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A composite coating for corrosion protection of AM60B magnesium alloy
Surface & Coatings Technology 205 (2011) 4459–4465
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Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
A composite coating for corrosion protection of AM60B magnesium alloy A. Mandelli c, M. Bestetti a,⁎, A. Da Forno a, N. Lecis b, S.P. Trasatti c, M. Trueba c a b c
Politecnico di Milano - Department of Chemistry, Materials and Chemical Engineering “G. Natta”, Via Mancinelli 7, 20131 Milan, Italy Politecnico di Milano - Department of Mechanics, Via La Masa 34, 20158 Milan, Italy Università degli Studi di Milano, Department of Physical Chemistry and Electrochemistry, Via Golgi 19, 20133 Milan, Italy
a r t i c l e
i n f o
Article history: Received 20 October 2010 Accepted in revised form 21 March 2011 Available online 26 March 2011 Keywords: Magnesium alloys Micro-arc anodic oxidation Spark discharge Silane-based coatings Corrosion resistance Scratch test
a b s t r a c t Oxide films have been produced on AM60B magnesium alloy by micro-arc anodic oxidation in an environmentally friendly alkaline solution, with and without addition of nanoparticles (TiO2, ZrO2 and Al2O3). Because of the anodic oxide porosity, inherent in the sparking process, organo-functional silanes top coat has been applied to seal pores and cracks, and achieve an efficient protective coating system. The surface and crosssection morphology of samples were analyzed by Scanning Electron Microscopy (SEM) coupled with Energy Dispersive Spectroscopy (EDS). Scratch tests were performed for evaluating the adhesion strength and scratch hardness of the anodic oxides to the AM60B substrate. The corrosion resistance of both anodic oxides and oxide/ silane composite coatings was evaluated in 0.6 M NaCl solution using potentiodynamic polarization tests. The addition of nanoparticles to the anodizing solution doesn't affect significantly the corrosion resistance in comparison with anodic oxides produced in nanoparticles free solutions. Conversely, the adhesion strength and scratch hardness of the anodic oxides to the substrate is quite scattered, and it is higher for the samples produced in ZrO2 and in Al2O3 rich solutions. For this reason specimens anodized in ZrO2 and Al2O3 containing solutions were chosen for silane deposition. Two silanes were used, namely octyltrimethoxysilane (OSi) and 1, 2-bis [triethoxysilyl] ethane (BTSE). The anodizing treatment carried out in oxides nanoparticles containing solutions (ZrO2 or Al2O3), followed by a silane top coat treatment performed using OSi precursor, is an interesting way, suitable for industrial applications, to synthesize adherent corrosion resistant coatings on magnesium alloy AM60B in a short process time. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Magnesium alloys are light structural and functional materials, increasingly used in the automotive, aerospace, and biomaterials industry. Unfortunately, magnesium is too reactive and generally exhibits a poor corrosion resistance because of high dissolution tendency by galvanic coupling or in chlorides environments. Moreover, impurities and second phases act as active cathodic sites causing local galvanic acceleration of corrosion of the matrix [1]. The alloys containing low amounts of iron, copper and nickel are more corrosion resistant, however a surface treatment for protection against corrosion is usually required for aggressive electrolytes exposure. In order to increase the working life of magnesium alloys products, several finishing techniques have been suggested to enhance corrosion resistance such as electroplating, conversion coatings and painting, gas-phase deposition and anodic oxidation [2]. Micro-arc anodic oxidation is a surface treatment technique and emphasis is drawn to its potential for surface modification and enhancement of final top coats adhesion without the necessity to introduce bilayer coatings in order to avoid significant differences in
⁎ Corresponding author. Tel.: + 39 0223993166; fax: + 39 0223993180. E-mail address: [email protected] (M. Bestetti). 0257-8972/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2011.03.066
mechanical properties between the coating and the substrate [3]. Electrolyte composition [4,5] plays a significant role in micro arc anodic oxidation and the oxides properties strongly depend on the electrolyte composition, concentration and conductivity. Recently many papers about plasma electrolytic oxidation have been published, but literature dealing with this process in presence of ceramic nanopowders in the anodizing solution is poor. According to Ko et al. [6] zirconia incorporation in oxides produced by plasma electrolytic oxidation improves oxidation resistance in comparison with oxides produced in zirconia free electrolyte. Liang et al. [7] investigated the properties of oxides produced in sodium phosphate and potassium hydroxide without and with addition of 4% titania sol, using a bi-polar pulsed electrical source. According to the authors, oxide films containing TiO2 compounds present a more uniform morphology than those formed in the titania free electrolyte. XPS and XRD analyses prove that crystalline rutile and anatase are present in the oxide films as well as periclase. The authors ascribe the better corrosion resistance to the TiO2 compounds Table 1 Chemical composition of AM60B alloy (wt.%). Al
Zn
Mn
Si
Fe
Cu
Ni
Mg
5.9
0.05
0.27
0.025
0.0017
0.0017
0.0004
Balance
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A. Mandelli et al. / Surface & Coatings Technology 205 (2011) 4459–4465
Fig. 1. SEM surface morphology of a sample produced in zirconia (left) and in alumina containing solution (right).
