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Corrosion behaviour of layers obtained by nitrogen implantation into Boron films deposited onto iron substrates
Materials Science and Engineering, 69 (1985) 289-295
289
Corrosion Behaviour of Layers Obtained by Nitrogen Implantation into Boron Films Deposited onto Iron Substrates* F. MARCHETTI, L. FEDRIZZI, F. GIACOMOZZI and L. GUZMAN Istituto per la Ricerca Scientifica e Tecnologica, 38050 Povo (Trento) (Italy) A. BORGESE Istituto Tecnico Industriale Statale B. Castelli, and Universitit di Brescia, 25100 Brescia (Italy) (Received September 17, 1984)
ABSTRACT
The electrochemical behaviour and corrosion resistance o f boron films deposited onto Armco iron after bombardment with 100 ke V N + ions were determined in various test solutions. The changes in the electrochemical parameters give evidence of lower anodic dissolution rates for the treated samples. Scanning electron microscopy and Auger analysis o f the corroded surfaces confirm the presence o f protective layers.
1. INTRODUCTION
It is well established that ion implantation may improve the resistance of metals to wear and corrosion. The more novel concept of ion beam mixing of a thin deposited film, including the formation of a buried mixed layer, extends the versatility of this technique considerably in altering the surface composition and properties of materials [ 1 ]. Boron nitride appears to be extremely interesting in view of its unique physical properties: high thermal conductivity, exceptional strength, low thermal expansion and high chemical inertness. The boron nitride coatings obtained by dipping or spraying retain many of the properties of boron nitride solids and powders, providing a more economical way of taking advantage of their excellent properties such as corrosion resistance, reduction in sticking and reaction, high temperature lubrication and mould release *Paper presented at the International Conference on Surface Modification of Metals by Ion Beams, Heidelberg, F.R.G., September 17-21, 1984. 0025-5416/85/$3.30
and indicating m a n y other applications in which boron nitride could be useful. There have been some recent attempts to obtain boron nitride coatings using deposition techniques such as plasma, ion beam or ionplating techniques, in which ionized and/or inert species of energy about I keV were involved [ 2 - 4 ] . These techniques yielded promising results, although microanalysis revealed that the films hitherto produced contained significant amounts of oxygen and carbon. In the present work we report on the corrosion behaviour of boron nitride layers created by N ÷ ion implantation of boron films deposited onto iron substrates.
2. EXPERIMENTAL PROCEDURES
Boron films were evaporated at 10 -4 Pa from an electron-gun-heated crucible onto Armco iron substrates to a thickness of 100200 nm and then implanted with 100 keV N ÷ ions at a fluence of 3 X 10 iv ions c m -2. The samples were then investigated by Auger electron spectroscopy combined with argon ion etching. The Auger spectra were obtained in the differential mode (Vmod = 1 V peak to peak) on standard equipment (PHI model 590 scanning Auger microprobe). The electron beam current was 0.3 pA at a primary beam energy of 3 keV. During the analysis the electron beam was scanned over an area of 100 pm by 100 pm. Sputtering was performed at an argon pressure of 1 X 10 -5 Pa in an ultrahigh vacuum system. The ion energy was 2 keV, and the area of the scanned ion beam was 1 mm X 1 mm. Q Elsevier Sequoia/Printed in The Netherlands
290
Electrochemical measurements were carried o u t to determine the general and localized corrosion behaviour of the treated samples in the following t w o solutions: (a) deaerated 1 iv1 NaC1 at pH 4 and (b) 0.3% Na2SO4. The tests consisted of obtaining (i) polarization curves at a voltage scan rate of 720 mV h -z, (ii) potentiostatic curves I(t) at - - 6 0 0 mV with respect to a saturated calomel electrode (SCE)) anodic polarization and (iii) polarization resistance Rp measurements as a function of time, in order to calculate the instantaneous corrosion rates [ 5, 6]. The cell used was o~ the ASTM type, the chosen geometry allowing a good uniformity of the electrical field and a strictly constant distance between the working and reference electrodes. The electrochemical apparatus used was an AMEL (Milan) Metalloscan. The area of the specimen exoosed to the electrolyte was approximately 0.5 c m 2. After each experiment the sample was rinsed and dried and then submitted to scanning electron microscopy and scanning Auger microprobe analysis.
