Corrosion Properties of Polydopamine Coatings Formed in One-Step

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Corrosion Properties of Polydopamine Coatings Formed in One-Step Immersion Process on Magnesium Ferdinand Singer,† Magdalena Schlesak,† Caroline Mebert,† Sarah Höhn,† and Sannakaisa Virtanen*,† †

Chair for Surface Science and Corrosion, Department for Materials Science, University of Erlangen-Nuremberg, Martensstr. 7, 91058 Erlangen, Germany S Supporting Information *

ABSTRACT: Polydopamine layers were polymerized directly from Tris(hydroxymethyl)aminomethane-buffered solution in a one-step immersion process onto magnesium surface. Scanning electron microscopy showed successful formation of a ∼1 μm thick layer. ASTM D3359−09 “Tape test” revealed excellent adhesion of the layer. X-ray induced photoelectron spectroscopy and Fourier transform infrared spectroscopy verified the presence of polydopamine on the surface. Corrosion measurements were performed in 0.1 M NaCl solution investigating the influence of coating parameters: dopamine concentration, immersion time, solution pH, and immersion angle. Tafel analysis revealed strong improvement of corrosion behavior compared to bare magnesium. Polydopamine layers prepared with optimized coating procedure showed promising corrosion properties in Dulbecco’s modified Eagle medium. In summary, polydopamine coatings offer a simple treatment for magnesium to improve the corrosion behavior and could further act as intermediate layer for further surface functionalization. KEYWORDS: magnesium, polydopamine, FTIR, corrosion, coating

1. INTRODUCTION

Materials adapted from nature, such as mussel inspired polydopamine (PD), offer an elegant method for biocompatible coatings.6 Dopamine is part of the human neurotransmitter system.7 By addition of alkaline buffered solutions, dopamine can easily be polymerized to PD.6 The exact polymerization mechanism and the resulting polymer structure are still a topic of discussion as well as the biocompatibility of PD.6,8−11 Nevertheless, PD exhibits excellent adhesion properties. Several studies suggest that PD coatings offer a variation of subsequent surface modification possibilities, for example, formation of hydrophobic monolayers and adsorption of proteins.12−16 Because of the alkaline polymerization process, PD is an interesting material for direct coating formation during polymerization by immersion of Mg in an aqueous solution, as the alkaline solution will decrease Mg corrosion during the coating process. For the first time, this study reports the formation of PD on magnesium by immersion technique in an alkaline Tris(hydroxymethyl)-aminomethane-buffered solution (TBS). The chemical composition of the formed coating was investigated with X-ray induced photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR). The surface structure and the adhesion were examined with scanning electron microscopy (SEM) and the ASTM “Tape

Magnesium (Mg) and its alloys are being considered as suitable materials for degradable implant applications such as stents or bone fixation screws or plates.1 The main advantages of Mg implants are the high biocompatibility, the favorable mechanical properties, and the possible application as temporary implants. The use of Mg as temporary biodegradable implant is possible due to the low corrosion resistance of Mg in aqueous solutions. The susceptibility of Mg to corrosion is a result of its high ionizing force to form Mg2+ ions; with a standard potential of −2.34 V versus SHE, it is the highest for industrially applied metals, leading also to a high sensitivity to internal galvanic corrosion between the Mg matrix and phases of alloying elements or impurities. In addition, the naturally grown porous Mg(OH)2/MgO surface layer offers little protection against dissolution induced by H2O or Cl−. As a drawback of the corrosion of Mg, evolution of hydrogen and the increase of pH in the vicinity of the surface raise concerns on the use as implant material.2 Although the corrosion rate of Mg decreases in neutral, static electrolytes due to the increase of pH and the formation of a partially protective corrosion product layer, control of Mg corrosion remains one of the main challenges to resolve. In general, three strategies are employed to overcome this: alloying, coating, or a combination of both.3,4 Coating with biocompatible materials offers the possibility to use Mg as implant material without the complications of alloying elements such as concerns regarding Al present in AZ alloys.5 © XXXX American Chemical Society

Received: September 17, 2015 Accepted: November 11, 2015

A

DOI: 10.1021/acsami.5b08760 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

PD formation on the achieved corrosion properties. The obtained polarization curves were analyzed using Tafel plots. For this, the lines were plotted at least 50−100 mV cathodic or anodic to Ecorr over at least one decade of current density.19

Test” method. The corrosion behavior of Mg, Mg coated by immersion in TBS only, and Mg coated by immersion in TBS with dopamine was explored by potentiodynamic polarization measurements in 0.1 M NaCl solution and in Dulbecco’s modified Eagle medium (DMEM). The influence of solution pH, immersion angle, dopamine concentration, and coating time on the achieved coating properties was investigated.

