gelatin composite coatings with anodized Mg surface

Journal Pre-proof Enhancing the biodegradability and surface protective performance of AZ31 Mg alloy using polypyrrole/gelatin composite coatings with anodized Mg surface

V. Jothi, Akeem Yusuf Adesina, A. Madhan Kumar, Mohammad Mizanur Rahman, J.S. Nirmal Ram PII:

S0257-8972(19)31130-2

DOI:

https://doi.org/10.1016/j.surfcoat.2019.125139

Reference:

SCT 125139

To appear in:

Surface & Coatings Technology

Received date:

20 September 2019

Revised date:

23 October 2019

Accepted date:

4 November 2019

Please cite this article as: V. Jothi, A.Y. Adesina, A.M. Kumar, et al., Enhancing the biodegradability and surface protective performance of AZ31 Mg alloy using polypyrrole/ gelatin composite coatings with anodized Mg surface, Surface & Coatings Technology (2019), https://doi.org/10.1016/j.surfcoat.2019.125139

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© 2019 Published by Elsevier.

Journal Pre-proof Enhancing the biodegradability and surface protective performance of AZ31 Mg alloy using polypyrrole/Gelatin composite coatings with anodized Mg surface V. Jothia, Akeem Yusuf Adesinab, A. Madhan Kumarb*, Mohammad Mizanur Rahmanb and J.S. Nirmal Rama* Center for Research and Development, PRIST University, Thanjavur, Tamil Nadu, India b

Center of Research Excellence in CORROSION, Research Institute,

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King Fahd University of Petroleum & Minerals, Dhahran, Saudi Arabia.

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Abstract The intention of the current study is to improve the biodegradability and corrosion resistant performance of AZ31 Mg substrates by anodization and electrodeposition of polypyrrole/gelatin composite coatings. The effect of anodization time, and the concentration of gelatin on the biodegradability and corrosion protective performance of coated AZ31 Mg substrates were

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systematically evaluated using different characterization techniques. Surface topographical and

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morphological studies of anodized substrates indicated that the surface roughness, distribution of

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pores and pore size increased with increasing the anodization time. Water contact angle studies

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confirmed the improved surface wettability of AZ31 Mg substrates after anodization treatment. Surface characterization results of PPy/Ge composite coatings revealed the significant influence

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of gelatin addition on the surface morphology of PPy coatings. Structural characterization results

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confirmed that intermolecular chemical interaction exists between the polypyrrole moiety and gelatin molecules. It was also found that the anodized layer has enhanced the adhesion of PPy

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coatings from 3B to 5B. In vitro corrosion analysis indicated that the anodization layer

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efficiently improved the corrosion resistance behavior of coated AZ31 Mg substrates and this performance was increased with the addition of gelatin up to 1 wt% and then slightly reduced. Hydrogen evolution measurements corroborated that the amount of hydrogen evolved is significantly reduced in the presence of anodization layer and gelatin addition into PPy coating. KEYWORDS: AZ31 Mg alloy; Polypyrrole coating; Corrosion; Biodegradability; SECM; ∗ Corresponding authors: [email protected], [email protected], Phone: +966538801789 Fax: +966538604818

Journal Pre-proof INTRODUCTION Magnesium (Mg) and its alloys are widely employed in various applications, owing to their salient characteristics like high strength to weight ratio, low density, good electromagnetic shielding and high thermal conductivity. Mg alloys have recently gained much consideration as viable orthopaedic and cardiac prosthesis owing to their similarity in mechanical characteristics with natural bone, low stress shielding and acceptable biocompatibility [1-3]. However, Mg and

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its alloys have few shortcomings such as low corrosion resistance and less bioactivity in human

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blood plasma, which further restrains its clinical applications. Also, the removal of an implant

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after sufficient recovery is not needed in the utilization of biodegradable Mg implantations, since

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the leached Mg+2 ions are assimilated through human metabolism [4]. Moreover, Mg is a necessary component to the human body for biological utilities with consistent intakes of 250-

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500 mg/day [5]. Conversely, the rapid corrosion/degradation of Mg implant leads to damage of

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mechanical integrity before the bone healing, which further limits the clinical practice of Mg implants. Hence, it is vital to regulate the biodegradability and enhance the surface protective

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performance of Mg alloy to confirm its suitability in clinical applications.

