Experimental studies of the effect of Ti interlayers on the corrosion resistance of TiN PVD coatings by using electrochemical methods

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Experimental studies of the effect of Ti interlayers on the corrosion resistance of TiN PVD coatings by using electrochemical methods ⁎

J. Vega , H. Scheerer, G. Andersohn, M. Oechsner State Materials Testing Institute Darmstadt, Chair and Institute for Materials Technology Technische Universität Darmstadt, Grafenstrasse 2, 64283, Darmstadt, Germany

A R T I C L E I N F O

A B S T R A C T

Keywords: B. Electrochemical methods B. Polarization B. EIS A. Sputtered coatings C. Anodic dissolution

The effect of Ti interlayers on the corrosion resistance of TiN PVD coatings is investigated. The coatings were deposited using direct current magnetron sputtering (DCMS) and high power impulse magnetron sputtering (HIPIMS). Ti interlayers with different thicknesses but same composition and deposition parameters were studied. The barrier effect was investigated using potentiodynamic polarization tests, electrochemical impedance spectroscopy (EIS) and scanning electrochemical microscopy (SECM) together with a chemical porosity test. Remarkable improvement of the corrosion resistance with increased thickness of the Ti interlayers was found. The results showed a good agreement between potentiodynamic polarization tests, EIS, SECM and the microscopic inspection.

1. Introduction PVD coatings present advanced properties such as high hardness [1–3], high melting point, low friction coefficient, high wear resistance [4–9], chemical stability [10] and corrosion resistance [11–13]. Uses of this coatings can be found in the metal-working industry [14–17], biomedical applications [10,18], micro-electronics industry [19] and for decorative purposes [20]. The electrochemical behaviour of a monolayer coating of TiN on steel substrate is similar to that in a system with an inert but porous layer overlying and active corroding substrate [21]. Non-metallic coating materials such as TiN or CrN consist of metals that have their chemical reactivity satisfied by the formation of bonds with other reactive ions and they are chemically unreactive [21–26]. The corrosion resistance of TiN PVD coatings is mainly controlled by the amount of open porosity in the TiN layer. The columnar structure and defects such as voids, pinholes, pores, cracks and even delamination lead to open paths between the substrate and the corrosive environment [20–23,27–29]. The TiN layer due to its more noble electrochemical behaviour seems to be intact whereas the substrate is severely corroded [23–25,27]. In order to improve the corrosion resistance of TiN coatings, the creation of an effective barrier between the corrosive environment and the substrate is required. For this purpose, multilayer coatings and monolayers of TiN with reduced microdefects, obtained by means of different techniques and deposition parameters have been reported [22–25,30–36]. Many PVD coatings use interlayers between the

substrate and the top layer to increase the adhesion by improving the chemical similarity and minimizing the internal stresses on the interfaces [19–21,37–43]. An alternative approach to improving the corrosion resistance of TiN coatings is the use of metallic Ti interlayers. These interlayers increase the corrosion resistance by preventing the corrosive medium from reaching the substrate through defects in the TiN layer [35,37,44] The effect of the coatings on the corrosion resistance can be studied through different methods. Chemical porosity tests for example, are designed to attack the substrate revealing the corrosion occurring trough the pores in the coating [45]. Major drawbacks of these visual tests are the restricted reproducibility and difficulties in quantitatively interpreting the results [46]. An alternative approach to investigating the protective efficiency is the use of electrochemical methods. Electrochemical methods such as potentiodynamic polarization tests [26–29,35–37,47], electrochemical impedance spectroscopy (EIS) [34–36,47–50] and more recently the scanning electrochemical microscopy (SECM) have been reported [51–55]. In this study, the effect of the Ti interlayers on the corrosion resistance of TiN PVD coatings was systematically investigated. For this purpose, Ti interlayers with different thicknesses but same composition and deposition parameters were deposited. Furthermore, TiN layers with different structures and different thicknesses were analyzed. The Ti/TiN PVD coatings were studied by potentiodynamic polarization curves, EIS and a novel hybrid method using SECM together with a chemical porosity test.

⁎

Corresponding author. E-mail addresses: [email protected] (J. Vega), [email protected] (H. Scheerer), [email protected] (G. Andersohn), [email protected] (M. Oechsner). https://doi.org/10.1016/j.corsci.2018.01.010 Received 26 July 2017; Received in revised form 21 December 2017; Accepted 18 January 2018 0010-938X/ © 2018 Elsevier Ltd. All rights reserved.

Please cite this article as: Vega, J., Corrosion Science (2018), https://doi.org/10.1016/j.corsci.2018.01.010

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2. Materials and methods

Table 1 Deposition parameters.

