- Email: [email protected]
Corrosion performance of layered coatings produced by physical vapour deposition
Surface and Coatings Technology, 43/44 (1990) 481—492
481
CORROSION PERFORMANCE OF LAYERED COATINGS PRODUCED BY PHYSICAL VAPOUR DEPOSITION M. J. PARK, A. LEYLAND and A. MA1~HEWS Research Centre in Surface Engineering, Department of Engineering Design and Manufacture, University of Hull, Hull HU6 7RX (tLK)
Abstract Potentiodynamic methods were used to investigate the relative corrosion protection provided on 304 stainless steel by various coating and pretreatment routes designed to isolate the substrate from the environment. These include combinations of plasma oxidizing, plasma nitriding and chromium, nickel and titanium nitride coatings produced by triode ion plating methods. It is shown that optimization of the layer structure and galvanic coupling are vital where a combination of wear and corrosion resistance is required. Nickel interlayers under TiN, for example, are shown to provide improved protection against corrosion. Attempts to increase corrosion resistance by producing a passive layer through a d.c. plasma processing stage on the substrate were not so successful, and the reasons for this are discussed.
1. Introduction Titanium nitride is now widely used as a thin protective coating, providing high surface hardness and good chemical stability. Its primary uses are therefore in applications requiring wear resistance [1—3],corrosion resistance [3] and/or diffusion barrier properties [4]. It has been reported that in certain corrosive environments, the service life of TiN coatings can be increased by reducing pinhole-type defects [5, 6]. When considering the corrosion protection, both galvanic and pitting corrosion mechanisms must be taken into account [7, 8]. TiN is an electrical conductor and electrochemically more noble than typical substrates (e.g. steels). Once substrates which are galvanically coupled with TiN are exposed to a corrosive atmosphere via pinholes, the exposed area will begin to dissolve anodically. Moreover, the small anodic area through the pinholes will cause a local increase in current density and thus accelerate the corrosion reaction at these points. Many attempts to reduce pinhole defects have been reported, through the improvement of TiN coating structures [5, 9—11] and the use of multilayer deposits produced by pulsing the reactive 0257-8972/901$3.50
© Elsevier Sequoia/Printed in The Netherlands
482
gas [12]. However, these approaches do not completely eliminate the corrosion problem. The intention of the work reported here was to isolate the substrate from the corrosive medium, rather than to modify the TiN structure. This was achieved by employing plasma diffusion pretreatments aimed at surface passivation, together with metallic coatings, both as single layers and as interlayers below TiN films. The relative corrosion performance was evaluated by potentiodynamic measurement techniques and scanning electron microscopy (SEM) studies before and after testing.
2. Experimental details Coating procedures The coatings employed in this work were deposited using a thermionically-enhanced plasma-assisted electron beam (EB) physical vapour deposition (PVD) system (Tecvac 1P35L). The plasma oxidizing and nitriding were also carried out using this equipment. The substrate material was stainless steel (AISI 304). Specimens were rectangular flat sheets (2 cm x 2 cm), 1 mm thick, ultrasonically degreased in freon and then washed in acetone. Substrates to be nitrided or oxidized were pretreated in a 10 mTorr enhanced triode discharge with a gas composition of 75% Ar and 25% H2 for 2 h at 2.1.
500 C. This was designed to aid the repeatability of the subsequent passivation treatments. Oxidizing was performed in a 10 mTorr mixture of 75% Ar and 25% 02. Nitriding was carried out at the same pressure with 75% N2 and 25% H2. In each case a triode discharge was employed, with the same power TABLE 1 Summary of treatments and coatings applied Specimen
Treatment conditions Sputter cleaning”
1. 2. 3. 4. 5. 6. 7. 8. 9. 10.
304 stainless steel substrate TiN on substrate TiN on Ni on substrate TiN on Cr on substrate TiN on nitrided substrate TiN on oxidized substrate TiN on Ni on oxidized substrate TiN on Cr on oxidized substrate Ni on substrate Cr on substrate
• •
“10 mTorr; 75% Ar + 25% H2 bias, 500 V; 500 ~C; 2 h. b10 mTorr; 75% N2 + 25% H2 bias, 500 V; 500 C; 2 h. ClO mTorr; 75% Ar + 25% 02; bias, 500 V; 500 ~C;2 h. d75 mTorr; bias, 120 V; 500 C.
