Effect of different post-treatments on the corrosion resistance and tribological properties of AZ91D magnesium alloy coated PEO

Surface & Coatings Technology 278 (2015) 99–107

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Effect of different post-treatments on the corrosion resistance and tribological properties of AZ91D magnesium alloy coated PEO A. Castellanos, A. Altube, J.M. Vega ⁎, E. García-Lecina, J.A. Díez, H.J. Grande Surfaces Division, IK4-CIDETEC, Paseo Miramón 196, 20009 Donostia-San Sebastián, Spain

a r t i c l e

i n f o

Article history: Received 15 May 2015 Revised 8 July 2015 Accepted in revised form 9 July 2015 Available online 1 August 2015 Keywords: AZ91D PEO UV curable sol–gel PTFE MPTES Dithiol

a b s t r a c t Plasma electrolytic oxidation (PEO) is widely used to improve the corrosion resistance and protection of magnesium and its alloys by the formation of ceramic oxide layers. However, this anodization processes generate porous and rough layers with poor tribological properties. In order to improve corrosion protection and tribological capabilities, three different post-treatments (novel sol–gel coating, polytetrafluoroethylene and acrylate-ethylene copolymer with nanoparticles) have been used as sealing agents on PEO anodized AZ91D magnesium alloys. Characterization of unsealed and sealed systems was made by Fourier transform IR (FTIR), scanning electron microscope (SEM) and energy dispersive X-ray spectrometry (EDX) in order to know the chemical structure, morphology and composition of PEO/coating systems. Corrosion resistance was evaluated by electrochemical measurements (open circuit potential and electrochemical impedance spectroscopy) in 5 wt.% NaCl solution at room temperature and by accelerated neutral salt spray test. Tribological properties were obtained using a ball-on-disk test. According to results, post-treatments have improved the corrosion protection performance of PEO systems. However, while internal physical interlocking of pores seemed to be crucial to enhance corrosion protection using polytetrafluoroethylene and sol–gel post-treatments, the formation of an external top layer was the main cause of the improvement of the barrier properties for acrylate–ethylene copolymer posttreatment. On the other hand, despite that tribological performance was mainly governed by the PEO coating, the formation of a top layer on the surface and its chemical composition can significantly reduce the friction coefficient. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Lightweight components with high strength [1] are increasing the industrial interest in sectors such as aeronautics, aerospace and automotive to the replacement of the conventional metals (iron and steel) by light metals, especially magnesium. AZ91D alloy is the most widely used magnesium die casting alloy for automotive components. This is because AZ91D high-purity alloy has an excellent combination of mechanical properties and castability. However, the use of magnesium (offering a low standard electrode potential −2.37 V) and its alloys in different applications is limited due to its low corrosion resistance [2–4], particularly in harsh conditions [5]. Thus, multiple surface treatments have been developed to improve the corrosion performance of these materials: chemical conversion layers [6–9], plasma electrolytic oxidation treatments (PEO, MAO) [10–12], physical vapor deposition (PVD) of organic coatings [13], laser surface treatments [14], ion implantation [15], electrochemical plating [16,17], electroless plating [18–20], cold spray coatings [21], etc.

⁎ Corresponding author. E-mail address: [email protected] (J.M. Vega).

http://dx.doi.org/10.1016/j.surfcoat.2015.07.017 0257-8972/© 2015 Elsevier B.V. All rights reserved.

PEO is the most useful process among them, considering its low cost and simplicity of use. It consists of growing a ceramic MgO layer by applying an anodic electrical current to the alloy. Several methods have been proposed after modification of electrolyte composition [22,23], power source (direct current (DC), alternating current (AC) or pulsed AC/DC) [24], using ultrasonic power during anodizing process [25], etc. Unfortunately, the presence of defects as pores and cracks in the PEO layer hinders or diminishes its ability to protect the alloy against corrosion [26–28]. Additionally, plasma electrolytic oxidation process increases surface roughness, which can be harmful if the alloy has to be used in sliding parts. In order to improve the corrosion performance, sealing agents are commonly used to fill pores and cracks. Rateick et al. [29] have applied parylene layers (grades C and HT) by chemical vapor deposition (CVD) in order to protect anodized WE43A magnesium alloy. Good adherence properties and a proper sealing of the pore structure of the PEO layer were found. Hara et al. [30] have obtained better corrosion protection on AZ91D magnesium alloy using a water-based cationic electrodeposition coating rather than using an organic sealing agent by spray. Wang et al. [31] have obtained a compact and smooth layer with enhanced protection against corrosion using low molecular-weight polymer on AZ31B magnesium alloy as a sealing agent. Then, although different