Fig. 2. SEM surface morphology of a sample produced in titania containing solution (left) and in nanoparticles free solution (right).
more stable than MgO in aqueous solutions. Recently, Guo et al. [8] have used electrolyte containing polytetrafluoroethylene nanoparticles to produce a novel multifunctional composite coating onto AM60B magnesium alloy with self-lubrication, hydrophobic and anticorrosion
properties. These papers show that by means of this process mixed oxides can be synthesised, containing the elements both from the substrate and from the electrolytic solution, which significantly affect the surface properties. Moreover, the anodic oxides favour the adhesion
Fig. 3. SEM surface and cross-section of an anodic oxide produced in zirconia containing solution.
A. Mandelli et al. / Surface & Coatings Technology 205 (2011) 4459–4465
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Fig. 4. SEM images (top) of track of anodic oxide produced in TiO2 containing solution at different magnifications with the line along which profiles of magnesium (-■-) and oxygen (―) were taken (bottom graph).
of post-treatment coatings, for example painting [9] and sol–gel coatings [10–13]. According to the literature, the effectiveness of the sol–gel coatings is attributed to the ability to seal pores and cracks
produced by micro-arc anodic oxidation. Moreover a multilayer approach (two or three layers) allows to reduce levels of porosity, thus eliminating diffusion paths of aggressive species to the substrate, as
Fig. 5. SEM images of tracks of anodic oxide produced in Al2O3 containing solution (left) and in ZrO2 containing solution (right).
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Table 2 Hardness scratch numbers and critical scratch load (LCN). Samples
LCN (N)
HSP (GPa)
Base solution (b.s.) b.s. + TiO2 b.s. + ZrO2 b.s. + Al2O3
17 15 22 21
1.19 0.87 2.03 1.25
proposed by Tan et al [12]. Recently Malayoglu et al. have shown that sol–gel post-treatment provides the most effective sealing when compared to alkaline phosphate and alkaline silicate coatings [14]. However, it is recalled that the thickness of silane sol–gel coatings is noticeable higher than those obtained by chemical surface conversion coatings or other surface pre-treatments [14,15]. Gnedenkov et al. [16] have suggested composite polymer containing coatings consisting of an anodic oxide produced by plasma electrolytic oxidation (PEO) and polymeric coating achieved using superdispersed polytetrafluoroethlylene (SPTFE). According to the authors this combination of PEO coating with SPTFE provides increased corrosion resistance of metals and alloys, suitable also for implantation surgery. Recently Minaev et al. [17] have proposed a composite coating consisting of a PEO oxide and a polymer layer produced using superdispersed polytetrafluoroethylene. Due to the good penetration into pores of a plasma electrolytic oxidation coating the system provides antiscale and anticorrosion protection. Treatments with hydro-alcoholic solutions of silanes, following the classical steps of immersion and curing, have been mainly used on bare magnesium alloys. N-alkyl-trimethoxy-silane on AZ60 alloys and on magnesium alloy composites reinforced with silicon carbide particles [18] have been studied. Other silane molecules that have been investigated are: γ-glycidoxypropyltrimethoxysilane (γ-GPS) on pure magnesium (99.8%) and commercial AZ31 [19], as well as on AZ91 [20]; 3-mercaptopropyltrimethoxysilane (MPTS) on WE43 [21] and AZ91 [22]; bis-1,2-(triethoxysilyl)ethane (BTSE) and γ-aminopropyltriethoxysilane (γ-APS) on AZ91 [23]; bis-(triethoxysilylpropyl)tetrasulfide (BTESPT) containing rare earth salts such as cerium and lanthanum nitrates [24], as well as bis-(triethoxysilylpropyl)amine (BAS) with carbon nanotubes on AZ31 [25]. A scrutiny of these studies discloses that the relevant aspects influencing performance of silane-based coatings on Mg alloys are the hydrophobicity of the silane molecule (monosilanes, like γ-APS and γ-GPS, exhibit less protection than bis-silanes, the latter giving better protection with the degree of hydrophobicity, e.g. BTSE and BAS) and the chemistry of the coating solution (silane concentration, pH, pre-hydrolysis time, presence of additives with sealing or inhibiting
properties, etc.). To our knowledge, just one very recent work reports on similar approaches to reinforce anodized magnesium [26]. We believe that our approach to the surface treatment of magnesium alloy have some elements of originality both for the use of three different ceramic nanopowders in the anodizing solutions and for the silane-based conversion post- treatments. In this study AM60B magnesium alloy substrates have been anodized by micro-arc regime in an environmentally friendly alkaline solution, with and without addition of nanoparticles (TiO2, ZrO2 and Al2O3). Silane-based solutions of BTSE and octylsilane (OSi) have been investigated as conversion post-treatments in order to produce a composite system able to improve both mechanical properties and corrosion resistance. 2. Experimental The chemical composition of die cast AM60B (Norsk Hydro) alloy used in the experiments is reported in Table 1. Specimens of 100 × 30× 3 mm were cut from Q-panels, polished with P1200 silicon carbide abrasive paper, degreased with acetone, thoroughly rinsed with distilled water and dried in air. The solutions were prepared by using analytical grade reagents and distilled water. The samples were anodized in alkaline electrolyte which consists of 100 g dm−3 sodium phosphate, 25 g dm−3 sodium borate, 20 g dm−3 sodium metasilicate (base solution), with or without addition of 4.0 g dm−3 titania (Degussa P-25) or zirconia (Tosoh TZ-3YS) or alumina (Buehler Ltd, 0.05 μm). Actually, as proved by diffraction laser particle size analyzer (laser granulometer CILAS 1180), the nanoparticles form agglomerates with micrometric size. Two AISI 316 panels were used as cathodes. The anodization process was carried out under voltage control and the initial temperature of the electrolytic solution was 30 °C. During anodizing the cell voltage raised linearly to the maintenance voltage in a fixed time ramp of 255 s, and then it was kept at maintenance voltage, with a total time of 10 min. Different voltages (60–160 V) were applied to the cell in order to assess the operating conditions suitable to produce, at visual inspection, a uniform oxide film. Silane-based coatings were obtained using two silanes, namely octyltrimethoxysilane (OSi) and 1,2-bis[triethoxysilyl]ethane (BTSE). Anodized specimens were dipped for 30 seconds in the 4% silane solution of methanol/water (95:5) with the pH adjusted to 4.3 by adding acetic acid (10 vol%). After dipping, specimens were dried in a hot-air stream and cured in an open-to-air sand oven for 30 min at 110 °C. The silane treatment was tested also on bare AM60B alloys samples. Surface morphology and cross-section of anodized coatings were observed by scanning electron microscopy (SEM) using a Zeiss EVO 50EP microscope at a chamber pressure of
Fig. 6. Anodic polarization scans for Mg substrates treated with: (left) BTSE and (right) OSi; (∙∙∙∙) bare alloy, (-□-) silane-coated alloy, (―) anodized Mg alloy, (-○-) anodized in solution containing ZrO2, (-◊-) anodized in solution containing Al2O3.
