A novel duplex PDMS/CrN coating with superior corrosion resistance for marine applications

Abstract

A new duplex PDMS/CrN coating was developed and the corrosion resistance of the system under seawater conditions was investigated. CrN coatings in four thicknesses were fabricated using a multi-arc ion plating system, and then the thin PDMS layers were covered on the coating surfaces by dip coating to fabricate the duplex PDMS/CrN coatings. Corrosion tests of the coatings were carried out using potentiodynamic polarization and electrochemical impedance spectroscopy in artificial seawater conditions. In a cross sectional image the PDMS layer was well adhered to the CrN layer. The corrosion current density for the duplex PDMS/CrN coating decreased by more than two orders of magnitude compared to the bare substrate, presenting the best corrosion protection efficiency. Furthermore, the impedance value was high to 10⁷ Ω cm², which improved by about three orders of magnitude compared to bare substrate. The excellent corrosion resistance of the duplex PDMS/CrN coating was attributed to the compact structure of the CrN coating combined with the good sealing barrier effect of the PDMS layer. Such duplex PDMS/CrN coating systems with superior corrosion resistance are considered as potential protective materials for mechanical assemblies for long-term service in marine applications.

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Introduction

The development of ocean exploitation required the mechanical assembly of marine equipment using high anti-corrosion materials for long-term operation. In particular for some crucial components in ocean-based equipment, such as pumps, gears and propellers, which directly face corrosion by seawater. Thus, it is imperative to enhance their corrosion performances for the reliable and safe operation of marine equipment.¹ Arc ion plating CrN coatings have been extensively known owing to their high hardness, chemical inertness, good wear resistance and superior oxidation resistance.^2–5 The mechanical and chemical durability of the CrN coating is excellent, while the intrinsic defects are detrimental. Pinholes, pores, columnar structures and/or porosities could extend through the coatings, providing direct routes for the solutions on the substrates and help the galvanic corrosion process. Multilayer, multi-component and thicken coating strategies were employed to inhibit such defects and obstruct the corrosive medium reach the substrate to improve the corrosion resistance of hard-coated parts.⁶ C/Cr multilayer coating exhibit high corrosion resistance by providing a barrier between substrate and corrosive medium to inhibit the diffusion of water to substrate.⁷ Cr/CrN multilayer coatings could inhibit columnar growth and compact the coatings to enhance the corrosion behaviors. Furthermore, the increase of the number of layers results in the greater corrosion resistances of the coated elements.^8–10 On the other hand, the additions of Zr, Si elements into CrN improved the densification and charge transfer resistances of coatings, resulting in the high anti-corrosion abilities.^11,12 In addition, thickness plays a significant role in the coating corrosion performance. It was reported that the corrosion properties of the TiN-coated samples were primarily determined by the synergetic effect of the packing factor and coating thickness. The former reflects the denseness of the coating, while the latter relates to the length of the diffusion path of the corrosive medium.¹³ And the packing factor is more effective than film thickness on the corrosion resistance of coatings. The thick multi-layer carbon coating owning high packing factor that exhibits better corrosion resistance compared with the simple monolayer coating.¹⁴ The high thickness coating would be more effective to safeguard against the corrosion attack on condition that the packing factor was sufficient high.¹⁵ The corrosion resistances of cerium oxide based coatings improve as the thickness increases from 100 nm to 900 nm, which decreases for 1400 nm thick coatings due to the increase of coating defects and porosity.¹⁶ Moreover, atomic layer deposition technique has been adopted to enhance the corrosion resistance of CrN coating by inserting a Al₂O₃ and/or Ta₂O₅ nanolaminate, which acts as a good sealing layer to inhibit charge transfer, dislocation motion and diffusion of corrosive medium.^17,18 Great achievements have been gained concerning the improvement of coatings corrosion performances. Nevertheless, the results are not always satisfactory since the microstructures and properties vary according to the deposition process. Thus, it is imperative to develop more simple and effective methods to seal or diminish defects to improve corrosion resistances of coatings.

