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An improved apparatus for the electrolytic isolation of phases in metals
Electroanalytical Chemistry and lnterfacial Electrochemistry Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
AN IMPROVED APPARATUS F O R THE ELECTROLYTIC ISOLATION OF PHASES IN METALS R. GRIMALDI and N. VANTINI
Centro Sperimentale Metallurgico S.p.A., 00129 Rome (Italy) (Received 5th January 1972)
INTRODUCTION
Potentiostatic and potentiodynamic methods, which are being developed in various electrochemical fields, are of considerable interest for the preliminary investigation of the conditions necessary for the electrolytic isolation of phases in a metal sample 1- 6. In order to associate a specific reaction with every state of polarization of an electrode, it is often necessary to examine the effects of the electrochemical action. For this, the diffractometric analysis of the treated surface 7,s is a convenient technique. and it is principally for this reason that the two techniques have been combined and an apparatus designed which considerably improves and facilitates electrochemical treatment in controlled potential and diffractometric analysis. APPARATUS
The sample-holding device, shown schematically in Fig. 1, for which a patent is pending (It. Pat. n. 813.820, filed 15/5/1968), consists of a Teflon support A, connected to one end of the cell L. This cell, of a type already known 9, consists of two cylindrical half-cells, separated by a semipermeable membrane, whose common axis is kept horizontal. In the center of this cover A, is a circular window f, 8mm in diameter, which
C
I I
o
LI
E
I
Fig. 1. Sample-holding device for electrochemical measurements and treatments: (A) Teflon support, (B) bearing shaft, (C) specimen, (D) pivot with locking pins and graduated drum, (E) compression spring controlling screw, (F) compression device, (L) extremity of cell, (f) circular window, (g) washing groove.
J. Electroanal. Chem., 40 (1972)
2
R. GRIMALDI, N. VANT1NI
allows electrolytic contact between the sample and the reference electrode by means of a salt bridge, and contact between the sample and the counter-electrode in the opposite half-cell. The specimen C is placed with the selected surface at the window and may be revolved freely about the axis of pivot D thus allowing measurements and treatments of different zones of the same surface. One end of this pivot, ~ i c h is connected to the sample, has two centering and locking pins, while the other end has a graduated drum for sample positioning. The pins of the pivot fit into two corresponding holes drilled in the sample, and are firmly held to the sample by means of a compression spring controlled through screw E. To achieve better adherence between the surface of the sample and that of the Teflon support, there is a compression device F by the window. The whole system is fitted on shaft B which is connected to the cover A. On the sample side of the cover there is a 1 mm deep, radial groove g oriented 60° with respect to the window, through a pair of holes drilled in the cover. This groove leads to two small pipes which permit washing and deaeration of the surface after electrochemical treatment. The shape and dimensions of window f were chosen so that electrochemical investigation could be done at relatively low electrolysis currents, which in turn allow faster responses by the potentiostat. This factor is especially important when fast potentiodynamic measurements are required.
C
s/"j
/h
l
I
I
Fig. 2. Sample adjusting device for diffractometric analysis of treated zones: (B)-(E) as in Fig. 1, (G) Teflon block, (H) and (I) inox supports, (11)and (i) centering screws.