boron nitride layer just below a surface layer of "untransformed" boron. Also we have shown previously that the adhesion between coating and substrate appears to be highly enhanced as a result of ion beam mixing at the B - F e interface after the implantation treatment [ 7 ]. Electrochemical characterization of the sample was carried out in order to determine the cathodic and anodic behaviour and the corrosion resistance in two solutions: NaC1 solution; Na2SO4 solution.
3.1. NaCl solution (pH 4) Figure 1 shows two potentiodynamic polarization curves in a deaerated 1 M NaC1 solution (pH 4 because of the addition of aqueous HC1). The current-potential plot for a treated sample is compared with that for a pure iron reference standard. There are major differences between the two curves. The first observation concerns the corrosion current i¢o~ values as extrapolated from the intersection of Tafel lines close to the free corrosion potential; the value corresponding to the treated sample is four times lower than that for Armco iron (0.4 pA compared with 1.6/~A). It is also important to note that the anodic currents for the Armco iron samples are always two orders of magnitude higher than those for the treated samples for the same anodic overvoltage.
3. R E S U L T S
The effect of the 100 keV N + ion implantation of boron deposits is the formation of a (my (SCE)) I
I
I
l
-400 Sd
~
I -600
e°oooe
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........
.-..~. ~-::~:¢,;
. o "~
-.~ . . . .
~-
Qeee
-100i
~
i 10 - 4
o
oe
o
oe
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o ..a •
:°oo
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10 - s
e
e
eoe
°
I 10 - 3
o
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• (mA) Fig. 1. P o l a r i z a t i o n c u r v e in 1 M NaC1 ( p H 4) s o l u t i o n ( d e a e r a t e d ) : . . . . . , N + - i o n - i m p l a n t e d b o r o n film o n i r o n ; . . . . . , iron.
291
The presence of an inflection point in the anodic range at about - - 6 5 0 mV (SCE) is particularly interesting, as it indicates a partial inhibition of the anodic reactions exerted by the multilayer of boron plus boron nitride plus ion-beam-mixed B - F e . In order to understand the decrease in the active corrosion, the samples were anodically polarized to a fixed potential of - - 6 0 0 mV (SCE) for various times and the anodic current changes were recorded (Figs. 2 and 3). The treated specimens showed a slowly increasing anodic current which reached 20 pA after 40 min, whereas for pure iron the anodic current exceeded 300 pA. The scanning electron micrographs of the treated samples included in Fig. 2 indicate generally undamaged surface layers after 20 min of corrosion and only a very few discontinuities after 40 min. We deduce from these results that for the treated samples we
do not have passive conditions {stable oxide films and constant currents) b u t rather films with a low reactivity which tend to decrease, however, in efficiency as time passes owing to the multilayer transformation. The corroded surfaces were examined by means of Auger microanalysis which not only permits the study of surface concentration changes b u t also gives spectroscopic information a b o u t the chemical environment of the single elements and their modification during depth profiling. The depth profiles in Figs. 4(a)-4(c) show that the multilayer created by the ion treatm e n t clearly changes with increasing corrosion time as follows: (i) nitrogen decreases progressively; (ii) oxygen increases at the interface region; (iii) near-surface boron is always present, but its total amount tends to decrease (in contrast, the boron profile in the interface region, which correlates well with
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Fig. 2. (a) Current v s . time plot for potentiostatic anodic polarization ( a t - 600 mV (SCE)) of the treated sample in 1 M NaC1 solution. The scanning electron micrographs obtained after an immersion time of (b) 20 min and (c) 40 rain are also shown.