Fig. 7. SEM cross-section examination of as-prepared anodized Mg substrates modified with silanes: left — BTSE on anodized Mg; middle — OSi on anodized Mg (the arrow points a crack filled with silane); right: OSi on anodized Mg with Al2O3. (1 — Mg substrate; 2 — oxide layer).
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50 Pa and 20 kV accelerating voltage. Scratch tests were performed using a scratch tester (Microcombi Platform — CSM Instruments) with a load range of 0.3–30 N. Scratches of length 3 mm were made using a diamond Rockwell indenter with a spherical tip radius of 200 μm sliding at a constant speed of 1.26 mm min−1. During the scratch tests the friction force was also measured. The tester was also equipped with acoustic sensors to detect the nucleation of the first cracks corresponding to the coating critical load. After scratch generation, the track width was measured by SEM observation. Protection performance of silane-based coatings was evaluated at room temperature in near-neutral naturally aerated 0.6 M NaCl solution by anodic polarization scan at 10 mVmin−1 after an open circuit equilibration of 15 min in a single-compartment O-ring cell with a working (active) surface of 1 cm2. A Pt sheet was used as a counter electrode, and an external SCE as a reference electrode, connected to the working compartment via a salt bridge containing the test solution and a Luggin capillary. Data were recorded by means of a PC driven Gamry potentiostat. Long-term immersion tests were performed at room temperature for a period of 7 days, according to ASTM G31 recommendations [27].
3. Results and discussion The anodic oxides grown in micro-arc regime are porous regardless of the cell voltage and the solution composition. SEM images (Fig. 1) of the samples produced in zirconia and alumina containing solutions show that the surfaces have a “fused aspect” due the high temperatures reached within the plasma [28]. Verdier et al. suggest that the pores are likely the traces of the sparks and the round shape structures are the result of bubbles expelled from the oxide during fusion, solidified after cooling. On the contrary, SEM images (Fig. 2) obtained in base solution and the in presence of titania nanoparticles show that oxides are porous but these round shape structures are less evident. It might be thought that lower plasma temperatures are achieved in absence of nanoparticles or in presence of titania, but further investigations are required to confirm and achieve a deep understanding of these results. Regardless of the presence of TiO2, ZrO2, Al2O3 nanoparticles, SEM cross-sections disclose that the coatings contain relatively large voids and the thickness of the anodic oxides is not homogeneous. The oxide thickness is in the range 5–12 μm for zirconia and titania containing solution, 7–18 μm for the oxides produced in alumina containing solution. Conversely, the oxides produced in nanoparticles free solution are thinner. The oxide nanoparticles are incorporated into the coating, as proved by EDS analysis that detects elements such as zirconium and titanium besides magnesium, aluminium and oxygen. As shown in Fig. 3 zirconia nanoparticles (discrete light regions) are not uniformly distributed, the cause of this heterogeneous distribution is uncertain. Zirconia nanoparticles are present between the barrier layer and the outer layer and at the surface of the coatings. According to Arrabal et al. the presence of these nanoparticles at the surface is due to the transport from the inner coating through the discharge channels or to an electrophoretic effect due to the electric field in the electrolyte close to the surface [29]. Fig. 4 shows the SEM tracks images after scratch tests of anodic oxides produced in titania containing solution. There was gross coating spallation detected and a large area delamination can be observed from the coating starting from a critical load LC = 15 N. In fact, when the total load reaches LC value, the substrate as well as the inner and the outer parts of the coating suffer from plastic deformation leading to the collapse of the outer part of the coating. For the inner part is more difficult to delaminate as shown by the line profile taken along the line in the right side of the figure. Actually, the oxygen signal is slightly higher in the area around the groove with respect to the groove track. In the grooves appear also semicircles which are typical of tensile cracks due to the plastic deformation of the more adherent inner part.
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Fig. 8. SEM surface examination after 7-day immersion in NaCl of Mg anodized with ZrO2 and treated with BTSE (top) and OSi (bottom).
