anodized layer.
160
L__J 0.050 #
Fig. 1. Polish in 80% HC104 + 20% CH3OH at 22 °C.
I
I
0,033 p
Fig. 2. Polish in 100% HC104 at 22 °C. C u f f a n d G r a n t [8] r e p o r t e d t h a t specific film m o r p h o l o g i e s w e r e a s s o c i a t e d w i t h c h e m i c a l polishing. This c o n c l u s i o n was m a d e f r o m e x a m i n a t i o n o f o n l y a small n u m b e r o f acid e l e c t r o l y t e s o f similar c o m p o s i t i o n . T h e q u e s t i o n r e m a i n i n g is w h e t h e r t h e polishing o f a l u m i n i u m is l i m i t e d t o a specific t y p e o f film m o r p h o l o g y or w h e t h e r t h e p h y s i c o -
161
I
I
0.033 #
Fig. 3. Polish in 150 g/1 Na2CO 3 + 50 g/1 Na3PO 4 + 30 g/1 NaOH at 65 °C. chemical p r o p e r t i e s o f the film are o f p r i m a r y i m p o r t a n c e . T h e w o r k r e p o r t e d describes e l e c t r o n m i c r o s c o p y studies o f e l e c t r o p o l i s h e d a l u m i n i u m films f o r m e d in a variety o f d i f f e r e n t e l e c t r o l y t e s . Experimental work
Determination of polarization curves A l u m i n i u m specimens were p u n c h e d f r o m 9 9 . 9 9 9 % p u r e foil, 0 . 0 0 5 in. thick, into circular discs 1 cm in d i a m e t e r . A n o d i c p o l a r i z a t i o n curves were d e t e r m i n e d u n d e r p o t e n t i o s t a t i c c o n t r o l for solutions given in Table 1, a t a sweep rate o f 1 V min - 1 . During the tests t h e specimens were r o t a t e d at a c o n s t a n t speed o f 8 5 0 rev. min - 1 one side o f t h e a l u m i n i u m being e x p o s e d t o t h e s o l u t i o n (1.57 cm2). F r o m the curves the range o f voltages w h i c h p r o d u c e d polishing were o b t a i n e d , and s u b s e q u e n t tests were p e r f o r m e d w i t h i n t h e polishing range f o r each e l e c t r o l y t e . A f t e r polishing f o r various times at f i x e d voltages w i t h i n t h e polishing range the specimens were r e m o v e d whilst the c u r r e n t was still flowing and w a s h e d with distilled water.
Obtaining the polishing film A f t e r t h e polishing t r e a t m e n t t h e specimens were b a c k e d on b o t h sides with a structureless barrier layer, d e p o s i t e d f r o m a b o r a x s o l u t i o n [ 9 ] . This gives s u p p o r t to t h e polishing film during the stripping and e x a m i n a t i o n process w i t h o u t a f f e c t i n g t h e s t r u c t u r e of the existing film. Areas f r o m the
TABLE 1 tO
Micrograph No.
S o l u t i o n details ( b y v o l u m e )
Temperature (°C )
T y p e o f surface
Current density ( m A / c m 2)
Voltage (V)
143.3
2
1
Perchloric acid (60 vol.%) 80% HC104 + 20% CH3OH
22
Polish
2
100% HCIO 4
22
Polish
3
150 g/1 Na2CO 3 + 50 g/1 Na3PO 4 + 30 g/1 NaOH
65
Polish
4
2% s o d i u m citrate + 10 g/1 N a O H
65
Polish
43.30 19.10
5
100% (sp. gray. 1.75) H3PO 4
65
Polish
6
80% H3PO 4 + 20% CH3OH
22
Semi-Polish
7
50% H3PO 4 + 50% H 2 0
65
Semi-Polish
8
53% H3PO 4 + 26% H 2 0 + 21% H2SO 4
65
Semi-Polish
9
P h o s b r i t e 159 ( c h e m i c a l Polish)
65
Polish
10
P h o s b r i t e 159 (chemical Polish)
100
Polish
79.60 105.0
Time (mins) 2
2
5
2
2
3
2
4
2
4.5
4
37.30
4
2
31.20
3
2
--
--
10
--
--
3
2.870
163
I _ _ l 0.033 #
Fig. 4. Polish in 2% sodium citrate + 10 g/1 NaOH at 65 °C.