The device in Fig. 2, similar to the one described above, allows the sample to be adjusted on an X-ray goniometer. By means of the two screws h and i the surface to be investigated can be centered vertically and horizontally with reference to the incident beam. The block G, bearing shaft B and the specimen C, are made of Teflon to minimize surface scratching during the centering of the surface and the rotation of the specimen. FUNCTIONING OF APPARATUS
For the preliminary operations, generally consisting of the filling of the cell, deaeration, setting of the stirring apparatus where necessary, pre-setting of the power supply, etc., the specimen is oriented so that one of the zones lying between those to be J. Electroanal. Chem. 40 (1972)
ELECTROLYTIC ISOLATION OF PHASES IN METALS
3
examined, faces the cell. This intermediate surface can be used for preliminary trials, such as the evaluation of the equilibrium potential, and the selection of a starting potential, thus making it possible to start with the cell switched on. To initiate the experiment proper, it is sufficient to move the specimen so that the next selected zone faces the cell. At this stage the current-voltage curves at the selected potential scanning rate i.e. potentiodynamic curves, or current-time curves at selected potentials, i.e. potentiostatic curves, can be measured, and from these the polarization curve can be traced. The first type:of measurement is particularly useful for selecting an appropriate potential for the electrolytic isolation of phases. For instance, if at a definite potential a component dissolves at the electrode, then electrolysis at such a potential produces an effective enrichrnent of other components in the zone undergoing this treatment. The portion of the surface thus treated may then be examined with X-rays, the reliability of the results being dependent on the preservation of the surface conditions in the period between the X-ray examination and the preceding electrochemical treatment. Surfaces which have undergone electrolysis are generally very active and this inconvenience can be minimized by the following precaution. After electrochemical treatment, the sample is moved to a position where the treated surface can be washed free of electrolyte and kept in a stream of inert gas. This washing operation can be carried out while the next zone is undergoing electrochemical treatment. When the treatment of all zones is complete, the sample is fitted to the X-ray
Fig. 3. Electrolytic cell with detail of device for insertion of sample. J. Electroanal. Chem., 40 (1972)
4
R. GRIMALDI, N. VANTINI
apparatus, and the various zones are carefully optically centered. The main advantages offered by such a device are: (i) a considerable reduction of time in the sample preparation stage, a single surface is sufficient for at least six tests, (ii) the possibility of operating with surfaces under the same initial conditionS, thus enabling the investigation to be carried out more easily and under well-defined conditions, (iii) the possibility of eliminating dead times between the electrochemical treatment and the washing of the treated surface. EXPERIMENTAL
A series of tests was carried out on binary alloys working in a solution of pH 8, containing 8 ~ HCI, 0.5~ citric acid, 10~ glycerine and NH~OH. This is the solution adopted by Klyachko and Baranova 1° for the electrolytic isolation of austenite from manganese steels.
iiiii
Fig. 4. Photograph of Fe-Ni alloy with 30~o Ni, after the following electrochemical treatments: (1) 5 min, - 100 mV; (2) 3 min, 0 mV; (3) 1 min, + 400 mV; (4) 20 min, - 300 mV; (5) 3 min, - 100 mV; (6) 1 rain, + 100 inV. For (1), (2) and (3) the selected potential was reached at 200 mV rain -1 starting from - 5 0 0 inV. - J. Electroanal. Chem., 40 (1972)
ELECTROLYTIC ISOLATION OF PHASES IN METALS
: 5
The samples were in the form of discs 6 mm in thickness and 50 mm in diameter with a surface polished to a 1 #m diamond paste finish. For any surface so prepared, it was possible to carry out at least six electrochemical treatments on circular areas of 8 mm diameter. A large saturated calomel electrode and salt bridge with a pipeshaped extremity 0.5 mm in diameter spaced 1 mm from the sample were used. Figure 3 shows the electrolytic cell. RESULTS
The first samples are two Fe-Ni alloys, one having 3070 Ni and the other 4 ~ Ni, quenched in water from 1000° C and 600° C, respectively. The structures of the two samples, determined by X-rays, are t w o - phase (0t+ ~) and a single phase (~-Fe) respectively. Figure 4 is a photograph of the 3070 Ni alloy and shows the six zones treated. The conditions adopted for each are given. The potentiodynamic curves of the samples, obtained with the pH 8 solution and a scanning rate of 200 mV min- 1, are compared in Fig. 5 with those of iron and nickel. These potentiodynamic curves have good reproducibility, but there are difficulties in interpretation with respect to the conditions necessary for the electrolytic isolation of phases. The first difficulty is that for a certain interval of potential, the curves show a different behaviour according to the electrochemical events at the electrode before scanning. This is confirmed by the behaviour of the current-time curves at constant potential. Also, if the potential is selected so that it falls outside the range of instability of the potentiodynamic curves, the current-time curves show a potentiostatic electrolysis current which is practically constant from the beginning
,1"
.i'i//' i/ i" '
~
/ //,~~/ /'/ / / / /,/
12¢ 6c !/
., -400
i -200
0 E/mY
i "~' +200 vs. ECS
Fig. 5. Comparison between potentiodynamic curves (200 mV m i n - l) of two alloys at (A) 3 0 ~ Ni and (B) 4 ~ Ni and those (. . . . . ) relating to iron and nickel. Branches with arrows refer to primary scanning.