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Numerous efforts have been completed to regulate the corrosion/degradation of Mg implants by adopting suitable surface treatment strategies including anodizing, conversion coating, electrodeposition, laser surface alloying and polymeric based coatings [6-10]. Among the investigated polymeric coatings, the utilization of polypyrrole (PPy) on Mg alloys for biomedical applications has been focused by many researchers owing to its distinct features, such as high stability, high electrical conductivity, ease of synthesis, redox properties and good biocompatibility [8, 10, 11]. Maryam Hatamin et al. prepared the PPy films on AZ31 Mg substrate through the electrochemical route and reported that the passivation pretreatment

Journal Pre-proof enhanced the adhesion strength and corrosion resistance of Mg alloy surface [11]. Pinto et al. prepared the silane films on anodized WE54 Mg alloy and found the synergetic effect between the anodized layer and silane films in improving the barrier effect and corrosion resistant performance by three orders of magnitude, compared to that of the blank silane coating [12]. Lu et al. prepared the epoxy coatings on anodized AZ91D alloy and revealed that the epoxy coating permeates the pores of the anodized film, thus improving the adhesion strength of the epoxy and

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Mg surface [13]. Arrabal et al. prepared the polymeric coatings on plasma anodized AZ31 Mg

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alloy and compared the results with the fluorotitanate–zirconate pre-treatment layer. Their results

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revealed that the PEO + polymer coating exhibited acceptable impact resistance, excellent

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adhesion, and enhanced corrosion resistance than the Ti/Zr + polymer coating [14]. A recent review highlighted the important role of plasma electrolytic oxidation (PEO) treatment on the

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corrosion resistant behavior of sol-gel, conversion, organic and electrophoretic coatings [15].

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However, to the best of our knowledge, the present investigation is the first, addressing the effect of anodization treatment of AZ31 Mg alloy before electrodeposition of polypyrrole coatings on

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AZ31 Mg alloy for biomedical applications.

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Wahid et al. have described that the PPy film with functionalization is more appropriate for covalent immobilization of a model protein, which further improves its biocompatibility [16]. However, the PPy has few shortcomings including poor mechanical characteristics and less biodegradability, which hinders its biomedical applications. Also, it is well recognized that PPy coating has less adherence to oxidizable metals. To overcome these shortcomings, the preparation of PPy composites with biopolymers are encouraged in recent years. Natural biopolymers have been recently considered as promising candidates in clinical applications due to their low cost, good biocompatibility and similarity with the extracellular matrix [17, 18].

Journal Pre-proof Among the various natural biopolymers, Gelatin as a collagen derivative has recently been utilized in various biomedical applications including controlled drug released, tissue engineering, wound dressing, and health caring devices due to its biological origin, biocompatibility, nonimmunogenicity, and biodegradability [19, 20]. Reported results in the literature reveal that gelatin improves the cell adhesion, migration, and proliferation of many cells [21]. Ge et al. prepared the PPy coatings through electrochemical route in the presence of gelatin and found that

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the surface microstructure of PPy coatings is effectively altered by the inclusion of gelatin [22].

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Harjo et al prepared the PPy coated glycose-gelatin scaffolds and their results revealed that the

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prepared gelatin scaffolds could be either utilized as scaffolds in tissue engineering, or

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biocompatible electrode material for energy storage [23]. However, there is no report on the electrochemical synthesis of composite coatings based on the polypyrrole and gelatin for

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biomedical applications.