A more recent, organic and inorganic sealants were used to close the high porosity of thermal-sprayed coatings to enhance the corrosion resistances.¹⁹ The high open porosity of plasma-sprayed ceramic coating is deleterious to their corrosion behavior in aggressive conditions. The sealing treatments by phenol and epoxy sealants could effectively enhance the corrosion resistance of plasma-sprayed alumina + titania coatings by decreasing the interconnected porosity.²⁰ When sealed by aluminum phosphates, the plasma-prayed Al₂O₃ and Cr₂O₃ coatings possessed good corrosion and abrasion wear resistances in both acidic and alkaline solutions.²¹ Furthermore, low surface energy poly(dimethylsiloxane) (PDMS) has been developed and used as a barrier layer to improve the corrosion resistance.²² Owing to the hydrophobicity, thermal stability, adhesivity and chemical inertness, PDMS is widely used in corrosion resistance.²² Sealing hard coatings by depositing a PDMS top layer to block pinholes and other defects has exhibited a potential to be a promising approach to improve the protection of hard coatings from corrosive attack.

In this study, a novel duplex approach combined hard CrN coating with PDMS layer was put forward to improve the drawbacks associated with current hard coating techniques. It is expected that CrN coating could provide superior mechanical property, with PDMS layer growing on and into coating defects to effectively seal them. CrN coatings with variable thickness were fabricated by multi-arc ion plating systems, followed by depositing a PDMS layer using dip coating. The corrosion behaviors of duplex PDMS/CrN coatings in artificial seawater environments were also investigated.

Experimental details

Coatings fabrication

CrN coatings with variable thicknesses were deposited on 316L steel substrates for corrosion tests and silicon wafers for microscopic observations by a multi-arc ion plating system (Hauzer Flexicoat 850). The 316 L samples with dimensions of 30 × 20 × 2 mm were ground and polished to a surface roughness of around 60 nm by a metallographic polishing machine. All the samples were ultrasonically cleaned in acetone and ethanol in succession and then were mounted 10 cm in front of the targets. The targets can focus on the samples by rotating the sample holder. When the chamber pressure was pumped down to 1 × 10⁻³ Pa by a turbomolecular pumping system, the samples were cleaned for 2 min by Ar+ bombardment with currents of 60 A and substrate bias voltage of −900 V, −1100 V and −1200 V, respectively, to remove the contaminants on the surfaces. The solid targets of Φ 63 × 32 mm (purity > 99.5 wt%) with current of 65 A were employed as the source of chromium (Cr). The Cr layer process was carried out in Ar atmosphere of 350 sccm, bias of −20 V for 10 min to improve the adhesion of the CrN coating. Then the CrN coatings were deposited at a bias voltage of −20 V, N₂ gas flow rate of 600 sccm. The pressure was controlled at around 0.6 Pa during the fabrication process. The deposition time of CrN layer was varied from 1.5 h to 6 h (1.5 h, 3 h, 4.5 h and 6 h) to achieve the different thicknesses coatings, which were labeled as T1, T2, T3 and T4 for convenience. The sample holder was clockwise rotated at 3 rpm and the temperature was controlled around 400 °C during fabrication process. In order to investigate the influence of interlayer texture on coatings, a thin Cr layer with the same parameters was fabricated.

Subsequently, the as-prepared coatings were dipped into a PDMS solution, which consisted of 3 g PDMS, 0.3 g curing agent and 100 mL ethylacetate. Both of PDMS and curing agent were purchased from Dow Corning. Upon completion, the samples were lifted from the solution and cured at 80 °C for 6 h. The duplex treatment coatings (PDMS/CrN coatings), which indicated the four CrN coatings with the PDMS modifications, were correspondingly simplified as D1, D2, D3 and D4, respectively.

Coatings characterization

The coatings phases were investigated by X-ray diffraction (Bruker D8 X-ray facility) with monochromatic Cu Kα (λ = 0.154 nm) radiation operated at 40 kV and 40 mA. The scan angle (2θ) was ranged from 20–90° at a scanning speed of 4° min⁻¹ with a 0.02° step size. Scanning electron microscope (SEM, Zeiss EV018) coupled with energy dispersive spectroscopy (EDS) was used to observe the surface, coatings thicknesses and cross-sectional morphologies information of the coatings. The surface morphologies of selected samples were also investigated using a confocal laser scanning microscope (CLSM). The surface composition of the duplex coating was examined by X-ray photoelectron spectroscopy (XPS, AXIS-ULTRA, Kratos) using an Al (mono) Kα X-ray source.