J. Electroanal. Chem., 40 (1972)
R. GRIMALDI, N. VANTINI
to the end of treatments of up to 20 min. Within the potential region in which there is variable behaviour of the potentiodynamic curves, the potentiostatic electrolysis current still tends towards a constant value, equal to that which may be deduced from the potentiodynamic curve obtained after activation of the surface, but this limiting value is reached only after about l min. It is interesting that this behaviour becomes more noticeable with increasing nickel content, almost to the point of characterizing different alloys. Because of this anomalous behaviour, and the proximity of the curves, the possibility of electrolytic isolation* of the structural components of these two alloys within a reasonable period of time, is unlikely given the particular solution used. It therefore seems necessary to choose a more suitable electrolytic solution. The diffractometric examinations carried out on the 30~o Ni alloy show, for an attack of 20 min at 300 mV, an actual increase in the surface austenite content (95~o) with respect to the value for the base alloy (80}/0). There is therefore a selective
iiiiiiii!i!
Fig. 6. Photograph of F e - C alloy with 4 ~ C, after the following treatments: (1) scanning from - 5 0 0 to + 800 mV, (2) the same after 10 rain at 0 mV, (3) the same after 10 min at + 200 mV, (4) the same after 5 min at + 600 mV, (5) 10 min at + 200 mV (only), (6) 10 min at + 600 mV (only). Scanning rate (when used), 200 mV m i n - 1. * Isolation is understood in a general sense here. In effect, the only phase isolation possible is that of the sample with 3 0 ~ Ni which is a two-phase structure. J. Electroanal. Chem., 40 (1972)
ELECTROLYTIC ISOLATION OF PHASES IN METALS
7
dissolution which, however, for reasons given above, may be rendered quantitative only with difficulty in this case. This is confirmed by the fact that true separation tests carried out on very much more extensive zones (30 mm) for times prolonged to 60 min did not completely eliminate the s-phase*. Another example of the application of the method is provided by an Fe-C alloy with 4 ~ carbon, studied with the same solution. The structure, from diffractometer results, is made up of ~'-Fe (or martensite) and FeaC (or cementite). On the diffraction pattern some lines (the most intense) of the carbide appear together with the (110) of the c(-Fe. The extremely high relative intensity of the latter, while effected by the superposition of the (002) + (103) of the carbide is, nevertheless, sufficient to prove the presence of the phase itself. Figure 6 shows the sample together with the conditions for the various treatments. The potentiodynamic curves (200 mV min- 1), shown in Fig. 7, were obtained in the following ways: (a) initial scanning starting from the equilibrium potential, (b) the same scanning but after a 10 min potentiostatic electrolysis at + 200 mV, and (c) the same after a 5 min electrolysis at + 600 mV. In the electrolyte used, this alloy shows a similar behaviour to one encountered in previous work on the electrochemical attack of cementite11- l a. In this case, given that the initial ascent of the curve corresponds to attack of the matrix only, it is possible
~///
t
lI
32Q
/ 24o
/I b
U
II I*'~ l
°~ <
~
,C
:4
//L; 80
III It, l l b,c
- 400
I II ~ / a
O
+ 400
E/mv vs.
+ 800
ECS
Fig. 7. Potentiodynamic curves (200 mV rain - ~) for 4~oC alloy, starting from - 500 mV in the usual soln. : (a) primary scanning, (b) after 3 rain at + 200 mY, (c) after 5 rain at + 600 mV.
* Cr radiation, which does not penetrate far, is used to ensure lack of interference from the base alloy lying below the anodically insoluble layer, which .adheres to the treated one. Debye--Scherrer diagrams of powders collected from the anodically insoluble products were used as a confirmatory test.