The behaviour of the coatings produced in Al2O3 containing solution and in ZrO2 containing solution is quite different (Fig. 5). As shown in Fig. 5 spallation is not present. However, at applied loads higher than the critical one, a series of cracks running perpendicular to the scratch edges are found in both samples. Their morphology can be associated to tensile cracking due to the redistribution of stress induced by plastic deformation. Small differences in the critical load values have been measured for the two samples, LC = 22 N for ZrO2 containing solution and LC = 21 N for Al2O3 rich solution, and the deformation mechanism is in both cases the plastic deformation, as suggested by surface topography. The scratch hardness numbers have been also calculated according to ASTM G-171-03 [30]. As shown in Table 2, samples produced in ZrO2 containing solutions have the highest hardness scratch number, on the contrary those produced in containing TiO2 solutions are softer than those obtained in nanoparticles free solution. In the light of these results, the efficacy of the nanoparticles oxides ZrO2 and Al2O3 addition is evident and for this reason the samples produced in anodizing solutions containing ZrO2 and Al2O3 have been further treated in order to seal the pores and increase the corrosion resistance. Fig. 6 shows anodic polarization curves recorded for AM60B samples treated with bis-ethane (BTSE) and octylsilane (OSi). The silane-based coatings provide some barrier action against corrosion of the bare alloy, as indicated by the electrochemical potential shift towards more positive values with respect to that of uncoated substrate. The higher shift achieved with BTSE could be attributed to more efficient Mg–O–Si bonding due to six hydroxyl groups per molecule available after hydrolysis with respect to octylsilane (three O–CH3 groups per molecule).
Opposite trend is obtained for anodized surfaces, particularly when magnesium oxide is grown in the presence of nanoparticles, as reflected by the decrease of about one order of magnitude of the current density in the passive region (Fig. 6 — right). Section SEM examination of OSiand BTSE-coated substrates showed that the former covers better the surface and penetrates deeper the cracks originated in the anodic oxidation process, as illustrated in Fig. 7. This observation supports the qualitative trend deduced from the polarization scans (Fig. 6). The different degree of adsorption/condensation is more likely attributed to differences in silane reactivity towards the magnesium surface rather than to possible steric hindrance for BTSE penetration taking into account substrates porosity (9.0% when magnesium is anodized in base solution, and 3.5% and 5.5% in solution containing ZrO2 and Al2O3, respectively). Thus, the more hydrophobic OSi is allowed to penetrate deeper the anodized substrates due to contemporary inhibition of magnesium dissolution, thus a better anchorage to the metal surface is obtained. The immersions in 3.5 wt.% NaCl for 7 days were carried out to qualitatively investigate the treatments efficiency under open circuit conditions. Visual examination of the specimens revealed a lower extent of uniform corrosion on the silane-treated bare substrates, in contrast to the un-covered alloy. The locally damaged areas progressively increased much faster on OSi-covered alloy, indicating poorer protection in support to the trend deduced from the polarization responses (Fig. 6). In the case of anodized specimens, no appreciable changes of the surface external appearance were observed, except for those modified with OSi. Randomly distributed small bubbles showed up after one day of immersion in the salt solution. These continue to grow with time and
A. Mandelli et al. / Surface & Coatings Technology 205 (2011) 4459–4465
finally stabilized at the 4th day of the test. This phenomenon was particularly intense for anodized-containing nanoparticles substrates. Bubbles formation is a clear indication of hydrogen evolution promoted by undercoating corrosion, but could suggest as well a more limited transport from the inside of the pores towards the solution. To better understand the nature of the corrosion attack, microscopic examination of silane-modified anodized specimens was carried out. Fig. 8 shows the SEM images of BTSE- (top) and OSi-coated (bottom) anodized alloys with ZrO2. Larger and more irregular pores (initially well-defined as produced in the anodization process, see Fig. 1 — left), are observed on the surface modified with BTSE, in contrast to OSi. Attack of the alloy at the bottom of largest defects can also be appreciated at higher magnifications (right side images). On this basis, the observed bubble formation on OSi-treated anodized substrates reflects an increasing wettability of the surface by contact with the electrolyte. But, their retention for long time on the surface shield part of the metal from the corrosive medium, thus lowering the rate of corrosion. The slow aging of the bubbles on vertically positioned specimens strongly suggest that well-adhered OSi layer with the alkylic tail directed towards the solution has been obtained [31]. These results confirm the best performance of OSi with respect to BTSE, in support to electrochemical tests. Many are the factors that could have influenced the differences between these silane molecules, but there is no doubt that the hydrophobicity of the OSi has played the major role under these experimental conditions. At present, other experiments are in course to more accurately investigate this aspect.