Investigations of the electrochemical corrosion of the deposited coatings were made in the artificial seawater solution in reference to the platinum electrode and to the saturated calomel electrode (SCE). The artificial seawater was prepared according to the standard ASTM D 1141-98, and the chemical composition was listed in Table 1. The tests were carried out on the potentiostat/galvanostat device (Modulab, Solartron Analytical) at room temperature. The polarization measurements were performed in the range of −1 V to 0.6 V at the scanning rate of 2 mV s⁻¹ after 30 min immersion in the seawater solutions. Corrosion potential and corrosion current density were achieved by means of Tafel's extrapolation. Electrochemical impedance spectroscopy (EIS) tests were conducted at open circuit potential by a frequency response analyzer. The spectrum was recorded in the frequency range of 10 mHz to 100 kHz. Impedance plot interpretation was based on the equivalent circuit obtained by a fitting procedure. The corrosion morphologies after the corrosion tests were examined by SEM.

Table 1 The chemical compositions of seawater

Constituent	Concentration (g L^−1)
NaCl	24.53
MgCl2	5.20
Na2SO4	4.09
CaCl2	1.16
KCl	0.695
NaHCO3	0.201
KBr	0.101
H3BO3	0.027
SrCl2	0.025
NaF	0.003

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Results and discussions

Structures and morphologies characterization

The X-ray diffraction spectra of four coatings and the Cr interlayer are shown in Fig. 1. It can be seen that only the CrN phase exists in the coatings, implying the phases in the coating did not alter with the coating thickness. The main phases in the four coatings have a face-centered cubic structure, with diffraction peaks of the (200) and (111) planes being identified. The reflection peak of (111) orientation became less obvious with the increase of coating thickness, implying the volume fraction of (111) orientation diminish. The integrated peak intensity ratio I₁₁₁/I₂₀₀ of the randomly oriented CrN powder is 0.8. While for the as-deposited coatings, the intensity ratio I₁₁₁/I₂₀₀ for T1, T2, T3 and T4 sample is 0.51, 0.59, 0.42 and 0.35, respectively. It indicated that four coatings all presented (200) preferred orientation. The (111) and (200) peaks are the dominant features in the patterns. The crystallography texture coefficient (T_c) of the (200) plane respecting to the (111) plane for the as-deposition coating is defined by the following eqn (1):

T_c = I₂₀₀/(I₂₀₀ + I₁₁₁) (1)

Fig. 1 XRD patterns of as-deposited coatings (a) and Cr layer and 316 substrate (b).

The integrated intensity (I) is given by the area under the curve of the diffraction peaks. The texture coefficient values for T1, T2, T3 and T4 samples are 0.66, 0.63, 0.70 and 0.74, respectively. It indicates the four coatings in different thicknesses are all highly textured in (200) orientation. From Fig. 1(b), it is seen that the Cr interlayer was highly textured in (200) orientation compared to the bare substrate. The dominant (200) texture of Cr layer may be derived from both the elevated temperature and high ion bombardment during the fabrication process.²³ The highly textured interlayer between substrate and the growing coating exerts a strong influence on the texture evolution of transition-metal nitride coating.^23,24 And the highly oriented Cr (200) layer would act as a crystallographic template and induce the CrN grains to develop in the (200) direction owing to the atoms requiring a low activation energy compared to a high energy barrier for the (111) plane.

Fig. 2 presents the XPS survey spectra of T4 and D4 coatings on Si substrates. For T4 coating, the mainly compositions are Cr, N, O and C elements. The C and O elements may be derived from the contamination carbon and the atmosphere, respectively. While for the duplex coating D4, besides the C and O elements, Si peaks appeared in the spectrum. The analysis depth for the XPS may be too thin to examine the compositions of CrN layer and Cr and N elements are absent in the survey spectrum of D4 coating. As the analysis depth can not reach the substrate, the obtained Si element can be assigned to the top PDMS layer, which confirmed the PDMS layer successfully covered on the CrN coating.

