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The electrochemical corrosion behavior of TiN and (Ti,Al)N coatings in acid and salt solution
Corrosion Science 45 (2003) 1367–1381 www.elsevier.com/locate/corsci
The electrochemical corrosion behavior of TiN and (Ti,Al)N coatings in acid and salt solution Ying Li *, Li Qu, Fuihui Wang State Key Laboratory for Corrosion and Protection, Institute of Metal Research, CAS, 62 Wencui Road Shenyang 110016, China Received 18 July 2001; accepted 21 October 2002
Abstract In this study, the corrosion properties of TiN and (Ti,Al)N coatings fabricated by Hollow Cathode Ionic Plating (HCIP) were studied by electrochemical techniques such as electrochemical impedance spectroscopy (EIS) measurement and potentiodynamic measurement in acid and salt solution. It was found that both coatings showed an excellent corrosion resistance in acid and salt solutions at the beginning of long-term immersing test. The corrosion resistance of TiN coating deteriorated rapidly after nearly 100 h immersion in both acid and salt solutions. In the contrast, the corrosion rate of (Ti,Al)N coating decreased a little and then kept at a stable value. For the TiN coating, the corrosion initiated from pinholes and the underlying corrosion was very similar to pitting corrosion. With the addition of aluminum to the TiN coating, the corrosion resistance was improved, especially in salt solution. The test results demonstrated that the (Ti,Al)N coating seemed to posses certain self-repairing function. The corrosion mechanism took the form of denudation corrosion, owing to deterioration of the adhesion of the coating. Ó 2003 Elsevier Science Ltd. All rights reserved. Keywords: TiN; (Ti,Al)N; Electrochemical corrosion
1. Introduction At present, TiN and (Ti,Al)N coatings have been used in many fields for their high wear resistance, hardness and low friction coefficient characteristics. With
*
Corresponding author. Fax: +86-24-2389-4149.
0010-938X/03/$ - see front matter Ó 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0010-938X(02)00223-8
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improvement in high temperature corrosion resistance, the applications of these coatings have been enlarged [1–5]. In recent years, Jin et al. [6–8] and others [9–14] reported that these coatings prepared by HCIP showed excellent high temperature corrosion resistance. These coatings may be used for compressor blade of aeroengine for their high wear resistance and high temperature corrosion resistance during 300–500 °C. Compressor blades of aero-engines, which service or stand in marine environment, may suffer from corrosion problems induced by chlorides. Chlorides may induce either accelerated corrosion at high temperature when chlorides are deposited at the surface of metallic materials or aqueous corrosion at ambient temperature when a aqueous thin film contain chloride forms on the surface of material. In the latter case, the electrochemical corrosion resistance of this coating is a very important factor. In the present work, a study of the corrosion behavior of these coatings in 0.5 mol/l NaCl and 1 mol/l H2 SO4 solution was carried out by electrochemical methods. The degradation mechanism of these coatings is also discussed.
2. Experimental details TiN and (Ti,Al)N coatings were prepared by a Hollow Cathode Ionic Plating (HCIP) apparatus IPB30/30T, ULVAC, Japan. N2 was used as the reactive gas. Pure Ti, and Ti–Al were used as evaporation sources. Substrate was 1Cr11Ni2W2MoV Martensitic stainless steel. Before deposition, coupons of dimension 15 10 2 mm were ground with 400–1000# SiC paper, cleaned ultrasonically in acetone. Being place in vacuum chamber of ion plating apparatus, the coupons were further cleaned by Arþ ions sputtering under 800 V bias potential for 5 min, and then, deposited at the selected parameters: temperature of coupons 450 °C, beam current 2–6 A, nitrogen partial press 0.133 Pa, negative bias 80 V and deposition time 50 min. The electrochemical experiments were performed in 0.5 mol/l NaCl and 1 mol/l H2 SO4 diluted aqueous solution at 25 °C, respectively. A classic three-electrode cell configuration was used. The counter electrode was Pt plate, and saturated calomel (SCE) was reference electrode. The corrosion rates of TiN and (Ti,Al)N coatings were inspected by CMB-1510 Portable Corrosion Rate Measurement Instrument. This instrument was produced by the Corrosion Inspecting Center in the Institute of Metal Research, CAS, China. The polarization potential was 40 mV. The time of measurement interval was 20 min. Potentiodynamic polarization was carried out using a PARC 273 Potentiostat manufactured by EG&G Company, USA. Polarization scale ranged from )150 to 1000 mV. Scan speed was set at 0.33 mV/s. Dates were automatically collected and analyzed with CORRVIEW software. Corrosion potential (Ecorr ), corrosion current (Icorr ), anodic Tafel slope (Ba ), cathodic Tafel slope (Bc ) were provided after analyzed by that software. Electrochemical impedance spectroscopy (EIS) measurement was carried out by PAR398 Electrochemical Measurement System consisting of Potentiostat/Galvanastat model 273 and model 5210 lock in amplifier, both manufactured by EG&G
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Company, USA. This electrochemical measurement system was controlled with a PC. This PC also served to acquire data using proprietary Z-plot software. All impedance measurements were performed at open circuit potential and used ac amplitude of 10 mV. The applied frequencies ranged from 105 to 102 Hz using five points/decade. The impedance data were analyzed by Z-View software. SEM observation was performed on Philip XL30 scanning electrode microscope manufactured by Philip Company, Netherlands. STM observation was performed on Topometrix Discoveror 2000 measurement system manufactured by Topometrix Company, USA. The operation parameters are given in the results shown below.
