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Surface analyses of anodic oxides films formed on Fe-3% Ti alloy
SURFACE
ANALYSES OF ANODIC OXIDE ON Fe-3 % Ti ALLOY MASAHIROSEO~~~ Faculty
of Engineering,
(Received
14 June
Hokkaido
FILMS
FORMED
NORIOSATO University,
i982: in revised form
Sapporo 20 Ocrober
040, Japan 1982)
Abstract-Surface analyses of the anochc oxide films formed on Fe-3% Ti alloy in deaerated pH 3.0 phosphate solution were performed with Auger electron spectroscopy (AES) and ellipsometry to evaluate the role of alloying titanium in the corrosion resistance of iron. The 3 “/,-addition of titanium reduced significantly the passivity-maintaining current density of iron as well as the maximum-active current density. Auger analysis revealed that titanium was enriched markedly in the anodic oxide films formed on Fe-3 “/, Ti alloy and that a significant amount of phosphorus was distributed in the whole range offilm thickness. Both theamount of titanium enriched in the filmand the film thickness (2C-40 nm) ellipsometrically obtained were proportIona to the amount ofelectric charge required for passivation. Atomic absorption analysis of the solution indicated that the formation of a titaniumenriched film resulted from a preferential dissolution of iron as ferrous ions. From the measurement of the potential decay cnrves of the Fe-3 “/, Ti alloy and pure iron electrodes passivated for direrent hours. it was concluded that the titanium-enriched layer promoted the passivity by suppressing, though incompletely, the active dissolution, whereas the substantial passivity of the alloy was attributed to the iron oxide film of barrier type formed at the interface of titanium-enriched layer/alloy substrate.
1. INTRODUCTlON By the use of recent surface analytical techniques such as Auger electron spectroscopy (AES)[l-31 and Xray-photoelectron spectroscopy[Qd] (XPS), it has been revealed that the high corrosion resistance of chromium-containing steels is mainly attributed to chromium-enrichment in the passive film on the steels. Titanium, like chromium, exhibits high-corrosion resistivity under severe corrosion environments and has been used as construction materials[‘i] of chemical plants and as substrate materials[S] of insoluble electrodes. It is expected that addition of a relevant amount of titanium to iron gives rise to titanium enrichment in anodic oxide films formed on the iron substrate and improves the corrosion resistance of iron. In this study, from the above viewpoint, surface analyses of anodic oxide films formed on Fe-3’% Ti alloy in deaerated pH 3.0 phosphate solution were performed with AES and ellipsometry.
2.
EXPERIMENTAL
The plates (0.1 x 1.0 x l.Ocm) of Fe-3 % Ti alloy and pure iron were used in this experiment. The composition of the specimen is shown in Table 1. The solubility limit[9] of titanium in a-iron extends over 5 wt ‘x above the temperature of 1400 K. The Fe-3 ‘x Ti alloy specimen was therefore heated in vacuum for 30 min at 1473 K and quenched into water to fix in CIphase, whereas the iron specimen was heated in vacuum for 30 min at 873 K and cooled in air. The specimens were mechanically polished with emery papers and with cc-Al,O, abrasives (0.5 pm), and subsequently washed with acetone using ultrasonic
techniques. The anodic oxidation of the Fe-3% Ti alloy specimen was performed in deaerated pH 3.0 phosphate solution containing phosphorus of 0.15 mol dm -3. The direct anodic oxidation of the iron specimen in pH 3.0 phosphate solution gives rise to the surface roughening due to active dissolution subjected when the specimen passing through the active region. The following two-step method, thus, was adopted for the anodic oxidation of the iron specimen; the specimen was anodically oxidised at O.OV (she) in the passive region for 1 h in pH 8.42 borate solution and then anodically oxidised in pH 3.0 phosphate solution without passing through the active region. The amount of iron and titanium dissolved into the solution from the specimen during anodic oxidation was analysed by an atomic absorption technique. Auger apparatus (PHI, 540 A) was used for the measurement of the depth-composition profile. The sputter-etching of the specimen anodically oxidised was performed at an argon pressure of 6.7 x lo- ’ Pa with a 0.5 keV argon ion beam at an emission current of 10 mA. The Auger spectra were taken by using a primary 3 kV, 40~A electron beam and a 4 V modulation amplitude at a frequency of 30 kHz. Further, the thickness and the refractive index of anodic oxide films were measured by a multiple-angle-of-incidence ellipsometry (R = 546.1 nm) in air after anodic oxidation.