Nitriding b
Oxidizing~
Coatingd
TiN TiN and TiN and TiN TiN TiN and TiN and Ni Cr
Ni Cr
Ni Cr
483
density and discharge parameters throughout, i.e. bias, 500 V; substrate temperature, 500 °C.The treatment duration was 2 h in each case. For all coatings (TiN, nickel and chromium), a substrate bias of 120 V was employed, with a background argon pressure of 7.5 mTorr. Again, the power densities were such that a deposition temperature of 500 ‘C was maintained. However, treatment times were varied to achieve the required coating thicknesses. Table 1 summarizes the coatings studied.
2.2. Potentiodynamic measurements A conventional three-electrode cell was used, the counterelectrode being a platinum sheet. The potential of the working electrode was measured by a saturated calomel electrode (SCE) through a solid salt bridge. A potentiostat (Thompson Ministat 402), sweep generator (Thompson Miniscan) and XY recorder (Servogor 210) were employed to obtain the potentio. dynamic polarization curves. The potential sweep rate was 15 mV min’. The electrolyte was an aerated 0.1 N H2S04 solution held at a temperature of 25 “C. The testpotentiodynamic specimens were measurement, masked with wax to expose a surfacewere areaset of 2. Before the working electrodes 1 cm—500 mV (SCE). to
3. Results 3.1. Potentiodynamic studies Figure 1 shows the anodic polarization curves of TiN-coated, metalinterlayered and uncoated 304 stainless steel. For the uncoated substrate there is an initial active region to about 0 mV (SCE) without any trace of an active peak, followed by a passive region up to 800 mV. Above 800 mV, a transpassive region develops in which the passivating film on the substrate is broken down. The most important aspects of anodic polarization curves in practical corrosion comparisons are the current density and voltage range of the passive region. The passivation current of the substrate varies from 3 x iO~mA to 102 mA. On TiN-coated stainless steel it is reduced to iO~mA and an improvement in corrosion resistance of TiN coatings can therefore be expected. The general pattern from the TiN-coated stainless steel is similar to that of the substrate, except for the “passive-like” behaviour at about 0 mV. Further improvement in corrosion resistance is evident in the TiN.coated/metal.interlayered stainless steel. Nickel- and chromium-interlayered TiN coatings show a slight reduction in passivation current compared with single-layered TiN. The passivation current ranges from 2 x iO~mA to 4 x 10~mA below 500 mV. There is typically an active peak, and the chromium-interlayered film shows a rapid increase in current over the transpassive region. The nickel-interlayered TiN coatings generally exhibit slightly better corrosion resistance (i.e. lower passivation current) than the chromium-interlayered TiN coatings.
484
2)
CURRENT DENSITY (mA/cm
/
0.1
0.01
/
*
1.000E-03
7
/~
1.000E-04
*
1.000E-05
*
-500
0
500 1000 1500 ELECTRODE POTENTIAL (mV,SCE)
2000
Fig. 1. Anodic polarization curves of TiN-coated, metal-interlayered and bulk 304 stainless steel: —*-—--, 304 stainless steel substrate; ~ TiN coating on the substrate; —*—, chromium-interlayered TiN coating on the substrate; —~—, nickel-interlayered TiN coating on the substrate.
CURRENT DENSITY (mA/cm2)
//
0.-i
*
0.01 *
1.000E-03
/
//
*
—~
*
1.000E-04=
1.000E-05[ -500
0
500
1000
1500
2000
ELECTRODE POTENTIAL (mV,SCE) Fig. 2. Anodic polarization curves of TiN coatings on untreated and passivation-treated 304 stainless steel: , 304 stainless steel substrate; —*—--, TiN coating on the substrate; —*—--, TiN coating on oxidized substrate; —~—, TiN coating on nitrided substrate.
485
Figure 2 shows the anodic polarization curves of TiN coatings on untreated and passivation-treated (oxidized and nitrided) substrates. The oxidized stainless steel coated with TiN has a passivation current of 3 x 10~mA. When compared with a single layer of TiN or untreated stainless steel (1 x 10 ~mA), the corrosion resistance is improved, but the passive region ends at about 300 mV. Above 600 mV the current density on the oxidized substrate coated with TiN is higher than that of the single-layer TiN-coated substrate. The nitrided substrate coated with TiN shows a sharp current increase in the initial active region, followed by strong passivation. However, in the practical potential range (below 1 V) it exhibits a higher current than even that of the untreated substrate. Further anodic polarization curves for TiN-coated, metal-interlayered and oxidized substrates are shown in Fig. 3. Nickel-interlayered TiN coatings on oxidized substrates show lower currents over the entire potential range (except for the active peak) than single TiN coatings on oxidized substrates. However, chromium.interlayered TiN coatings on oxidized substrates exhibit a high current active peak and a passivation current of about 2 x 10~mA. Again, there is a rapid current increase in the transpassive region (above 800 mV). This was found to be due to dissolution of Cr6 which produced a yellow colour in solution and led to spalling of the TiN coating. +
~CURRENT DENSITY (mA/cm2) 0.1
/
0.O1L
1.000E-03
1.000E-04~
/
/ I /~
l.OOOE~O 5Li±~_ -500
-~
0
500
1000
1500
2000
ELECTRODE POTENTIAL (mV,SCE) Fig. 3. Anodic polarization curves of TiN.coated, interlayered and oxidized 304 stainless steel: —*—, stainless steel substrate; —*—, TiN coating on oxidized substrate; —*—, chromiuminterlayered TiN coating on oxidized substrate; , nickel-interlayered TiN coating on oxidized substrate.