2.1. Coating deposition and characterisation PVD TiN coatings were applied on round steel substrates of X153CrMoV12 with 13 mm in diameter using a CC800/9 coating system (CemeCon). The steel substrates were grinded with SiC abrasive paper up to grit 1200. Afterwards, the samples were polished using a diamond suspension with particle size of 6 μm, finishing with particle size of 3 μm. Prior to the coating process, the samples were cleaned using a multistage ultrasonic bath with n-heptane and ethanol for 15 min. Before the coatings were deposited, the surface of the samples was cleaned using sputtering etching with 650 V, 240 kHz, 1600 ns for 15 min. The total pressure was 350 mPa. The sputtering targets were Ti (> 99.8%). The coatings were obtained by using a target with high power impulse magnetron sputtering (HIPIMS) with 2000 W, 500 Hz, 200 ns and three targets with direct current magnetron sputtering (DCMS) with 2500 W on each. Interlayers of pure Ti were deposited with 400 V bias-voltage on all substrates. TiN layers were deposited by using bias-voltages of 75 V and 150 V. The thickness of the interlayer was 75 nm, 150 nm and 1 μm. The TiN layers had thicknesses of approximately 0.5 μm and 1 μm. The coatings were deposited at 450 °C with an Argon/Nitrogen atmosphere and total pressure of 350 mPa. For the comparison purpose, 10 different types of samples were fabricated. In these samples, the structure of the TiN layer, the thicknesses of both TiN layer and interlayer and their positions were controlled, Fig. 1. The proposed coatings are not intended to replicate the available coatings in industrial applications. The structure and thickness were selected specifically to evaluate and validate the results obtained through the electrochemical methods. Table 1 summarizes the deposition parameters. The cross-section morphology and the surfaces of the coatings were observed with a high resolution scanning electron microscope (SEM) JSM 7600F (Jeol). Additional cross-section characterizations were done by an automated optical microscope DM LA (Leica). The topography of the coatings was measured with a confocal microscope DCM 3D (Leica). The roughness (Ra, Rz) was characterized with a measuring system Hommel-Etamic T8000 (Jenoptic).

Coating Parameters

Interlayer (Ti)

Top layer TiN (Structure 1)

Top layer TiN (Structure 2)

Bias-Voltage (V) HIPIMS-Frequency (Hz) HIPIMS-Pulse time (μs) Thickness (μm) HIPIMS-Power (W) DCMS-Power (W) Pressure (mPa) Temperature (°C) Ar:N2 Ratio

400

75 500 200

150

0.075–0.150–1

1:0

0.5–1 2000 (1 Target) 2500 (3 Targets) 350 450 24:5

and working surface were kept identical as already described for the potentiodynamic polarization tests. All measurements were done after one hour of stabilization at OCP. The frequencies were swept between 100 kHz and 10 mHz. The AC voltage was 10 mV (rms). 2.4. Scanning electrochemical microscopy (SECM) SECM measurements were done in a four electrode electrochemical cell. A M370 scanning electrochemical workstation (Uniscan Instruments) was used. An Ag/AgCl electrode (KCl saturated) was used as reference electrode, a platinum mesh as counter electrode, a 15 μm platinum microelectrode as probe (Pt-microelectrode) and the samples as working electrodes. The electrochemical cell consists of a glass cylinder with 7 cm in diameter, with a base of polytetrafluoroethylene (PTFE). The samples were placed inside the PTFE base and its surface was covered with an adhesive foil with thickness of 20 μm. In the center, a perforation of 5.5 mm in diameter exposed the surface of the coating, Fig. 2. The adhesive foil was used to prevent leaks through the space between sample and the PTFE base and to reduce the edge effect. The test solution was 5 mM potassium hexacyanoferrate(III) (K3[Fe (CN)6]) and 100 mM potassium chloride (KCl) with pH 7.2 at room temperature. The solution can be used as indicator for iron. It can also be used for SECM measurements by using the redox wave [Fe(CN)6]3- /[Fe(CN)6]4- , Eq. (1) [56].

2.2. Potentiodynamic polarization tests

[Fe(CN)6)]3- + e- → [Fe(CN)6)]4-

(1)

Potentiodynamic polarization curves in 3% NaCl at room temperature were obtained by using a potentiostat/galvanostat reference 600 (Gamry instruments). The measurements were done after one hour of stabilization at the open circuit potential (OCP). A three electrode electrochemical cell was used. The reference electrode was Ag/AgCl (KCl saturated), the counter electrode was a platinum mesh. All samples had a working surface of 1.038 cm2. They were polarized −250 mV in cathodic direction and 1500 mV in anodic direction related to the OCP. The scan rate was 1000 mV/hour.

Fe2++[Fe(CN)6)]3- ↔ Fe3+ [Fe(CN)6)]4-

(3)

Fe2++[Fe(CN)6)]3- +K+ → KFe[Fe(CN)6)]

(4)

2.3. Electrochemical impedance spectroscopy (EIS)

Fe3++[Fe(CN)6)]4- +K+ → KFe[Fe(CN)6)]

(5)

The test solution will dissolve in water according to Eq. (2).