L m J 0.125 p
Fig. 5. Polish in 100% H3PO 4 at 65 °C. n o n - p o l i s h e d side o f the s p e c i m e n were t h e n a b r a d e d t o s u p p l y access t o the i o d i n e / m e t h a n o l film s e p a r a t i n g solution, in w h i c h t h e m e t a l was s u s p e n d e d . T e n m i n u t e s e x p o s u r e t o this s o l u t i o n at 60 °C s u c c e e d e d in dissolving a w a y t h e m e t a l f r o m the a b r a d e d areas and e x p o s i n g w i n d o w s of polishing film.
164
I
0.833
]
Fig. 6. Semi-polish in 80% H3PO 4 + 20% CH3OH at 22 °C.
~0.060 #~ Fig. 7. Semi-polish in 50% H3PO 4 + 50% H20 at 65 °C. These windows were transferred to electron microscope grids and examined in transmission at 100 kV. The electron micrographs obtained a r e s h o w n in Figs. 1 - 1 0 f o r s o l u t i o n s 1 - 1 0 r e s p e c t i v e l y o f T a b l e 1.
165
I
i
0.050 p Fig. 8. Semi-polish in 53% H3PO 4 + 26% H 2 0 + 21% H2SO 4 at 65 °C.
Included in the results are films p r o d u c e d by the chemical polishing solution, Phosbrite 159. Results and Discussion In all cases o f polishing it was possible to separate a solid film f r o m the surface. The electron micrographs show clearly that each solution p r o d u c e d a film o f d if f er en t m o r p h o l o g y , ranging f r om the pore and cell n e t w o r k in Fig. 5 to the diffuse and ill-defined structure in Fig. 2. The average pore diameter in Fig. 5 is 0.0125 pm, contained in cells o f diameter 0.120 pm when fully established. A small n u m b e r of cells which are still growing, and have been overshadowed b y their faster growing neighbours are shown at the b o t t o m o f the picture. At a relatively low current density of 19.10 mA cm - 2 the surface was satisfactorily polished in t w o minutes for this electrolyte. On adding 50% water the structure was modified (Fig. 7). For polishing at the same voltage f or the same period of time the current density was 37.20 mA cm - 2 , due to the presence o f the added water. There are no pores to be seen, although the picture m a y be o f the establishment o f a cell structure, modified by the presence of water. A m i x t u r e of p h o s p h o r i c sulphuric acids and water, Fig. 8, p r o d u c e d the pore and cell structure again at a cu r r en t density of 31.20 m A cm - 2 . There m a y well be a relationship
166
I
I
0.033 p
Fig. 9. Chemical polish in Phosbrite 159 at 65 °C. between the production of pores, the water content of the bath, and the quality of polish attained in these phosphoric acid based solutions, since electrolyte (7} did n o t polish as well as the combined acid and water electrolyte (8) which in turn did not polish to the same degree as 100% phosphoric acid (5). The pore diameter of (8) was found to be 0.0150 pm and the average cell size 0.200 pm. Figure 6 shows a semi-polished specimen obtained from a phosphoric acid/methanol electrolyte, having etch pits in a directionally furrowed surface. A current density of 2.87 mA cm - 2 only was possible, due to the presence of methanol; which for phosphoric acid based solutions seems to act in an opposing way to the addition of water. It is possible that the semibright finish obtained was due to smoothing of the surface during etching; brightening n o t being attained under these conditions. The alkaline electrolytes (3) and (4) produced films similar to those reported in sulphuric acid anodizing [10]. The perchloric acid electrolyte gave no well defined film structure, but had the most rapid polishing action of all the solutions tested and also gave the best quality of polish. It is interesting to note t h a t the films from this solution were extremely difficult to separate without damage, perhaps due to their extreme thinness. On adding 20% methanol to perchloric acid a good polish was still attained, but unlike the phosphoric based solutions the methanol caused an increase in the current density to 143.3 mA cm - 2 , at
167
10.033/~J Fig. 10. Chemical polish in Phosbrite 159 at 100 °C.