304
486 2)
CURRENT DENSITY (mA/cm 1~ -
-.._-____
/
0.1
0.01
1.000E-03~
*_
/
*
1.000E_04j
1.000 E -05 -500
*
*
j-----_____
0
---—-—
500
—-
--
- —--——-—
1000
-
-
1500
2000
ELECTRODE POTENTIAL (mV,SCE) Fig. 4. Anodic polarization curves of metal-coated and uncoated 304 stainless steel: stainless steel substrate; substrate.
—*—-,
nickel coating on the substrate;
—*—-,
, 304
chromium coating on the
Figure 4 shows the anodic polarization curves for chromium and nickel coatings compared with the uncoated substrate. Both of the metal-coated substrates exhibit typical ‘S’ curves and a strong active peak. As expected, the chromium coating gives the lowest passivation current due to the formation of a strong oxide film, but the passive region is short; the onset of transpassive behaviour starts at about 500 mV. Conversely, the nickel coating shows a much higher passivation current, but the passive region is quite wide and stable. 3.2. SEM studies Figure 5 shows fracture cross-sections through some of the coatings. Points to note are that the thicknesses of the TiN, chromium and nickel layers are similar, i.e. about 0.7 pm. In addition, all coatings exhibit dense columnar growth morphologies, with no apparent effect of pretreatments, such as nitriding, on the structure. Figure 6 shows typical surface appearances of the three coating types. The TiN and chromium coatings are somewhat similar, whilst the nickel coating has a more faceted appearance. Figure 6(d) reveals how the corrosion has preferentially attacked the nickel grain or column boundaries, whereas chromium and TiN are not affected in this way. The explanation for this is that TiN is relatively inert, and chromium forms a strong oxide film which provides protection and results in
487
(a)
(b)
(c)
Fig. 5. Scanning electron micrographs of cross-sections of coatings: (a) TiN coating on 304 stainless steel; (b) TiN coating and chromium interlayer coating on 304 stainless steel; (c) TiN coating on nitrided 304 stainless steel.
488
a a
~ a a
—
a
489
(a)
4.
(b) Fig. 7. Surface morphology of TiN coatings with pinhole defects (SEM): (a) as.deposited TiN coating; (b) TiN coating after potentiodynaniic test.
more uniform corrosion (except where there are defects in the inert layer, i.e. cracks or pinholes). Figure 7 shows typical pinholes in a TiN coating, before and after corrosion testing. The pinhole sizes vary, but are slightly larger after testing. This appears to confirm that a pitting corrosion mechanism is operating. Figure 8 shows the pitting pattern observed in a chromium-interlayered TiN coating, after potentiodynamic testing. In Fig. 8(a) the average diameter of the pits is about 120 p.m. The figure shows serious spalling of the TiN layer, as a consequence of chromium dissolution. Figure 8(b) shows the pit interior with traces of chromium residues on the substrate, with extensive intergranular attack and some pitting corrosion of the substrate.
490
(a)
(b~ Fig. 8. Surface morphology of TiN-coated and chromium-interlayered 304 stainless steel after potentiodynamic test (SEM): (a) pit pattern; (b) inside the pit.
4. Discussion Our potentiodynamic measurements have shown that interlayered TiN coatings can exhibit better corrosion resistance (i.e. reduced passivation currents in the voltage range of interest) than conventional single-layer TiN coatings. There are a number of possible explanations for the improvements observed. Firstly, the introduction of less refractory metallic interlayers such as chromium or nickel provides a dense intermediate coating layer under given deposition conditions, reducing the defects which can lead to corrosive attack. However, it should be noted that the increase in total coating thickness provided by the interlayer could, in itself, be enhancing the corrosion performance.