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The current study is aimed to examine the biodegradability and surface protective performance of AZ31 Mg alloys using PPy coating with anodized Mg surface. Surface and

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structural characterization of the anodized layer and PPy composite coated Mg substrate were

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performed using the various analysis. In vitro corrosion behavior of AZ31 Mg substrates was assessed in simulated body fluid (SBF) through electrochemical frequency modulation (EFM) and electrochemical impedance spectroscopy (EIS) tests. The hydrogen evolution from coated AZ31 Mg substrates in SBF is assessed by Scanning Electrochemical Microscopic (SECM) techniques.

Journal Pre-proof 2. Experimental Procedure 2.1 Materials and methods AZ31 Mg alloy of elemental composition (wt %): 2.5−3.5 Al, 0.6−1.4 Zn,0.2−1 Mn, 0.1 Si, 0.05 Cu, and Mg remaining, with dimensions of 25 mm × 25 mm× 2.5mm was used as a base substrate. The substrate was mechanically ground by various girt size silicon carbide papers ranging from 240-2000. After polishing, the substrate surface was rinsed with ethanol to remove

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the oil, greases, impurities, and dust on it. Before anodization, the substrate was subjected to

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pretreatment to activate the Mg surface. The substrate is dipped in a mixture of NaOH (50 g/l)

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and Na3PO4 (10 g/l) for 5 min. about 70 °C. After the alkaline cleaning, the substrate was dipped

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in 100 ml/l of HNO3 for about 3 min. Afterward, the substrate is washed using distilled water and dried in air. Anodization process was performed using a DC Power supply with the base

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substrate and graphite rod acting as the anode and the cathode, respectively. The electrolytes are

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the aqueous solution containing NaOH (25 g/l), KOH (25 g/l) and Na2SiO3 (50 g/l). Anodization was carried out at room temperature for different anodization time (20, 40 and 60 min.) at a

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constant voltage of 5 V [24-26].

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2.2 Synthesis of polypyrrole/gelatin composite coatings on anodized AZ31 Mg substrates PPy and its composite coatings were electrodeposited on anodized Mg alloy substrates by the Cyclic Voltammeteric (CV) method, based on a three-electrode assembly with Pt mesh, Saturated Calomel Electrode (SCE), and the anodized Mg substrate as an auxiliary, reference, and working electrodes, respectively. PPy and its composite coatings were electrochemically deposited on anodized substrates using the sodium salicylate solution (0.5 M) containing pyrrole (0.25 M) without and with the addition of gelatin (0.5, 1, and 2 wt.%) by applying the potential between −0.75 and 1 V vs. SCE for 20 cycles with a scan rate of 20 mV/s. For comparative

Journal Pre-proof purposes, pure PPy coating was also electrodeposited on bare AZ31 Mg substrates. In this case, bare AZ31 Mg substrate needs the surface pretreatment to make the Mg surface partially passive to get the uniform polypyrrole coatings. Hence, the pretreatment was done in sodium salicylate solution (0.5 M) through linear sweep voltammetric method by applying the potential from -0.1 to 1 V vs. SCE with the slow scan rate of 10 mV/s. After the electrochemical synthesis, coated Mg substrates were rinsed with distilled water and then kept at 60 °C overnight. PPy without and

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with the addition of gelatin content (0.5, 1, and 2wt.%) were labeled as pure PPy, PPy/Ge1,

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PPy/Ge2 and PPy/Ge3 coatings. The thickness of the coatings was measured using the Elcometer

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thickness gauge at five different sites and the average thickness values are presented in Table 2.