the same setting o f 2v. The very fine porous structure p r o d u c e d is seen in Fig. 1,, Chemical polishing in Phosbrite 159 p r o d u c e d a semi-polished surface at 65 °C, similar to the semipolish shown in Fig. 7. When the t e m p e r a t u r e was raised to 100 °C a b e t t e r polish was attained, and a porous t y p e of structure developed, Fig. 10. Conclusion Polishing in the various electrolytes produces films which have different morphologies depending on the el ect r ol yt e used. These results show t h a t polishing can occur via films which have no c o m m o n or specific structure, e x c e p t t h a t t h e y all show t o a greater or lesser degree the porosity t h a t is c o m m o n to all well-conducting anodic films on aluminium. Cuff and Grant [8] f o u n d a c o m m o n structure which t h e y a t t r i b u t e d t o a surface manifestation of the underlying three-dimensional structure, and co n clu d ed t h a t the polishing action was d e t e r m i n e d by some quality of the crystal lattice. The present results do n o t s u p p o r t such a view w hen applied to electropolishing in electrolytes of varying characteristics. A film forms which has no c o m m o n structure for the range of solutions tested. It seems t h a t the electrolyte needs only to be able t o induce adequate ionic c o n d u ctiv ity in the surface film for the p r o d u c t i o n of a s m o o t h and bright surface to be attained.
168 Acknowledgement T h e a u t h o r s a r e g r e a t f u l t o Mr. G. I s s e r l i s , H e a d o f t h e M e t a l S c i e n c e Division, (Polytechnic of the South Bank), for providing facilities for the work and for his continued encouragement. References 1 2 3 4 5 6 7 8 9 10
T. P. Hoar and J. A. S. Mowat, Nature, 165 (1950) 64. M. Turner and P. A. Brook, Electrodep. Surface Treat., 2 (1973/74) 245. A. K. Vijh, Electrodep. Surface Treat., 2 (1973/74) 461. J. Allen, Trans. Faraday Soc., 48 (1952) 273. M. Cohen, J. Phys. Chem., 56 (1952) 451. P. Neufeld and H. O. Ali, J. Electrochem. Soc., 120 (1973) 479. M. Gy. Hollo, First Intern. Congress Met. Corr., 1961, p. 45. Cuff and Grant, J. Inst. Metals, 87 (1959) 248. P. Neufeld and C. White, Unpublished Work. H. O. Ali, Ph.D. Thesis, Univ. London, 1972.
159
© Elsevier Sequoia S.A., Lausanne -- Printed in Switzerland
T H E E L E C T R O P O L I S H I N G OF ALUMINIUM P. NEUFELD and D. M. SOUTHALL Division of Metal Science, Polytechnic of the South Bank, Borough Road, London, S E 1 0 A A (Gt. Britain)
(Received January 22, 1975)
Summary An electron microscopical study of electropolished aluminium films f o r m e d in a variety of different electrolytes is reported. Polishing in various electrolytes produces films which have different morphologies d e p e n d e n t u p o n the electrolyte used. The films have no c o m m o n structure for the range of electrolytes used. It appears that the electrolyte needs only to be able to induce adequate ionic conductivity in the surface film for the p r o d u c t i o n of a s m o o t h and bright surface.