K3Fe(CN)6 +KCl → [Fe(CN)6)]3- +4K++Cl-

(2)

In presence of iron ions, the solution reacts according to Eqs. (3)–(5) to form “soluble Prussian blue” Eqs. (4) and (5) [57–60].

With excess of iron ions, the “insoluble Prussian blue” will be

The three electrode electrochemical cell, test solution, temperature

Fig. 1. Designation of the coatings and related information.

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Fig. 3. SEM images of cross-section morphologies. a) Coating 2 (structure 1). b) Coating 7 (structure 2).

Fig. 2. Electrochemical cell: 1) Pt-microelectrode (tip: 15 μm). 2) Glass body. 3) Electrolyte 4) Counter electrode (Platinum mesh). 5) Sample. 6) PTFE base. 7) Reference electrode Ag/AgCl (KCl sat.). 8) Sample (measuring surface). 9) Adhesive (20 μm thick). 10) Camera-Microscope.

The bigger grain size and the presence of peaks in the TiN layer of the coating 2 suggest a rougher surface compared to the coating 7. The roughness of the coatings 2 and 7 coincide with this observations, however, no clear correlation between the structures or the thicknesses is observed on the other coatings, Fig. 5 and Table 2. The coatings 2 and 5 with structure 1 present the highest mean Rz and Ra roughness. The mean Rz roughness is smaller than 150 nm. In the same way, the mean Ra roughness is smaller than 13 nm. According to the characterisation of the coatings, two different TiN layers were obtained. The defect densities on both structures are comparable. All samples present a smooth surface in terms of roughness. The surface morphologies and the cross-sections of the samples exhibit an open structure of the TiN layer with structure 1 and a denser TiN layer with the structure 2 (coatings 2 and 7 respectively). This higher density was expected due to the increased atom mobility and surface diffusion that is associated with a higher bias-voltage [47,62]. The TiN layers were designed for comparison purposes to study the effect of the Ti interlayers on the corrosion resistance of coatings with different permeability.

formed, Eq. (6) [61].

4Fe3++3[Fe(CN)6)]4- → Fe4 [Fe(CN)6]3

(6)

When the Pt-microelectrode is polarized in the positions where Prussian blue is formed, the redox reaction described in Eq. (1) will not happen, because no free ions of [Fe(CN)6)]3- are available for the reaction. Due to the fact that neither the Ti interlayer nor the TiN layer participate in Eqs. (3)–(6), only the iron ions from the substrate are responsible for the changes in reduction currents. Hence the interaction between the substrate and the solution through the coating can be evaluated. The Pt-microelectrode was placed 5 μm above the adhesive foil. It was polarized at −250 mV for 5 min prior to every measurement to stabilize the electrochemical reaction. It remained polarized until the measurement was completed. The scan velocity was 1000 μm/s. The reduction current that is related to Eq. (1) was measured in X-direction every 50 μm and 100 μm in Y-direction. The samples were not polarized, they remained at OCP. All potential values presented in this document are related to the reference electrode Ag/AgCl (KCl saturated).

3.2. Potentiodynamic polarization tests The potentiodynamic polarization curve of the reference (uncoated) shows a steep anodic slope at the beginning of the anodic polarization (−543 to −300 mV), Fig. 6. The sample is active and is anodic dissolved. The slope of the anodic region is less steep at potentials higher than −300 mV and nearly the same until the end of the polarization. The behaviour can be related to a lower oxygen diffusion due to the oxide layer on the surface [25]. The coatings with a thin interlayer present a slightly more positive (noble) OCP compared with the reference (−426 mV for the coating 1 and −375 mV for the coating 2). Both coatings present a less pronounced current density in the anodic region compared with the reference, indicating that the substrate is being partially protected by the coating. Due to the increased physical barrier between the substrate and the solution, the coating 2 with 1 μm TiN presents lower current density compared with the coating 1 with 0.5 μm TiN. The displacement of the OCP to nobler potentials is observed with the increased thickness of the Ti interlayer from 150 nm to 1 μm (−225 mV for the coating 3 and −185 mV for the coating 4). The current densities in these cases are quite lower compared with the reference and the coatings 1 and 2. They indicate an improvement on the corrosion resistance with the thick Ti interlayer. The curves of the coatings 3 and 4 exhibit a different behaviour compared with the coatings with thin interlayer. The anodic slope is less steep, indicating a reduced anodic dissolution. The concavity is different, due to the high density and increased thickness of the Ti interlayer, also its ability to