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2.3 Surface examination

The morphological evolution of the anodized and coated Mg substrate was characterized by

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Field Emission-Scanning Electron Microscopy (FE-SEM, Tescan microscope, and accelerated

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voltage of 20 kV and irradiation current of 10 µA). The optical profilometer (Contour GT-K, Bruker Nano GmBH, Germany) was utilized to evaluate the surface topography of the anodized

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Mg substrates. Water Contact angles (WCA) of anodized Mg substrate were monitored through a

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contact angle meter (VCA OPTIMA, AST products Inc. USA) using a sessile drop method. The chemical structure of anodized and coated Mg substrate was studied by Attenuated Total Reflectance-Infrared spectrometer (Thermo scientific, with universal ATR accessory region of 400-4000 cm-1). 2.4 In vitro corrosion measurements In vitro corrosion tests were done using Gamry Reference 3000 Potentiostat/Galvanostat. EIS and EFM tests were utilized to inspect the in vitro corrosion resistant behavior of anodized and coated Mg substrates in SBF. The three-electrode cell consisting of an AZ31 Mg substrate, SCE,

Journal Pre-proof and graphite is acting as working, reference and counter electrodes, respectively. EFM test was carried out by sweeping a potential perturbation signal with two sine waves of 2 and 5 Hz with an amplitude of 10 mV vs. SCE. Higher intensity peaks were selected to calculate Tafel constants, the corrosion current density (icorr), and causality factors (CF2 and CF3). The EIS data was obtained in the frequency region of 100 kHz–1 mHz with a 10 mV vs SCE amplitude in OCP. All the obtained EFM and EIS data were analyzed through Gamry Echem Analyst, version

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6.03 software packages. The attained EIS curves were further evaluated through fitting procedure

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using inbuilt software and all electrochemical tests were repeated three times to authenticate the

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obtained results. All the data are presented as means ± standard deviation with n = 3. The statistical analysis was performed using Student's t-test and one-way analysis of variance

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(ANOVA) and statistical significance was achieved when “p” value was less than 0.05.

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SECM measurements were performed using M370 scanning electrochemical workstation. SECM

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ultra-microelectrode (UME) made up of Pt tip with a diameter of 10 µm was used as working electrode, whereas SCE and graphite rod were utilized as reference and auxiliary electrodes,

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respectively. A region of 1000 × 1000 µm was monitored using the scan rate of 25 µm/s. The

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substrate-UME tip distance was fixed at a constant distance with the aid of a digital microscope. The distance between the substrate and UME tip was kept at a constant distance of 50 µm by approaching the UME tip into contact with the substrate’s surface and then, the UME tip was upraised by the positioning system of the SECM to the chosen height above the substrate. Substrate generation/Tip collection (SG/TC) mode was utilized to analyze the hydrogen evolution reaction that occurred at Mg surface by applying the UME tip potential of 0.0 V vs. SCE. At this tip potential, the H2 released at the cathodic regions on the Mg surface is oxidized at a UME tip as,

Journal Pre-proof H2 → 2H+ + 2e- ---------------- (1) As this reaction at the UME tip is diffusion controlled, the current measured at the UME tipis a direct measure of the local concentration of H2.

3. Results and Discussions 3.1 Surface examination results of anodized AZ31 Mg substrates

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Figure 1(a-d) represent the SEM micrographs of anodized AZ31 Mg substrates in different

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anodization time. All the anodized surface exhibited the typical porous microstructure with the

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different shapes and sizes of the pores, which depends on the anodization time. The obtained

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porous morphology with many craters is generally observed for the anodized Mg surface, which is ascribed to erosion and gas evolution during sparking [27]. At 20 min. (Fig.1a) anodization

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time, a compact surface with limited pores with small size is found due to an insufficient

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anodization time. The anodized surface produced at 40 min. (Fig.1b) showed the homogeneous scattering of the pores and the diameter of the pores increases with increasing anodization time

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upto 1 hr. With increasing anodization time, the number of pores was found to decrease;

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however, the pore size increased and the coating’s surface turns into more rougher. The SEM images of the anodized Mg substrates were examined by Image J software version 1.52a to determine the average pore size with its distribution and the obtained results are given in Figure S1 as supporting information. The porosity was estimated using image analysis as the percentage of area occupied by the pores on the images. For an anodized layer processed at 20 minutes, it exhibits an average pore size of 1.78 µm and a porosity of 8.25 %. Both pore density NP and pore diameter dP depend exponentially on the anodization time. The pore size is increased, whereas the pore density is decreased as the anodization time increased. The average pore size