3. Results 3.1. Microstructure of HCIP TiN and (Ti,Al)N coatings Observed by eye, both TiN and (Ti,Al)N coatings prepared by HCIP were very smooth, uniform and shining golden color. The cross-section morphologies of both coatings observed by SEM shown that both coatings were about 5 lm thick without any obvious difference between them. The component and the phases of TiN and (Ti,Al)N coatings were analyzed by X-ray diffraction (XRD) analysis and electron probe microanalysis (EPMA). The results are shown in Figs. 1 and 2, which revealed that, for TiN coating, the titanium and nitrogen distributed in the coating uniformly existing as TiN and Ti2 N phases. For (Ti,Al)N coating, except those two phases, Al phase appeared in XRD. From EPMA results, it can be confirmed that Al entirely enriched at the interface of coating and substrate continuously. The STM surface morphologies of both coatings are shown in Fig. 3, where the brighter areas of the picture correspond to the highest positions on the sample surface. The roughness described by Rt (the average diameter of the particle on the surface) can be calculated by line analysis. The result reveals that the surface of the (Ti,Al)N coating was made of nano-sized particles. From the above result, it can be seen that the microstructure of TiN and (Ti,Al)N coating are different. For the TiN coating, the film is uniform and compact with some pinhole on it. But after the addition of Al element in TiN coating, there is much more grain boundary on the surface with provides more sites for corrosion. 3.2. The corrosion resistance of both TiN and (Ti,Al)N coatings The corrosion rates of TiN and (Ti,Al)N coatings in 1 mol/l H2 SO4 solution measured in situ by CMB-1510 Portable Corrosion Rate Measurement Instrument are shown in Fig. 4. During the first 35 h immersion period, both coatings showed an excellent corrosion resistance with the low corrosion rate which means that TiN and (Ti,Al)N coatings were a good protection coating in this environment. But the corrosion rate of TiN was lower in comparison with TiAlN coating. As the immersion time was extending, the corrosion rate of the TiN coating went up rapidly. However, the corrosion rate of (Ti,Al)N coating decreased to some degree in
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700
TiN coating
TiN Ti2N α Fe
(110)
600
Intensity
500
(204) (220)
400
(211) 300
(200) (220)
200
(112) (111) (200)
100 0 10
20
30
40
50
60
70
80
90
2θ 1200
(Ti,Al)N coating
(111) (112)
TiN Ti2N
α Fe
1000
Al
Intensity
800
600
400
200
(220) (200) (110)
(204) (200) (220) (200)
(311) (222)
(215) (311) (211)
0 10
20
30
40
50
60
70
80
90
Fig. 1. XRD patterns of TiN and (Ti,Al)N coatings.
contrast and then stayed at a stable value after 38 h immersion which means that the (Ti,Al)N coating possesses certain self-repairing ability due to the introduction of Al, thus has a higher corrosion resistance than TiN coating. The corrosion rates of TiN and (Ti,Al)N coatings in 0.5 mol/l NaCl solution were also measured. The results are shown in Fig. 5. It can be seen that the corrosion rates of TiN and (Ti,Al)N coatings were very much lower than that of substrate which implied the higher resistance of the two coatings. However, the corrosion rate of TiN coating increased sharply after 100 h immersion and the corrosion rate of (Ti,Al)N stayed at a stable value in contrast, which revealed that the (Ti,Al)N coating was a better protective coating than TiN in salt solution.
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Fig. 2. Electron probe microanalysis results for the cross-section of (Ti,Al)N coating––8 lm.
3.3. EIS of TiN and (Ti,Al)N coatings during long-term immersion test The typical EIS spectra of TiN and (Ti,Al)N coating during 100 h immersion test in 1 mol/l H2 SO4 solution are shown in Fig. 6. The equivalent circuits for each EIS spectrum are also shown along with. For TiN coating, there were two typical EIS spectra which were single capacitance arc at the beginning and capacitance
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Fig. 3. STM morphologies of TiN and (Ti,Al)N coatings.
arc þ inductance arc after long-term immersion. Different from TiN coating, the typical EIS spectrum of (Ti,Al)N after long-term immersion was double capacitance arc even thought the EIS spectrum was also single capacitance arc at the beginning. According to the equivalent circuits, some parameters such as the polarization resistance and the double-layer capacitance can be calculated by Z-View software.
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0.6
1mol/L H2SO4
blank
icorr /mA.cm-2
0.4
0.2
(Ti,Al)N TiN
0.0 0
10
20
30
40
50
Fig. 4. The corrosion rate of TiN and (Ti,Al)N coatings during the immersion in 1 mol/l H2 SO4 solution.