Introduction The c o n c e p t t ha t a solid surface film may be present during anodic brightening was confirmed in 1952 [ 1 ] . Recent work [2, 3] has indicated t h a t a thin solid film m a y be i m p o r t a n t in m any practical instances of electropolishing. Evidence of the general occurrence of these films during electropolishing has been provided by the thickening of the film on zinc to interference tint dimensions, and there is considerable evidence for the presence of films on c oppe r and iron [4, 5]. Electropolishing on a " m i c r o " scale (microsmoothing, as opposed to levelling) depends on the uni f or m anodic dissolution of the metal. During etching the attack is directed to the high energy sites on the metal surface, and so is d e p e n d e n t upon crystal orientation. The distinction between etching and polishing in the majority of practical cases may well be d e p e n d e n t u p o n the presence of a solid film. The film into which the oxidized metal ions pass reduces the energy differentials between various parts o f the surface and thus p r o m o t e s uniform dissolution. Aluminium provides well d o c u m e n t e d evidence of a substantial solid film. Neufeld and Ali [6] observed a transition from anodizing to polishing film in citrate solutions, due to an increase in pore wall dissolution, brought a b o u t by increasing the solvent pow e r of the solution, and n o t because o f any f u n dam e nt a l alteration in film m orphol ogy. The self structure observed on aluminium [7] has a close resemblance to the "porebase" p atter n which is left after dissolving o f f the anodized layer.
160
L__J 0.050 #
Fig. 1. Polish in 80% HC104 + 20% CH3OH at 22 °C.
I
I
0,033 p
Fig. 2. Polish in 100% HC104 at 22 °C. C u f f a n d G r a n t [8] r e p o r t e d t h a t specific film m o r p h o l o g i e s w e r e a s s o c i a t e d w i t h c h e m i c a l polishing. This c o n c l u s i o n was m a d e f r o m e x a m i n a t i o n o f o n l y a small n u m b e r o f acid e l e c t r o l y t e s o f similar c o m p o s i t i o n . T h e q u e s t i o n r e m a i n i n g is w h e t h e r t h e polishing o f a l u m i n i u m is l i m i t e d t o a specific t y p e o f film m o r p h o l o g y or w h e t h e r t h e p h y s i c o -
161
I
I
0.033 #
Fig. 3. Polish in 150 g/1 Na2CO 3 + 50 g/1 Na3PO 4 + 30 g/1 NaOH at 65 °C. chemical p r o p e r t i e s o f the film are o f p r i m a r y i m p o r t a n c e . T h e w o r k r e p o r t e d describes e l e c t r o n m i c r o s c o p y studies o f e l e c t r o p o l i s h e d a l u m i n i u m films f o r m e d in a variety o f d i f f e r e n t e l e c t r o l y t e s . Experimental work
Determination of polarization curves A l u m i n i u m specimens were p u n c h e d f r o m 9 9 . 9 9 9 % p u r e foil, 0 . 0 0 5 in. thick, into circular discs 1 cm in d i a m e t e r . A n o d i c p o l a r i z a t i o n curves were d e t e r m i n e d u n d e r p o t e n t i o s t a t i c c o n t r o l for solutions given in Table 1, a t a sweep rate o f 1 V min - 1 . During the tests t h e specimens were r o t a t e d at a c o n s t a n t speed o f 8 5 0 rev. min - 1 one side o f t h e a l u m i n i u m being e x p o s e d t o t h e s o l u t i o n (1.57 cm2). F r o m the curves the range o f voltages w h i c h p r o d u c e d polishing were o b t a i n e d , and s u b s e q u e n t tests were p e r f o r m e d w i t h i n t h e polishing range f o r each e l e c t r o l y t e . A f t e r polishing f o r various times at f i x e d voltages w i t h i n t h e polishing range the specimens were r e m o v e d whilst the c u r r e n t was still flowing and w a s h e d with distilled water.
Obtaining the polishing film A f t e r t h e polishing t r e a t m e n t t h e specimens were b a c k e d on b o t h sides with a structureless barrier layer, d e p o s i t e d f r o m a b o r a x s o l u t i o n [ 9 ] . This gives s u p p o r t to t h e polishing film during the stripping and e x a m i n a t i o n process w i t h o u t a f f e c t i n g t h e s t r u c t u r e of the existing film. Areas f r o m the
TABLE 1 tO
Micrograph No.