491
In view of the strong passivation behaviour demonstrated by the chromium coatings produced here (Fig. 4), it was expected that chromiuminterlayered TiN coatings would give better corrosion resistance than their nickel counterparts (see Figs. 1 and 3). It might be expected that any corrosion of the chromium layer would take place via general galvanic attack rather than locally around pinholes. However, the passivation region displayed by the chromium is comparatively narrow and unstable (Fig. 4), with a rapid rise in dissolution current at the onset of transpassive behaviour. In our potentiodynamic tests, this effect was manifested by a rapid dissolution of the chromium layer (Figs. 1(d), 3(d) and 4(c)). From Fig. 3 it can be seen that chromium-interlayered TiN coatings show a relatively high current over the entire potential range employed. This could result from strong galvanic coupling between the TiN and the chromium, caused by electrical isolation of the substrate by the oxide layer. Since the metal (chromium) interlayer has a lower standard electrode potential than that of the substrate, it will tend to reduce the corrosion resistance of the TiN overlayer. In addition, if the passivating film on the chromium interlayer breaks down (i.e. at the onset of the transpassive region), spallation of the TiN layer will occur (Fig. 8). The nitriding treatments employed in this work appeared to reduce the corrosion performance of the TiN coating (Fig. 2(d)) and, indeed, of the substrate. This result was unexpected; however, it may be that the nitrided layer itself contained similar defects to those observed in the coating (i.e. cracks and/or pinholes). The incorporation of a hydrogen preclean stage to remove oxide contamination on substrates (to improve treatment repeatability) may have accelerated corrosive attack through these defects by exposing an active surface. This could be accentuated by any tensile stresses produced in the treated substrate surface as a result of residual compressive stresses in the coating. Oxidizing treatments appeared to produce a slight increase in the corrosion protection afforded by the TiN layer (at least below a working potential of 600 mV). However, the oxide layer was unstable above 300 mV. This was again unexpected since the onset of transpassive behaviour in oxide films on 304 stainless steel is typically observed only above 800 mV. It may therefore be possible to improve the overall corrosion resistance further by modifying the oxidation treatment.
5. Conclusions In the coating systems discussed here, corrosion of the TiN layer itself was not a significant factor in the overall performance. As expected, corrosive attack via pinhole defects was almost without exception the primary mechanism for coating failure. However, it was demonstrated that corrosion resistance could be improved by metallic interlayers, reducing pinhole densities and modifying galvanic coupling effects. Nickel interlayers appeared to
492
provide the optimum protection in these respects. Plasma nitriding of the substrate prior to coating worsened the overall corrosion performance, whilst plasma oxidizing caused a slight improvement. However, these treatments have the potential, with further development, to provide more effective protection.
Acknowledgments M.J.P. acknowledges the support of an Overseas Research Studentship (ORS) award. The authors thank Dr. G. Thompson (University of Manchester Institute of Science and Technology) for the provision of potentiodynamic test equipment and E. Namgoong for help with the testing. The ongoing support of the Science and Engineering Research Council for the Research Centre in Surface Engineering is also acknowledged.
References 1 2 3 4 5 6 7 8 9 10 11 12
A. Matthews, Surf. Eng., 1 (2) (1985) 93. J. Vogel and E. Bergman, J. Vac. Sci. Technol. A, 4 (6) (1986) 2731. H. Jacobs, Surf. Coat. Technol., 29 (1986) 221. C. Y. Ting, J. Vac. Sci. Technol., 21 (1) (1982) 14. T. A. Mantyla, P. J. Helevirta, T. T. Lepisto and P. T. Siitonen, Thin Solid Films, 126 (1985) 275. Y. Massiani, J. Crousier, L. Fedrizzi, A. Calvaleri and P. L. Bonora, Surf. Coat. Technol., 33 (1989) 309. M. J. Pryor, in L. L. Shreir (ed), Corrosion, Vol. 1, Newnes—Butterworths, London, 2nd edn., 1976, p. 1192. L. L. Shrier, in L. L. Shreir (ed.), Corrosion, Vol. 1, Newnes—Butterworths, London, 2nd edn., 1976, p. 1150. A. Erdedemir, W. B. Carter, R. F. Hochman and E. I. Meletis, Mater. Sci. Eng., 69(1985) 89. A. Telama, T. Mantyla and P. Kettunen, J. Vac. Sci. Technol. A, 4 (6) (1986) 2911. M. Tavi, J. Aromaa, 0. Forsen and S. Ylasaari, 1~h World Congress on Surface Finishing, Paris, France, October 1988, AITE, France, 1988, p. 955. U. K. Wiiala, I. M. Penttinen, A. S. Korhonen, J. Aromaa and E. Ristolainen, Surf. Coat. Technol., to be published.