Journal Pre-proof increased from ~1.78 µm to 3.45 µm, when the anodizing time increased from 20 to 60 min. With the increasing anodization time, the discharge intensity of the single spark was improved, thus the pore size of the anodized layer increased at prolonged anodization time. The variation in the degree of porosity with anodization time is ascribed to the slight difference in discharge characteristics. The obtained behavior is accompanied with the spark discharge tended to be more energetic with an increased anodization time, hence forming the rough anodized layer with

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larger pores [28, 29]. Guo et al have also reported the similar trend in the MAO films processed

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in different treatment time [30].

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Further, The EDS analysis for the given region as shown in Fig. 1d, revealing the existence of

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Mg, O and Si as the main elements with a few of K and Na also identified on the anodized surface. The elemental compositions of the anodized Mg substrates (Figure S2) exhibit minor

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changes at different anodization time, revealing that the anodization time mainly influences the

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surface morphology and topography of the anodized films. Gu et al. have also found the similar trend that the processing time showed minor impact on the elemental compositions of the MAO

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films processed in different time [31].

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Figure 2 (a-d) shows the surface topographic profiles of anodized substrates with different anodization time. The un-anodized substrate exhibited a flat surface with unidirectional grooves produced during mechanical grinding, whereas, all the anodized substrates displayed plenty of hills, pores and valleys. Based on the surface profiles, it is clear that the surface roughness of the un-anodized substrate is lower than the anodized substrates. In general, the surface roughness is effectively influenced by the difference in the distribution of micro cracks and pores on its surfaces. Table 1 displays the calculated surface roughness values. In general, Rq (root mean square, RMS) and Ra (average surface roughness) are considered as vital factors for describing

Journal Pre-proof the surface roughness as these parameters give information about the predictability of variation from an even surface by scanning a continuous surface profile [32]. From Table 1, the Ra and Rq values of un-anodized substrate were found to be low (between) and were increased to 5-10 mm for anodized substrates. Further, the surface roughness values of anodized substrates were increased with increasing the anodization time. From the recent report, it’s revealed that the higher surface roughness enables the formation of physical interfaces of polymer molecules with

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the Mg surface and hence enhances adhesion strength viz. mechanical interlocking theory [33].

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Determining the surface wettability is considered one of the major interpretations to explain the

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suitability of the surfaces for interacting with other materials. The effect of anodization time on

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the hydrophobicity of the Mg substrates was examined through recording the water contact angle and the obtained data are displayed in Figure 3(a-d). An un-anodized Mg substrate displayed a

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WCA of 80.50°, which indicated the hydrophobicity of the un-anodized Mg surface. On the

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other hand, the WCA of Mg substrates after anodization was significantly reduced (nearly 30°), which is attributed to the higher surface roughness. The achieved surface wettability of Mg

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substrates is expected to facilitate the adhesion and more interaction with polymer molecules

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during electrochemical synthesis [34].

3.2 Electrochemical synthesis of Polypyrrole/Gelatin composite coatings on anodized AZ31Mg substrates Figure 4 (a-d) displays the cyclic voltammetric curves obtained on the un-anodized and anodized Mg substrates in 0.5 M sodium salicylate (SS) solution containing 0.2 M pyrrole with and without the different amount of gelatin. For the un-anodized substrates, the pretreatment of Mg substrate is needed before electropolymerisation as the Mg substrates are readily oxidizable

Journal Pre-proof metal and possess a faster rate of dissolution in aqueous mediums [35]. Hence, the Mg substrate was passivated in 0.5 M SS solution without pyrrole by applying the potential between -0.1 and 1 V with the scan rate of 10 mV/ s (Fig.1a). The passivation of Mg substrates by depositing a salicylate layer has been previously reported in the literature [36]. Mg2+ ions released from the dissolution reaction beginning around 0.2 V vs. SCE react with the SS ions to produce an insoluble Mg (II) salicylate protective layer that attaches to the substrate surface through the

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passivation procedure at very low scan rates [37, 38]. CV obtained in the presence of py display

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similar behavior in initial cycles (Fig.1b), with anodic peaks encountered nearly 0.2 and 0.7 V

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vs. SCE related to the anodic dissolution of Mg and oxidation of salicylate, respectively [39].