Fig. 2 XPS survey spectra of T4 and D4 coatings on Si substrates.

Fig. 3 shows the surface morphologies of CrN coatings and the duplex PDMS/CrN coatings. Macro-particles and pores are shown in the T1 coating (Fig. 3 (a)). With the increase of coating thickness, seeing Fig. 3(b), the amount and sizes of macro-particles sharply increased due to coalescence. The macro-particles were derived from droplets emitted from the arc spots in the chromium target and mainly rich in Cr composition. It is reasonable to understand that the macro-particles would be increased and reunited to large sizes for thick coatings due to the increase of deposition time, high temperature and high energy bombardment during deposition process. Furthermore, the pores were hardly to find for the thick coatings likely account for the agglomeration of droplets blocked the pores. While for the duplex PDMS/CrN coating, a thin transparent layer can be seen on the coating surface. Moreover, the macro-particles were apparently covered and/or embedded by the PDMS layer. Fig. 4 presents the morphologies and surface roughnesses of T4 and D4 coatings obtained by CLSM. The surface of T4 coating is rough accompanied with large amounts of voids. While after covered with a PDMS layer, most of voids were sealed and the surface roughness decreased from 0.819 μm for T4 coating to 0.593 μm for D4 coating. The surface roughness values of others coatings obtained by CLSM were also listed in Table 2. It can be seen that the surface roughness increased as the increase of coatings thicknesses. However, the roughness values of duplex PDMS/CrN coatings were decreased compared to the corresponding single CrN coatings.

Fig. 3 Surfaces morphologies of as-deposited coatings: (a) T1 coating; (b) T4 coating; (c) D1 coating; (d) D4 coating.

Fig. 4 CLSM morphologies and surface roughness of T4 coating (a) and D4 coating (b).

Table 2 Surface roughness (R_a) values of different coatings obtained by CLSM

Samples	T1	T2	T3	T4	D1	D2	D3	D4
Ra (μm)	0.261	0.413	0.579	0.819	0.212	0.31	0.476	0.593

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The cross section morphologies of the CrN coatings and the selected duplex PDMS/CrN coating on Si substrates are shown in Fig. 5. As evident from Fig. 5(a–d), the CrN coatings demonstrated columnar structures and the coatings were strongly adhesive on the substrates. The total thicknesses for the four coatings are around 4.8 μm, 9.7 μm, 14.8 μm and 18.6 μm for T1, T2, T3 and T4 coatings, respectively. The four coatings were dense and compact. However, there are macro-droplets embedded within the coating and resulted in the crooked cracks towards the coating surface. The phenomena are distinctly apparent for the thick coating, because the long deposited time increased the likelihood for the existence of embedded macro-particles. These crooked micro-cracks resulted from the macro-particles, which were firstly bumped into the coating surface and cooled down rapidly.²⁵ The difference of expansive coefficient between macro-particle and CrN coating contributed to the generation of crooked micro-cracks. Moreover, the solidified macro-particles would shadow the adjacent regions did not form CrN crystallites, causing the formation of a voided boundary region. These cracks and voids, where the physical bonding to the coating matrix was poor, may be detrimental for the coating corrosion resistance. As Fig. 5(e) shown, there is a PDMS layer with thickness of approximately 2.5 μm well adhered on the CrN layer for the duplex coating (D4 coating). Fig. 5(f) was the EDS line scans of Cr, N and Si elements along depth. The element profile along with the scan direction was changed from Si to Cr enrichment, confirming the duplex PDMS/CrN coating consisted of top PDMS layer and subsequent CrN layer.

Fig. 5 Cross section morphologies of as-deposited coatings on Si substrates: (a) T1 coating; (b) T2 coating; (c) T3 coating; (d) T4 coating; (e) D4 coating; (f) EDS analysis of D4 coating.