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strategies can be used, the application of organic coatings seems to be good enough, easier and cheaper to obtain good corrosion protection. On the other hand, several studies have applied sol–gel technology to improve the corrosion resistance of anodized magnesium alloys [32–34]. Malayoglu et al. [33] have compared phosphate, silicate and sol–gel sealing agents for AM50B and AM60B magnesium alloys. Potentiodynamic polarization curves and surface evaluation by SEM images support a better sealing using sol–gel coating. Tan et al. [34] have applied multilayer sol–gel coating on AZ91D magnesium alloy by spraying. Authors have claimed significant improvement in corrosion protection by filling of the pores physically, due to the combination of the PEO layer and sol–gel coating. However, there are not too many studies comparing existing sealing compounds with sol–gel systems in order to study the effectiveness of the sol–gel post-treatment as sealers. Moreover, the attention in literature is not focused to enhance simultaneously tribological properties of anodized magnesium alloys. The main goal of this paper is to study the sealing effect of three different post-treatments with diverse chemical composition on anodized AZ91D magnesium alloy. A novel UV-curable sol–gel modified with dithiol, a water-based polytetrafluoroethylene (PTFE, ALFIFLON 39) polymer and a water-based acrylate-ethylene copolymer with nanoparticles (PA/PE-Si, HESSOTOP Si 3000 N). The last two compounds are designed to be used as sealers for hard anodized aluminum alloys and for trivalent passivation on zinc alloys, respectively. So far, none information was found about their performance as sealers for anodized magnesium. A morphological and structural characterization of unsealed and sealed specimens was performed. The corrosion protection was evaluated by electrochemical tests (open circuit potential (OCP) and electrochemical impedance spectroscopy (EIS)) using 5 wt.% NaCl solution and by salt spray accelerated test. The tribological properties of the coatings (coefficient of friction evolution, wear track, etc.) were studied by ball-on-disk test and optical and scanning electron microscopy.

air, (2) degreased in a NaOH/Na3PO4 solution at 60 °C during 10 min, and (3) rinsed in distilled water. Ceramic films were grown by PEO process in an alkaline electrolyte solution containing several compounds as Na2SiO3, NaOH and Na3PO4. Anodizing was carried out in a two-electrode thermostatized cell with a stainless steel cathode and air stirring. A constant current density of 20 mA cm−2 was applied by a DC power supply (Sorensen SGI series). The temperature of the electrolyte was kept constant at 35 °C using a water cooling system. PEO process was carried up to 16 min and a nominal thickness of 18 μm was obtained. Once PEO process was completed, samples were cleaned with deionized water and dried. 2.3. Post-treatments Anodic layers were sealed by sample immersion into the corresponding solution according to the conditions showed in Table 1. Three different sealing agents were used: (i) PA/PE-Si. Inorganic–organic sealing compound (HESSOTOP ® Si 3000 N) with the following composition: a polymeric matrix (polyacrylate and polyethylene) and nanoparticles, according to the International Material Data System (IMDS) ID 976386. (ii) PTFE. Water-based polytetrafluoroethylene (ALFIFLON 39). (iii) TEOS:MPTES:Dithiol. Hybrid organic–inorganic silica sol was synthesized at room temperature by hydrolysis and polycondensation of ethanol-diluted tetraethylorthosilicate (TEOS) in the presence of methacryloxypropyltriethoxysilane (MPTES) as a co-precursor. To increase the performance, 1–5 pentanedithiol was added to the solution to promote crosslinking. TEOS, MPTES and 1–5 pentanedithiol were mixed in a molar ratio 4:1:0.5 and HCl 0.01 M was used as a catalyst. The solution was hydrolysed adding the stoichiometric amount of water. Subsequently, the transparent sol solution was obtained without any phase separation. The resultant solution was stirred at a rate of 200 rpm during 1 h.

2. Experimental 2.1. Materials

Finally, after dip-coating application, samples (i) and (ii) were cured using an oven and sample (iii) using UV light [35].