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However, a dissimilar CVs were obtained in consequent cycles, depending on the anodized surface and gelatin content.

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The CV curves of electropolymersation of py on the anodized Mg alloys in 0.5 M SS comprising

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of 0.25M py for the first four cycles are displayed in Fig.1c. Noticeably, the anodic current peak was found to be lower and sharper compared to that of un-anodized substrates, possibly due to

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the presence of the anodized layer, which reduces the Mg dissolution. Moreover, this anodic

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current peak was disappeared for anodized substrates processed at 40 min and 60 min. Figure 1d displays the influence of gelatin addition on the CV curves of electropolymerisation of py from the 0.5 M SS medium comprising of 0.25 M py with different amounts of gelatin (0.5, 1 and 2 wt%) content. The monitored CVs for PPy composite coatings were slightly different to those attained for pure PPy and displayed a lesser oxidation current with a beginning potential of 0.85 V vs. SCE. On the other hand, the peak current was found to be higher, probably due to the interaction of the gelatin molecules on Mg surface. It has been already that gelatin is an amphoteric polyelectrolyte with a long chain comprising of many amino acids. Amino acids own

Journal Pre-proof –NH2 and –COOH functional groups, both of which offer the active sites for the adsorption of pyrrole monomer thorough hydrogen bond [40]. The visual observation of the Mg substrate after removing from the electropolymerization medium indicated the existence of greenish-black film, validating the formation of a PPy and PPy-Ge composite coating. To acquire the evident insight on the adhesion strength between the investigated polymeric films and the Mg substrates, the scratch tape adhesion tests according to ASTM standard D 3359-02

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were performed for pure PPy and PPy/Ge composite coatings on bare and anodized Mg

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substrates. PPy coating on bare AZ31 Mg alloy substrate exhibited coatings delamination of 10–

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15%, equivalent to a rank of 3B. Whereas, PPy coating on anodized Mg substrates showed

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coatings delamination of less than 5%, and graded 4B. With the addition of gelatin into PPy coatings, the adhesion strength is improved by exhibiting 0% delamination areas and graded to

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5B. In general, the adhesion between the polymeric coating and metallic surface occurs either

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mechanically or chemically. The mechanical adhesion occurs through the penetration of polymeric coatings on the surface defects as pits, pores, and crevices that establish the bond due

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to the mechanical interlocking theory of adhesion which states that the adhesion strength can be

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enhanced by increasing the surface roughness. Whereas, the chemical adhesion occurs through the formation of interatomic bonds which can be primary (ionic or covalent bonds), secondary (dipole interactions, dispersion forces or van der Waals forces) or hydrogen bridge-type [41]. In the present investigation, adhesion strength is improved by adopting both mechanical and chemical adhesion aspects by creating high surface roughness using anodization treatment and enhancing the functional groups in the polymeric coatings using the addition of gelatin to PPy coatings.

Journal Pre-proof As presented in Figure 1, anodized layer possesses micro-rough surface with pores in which the bonding area is evidently increased and the PPy is embedded into the pores, leading to the improvement in bonding strength of the surface. Whereas, gelatin contains many functional groups including amino and carbonyl linkages, that can interact with a variety of substrates to improve the adhesion strength between the base substrate and coatings.