3. Results 3.1. Characterisation of the coatings In the SEM images it is possible to identify the substrate at the lower part, the Ti interlayer on the substrate and the TiN layer on the top of the Ti interlayer, Fig. 3. The TiN layer in the coating 2 is characterized by the presence of voids inside the structure, Fig. 3a). This voids are especially visible at the top of the TiN layer (white ovals). The open structure suggests a higher permeability of the coating compared with the TiN layer of the coating 7, Fig. 3b). In this case, the coating 7 presents a denser structure and no voids inside the coating are visible. The observed amount of defects and their distribution is comparable on the surfaces of the coatings 2 and 7, Fig. 4a) and e). The dark blue points represent defects that are deeper than the surface of the coating. The red-white points represent defects on the surface. Nodules, pinholes and craters are visible, Fig. 4c), d), g) and h). The TiN layer of the coating 2 presents bigger grain size and more open structure compared with the TiN layer of the coating 7. Voids between the grains are also recognizable in the coating 2, Fig. 4c) and d). The surface of the TiN layer of the coating 7 do not present an open structure, Fig. 4g) and h). In this case, the microstructure is denser compared with the TiN layer in the coating 2 and no voids between the grains are recognizable. 3

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Fig. 4. Surface of the coatings 2 (a, b, c, d) and 7 (e, f, g, h). a, e) Topography. b, f) Global view of the surface. c, g) Nodular defects. d) Pin-hole. h) Crater.

Table 2 Roughness Ra, Rz. Coating 1 2 3 4 5 6 7 8 9 10

Fig. 5. Roughness Ra, Rz.

Ra (μm)

Rz (μm)

5.1E-3 ± 2.5E-4 13E-3 ± 3.21E-3 5.89E-3 ± 3.5E-4 6E-3 ± 4.5E-4 12.6E-3 ± 2.7E-3 3.7E-3 ± 2.5E-4 7.2E-3 ± 8.6E-4 7.5E-3 ± 3.3E-4 5.62E-3 ± 4.6E-4 4.8E-3 ± 5.8E-4

32.7E-3 ± 2.2E-3 147E-3 ± 33E-3 33.6E-3 ± 2E-3 33.5E-3 ± 1.9E-3 97.6E-3 ± 29.6E-3 23E-3 ± 2.1E-3 69.8E-3 ± 5.1E-3 49.2E-3 ± 3.8E-3 42.1E-3 ± 4.3E-3 40.8E-3 ± 7.8E-3

sudden increase and fast decrease in the current density. This behaviour could be associated with the anodic dissolution of the substrate in the corrosion pits or the presence of local corrosion phenomenon in the interlayer, due to partial destruction of the passive layer. At higher polarization values, exceeding 700 mV from OCP in anodic direction,

passivate [63], minimizing the ion exchange with the solution. The protective effect can be seen even with a broad polarization of about 600 mV from OCP in the anodic region. The curves present some 4

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due to the corrosive process underneath the coating. This theory will be discussed later in this section. The coatings 8 and 9 with thick interlayer present a remarkable reduction of the current density and displacement to nobler OCP compared with the reference and the coatings 6 and 7, Fig. 7. The concavity of the curves indicate a different corrosion behaviour compared with the reference and the coatings with thin Ti interlayer. The curves present smaller slopes than those of the coatings with thin interlayer and the reference. This behaviour evidences a decreased anodic dissolution due to the passivation of the interlayer, indicating a good coverage of the substrate and a reduced open porosity. The current densities are smaller than those obtained with the same interlayer but TiN layer with structure 1. It indicates better protection with the denser TiN layer. The coating 9 presents a reduced current density since the beginning of the anodic polarization until approximately 1000 mV compared with the coating 8. After this point the current densities remain almost the same for both coatings. The difference could be explained due to a slightly decreased amount of trough-thickness defects with the thick TiN layer. The current density potentiodynamic curves of coatings with structure 1 and 2 (samples 5 and 10) present a similar behaviour and in comparison with the reference, Fig. 8. Due to the design of the coatings, comparable current densities were expected. The reduced current densities of the coatings compared to the reference, especially in the region between the OCP until about 100 mV, indicate that the corrosion processes are similar but the reaction is hindered because the coating decreases the reaction surface. The protective effect is improved by the presence of the thin but dense Ti interlayer. The surfaces of the samples after the measurements are similar to those of the coatings 6 and 7. Big areas of the substrate are exposed due to the destruction of the coating and the current densities are comparable with the reference. It is possible to have comparable current densities even with the coating partially intact because of the increased reaction surface of the substrate, Fig. 9. As already discussed, the current densities corresponding to the coatings 6 and 7 are unexpectedly high. The current densities do not reflect the expected trend according to the coatings structures and thicknesses. The coatings 1 and 2 with a less dense microstructure, present lower current densities than the denser coatings 6 and 7. Due to the bias voltage of 150 V used to obtain the denser TiN structure, a higher adatom mobility was expected. Hence, the defect density should not be higher than those of the coatings with less dense structure. As already showed, the defect density on the coatings 2 and 7 with different TiN structures is comparable, Fig. 2. Thus another hypothesis is required to explain the poor corrosion protection of the coatings 6 and 7. A second hypothesis to explain the high current densities of the