481
CORROSION PERFORMANCE OF LAYERED COATINGS PRODUCED BY PHYSICAL VAPOUR DEPOSITION M. J. PARK, A. LEYLAND and A. MA1~HEWS Research Centre in Surface Engineering, Department of Engineering Design and Manufacture, University of Hull, Hull HU6 7RX (tLK)
Abstract Potentiodynamic methods were used to investigate the relative corrosion protection provided on 304 stainless steel by various coating and pretreatment routes designed to isolate the substrate from the environment. These include combinations of plasma oxidizing, plasma nitriding and chromium, nickel and titanium nitride coatings produced by triode ion plating methods. It is shown that optimization of the layer structure and galvanic coupling are vital where a combination of wear and corrosion resistance is required. Nickel interlayers under TiN, for example, are shown to provide improved protection against corrosion. Attempts to increase corrosion resistance by producing a passive layer through a d.c. plasma processing stage on the substrate were not so successful, and the reasons for this are discussed.
1. Introduction Titanium nitride is now widely used as a thin protective coating, providing high surface hardness and good chemical stability. Its primary uses are therefore in applications requiring wear resistance [1—3],corrosion resistance [3] and/or diffusion barrier properties [4]. It has been reported that in certain corrosive environments, the service life of TiN coatings can be increased by reducing pinhole-type defects [5, 6]. When considering the corrosion protection, both galvanic and pitting corrosion mechanisms must be taken into account [7, 8]. TiN is an electrical conductor and electrochemically more noble than typical substrates (e.g. steels). Once substrates which are galvanically coupled with TiN are exposed to a corrosive atmosphere via pinholes, the exposed area will begin to dissolve anodically. Moreover, the small anodic area through the pinholes will cause a local increase in current density and thus accelerate the corrosion reaction at these points. Many attempts to reduce pinhole defects have been reported, through the improvement of TiN coating structures [5, 9—11] and the use of multilayer deposits produced by pulsing the reactive 0257-8972/901$3.50
© Elsevier Sequoia/Printed in The Netherlands
482
gas [12]. However, these approaches do not completely eliminate the corrosion problem. The intention of the work reported here was to isolate the substrate from the corrosive medium, rather than to modify the TiN structure. This was achieved by employing plasma diffusion pretreatments aimed at surface passivation, together with metallic coatings, both as single layers and as interlayers below TiN films. The relative corrosion performance was evaluated by potentiodynamic measurement techniques and scanning electron microscopy (SEM) studies before and after testing.
2. Experimental details Coating procedures The coatings employed in this work were deposited using a thermionically-enhanced plasma-assisted electron beam (EB) physical vapour deposition (PVD) system (Tecvac 1P35L). The plasma oxidizing and nitriding were also carried out using this equipment. The substrate material was stainless steel (AISI 304). Specimens were rectangular flat sheets (2 cm x 2 cm), 1 mm thick, ultrasonically degreased in freon and then washed in acetone. Substrates to be nitrided or oxidized were pretreated in a 10 mTorr enhanced triode discharge with a gas composition of 75% Ar and 25% H2 for 2 h at 2.1.
500 C. This was designed to aid the repeatability of the subsequent passivation treatments. Oxidizing was performed in a 10 mTorr mixture of 75% Ar and 25% 02. Nitriding was carried out at the same pressure with 75% N2 and 25% H2. In each case a triode discharge was employed, with the same power TABLE 1 Summary of treatments and coatings applied Specimen
Treatment conditions Sputter cleaning”
1. 2. 3. 4. 5. 6. 7. 8. 9. 10.
304 stainless steel substrate TiN on substrate TiN on Ni on substrate TiN on Cr on substrate TiN on nitrided substrate TiN on oxidized substrate TiN on Ni on oxidized substrate TiN on Cr on oxidized substrate Ni on substrate Cr on substrate
• •
“10 mTorr; 75% Ar + 25% H2 bias, 500 V; 500 ~C; 2 h. b10 mTorr; 75% N2 + 25% H2 bias, 500 V; 500 C; 2 h. ClO mTorr; 75% Ar + 25% 02; bias, 500 V; 500 ~C;2 h. d75 mTorr; bias, 120 V; 500 C.