synthesize adherent corrosion resistant coatings on magnesium alloy AM60B in a short process time. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]
[18] [19]
4. Conclusions In this study a composite anodic-oxide silane-based top coat has been produced. The anodic oxides have been grown in an environmentally friendly solution with or without addition of oxides nanoparticles using a micro-arc anodic oxidation technique. Experimental results showed the efficacy of the nanoparticles oxides ZrO2 and Al2O3 in increasing both the adhesion to the substrate and the scratch hardness of the micro arc anodic oxides. Subsequent sealing of the porosity was effectively obtained through silanization procedure using two different silanes, namely octyltrimethoxysilane (OSi) and 1, 2-bis [triethoxysilyl] ethane (BTSE). The silane post-treatments also provided corrosion protection. However, the hydrophobic nature of OSi accounted for a better barrier action of the anodized substrates. The anodizing in oxides nanoparticles rich solutions (ZrO2 or Al2O3) followed by a silane top coat treatment performed using OSi as precursor is an interesting way to
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[20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31]
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Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
A composite coating for corrosion protection of AM60B magnesium alloy A. Mandelli c, M. Bestetti a,⁎, A. Da Forno a, N. Lecis b, S.P. Trasatti c, M. Trueba c a b c
Politecnico di Milano - Department of Chemistry, Materials and Chemical Engineering “G. Natta”, Via Mancinelli 7, 20131 Milan, Italy Politecnico di Milano - Department of Mechanics, Via La Masa 34, 20158 Milan, Italy Università degli Studi di Milano, Department of Physical Chemistry and Electrochemistry, Via Golgi 19, 20133 Milan, Italy
a r t i c l e
i n f o
Article history: Received 20 October 2010 Accepted in revised form 21 March 2011 Available online 26 March 2011 Keywords: Magnesium alloys Micro-arc anodic oxidation Spark discharge Silane-based coatings Corrosion resistance Scratch test
a b s t r a c t Oxide films have been produced on AM60B magnesium alloy by micro-arc anodic oxidation in an environmentally friendly alkaline solution, with and without addition of nanoparticles (TiO2, ZrO2 and Al2O3). Because of the anodic oxide porosity, inherent in the sparking process, organo-functional silanes top coat has been applied to seal pores and cracks, and achieve an efficient protective coating system. The surface and crosssection morphology of samples were analyzed by Scanning Electron Microscopy (SEM) coupled with Energy Dispersive Spectroscopy (EDS). Scratch tests were performed for evaluating the adhesion strength and scratch hardness of the anodic oxides to the AM60B substrate. The corrosion resistance of both anodic oxides and oxide/ silane composite coatings was evaluated in 0.6 M NaCl solution using potentiodynamic polarization tests. The addition of nanoparticles to the anodizing solution doesn't affect significantly the corrosion resistance in comparison with anodic oxides produced in nanoparticles free solutions. Conversely, the adhesion strength and scratch hardness of the anodic oxides to the substrate is quite scattered, and it is higher for the samples produced in ZrO2 and in Al2O3 rich solutions. For this reason specimens anodized in ZrO2 and Al2O3 containing solutions were chosen for silane deposition. Two silanes were used, namely octyltrimethoxysilane (OSi) and 1, 2-bis [triethoxysilyl] ethane (BTSE). The anodizing treatment carried out in oxides nanoparticles containing solutions (ZrO2 or Al2O3), followed by a silane top coat treatment performed using OSi precursor, is an interesting way, suitable for industrial applications, to synthesize adherent corrosion resistant coatings on magnesium alloy AM60B in a short process time. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Magnesium alloys are light structural and functional materials, increasingly used in the automotive, aerospace, and biomaterials industry. Unfortunately, magnesium is too reactive and generally exhibits a poor corrosion resistance because of high dissolution tendency by galvanic coupling or in chlorides environments. Moreover, impurities and second phases act as active cathodic sites causing local galvanic acceleration of corrosion of the matrix [1]. The alloys containing low amounts of iron, copper and nickel are more corrosion resistant, however a surface treatment for protection against corrosion is usually required for aggressive electrolytes exposure. In order to increase the working life of magnesium alloys products, several finishing techniques have been suggested to enhance corrosion resistance such as electroplating, conversion coatings and painting, gas-phase deposition and anodic oxidation [2]. Micro-arc anodic oxidation is a surface treatment technique and emphasis is drawn to its potential for surface modification and enhancement of final top coats adhesion without the necessity to introduce bilayer coatings in order to avoid significant differences in
⁎ Corresponding author. Tel.: + 39 0223993166; fax: + 39 0223993180. E-mail address: [email protected] (M. Bestetti). 0257-8972/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2011.03.066
mechanical properties between the coating and the substrate [3]. Electrolyte composition [4,5] plays a significant role in micro arc anodic oxidation and the oxides properties strongly depend on the electrolyte composition, concentration and conductivity. Recently many papers about plasma electrolytic oxidation have been published, but literature dealing with this process in presence of ceramic nanopowders in the anodizing solution is poor. According to Ko et al. [6] zirconia incorporation in oxides produced by plasma electrolytic oxidation improves oxidation resistance in comparison with oxides produced in zirconia free electrolyte. Liang et al. [7] investigated the properties of oxides produced in sodium phosphate and potassium hydroxide without and with addition of 4% titania sol, using a bi-polar pulsed electrical source. According to the authors, oxide films containing TiO2 compounds present a more uniform morphology than those formed in the titania free electrolyte. XPS and XRD analyses prove that crystalline rutile and anatase are present in the oxide films as well as periclase. The authors ascribe the better corrosion resistance to the TiO2 compounds Table 1 Chemical composition of AM60B alloy (wt.%). Al
Zn
Mn
Si
Fe
Cu
Ni
Mg
5.9
0.05
0.27
0.025
0.0017
0.0017
0.0004
Balance
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Fig. 1. SEM surface morphology of a sample produced in zirconia (left) and in alumina containing solution (right).