Fig. 6. Left: Current density potentiodynamic curves of coatings with TiN layer structure 1 and reference (black line). Right: Sample after test. Test parameters: 3% NaCl, room temperature, scan velocity: 1 V/h, polarization: −0.25 V/ + 1.5 V vs Ag/AgCl (KCl Sat.).

the coatings 3 and 4 exhibit an increasing and stable current density. The increased thickness of the TiN layer decreases the current density but the corrosion behaviour remains similar. Similar corrosion behaviours and lower current density with the thicker TiN layer indicates that the thicker coating is increasing the electrochemical diffusion layer thickness [47,50]. This behaviour is observed until the end of the polarization. The stable trend indicates a progressive anodic dissolution of the substrate in the corrosion pits. The effect of the increased thickness of the top layer is not easily recognizable in the coatings 6 and 7 with TiN layer with structure 2 and thin Ti interlayer. Both coatings exhibit almost the same corrosive behaviour and the current densities are comparable. The current densities show two recognizable zones in the anodic region, Fig. 7. The first zone starts at the OCP, about −505 mV and −492 mV for the coating 6 and for the coating 7 respectively. In this zone a high anodic dissolution is observed in potentials close to the OCP. With increasing anodic polarization the behaviour of the current density changes. The slope decreases, probably due to clogging of the corrosion pits with corrosion products [47,50]. This behaviour is observed up to a point at about −150 mV. After this point is reached, the form of the curves changes drastically. The current density increases quickly and reaches values close to those of the reference. The two regions can be explained by a quick corrosive attack on the substrate through the pores and defects in the coatings at the beginning of the anodic polarization. After this stage, the pores in the coating are probably partially clogged with corrosion products. This products hinder the anodic dissolution, protecting the substrate and the slope of the curve decreases. The sudden increase in the current density after −150 mV can be explained by the physical destruction of the coating

Fig. 7. Left: Current density potentiodynamic curves of coatings with TiN layer structure 2 and reference (black line). Right: Sample after test. Test parameters: 3% NaCl, room temperature, scan velocity: 1 V/h, polarization: −0.25 V/ + 1.5 V vs Ag/AgCl (KCl Sat.).

Fig. 8. Left: Current density potentiodynamic curves of coatings 5, 10 and reference (black line). Right: Sample after test. Test parameters: 3% NaCl, Room temperature, scan velocity: 1 V/h, polarization: −0.25 V/ + 1.5 V vs Ag/AgCl (KCl Sat.)

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Fig. 9. Cross-section of the coating 10 after potentiodynamic polarization.

coatings 6 and 7 is related to their mechanical stability due to the internal stresses. This coatings with a denser TiN layer, should have higher compressive stresses than the coatings 1 and 2 because of the more energetic ion bombardment during the deposition process [64,65]. Thus the mechanical stability of the dense TiN layer should be lower, especially if the substrate is dissolved. In order to study this hypothesis, the cross-section morphologies of the coatings 2 and 7 after the current density polarization measurements were observed, Fig. 10. According to the micrographs, the electrolyte is able to penetrate through both coatings dissolving the substrate, Fig. 10a), c). The anodic dissolution of the substrate occurs whereas both the TiN layer and the Ti interlayer remain intact, Fig. 10b). The dissolution of the substrate is preferential due to the formation of a galvanic cell between the substrate and the coating when the electrolyte reaches the substrate. Many authors suggest that this penetration is possible due to the open structure of the TiN layer and specially trough pin-hole defects [21–25]. The coating 2 with less dense TiN layer is able to maintain his integrity without mechanical support of the substrate, Fig. 10a) and b). The denser TiN layer of the coating 7 presents buckling even with less extended corrosion of the substrate than the coating 2, Fig. 10c). After the buckling, the coating collapses leaving big areas of the substrate

Fig. 11. Bode diagrams of coatings with structure 1. Test parameters: 3% NaCl, room temperature, AC voltage: 10 mV rms, frequency sweep: 10 mHz–100 kHz, immersion time: 1 h.

exposed to the electrolyte, Fig. 10d). According to the images of the samples after the potentiodynamic polarization, the described phenomenon does not occur in coatings with thick Ti interlayer (coatings 3, Fig. 10. Cross-section of the coatings 2 (a, b) and coating 7 (c, d) after polarization measurements.