S o l u t i o n details ( b y v o l u m e )
Temperature (°C )
T y p e o f surface
Current density ( m A / c m 2)
Voltage (V)
143.3
2
1
Perchloric acid (60 vol.%) 80% HC104 + 20% CH3OH
22
Polish
2
100% HCIO 4
22
Polish
3
150 g/1 Na2CO 3 + 50 g/1 Na3PO 4 + 30 g/1 NaOH
65
Polish
4
2% s o d i u m citrate + 10 g/1 N a O H
65
Polish
43.30 19.10
5
100% (sp. gray. 1.75) H3PO 4
65
Polish
6
80% H3PO 4 + 20% CH3OH
22
Semi-Polish
7
50% H3PO 4 + 50% H 2 0
65
Semi-Polish
8
53% H3PO 4 + 26% H 2 0 + 21% H2SO 4
65
Semi-Polish
9
P h o s b r i t e 159 ( c h e m i c a l Polish)
65
Polish
10
P h o s b r i t e 159 (chemical Polish)
100
Polish
79.60 105.0
Time (mins) 2
2
5
2
2
3
2
4
2
4.5
4
37.30
4
2
31.20
3
2
--
--
10
--
--
3
2.870
163
I _ _ l 0.033 #
Fig. 4. Polish in 2% sodium citrate + 10 g/1 NaOH at 65 °C.
L m J 0.125 p
Fig. 5. Polish in 100% H3PO 4 at 65 °C. n o n - p o l i s h e d side o f the s p e c i m e n were t h e n a b r a d e d t o s u p p l y access t o the i o d i n e / m e t h a n o l film s e p a r a t i n g solution, in w h i c h t h e m e t a l was s u s p e n d e d . T e n m i n u t e s e x p o s u r e t o this s o l u t i o n at 60 °C s u c c e e d e d in dissolving a w a y t h e m e t a l f r o m the a b r a d e d areas and e x p o s i n g w i n d o w s of polishing film.
164
I
0.833
]
Fig. 6. Semi-polish in 80% H3PO 4 + 20% CH3OH at 22 °C.
~0.060 #~ Fig. 7. Semi-polish in 50% H3PO 4 + 50% H20 at 65 °C. These windows were transferred to electron microscope grids and examined in transmission at 100 kV. The electron micrographs obtained a r e s h o w n in Figs. 1 - 1 0 f o r s o l u t i o n s 1 - 1 0 r e s p e c t i v e l y o f T a b l e 1.
165
I
i
0.050 p Fig. 8. Semi-polish in 53% H3PO 4 + 26% H 2 0 + 21% H2SO 4 at 65 °C.
Included in the results are films p r o d u c e d by the chemical polishing solution, Phosbrite 159. Results and Discussion In all cases o f polishing it was possible to separate a solid film f r o m the surface. The electron micrographs show clearly that each solution p r o d u c e d a film o f d if f er en t m o r p h o l o g y , ranging f r om the pore and cell n e t w o r k in Fig. 5 to the diffuse and ill-defined structure in Fig. 2. The average pore diameter in Fig. 5 is 0.0125 pm, contained in cells o f diameter 0.120 pm when fully established. A small n u m b e r of cells which are still growing, and have been overshadowed b y their faster growing neighbours are shown at the b o t t o m o f the picture. At a relatively low current density of 19.10 mA cm - 2 the surface was satisfactorily polished in t w o minutes for this electrolyte. On adding 50% water the structure was modified (Fig. 7). For polishing at the same voltage f or the same period of time the current density was 37.20 mA cm - 2 , due to the presence o f the added water. There are no pores to be seen, although the picture m a y be o f the establishment o f a cell structure, modified by the presence of water. A m i x t u r e of p h o s p h o r i c sulphuric acids and water, Fig. 8, p r o d u c e d the pore and cell structure again at a cu r r en t density of 31.20 m A cm - 2 . There m a y well be a relationship
166
I
I
0.033 p