3. RESULTS AND DISCUSSION 3.1. Surface Characterization and Adhesion. For SEM and XPS investigation, coatings were prepared in 50 mmol/L of TBS solution with a pH of 10 and a dipping angle of 0° for 2 h of immersion time without and with 2 mg/mL of dopamine. Figure 1, panels a and b show characteristic features of magnesium immersed in an alkaline solution.

2. EXPERIMENTAL SECTION A commercially pure magnesium rod (25.4 mm diameter, 99.9% purity, Chempur Feinchemikalien and Forschungsbedarf GmbH) was employed. The rod was cut into 2 mm thick slices and ground on Microcut 1200 paper disc (Bühler GmbH) using ethanol as lubricant. The samples were cleaned in ethanol in an ultrasonic bath and dried under hot air. All chemicals were obtained from Sigma-Aldrich, used without further purification, and were of reaction grade purity. Highpurity water with a resistivity of 18.2 MΩ cm was used as a solvent. The PD coating was achieved by immersing the Mg samples in 70 mL of 50 mmol/L of TBS (pH = 10) containing dopamine. Upon addition of dopamine to the base solution, the solution immediately began to color to an increasingly dark brown. Three samples were coated simultaneously in 70 mL of immersion solution. To investigate the influence of the different immersion coating parameters on the layer formation and resulting corrosion properties, the parameters were adjusted as described in the following. The pH of the TBS solution was adjusted using concentrated (70%) nitric acid or sodium hydroxide. Different coating angles of the surface were achieved by tilting the sample (at 0° the sample was lying on the bottom of the employed glass beaker and the upper side was the investigated surface) to investigate the influence of precipitation of PD, polymerized in solution, on the surface. Furthermore, different coating times and dopamine concentrations were investigated. Mg samples were prepassivated in 1 M NaOH solution by immersion for 1, 6, and 24 h prior to immersion in ether TBS with pH 10 or TBS with pH 10 + 2 mg/mL dopamine for 2 h to investigate the influence of Mg(OH)2 layers on the PD formation. The surface morphology was investigated with a field emission scanning electron microscope (Hitachi, FE-SEM S-4800). Crosssection of PD coating was prepared with an IM4000 (Hitachi) ion milling system. The chemical composition of the surface was characterized by XPS (PHI-5600), using Al Kα radiation, and FTIR (Thermo Scientific, Nicolet 6700 FT-IR with ATR-unit). Adhesion measurements were carried out following ASTM D3359−09 “Standard Test Methods for Measuring Adhesion by Tape Test”17 on PD coated magnesium samples. The corrosion properties of the immersed samples were determined using a three-electrode-system and a bottom hole cell (diameter = 1.5 cm) consisting of a Pt disc (1 cm2) as a counter electrode, an Ag/AgCl (3 M KCl) reference electrode, and the coated samples as working electrode. The Ag/AgCl electrode was composed of two Haber−Luggin capillaries. The first was filled with 3 M KCl and the second one with the test electrolyte. Thus, diffusion of Cl− into the test electrolyte in the proximity of the surface was hindered and an increased corrosion of the surface due to a surplus of Cl− prevented. The test electrolytes were 0.1 M NaCl solution and DMEM (Biochrom AG; composition, ref 18). DMEM was used to simulate human body environment, and thus the experiments with DMEM were carried out at 37 °C. A Zennium potentiostat (Zahner Elektrik GmbH and Co. KG) was employed to conduct polarization measurements. These were performed after 15 min at open-circuit in the electrolyte. The scan started −300 mV relative to OCP to 0 V with a rate of 1 mV/s. The scan was stopped when the current density exceeded 10 mA/cm2. All experiments were repeated three times, and the average is shown. All measurements are shown with respect to the exposed surface area. Since immersion in pure TBS solution could already improve the corrosion properties of Mg due to its alkaline nature, additional reference samples were prepared by immersing Mg samples in the pure TBS solution. The behavior of these samples is compared with samples coated by immersion in TBS solution with dopamine monomer to enable a differentiation between hydroxide and

Figure 1. SEM images of (a and b) magnesium immersed for 2 h in TBS and (c and d) magnesium coated with PD for 2 h.