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3.3 Surface characterization results of PPy/Ge composite coatings

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SEM images in Figure 5(a-f) provide clear evidence that important variances present between

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surface microstructures of pure PPy and PPy composite coatings. Pure PPy coating on anodized

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substrates exhibited a compact surface with uniformly spread cauliflower-shaped grains, whereas on un-anodized substrates, the size of the PPy grains was not uniform and ranging from 2 µm to

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20 µm, which is in good consistent with the previous reports [42, 43]. It has been already

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reported that the change in the surface morphology of PPy coatings is attributed to the slight alteration in the growth mechanism of polymeric films [44]. The influence of gelatin addition on

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the surface microstructures of PPy coatings is revealed by the comparison of SEM images of PPy

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and PPy/Ge composite coatings. Surface morphology of PPy composite coatings (Fig.5d-f) exhibited the presence of globular like particles with the grain size of about 2 µm, which increases with increasing the amount of gelatin in the electrolytic solution further confirmed the influence of gelatin addition on PPy morphology. Further, to inspect the effect of gelatin addition on the thickness of PPy composite coating, the thickness of the prepared coatings was examined using the cross sectional SEM analysis and the results are presented in Figure S3 as supporting information. The average thickness of the investigated coatings was found to be in the range of

Journal Pre-proof 10-12 µm ± 0.25 µm. As expected, because of the similar experimental conditions, the addition of gelatin into the PPy coatings had almost no significant influence on their thickness.

3.4 Structural characterization results of PPy/Ge composite coatings Figure 6 (a-d) displayed the IR curves of PPy and PPy/Ge composite coated Mg substrates. In the case of PPy coatings on anodized substrates (Fig.6b), the peaks appeared at 675 cm−1, 868

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cm−1 and 1009 cm−1 are ascribed to out of the plane and in-plane C-H bending vibration of the

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aromatic rings, respectively. Additional representative peaks obtained around 1138 cm−1, 1415

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cm−1, and 1560 cm−1 are ascribed to the breathing and the stretching vibration of C-C, C=C and

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=C-H in the pyrrole ring, respectively [45]. No noticeable difference is obtained for PPy coatings on un-anodized (Fig.6a) and anodized Mg substrates, validating that the chemical structure of

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PPy coatings obtained in both conditions is similar.

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PPy/Ge composite coating (Fig.6c-d) showed all the peaks related to polypyrrole moieties with the additional peak of 1650 cm-1 correspond to the carbonyl stretch of amide I linkages in

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gelatin. The appeared peak at 1545 cm-1 is attributed to N-H bending and C-H stretching

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vibration of amide II linkages. Also, the peaks appeared around 1210 (amide III) are attributed to the C-N stretch plus N-H in phase bending of amide III linkages. Further, the peak at 1060 cm-1 represents stretching vibration of –C-N group [46]. The obtained peak around 3300 cm−1 is attributed to the N–H stretching vibration, representing the inherent feature of gelatin [41, 47]. The molecular structure of gelatin and polypyrrole consists of polarized functional groups including aromatic amine and carbonyl groups, which possess a higher possibility of intermolecular chemical interaction as verified using the IR spectra [48].

Journal Pre-proof Figure 7 represents the Raman spectra of coated Mg substrates. Raman spectrum of pure PPy coatings exhibited the representative peaks at 936/975, 1043, 1225, and 1337, which are accompanied with the ring deformation vibrations indication units and radical cation, symmetrical C−H in-plane bending indication units and radical cation, antisymmetric C−H deformation vibrations, respectively [49]. Moreover, the obtained peaks at 1375 and 1599 cm-1are recognized to the C-N stretching and the backbone stretching of C=C bonds of

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PPy, respectively. In the case of PPy/Ge2 composite coatings, its distinguished peaks are well

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agreement with the polypyrrole and gelatin in addition to a few peaks with slight shifting and

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overlapping [50]. Particularly, the peak appeared at 1590 cm-1 validate the strong amide I absorption due to the stretching vibration of the C=O bond. Additional distinctive peak around

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1250 cm-1 is recognized for the amide III relating the C-N stretching and N-H in-plane bending

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vibrations of the peptide bond as well as contributions from C-C stretching and C=O in-plane

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bending, respectively [51]. The obtained data also validates the interaction between the π-

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conjugated structure of PPy and the gelatin, which is analogous to those obtained in IR spectra.