0.010
TiN icorr /mA.cm-2
TiAlN 0.005
0.000
0
50
100
150
200
250
300
Fig. 5. The corrosion rate of TiN and (Ti,Al)N coatings during the immersion in 1 mol/l NaCl solution.
As it is well known, when the other corrosion process such as pitting corrosion, adsorption of hydrogen or inhibitor on the surface takes place, the EIS shows more than two time-constants. For the double capacitance arcs on (Ti,Al)N coating, the capacitance arc at higher frequency was related to the double-layer of the coating, therefore, from which the polarization resistance and the double-layer capacitance of the coating can be calculated. The other capacitance arc was related to the dissolving process of Al at the interface between coating and substrate. For the EIS of TiN coating, the capacitance arc was related to the double-layer of the TiN coating from
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Fig. 6. Typical EIS of TiN (a) and (Ti,Al)N (b) coatings during immersion in 1 mol/l H2 SO4 solution.
which the polarization resistance and double-layer capacitance can be fitted. The inductance arc was related to another corrosion process that will be discussed later. The polarization resistance and double-layer capacitance of each coating during immersion in 1 mol/l H2 SO4 solution are shown in Figs. 7 and 8, respectively. The polarization resistance is a parameter correlated to the corrosion rate. The higher the polarization resistance, the lower is the corrosion rate. For TiN coating, the reaction transfer resistance decreased with the immersion time during the initial 30 h immersion, and then stayed at a stable value. However, the polarization resistance of
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Fig. 6 (continued)
(Ti,Al)N coating decreased sharply for the first 5 h and then raised gradually which revealed the improvement of the corrosion resistance of the coating. As the immersion test reached 75 h, the polarization resistance began to decrease and then gradually came to a stable value higher than that of TiN coating as the immersion time reached 90 h. The double-layer capacitance is another parameter embodied some characters of double layer, such as its component and the number of charge. From Fig. 8, it can be seen that the capacitance of TiN coating increased in three steps. At the first 25 h
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Fig. 7. Rp value vs. time of TiN and (Ti,Al)N coatings in 1 mol/l H2 SO4 solution (a), (Ti,Al)N, (b), TiN. 1000
1mol/L H 2SO4 800
C p /µF.cm-2
T iN 600
400
(Ti,Al)N
200
0 0
10
20
30
40
50
60
70
80
90
time/h Fig. 8. Cp value vs. time of TiN and (Ti,Al)N coatings in 1 mol/l H2 SO4 solution.
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period, the capacitance stay at lower value. At the second period from 25 to 70 h, the capacitance increased bit by bit. At the third period, the capacitance increased sharply which revealed the big change of the double layer. As for the (Ti,Al)N coating, the capacitance did not change so much which imply a fairy good stability of the double-layer during long-term immersion.
4. Polarization curves of TiN and (Ti,Al)N coatings in acid and salt solution The polarization curves of TiN and (Ti,Al)N coatings in 1 mol/l H2 SO4 solution after 2 h immersion are shown in Fig. 9. It can be seen that a passive film automatically forms on the TiN coating. However, the (Ti,Al)N coating dissolved at low rate without forming a passive film on its surface. Even though the passive film formed as the polarization potential raising up to a certain value, the dissolution rate of this passive film was higher than that on TiN coating. From the above results, it can be confirmed that the corrosion behavior of TiN and (Ti,Al)N coatings were different. The corrosion resistance of TiN coating was higher for the passive film forming automatically on it. The polarization curves of TiN and (Ti,Al)N coatings in 0.5 mol/l NaCl solution after 2 h immersion are shown in Fig. 10. The parameters fitted by CORRVIEW software are listed in Table 1. From the above results, it can be seen that both coatings dissolved actively in salt solution without forming a passive film on their surfaces. The corrosion rates of two coatings were at the same level. However, the corrosion process for TiN and (Ti,Al)N is different according to the parameter icorr and ba , respectively.
2.0
TiN 1.5
(Ti,Al)N
E SCE /V
1.0
0.5
0.0
-0.5
-1.0 1E-7
1E-6
1E-5
i/A.cm
1E-4
1E-3
-2
Fig. 9. Polarization curve of TiN and (Ti,Al)N coatings in 1 mol/l H2 SO4 solution.
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TiN
0.0
(Ti,Al)N
E SCE /V
-0.2
substract
-0.4
-0.6
0.5mol/L NaCl -0.8 1E-9
1E-8
1E-7
1E-6
i/A.cm
1E-5
1E-4
-2
Fig. 10. Polarization curve of TiN and (Ti,Al)N coatings in 1 mol/l NaCl solution.