The formed Mg(OH)2/MgO layer (Figure 1a,b; Mg immersed in pure TBS) shows a similar topography as the magnesium surface beneath, showing cracks aligned by the underlying grinding grooves. With higher magnification, the beginning of typical Mg(OH)2/MgO platelet structure growth20−22 can be seen. In contrast to the formed layer in TBS, the PD-coated samples in Figure 1, panels c and d show a smoother surface indicating the presence of PD. Nevertheless, the coating is still aligned with the grinding grooves underneath and shows aligned cracks; this indicates a relative thin coating. By higher magnification, it can be seen that the presence of PD reduces the amount of Mg(OH)2/MgO platelets on the surface. In the vicinity of cracks, the platelet structure becomes more obvious. This indicates that the PD does not form a homogeneous layer on the surface but rather a mixture of Mg(OH)2/MgO/PD. This type of coating formation may be due to the simultaneous formation of PD and Mg(OH)2 during the coating process, as the employed solution has a pH of 10. That is, Mg dissolution (production of Mg2+) is not completely prevented, and the alkaline pH favors formation of Mg(OH)2. The formation of Mg(OH)2 could also explain the presence of cracks in addition to the grinding grooves. As Mg(OH)2 is formed, H2 evolves forming gas bubbles on the metal surface, eventually detaching and rupturing the coating at the weakest points. Figure 2, panel a shows a cross-section of PD coated magnesium sample with the coating prepared as described earlier. The layer thickness was found to be 977 nm ±123 nm thick. The layer, as can be seen in Figure 2, panel b, seems to be nonporous and smoothly attached to the magnesium surface. Nevertheless, as already observed in Figure 1, panels c and d, B


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Figure 2. SEM pictures of (a) cross-section and (b) detailed picture of ruptured PD layer on magnesium.

the coating shows cracks in the layer, most likely aligned with the grinding grooves. The cracks could be the result of H2 formation on the ridges of the grinding grooves. Adhesion was tested on three different samples, and the average behavior can be seen in Figure 3. The PD layers were

Figure 4. XPS of magnesium (a) immersed for 2 h in TBS and (b) coated with PD for 2 h.

of TRIS as well as to the adsorption of carbon containing contaminations from atmosphere, as was shown by Fotea et al.25 Dominating is the presence of oxygen and magnesium at the surface. Additionally, the Mg 2p peak shows no metallic part (in the high-resolution spectra, not shown). Thus, the formed layer, as well as the amount of measured Mg, seems to be mostly bound in either in Mg(OH)2 or MgO, and the layer thickness exceeds the maximal measurement depth possible with XPS. By addition of PD to the coating solution, the amount of N measured on the coated magnesium increases as well as the amount of carbon. This and the reduced amount of Mg and O present at the surface indicates the formation of PD at the surface. Since the amount of Mg and O is still high, the layer seems to be consist with a mixture Mg(OH)2/MgO and PD, as already indicated by the morphology of the surface. To investigate the formation of PD at the magnesium surface, FTIR was carried out. Figure 5, panel a shows FTIR spectra for Mg immersed for 2 h in 50 mmol/L of TBS solution with a pH of 10 and a dipping angle of 0°; Figure 5, panel b shows PD derived from 50 mmol/L of TBS with 2 mg/mL of dopamine without the presence of Mg in the solution. The formed PD was centrifugalized from solution and washed with isopropanol. Figure 5, panel c shows the result for FTO coated with 2 mg/mL of PD in 50 mmol/L of TBS solution with a pH of 10 and an immersion angle of 0°. Finally, Figure 5, panel d shows the data obtained for PD coated magnesium. The coating was achieved with the same procedure as for SEM and XPS. Table S2 (Supporting Information) shows the assigned bonds to the occurring bands for the different recorded spectra. Magnesium immersed in TBS solution shows a characteristic band at 3696 cm−1 that is assigned to the ν(O−H) stretching modes of free −OH occurring in Mg(OH)2. A shallow band

Figure 3. Macroscopic pictures of PD coated Mg (a) as received and (b) after adhesion test.

produced by applying the same procedure as for the SEM investigation. The PD appears as a brownish layer on the surface, as can be seen in Figure 3, panel a. The brownish color indicates either a very thin layer or a mixed layer, as literature reports PD layers to range from a dark brown to black in color.23,24 Also, the grinding grooves are still visible, and the layer appears to be slightly inhomogeneous. As already mentioned, this could be a result from H2 bubbles formed and adhered temporarily to the surface, hindering the growth of PD. ASTM “Tape Test” appears to leave the surface unchanged indicating excellent adhesion with less than 5% removal of coating, thus confirming the already proposed excellent adhesion.6,24 3.2. Chemical Composition of PD Layers Fabricated on Magnesium. The presence of PD and the absence of TBS on the surface were investigated by XPS. Figure 4, panel a shows the obtained spectra for Mg immersed in TBS solution, and Figure 4, panel b depicts the spectra for PD-coated Mg. Table S1 (Supporting Information) shows the gained amount of elements in weight percent (wt %). Magnesium immersed in TBS shows 1 wt % of N present on the topmost surface layer indicating adsorption or incorporation of TRIS in the formed Mg(OH)2/MgO layer. The carbon content of 10 wt % can be partially due to the presence C