3.
RESULTS
AND
DISCUSSION
A ES Analysis
Figure 1 shows the polarisation curves of Fe-3 ‘x Ti alloy and pure iron at 1 h steady state in pH 3.0 phosphate solution. It is seen that the 3 ‘i/,-addition of titanium reduces significantly both the maximum
723
724
MASAHLRO
SEO AND NORIO SATO
Fe-3t,
Alloy
pH 30
Phosphate
O.GOV(SHE).
S~lulion
Ih
pH 3.0 Phosphate Solution
Fig. 2. Depth-profile of anodic oxide formed on Fe-3 7; Ti alloy.
-31
-05
0
1 I
1
0.5 E/V
10
15
20
[SHEI
Fig. I. Polarization curves of F-3 “/d Ti alloy and pure iron at 1 h-steady state in pH 3.0 phosphate solution. density in the active region and the passivitymaintaining current density of iron. Figure 2 shows the depth-profile of Fe-3 7; Ti alloy anodically oxidised for I h at 0.60 V (she)after natural immersion for 5 min in pH 3.0 phosphate solution. No iron is present on the uppermost surface but titanium is markedly enriched in the anodic oxide film. A significant amount of phosphorus is distributed in the whole depth of the film. The amount of titanium enriched in the film may be defined by the value of AM,, obtained from the depth-profile in Fig. 2, using the following equation; current
AM, = A&f;; (1) where A, is the hatched area of titanium-profile and H$ the titanium Auger peak amplitude corresponding to the bulk composition. The specimen anodically oxidised for 1 h at a fixed potential in the passive region, however, indicated some amount of scatter in the calculated values of AM,;. The solid circles (a) in
Fig. 3 shows the relation between the value of AMTi and the electric charge passed when the specimen was subjected to active dissolution at -0.20 V (she) for different time periods. The value of AMri is proportional to the electric charge passed during the active dissolution. The scattered values of AM,, for the specimen passivated for 1 h at 1.00 V (she) are also plotted us the electric charge required for passivation as shown by open circles (o)in Fig. 3. As seen from the i us t curve in Fig. 3, the most part of the electric charge for passivation is consumed for an initial short period of time (several seconds) when the specimen was subjected to the active dissolution before passivation. The magnitude of the limiting dissolution current and the time period of active dissolution are controlled by the performance of the potentiostat and the iR drop between the test and counter electrodes. The scatter of the values of AMTt is mainly originated from the scatter of the time period of active dissolution. Both solid and open circles in Fig. 3 are located on the same straight line, indicating that the titanium-enrichment in the anodic oxide film is directly related to the amount of active dissolution of the specimen. Eliipsometry
and solution analysis
The film thickness ellipsometrically obtained for the specimen anodically oxidised for 1 h at I .OOV (she) is 1
14 -
Fe-3
Ti Alloy
pH 3.0 Phosphnte
L ? 6 <
IO-
. - 0.20 v (SHE) 0 I.OVlSHE).
S~lutlOn
Ih
a-
Fig. 3. Relation between the amount of titanium, AMTi,ennched m the film and the electriccharge, Q. passed during anodic oxidation.
Surface analyses of anodic oxide films
725
Fig. 4. Film thickness, L,, eliipsomelrically obtained as a function of the electric charge, Q, passed during anodic oxidation.
plotted us the electric charge required for passivation in Fig. 4. The film thickness (2WO nm) increases with the electric charge. The measured refractive index (Nf = I .6+0.01 i) of the anodic oxide film on Fe3 7; Ti alloy differs from that[ lo] (Nr = 2.10 - 0.03i) of the anodic oxide films on pure titanium in phosphate solution. The low value of the real part (1.44) in the refractive index means that the anodic oxide film on Fe-3 y; Ti alloy is a low density-film, that is, a hydrous oxide film. Figure 5 shows the results of atomic absorption analysis of the solution in which the specimen was subjected to active dissolution at -0.2OV (she) for different time periods. The dissolved amount of iron increases linearly with the electric charge passed during the active dissolution. The detection limit of atomic absorption analysis for titanium is 0.2 p-pm. No titanium was detected in the solution; if iron and titanium was dissolved at the same rate as the bulk composition, the solution would contain the amount of titanium equal to three times the detection limit. All experimental data points shown in Fig. 5 lie on the theoretical dissolution line of ferrous ions, indicating that iron is preferentially dissolved as ferrous ions from the specimen in the active dissolution region. The formation of the titanium-enriched film, therefore,
0
2
4
6
6
IO
12
0 1 kC ,,-’
Fig. 5. The dissolved amount of iron, W,,, measured by atomic absorption analysis as a function of the electric charge, Q.