Corrosion behaviors

Fig. 6 compares the polarization curves between the single CrN coatings and the duplex PDMS/CrN coatings in artificial seawater solutions. The bare 316L substrate was also used as a reference. The corrosion potentials (E_corr) and corrosion current densities (i_corr) obtained from the Tafel extrapolation are illustrated in Table 3. It can be seen that four coatings all exhibit higher corrosion resistance with noble corrosion potential and lower corrosion current density compared to bare substrate. However, the i_corr was not distinctly diminished as the increase of coating thickness. T3 and T4 presented more positive E_corr, yet the i_corr was slightly higher compared to T1 and T2 coatings. The slight higher corrosion rate was presumably ascribed to the coating defects such as voids and crooked cracks induced by droplets. It was also reported that the corrosion resistance of TiN coating with 30 μm decreased due to the increased number of defects compared to that with 12 and 18 μm.²⁵ At a smaller thickness, the drop-phase favors the porosity owing to a shrinkage at a following solidification.²⁶ The thin coating owns a high porosity causing an electrochemical heterogeneity of the surface contacting the electrolyte, which results in a lower value E_corr. While the piercing pores are no longer formed for a TiN coating thickness more than 8 μm.²⁶ The protective ability of high thickness coating is possibly determined not only by the thickness, but also by the high drop-phase contents. As for CrN coating, the critical thickness for piercing pores may be more than 10 μm. Since the drop-phase is rich in chromium and is resistant in the seawater medium, leads to a shifting of the corrosion potential towards greater values. Nevertheless, after PDMS modification, the i_corr of substrate/PDMS and duplex PDMS/CrN coatings all decreased, attaining up to one order lower magnitude compared to the bare substrate and single CrN coatings. In particular, the i_corr of D4 coating (3.3 × 10⁻⁹ A cm⁻²) decreased by more than two orders of magnitude compared to the bare substrate. Furthermore, passive behaviors were observed in the anodic potential range for the duplex PDMS/CrN coatings, which was assumed to the accumulation of substrate corrosion products at the bottom of some pinholes not perfectly closed by the PDMS layer.^18,27 The lower corrosion current density indicated the excellent anticorrosion performances of duplex coatings, especially for the D4 coating. The protection efficiency (η) was listed in Table 3 to present the protective capability of the coatings against corrosion. η can be calculated by the following eqn (2):where i_corr and i⁰_corr is the corrosion current density of the coated sample and bare substrate, respectively. It can be clearly seen that CrN coatings present high protection efficiency in spite of the slightly decrease of corrosion current density. Furthermore, by adding the PDMS layer, the protection efficiencies of duplex coatings are further increased. η increases to 99.7% for D4 sample, which indicates that the addition of the PDMS layer improved the corrosion resistance and the protective capability of the CrN coatings significantly.

Fig. 6 Potentiodynamic polarization curves for the deposited CrN coatings (a) and duplex PDMS/CrN coatings (b), and the bare substrate was used as a reference.

Table 3 Electrochemical parameters of the substrate and the coated samples obtained from potentiodynamic polarization curves in artificial seawater solution

Samples	Ecorr (V)	icorr (A cm^2)	η (%)
Substrate	−0.59	1.23 × 10^−6	—
T1	−0.45	4.6 × 10^−8	96.3
T2	−0.46	4.4 × 10^−8	96.4
T3	−0.37	2.15 × 10^−7	82.5
T4	−0.38	8.3 × 10^−8	93.3
Substrate/PDMS	−0.37	1.7 × 10^−7	86.2
D1	−0.32	3.2 × 10^−8	97.4
D2	−0.31	6.8 × 10^−9	99.4
D3	−0.37	6.87 × 10^−9	99.4
D4	−0.31	3.3 × 10^−9	99.7