Nitriding b
Oxidizing~
Coatingd
TiN TiN and TiN and TiN TiN TiN and TiN and Ni Cr
Ni Cr
Ni Cr
483
density and discharge parameters throughout, i.e. bias, 500 V; substrate temperature, 500 °C.The treatment duration was 2 h in each case. For all coatings (TiN, nickel and chromium), a substrate bias of 120 V was employed, with a background argon pressure of 7.5 mTorr. Again, the power densities were such that a deposition temperature of 500 ‘C was maintained. However, treatment times were varied to achieve the required coating thicknesses. Table 1 summarizes the coatings studied.
2.2. Potentiodynamic measurements A conventional three-electrode cell was used, the counterelectrode being a platinum sheet. The potential of the working electrode was measured by a saturated calomel electrode (SCE) through a solid salt bridge. A potentiostat (Thompson Ministat 402), sweep generator (Thompson Miniscan) and XY recorder (Servogor 210) were employed to obtain the potentio. dynamic polarization curves. The potential sweep rate was 15 mV min’. The electrolyte was an aerated 0.1 N H2S04 solution held at a temperature of 25 “C. The testpotentiodynamic specimens were measurement, masked with wax to expose a surfacewere areaset of 2. Before the working electrodes 1 cm—500 mV (SCE). to
3. Results 3.1. Potentiodynamic studies Figure 1 shows the anodic polarization curves of TiN-coated, metalinterlayered and uncoated 304 stainless steel. For the uncoated substrate there is an initial active region to about 0 mV (SCE) without any trace of an active peak, followed by a passive region up to 800 mV. Above 800 mV, a transpassive region develops in which the passivating film on the substrate is broken down. The most important aspects of anodic polarization curves in practical corrosion comparisons are the current density and voltage range of the passive region. The passivation current of the substrate varies from 3 x iO~mA to 102 mA. On TiN-coated stainless steel it is reduced to iO~mA and an improvement in corrosion resistance of TiN coatings can therefore be expected. The general pattern from the TiN-coated stainless steel is similar to that of the substrate, except for the “passive-like” behaviour at about 0 mV. Further improvement in corrosion resistance is evident in the TiN.coated/metal.interlayered stainless steel. Nickel- and chromium-interlayered TiN coatings show a slight reduction in passivation current compared with single-layered TiN. The passivation current ranges from 2 x iO~mA to 4 x 10~mA below 500 mV. There is typically an active peak, and the chromium-interlayered film shows a rapid increase in current over the transpassive region. The nickel-interlayered TiN coatings generally exhibit slightly better corrosion resistance (i.e. lower passivation current) than the chromium-interlayered TiN coatings.
484
2)
CURRENT DENSITY (mA/cm
/
0.1
0.01
/
*
1.000E-03
7
/~
1.000E-04
*
1.000E-05
*
-500
0
500 1000 1500 ELECTRODE POTENTIAL (mV,SCE)
2000
Fig. 1. Anodic polarization curves of TiN-coated, metal-interlayered and bulk 304 stainless steel: —*-—--, 304 stainless steel substrate; ~ TiN coating on the substrate; —*—, chromium-interlayered TiN coating on the substrate; —~—, nickel-interlayered TiN coating on the substrate.
CURRENT DENSITY (mA/cm2)
//
0.-i
*
0.01 *
1.000E-03
/
//
*
—~
*
1.000E-04=
1.000E-05[ -500
0
500
1000
1500
2000
ELECTRODE POTENTIAL (mV,SCE) Fig. 2. Anodic polarization curves of TiN coatings on untreated and passivation-treated 304 stainless steel: , 304 stainless steel substrate; —*—--, TiN coating on the substrate; —*—--, TiN coating on oxidized substrate; —~—, TiN coating on nitrided substrate.
485
Figure 2 shows the anodic polarization curves of TiN coatings on untreated and passivation-treated (oxidized and nitrided) substrates. The oxidized stainless steel coated with TiN has a passivation current of 3 x 10~mA. When compared with a single layer of TiN or untreated stainless steel (1 x 10 ~mA), the corrosion resistance is improved, but the passive region ends at about 300 mV. Above 600 mV the current density on the oxidized substrate coated with TiN is higher than that of the single-layer TiN-coated substrate. The nitrided substrate coated with TiN shows a sharp current increase in the initial active region, followed by strong passivation. However, in the practical potential range (below 1 V) it exhibits a higher current than even that of the untreated substrate. Further anodic polarization curves for TiN-coated, metal-interlayered and oxidized substrates are shown in Fig. 3. Nickel-interlayered TiN coatings on oxidized substrates show lower currents over the entire potential range (except for the active peak) than single TiN coatings on oxidized substrates. However, chromium.interlayered TiN coatings on oxidized substrates exhibit a high current active peak and a passivation current of about 2 x 10~mA. Again, there is a rapid current increase in the transpassive region (above 800 mV). This was found to be due to dissolution of Cr6 which produced a yellow colour in solution and led to spalling of the TiN coating. +
~CURRENT DENSITY (mA/cm2) 0.1
/
0.O1L
1.000E-03
1.000E-04~
/
/ I /~
l.OOOE~O 5Li±~_ -500
-~
0
500
1000
1500
2000
ELECTRODE POTENTIAL (mV,SCE) Fig. 3. Anodic polarization curves of TiN.coated, interlayered and oxidized 304 stainless steel: —*—, stainless steel substrate; —*—, TiN coating on oxidized substrate; —*—, chromiuminterlayered TiN coating on oxidized substrate; , nickel-interlayered TiN coating on oxidized substrate.