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face is associated with the oxidation of the boron released from the boron nitride (Fig. 4(b)) (it is important to note that, in spite of the presence of oxygen at the interface, the Auger spectroscopy analysis did not show any evidence of oxidation of the iron substrate [9]) and (iv) if the corrosion is pushed further then the multicomponent film tends to be partially removed but, in the zone where the film remains, we find boron also in a new chemical state (Fig. 4(c)) which has not been identified unequivocally and is probably a borate or a ternary oxide.
o
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101
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the nitrogen profile before the corrosion test, tends progressively to follow the oxygen profile). The monitoring of the Auger signal (Fig. 4(d)) indicates a dramatic change in the boron Auger line shape across the profile. This fact is associated with a change in the chemical eno vironment of boron, in good agreement with the spectra reported in the literature (and reproduced also in our laboratory) for commercially available boron nitride and boron Oxide [8]. Therefore the Auger line shape analysis becomes a very useful tool for studying in more detail the Auger depth profiles of Figs. 4(a)4(c). We find that with increasing corrosion time (i) the surface elemental boron tends to decrease, (ii) the thickness of the ion-induced boron nitride layer also tends to decrease but a certain amount of boron is still bonded to nitrogen even after severe corrosion attack, (iii) the appearance of oxygen at the inter-
A further electrochemical characterization of the treated samples for comparison with pure Armco iron was carried out by means of polarization resistance Rp measurements in the neutral 0.3% Na2SO4 solution in order to evaluate their behaviour under free-corrosion conditions. The effect of the surface layer is evident from the low initial corrosion rate value (Fig. 5). This value tends to increase rapidly but stabilizes at a value corresponding to a corrosion rate 30% smaller than that of Armco iron. The surface morphologies of treated and untreated samples after immersion for 60 h in the electrolyte are also shown in Fig. 5. The Armeo iron is uniformly attacked whereas the treated sample is generally undamaged, showing localized attack, however, which is probably due to faults present in the deposited layer.
4. DISCUSSION AND CONCLUSIONS
The implantation of 100 keV N ÷ ions in boron deposited onto iron substrates results in the formation of a B - F e mixed layer and a boron nitride layer just below a surface layer of " u n t r a n s f o r m e d " boron. The results obtained in our experiments show clearly the efficacy of the multicomponent layer in terms of improved corrosion behaviour of Armco iron in NaC1 and Na2SO4 solutions. An unambiguous determination of the protection mechanism is rather difficult, however, because of the complexity of the multilayer. Indeed, many phenomena may operate simul-
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150 170 190 ENERGY (eV)
Fig. 4. Auger characterization of N+-ion-implanted boron film on iron: (a) depth concentration profiles of asimplanted samples; (b) after 40 rain in an NaC1 solution a t - - 6 0 0 mV (SCE); (c) after more severe corrosion attack; (d) B KLL Auger line shape corresponding to the regions indicated in the depth profiles (m, elemental boron (deposit); - , boron nitride (BN); e, oxidized boron (B203); , , unidentified compound, probably iron borate or ternary oxide).
t a n e o u s l y or in successive steps, c o n t r i b u t i n g to t h e decrease in t h e iron c o r r o s i o n . (1) T h e s u r f a c e e l e m e n t a l b o r o n , w h i c h is relatively inert, m a y give a p r i m a r y p r o t e c t i o n ; it s e e m s to a c t as a sacrificial coating. (2) T h e p r e s e n c e o f b o r o n nitride c e r t a i n l y plays an i m p o r t a n t role b e c a u s e o f its r e m a r k -
able p r o p e r t i e s ; it is m a n i f e s t l y n o n - r e a c t i v e in m o s t cases. H o w e v e r , t h e r e is e v i d e n c e t h a t b o r o n n i t r i d e d e c o m p o s e s v e r y s l o w l y in extreme conditions [10]. (3) A t a f u r t h e r c o r r o s i o n stage t h e B - F e m i x e d l a y e r m a y give i m p r o v e d resistance t o c o r r o s i o n if we r e f e r t o results o b t a i n e d in
294
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15
25
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45
Time (h)
Fig. 5. (a) Corrosion rate vs. time curve for untreated (e) and treated (A) samples; (b) morphology of untreated samples; (c) morphology of treated samples.
boron-implanted iron [11] or boronized steel [12]. The extreme thinness of the mixed layer is at present a major problem, because erosion or damage will result in the establishm e n t of a galvanic couple and accelerate local corrosion. (4) Finally, if the boron oxide is further converted, a classical borate inhibition mechanism m a y operate [13]. It is important to improve the preparation technique of our samples in order to obtain more homogeneous and reproducible surfaces t h a t are free of faults which have a deleterious effect on the protective action.