may be caused by a preferential dissolution of iron as ferrous ions from the alloy in the active region. Open circuit-potential decay curves of passivated Fe-3 “/d. Ti niloy and pure iron electrodes Figures 6 and 7 show the open circuit-potential decay curves of the Fe-3 % Ti alloy and pure iron electrodes in the renewed pH 3.0 phosphate solution after passivation at 1.32 V (she) for different hours in the same solution, respectively. The time period, r, required for the activation of the Fe-3% Ti alloy electrode increases with the passivation time, whereas the pure iron electrode is not sensitive to the passivation time except for the abnormal value of 5 at 1 hpassivation. As described in the experimental section, the two-step anodic oxidation was adopted for the pure iron electrode. According to the previous reports[ 11, 121 concerning the two-step anodic oxidation of pure iron, the outer layer of the anodic oxide film formed by the first step in pH 8.4 borate solution dissolves during the second step anodic oxidation in pH 3.0 solution. The abnormal value of r at 1 hpassivation in Fig. 7 may be related to the slow dissolution process of the outer layer in pH 3.0 phosphate solution since the remainder of the outer layer undissolved at 1 h-passivation is likely to prolong the value of 7. The Flade potential, EP, of the pure iron electrode obtained from the potential arrest in the decay curves is 0.42 V (she), which is independent of the passivation time and consistent with the calculated value from the pH dependencell3, 141 of the Flade potential explained in terms of the equilibrium potential of y-Fe, O,/Fe,O,. The Flade potential, EP, of the F+3”/d Ti alloy electrode is close to that of the pure iron electrode, although it increases from 0.42 to 0.53 V (she) with the passivation time. According to Pourbaix diagram[ 151, the equilibrium potentials of Ti (OH),/Ti02H,0 and Ti,OJTiO, in pH 3.0 solution are - 0.2 and - 0.73 V (she), respectively, both of which are not consistent with the values of EP of the Fe-3 % Ti alloy electrode, suggesting that the passivity of the Fe-3 7; Ti alloy is not sustained by the titanium-enriched layer. The titanium-enriched layer seems to differ from the compact film of barrier type formed on pure titanium and to contain a considerable
726
MASAHIROSEO AND
Fig. 6. Potential decay curves of Fe-3 “/, Ti alloy passlvated for different hours in pH 3 0 phosphate solution.
Time / mm
NORIO
SATO
ution were performed with AES and ellipsometry to evaluate the roIe of alloying titanium in the corrosion resistance of iron. The results are summarised as follows: (1) The 3”/,-addition of titanium reduces significantly both the maximum-active dissolution current density and the passivity-maintaining current density of iron. (2) Titanium is enriched markedly in the anodic oxide films formed on Fe-3 “/, Ti alloy. A significant amount of phosphorus originated from the solution is distributed in the whole range of the film thickness. (3) Both the amount of titanium enriched in the film and the film thickness (2CMOnm) increase with the amount of electric charge required for passivation, the most part of which is consumed when the alloy is subjected to active dissolution before passivation. (4) The formation of titanium-enriched anodic oxide film results from a preferential dissolution of iron as ferrous ions from the alloy in the active region. (5) The titanium-enriched layer plays a role of passivity promotion in that it covers the alloy surface to suppress, though incompletely, the active dissolution. The passivity of Fe-3 “/, Ti alloy is substantially attributed to the passive film formed at the interface between the titanium-enriched layer and the alloy substrate. The passive film formed at the interface is nearly the same as that formed on pure iron. (6) The aging of Fe-3% Ti alloy in the passive region makes significant alteration of the passive film improving the corrosion resistance.