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aromatic amino groups. At 1375 cm−1, the band is allocated to indole ring νring(CNC) stretching modes. Some δas(C−H) asymmetric bending vibrations were observed at 1294 cm−1. Between 1225 and 950 cm−1, the fingerprint area of aromatic molecules can be seen. At 886 cm−1, δring(C−H) deformation modes for an isolated hydrogen occur.15,26,27 If the presence of the hydrogen bonds or the presence of indole features indicate a tendency to any of the proposed PD structures, no definite conclusions can be derived from the available data. The absence of bands around 1650 cm−1 is possibly a cue to the absence of TBS or the complete polymerization of dopamine, which came in contact with the surface, if the moderate sensitivity of FTIR is considered. FTIR spectra of PD-coated FTO show the same bands as described earlier for pure PD. The only difference is a band appearing at 743 cm−1 assigned to γ(C−H) out of plane deformation modes. Magnesium coated with PD shows similar bands as PD derived without the presence of magnesium. The characteristic amino and indole features in the area around 1550−1300 cm−1 indicate a successful coating with PD. The difference between Figure 5, panels b and c can be seen at 3690 cm−1. This band is allotted to ν(O−H) stretching modes for free OH− groups similar to Mg immersed in TBS solution (Figure 5a). These −OH groups can be assigned to the presence of Mg(OH)2 caused by the alkaline coating condition. The presence of hydroxide groups and the broad band around 3200 cm−1 could not only derive from inter- or intramolecular interaction, but also could be a cue to the adsorption mechanism of PD to the magnesium surface via hydrogen bonds. 3.3. Corrosion Behavior of PD-Coated Magnesium. 3.3.1. Influence of Coating Parameters on the Corrosion Behavior of Magnesium. The influence of the different coatings parameters in the corrosion behavior was studied by polarization measurements in 0.1 M NaCl solution. Figure 6, panels a, c, and e show the corrosion behavior of coatings formed on Mg after immersion in TBS for different pH, dipping angle, and immersion time in the presence of dopamine in the immersion solution. Figure 6, panels b, d, and f show, respectively, the results for coatings formed by immersion of Mg in TBS without the addition of dopamine. Figure 6, panel g displays the obtained polarization behavior in 0.1 M NaCl for bare Mg, Mg after immersion in TBS, and Mg coated with PD for different PD concentrations. A representative curve for the three measurements performed for each parameter setting is shown. 3.3.1.1. Corrosion Behavior of Mg Coated with PD in Solutions of Different pH Values. Different pH values for the coating solution were achieved by addition of nitric acid or aqueous sodium hydroxide to 50 mM TBS solution. The investigated pH values were 8, 9, 10, 11, and 12, as can be seen in Figure 6, panels a and b. Besides the coating solution pH, other parameters of immersion conditions studied were a dipping angle of 0°, 2 h (Figure 6b) and for PD coating the addition of 2 mg/mL dopamine (Figure 6a). The resulting corrosion potential (Ecorr) and corrosion current density (icorr) are depicted in Table 1. Polarization experiments show that the corrosion behavior for Mg immersed in TBS and in TBS/dopamine is similar to behavior for pH values of 9−12. The cathodic as well as the anodic current density shows charge transfer controlled reaction behavior in a similar range for all coating conditions. The anodic current density also shows diffusion-controlled dissolution as the current density approaches the measurement

Figure 5. FTIR spectra of (a) magnesium immersed for 2 h in TBS, (b) PD produced without the presence of magnesium in TBS for 2 h, (c) PD coated FTO glass, and (d) PD-coated magnesium.

appears at 1659 cm−1 related to δ(N−H) deformation modes resulting from the −NH2 groups of the Tris(hydroxymethyl)aminomethane (TRIS) molecule. The wide shallow bands at 1160 and 1031 cm−1 are allocated to ν(C−O) stretching modes, and the band 862 cm−1 is assigned to δ(C−H) out of plane modes. As indicated by Fotea et al.,25 the referring bands could as well occur due to the presence of atmospheric contaminations, but in combination with XPS data, it is more likely that TRIS was partially incorporated into the surface layer alongside with the formation of Mg(OH)2. PD produced without the presence of magnesium shows a quite different band structure compared to Mg immersed in TBS. The broad band between 3300 and 3000 cm−1 is assigned to ν(O−H) and ν(N−H) stretching modes possibly resulting from intra hydrogen bonds of PD molecules. The FTIR results and the resulting information on chemical bonds of the formed PD can only be interpreted with respect to the unsettled topic of the structure of PD. A polymer structure featuring a C−C intermonomer bond was suggested previously.6,26 According to these structures, the formation of hydrogen bonds would originate in the tertiary or quaternary structuring of dopamine oligomers. According to Dreyer et al.,8 the hydrogen bonds could also be the link between monomers forming the polymer. Since the exact structure is not fully understood, the origin of the measured hydrogen bonds remains a topic of discussion. The bands appearing between 2955 and 2853 cm−1 are assigned to ν(C−H) aliphatic stretching modes. The bands at 1575, 1552, and 1460 cm−1 represent νring(CC) and at 1514 cm−1 νring(CN) stretching modes indicating the presence of D