AZ91D magnesium alloy was provided by GRUPO ANTOLÍN-IRAUSA, S.A.. The chemical composition wt.% was 12.61% of Al; 0.89% of Zn; 0.65% of Si; 0.24% of Mn; 0.015% of Fe and 0.0015% of Cu. Na2SiO3, NaOH and Na3PO4 were purchased from Sigma-Aldrich as main components to formulate the electrolyte for anodization. Commercial post-treatments ALFIFLON 39 and HESSOTOP ® Si 3000 N were provided by Alufinish and Dr. Hesse, respectively. Sol–gel reagents were purchased from different providers: TEOS (98%) was purchased from Sigma-Aldrich, MPTES (97%) was purchased from ABCR and 1–5 pentanedithiol (96%) from Alfa Aesar. All the solutions were prepared from deionized water and sodium chloride analytical grade.

2.4. Sample characterization The surface morphology of the sealed samples was observed by means of a scanning electron microscope (SEM) (JEOL JSM 550 LV). The attached EDX detector was used to determine the atomic composition of the coating. The thickness of the anodized and sealed samples was determined using a coating thickness measuring instrument (Fischer Dualscope MP20). The thicknesses were measured at 10 different points of each sample. FTIR spectroscopy was performed on a JASCO FT-IR 4100 spectrometer collecting 160 scans in the 4000–650 cm−1 range with 4 cm−1 resolution. The surface roughness of the coatings was measured using a Taylor-Hobson model Talysurf 50 profilometer. The arithmetic average of the absolute values (Ra) has been obtained. Two perpendicular measures of 10 mm in each side of the samples were carried out.

2.2. PEO process Prior to the PEO process, all the samples were pre-treated in three steps: (1) machined into a vibratory bowl and dried with compressed

Table 1 Experimental parameters for the three sealing systems. Sealing compound

Concentration

PA/PE-Si

Part 1: 50 g/L Part 2: 213 g/L 9 g/L 4:1:0.5 mol

PTFE TEOS:MPTES:Dithiol

Immersion time/min

Immersion temperature/°C

Curing time/min

Curing temperature/°C

1

35

15

80

10 5

60 25

15 30

80 UV light

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Fig. 1. Surface micrographs by SEM on AZ91D magnesium alloy: (a) PEO, (b) PEO + PTFE coating, (c) PEO + PA/PE-Si coating and (d) PEO + TEOS:MPTES:Dithiol coating.

2.5. Electrochemical tests Electrochemical tests were performed using a multichannel potentiostat BIO-LOGIC VMP3 to evaluate corrosion behavior of the systems. A typical three electrode cell, with a saturated Ag/AgCl (saturated with KCl) as reference electrode, a platinum mesh counter electrode and the different sealed and unsealed specimens as working electrode were used. Electrochemical experiments were carried out at least by triplicate using an area of 1 cm2. The electrochemical tests were conducted in 5 wt.% NaCl solution at room temperature. OCP was measured during 3 h before to perform EIS measurements. Frequency scans were carried out by applying ± 10 mV sinusoidal wave perturbation versus OCP. The frequency range was from 200 kHz to 100 mHz, obtaining 8 points per decade.

2.6. Accelerated corrosion test Three specimens of 70 × 50 × 5 mm were used to perform neutral salt spray tests (NSS) for unsealed and sealed systems. Experiment was conducted using a DYCOMETAL MODEL SCC-400 salt spray chamber during 1056 h. Test parameters were set according to ASTM B117 standard. Visual evaluation was carried out according to ISO 10289:1999 standard.

2.7. Tribological test The coefficient of friction (COF) was obtained using a CSM Instruments rotational tribometer on rectangular specimens with dimensions of 20 × 30 × 5 mm for at least three specimens for each system. Friction tests were performed with a 6 mm steel ball outer diameter at 40 mm s−1 with 2 N normal load in a circular motion of 5.54 mm radius. Friction test were conducted during 150 m. The coefficient of friction was recorded during all tests.