ACKNOWLEDGMENTS
The authors are indebted to Professor F. Ferrari and Professor I. Scotoni for encouragement and are grateful to P. L. Bonora,
G. Cerisola, F. Defrancesco and F. Rabbi for valuable discussions.
REFERENCES 1 G. Dearnaley, Thin Solid Films, 107 (1983) 315. 2 C, Weissmantel, J. Vac. Sci. Technol., 18 (1981) 179. 3 C. Weissmantel, K. Bewilogua, K. Brener, D. Dietrich, U. Ebersbach, H. J. Erler, B. Rau and G. Reisse, Thin Solid Films, 96 (1982) 31. 4 S. Shanfield and R. Wolfson, J. Vac. Sc£ Technol., 41 (1983) 323. 5 E. Heitz and W. Schwenk, Br. Corros. J., 11 (1976) 74. 6 P. L. Bonora, G. Cerisola and G. Trombetti, Mater. Chem., 2 (1977) 221. 7 L. Guzman, F. Marchetti, L. Calliari, I. Scotoni and F. Ferrari, Thin Solid Films, 117 (1984) L63. 8 D. J. Joyner and D. M. Hercules, J. Chem. Phys., 72 (1980) 1095. 9 G. Ertl and K. Wandelt, Surf. Sci., 50 (1975) 479.
295 10 R. De Keermaecker, Rev. Sci. Instrum., 45 (1975) 421. 11 P. L. Bonora, M. Bassoli, G. Cerisola, P. L. De Anna, S. Lo Russo, P. Mazzoldi, B. Tiveron, I. Scotoni, C. Tosello and A. Bernard, in R. E. Benenson, E. H. Kaufmann, G. L. Miller and W. W. Scholz (eds.), Proc. 2nd Int. Conf. on Ion
Beam Modification o f Materials. Albany, NY, 1980, in Nucl. Instrum. Methods, 182-183 (1981) 1001. 12 P. Siiry, Br. Corros. J., 1 (1978) 13. 13 A. D. Mercer, in L. L. Shreier (ed.), Corrosion, Vol. 1, Butterworths, London, 1979, Chapter 18, p. 12.
289
Corrosion Behaviour of Layers Obtained by Nitrogen Implantation into Boron Films Deposited onto Iron Substrates* F. MARCHETTI, L. FEDRIZZI, F. GIACOMOZZI and L. GUZMAN Istituto per la Ricerca Scientifica e Tecnologica, 38050 Povo (Trento) (Italy) A. BORGESE Istituto Tecnico Industriale Statale B. Castelli, and Universitit di Brescia, 25100 Brescia (Italy) (Received September 17, 1984)
ABSTRACT
The electrochemical behaviour and corrosion resistance o f boron films deposited onto Armco iron after bombardment with 100 ke V N + ions were determined in various test solutions. The changes in the electrochemical parameters give evidence of lower anodic dissolution rates for the treated samples. Scanning electron microscopy and Auger analysis o f the corroded surfaces confirm the presence o f protective layers.