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This might be a result of the coating process where a slightly increased H2 formation compared to lower pH might randomize a homogeneous coating formation. This increase of H2 evolution might be a result of an increased Mg(OH)2 formation due to higher OH− concentration, consuming the formed Mg2+ ions. As Mg(OH)2 precipitates from solution, the equilibrium of this reaction forces new formation of Mg(OH)2, and thereby the redox reaction is excited to form new Mg2+ (to gain the equilibrium state again) and results in an increased Mg(OH)2 and H2 formation in the same time period compared to lower pH values (based on the principle of Le Chatelier). The most noteworthy results are observed for surface modification by immersion in solution of pH 8. In this case, the corrosion behavior significantly changes with and without presence of dopamine in the coating solution. The cathodic current density is similar compared to higher pH values. The anodic behavior however shows a significant decrease of current densities, indicating a slower charge transfer controlled dissolution until a potential of approximately −1.3 V. At this point, the current drastically increases until measurement breakoff. The slower dissolution behavior looks similar to spontaneously passive surfaces with a breakthrough of protective layer at a critical potential. Nevertheless, the currents in the range from 10−100 μA/cm2 are still rather high and the “passive range” too narrow for true passivity. As PD formation takes place and Mg(OH)2 is formed on Mg in alkaline conditions, an increase of corrosion resistance could be expected with increasing pH value. A pH value of 12 was the highest tested in this study, as Bernsmann et al.28 reported that PD dissolves in solutions with pH values of 13 or higher. Lower pH values were not tested, as corrosion of Mg during the coating process would increase with lower pH values, hindering the formation of a homogeneous layer due to an intensification of H2 evolution on the Mg surface. Prepassivated Mg surfaces in 1 M NaOH solution at room temperature could offer less H2 evolution and Mg(OH)2 formation during coating process. The corrosion resistance indeed increased with elongated prepassivation time after immersion of Mg in TBS but decreased for PD coated Mg (see Supporting Information Figure S1 and Table S3). The findings in Figure 6, panels a and b do not support the expectation of better corrosion protection achieved after immersion of Mg in solutions of higher pH values. The results for Mg immersed in TBS indicate that the increase of pH does not increase the amount of Mg(OH)2 formed or decreased the density of the formed layer and thus the corrosion resistance. Another reason for the similar corrosion resistance after immersion in solution with pH 9−12 could be the unfavorable Pilling−Bedworth ratio of Mg(OH)2 to Mg, which results in a more porous (or “cracked”) Mg(OH)2 layer with increasing thickness due to higher internal stresses. As mentioned earlier, the lowest icorr value was determined for

Figure 6. Corrosion behavior measured in 0.1 M NaCl solution of Mg either immersed in TBS + dopamine (a, c, e) or in TBS only (b, d, f). Influence of the pH value (a) TBS + dopamine, (b) TBS only. Influence of the dipping angle (c) TBS + dopamine, (d) TBS only. Influence of the coating time (e) TBS + dopamine, (f) TBS only. (g) Influence of the dopamine concentration.

breakoff criteria of 10 mA/cm2. The corrosion current density is slightly decreasing upon addition of dopamine, indicating the above-discussed formation of a PD layer. The change of pH of the immersion solution and the addition of PD appears to be without consistent influence on the corrosion potential. Ecorr for pH between 9 and 12 varies from −1.5 V to −1.55 V. At pH 10, the results show a slightly lower icorr than for all other pH values, but here a highest standard deviation of all PD coated Mg samples is observed.

Table 1. Ecorr and icorr in 0.1 M NaCl of Mg Either Immersed in TBS or in TBS + 2 mg/mL of Dopamine for 2 h in Dependency of pH Value TBS

PD

pH

Ecorr (V)

±

icorr (μA/cm )

±

Ecorr (V)

±

icorr (μA/cm2)

±

8 9 10 11 12

−1.49 −1.51 −1.54 −1.51 −1.53

0.01 0.01 0.01 0.01 0.02

15.8 32.7 33.8 23.1 52.4

0.62 1.4 3.3 7.9 17.2

−1.50 −1.54 −1.50 −1.54 −1.56

0.01 0.00 0.05 0.01 0.01

6.8 21.1 10.9 21.6 24.9

1.9 1.32 5.6 1.6 2.5

2

E


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Table 2. Ecorr and icorr in 0.1 M NaCl of Mg Either Immersed in TBS or in TBS + 2 mg/mL of Dopamine for 2 h in Dependency of Dipping Angle TBS

PD

angle (°)

Ecorr (V)

±

icorr (μA/cm )

±

Ecorr (V)

±

icorr (μA/cm2)