Fig. 7. Potential decay curves of pure iron passivated for different hours in pH 3.0 phosphate solution. REFERENCES density of active dissolution path for ferrous ions. The passivation of this aIloy, therefore, results from the passive film formation at the interface between the titanium-enriched layer and the alloy substrate. The passive film formed at the interface seems to be nearly the same as that formed on pure iron, as expected from a fairly good agreement between the values of EP of the Fe-3 ‘x Ti alloy and that of pure iron. Accordingly, the titanium-enriched layer plays a role of passivity promotion in that it covers the alloy surface to suppress, though incompletely, the active dissolution. The remarkable increase in the values of r and EP with increasing passivation time for the Fe-3 ‘x Ti alloy proves that the aging of the alloy in the passive region significantly improves the corrosion resistance.
4, CONCLUSlON Surface analyses of the anodic oxide films formed on Fe-3% Ti alloy in deaerated pH 3.0 phosphate sol-
1. J. B. Lumsden and R. W. Staehle. Ser. melall. 6, 1205 ( 1972). 2. H. Ogawa, H. Omata, 1. Itohand H. Okada,Corrosion 34, 52 (1978). 3. M. Seo and N. Sate, Surf SCI. 86, 601 (1979). 4. G. Okamoto and T. Shibata, Corros. Sri. 10, 37 (1970). 5. K. Asami, K. Hashimoto and S. Shimodaira, Corros. Sci. 18, 151 (1978). 6. 1 Olefjord, Marer. Sci. Engng 42, 161 (1980). 7. T. Nishimura, J. Metal Finish. Sm. Japan 31,625 (1980). 8. M. A. Warne, Mater. Perform. 18, 32 (1979). 9. R. P. Elliott, Cons~ution of Binary Alloys. First Supgl., p. 438. McGraw-Hill, New York (1965). 10. T. Ohtsuka and N. Sato, Unpubhshed data. II. N. Sate. T. Noda and K. Kudo, Elecrroclzim. Acra 19,471 (1974). 12. N. Sato, K. Kudo and T. Noda, 2. phys. Chem. N.F. 98, 271 (1975) 13. U. F. Franck, 2. Naturj. 49, 378 (1949). 61, 1291 (1957). 14. H. Gohr and E. Lange, 2. Eiektrochem. 15. M. Pourbaix, Atlas I$ Electrochemical Equilibria in ngurous Solutions, p. 213 Pergamon Press, Oxford ( 1966).
ANALYSES OF ANODIC OXIDE ON Fe-3 % Ti ALLOY MASAHIROSEO~~~ Faculty
of Engineering,
(Received
14 June
Hokkaido
FILMS
FORMED
NORIOSATO University,
i982: in revised form
Sapporo 20 Ocrober
040, Japan 1982)
Abstract-Surface analyses of the anochc oxide films formed on Fe-3% Ti alloy in deaerated pH 3.0 phosphate solution were performed with Auger electron spectroscopy (AES) and ellipsometry to evaluate the role of alloying titanium in the corrosion resistance of iron. The 3 “/,-addition of titanium reduced significantly the passivity-maintaining current density of iron as well as the maximum-active current density. Auger analysis revealed that titanium was enriched markedly in the anodic oxide films formed on Fe-3 “/, Ti alloy and that a significant amount of phosphorus was distributed in the whole range offilm thickness. Both theamount of titanium enriched in the filmand the film thickness (2C-40 nm) ellipsometrically obtained were proportIona to the amount ofelectric charge required for passivation. Atomic absorption analysis of the solution indicated that the formation of a titaniumenriched film resulted from a preferential dissolution of iron as ferrous ions. From the measurement of the potential decay cnrves of the Fe-3 “/, Ti alloy and pure iron electrodes passivated for direrent hours. it was concluded that the titanium-enriched layer promoted the passivity by suppressing, though incompletely, the active dissolution, whereas the substantial passivity of the alloy was attributed to the iron oxide film of barrier type formed at the interface of titanium-enriched layer/alloy substrate.
1. INTRODUCTlON By the use of recent surface analytical techniques such as Auger electron spectroscopy (AES)[l-31 and Xray-photoelectron spectroscopy[Qd] (XPS), it has been revealed that the high corrosion resistance of chromium-containing steels is mainly attributed to chromium-enrichment in the passive film on the steels. Titanium, like chromium, exhibits high-corrosion resistivity under severe corrosion environments and has been used as construction materials[‘i] of chemical plants and as substrate materials[S] of insoluble electrodes. It is expected that addition of a relevant amount of titanium to iron gives rise to titanium enrichment in anodic oxide films formed on the iron substrate and improves the corrosion resistance of iron. In this study, from the above viewpoint, surface analyses of anodic oxide films formed on Fe-3’% Ti alloy in deaerated pH 3.0 phosphate solution were performed with AES and ellipsometry.