Table 1 Electrochemical parameters of TiN and (Ti,Al)N coatings in 0.5 mol/l NaCl solution Sample
Ba (V)
Bc (V)
Ecorr (V)
icorr (A cm2 )
Substrate (Ti,Al)N TiN
0.276 0.101 0.106
0.166 0.160 0.187
)0.521 )0.378 )0.286
2.38e)6 1.71e)7 3.35e)7
5. Discussion From the above results, it followed that the corrosion behaviour of TiN and (Ti,Al)N coatings were different. As it is well known, the composition and microstructure of coatings are the main factors that affect the corrosion behavior. The structure of TiN and (Ti,Al)N coatings are different even through the composition on the top surface of these coatings are the same. For the TiN coating, the structure was compact columnar crystal with defects such as pinholes or pores in it [6–8]. Many investigations have revealed that TiN coating is an inert coating and does not take part in the corrosion reaction [15,16]. In that case, the defects, the only path by which the corrosion agent transferred to the substrate, are the main factor that decides the corrosion resistance of the coating. Pitting corrosion occurred at this site. According to the corrosion current and time curve, Cp and time curve and the SEM morphology of TiN coating after one week immersion in 0.5 mol/l NaCl (Fig. 11a), It can be confirmed that the corrosion process was pitting corrosion and the pinhole acted as ‘‘occluded cell’’ [17] which had catalytic effects on the corrosion process. It took several hours for corrosion agent to transfer to the substrate and induced the corrosion process. The corrosion process went on smoothly until the environment in
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Fig. 11. SEM morphologies of TiN and (Ti,Al)N coating after one week immersion in 0.5 mol/l NaCl solution.
the pinhole became so hostile that the catalytic corrosion process began. In this case, corrosion rate increased rapidly and large amount of charge accumulated on the double-layer. As a result, capacitance of the double-layer increased obviously too. On an other hand, the structure of (Ti,Al)N coating changed for the different deposition process by the addition of Al. During depositing process, Al evaporated first for its lower saturated vapor tension even though Al and Ti have mixed together uniformly. Al accumulated on the substrate preferentially in deposition process and Ti deposited later as Al was exhausted. In this case, the structure of (Ti,Al)N layer was equi-axied nanocrystal. The grain boundary on its surface was higher than that of the TiN coating. The amount of the active atoms that took part in corrosion process was higher than that of TiN coating. In this case, the corrosion rate of the (Ti,Al)N coating was higher than the TiN coating. However, for (Ti,Al)N coating,
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Al enriches on the interface between coating and substrate. Al was an active element in salt and sulfuric acid solution. When the corrosion agent reached at this site, Al dissolved with aluminum hydroxide taps formed. The transfer path was jammed for the higher volume aluminum hydroxide tap has. The corrosion process was obstructed. That caused the corrosion rate and Rt to be reduced, and the double-layer capacitance did not change so much for long-term immersion. As more corrosion products accumulated, the microstress was induced into the coating that reduced the adhesion of coating. The coating started to spall off. This was verified by the SEM observation after one week immersion in 0.5 mol/l NaCl in Fig. 11b. Basing on above analysis, the conclusion can be drawn that the enrichment of the Al at coating/ substrate interface was the reason why (Ti,Al)N coating had higher corrosion resistance for long-term immersion test. The main corrosion form of (Ti,Al)N coating was denudation.
6. Summary The above results revealed that both TiN and (Ti,Al)N coatings had a high corrosion resistance in acid and salt solution at the beginning of immersion test. However, the corrosion resistances for long-term immersion were different for the difference microstructure they had. The corrosion resistance of TiN coating deteriorated rapidly after nearly one hundred hour immersion in both acid and salt environment for the pitting corrosion occurred in the pinholes. In contrast, the corrosion rate of (Ti,Al)N coating showed an excellent corrosion resistance during a long-term immersion test. This coating seemed to posses certain self-repairing function because the corrosion process was obstructed by the corrosion product of Al on the interface between the coating and substrate.
Acknowledgements The project is supported by the national natural science fund of China under the contract No.50001013. This work is also subsidized with the Special Fund for the Major State Basic Research Projects G19990650. The author is also pleased to show her deeply thank to Professor Wu Weitao for the modification of English.
References [1] M. Bin-Sudin, A. Leyland, A.S. James, A. Mattews, J. Housden, B. Garside, Surface and Coating Technology 81 (1996) 215. [2] I.M. Penttinen, A.S. Korhonen, E. Harju, M.A. Turkia, O. Forsen, E.O. Ristolainen, Surface and Coating Technology 50 (1992) 161. [3] I. Wadsworth, I.J. Smith, L.A. Donohue, W.-D. Munz, Surface and Coating Technology 94–95 (1997) 315.