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4, 8 and 9) due to the minimized interaction between electrolyte and substrate. The Ti interlayers also allow extensive plastic deformation improving the fracture resistance [32,43]. The results of the potentiodynamic polarization tests and observations indicate that the anodic dissolution of the substrate and the mechanical stress in the coating is needed in order to collapse the coatings with dense TiN structure. 3.3. Electrochemical impedance spectroscopy (EIS) Two different behaviours of the coatings with TiN layer with structure 1 depending on the thickness of the interlayer are recognizable in the Bode diagrams, Fig. 11. At high frequencies all coatings present similar impedance values related to the resistance of the solution. At middle and low frequencies the curves have a linear region dependent on the frequency. This region represents a mostly capacitive behaviour at the coating-electrolyte interface. It is related to passivation of the sample [66]. At lower frequencies the curves with thin Ti interlayer (coatings 1 and 2) present a non-linear behaviour. This is a sign of a poor corrosion protection, normally associated with permeability of the coating through pin-holes and imperfections [34], Fig. 11a). This behaviour can be also observed on the phase angle, Fig. 11b). The curves of the coatings with thin Ti interlayers present a shift in the phase at low frequencies, indicating a formation of the substrate-electrolyte interface [63]. Due to the short immersion time (1 h), this second interface cannot be attributed to the anodic dissolution of the interlayer. The creation of this interface is just possible because of the presence of trough-thickness defects. In the case of coatings with thick interlayers (coatings 3 and 4) the shift of the phase is lower, indicating a better corrosion protection. However, the shift at the lower frequencies indicates that the corrosion protection of the substrate is not perfect. The improvement of the corrosion protection is achieved because the Ti interlayer minimizes the trough-thickness defects. It has been also proposed that the Ti interlayers may prevent corrosion by forming a protective Ti oxide at the top of the defects [34]. Bode diagrams of coatings with structure 2 and thin interlayer (coatings 6 and 7) present two regions with different linear behaviour at middle and low frequencies, Fig. 12a). The shift of the phase angle indicates the presence of two time constants, Fig. 12b). The first is formed at the coating-solution interface and the second at the substratecoating interface. The clear resolution of two time constants indicates high porosity and reduced protection. The coatings 6 and 7 exhibit even lower impedance values than the same coating with structure 1 (coatings 1 and 2). The mechanism to explain the poor corrosion protection of this coatings have been already discussed on the Section 3.2. It is related to corrosion processes underneath the coating, the substrate is dissolved and the coatings collapse due to the high compressive stress in the dense TiN layer. Due to the linear behaviour of the impedance even at low frequencies, the behaviour of the coatings with thick interlayer (8 and 9) can be described as predominantly capacitive Fig. 12a) and b). The capacitive behaviour as already described for the coatings 3 and 4, is related to the passivation of the sample. On the Bode diagram of impedance, no clear evidence of a second capacitive effect at low frequencies is observed, Fig. 12a). The coating system (interlayer + top layer) seems to be effectively minimizing the formation of the solutionsubstrate interface. Nevertheless, the phase angles present a small shift at low frequencies, Fig. 12b). It indicates a small penetration of electrolyte reaching the substrate. The behaviour of coatings with mixed structure presented in Fig. 13 is comparable to those of coatings with thin interlayer and structure 1 and 2. At middle frequencies a linear region is associated with a double layer at the coating-solution interface. At lower frequencies the decrease of the impedance indicates that the electrolyte is able to originate a second interface on the substrate. No clear evidence of another capacitive reactance is visible and it is not possible to distinguish

Fig. 12. Bode diagrams of coatings with structure 2. Test parameters: 3% NaCl, room temperature, AC voltage: 10 mV rms, frequency sweep: 10 mHz–100 kHz, immersion time: 1 h.

Fig. 13. Bode diagrams of coatings with mixed structure. Test parameters: 3% NaCl, room temperature, AC Voltage: 10 mV rms, frequency sweep: 10 mHz–100 kHz, immersion time: 1 h.

between the individual interlayers. The differences between the impedance values of coatings 5 and 10 could be attributed to the presence of a slightly different defect density on the structure of the coatings. The EIS measurements indicate a remarkable higher impedance of the coatings with a combination of dense TiN structure and thick Ti interlayer, Table 3. The high impedances of the coatings 8 and 9 indicate that if no delamination is present, the coatings with structure 2 7