J. Electroanal. Chem., 40 (1972)
8
R. GRIMALDI, N. VANTINI
to choose conditions for selective dissolution of this phase fairly precisely thereby making it possible to isolate the carbides. This is confirmed by diffractometric studies which for 3 min treatments at + 200 mV show the presence of iron carbide lines (Fe3C) only. CONCLUSION
The apparatus and methodology developed are particularly well adapted for choosing suitable conditions for electrolytic isolation of phases. They result in simple, rapid and economic preparation of samples and the obtaining of a considerable amount of information in a very short time. SUMMARY
An apparatus has been developed to study conditions for the electrolytic isolation of phases in metals. Information can be obtained in a very short time due to the ease of handling and the possibility of working with surfaces that have been subjected to the same initial conditions. Examples of applications.are given. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13
W. Kock and H. Liidering, Arch. Eisenhi~ttenw., 28 (1957) 201. Yu. A. Klyachko and O. D. Larina, Zavod. Lab., 22 (1956) 12. W. Sehaarw~ichter, H. Liidering and F. K. Naumann, Arch. Eisenhiittenw., 31 (1960) 385. Yu. A. Klyachko and V. S. Mal'tseva, Zavod. Lab., 27 (1964) 1182. M. Froment, H. Morel and I. Epelboin, Mere. Sci. Rev. Metall., 62 (1965) 125. W. Kock, Metallkundliche Analyse, Verlag Stahleisen m.b.H., D~isseldorf, 1965. K. V. Andrews and H. Hughes in W. M. Mueller (Ed.), Advances in X-Ray Analysis Vol. 1, Plenum Press, New York (1957) p. 101. J. Burbank and E. J. Ritchie, J. Electrochem. Soc., 116 (1969) 125. A. Wittmoser and H. Bockshammer, Arch. Eisenhiittenw., 26 (1955) 319. Yu. A. Klyachko and G. K. Baranova, Zavod. Lab., 30 (1965) 646. L. B~icker, R. Bigot and E. Herzog, Mere. Sci. Rev. Metall., 62 (1960) 527. A. P. Gulyaev, I. K. Kupalova and V. A. Landa, Zavod. Lab., 31 (1965) 298. R. Grimaldi, S. Maneschi and N. Vantini, Arch. Eisenhiittenw., 38 (1967) 407.
J. Electroanal. Chem., 40 (1972)
AN IMPROVED APPARATUS F O R THE ELECTROLYTIC ISOLATION OF PHASES IN METALS R. GRIMALDI and N. VANTINI
Centro Sperimentale Metallurgico S.p.A., 00129 Rome (Italy) (Received 5th January 1972)
INTRODUCTION
Potentiostatic and potentiodynamic methods, which are being developed in various electrochemical fields, are of considerable interest for the preliminary investigation of the conditions necessary for the electrolytic isolation of phases in a metal sample 1- 6. In order to associate a specific reaction with every state of polarization of an electrode, it is often necessary to examine the effects of the electrochemical action. For this, the diffractometric analysis of the treated surface 7,s is a convenient technique. and it is principally for this reason that the two techniques have been combined and an apparatus designed which considerably improves and facilitates electrochemical treatment in controlled potential and diffractometric analysis. APPARATUS
The sample-holding device, shown schematically in Fig. 1, for which a patent is pending (It. Pat. n. 813.820, filed 15/5/1968), consists of a Teflon support A, connected to one end of the cell L. This cell, of a type already known 9, consists of two cylindrical half-cells, separated by a semipermeable membrane, whose common axis is kept horizontal. In the center of this cover A, is a circular window f, 8mm in diameter, which
C
I I
o
LI
E
I
Fig. 1. Sample-holding device for electrochemical measurements and treatments: (A) Teflon support, (B) bearing shaft, (C) specimen, (D) pivot with locking pins and graduated drum, (E) compression spring controlling screw, (F) compression device, (L) extremity of cell, (f) circular window, (g) washing groove.