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[4] A. Mitsuo, S. Uchida, N. Nihira, M. Iwaki, Surface and Coating Technology 103–104 (1998) 98. [5] L. Cunha, M. Andritschky, L. Rebouta, K. Pischow, Surface and Coating Technology 116–119 (1999) 1152. [6] Z. Jin, C. Liu, W. Wu, Journal of Chinese Society for Corrosion and Protection 10 (1990) 18. [7] Z. Jin, C. Liu, W. Wu, Surface and Coating Technology 46 (1991) 307. [8] Y. Li, F. Lang, Z. Yu, J. Xu, Z. Jin, Corrosion Science and Protection Technology 11 (1999) 84. [9] M.J. Park, A. Leyland, A. Matthews, Surface and Coating Technology 43/44 (1990) 481. [10] B. Matthes, E. Broszeit, J. Aromaa, H. Ronkainen, S.-P. Hannula, A. Leyland, A. Matthews, Surface and Coating Technology 49 (1991) 489. [11] C. Liu, A. Leyland, S. Lyon, A. Matthews, Surface and Coating Technology 76–77 (1995) 615. [12] C. Liu, A. Leyland, S. Lyon, A. Matthews, Surface and Coating Technology 76–77 (1995) 623. [13] H.-S. Park, H. Kappl, K.H. Lee, J.-J. Lee, H.A. Jehn, M. Fenker, Surface and Coating Technology 133–134 (2000) 176. [14] C. Liu, A. Leyland, Q. Bi, A. Matthews, Surface and Coating Technology 141 (2001) 164. [15] M. Lakatos-Varsanyi, D. Hanzel, Corrosion Science 41 (1999) 1585. [16] S. Rudenja, J. Pan, I.O. Wallinder, C. Leygraf, P. Kulu, Journal of the Electrochemical Society 146 (1999) 4082. [17] W. Lynes, Journal of the Electrochemical Society 103 (1956) 467.
The electrochemical corrosion behavior of TiN and (Ti,Al)N coatings in acid and salt solution Ying Li *, Li Qu, Fuihui Wang State Key Laboratory for Corrosion and Protection, Institute of Metal Research, CAS, 62 Wencui Road Shenyang 110016, China Received 18 July 2001; accepted 21 October 2002
Abstract In this study, the corrosion properties of TiN and (Ti,Al)N coatings fabricated by Hollow Cathode Ionic Plating (HCIP) were studied by electrochemical techniques such as electrochemical impedance spectroscopy (EIS) measurement and potentiodynamic measurement in acid and salt solution. It was found that both coatings showed an excellent corrosion resistance in acid and salt solutions at the beginning of long-term immersing test. The corrosion resistance of TiN coating deteriorated rapidly after nearly 100 h immersion in both acid and salt solutions. In the contrast, the corrosion rate of (Ti,Al)N coating decreased a little and then kept at a stable value. For the TiN coating, the corrosion initiated from pinholes and the underlying corrosion was very similar to pitting corrosion. With the addition of aluminum to the TiN coating, the corrosion resistance was improved, especially in salt solution. The test results demonstrated that the (Ti,Al)N coating seemed to posses certain self-repairing function. The corrosion mechanism took the form of denudation corrosion, owing to deterioration of the adhesion of the coating. Ó 2003 Elsevier Science Ltd. All rights reserved. Keywords: TiN; (Ti,Al)N; Electrochemical corrosion
1. Introduction At present, TiN and (Ti,Al)N coatings have been used in many fields for their high wear resistance, hardness and low friction coefficient characteristics. With
*
Corresponding author. Fax: +86-24-2389-4149.
0010-938X/03/$ - see front matter Ó 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0010-938X(02)00223-8
1368
Y. Li et al. / Corrosion Science 45 (2003) 1367–1381
improvement in high temperature corrosion resistance, the applications of these coatings have been enlarged [1–5]. In recent years, Jin et al. [6–8] and others [9–14] reported that these coatings prepared by HCIP showed excellent high temperature corrosion resistance. These coatings may be used for compressor blade of aeroengine for their high wear resistance and high temperature corrosion resistance during 300–500 °C. Compressor blades of aero-engines, which service or stand in marine environment, may suffer from corrosion problems induced by chlorides. Chlorides may induce either accelerated corrosion at high temperature when chlorides are deposited at the surface of metallic materials or aqueous corrosion at ambient temperature when a aqueous thin film contain chloride forms on the surface of material. In the latter case, the electrochemical corrosion resistance of this coating is a very important factor. In the present work, a study of the corrosion behavior of these coatings in 0.5 mol/l NaCl and 1 mol/l H2 SO4 solution was carried out by electrochemical methods. The degradation mechanism of these coatings is also discussed.