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Electrochemical impedance spectroscopy was performed to gain additional insight into the electrochemical mechanism of the duplex PDMS/CrN coatings. Fig. 7 depicts the Bode plots (frequency vs. impedance values) for the CrN and the duplex coatings. The impedance modulus at low frequency (e.g. |Z|_{0.01 Hz}) was usually used to represent the anti-corrosion abilities of coatings.^22,28 The higher values of |Z|_{0.01 Hz} is, the lower corrosion rate is, resulting in the better anti-corrosion performance of the coating. The impedance values at 0.01 Hz for four CrN coatings were in the 10⁵ Ω cm² orders of magnitude, slightly higher than that of the bare substrate. While for the duplex PDMS/CrN coatings, the values sharply increased to 10⁷ Ω cm² orders of magnitude, improved more than two orders of magnitude. D4 coating presents the highest impedance values (8.6 × 10⁷ Ω cm²) at 0.01 Hz among the coatings, implying the lowest corrosion rate and the best corrosion resistance. Nyquist diagrams for the coatings are presented in Fig. 8. The global impedances of the coated samples were larger compared to the bare substrate, whereas the duplex PDMS/CrN coating exhibited significantly increased impedance than single CrN coating, especially for the D4 coating. Equivalent circuit was also used to analyze the EIS data. As Fig. 9 shown, the equivalent circuit models (a–c) were used for the bare substrate, single CrN coatings and the duplex PDMS/CrN coatings, respectively. A constant phase element (CPE), expressed as Y_CPE = Y_O(jω)⁻ⁿ, was introduced for better data fitting.^10,29 Y_O and n are frequency – independent fit parameters, j = (−1)^1/2 and ω is angular frequency. One time constant was used to describe the bare substrate, corresponding to the corrosion process occurring on the surface.³⁰ While two time constants were used to model the CrN coated samples. The one in high frequency represented the coating dielectric pattern and the other at low frequency represented the coating/substrate interface properties. However, for the duplex PDMS/CrN coating, better fitting results could be obtained by one time constant according to the bode plots, which was presumably ascribed to the well sealing properties of PDMS layer restricted the corrosion behaviors occurred in the coating. In these circuit models, R_s is the solution electrolyte resistance. A couple of CPE_dl and R_ct elements are used to describe the charge transfer at the electrolyte–metal interface or coating–substrate interface. While for the CrN coated systems, CPE_c and R_p in parallel were adopted to represent the capacitance of the coating and the resistance of the electrolyte in the channels and defects, respectively. Warburg impedance (W) was also introduced to express the diffusion resistance for PVD coatings with columnar structure.³¹ Results of the fitting values for bare substrate, T4 and D4 coatings are summarized in Table 4. It was observed that T4 coating showed higher R_ct values compared to bare substrate, whereas the D4 coating presented a further increase, more than three orders of magnitude, in R_ct compared to the pure CrN coating. CrN coating is inert to chemical attacks and could form a thin oxide layer which prevents the corrosive medium reach the substrate and results in the high R_ct values.³² The dense and compact structure also contributed to the high anti-corrosion performance. However, the coatings defects restrain the further improvement of corrosion resistance. The PDMS layer on the CrN coating could act as a good barrier layer to block the pores and pinholes and defense against the diffusion of corrosive medium, which significantly improve the charge transfer resistor in the interface.

Fig. 7 Bode plots (frequency vs. impedance modulus) for the single CrN coatings (a) and duplex PDMS/CrN coatings (b), and the uncoated substrate was used as a reference.

Fig. 8 Nyquist plots for the single CrN coatings (a) and duplex PDMS/CrN coatings (b), the uncoated substrate was used as a reference.

Fig. 9 Equivalent circuit for bare substrate (a), CrN coated substrates samples (b) and the duplex PDMS/CrN coated samples (c).

Table 4 Optimized values for the equivalent circuit parameters obtained by ZSimpWin program simulation

Samples	Rs (Ω cm^2)	CPEc (μF cm^−2)	nc	Rp (Ω cm^2)	CPEdl (μF cm^−2)	ndl	Rct (Ω cm^2)	W (μF cm^−2)
Substrate	58.49	—	—	—	3.23 × 10^−5	0.8	2.25 × 10^4	—
T4	90.44	1.37 × 10^−6	0.906	1.4 × 10^4	4.84 × 10^−6	0.614	2.73 × 10^4	3.35 × 10^−5
D4	7037	—	—	—	5.83 × 10^−8	0.803	5.91 × 10^7	2.43 × 10^−8