1. INTRODUCTION
It is well established that ion implantation may improve the resistance of metals to wear and corrosion. The more novel concept of ion beam mixing of a thin deposited film, including the formation of a buried mixed layer, extends the versatility of this technique considerably in altering the surface composition and properties of materials [ 1 ]. Boron nitride appears to be extremely interesting in view of its unique physical properties: high thermal conductivity, exceptional strength, low thermal expansion and high chemical inertness. The boron nitride coatings obtained by dipping or spraying retain many of the properties of boron nitride solids and powders, providing a more economical way of taking advantage of their excellent properties such as corrosion resistance, reduction in sticking and reaction, high temperature lubrication and mould release *Paper presented at the International Conference on Surface Modification of Metals by Ion Beams, Heidelberg, F.R.G., September 17-21, 1984. 0025-5416/85/$3.30
and indicating m a n y other applications in which boron nitride could be useful. There have been some recent attempts to obtain boron nitride coatings using deposition techniques such as plasma, ion beam or ionplating techniques, in which ionized and/or inert species of energy about I keV were involved [ 2 - 4 ] . These techniques yielded promising results, although microanalysis revealed that the films hitherto produced contained significant amounts of oxygen and carbon. In the present work we report on the corrosion behaviour of boron nitride layers created by N ÷ ion implantation of boron films deposited onto iron substrates.
2. EXPERIMENTAL PROCEDURES
Boron films were evaporated at 10 -4 Pa from an electron-gun-heated crucible onto Armco iron substrates to a thickness of 100200 nm and then implanted with 100 keV N ÷ ions at a fluence of 3 X 10 iv ions c m -2. The samples were then investigated by Auger electron spectroscopy combined with argon ion etching. The Auger spectra were obtained in the differential mode (Vmod = 1 V peak to peak) on standard equipment (PHI model 590 scanning Auger microprobe). The electron beam current was 0.3 pA at a primary beam energy of 3 keV. During the analysis the electron beam was scanned over an area of 100 pm by 100 pm. Sputtering was performed at an argon pressure of 1 X 10 -5 Pa in an ultrahigh vacuum system. The ion energy was 2 keV, and the area of the scanned ion beam was 1 mm X 1 mm. Q Elsevier Sequoia/Printed in The Netherlands
290
Electrochemical measurements were carried o u t to determine the general and localized corrosion behaviour of the treated samples in the following t w o solutions: (a) deaerated 1 iv1 NaC1 at pH 4 and (b) 0.3% Na2SO4. The tests consisted of obtaining (i) polarization curves at a voltage scan rate of 720 mV h -z, (ii) potentiostatic curves I(t) at - - 6 0 0 mV with respect to a saturated calomel electrode (SCE)) anodic polarization and (iii) polarization resistance Rp measurements as a function of time, in order to calculate the instantaneous corrosion rates [ 5, 6]. The cell used was o~ the ASTM type, the chosen geometry allowing a good uniformity of the electrical field and a strictly constant distance between the working and reference electrodes. The electrochemical apparatus used was an AMEL (Milan) Metalloscan. The area of the specimen exoosed to the electrolyte was approximately 0.5 c m 2. After each experiment the sample was rinsed and dried and then submitted to scanning electron microscopy and scanning Auger microprobe analysis.
boron nitride layer just below a surface layer of "untransformed" boron. Also we have shown previously that the adhesion between coating and substrate appears to be highly enhanced as a result of ion beam mixing at the B - F e interface after the implantation treatment [ 7 ]. Electrochemical characterization of the sample was carried out in order to determine the cathodic and anodic behaviour and the corrosion resistance in two solutions: NaC1 solution; Na2SO4 solution.
3.1. NaCl solution (pH 4) Figure 1 shows two potentiodynamic polarization curves in a deaerated 1 M NaC1 solution (pH 4 because of the addition of aqueous HC1). The current-potential plot for a treated sample is compared with that for a pure iron reference standard. There are major differences between the two curves. The first observation concerns the corrosion current i¢o~ values as extrapolated from the intersection of Tafel lines close to the free corrosion potential; the value corresponding to the treated sample is four times lower than that for Armco iron (0.4 pA compared with 1.6/~A). It is also important to note that the anodic currents for the Armco iron samples are always two orders of magnitude higher than those for the treated samples for the same anodic overvoltage.
3. R E S U L T S
The effect of the 100 keV N + ion implantation of boron deposits is the formation of a (my (SCE)) I
I
I
l
-400 Sd
~
I -600
e°oooe
-80(
........