Fig. 2. SEM surface morphology of a sample produced in titania containing solution (left) and in nanoparticles free solution (right).
more stable than MgO in aqueous solutions. Recently, Guo et al. [8] have used electrolyte containing polytetrafluoroethylene nanoparticles to produce a novel multifunctional composite coating onto AM60B magnesium alloy with self-lubrication, hydrophobic and anticorrosion
properties. These papers show that by means of this process mixed oxides can be synthesised, containing the elements both from the substrate and from the electrolytic solution, which significantly affect the surface properties. Moreover, the anodic oxides favour the adhesion
Fig. 3. SEM surface and cross-section of an anodic oxide produced in zirconia containing solution.
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Fig. 4. SEM images (top) of track of anodic oxide produced in TiO2 containing solution at different magnifications with the line along which profiles of magnesium (-■-) and oxygen (―) were taken (bottom graph).
of post-treatment coatings, for example painting [9] and sol–gel coatings [10–13]. According to the literature, the effectiveness of the sol–gel coatings is attributed to the ability to seal pores and cracks
produced by micro-arc anodic oxidation. Moreover a multilayer approach (two or three layers) allows to reduce levels of porosity, thus eliminating diffusion paths of aggressive species to the substrate, as
Fig. 5. SEM images of tracks of anodic oxide produced in Al2O3 containing solution (left) and in ZrO2 containing solution (right).
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Table 2 Hardness scratch numbers and critical scratch load (LCN). Samples
LCN (N)
HSP (GPa)
Base solution (b.s.) b.s. + TiO2 b.s. + ZrO2 b.s. + Al2O3
17 15 22 21
1.19 0.87 2.03 1.25
proposed by Tan et al [12]. Recently Malayoglu et al. have shown that sol–gel post-treatment provides the most effective sealing when compared to alkaline phosphate and alkaline silicate coatings [14]. However, it is recalled that the thickness of silane sol–gel coatings is noticeable higher than those obtained by chemical surface conversion coatings or other surface pre-treatments [14,15]. Gnedenkov et al. [16] have suggested composite polymer containing coatings consisting of an anodic oxide produced by plasma electrolytic oxidation (PEO) and polymeric coating achieved using superdispersed polytetrafluoroethlylene (SPTFE). According to the authors this combination of PEO coating with SPTFE provides increased corrosion resistance of metals and alloys, suitable also for implantation surgery. Recently Minaev et al. [17] have proposed a composite coating consisting of a PEO oxide and a polymer layer produced using superdispersed polytetrafluoroethylene. Due to the good penetration into pores of a plasma electrolytic oxidation coating the system provides antiscale and anticorrosion protection. Treatments with hydro-alcoholic solutions of silanes, following the classical steps of immersion and curing, have been mainly used on bare magnesium alloys. N-alkyl-trimethoxy-silane on AZ60 alloys and on magnesium alloy composites reinforced with silicon carbide particles [18] have been studied. Other silane molecules that have been investigated are: γ-glycidoxypropyltrimethoxysilane (γ-GPS) on pure magnesium (99.8%) and commercial AZ31 [19], as well as on AZ91 [20]; 3-mercaptopropyltrimethoxysilane (MPTS) on WE43 [21] and AZ91 [22]; bis-1,2-(triethoxysilyl)ethane (BTSE) and γ-aminopropyltriethoxysilane (γ-APS) on AZ91 [23]; bis-(triethoxysilylpropyl)tetrasulfide (BTESPT) containing rare earth salts such as cerium and lanthanum nitrates [24], as well as bis-(triethoxysilylpropyl)amine (BAS) with carbon nanotubes on AZ31 [25]. A scrutiny of these studies discloses that the relevant aspects influencing performance of silane-based coatings on Mg alloys are the hydrophobicity of the silane molecule (monosilanes, like γ-APS and γ-GPS, exhibit less protection than bis-silanes, the latter giving better protection with the degree of hydrophobicity, e.g. BTSE and BAS) and the chemistry of the coating solution (silane concentration, pH, pre-hydrolysis time, presence of additives with sealing or inhibiting
properties, etc.). To our knowledge, just one very recent work reports on similar approaches to reinforce anodized magnesium [26]. We believe that our approach to the surface treatment of magnesium alloy have some elements of originality both for the use of three different ceramic nanopowders in the anodizing solutions and for the silane-based conversion post- treatments. In this study AM60B magnesium alloy substrates have been anodized by micro-arc regime in an environmentally friendly alkaline solution, with and without addition of nanoparticles (TiO2, ZrO2 and Al2O3). Silane-based solutions of BTSE and octylsilane (OSi) have been investigated as conversion post-treatments in order to produce a composite system able to improve both mechanical properties and corrosion resistance. 2. Experimental The chemical composition of die cast AM60B (Norsk Hydro) alloy used in the experiments is reported in Table 1. Specimens of 100 × 30× 3 mm were cut from Q-panels, polished with P1200 silicon carbide abrasive paper, degreased with acetone, thoroughly rinsed with distilled water and dried in air. The solutions were prepared by using analytical grade reagents and distilled water. The samples were anodized in alkaline electrolyte which consists of 100 g dm−3 sodium phosphate, 25 g dm−3 sodium borate, 20 g dm−3 sodium metasilicate (base solution), with or without addition of 4.0 g dm−3 titania (Degussa P-25) or zirconia (Tosoh TZ-3YS) or alumina (Buehler Ltd, 0.05 μm). Actually, as proved by diffraction laser particle size analyzer (laser granulometer CILAS 1180), the nanoparticles form agglomerates with micrometric size. Two AISI 316 panels were used as cathodes. The anodization process was carried out under voltage control and the initial temperature of the electrolytic solution was 30 °C. During anodizing the cell voltage raised linearly to the maintenance voltage in a fixed time ramp of 255 s, and then it was kept at maintenance voltage, with a total time of 10 min. Different voltages (60–160 V) were applied to the cell in order to assess the operating conditions suitable to produce, at visual inspection, a uniform oxide film. Silane-based coatings were obtained using two silanes, namely octyltrimethoxysilane (OSi) and 1,2-bis[triethoxysilyl]ethane (BTSE). Anodized specimens were dipped for 30 seconds in the 4% silane solution of methanol/water (95:5) with the pH adjusted to 4.3 by adding acetic acid (10 vol%). After dipping, specimens were dried in a hot-air stream and cured in an open-to-air sand oven for 30 min at 110 °C. The silane treatment was tested also on bare AM60B alloys samples. Surface morphology and cross-section of anodized coatings were observed by scanning electron microscopy (SEM) using a Zeiss EVO 50EP microscope at a chamber pressure of
Fig. 6. Anodic polarization scans for Mg substrates treated with: (left) BTSE and (right) OSi; (∙∙∙∙) bare alloy, (-□-) silane-coated alloy, (―) anodized Mg alloy, (-○-) anodized in solution containing ZrO2, (-◊-) anodized in solution containing Al2O3.