±

0 30 45 90 135

−1.54 −1.51 −1.51 −1.49 −1.50

0.01 0.01 0.01 0.01 0.01

33.8 23.1 32.7 32.7 71.9

3.3 7.9 1.4 1.4 20.4

−1.50 −1.54 −1.54 −1.55 −1.52

0.05 0.01 0.00 0.01 0.01

10.9 21.6 21.1 17.7 48.7

5.6 1.6 1.3 1.1 2.9

2

Table 3. Ecorr and icorr in 0.1 M NaCl of Mg Either Immersed in TBS or in TBS + 2 mg/mL of Dopamine in Dependency of Immersion Time TBS

PD

coating time

Ecorr (V)

±

icorr (μA/cm2)

±

Ecorr (V)

±

icorr (μA/cm2)

±

10 min 1h 2h 4h 6h 24 h 48 h

−1.5 −1.52 −1.65 −1.45 −1.47 −1.49 −1.56

0.04 0.01 0.02 0.01 0.02 0.01 0.00

44.7 15.9 22 10.3 20.6 33.5 60.4

18.4 4.4 4.9 2.6 4.7 6.6 17.5

−1.55 −1.56 −1.50 −1.52 −1.54 −1.49 −1.51

0.00 0.00 0.00 0.03 0.02 0.01 0.01

18.5 19.2 10.9 18.2 19.8 18.9 13.1

2.3 2.5 5.6 5.9 2.3 5.2 2.8

surface treatments carried out at pH 8. Presence of dopamine in the coating solution generally improves the corrosion properties of the coatings slightly. Coatings obtained at a pH of 8 and 10 showed the best performance. PD formation at pH 8 mimicks the natural formation conditions present in seawater.6,11 The increase of corrosion resistance at pH 10 could result from an increased formation rate of PD on the surface, as the measurements showed that Mg immersed in pH 10 TBS does not increase icorr. This is somewhat contradictory to literature, as the solubility of PD is supposed to increase with pH,29 and therefore the deposition of PD on the surface should decrease. One explanation applicable for surface layers obtained with and without PD could be the slight increase of H2 evolution with rising pH as discussed earlier, which results in a more porous and thicker Mg(OH)2 layer (for TBS immersion) or a more inhomogeneous PD layer formation (for PD coating) or more cracked surface layers due to increasing internal stresses with increasing layer thickness (for both). The decrease of icorr for Mg coated with PD at pH 10 could be the result of an optimal combination of concurrent formation of Mg(OH)2 and PD. 3.3.1.2. Corrosion Resistance of Coated Mg in Dependence of the Immersion Angle. Figure 6, panels c and d show the corrosion behavior of Mg immersed in TBS with addition of dopamine (Figure 6c) and in TBS only (Figure 6d) in connection with immersion angles of 0, 30, 45, 90, and 135°. Table 2 pictures the obtained Ecorr and icorr values for the described measurements. The remaining coating parameters were: 2 h of immersion, pH 10, and for PD coating with the addition of 2 mg/mL of dopamine. The corrosion behavior of Mg coated by immersion in pure TBS appears to be independent of the dipping angle, as can be seen in Figure 6, panel d and Table 2. Ecorr as well as icorr only slightly alter with respect to the coating angle. Only “backside” coating at 135°, where the investigated surface faces the beaker bottom and not the solution surface as for the other angles, appears to exhibit worse corrosion behavior. In presence of dopamine in the coating solution, the formed PD coatings show for all cases some improvement of the corrosion resistance. Ecorr

shows similar behavior as for TBS. The corrosion current density is slightly decreasing with decreasing dipping angle. In general, the measured current density displays charge transfer controlled reaction behavior for both cathodic and anodic reactions independent of the dipping angle and the presence of dopamine during immersion. All investigated angles, regardless of the presence of dopamine in solution, show diffusion controlled dissolution mechanism while approaching the breakoff criteria. The improved corrosion behavior of PD-coated Mg with an immersion angle of 0° is in agreement with literature.11 The formation of PD on the surface seems to depend on the deposition of already polymerized dopamine on the surface by gravitation, independent of the base material. This would also explain, besides the effect of permanent remaining H2 bubbles, the strongly decreased corrosion resistance of “backside”-coated samples. 3.3.1.3. Influence of Immersion Time on the Corrosion Behavior of PD-Coated Mg. Corrosion properties of coatings formed by varying immersion times were investigated by coating Mg in 50 mM TBS at pH 10, a dipping angle of 0°, and 2 mg/mL of dopamine for up to 48 h (Figure 6e) as well as with the same parameters without dopamine (Figure 6f). Ecorr and icorr determined by applying Tafel equation are shown in Table 3 (Supporting Information). The corrosion resistance of Mg treated in TBS solution shows no clear dependency on the immersion time. Corrosion potential as well as corrosion current density are in a similar range for all immersion times. For 4, 6, 24, and 48 h, the anodic current density shows, increasing with time, a growing current plateau similar to spontaneously passive materials and “passivity” breakdown. Still, currents are too high for true passivity. The current flow probably is somewhat hindered by a formed Mg(OH)2 layer. As this plateau becomes more distinct with longer immersion times, the corrosion current density increases. This could result from increased Mg(OH)2 layer formation but higher porosity due to simultaneous H 2 evolution. For PD-coated Mg, no immersion time results in current density plateau formation. Instead, although icorr is F