2. Experimental details TiN and (Ti,Al)N coatings were prepared by a Hollow Cathode Ionic Plating (HCIP) apparatus IPB30/30T, ULVAC, Japan. N2 was used as the reactive gas. Pure Ti, and Ti–Al were used as evaporation sources. Substrate was 1Cr11Ni2W2MoV Martensitic stainless steel. Before deposition, coupons of dimension 15 10 2 mm were ground with 400–1000# SiC paper, cleaned ultrasonically in acetone. Being place in vacuum chamber of ion plating apparatus, the coupons were further cleaned by Arþ ions sputtering under 800 V bias potential for 5 min, and then, deposited at the selected parameters: temperature of coupons 450 °C, beam current 2–6 A, nitrogen partial press 0.133 Pa, negative bias 80 V and deposition time 50 min. The electrochemical experiments were performed in 0.5 mol/l NaCl and 1 mol/l H2 SO4 diluted aqueous solution at 25 °C, respectively. A classic three-electrode cell configuration was used. The counter electrode was Pt plate, and saturated calomel (SCE) was reference electrode. The corrosion rates of TiN and (Ti,Al)N coatings were inspected by CMB-1510 Portable Corrosion Rate Measurement Instrument. This instrument was produced by the Corrosion Inspecting Center in the Institute of Metal Research, CAS, China. The polarization potential was 40 mV. The time of measurement interval was 20 min. Potentiodynamic polarization was carried out using a PARC 273 Potentiostat manufactured by EG&G Company, USA. Polarization scale ranged from )150 to 1000 mV. Scan speed was set at 0.33 mV/s. Dates were automatically collected and analyzed with CORRVIEW software. Corrosion potential (Ecorr ), corrosion current (Icorr ), anodic Tafel slope (Ba ), cathodic Tafel slope (Bc ) were provided after analyzed by that software. Electrochemical impedance spectroscopy (EIS) measurement was carried out by PAR398 Electrochemical Measurement System consisting of Potentiostat/Galvanastat model 273 and model 5210 lock in amplifier, both manufactured by EG&G
Y. Li et al. / Corrosion Science 45 (2003) 1367–1381
1369
Company, USA. This electrochemical measurement system was controlled with a PC. This PC also served to acquire data using proprietary Z-plot software. All impedance measurements were performed at open circuit potential and used ac amplitude of 10 mV. The applied frequencies ranged from 105 to 102 Hz using five points/decade. The impedance data were analyzed by Z-View software. SEM observation was performed on Philip XL30 scanning electrode microscope manufactured by Philip Company, Netherlands. STM observation was performed on Topometrix Discoveror 2000 measurement system manufactured by Topometrix Company, USA. The operation parameters are given in the results shown below.
3. Results 3.1. Microstructure of HCIP TiN and (Ti,Al)N coatings Observed by eye, both TiN and (Ti,Al)N coatings prepared by HCIP were very smooth, uniform and shining golden color. The cross-section morphologies of both coatings observed by SEM shown that both coatings were about 5 lm thick without any obvious difference between them. The component and the phases of TiN and (Ti,Al)N coatings were analyzed by X-ray diffraction (XRD) analysis and electron probe microanalysis (EPMA). The results are shown in Figs. 1 and 2, which revealed that, for TiN coating, the titanium and nitrogen distributed in the coating uniformly existing as TiN and Ti2 N phases. For (Ti,Al)N coating, except those two phases, Al phase appeared in XRD. From EPMA results, it can be confirmed that Al entirely enriched at the interface of coating and substrate continuously. The STM surface morphologies of both coatings are shown in Fig. 3, where the brighter areas of the picture correspond to the highest positions on the sample surface. The roughness described by Rt (the average diameter of the particle on the surface) can be calculated by line analysis. The result reveals that the surface of the (Ti,Al)N coating was made of nano-sized particles. From the above result, it can be seen that the microstructure of TiN and (Ti,Al)N coating are different. For the TiN coating, the film is uniform and compact with some pinhole on it. But after the addition of Al element in TiN coating, there is much more grain boundary on the surface with provides more sites for corrosion. 3.2. The corrosion resistance of both TiN and (Ti,Al)N coatings The corrosion rates of TiN and (Ti,Al)N coatings in 1 mol/l H2 SO4 solution measured in situ by CMB-1510 Portable Corrosion Rate Measurement Instrument are shown in Fig. 4. During the first 35 h immersion period, both coatings showed an excellent corrosion resistance with the low corrosion rate which means that TiN and (Ti,Al)N coatings were a good protection coating in this environment. But the corrosion rate of TiN was lower in comparison with TiAlN coating. As the immersion time was extending, the corrosion rate of the TiN coating went up rapidly. However, the corrosion rate of (Ti,Al)N coating decreased to some degree in
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700
TiN coating
TiN Ti2N α Fe
(110)
600
Intensity
500
(204) (220)
400
(211) 300
(200) (220)
200
(112) (111) (200)
100 0 10
20
30
40
50
60
70
80
90
2θ 1200
(Ti,Al)N coating
(111) (112)
TiN Ti2N
α Fe
1000
Al
Intensity
800
600
400
200
(220) (200) (110)
(204) (200) (220) (200)
(311) (222)
(215) (311) (211)
0 10
20
30
40
50
60
70
80
90
Fig. 1. XRD patterns of TiN and (Ti,Al)N coatings.
contrast and then stayed at a stable value after 38 h immersion which means that the (Ti,Al)N coating possesses certain self-repairing ability due to the introduction of Al, thus has a higher corrosion resistance than TiN coating. The corrosion rates of TiN and (Ti,Al)N coatings in 0.5 mol/l NaCl solution were also measured. The results are shown in Fig. 5. It can be seen that the corrosion rates of TiN and (Ti,Al)N coatings were very much lower than that of substrate which implied the higher resistance of the two coatings. However, the corrosion rate of TiN coating increased sharply after 100 h immersion and the corrosion rate of (Ti,Al)N stayed at a stable value in contrast, which revealed that the (Ti,Al)N coating was a better protective coating than TiN in salt solution.