Fig. 7. SEM cross-section examination of as-prepared anodized Mg substrates modified with silanes: left — BTSE on anodized Mg; middle — OSi on anodized Mg (the arrow points a crack filled with silane); right: OSi on anodized Mg with Al2O3. (1 — Mg substrate; 2 — oxide layer).
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50 Pa and 20 kV accelerating voltage. Scratch tests were performed using a scratch tester (Microcombi Platform — CSM Instruments) with a load range of 0.3–30 N. Scratches of length 3 mm were made using a diamond Rockwell indenter with a spherical tip radius of 200 μm sliding at a constant speed of 1.26 mm min−1. During the scratch tests the friction force was also measured. The tester was also equipped with acoustic sensors to detect the nucleation of the first cracks corresponding to the coating critical load. After scratch generation, the track width was measured by SEM observation. Protection performance of silane-based coatings was evaluated at room temperature in near-neutral naturally aerated 0.6 M NaCl solution by anodic polarization scan at 10 mVmin−1 after an open circuit equilibration of 15 min in a single-compartment O-ring cell with a working (active) surface of 1 cm2. A Pt sheet was used as a counter electrode, and an external SCE as a reference electrode, connected to the working compartment via a salt bridge containing the test solution and a Luggin capillary. Data were recorded by means of a PC driven Gamry potentiostat. Long-term immersion tests were performed at room temperature for a period of 7 days, according to ASTM G31 recommendations [27].
3. Results and discussion The anodic oxides grown in micro-arc regime are porous regardless of the cell voltage and the solution composition. SEM images (Fig. 1) of the samples produced in zirconia and alumina containing solutions show that the surfaces have a “fused aspect” due the high temperatures reached within the plasma [28]. Verdier et al. suggest that the pores are likely the traces of the sparks and the round shape structures are the result of bubbles expelled from the oxide during fusion, solidified after cooling. On the contrary, SEM images (Fig. 2) obtained in base solution and the in presence of titania nanoparticles show that oxides are porous but these round shape structures are less evident. It might be thought that lower plasma temperatures are achieved in absence of nanoparticles or in presence of titania, but further investigations are required to confirm and achieve a deep understanding of these results. Regardless of the presence of TiO2, ZrO2, Al2O3 nanoparticles, SEM cross-sections disclose that the coatings contain relatively large voids and the thickness of the anodic oxides is not homogeneous. The oxide thickness is in the range 5–12 μm for zirconia and titania containing solution, 7–18 μm for the oxides produced in alumina containing solution. Conversely, the oxides produced in nanoparticles free solution are thinner. The oxide nanoparticles are incorporated into the coating, as proved by EDS analysis that detects elements such as zirconium and titanium besides magnesium, aluminium and oxygen. As shown in Fig. 3 zirconia nanoparticles (discrete light regions) are not uniformly distributed, the cause of this heterogeneous distribution is uncertain. Zirconia nanoparticles are present between the barrier layer and the outer layer and at the surface of the coatings. According to Arrabal et al. the presence of these nanoparticles at the surface is due to the transport from the inner coating through the discharge channels or to an electrophoretic effect due to the electric field in the electrolyte close to the surface [29]. Fig. 4 shows the SEM tracks images after scratch tests of anodic oxides produced in titania containing solution. There was gross coating spallation detected and a large area delamination can be observed from the coating starting from a critical load LC = 15 N. In fact, when the total load reaches LC value, the substrate as well as the inner and the outer parts of the coating suffer from plastic deformation leading to the collapse of the outer part of the coating. For the inner part is more difficult to delaminate as shown by the line profile taken along the line in the right side of the figure. Actually, the oxygen signal is slightly higher in the area around the groove with respect to the groove track. In the grooves appear also semicircles which are typical of tensile cracks due to the plastic deformation of the more adherent inner part.
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Fig. 8. SEM surface examination after 7-day immersion in NaCl of Mg anodized with ZrO2 and treated with BTSE (top) and OSi (bottom).