304
486 2)
CURRENT DENSITY (mA/cm 1~ -
-.._-____
/
0.1
0.01
1.000E-03~
*_
/
*
1.000E_04j
1.000 E -05 -500
*
*
j-----_____
0
---—-—
500
—-
--
- —--——-—
1000
-
-
1500
2000
ELECTRODE POTENTIAL (mV,SCE) Fig. 4. Anodic polarization curves of metal-coated and uncoated 304 stainless steel: stainless steel substrate; substrate.
—*—-,
nickel coating on the substrate;
—*—-,
, 304
chromium coating on the
Figure 4 shows the anodic polarization curves for chromium and nickel coatings compared with the uncoated substrate. Both of the metal-coated substrates exhibit typical ‘S’ curves and a strong active peak. As expected, the chromium coating gives the lowest passivation current due to the formation of a strong oxide film, but the passive region is short; the onset of transpassive behaviour starts at about 500 mV. Conversely, the nickel coating shows a much higher passivation current, but the passive region is quite wide and stable. 3.2. SEM studies Figure 5 shows fracture cross-sections through some of the coatings. Points to note are that the thicknesses of the TiN, chromium and nickel layers are similar, i.e. about 0.7 pm. In addition, all coatings exhibit dense columnar growth morphologies, with no apparent effect of pretreatments, such as nitriding, on the structure. Figure 6 shows typical surface appearances of the three coating types. The TiN and chromium coatings are somewhat similar, whilst the nickel coating has a more faceted appearance. Figure 6(d) reveals how the corrosion has preferentially attacked the nickel grain or column boundaries, whereas chromium and TiN are not affected in this way. The explanation for this is that TiN is relatively inert, and chromium forms a strong oxide film which provides protection and results in
487
(a)
(b)
(c)
Fig. 5. Scanning electron micrographs of cross-sections of coatings: (a) TiN coating on 304 stainless steel; (b) TiN coating and chromium interlayer coating on 304 stainless steel; (c) TiN coating on nitrided 304 stainless steel.
488
a a
~ a a
—
a
489
(a)
4.
(b) Fig. 7. Surface morphology of TiN coatings with pinhole defects (SEM): (a) as.deposited TiN coating; (b) TiN coating after potentiodynaniic test.
more uniform corrosion (except where there are defects in the inert layer, i.e. cracks or pinholes). Figure 7 shows typical pinholes in a TiN coating, before and after corrosion testing. The pinhole sizes vary, but are slightly larger after testing. This appears to confirm that a pitting corrosion mechanism is operating. Figure 8 shows the pitting pattern observed in a chromium-interlayered TiN coating, after potentiodynamic testing. In Fig. 8(a) the average diameter of the pits is about 120 p.m. The figure shows serious spalling of the TiN layer, as a consequence of chromium dissolution. Figure 8(b) shows the pit interior with traces of chromium residues on the substrate, with extensive intergranular attack and some pitting corrosion of the substrate.
490
(a)
(b~ Fig. 8. Surface morphology of TiN-coated and chromium-interlayered 304 stainless steel after potentiodynamic test (SEM): (a) pit pattern; (b) inside the pit.
4. Discussion Our potentiodynamic measurements have shown that interlayered TiN coatings can exhibit better corrosion resistance (i.e. reduced passivation currents in the voltage range of interest) than conventional single-layer TiN coatings. There are a number of possible explanations for the improvements observed. Firstly, the introduction of less refractory metallic interlayers such as chromium or nickel provides a dense intermediate coating layer under given deposition conditions, reducing the defects which can lead to corrosive attack. However, it should be noted that the increase in total coating thickness provided by the interlayer could, in itself, be enhancing the corrosion performance.