.-..~. ~-::~:¢,;
. o "~
-.~ . . . .
~-
Qeee
-100i
~
i 10 - 4
o
oe
o
oe
e''m
o ..a •
:°oo
% ~
10 - s
e
e
eoe
°
I 10 - 3
o
I 10 - 2
I 10 - 1
• (mA) Fig. 1. P o l a r i z a t i o n c u r v e in 1 M NaC1 ( p H 4) s o l u t i o n ( d e a e r a t e d ) : . . . . . , N + - i o n - i m p l a n t e d b o r o n film o n i r o n ; . . . . . , iron.
291
The presence of an inflection point in the anodic range at about - - 6 5 0 mV (SCE) is particularly interesting, as it indicates a partial inhibition of the anodic reactions exerted by the multilayer of boron plus boron nitride plus ion-beam-mixed B - F e . In order to understand the decrease in the active corrosion, the samples were anodically polarized to a fixed potential of - - 6 0 0 mV (SCE) for various times and the anodic current changes were recorded (Figs. 2 and 3). The treated specimens showed a slowly increasing anodic current which reached 20 pA after 40 min, whereas for pure iron the anodic current exceeded 300 pA. The scanning electron micrographs of the treated samples included in Fig. 2 indicate generally undamaged surface layers after 20 min of corrosion and only a very few discontinuities after 40 min. We deduce from these results that for the treated samples we
do not have passive conditions {stable oxide films and constant currents) b u t rather films with a low reactivity which tend to decrease, however, in efficiency as time passes owing to the multilayer transformation. The corroded surfaces were examined by means of Auger microanalysis which not only permits the study of surface concentration changes b u t also gives spectroscopic information a b o u t the chemical environment of the single elements and their modification during depth profiling. The depth profiles in Figs. 4(a)-4(c) show that the multilayer created by the ion treatm e n t clearly changes with increasing corrosion time as follows: (i) nitrogen decreases progressively; (ii) oxygen increases at the interface region; (iii) near-surface boron is always present, but its total amount tends to decrease (in contrast, the boron profile in the interface region, which correlates well with
(laA) I
4f
I
40
30
20
.$. .$. .$.
10 .$. .$. O oo
0 (a)
~
I
I
I
10
20
30
,
40
T i m e (min)
Fig. 2. (a) Current v s . time plot for potentiostatic anodic polarization ( a t - 600 mV (SCE)) of the treated sample in 1 M NaC1 solution. The scanning electron micrographs obtained after an immersion time of (b) 20 min and (c) 40 rain are also shown.
292
(~A) I
II
II
400
O
¢.
300 ,
t
e o
face is associated with the oxidation of the boron released from the boron nitride (Fig. 4(b)) (it is important to note that, in spite of the presence of oxygen at the interface, the Auger spectroscopy analysis did not show any evidence of oxidation of the iron substrate [9]) and (iv) if the corrosion is pushed further then the multicomponent film tends to be partially removed but, in the zone where the film remains, we find boron also in a new chemical state (Fig. 4(c)) which has not been identified unequivocally and is probably a borate or a ternary oxide.
o
200
3.2. NaeSO4 solution
101
0
I
I
!
10
20
30
T i m e (rain) Fig. 3. As for Fig. 2(a), bu~ for an untreated Armco iron sample.