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The surface morphologies of corroded samples (T4 and D4 coatings) after potentiodynamic polarization measurements are presented in Fig. 10. Compared to Fig. 3(b), it can be confirmed that pits are formed on the T4 coating surface as a result of corrosion attack. The pits were presumably resulted from the full corrosion or peeling of droplets, which may be initiated from the pores or droplets. It was reported that the droplets were compositionally metal rich and could induce crevice corrosion with respect to surrounding coating.³³ The galvanic corrosion with the ejection of droplets may occur by the corrosive attack, and the leaving crater may provide direct channels for the corrosive media to reach the substrate. Nevertheless, the tiny pits were not found on the D4 coating, indicating that the outermost PDMS layer played a barrier role in significantly blocking the diffusion of corrosive medium.

Fig. 10 SEM surface morphologies of T4 (a) and D4 coating (b) after potentiodynamic polarization tests on 316 substrates.

Discussions

A schematic diagram was proposed to illustrate the corrosion mechanisms for the single CrN and duplex PDMS/CrN coatings, shown in Fig. 11. As Fig. 11(a) shown, transition metal nitride coatings, such as CrN coatings, have noble corrosion resistances in aggressive solutions, which could effectively improve the corrosion resistances of substrates from chemical attack. Yet the growth defects (e.g. droplets, pinholes, pores and cracks and so on) would be deleterious, as the under dense adjacent region in the lower region is a solution-pathway to the substrate surface and hence a potential region of pitting corrosion.³³ The different corrosion behaviors of coatings with variable thicknesses may be derived from the defects densities in the coatings. For thin coatings, despite of the macro-particles on the surface, the dense and compact microstructures inhibited the corrosion media throughout coatings and slowed down the corrosion rate. As the thickness increased over 10 μm, the long diffusion path and compact structure contributed to the higher corrosion potential. However, the macro-particles were able to aggregate into large sizes and embed in the coating, which not only produce shadow effect to adjacent region leaving voids, but also introduce crooked cracks. Both of aspects would provide pathways for the solution deteriorate the coating corrosion resistance and accelerate the corrosion rate. Thus, the corrosion current density for T3 and T4 coatings was slightly increased. However, after the PDMS modification, the corrosion resistances of duplex PDMS/CrN coatings distinctly improved, especially for the thick coating. The improvement of the duplex PDMS/CrN coatings on the corrosion behaviors may be ascribed to two major reasons. On the one hand, the compact and dense CrN layer could inhibit the permeation of seawater and slow down the diffusion rate; on the other hand, the low surface energy PDMS layer not only acted as a perfect barrier for blocking charge transport, but also could seal the pinholes and pores on the coatings surfaces. Since the small ionic radius ions could easily diffuse through columnar grain boundaries, intrinsic pinholes to corrode the metal substrates, the sealed duplex PDMS/CrN coating would blocked the diffusion paths and inhibited the corrosion media throughout coatings and improved the corrosion resistance of coatings. The thicker the coating is, the longer diffusion paths for the corrosive media, which leads to the higher corrosion resistance of coating. Thus, the D4 coating with higher thickness presented excellent corrosion resistance.

Fig. 11 Schematic diagrams for the possible corrosion mechanism of single CrN coatings (a) and the duplex PDMS/CrN coatings (b).

Conclusions

From what discussed above, the following conclusions can be drawn:

(1) A novel duplex PDMS/CrN coating was successfully fabricated composed of CrN coating as a supporting layer and a PDMS as upper layer.

(2) The corrosion current density and impedance value of duplex PDMS/CrN coating was up to 10⁻⁹ A cm⁻² and 10⁷ Ω cm², respectively, presenting superior corrosion resistance under seawater conditions.

(3) The excellent corrosion resistance of duplex PDMS/CrN coating was ascribed to not only the compact structure of CrN coating, but also to the perfect barrier effect of PDMS layer.

(4) The excellent corrosion resistance of duplex PDMS/CrN coating made it the good candidate protective material in a wide range of marine applications.

Acknowledgements

The authors are grateful for financial supported from the National Natural Science Foundation of China (Grant no. 51475449 and 51202261), the National Basic Research Program of China (973 Program) (Grant no. 2013CB632302) and China Postdoctoral Science Foundation Grant (2014M561802).

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