Images of the wear track were obtained by optical photograph and SEM. The atomic composition on the track was determined by EDX analysis during SEM. 3. Results and discussion 3.1. Coating characterization 3.1.1. Morphology and composition SEM images in Fig. 1 are indicating the surface morphology of unsealed and sealed PEO coatings. Unsealed PEO, sealed PEO + PTFE and PEO + TEOS:MPTES:Dithiol coatings show open pores on the surface. Similar pore density and morphology is observed comparing Fig. 1(a), (b) and (d). However, pores are not visible on the surface of PEO + PA/PE-Si coatings. Instead, a homogeneous top layer was observed (Fig. 1(c)), which composition, determined by EDX analysis, showed the presence of Si and C (Table 2). In contrast, wt.% of carbon for the other two coatings was similar to the unsealed PEO system, being their main difference the presence of fluorine for PEO + PTFE coating and the higher amount of Si for PEO + TEOS:MPTES:Dithiol coating (15 wt.% instead of 8 wt.%). Finally, the presence of Mg, O and Si in the composition of unsealed specimens suggests the presence of Mg2SiO4 on the surface as reported in the literature [30,36]. Table 2 EDX analysis for unsealed and sealed systems on AZ91D magnesium alloys. Specimen

PEO PEO + PTFE PEO + PA/PE-Si PEO + TEOS:MPTES:Dithiol

Composition/wt.% C

O

Na

Mg

Al

Si

F

4 5 40 5

48 40 31 49

2 3 1 5

35 28 4 24

3 2 0.5 2

8 10 23 15

– 11 – –

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of PEO + PA/PE-Si coating are showing a wide band at 1100–1000 cm−1 assigned to the stretching of Si–O–Si, which is due to the presence of nanoparticles of SiO2 [38]. On the other hand, spectra of PEO + PTFE coating are showing a wide band at 1350 cm− 1 due to the C–F stretching vibration [39], which is easily affected by the adjacent atoms or groups. Finally, spectra of PEO + TEOS:MPTES:Dithiol coating is showing a band at 868 cm−1 assigned to the stretching vibration of Si–O [40], due to the sol–gel post-treatment. Therefore, FTIR shows the chemical features on the surface of PEO coatings depending on each post-treatment and it reveals the different structure of silicon (presence of nanoparticles or sol–gel coating). 3.1.3. Electrochemical measurements In order to obtain complementary information to optical and structural characterization, different electrochemical parameters have been studied to know the corrosion performance of sealed PEO.

Fig. 2. FTIR spectra of unsealed and sealed PEO specimens from 1600 to 650 cm−1.

According to this surface evaluation by SEM images, PEO + PA/PE-Si coating might behave much better than the other two systems. Malayoglu et al. made a statistical distribution of the pore size using SEM images and certain correlation with potentiodynamic polarization curves was found [33]. However, surface evaluation by images does not provide enough information about the internal sealing of porous and may drive to wrong conclusions. Therefore, electrochemical measurements must be used to provide additional information related to physical interlocking of pores internally. 3.1.2. Structural characterization The chemical structure of the surface after different post-treatments has been explored using FTIR spectroscopy. Fig. 2 shows the transmittance bands within the wavelength range from 1600 to 650 cm−1. Despite to the low intensity of the bands and the noise of the signal, it can provide information about the different post-treatments, especially for the systems containing silicon. Unsealed PEO is showing two bands at 1512 and 1005 cm−1 which could be assigned to the presence of Mg2SiO4 [37] or a similar magnesium silicate according to the composition by SEM-EDX (Table 2). The band around 1005 cm−1 also appears in PEO + PTFE and TEOS:MPTES:Dithiol coatings (located in the range of 993–983 cm−1). The presence of this band confirms the absence of a continuous layer of these two sealing agents (in agreement with SEM images in Fig. 1). If FTIR spectra are compared for each post-treatment, bands appear at different wavelength according to the chemical composition. Spectra

Fig. 3. OCP variation with time of immersion in 5 wt.% NaCl solution for bare AZ91D and anodized specimens (unsealed and sealed).