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Bare Mg exhibits the lowest corrosion resistance with an icorr of 50 μA/cm2. The cathodic reaction is charge transfer controlled. The anodic reaction is charge transfer controlled at the beginning but changes to diffusion-controlled dissolution in proximity to breakoff criteria of 10 mA/cm2. The measurements for bare Mg also display the highest deviation, which could be due to the increased influence of varying surface conditions, such as the distribution of impurities,2 as the employed Mg samples were only of 99.8% purity. Corrosion potential of untreated Mg is similar to −1.53 V to Ecorr obtained for the treated Mg samples. Mg immersed in TBS has a slightly increased corrosion resistance. Cathodic reaction shows the same behavior and almost the same reaction rates as for the bare sample. The anodic behavior is slightly changed; still, charge transfer controlled dissolution is the dominant mechanism, but the slope of current increase is lower. This could indicate a deceleration of dissolution due to the presence of a Mg(OH)2 layer. For PD-coated Mg, a maximum of corrosion resistance is obtained for a dopamine concentration of 1 mg/mL with an icorr of 4.3 μA/cm2. The cathodic and anodic behavior for the other investigated dopamine concentrations is similar to Mg immersed in TBS. The corrosion rate of Mg coated in the optimum concentrated dopamine solution hence decreased by a factor of about 10 as compared to bare Mg. For 1 mg/mL, a narrow anodic passivity region, as already discussed, followed by some kind of layer breakdown and rapid increase of current is observed. Corrosion potential is shifted to a more anodic potential of −1.45 V. Dopamine concentration of 1 mg/mL representing the best concentration for PD layer formation is controversial to the literature. While Bernsmann et al.28 appoint it as a sufficient concentration for PD layer formation, Liu et al.24 claim that 2 mg/mL of dopamine in solution is necessary to form a sufficient layer. As already discussed earlier, analyzing FTIR on Mg, the most likely formed layer is a combination of PD and Mg(OH)2. Under these circumstances, it is possible that an equilibrium between Mg(OH)2 and PD formation on the surface leads to optimized corrosion resistance. Thus, concentrations below 1 mg/mL could lead to incomplete layer formation, whereas a

slightly lower in general, the maximum corrosion resistance is achieved after 2 h coating procedure with an icorr of 8.3 μA/cm2. A coating time of 2 h seems to lead to the most favorable results regarding the formation of a corrosion lowering PD layer on Mg. As already discussed analyzing FTIR measurements, the most likely formed layer appears to be a mix of PD and Mg(OH)2. As a consequence, 2 h immersion time appears to form the most favorable combination of both. Incomplete layer formation could be the reason for higher corrosion rates with lower submerging times. For increasing process times, the combination of two mechanisms could lead to a decrease of corrosion resistance. As more Mg(OH)2 is formed, the simultaneously formed H2 might lead to a more porous coating or might detach the formed PD of the surface. In combination with the formed porous Mg(OH)2 layer, a reduction of PD formation rate due to O2 lack in solution24,30 or depletion of dopamine monomer11,24 could lead to decreasing corrosion resistance for longer immersion times. 3.3.1.4. Effect of Dopamine Concentration during Coating Process on the Corrosion Properties of Magnesium. Figure 6, panel g shows the polarization measurements in 0.1 M NaCl of bare Mg, Mg immersed in TBS at pH 10 with a dipping angle of 0° for 2 h, and for various coatings produced by adding different amounts of dopamine (in mg/mL) to TBS solution for 2 h. Table 4 shows the corresponding Ecorr and icorr values obtained by applying Tafel analysis. Table 4. Ecorr and icorr in 0.1 M NaCl of Bare Mg, Mg Immersed in TBS, and Mg Coated with Different Dopamine Concentrations concentration (mg/mL)

Ecorr (V)

±

icorr (μA/cm2)

±

reference TBS 0.5 1 2 3 4 8

−1.53 −1.54 −1.53 −1.45 −1.50 −1.56 −1.55 −1.54

0.02 0.01 0.00 0.01 0.05 0.01 0.00 0.01

50.2 33.8 26.4 4.3 10.9 30.9 18.5 22.9

15.9 3.3 3.5 1.4 5.6 3.7 1.8 2.9

Figure 7. Mg coated with PD (a) under two different optimized conditions tested in 0.1 M NaCl and (b) combination 2 tested in DMEM. G


Research Article


coating on Mg to improve the corrosion resistance for biomedical implant applications. For the coating formed under optimized conditions, a reduction of the corrosion rate, based on potentiodynamic polarization curves, by a factor of about 12 in 0.1 M NaCl can be observed.