2.
EXPERIMENTAL
The plates (0.1 x 1.0 x l.Ocm) of Fe-3 % Ti alloy and pure iron were used in this experiment. The composition of the specimen is shown in Table 1. The solubility limit[9] of titanium in a-iron extends over 5 wt ‘x above the temperature of 1400 K. The Fe-3 ‘x Ti alloy specimen was therefore heated in vacuum for 30 min at 1473 K and quenched into water to fix in CIphase, whereas the iron specimen was heated in vacuum for 30 min at 873 K and cooled in air. The specimens were mechanically polished with emery papers and with cc-Al,O, abrasives (0.5 pm), and subsequently washed with acetone using ultrasonic
techniques. The anodic oxidation of the Fe-3% Ti alloy specimen was performed in deaerated pH 3.0 phosphate solution containing phosphorus of 0.15 mol dm -3. The direct anodic oxidation of the iron specimen in pH 3.0 phosphate solution gives rise to the surface roughening due to active dissolution subjected when the specimen passing through the active region. The following two-step method, thus, was adopted for the anodic oxidation of the iron specimen; the specimen was anodically oxidised at O.OV (she) in the passive region for 1 h in pH 8.42 borate solution and then anodically oxidised in pH 3.0 phosphate solution without passing through the active region. The amount of iron and titanium dissolved into the solution from the specimen during anodic oxidation was analysed by an atomic absorption technique. Auger apparatus (PHI, 540 A) was used for the measurement of the depth-composition profile. The sputter-etching of the specimen anodically oxidised was performed at an argon pressure of 6.7 x lo- ’ Pa with a 0.5 keV argon ion beam at an emission current of 10 mA. The Auger spectra were taken by using a primary 3 kV, 40~A electron beam and a 4 V modulation amplitude at a frequency of 30 kHz. Further, the thickness and the refractive index of anodic oxide films were measured by a multiple-angle-of-incidence ellipsometry (R = 546.1 nm) in air after anodic oxidation.
3.
RESULTS
AND
DISCUSSION
A ES Analysis
Figure 1 shows the polarisation curves of Fe-3 ‘x Ti alloy and pure iron at 1 h steady state in pH 3.0 phosphate solution. It is seen that the 3 ‘i/,-addition of titanium reduces significantly both the maximum
723
724
MASAHLRO
SEO AND NORIO SATO
Fe-3t,
Alloy
pH 30
Phosphate
O.GOV(SHE).
S~lulion
Ih
pH 3.0 Phosphate Solution
Fig. 2. Depth-profile of anodic oxide formed on Fe-3 7; Ti alloy.
-31
-05
0
1 I
1
0.5 E/V
10
15
20
[SHEI
Fig. I. Polarization curves of F-3 “/d Ti alloy and pure iron at 1 h-steady state in pH 3.0 phosphate solution. density in the active region and the passivitymaintaining current density of iron. Figure 2 shows the depth-profile of Fe-3 7; Ti alloy anodically oxidised for I h at 0.60 V (she)after natural immersion for 5 min in pH 3.0 phosphate solution. No iron is present on the uppermost surface but titanium is markedly enriched in the anodic oxide film. A significant amount of phosphorus is distributed in the whole depth of the film. The amount of titanium enriched in the film may be defined by the value of AM,, obtained from the depth-profile in Fig. 2, using the following equation; current
AM, = A&f;; (1) where A, is the hatched area of titanium-profile and H$ the titanium Auger peak amplitude corresponding to the bulk composition. The specimen anodically oxidised for 1 h at a fixed potential in the passive region, however, indicated some amount of scatter in the calculated values of AM,;. The solid circles (a) in
Fig. 3 shows the relation between the value of AMTi and the electric charge passed when the specimen was subjected to active dissolution at -0.20 V (she) for different time periods. The value of AMri is proportional to the electric charge passed during the active dissolution. The scattered values of AM,, for the specimen passivated for 1 h at 1.00 V (she) are also plotted us the electric charge required for passivation as shown by open circles (o)in Fig. 3. As seen from the i us t curve in Fig. 3, the most part of the electric charge for passivation is consumed for an initial short period of time (several seconds) when the specimen was subjected to the active dissolution before passivation. The magnitude of the limiting dissolution current and the time period of active dissolution are controlled by the performance of the potentiostat and the iR drop between the test and counter electrodes. The scatter of the values of AMTt is mainly originated from the scatter of the time period of active dissolution. Both solid and open circles in Fig. 3 are located on the same straight line, indicating that the titanium-enrichment in the anodic oxide film is directly related to the amount of active dissolution of the specimen. Eliipsometry
and solution analysis
The film thickness ellipsometrically obtained for the specimen anodically oxidised for 1 h at I .OOV (she) is 1
14 -
Fe-3
Ti Alloy
pH 3.0 Phosphnte
L ? 6 <
IO-
. - 0.20 v (SHE) 0 I.OVlSHE).