3.1.3.1. Open circuit potential (OCP). Variation of the OCP as a function of the immersion time can be used to monitor the stability under aggressive conditions using sodium chloride as aggressive electrolyte. Fig. 3 shows the evolution of the OCP during 3 h exposure in 5 wt.% NaCl solution. Bare AZ91D magnesium alloy is showing a quick stabilization of the potential value. This value is close to the reported corrosion potential of common magnesium alloys in 3–6 wt.% NaCl solution (−1.53 V Ag/AgCl) [41]. The stabilization of the potential indicates the formation of corrosion products acting as a barrier layer [42]. If this variation is compared once AZ91D is anodized, the oxide layer shifts the potential to the positive direction showing an irregular variation. Potential drops rapidly from −1.45 V to −1.55 V and starts to fluctuate between −1.50 and −1.55 V. This may be due to that electrolyte infiltrate through the porous PEO layer reaching the interface between the inner barrier layer and the outer porous layer, as it was suggested elsewhere [43]. The fluctuation can be assigned to several factors: chemical dissolution, electrolyte breakthrough and electrochemical corrosion [44]. In contrast, PEO + PTFE and PEO + PA/PE-Si coatings show a smooth evolution of the OCP. Potential is increasing slightly and placidly until it reaches −1.45 V after 1 h. It could reflect a continuous absorption of the electrolyte until a saturated state is reached [36]. Finally, PEO + TEOS:MPTES:Ditihol coating shows a similar variation of the OCP than PEO although potential values are shifted to more noble ones. Potential fluctuation is located in the range of − 1.42 and − 1.52 V, where its less negative value (− 1.42 V) is slightly nobler than the other two sealed PEO and its most negative potential

Fig. 4. Nyquist diagram after 3 h of immersion in 5 wt.% NaCl solution.

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Fig. 5. Zoom of Nyquist diagram after 3 h of immersion in 5 wt.% NaCl solution: (a) AZ91D, and (b) PEO.

(−1.52 V) does not reach the one corresponding to general corrosion of AZ91D alloy (− 1.55 V). Then, apparently, heterogeneous sealing has been provided by the sol–gel system according to the intermediate behavior of its OCP. Taking account the OCP variation, the stability should increase as follows: bare AZ91D alloy ≪ PEO and PEO + TEOS:MPTES:Dithiol ≪ PEO + PTFE and PEO + PA/PE-Si.

3.1.3.2. Electrochemical impedance spectroscopy (EIS). In order to understand the behavior of these coatings as barrier systems, EIS can provide complementary information (e.g. internal sealing of the pores) to OCP measurements and surface evaluation by SEM images. Impedance spectra measured at the stable OCP for each system are shown in Fig. 4 (the curve of AZ91D alloy is not visible here due to its low impedance value at the scale used in this graph). Impedance spectra of bare Mg alloy may consist of one or two semicircles (corresponding to different time constant) depending on the surface concentration of [Mg+]ads followed by a pseudo-inductive loop which may corresponds to the so called Negative Difference Effect (NDE) and relaxation of adsorbed [Mg+]ads according to literature [45]. Results for AZ91D and PEO shown in Fig. 5

indicate the presence of two not well defined semicircles and the pseudo-inductive loop, where the high frequency loop is the result of the charge transfer resistance (Rct) and film effect (hydroxide/oxide layers) [45]. Similar spectra were obtained for PEO systems, where the film effect (corrosion products for AZ91D and ceramic oxide layer for PEO system) is much higher. Generally, higher resistance indicates lower corrosion rate (e.g. value of the Rct). Here, the oxide layer has been grown to promote the corrosion resistance and barrier protection. However, although impedance has been increased for both semicircles in the PEO system, the charge transfer process is still taking place due to the high porosity which creates an easy path to the penetration of water and aggressive ions like chloride, promoting the corrosion process. As it was expected, the impedance value was increased after the sealing processes (Fig. 4). Nyquist plots are showing a semicircle for these systems in comparison with bare AZ91D alloy and PEO system. This behavior suggests that all post-treatments were able to block the pores (in different manner) and, thus, slow down the corrosion process of the magnesium. Sealed PEO coating systems are formed (from top to bottom) of the sealing layer, the outer porous layer, the inner barrier layer and the substrate [36]. Here, high frequency impedance provides

Fig. 6. Equivalent circuits for (a) a single time constant for sealed systems and (b) two time constants for unsealed systems.

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Table 3 Impedance values using equivalent circuit for fitting considering one or two time constants, depending on the impedance spectra. Sample