J. Electroanal. Chem., 40 (1972)
2
R. GRIMALDI, N. VANT1NI
allows electrolytic contact between the sample and the reference electrode by means of a salt bridge, and contact between the sample and the counter-electrode in the opposite half-cell. The specimen C is placed with the selected surface at the window and may be revolved freely about the axis of pivot D thus allowing measurements and treatments of different zones of the same surface. One end of this pivot, ~ i c h is connected to the sample, has two centering and locking pins, while the other end has a graduated drum for sample positioning. The pins of the pivot fit into two corresponding holes drilled in the sample, and are firmly held to the sample by means of a compression spring controlled through screw E. To achieve better adherence between the surface of the sample and that of the Teflon support, there is a compression device F by the window. The whole system is fitted on shaft B which is connected to the cover A. On the sample side of the cover there is a 1 mm deep, radial groove g oriented 60° with respect to the window, through a pair of holes drilled in the cover. This groove leads to two small pipes which permit washing and deaeration of the surface after electrochemical treatment. The shape and dimensions of window f were chosen so that electrochemical investigation could be done at relatively low electrolysis currents, which in turn allow faster responses by the potentiostat. This factor is especially important when fast potentiodynamic measurements are required.
C
s/"j
/h
l
I
I
Fig. 2. Sample adjusting device for diffractometric analysis of treated zones: (B)-(E) as in Fig. 1, (G) Teflon block, (H) and (I) inox supports, (11)and (i) centering screws.
The device in Fig. 2, similar to the one described above, allows the sample to be adjusted on an X-ray goniometer. By means of the two screws h and i the surface to be investigated can be centered vertically and horizontally with reference to the incident beam. The block G, bearing shaft B and the specimen C, are made of Teflon to minimize surface scratching during the centering of the surface and the rotation of the specimen. FUNCTIONING OF APPARATUS
For the preliminary operations, generally consisting of the filling of the cell, deaeration, setting of the stirring apparatus where necessary, pre-setting of the power supply, etc., the specimen is oriented so that one of the zones lying between those to be J. Electroanal. Chem. 40 (1972)
ELECTROLYTIC ISOLATION OF PHASES IN METALS
3
examined, faces the cell. This intermediate surface can be used for preliminary trials, such as the evaluation of the equilibrium potential, and the selection of a starting potential, thus making it possible to start with the cell switched on. To initiate the experiment proper, it is sufficient to move the specimen so that the next selected zone faces the cell. At this stage the current-voltage curves at the selected potential scanning rate i.e. potentiodynamic curves, or current-time curves at selected potentials, i.e. potentiostatic curves, can be measured, and from these the polarization curve can be traced. The first type:of measurement is particularly useful for selecting an appropriate potential for the electrolytic isolation of phases. For instance, if at a definite potential a component dissolves at the electrode, then electrolysis at such a potential produces an effective enrichrnent of other components in the zone undergoing this treatment. The portion of the surface thus treated may then be examined with X-rays, the reliability of the results being dependent on the preservation of the surface conditions in the period between the X-ray examination and the preceding electrochemical treatment. Surfaces which have undergone electrolysis are generally very active and this inconvenience can be minimized by the following precaution. After electrochemical treatment, the sample is moved to a position where the treated surface can be washed free of electrolyte and kept in a stream of inert gas. This washing operation can be carried out while the next zone is undergoing electrochemical treatment. When the treatment of all zones is complete, the sample is fitted to the X-ray
Fig. 3. Electrolytic cell with detail of device for insertion of sample. J. Electroanal. Chem., 40 (1972)
4
R. GRIMALDI, N. VANTINI
apparatus, and the various zones are carefully optically centered. The main advantages offered by such a device are: (i) a considerable reduction of time in the sample preparation stage, a single surface is sufficient for at least six tests, (ii) the possibility of operating with surfaces under the same initial conditionS, thus enabling the investigation to be carried out more easily and under well-defined conditions, (iii) the possibility of eliminating dead times between the electrochemical treatment and the washing of the treated surface. EXPERIMENTAL
A series of tests was carried out on binary alloys working in a solution of pH 8, containing 8 ~ HCI, 0.5~ citric acid, 10~ glycerine and NH~OH. This is the solution adopted by Klyachko and Baranova 1° for the electrolytic isolation of austenite from manganese steels.