Fig. 9. Chemical polish in Phosbrite 159 at 65 °C. between the production of pores, the water content of the bath, and the quality of polish attained in these phosphoric acid based solutions, since electrolyte (7} did n o t polish as well as the combined acid and water electrolyte (8) which in turn did not polish to the same degree as 100% phosphoric acid (5). The pore diameter of (8) was found to be 0.0150 pm and the average cell size 0.200 pm. Figure 6 shows a semi-polished specimen obtained from a phosphoric acid/methanol electrolyte, having etch pits in a directionally furrowed surface. A current density of 2.87 mA cm - 2 only was possible, due to the presence of methanol; which for phosphoric acid based solutions seems to act in an opposing way to the addition of water. It is possible that the semibright finish obtained was due to smoothing of the surface during etching; brightening n o t being attained under these conditions. The alkaline electrolytes (3) and (4) produced films similar to those reported in sulphuric acid anodizing [10]. The perchloric acid electrolyte gave no well defined film structure, but had the most rapid polishing action of all the solutions tested and also gave the best quality of polish. It is interesting to note t h a t the films from this solution were extremely difficult to separate without damage, perhaps due to their extreme thinness. On adding 20% methanol to perchloric acid a good polish was still attained, but unlike the phosphoric based solutions the methanol caused an increase in the current density to 143.3 mA cm - 2 , at
167
10.033/~J Fig. 10. Chemical polish in Phosbrite 159 at 100 °C.
the same setting o f 2v. The very fine porous structure p r o d u c e d is seen in Fig. 1,, Chemical polishing in Phosbrite 159 p r o d u c e d a semi-polished surface at 65 °C, similar to the semipolish shown in Fig. 7. When the t e m p e r a t u r e was raised to 100 °C a b e t t e r polish was attained, and a porous t y p e of structure developed, Fig. 10. Conclusion Polishing in the various electrolytes produces films which have different morphologies depending on the el ect r ol yt e used. These results show t h a t polishing can occur via films which have no c o m m o n or specific structure, e x c e p t t h a t t h e y all show t o a greater or lesser degree the porosity t h a t is c o m m o n to all well-conducting anodic films on aluminium. Cuff and Grant [8] f o u n d a c o m m o n structure which t h e y a t t r i b u t e d t o a surface manifestation of the underlying three-dimensional structure, and co n clu d ed t h a t the polishing action was d e t e r m i n e d by some quality of the crystal lattice. The present results do n o t s u p p o r t such a view w hen applied to electropolishing in electrolytes of varying characteristics. A film forms which has no c o m m o n structure for the range of solutions tested. It seems t h a t the electrolyte needs only to be able t o induce adequate ionic c o n d u ctiv ity in the surface film for the p r o d u c t i o n of a s m o o t h and bright surface to be attained.
168 Acknowledgement T h e a u t h o r s a r e g r e a t f u l t o Mr. G. I s s e r l i s , H e a d o f t h e M e t a l S c i e n c e Division, (Polytechnic of the South Bank), for providing facilities for the work and for his continued encouragement. References 1 2 3 4 5 6 7 8 9 10
T. P. Hoar and J. A. S. Mowat, Nature, 165 (1950) 64. M. Turner and P. A. Brook, Electrodep. Surface Treat., 2 (1973/74) 245. A. K. Vijh, Electrodep. Surface Treat., 2 (1973/74) 461. J. Allen, Trans. Faraday Soc., 48 (1952) 273. M. Cohen, J. Phys. Chem., 56 (1952) 451. P. Neufeld and H. O. Ali, J. Electrochem. Soc., 120 (1973) 479. M. Gy. Hollo, First Intern. Congress Met. Corr., 1961, p. 45. Cuff and Grant, J. Inst. Metals, 87 (1959) 248. P. Neufeld and C. White, Unpublished Work. H. O. Ali, Ph.D. Thesis, Univ. London, 1972.