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Fig. 2. Electron probe microanalysis results for the cross-section of (Ti,Al)N coating––8 lm.
3.3. EIS of TiN and (Ti,Al)N coatings during long-term immersion test The typical EIS spectra of TiN and (Ti,Al)N coating during 100 h immersion test in 1 mol/l H2 SO4 solution are shown in Fig. 6. The equivalent circuits for each EIS spectrum are also shown along with. For TiN coating, there were two typical EIS spectra which were single capacitance arc at the beginning and capacitance
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Fig. 3. STM morphologies of TiN and (Ti,Al)N coatings.
arc þ inductance arc after long-term immersion. Different from TiN coating, the typical EIS spectrum of (Ti,Al)N after long-term immersion was double capacitance arc even thought the EIS spectrum was also single capacitance arc at the beginning. According to the equivalent circuits, some parameters such as the polarization resistance and the double-layer capacitance can be calculated by Z-View software.
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0.6
1mol/L H2SO4
blank
icorr /mA.cm-2
0.4
0.2
(Ti,Al)N TiN
0.0 0
10
20
30
40
50
Fig. 4. The corrosion rate of TiN and (Ti,Al)N coatings during the immersion in 1 mol/l H2 SO4 solution.
0.010
TiN icorr /mA.cm-2
TiAlN 0.005
0.000
0
50
100
150
200
250
300
Fig. 5. The corrosion rate of TiN and (Ti,Al)N coatings during the immersion in 1 mol/l NaCl solution.
As it is well known, when the other corrosion process such as pitting corrosion, adsorption of hydrogen or inhibitor on the surface takes place, the EIS shows more than two time-constants. For the double capacitance arcs on (Ti,Al)N coating, the capacitance arc at higher frequency was related to the double-layer of the coating, therefore, from which the polarization resistance and the double-layer capacitance of the coating can be calculated. The other capacitance arc was related to the dissolving process of Al at the interface between coating and substrate. For the EIS of TiN coating, the capacitance arc was related to the double-layer of the TiN coating from
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Fig. 6. Typical EIS of TiN (a) and (Ti,Al)N (b) coatings during immersion in 1 mol/l H2 SO4 solution.
which the polarization resistance and double-layer capacitance can be fitted. The inductance arc was related to another corrosion process that will be discussed later. The polarization resistance and double-layer capacitance of each coating during immersion in 1 mol/l H2 SO4 solution are shown in Figs. 7 and 8, respectively. The polarization resistance is a parameter correlated to the corrosion rate. The higher the polarization resistance, the lower is the corrosion rate. For TiN coating, the reaction transfer resistance decreased with the immersion time during the initial 30 h immersion, and then stayed at a stable value. However, the polarization resistance of
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Fig. 6 (continued)
(Ti,Al)N coating decreased sharply for the first 5 h and then raised gradually which revealed the improvement of the corrosion resistance of the coating. As the immersion test reached 75 h, the polarization resistance began to decrease and then gradually came to a stable value higher than that of TiN coating as the immersion time reached 90 h. The double-layer capacitance is another parameter embodied some characters of double layer, such as its component and the number of charge. From Fig. 8, it can be seen that the capacitance of TiN coating increased in three steps. At the first 25 h
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Fig. 7. Rp value vs. time of TiN and (Ti,Al)N coatings in 1 mol/l H2 SO4 solution (a), (Ti,Al)N, (b), TiN. 1000
1mol/L H 2SO4 800
C p /µF.cm-2
T iN 600
400
(Ti,Al)N
200
0 0
10
20
30
40
50
60
70
80
90
time/h Fig. 8. Cp value vs. time of TiN and (Ti,Al)N coatings in 1 mol/l H2 SO4 solution.
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period, the capacitance stay at lower value. At the second period from 25 to 70 h, the capacitance increased bit by bit. At the third period, the capacitance increased sharply which revealed the big change of the double layer. As for the (Ti,Al)N coating, the capacitance did not change so much which imply a fairy good stability of the double-layer during long-term immersion.
4. Polarization curves of TiN and (Ti,Al)N coatings in acid and salt solution The polarization curves of TiN and (Ti,Al)N coatings in 1 mol/l H2 SO4 solution after 2 h immersion are shown in Fig. 9. It can be seen that a passive film automatically forms on the TiN coating. However, the (Ti,Al)N coating dissolved at low rate without forming a passive film on its surface. Even though the passive film formed as the polarization potential raising up to a certain value, the dissolution rate of this passive film was higher than that on TiN coating. From the above results, it can be confirmed that the corrosion behavior of TiN and (Ti,Al)N coatings were different. The corrosion resistance of TiN coating was higher for the passive film forming automatically on it. The polarization curves of TiN and (Ti,Al)N coatings in 0.5 mol/l NaCl solution after 2 h immersion are shown in Fig. 10. The parameters fitted by CORRVIEW software are listed in Table 1. From the above results, it can be seen that both coatings dissolved actively in salt solution without forming a passive film on their surfaces. The corrosion rates of two coatings were at the same level. However, the corrosion process for TiN and (Ti,Al)N is different according to the parameter icorr and ba , respectively.