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reduction current over the entire surface, Fig. 15 Right. Almost no differences between the coatings with thick interlayer are observed. High current values indicate low permeability of the coating and no electrolyte or just small amounts of it are reaching the substrate. The SECM measurements indicate that all the coatings with a thick Ti interlayer offer better barrier effect between the solution and the substrate compared with the coatings with thin Ti interlayer. The coating 9 presents a slight smaller current than the other coatings with thick interlayer, although a higher current was expected in this case, Fig. 13 Right d). This can be explained due to possible small differences in the distance between the Pt-microelectrode and the sample, because of the indirect positioning using the adhesive layer as reference. The coatings 5 and 10 with mixed structure present a similar behaviour to the coatings with thin interlayers, Fig. 16. Reduction currents close to 0 nA indicate the reaction of the substrate with the electrolyte. The localized reaction means that the distribution of defects and permeable areas are not completely uniform over the entire surface. In order to determine how the coating system reacts with the test solution in the SECM measurements, two samples with oppositional behaviour were analyzed, Fig. 17. Dissolution of the substrate is recognizable in the cross-section of the coating system 7, Fig. 17c). The localized dissolution indicates that just some preferential zones are highly permeable. It is possible due to imperfections, defects and the own permeability of the structure of the coating. The initial size of the defect is not possible to determine because of the extensive damage in the coating due to the formation of Prussian blue. The detection of small reduction currents where the coating is not damaged can be explained by two phenomena, Fig. 17a). The first is the nature of the insoluble Prussian blue which increases its volume due to the incorporation of water in the molecular structure. The second effect is due to the small distance between the Pt-microelectrode and the sample. If the insoluble Prussian blue is formed, the Pt-microelectrode will drag a certain amount of the compound over the surface, measuring small currents close to positions where the formation of Prussian blue is taking place. The cross section of the coating 9 presents no visible reaction on the substrate, Fig. 17d). An extensive scan was done, but no evidence was found. The coating and substrate appear to be intact after the SECM measurement. This result is an indication of a good barrier effect with the thick Ti interlayer.

Table 3 Impedance at 10 mHz. Test parameters: Room temperature, AC Voltage: 10 mV rms, frequency sweep: 10 mHz–100 kHz, immersion time: 1 h. Sample

Impedance at 10 mHz (ohm cm2)

1 2 3 4 5 6 7 8 9 10

1.761 3.960 6.278 19.830 1.974 975 1.309 262.100 271.500 3.040

and thick Ti interlayer offer the better barrier effect. This improvement can be attributed to a decreased diffusion coefficient as well as the increase in the diffusion layer thickness [47,50].

3.4. Scanning electrochemical microscopy (SECM) A cyclic polarization (3 cycles) of the Pt-microelectrode far away from the sample was done to obtain the redox wave [Fe(CN)6]3- / [Fe(CN)6]4- , Eq. (1). The distance ensures a hemispherical distribution of the ions at the tip of the Pt-microelectrode without effect of the surface of the samples. During the measurements, swelling is not expected and the surfaces are very smooth in terms of roughness, Fig. 5. and Table 2. These properties result in a constant distance between the Pt-Microelectrode and the sample. Hence the contribution of the topography of the surfaces to the SECM measurements is considered to have only a marginal impact compared to the reactions between the Pt-microelectrode and the sample. The Pt-microelectrode was polarized between −0.25 V and +0.65 V, Fig. 14. The cyclic polarization indicates a maximum of current with a polarization of −0.25 V corresponding to the described redox wave. This polarization was used for all SECM measurements. SECM measurements of coatings with thin interlayer and structures 1 and 2 do not present a constant value of reduction current, Fig. 15 Left. Currents between 0 nA and 11 nA are observed. Small currents are associated with porosities and defects in the coating. Iron ions from the substrate react with the solution according to Eqs. (3)–(6). The reduction currents close to 0 nA can be explained because no free ions of [Fe(CN)6)]3- are available for the reaction. Coatings with thick interlayers (3, 4, 8 and 9) present uniform

4. Discussion The TiN layer can be considered in most of the cases inert in terms of corrosion. Hence a perfect TiN layer should protect the substrate from corrosion, independent of its thickness. However, TiN coatings are in general characterized for the presence of defects an open microstructures. Thus the corrosion resistance of TiN PVD coatings is highly dependent on the porosity of the coating. Due to the localized corrosion of the substrate underneath the PVD coating, global electrochemical methods such as potentiodynamic polarization tests and EIS must be carefully used. In the case of potentiodynamic curves, the extrapolation of results to obtain corrosion rates using the corrosion current is not recommended. The corrosion rates are usually calculated by using the apparent reaction surface. As already discussed, this area do not correspond with the effective reaction area. The effective reaction area also depends on the exposition time and polarization of the sample. Moreover, the corrosion rates mostly assume uniform corrosion over the entire surface. The SECM measurements showed clearly the localized nature of the reactive areas. This areas are located in trough-thickness defects and they are no uniformly distributed over the entire surface. The potentiodynamic polarization curves indicated an improvement of the corrosion resistance by using thick Ti interlayers. The beneficial effects on the corrosion resistance of the thick TiN layers were also

Fig. 14. Cyclic polarization of the 15 μm Pt-microelectrode in 5 mM K3[Fe(CN)6] and 100 mM KCl. Sweep rate 50 mV/s. Sweep potential −0.25 V/+0.65 V vs Ag/AgCl (KCl Sat.).