higher amount of dopamine could lead to an increased polymerization rate of PD. This could lead to depletion of O2 or monomer, before the end of the coating procedure, while ongoing H2 evolution could damage the PD layer or increase the porosity of the formed Mg(OH)2 layer until the end of coating procedure, resulting in lower corrosion resistance. 3.3.2. Corrosion Behavior of Mg Coated with Optimized Procedure in NaCl and in DMEM. Figure 7, panel a shows the corrosion behavior of two different optimized PD coatings on Mg in 0.1 M NaCl (Combination 1 and 2). Combination 1 was produced with 1 mg/mL of dopamine in 50 mM TBS with a pH of 8 and an immersion angle of 0° for 2 h. Combination 2 was achieved with 1 mg/mL of dopamine in 50 mM TBS with pH of 10 and an immersion angle of 0° for 2 h. Figure 7, panel b shows the corrosion behavior in DMEM of Combination 2 with and without PD. Table 5 shows the

4. CONCLUSIONS As shown by XPS, SEM, and FTIR, PD coatings were successfully prepared on magnesium surface in a simple immersion treatment. ASTM “Tape Test” showed excellent adhesion properties of PD on Mg surface. Polarization measurements showed that an immersion angle of 0° and a coating time of 2 h provided best corrosion protection in 0.1 M NaCl solution. FTIR showed that the formed coating on the surface consists most likely of a mixture of Mg(OH)2 and PD. Mg(OH)2 is formed due to the alkaline nature of PD formation on Mg surface. PD formation seems to depend on the availability of O2 and monomer. In this case, optimum corrosion resistance was achieved with both a pH of 8 and 2 mg/mL of dopamine, or pH 10 and 1 mg/mL of dopamine. The latter coating conditions also led to improved corrosion resistance in DMEM at 37 °C, indicating suitability for biomedical applications. Applying PD as intermediate layer for sandwich-like coating structures, improving the adhesion of the second layer to Mg, will be a further topic of research.

Table 5. Ecorr and icorr in 0.1M NaCl of Bare and Coated Mg with PD Derived under Optimized Conditions and in DMEM of Bare Mg, Mg Immersed in TBS, and Mg Coated with PD Derived from Combination 2 Ecorr (V) Mg Combination 1 Combination 2 Mg TBS PD

±

In 0.1 M NaCl −1.53 0.02 −1.48 0 −1.45 0.01 In DMEM −1.68 0.03 −1.64 0.02 −1.62 0.01

icorr (μA/cm2)

±

50.2 16.7 4.2

15.9 7.2 1.4

35.27 31 14.2

10.3 8.2 5

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b08760. Obtained amount in wt % of elements by XPS on Mg immersed in TBS and coated with PD; assigned chemical bonds for the tested surface; corrosion results measured in 0.1 M NaCl for prepassivated Mg surfaces subsequently immersed in TBS with pH 10 or coated with PD in TBS with pH 10 + 2 mg/mL of dopamine for 2 h (PDF)

obtained Ecorr and icorr values for the above-described coatings in 0.1 M NaCl solution and DMEM at 37 °C. An immersion angle of 0°, a dopamine concentration of 1 mg/mL, and a coating time of 2 h were chosen, as experiments showed that coatings prepared with these parameters led to the best corrosion protection. Since the pH dependency was not as obvious as the other parameters, we here additionally compared coating formation in pH 8 and 10 solution. The results indicate that a pH of 10 results in the lowest icorr value, as can be seen in Figure 7, panel a and Table 5. Since coatings obtained in a solution pH of 8 and dopamine concentration of 2 mg/mL led to similar results, it is conceivable that the formation of Mg(OH)2 and simultaneously PD is the key factor to improve corrosion resistance with PD coatings on Mg. Figure 7, panel b and Table 5 show the obtained corrosion behavior of bare Mg, immersed in TBS, and coated with PD in DMEM at 37 °C. The cathodic reaction is in a similar range independent of the surface treatment. The anodic reaction decreases from bare Mg to TBS immersion slightly and further decreases with an applied PD layer. Ecorr also increases slightly in the above-mentioned succession. Formation of a protective corrosion product layer after 15 min of immersion in DMEM at 37 °C for bare Mg can be expected, as previously discussed by Degner et al. and Singer et al.18,31 This layer could explain the narrow anodic plateau on all tested samples, indicating deceleration of dissolution in DMEM as compared with NaCl, and explaining the “breakthrough” potential at about −1.4 V. PD increases the corrosion resistance additionally to the corrosion product layer formed in DMEM. These results indicate that PD could form a suitable

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support provided by DFG (German Research Foundation). The authors also gratefully acknowledge Helga Hildebrand for conducting the XPS measurements.

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REFERENCES

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Corrosion Properties of Polydopamine Coatings Formed in One-Step

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