iiiii
Fig. 4. Photograph of Fe-Ni alloy with 30~o Ni, after the following electrochemical treatments: (1) 5 min, - 100 mV; (2) 3 min, 0 mV; (3) 1 min, + 400 mV; (4) 20 min, - 300 mV; (5) 3 min, - 100 mV; (6) 1 rain, + 100 inV. For (1), (2) and (3) the selected potential was reached at 200 mV rain -1 starting from - 5 0 0 inV. - J. Electroanal. Chem., 40 (1972)
ELECTROLYTIC ISOLATION OF PHASES IN METALS
: 5
The samples were in the form of discs 6 mm in thickness and 50 mm in diameter with a surface polished to a 1 #m diamond paste finish. For any surface so prepared, it was possible to carry out at least six electrochemical treatments on circular areas of 8 mm diameter. A large saturated calomel electrode and salt bridge with a pipeshaped extremity 0.5 mm in diameter spaced 1 mm from the sample were used. Figure 3 shows the electrolytic cell. RESULTS
The first samples are two Fe-Ni alloys, one having 3070 Ni and the other 4 ~ Ni, quenched in water from 1000° C and 600° C, respectively. The structures of the two samples, determined by X-rays, are t w o - phase (0t+ ~) and a single phase (~-Fe) respectively. Figure 4 is a photograph of the 3070 Ni alloy and shows the six zones treated. The conditions adopted for each are given. The potentiodynamic curves of the samples, obtained with the pH 8 solution and a scanning rate of 200 mV min- 1, are compared in Fig. 5 with those of iron and nickel. These potentiodynamic curves have good reproducibility, but there are difficulties in interpretation with respect to the conditions necessary for the electrolytic isolation of phases. The first difficulty is that for a certain interval of potential, the curves show a different behaviour according to the electrochemical events at the electrode before scanning. This is confirmed by the behaviour of the current-time curves at constant potential. Also, if the potential is selected so that it falls outside the range of instability of the potentiodynamic curves, the current-time curves show a potentiostatic electrolysis current which is practically constant from the beginning
,1"
.i'i//' i/ i" '
~
/ //,~~/ /'/ / / / /,/
12¢ 6c !/
., -400
i -200
0 E/mY
i "~' +200 vs. ECS
Fig. 5. Comparison between potentiodynamic curves (200 mV m i n - l) of two alloys at (A) 3 0 ~ Ni and (B) 4 ~ Ni and those (. . . . . ) relating to iron and nickel. Branches with arrows refer to primary scanning.
J. Electroanal. Chem., 40 (1972)
R. GRIMALDI, N. VANTINI
to the end of treatments of up to 20 min. Within the potential region in which there is variable behaviour of the potentiodynamic curves, the potentiostatic electrolysis current still tends towards a constant value, equal to that which may be deduced from the potentiodynamic curve obtained after activation of the surface, but this limiting value is reached only after about l min. It is interesting that this behaviour becomes more noticeable with increasing nickel content, almost to the point of characterizing different alloys. Because of this anomalous behaviour, and the proximity of the curves, the possibility of electrolytic isolation* of the structural components of these two alloys within a reasonable period of time, is unlikely given the particular solution used. It therefore seems necessary to choose a more suitable electrolytic solution. The diffractometric examinations carried out on the 30~o Ni alloy show, for an attack of 20 min at 300 mV, an actual increase in the surface austenite content (95~o) with respect to the value for the base alloy (80}/0). There is therefore a selective
iiiiiiii!i!