χ2

Roxide/Ω·cm2

Error/%

Rc/Ω·cm2

Error/%

Rct/Ω·cm2

Error/%

AZ91D PEO PEO + PA/PE-Si PEO + PTFE PEO + TEOS:MPTES:Dithiol PEO + TEOS:MPTES

0.001 0.002 0.008 0.001 0.002 0.008

201 1.88 · 104

2 4 – – – –

–

– – 5 2 2 2

27 2.09 · 104 – – – –

17 8 – – – –

– – – –

information about the outer layer properties, while the low frequency range characterizes the inner layer properties. In general, impedance spectra are able to provide information of all these layers within a frequency range. However, depending on the systems the time constant of the different processes may overlap and they cannot be tackled separately. The present study has been focused for simple comparative discussion of various protection systems. Equivalent circuits are widely used to fit the impedance data in order to know capacitance and resistance values. Although complex equivalent circuits can be used to fit the impedance spectra [36], simplified equivalent circuits with a physical meaning can be used derived for a typical Randles circuit [46]. Fig. 6 shows two equivalent circuits with a single time constant (e.g. sealed systems) or two time constants (e.g. unsealed systems without considering the pseudo-inductive loop), where Rs is the resistance of the electrolyte, Rc and CPEc are the resistance and constant phase element of the coating or sealing system in this study, respectively; Roxide and CPEoxide the resistance and the constant phase element of the oxide layer, respectively; and Rct and CPEdl are the charge transfer resistance and constant phase element of the double layer of the metal. Table 3 has summarized the impedance value for each system after fitting by the corresponding equivalent circuit depending on the number of time constants. The criteria used to estimate the quality of the fit were focused on the low chi-square (χ2) value the low error (%) of each specimen. Interestingly, comparing all sealed systems, PEO + TEOS:MPTES:Dithiol coating is showing the highest impedance value despite its high superficial porosity (Fig. 1) and the oscillatory variation of the OCP (Fig. 3). In contrast, PEO + PTFE coating only increases 4 times the impedance (from 1.88 · 104 to 7.46 · 104 Ω·cm2), despite that it is offering visually a similar porosity on the surface (Fig. 1). It might indicate that the physical–chemical properties of the sealing compounds (sol–gel and PTFE) play an important role to promote an internal physical interlocking of pores. Finally, as was expected, PEO + PA/PE-Si coating has provided

–

1.54E·105 7.46 · 104 1.61 · 105 2.23 · 104

a high impedance value (similar to PEO + TEOS:MPTES:Dithiol coating) most probably due to the presence of a homogenous film on the surface (Fig. 1). Therefore, morphological and surface evaluation by SEM images (e.g. statistical distribution of pore sizes) seems not be enough to discriminate the corrosion protection performance of each posttreatment. Although the presence of a single time constant in impedance spectra indicates certain barrier protection from all sealed systems, results have shown that PEO + TEOS:MPTES:Dithiol and PEO + PA/PESi coatings provide the best corrosion resistance despite their different morphology on the surface. Finally, in addition to the analysis above, the effect of 1–5 pentanedithiol in the sol–gel coating has been briefly discussed. Fig. 7 is showing a single time constant for PEO + TEOS:MPTES coating in absence of dithiol in the coating composition. Despite certain sealing is provided with TEOS and MPTES, this resistance value (2.23 · 104 Ω·cm2, Table 3) is quite similar to the Roxide of PEO systems (2.09 · 104 Ω·cm2, Table 3) and smaller than the Rc for PEO + PTFE coatings (7.46 · 104 Ω·cm2, Table 3). Therefore, the corrosion protection performance for this sol–gel formulation is unable to improve the protection of the PTFE and PA/PE-Si sealers. Then, in order to improve this result, the addition of 1–5 pentanedithiol was carried out aiming to enhance the corrosion protection behavior. In fact, the presence of dithiol in the sol–gel formulation was able to increase impedance one order of magnitude (1.61 · 105 Ω·cm2, Table 3), indicating the enhancement of the barrier protection of the sol–gel system as a sealer. It can be justified by the crosslinking effect between the thiol and methacryloxy groups, activated by UV light. 3.2. Neutral salt spray chamber Usually, the corrosion protection performance of coatings and protection layers is evaluated using accelerated corrosion, especially in the industry. Here, salt spray test was carried out in a fog chamber for unsealed and sealed specimens (AZ91D was not studied due to its poor corrosion resistance) to study the corrosion protection simulating a marine environment. Visual evaluation was carried out according to ISO 10289:1999 standard, where degradation is rated from 10 (corroded area = 0%) to 0 (corroded area N 50%). Table 4 is showing the behavior of the coatings at different time: 72 h, 240 h, 504 h, 788 h, 956 h and 1056 h, respectively. Results within 72 h of exposure have shown certain difference between coatings. Absence of corrosion was observed for PEO + PA/PESi coatings (10). PEO + TEOS:MPTES:Dithiol coating has shown low degradation (9). In contrast, the other two systems are showing damage of 8 (0.1% b corroded area b 0.25%), where corrosion spots along the

Table 4 Visual evaluation at different time of exposure in accelerated salt spray test according to standard ISO 10289:1999 for unsealed and sealed specimens.