S~lutlOn
Ih
a-
Fig. 3. Relation between the amount of titanium, AMTi,ennched m the film and the electriccharge, Q. passed during anodic oxidation.
Surface analyses of anodic oxide films
725
Fig. 4. Film thickness, L,, eliipsomelrically obtained as a function of the electric charge, Q, passed during anodic oxidation.
plotted us the electric charge required for passivation in Fig. 4. The film thickness (2WO nm) increases with the electric charge. The measured refractive index (Nf = I .6+0.01 i) of the anodic oxide film on Fe3 7; Ti alloy differs from that[ lo] (Nr = 2.10 - 0.03i) of the anodic oxide films on pure titanium in phosphate solution. The low value of the real part (1.44) in the refractive index means that the anodic oxide film on Fe-3 y; Ti alloy is a low density-film, that is, a hydrous oxide film. Figure 5 shows the results of atomic absorption analysis of the solution in which the specimen was subjected to active dissolution at -0.2OV (she) for different time periods. The dissolved amount of iron increases linearly with the electric charge passed during the active dissolution. The detection limit of atomic absorption analysis for titanium is 0.2 p-pm. No titanium was detected in the solution; if iron and titanium was dissolved at the same rate as the bulk composition, the solution would contain the amount of titanium equal to three times the detection limit. All experimental data points shown in Fig. 5 lie on the theoretical dissolution line of ferrous ions, indicating that iron is preferentially dissolved as ferrous ions from the specimen in the active dissolution region. The formation of the titanium-enriched film, therefore,
0
2
4
6
6
IO
12
0 1 kC ,,-’
Fig. 5. The dissolved amount of iron, W,,, measured by atomic absorption analysis as a function of the electric charge, Q.
may be caused by a preferential dissolution of iron as ferrous ions from the alloy in the active region. Open circuit-potential decay curves of passivated Fe-3 “/d. Ti niloy and pure iron electrodes Figures 6 and 7 show the open circuit-potential decay curves of the Fe-3 % Ti alloy and pure iron electrodes in the renewed pH 3.0 phosphate solution after passivation at 1.32 V (she) for different hours in the same solution, respectively. The time period, r, required for the activation of the Fe-3% Ti alloy electrode increases with the passivation time, whereas the pure iron electrode is not sensitive to the passivation time except for the abnormal value of 5 at 1 hpassivation. As described in the experimental section, the two-step anodic oxidation was adopted for the pure iron electrode. According to the previous reports[ 11, 121 concerning the two-step anodic oxidation of pure iron, the outer layer of the anodic oxide film formed by the first step in pH 8.4 borate solution dissolves during the second step anodic oxidation in pH 3.0 solution. The abnormal value of r at 1 hpassivation in Fig. 7 may be related to the slow dissolution process of the outer layer in pH 3.0 phosphate solution since the remainder of the outer layer undissolved at 1 h-passivation is likely to prolong the value of 7. The Flade potential, EP, of the pure iron electrode obtained from the potential arrest in the decay curves is 0.42 V (she), which is independent of the passivation time and consistent with the calculated value from the pH dependencell3, 141 of the Flade potential explained in terms of the equilibrium potential of y-Fe, O,/Fe,O,. The Flade potential, EP, of the F+3”/d Ti alloy electrode is close to that of the pure iron electrode, although it increases from 0.42 to 0.53 V (she) with the passivation time. According to Pourbaix diagram[ 151, the equilibrium potentials of Ti (OH),/Ti02H,0 and Ti,OJTiO, in pH 3.0 solution are - 0.2 and - 0.73 V (she), respectively, both of which are not consistent with the values of EP of the Fe-3 % Ti alloy electrode, suggesting that the passivity of the Fe-3 7; Ti alloy is not sustained by the titanium-enriched layer. The titanium-enriched layer seems to differ from the compact film of barrier type formed on pure titanium and to contain a considerable
726
MASAHIROSEO AND
Fig. 6. Potential decay curves of Fe-3 “/, Ti alloy passlvated for different hours in pH 3 0 phosphate solution.