2.0
TiN 1.5
(Ti,Al)N
E SCE /V
1.0
0.5
0.0
-0.5
-1.0 1E-7
1E-6
1E-5
i/A.cm
1E-4
1E-3
-2
Fig. 9. Polarization curve of TiN and (Ti,Al)N coatings in 1 mol/l H2 SO4 solution.
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TiN
0.0
(Ti,Al)N
E SCE /V
-0.2
substract
-0.4
-0.6
0.5mol/L NaCl -0.8 1E-9
1E-8
1E-7
1E-6
i/A.cm
1E-5
1E-4
-2
Fig. 10. Polarization curve of TiN and (Ti,Al)N coatings in 1 mol/l NaCl solution.
Table 1 Electrochemical parameters of TiN and (Ti,Al)N coatings in 0.5 mol/l NaCl solution Sample
Ba (V)
Bc (V)
Ecorr (V)
icorr (A cm2 )
Substrate (Ti,Al)N TiN
0.276 0.101 0.106
0.166 0.160 0.187
)0.521 )0.378 )0.286
2.38e)6 1.71e)7 3.35e)7
5. Discussion From the above results, it followed that the corrosion behaviour of TiN and (Ti,Al)N coatings were different. As it is well known, the composition and microstructure of coatings are the main factors that affect the corrosion behavior. The structure of TiN and (Ti,Al)N coatings are different even through the composition on the top surface of these coatings are the same. For the TiN coating, the structure was compact columnar crystal with defects such as pinholes or pores in it [6–8]. Many investigations have revealed that TiN coating is an inert coating and does not take part in the corrosion reaction [15,16]. In that case, the defects, the only path by which the corrosion agent transferred to the substrate, are the main factor that decides the corrosion resistance of the coating. Pitting corrosion occurred at this site. According to the corrosion current and time curve, Cp and time curve and the SEM morphology of TiN coating after one week immersion in 0.5 mol/l NaCl (Fig. 11a), It can be confirmed that the corrosion process was pitting corrosion and the pinhole acted as ‘‘occluded cell’’ [17] which had catalytic effects on the corrosion process. It took several hours for corrosion agent to transfer to the substrate and induced the corrosion process. The corrosion process went on smoothly until the environment in
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Fig. 11. SEM morphologies of TiN and (Ti,Al)N coating after one week immersion in 0.5 mol/l NaCl solution.
the pinhole became so hostile that the catalytic corrosion process began. In this case, corrosion rate increased rapidly and large amount of charge accumulated on the double-layer. As a result, capacitance of the double-layer increased obviously too. On an other hand, the structure of (Ti,Al)N coating changed for the different deposition process by the addition of Al. During depositing process, Al evaporated first for its lower saturated vapor tension even though Al and Ti have mixed together uniformly. Al accumulated on the substrate preferentially in deposition process and Ti deposited later as Al was exhausted. In this case, the structure of (Ti,Al)N layer was equi-axied nanocrystal. The grain boundary on its surface was higher than that of the TiN coating. The amount of the active atoms that took part in corrosion process was higher than that of TiN coating. In this case, the corrosion rate of the (Ti,Al)N coating was higher than the TiN coating. However, for (Ti,Al)N coating,
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Al enriches on the interface between coating and substrate. Al was an active element in salt and sulfuric acid solution. When the corrosion agent reached at this site, Al dissolved with aluminum hydroxide taps formed. The transfer path was jammed for the higher volume aluminum hydroxide tap has. The corrosion process was obstructed. That caused the corrosion rate and Rt to be reduced, and the double-layer capacitance did not change so much for long-term immersion. As more corrosion products accumulated, the microstress was induced into the coating that reduced the adhesion of coating. The coating started to spall off. This was verified by the SEM observation after one week immersion in 0.5 mol/l NaCl in Fig. 11b. Basing on above analysis, the conclusion can be drawn that the enrichment of the Al at coating/ substrate interface was the reason why (Ti,Al)N coating had higher corrosion resistance for long-term immersion test. The main corrosion form of (Ti,Al)N coating was denudation.
6. Summary The above results revealed that both TiN and (Ti,Al)N coatings had a high corrosion resistance in acid and salt solution at the beginning of immersion test. However, the corrosion resistances for long-term immersion were different for the difference microstructure they had. The corrosion resistance of TiN coating deteriorated rapidly after nearly one hundred hour immersion in both acid and salt environment for the pitting corrosion occurred in the pinholes. In contrast, the corrosion rate of (Ti,Al)N coating showed an excellent corrosion resistance during a long-term immersion test. This coating seemed to posses certain self-repairing function because the corrosion process was obstructed by the corrosion product of Al on the interface between the coating and substrate.
Acknowledgements The project is supported by the national natural science fund of China under the contract No.50001013. This work is also subsidized with the Special Fund for the Major State Basic Research Projects G19990650. The author is also pleased to show her deeply thank to Professor Wu Weitao for the modification of English.
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