Fig. 7. Nyquist diagram of PEO + two different sol–gel systems (with and without 1–5 pentanedithiol) after 3 h of immersion in 5 wt.% NaCl solution.

Sample

72 h

240 h

504 h

788 h

1056 h

PEO PEO + PA/PE-Si PEO + PTFE PEO + TEOS:MPTES:Dithiol

8 10 8 9

8 9 8 9

7 9 6 8

7 8 6 8

7 8 6 8

A. Castellanos et al. / Surface & Coatings Technology 278 (2015) 99–107 Table 5 Roughness and COF of bare AZ91D and anodized (unsealed and sealed) systems. Sample

Roughness/μm

Coefficient of friction (COF)

AZ91D PEO PEO + PA/PE-Si PEO + PTFE PEO + TEOS:MPTES:Dithiol

0.12 ± 0.09 2.45 ± 0.24 1.11 ± 0.11 1.17 ± 0.04 1.61 ± 0.06

0.32 ± 0.02 0.95 ± 0.08 0.15 ± 0.01 0.52 ± 0.01 0.88 ± 0.03

sample are observed. Apparently, PTFE as a sealing agent is showing the lowest performance since the beginning of the test while PA/PE-Si sealing agent is providing the best protection. This good performance may be due to the thicker layer on the surface (Fig. 1). Exposure during 240 h has promoted the damage (9) on PA/PESi coatings, which is equal to the one observed for PEO + TEOS:MPTES:Dithiol coatings. Both coatings have also shown the best results in terms of impedance (Fig. 4), indicating a correlation between electrochemical test and salt spray. On the other hand, the degree of degradation (8) for PEO and PEO + PTFE remains constant compared with the previous evaluation period. The corrosion resistance performance was decreasing at longer time of exposure, until the end of the test (1056 h). The final corrosion resistance is decreasing as follows: PEO + PA/PE-Si and PEO + TEOS:MPTES:Dithiol N unsealed PEO N PEO + PTFE. In general, correlation between salt spray and electrochemical results was observed, indicating that PA/PE-Si and TEOS:MPTES:Dithiol are showing the best performance as sealers. However, the huge aggressiveness of this accelerated test makes difficult to quantify the corrosion resistance compared with electrochemical evaluation.

105

3.3. Roughness and tribological behavior Besides to the corrosion protection performance, certain applications demand enhanced tribological properties of surfaces. Therefore, several parameters like COF and roughness can be measured to estimate tribological capabilities. Table 5 shows a summary of Ra and the average value of COF for bare AZ91D magnesium and anodized samples (unsealed and sealed). Anodization of bare AZ91D increased Ra one order of magnitude (from 0.12 μm to 2.45 μm) and three times the COF. Although all post-treatments have decreased Ra and COF, similar roughness does not provide similar COF. Therefore, variation of Ra cannot be used straightforward to predict the sliding behavior (COF) of anodized AZ91D magnesium [47]. The nature of the coating layer and its composition play an important role as can be observed below. Fig. 8 is clearly showing the visual appearance of the unsealed and sealed specimens after the tribological test. All specimens are showing the presence of ball footprint except PEO + PA/PE-Si coating (Fig. 8(c)). In order to obtain more information about the wear mechanism, the friction coefficient variation versus the sliding distance (Fig. 9) was analyzed and SEM images from the wear track (Fig. 10) were obtained. The curve for AZ91D alloy reveals that the friction coefficient values are fluctuating in the range of 0.12–0.44. In the case of unsealed PEO system, two different behaviors have been observed: i) “PEO” and ii) “PEO with transition” curves. COF was observed to increase steadily, before reaching steady state value around 0.95 for scenario i). Fig. 10(a) is showing wear track image (see width of the arrow), indicating that unsealed PEO coating was not removed completely. In fact, EDX analysis (not shown) of wear debris consists of elements from the steel counterpart (Fe, Cr, Mn) as well as the

Fig. 8. Visual images of the ball track after the tribological test for unsealed and sealed specimens: a) PEO, (b) PEO + PTFE coating, (c) PEO + PA/PE-Si coating and (d) PEO + TEOS:MPTES:Dithiol coating.