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Fig. 15. Left: SECM measurements of coatings with thin Ti interlayer (1, 2, 6 and 7) Right: SECM measurements of coatings with thick Ti interlayer (3, 4, 8 and 9). Test parameters: 5 mM K3[Fe(CN)6] and 100 mM KCl, Pt-microelectrode: 15 μm, tip-potential: −0.25 V vs Ag/ AgCl (KCl Sat.).

Fig. 16. SECM measurements of coatings 5 and 10. Test parameters: 5 mM K3[Fe(CN)6] and 100 mM KCl, Pt-microelectrode: 15 μm, tippotential: −0.25 V vs Ag/AgCl (KCl Sat.).

penetration of the solution trough the coating reaching the substrate. The fast penetration cannot be explained by the anodic dissolution of the Ti interlayers or the TiN layer, due to their high corrosion resistance. As already presented in Section 3.2, both TiN layer and Ti interlayer remain intact while the substrate is dissolved. Hence the trough-thickness defects must be responsible for the paths between the substrate and the solution. This result was confirmed by SECM measurements. They showed the interaction between the substrate and the test solution after 5 min. This interaction was clearly detected in coatings with thin Ti interlayers, whereas the penetration of the solution in coatings with thick Ti interlayers was highly decreased. Both thin and thick Ti interlayers were obtain by the same deposition parameters. Thus the corrosion resistance of a defect free Ti layer should be the same independent of the thickness. As already described, that was not the case in this investigation. The corrosion resistance was considerably higher with thick Ti interlayers. The improvement of the corrosion with thick Ti interlayers can be explain through a combined mechanism. The mechanism consist of an increased diffusion layer thickness combined with a reduced diffusion coefficient. The diffusion layer thickness is directly related to the thickness of the Ti interlayer [47,50]. In this case,

observed. Nevertheless this effect was smaller compared with both the effect of the thick Ti interlayer and the effect of the structure of the TiN layer. This trend was clearly demonstrated with a combination of thick interlayer and both structures of the TiN layers. It was observed that the best corrosion protection can be obtained with a thick Ti interlayer and a thick and dense TiN layer. However, to achieve the corrosion resistance under long term exposures, the mechanical stability of the dense TiN layer under anodic dissolution of the substrate has to be guaranteed. For instance, the polarization curves and the EIS results showed unexpected poor corrosion protection for the coatings 2 and 7 with dense TiN layers. In this cases, the presence of trough-thickness defects lead to the formation of a galvanic cell with preferential dissolution of the substrate and unfavourable anode/cathode ratio. The localized anodic dissolution of the substrate creates deep voids below the coating. As a result, the denser TiN layers collapse due to their higher compressive stresses. Consequently bigger areas of the substrate are exposed to the corrosive environment and the corrosion resistance decreases drastically. The EIS measurements indicate the formation of two interfaces in most of the coatings after 1 h of exposition. This results indicate a fast 9

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Fig. 17. a, b) SECM measurements of the coatings 7 and 9. c, d) crosssection of the coatings 7 and 9 after SECM measurements.

References

the increased thickness hinder the mass transport between the substrate and the environment. The reduced diffusion coefficient is result of the reduction of the trough-thickness defects [47,50].

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5. Conclusions Electrochemical methods such as potentiodynamic polarization tests, EIS and SECM allowed the investigation of the corrosion resistance of TiN coatings with Ti interlayers. The SECM measurements show clearly the localized nature of the reactive areas on the substrate. This areas are located in trough-thickness defects and they are no uniformly distributed over the entire surface. If this type of defects are present, the solution quickly reaches the substrate. A galvanic cell between the substrate and the coating will be established, the substrate is preferentially dissolved and the corrosion resistance decreases drastically. The results indicated an improvement of the corrosion resistance by using thick Ti interlayers. The improvement can be explain through a combination of an increased diffusion layer and a reduced diffusion coefficient. The increased thickness of the TiN layer improves the corrosion resistance. However, the increased thickness of the Ti interlayer and the structure of the TiN layer had a bigger impact on the corrosion resistance. The best corrosion protection was obtained with a combination of thick Ti interlayer and thick and dense TiN layer. It was observed that the corrosion resistance not only depends on the density of the coating but also on its mechanical stability. In long term exposures, the mechanical stability of the TiN layer must be guaranteed even if the substrate underneath the coating is dissolved.

Acknowledgements The IGF project (18561N) of the Research Association of Plastics (FGK) was supported by the German Federation of Industrial Research Associations (AiF) under the program for the promotion of industrial joint research and development (IGF). The authors would like to thank the FGK for the opportunity to carry out the project as well as the AiFIGF for the financial support. A working group of the FGK accompanied the project. This working group deserves our gratitude for the great support.

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