the nitrogen profile before the corrosion test, tends progressively to follow the oxygen profile). The monitoring of the Auger signal (Fig. 4(d)) indicates a dramatic change in the boron Auger line shape across the profile. This fact is associated with a change in the chemical eno vironment of boron, in good agreement with the spectra reported in the literature (and reproduced also in our laboratory) for commercially available boron nitride and boron Oxide [8]. Therefore the Auger line shape analysis becomes a very useful tool for studying in more detail the Auger depth profiles of Figs. 4(a)4(c). We find that with increasing corrosion time (i) the surface elemental boron tends to decrease, (ii) the thickness of the ion-induced boron nitride layer also tends to decrease but a certain amount of boron is still bonded to nitrogen even after severe corrosion attack, (iii) the appearance of oxygen at the inter-
A further electrochemical characterization of the treated samples for comparison with pure Armco iron was carried out by means of polarization resistance Rp measurements in the neutral 0.3% Na2SO4 solution in order to evaluate their behaviour under free-corrosion conditions. The effect of the surface layer is evident from the low initial corrosion rate value (Fig. 5). This value tends to increase rapidly but stabilizes at a value corresponding to a corrosion rate 30% smaller than that of Armco iron. The surface morphologies of treated and untreated samples after immersion for 60 h in the electrolyte are also shown in Fig. 5. The Armeo iron is uniformly attacked whereas the treated sample is generally undamaged, showing localized attack, however, which is probably due to faults present in the deposited layer.
4. DISCUSSION AND CONCLUSIONS
The implantation of 100 keV N ÷ ions in boron deposited onto iron substrates results in the formation of a B - F e mixed layer and a boron nitride layer just below a surface layer of " u n t r a n s f o r m e d " boron. The results obtained in our experiments show clearly the efficacy of the multicomponent layer in terms of improved corrosion behaviour of Armco iron in NaC1 and Na2SO4 solutions. An unambiguous determination of the protection mechanism is rather difficult, however, because of the complexity of the multilayer. Indeed, many phenomena may operate simul-
293 160
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40 (c)
80 SPUTT.
120 T I M E (min}
160 (d)
150 170 190 ENERGY (eV)
Fig. 4. Auger characterization of N+-ion-implanted boron film on iron: (a) depth concentration profiles of asimplanted samples; (b) after 40 rain in an NaC1 solution a t - - 6 0 0 mV (SCE); (c) after more severe corrosion attack; (d) B KLL Auger line shape corresponding to the regions indicated in the depth profiles (m, elemental boron (deposit); - , boron nitride (BN); e, oxidized boron (B203); , , unidentified compound, probably iron borate or ternary oxide).
t a n e o u s l y or in successive steps, c o n t r i b u t i n g to t h e decrease in t h e iron c o r r o s i o n . (1) T h e s u r f a c e e l e m e n t a l b o r o n , w h i c h is relatively inert, m a y give a p r i m a r y p r o t e c t i o n ; it s e e m s to a c t as a sacrificial coating. (2) T h e p r e s e n c e o f b o r o n nitride c e r t a i n l y plays an i m p o r t a n t role b e c a u s e o f its r e m a r k -
able p r o p e r t i e s ; it is m a n i f e s t l y n o n - r e a c t i v e in m o s t cases. H o w e v e r , t h e r e is e v i d e n c e t h a t b o r o n n i t r i d e d e c o m p o s e s v e r y s l o w l y in extreme conditions [10]. (3) A t a f u r t h e r c o r r o s i o n stage t h e B - F e m i x e d l a y e r m a y give i m p r o v e d resistance t o c o r r o s i o n if we r e f e r t o results o b t a i n e d in
294
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E
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•
A
~.
,..,- 0.4
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0.2
5
(a)
15
25
35
45
Time (h)
Fig. 5. (a) Corrosion rate vs. time curve for untreated (e) and treated (A) samples; (b) morphology of untreated samples; (c) morphology of treated samples.
boron-implanted iron [11] or boronized steel [12]. The extreme thinness of the mixed layer is at present a major problem, because erosion or damage will result in the establishm e n t of a galvanic couple and accelerate local corrosion. (4) Finally, if the boron oxide is further converted, a classical borate inhibition mechanism m a y operate [13]. It is important to improve the preparation technique of our samples in order to obtain more homogeneous and reproducible surfaces t h a t are free of faults which have a deleterious effect on the protective action.
ACKNOWLEDGMENTS
The authors are indebted to Professor F. Ferrari and Professor I. Scotoni for encouragement and are grateful to P. L. Bonora,
G. Cerisola, F. Defrancesco and F. Rabbi for valuable discussions.
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