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Fig. 9. Friction coefficients as a function of sliding distance measured from AZ91D alloy, untreated and treated PEO systems.

coating itself (Mg, O, Si,). On the other hand (scenario ii), at the beginning of the test COF is increasing steadily until a friction transition occurs at 25–45 m, where COF decreases from 0.94 to the range of 0.5– 0.7. This friction transition can be attributed to the displacement of the steel counterpart through the coating-substrate interface [48], indicating the end of the lifetime of the PEO coating and reaching the bare substrate which was confirmed by EDX analysis (not shown). This effect could be attributed to cavities and heterogeneities of the PEO layer structure produced with this kind of anodizing processes [49], which could generate areas with different mechanical properties (e.g. friction resistance, and hardness). The wear behavior of all sealed PEO systems was distinctly different from each other. PEO + PTFE system underwent an abrupt friction

transition at 22 m, where COF decreases down (0.34 - 0.60) to comparable values to those measured for unsealed “PEO with transition”. The bare substrate was reached as can be observed in Fig. 10(b). Therefore, the drop of average value COF to 0.52 (Table 5) using PTFE as sealer can be assigned to the friction with bare AZ91D substrate (once PEO + PTFE coating is removed) rather that the role shown by PTFE as solid lubrication agent [48]. Further investigations should be done to understand this phenomenon and to know if PTFE as sealer could promote this transition by modification of the PEO coating. On the other hand, PEO + PA/PE-Si coating is showing a smooth variation of the COF along the total sliding distance, providing the lowest average value COF (0.15). Under these experimental conditions, this post-treatment is indicating good wear resistance as can be observed in Fig. 10(c), where the wear track is the narrowest (see width of the arrow). This behavior can be justified thanks to the total covering of the anodized surface together to the presence of SiO2 nanoparticles within its composition, as it was shown by Kang et al. using silica nanoparticles on epoxy as a matrix [50]. Finally, a similar trend (Fig. 9) and wear track (Fig. 10) to PEO curve was observed for PEO + TEOS:MPTES:Dithiol, where the wear debris also consisted of elements from the steel counterpart and PEO coating. The main difference was observed on the running-in distance (much longer) and the steady state value (reached at approximately 50–60 m). It might indicate the presence of sealer within the pores and close to the anodized surface. However, this modification barely affects the average value of COF despite its good corrosion protection (Fig. 4). As a conclusion, PTFE is providing a modest improvement in terms of corrosion protection and tribological properties. Sol–gel post-treatment has improved the corrosion protection although barely affects the friction properties (governed mainly for the PEO coating itself). PA/PE-Si post-treatment was able to enhance both tribological and corrosion protection capabilities, mainly due to the formation of a top layer and the presence of nanoparticles within its composition.

Fig. 10. Wear track micrographs obtained by SEM from: (a) PEO, (b) PEO + PTFE coating, (c) PEO + PA/PE-Si coating and (d) PEO + TEOS:MPTES:Dithiol coating.

A. Castellanos et al. / Surface & Coatings Technology 278 (2015) 99–107

4. Conclusions A porous ceramic layer was obtained using the spark process, where pores are randomly distributed: small size pores (≈3 μm) and large size pores (≈10 μm). Further corrosion protection to 18 μm PEO coatings on AZ91D is achieved by applying PTFE, PA/PE-Si and sol–gel based sealing treatments. Electrochemical tests have provided complementary and valuable information about the corrosion resistance and sealing efficiency of each coating. PTFE sealer has slightly improved corrosion resistance and tribological properties of anodized magnesium. However, its performance was far from the other systems, especially in salt spray test. TEOS:MPTES:Dithiol has provided barrier protection by physical interlocking of pores, despite the presence of open micro-pores on the surface observed by SEM images. PA/PE-Si was successfully applied as sealer on PEO magnesium alloy. It has shown enhanced barrier protection according to the electrochemical results and accelerated salt spray performance. Although PA/PE-Si and TEOS:MPTES:Dithiol were able to effectively seal pores, only PA/PE-Si has improved the tribological properties thanks to the synergy of a top layer covering the porous surface together to the presence of nanoparticles within its composition.

Acknowledgments Authors gratefully acknowledge to MUGAPE S.A. which has provided financial support through MAGNO 2008 — CENIT program, funded by the Spanish Ministry of Science and Innovation (MAGNO2008-1028CENIT).

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