The behaviour of the coatings produced in Al2O3 containing solution and in ZrO2 containing solution is quite different (Fig. 5). As shown in Fig. 5 spallation is not present. However, at applied loads higher than the critical one, a series of cracks running perpendicular to the scratch edges are found in both samples. Their morphology can be associated to tensile cracking due to the redistribution of stress induced by plastic deformation. Small differences in the critical load values have been measured for the two samples, LC = 22 N for ZrO2 containing solution and LC = 21 N for Al2O3 rich solution, and the deformation mechanism is in both cases the plastic deformation, as suggested by surface topography. The scratch hardness numbers have been also calculated according to ASTM G-171-03 [30]. As shown in Table 2, samples produced in ZrO2 containing solutions have the highest hardness scratch number, on the contrary those produced in containing TiO2 solutions are softer than those obtained in nanoparticles free solution. In the light of these results, the efficacy of the nanoparticles oxides ZrO2 and Al2O3 addition is evident and for this reason the samples produced in anodizing solutions containing ZrO2 and Al2O3 have been further treated in order to seal the pores and increase the corrosion resistance. Fig. 6 shows anodic polarization curves recorded for AM60B samples treated with bis-ethane (BTSE) and octylsilane (OSi). The silane-based coatings provide some barrier action against corrosion of the bare alloy, as indicated by the electrochemical potential shift towards more positive values with respect to that of uncoated substrate. The higher shift achieved with BTSE could be attributed to more efficient Mg–O–Si bonding due to six hydroxyl groups per molecule available after hydrolysis with respect to octylsilane (three O–CH3 groups per molecule).
Opposite trend is obtained for anodized surfaces, particularly when magnesium oxide is grown in the presence of nanoparticles, as reflected by the decrease of about one order of magnitude of the current density in the passive region (Fig. 6 — right). Section SEM examination of OSiand BTSE-coated substrates showed that the former covers better the surface and penetrates deeper the cracks originated in the anodic oxidation process, as illustrated in Fig. 7. This observation supports the qualitative trend deduced from the polarization scans (Fig. 6). The different degree of adsorption/condensation is more likely attributed to differences in silane reactivity towards the magnesium surface rather than to possible steric hindrance for BTSE penetration taking into account substrates porosity (9.0% when magnesium is anodized in base solution, and 3.5% and 5.5% in solution containing ZrO2 and Al2O3, respectively). Thus, the more hydrophobic OSi is allowed to penetrate deeper the anodized substrates due to contemporary inhibition of magnesium dissolution, thus a better anchorage to the metal surface is obtained. The immersions in 3.5 wt.% NaCl for 7 days were carried out to qualitatively investigate the treatments efficiency under open circuit conditions. Visual examination of the specimens revealed a lower extent of uniform corrosion on the silane-treated bare substrates, in contrast to the un-covered alloy. The locally damaged areas progressively increased much faster on OSi-covered alloy, indicating poorer protection in support to the trend deduced from the polarization responses (Fig. 6). In the case of anodized specimens, no appreciable changes of the surface external appearance were observed, except for those modified with OSi. Randomly distributed small bubbles showed up after one day of immersion in the salt solution. These continue to grow with time and
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finally stabilized at the 4th day of the test. This phenomenon was particularly intense for anodized-containing nanoparticles substrates. Bubbles formation is a clear indication of hydrogen evolution promoted by undercoating corrosion, but could suggest as well a more limited transport from the inside of the pores towards the solution. To better understand the nature of the corrosion attack, microscopic examination of silane-modified anodized specimens was carried out. Fig. 8 shows the SEM images of BTSE- (top) and OSi-coated (bottom) anodized alloys with ZrO2. Larger and more irregular pores (initially well-defined as produced in the anodization process, see Fig. 1 — left), are observed on the surface modified with BTSE, in contrast to OSi. Attack of the alloy at the bottom of largest defects can also be appreciated at higher magnifications (right side images). On this basis, the observed bubble formation on OSi-treated anodized substrates reflects an increasing wettability of the surface by contact with the electrolyte. But, their retention for long time on the surface shield part of the metal from the corrosive medium, thus lowering the rate of corrosion. The slow aging of the bubbles on vertically positioned specimens strongly suggest that well-adhered OSi layer with the alkylic tail directed towards the solution has been obtained [31]. These results confirm the best performance of OSi with respect to BTSE, in support to electrochemical tests. Many are the factors that could have influenced the differences between these silane molecules, but there is no doubt that the hydrophobicity of the OSi has played the major role under these experimental conditions. At present, other experiments are in course to more accurately investigate this aspect.
synthesize adherent corrosion resistant coatings on magnesium alloy AM60B in a short process time. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]
[18] [19]
4. Conclusions In this study a composite anodic-oxide silane-based top coat has been produced. The anodic oxides have been grown in an environmentally friendly solution with or without addition of oxides nanoparticles using a micro-arc anodic oxidation technique. Experimental results showed the efficacy of the nanoparticles oxides ZrO2 and Al2O3 in increasing both the adhesion to the substrate and the scratch hardness of the micro arc anodic oxides. Subsequent sealing of the porosity was effectively obtained through silanization procedure using two different silanes, namely octyltrimethoxysilane (OSi) and 1, 2-bis [triethoxysilyl] ethane (BTSE). The silane post-treatments also provided corrosion protection. However, the hydrophobic nature of OSi accounted for a better barrier action of the anodized substrates. The anodizing in oxides nanoparticles rich solutions (ZrO2 or Al2O3) followed by a silane top coat treatment performed using OSi as precursor is an interesting way to
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