Time / mm
NORIO
SATO
ution were performed with AES and ellipsometry to evaluate the roIe of alloying titanium in the corrosion resistance of iron. The results are summarised as follows: (1) The 3”/,-addition of titanium reduces significantly both the maximum-active dissolution current density and the passivity-maintaining current density of iron. (2) Titanium is enriched markedly in the anodic oxide films formed on Fe-3 “/, Ti alloy. A significant amount of phosphorus originated from the solution is distributed in the whole range of the film thickness. (3) Both the amount of titanium enriched in the film and the film thickness (2CMOnm) increase with the amount of electric charge required for passivation, the most part of which is consumed when the alloy is subjected to active dissolution before passivation. (4) The formation of titanium-enriched anodic oxide film results from a preferential dissolution of iron as ferrous ions from the alloy in the active region. (5) The titanium-enriched layer plays a role of passivity promotion in that it covers the alloy surface to suppress, though incompletely, the active dissolution. The passivity of Fe-3 “/, Ti alloy is substantially attributed to the passive film formed at the interface between the titanium-enriched layer and the alloy substrate. The passive film formed at the interface is nearly the same as that formed on pure iron. (6) The aging of Fe-3% Ti alloy in the passive region makes significant alteration of the passive film improving the corrosion resistance.
Fig. 7. Potential decay curves of pure iron passivated for different hours in pH 3.0 phosphate solution. REFERENCES density of active dissolution path for ferrous ions. The passivation of this aIloy, therefore, results from the passive film formation at the interface between the titanium-enriched layer and the alloy substrate. The passive film formed at the interface seems to be nearly the same as that formed on pure iron, as expected from a fairly good agreement between the values of EP of the Fe-3 ‘x Ti alloy and that of pure iron. Accordingly, the titanium-enriched layer plays a role of passivity promotion in that it covers the alloy surface to suppress, though incompletely, the active dissolution. The remarkable increase in the values of r and EP with increasing passivation time for the Fe-3 ‘x Ti alloy proves that the aging of the alloy in the passive region significantly improves the corrosion resistance.
4, CONCLUSlON Surface analyses of the anodic oxide films formed on Fe-3% Ti alloy in deaerated pH 3.0 phosphate sol-
1. J. B. Lumsden and R. W. Staehle. Ser. melall. 6, 1205 ( 1972). 2. H. Ogawa, H. Omata, 1. Itohand H. Okada,Corrosion 34, 52 (1978). 3. M. Seo and N. Sate, Surf SCI. 86, 601 (1979). 4. G. Okamoto and T. Shibata, Corros. Sri. 10, 37 (1970). 5. K. Asami, K. Hashimoto and S. Shimodaira, Corros. Sci. 18, 151 (1978). 6. 1 Olefjord, Marer. Sci. Engng 42, 161 (1980). 7. T. Nishimura, J. Metal Finish. Sm. Japan 31,625 (1980). 8. M. A. Warne, Mater. Perform. 18, 32 (1979). 9. R. P. Elliott, Cons~ution of Binary Alloys. First Supgl., p. 438. McGraw-Hill, New York (1965). 10. T. Ohtsuka and N. Sato, Unpubhshed data. II. N. Sate. T. Noda and K. Kudo, Elecrroclzim. Acra 19,471 (1974). 12. N. Sato, K. Kudo and T. Noda, 2. phys. Chem. N.F. 98, 271 (1975) 13. U. F. Franck, 2. Naturj. 49, 378 (1949). 61, 1291 (1957). 14. H. Gohr and E. Lange, 2. Eiektrochem. 15. M. Pourbaix, Atlas I$ Electrochemical Equilibria in ngurous Solutions, p. 213 Pergamon Press, Oxford ( 1966).