Fig. 6. Photograph of F e - C alloy with 4 ~ C, after the following treatments: (1) scanning from - 5 0 0 to + 800 mV, (2) the same after 10 rain at 0 mV, (3) the same after 10 min at + 200 mV, (4) the same after 5 min at + 600 mV, (5) 10 min at + 200 mV (only), (6) 10 min at + 600 mV (only). Scanning rate (when used), 200 mV m i n - 1. * Isolation is understood in a general sense here. In effect, the only phase isolation possible is that of the sample with 3 0 ~ Ni which is a two-phase structure. J. Electroanal. Chem., 40 (1972)
ELECTROLYTIC ISOLATION OF PHASES IN METALS
7
dissolution which, however, for reasons given above, may be rendered quantitative only with difficulty in this case. This is confirmed by the fact that true separation tests carried out on very much more extensive zones (30 mm) for times prolonged to 60 min did not completely eliminate the s-phase*. Another example of the application of the method is provided by an Fe-C alloy with 4 ~ carbon, studied with the same solution. The structure, from diffractometer results, is made up of ~'-Fe (or martensite) and FeaC (or cementite). On the diffraction pattern some lines (the most intense) of the carbide appear together with the (110) of the c(-Fe. The extremely high relative intensity of the latter, while effected by the superposition of the (002) + (103) of the carbide is, nevertheless, sufficient to prove the presence of the phase itself. Figure 6 shows the sample together with the conditions for the various treatments. The potentiodynamic curves (200 mV min- 1), shown in Fig. 7, were obtained in the following ways: (a) initial scanning starting from the equilibrium potential, (b) the same scanning but after a 10 min potentiostatic electrolysis at + 200 mV, and (c) the same after a 5 min electrolysis at + 600 mV. In the electrolyte used, this alloy shows a similar behaviour to one encountered in previous work on the electrochemical attack of cementite11- l a. In this case, given that the initial ascent of the curve corresponds to attack of the matrix only, it is possible
~///
t
lI
32Q
/ 24o
/I b
U
II I*'~ l
°~ <
~
,C
:4
//L; 80
III It, l l b,c
- 400
I II ~ / a
O
+ 400
E/mv vs.
+ 800
ECS
Fig. 7. Potentiodynamic curves (200 mV rain - ~) for 4~oC alloy, starting from - 500 mV in the usual soln. : (a) primary scanning, (b) after 3 rain at + 200 mY, (c) after 5 rain at + 600 mV.
* Cr radiation, which does not penetrate far, is used to ensure lack of interference from the base alloy lying below the anodically insoluble layer, which .adheres to the treated one. Debye--Scherrer diagrams of powders collected from the anodically insoluble products were used as a confirmatory test.
J. Electroanal. Chem., 40 (1972)
8
R. GRIMALDI, N. VANTINI
to choose conditions for selective dissolution of this phase fairly precisely thereby making it possible to isolate the carbides. This is confirmed by diffractometric studies which for 3 min treatments at + 200 mV show the presence of iron carbide lines (Fe3C) only. CONCLUSION
The apparatus and methodology developed are particularly well adapted for choosing suitable conditions for electrolytic isolation of phases. They result in simple, rapid and economic preparation of samples and the obtaining of a considerable amount of information in a very short time. SUMMARY
An apparatus has been developed to study conditions for the electrolytic isolation of phases in metals. Information can be obtained in a very short time due to the ease of handling and the possibility of working with surfaces that have been subjected to the same initial conditions. Examples of applications.are given. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13
W. Kock and H. Liidering, Arch. Eisenhi~ttenw., 28 (1957) 201. Yu. A. Klyachko and O. D. Larina, Zavod. Lab., 22 (1956) 12. W. Sehaarw~ichter, H. Liidering and F. K. Naumann, Arch. Eisenhiittenw., 31 (1960) 385. Yu. A. Klyachko and V. S. Mal'tseva, Zavod. Lab., 27 (1964) 1182. M. Froment, H. Morel and I. Epelboin, Mere. Sci. Rev. Metall., 62 (1965) 125. W. Kock, Metallkundliche Analyse, Verlag Stahleisen m.b.H., D~isseldorf, 1965. K. V. Andrews and H. Hughes in W. M. Mueller (Ed.), Advances in X-Ray Analysis Vol. 1, Plenum Press, New York (1957) p. 101. J. Burbank and E. J. Ritchie, J. Electrochem. Soc., 116 (1969) 125. A. Wittmoser and H. Bockshammer, Arch. Eisenhiittenw., 26 (1955) 319. Yu. A. Klyachko and G. K. Baranova, Zavod. Lab., 30 (1965) 646. L. B~icker, R. Bigot and E. Herzog, Mere. Sci. Rev. Metall., 62 (1960) 527. A. P. Gulyaev, I. K. Kupalova and V. A. Landa, Zavod. Lab., 31 (1965) 298. R. Grimaldi, S. Maneschi and N. Vantini, Arch. Eisenhiittenw., 38 (1967) 407.
J. Electroanal. Chem., 40 (1972)