491
In view of the strong passivation behaviour demonstrated by the chromium coatings produced here (Fig. 4), it was expected that chromiuminterlayered TiN coatings would give better corrosion resistance than their nickel counterparts (see Figs. 1 and 3). It might be expected that any corrosion of the chromium layer would take place via general galvanic attack rather than locally around pinholes. However, the passivation region displayed by the chromium is comparatively narrow and unstable (Fig. 4), with a rapid rise in dissolution current at the onset of transpassive behaviour. In our potentiodynamic tests, this effect was manifested by a rapid dissolution of the chromium layer (Figs. 1(d), 3(d) and 4(c)). From Fig. 3 it can be seen that chromium-interlayered TiN coatings show a relatively high current over the entire potential range employed. This could result from strong galvanic coupling between the TiN and the chromium, caused by electrical isolation of the substrate by the oxide layer. Since the metal (chromium) interlayer has a lower standard electrode potential than that of the substrate, it will tend to reduce the corrosion resistance of the TiN overlayer. In addition, if the passivating film on the chromium interlayer breaks down (i.e. at the onset of the transpassive region), spallation of the TiN layer will occur (Fig. 8). The nitriding treatments employed in this work appeared to reduce the corrosion performance of the TiN coating (Fig. 2(d)) and, indeed, of the substrate. This result was unexpected; however, it may be that the nitrided layer itself contained similar defects to those observed in the coating (i.e. cracks and/or pinholes). The incorporation of a hydrogen preclean stage to remove oxide contamination on substrates (to improve treatment repeatability) may have accelerated corrosive attack through these defects by exposing an active surface. This could be accentuated by any tensile stresses produced in the treated substrate surface as a result of residual compressive stresses in the coating. Oxidizing treatments appeared to produce a slight increase in the corrosion protection afforded by the TiN layer (at least below a working potential of 600 mV). However, the oxide layer was unstable above 300 mV. This was again unexpected since the onset of transpassive behaviour in oxide films on 304 stainless steel is typically observed only above 800 mV. It may therefore be possible to improve the overall corrosion resistance further by modifying the oxidation treatment.
5. Conclusions In the coating systems discussed here, corrosion of the TiN layer itself was not a significant factor in the overall performance. As expected, corrosive attack via pinhole defects was almost without exception the primary mechanism for coating failure. However, it was demonstrated that corrosion resistance could be improved by metallic interlayers, reducing pinhole densities and modifying galvanic coupling effects. Nickel interlayers appeared to
492
provide the optimum protection in these respects. Plasma nitriding of the substrate prior to coating worsened the overall corrosion performance, whilst plasma oxidizing caused a slight improvement. However, these treatments have the potential, with further development, to provide more effective protection.
Acknowledgments M.J.P. acknowledges the support of an Overseas Research Studentship (ORS) award. The authors thank Dr. G. Thompson (University of Manchester Institute of Science and Technology) for the provision of potentiodynamic test equipment and E. Namgoong for help with the testing. The ongoing support of the Science and Engineering Research Council for the Research Centre in Surface Engineering is also acknowledged.
References 1 2 3 4 5 6 7 8 9 10 11 12
A. Matthews, Surf. Eng., 1 (2) (1985) 93. J. Vogel and E. Bergman, J. Vac. Sci. Technol. A, 4 (6) (1986) 2731. H. Jacobs, Surf. Coat. Technol., 29 (1986) 221. C. Y. Ting, J. Vac. Sci. Technol., 21 (1) (1982) 14. T. A. Mantyla, P. J. Helevirta, T. T. Lepisto and P. T. Siitonen, Thin Solid Films, 126 (1985) 275. Y. Massiani, J. Crousier, L. Fedrizzi, A. Calvaleri and P. L. Bonora, Surf. Coat. Technol., 33 (1989) 309. M. J. Pryor, in L. L. Shreir (ed), Corrosion, Vol. 1, Newnes—Butterworths, London, 2nd edn., 1976, p. 1192. L. L. Shrier, in L. L. Shreir (ed.), Corrosion, Vol. 1, Newnes—Butterworths, London, 2nd edn., 1976, p. 1150. A. Erdedemir, W. B. Carter, R. F. Hochman and E. I. Meletis, Mater. Sci. Eng., 69(1985) 89. A. Telama, T. Mantyla and P. Kettunen, J. Vac. Sci. Technol. A, 4 (6) (1986) 2911. M. Tavi, J. Aromaa, 0. Forsen and S. Ylasaari, 1~h World Congress on Surface Finishing, Paris, France, October 1988, AITE, France, 1988, p. 955. U. K. Wiiala, I. M. Penttinen, A. S. Korhonen, J. Aromaa and E. Ristolainen, Surf. Coat. Technol., to be published.