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3.5 In vitro corrosion test results of PPy/Ge composite coatings Corrosion resistant behavior of uncoated and coated Mg substrates in SBF was initially examined by monitoring the OCP values with immersion period and the results are shown in Figure 8. The OCP values of uncoated Mg substrates increased promptly to about -1.65 V vs. SCE in initial immersion and then decreased gradually, reaching a constant comparative value of about -1.75 V vs. SCE after 48 hrs of immersion. All coated samples exhibited a quick drop of the OCP in the initial hours of immersion in SBF, which is associated with the permeation of solution through the micropores of the polymer to the Mg substrates [52]. However, compared to

Journal Pre-proof the bare Mg substrate, all coated samples displayed OCP value of -1.450 V vs. SCE in the first 24 h immersion and exhibited slight variations around this OCP in the remain exposure period. Further, PPy and PPy/Ge composite coatings on anodized substrates display higher and more stable OCP values than the pure PPy as well as uncoated substrate. Particularly, the PPy/Ge2 substrate exhibited the noblest shift in the OCP value among the investigated samples. Further, the highest and lowest reduction in the OCP was observed for the uncoated and PPy/Ge2 coated

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substrates, respectively. The identified results designate that the anodized film and gelatin

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addition could improve the corrosion propensity of the PPy coatings on Mg substrates.

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EFM graphs of uncoated and coated Mg substrates in SBF are shown in Figure 9 and the

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calculated parameters are presented in Table 2. Causality factors, CF-2 and CF-3 values are almost same to the theoretical values of 2.0 and 3.0, authenticating the experimentally acquired

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values [53, 54]. In comparison with the uncoated substrates (Figure S4 as supporting

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information), the Ecorr values of the Mg substrates with the PPy and PPy composite coating increased from -1.702 V vs. SCE to about -1.40 V vs. SCE and -1.30 V vs. SCE, and the icorr

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decreased from 44.52 µA/cm2 to 0.12 µA/cm2 and 0.01 µA/cm2, respectively. The positive shift

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in Ecorr and the reduction in icorr values for the coated substrates validated that the synthesized coatings on anodized Mg substrate could deliver improved surface protection against corrosion [55, 56]. All the coated Mg substrates show lower icorr than uncoated, pointedly, PPy/Ge2 display lowest icorr value, representing that the PPy coating formed on the Mg substrate with anodized layer and gelatin addition considerably enhances its corrosion resistance. EIS results of the uncoated and coated Mg substrates in SBF medium are displayed in Figure 10 (a-b). It is obvious that all the investigated coated substrates show a predominantly capacitive behavior at the investigated frequency region [57]. Also, the low frequency impedance value of

Journal Pre-proof all the investigated substrates is found to be close to 106 Ω cm2, which reveals a high corrosion resistance as a result of the strong barrier performance of PPy coatings. Particularly, PPy/Ge2 coated substrates exhibited a significant increase in the low frequency impedance, which possibly due to the formation of denser and compact coating films compared to that of pure PPy coating. Phase angle in bode plots shows the surface state of the substrate and its structural variations and

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the obtained results displayed the capacitive response in the investigated frequency region [58].

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The trend of the phase angle plots for uncoated and coated samples is found to be distinct,

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indicating that Mg substrate experience different corrosion phenomena with and without

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coatings. The phase angle values of coated substrates were moved to negative frequencies, revealing the strong barrier performance due to the compact polymeric layer [59].

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To evaluate the corrosion resistant performance in quantitative route, electrical circuit fitting was

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performed using the proper electrical circuit which displayed in Figure 11 (a-b) and the acquired values are displayed in Table 3. Rs, Rf and Rct represent the electrolytic, film and charge transfer

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resistance, respectively. CPEdl and CPEf denote the double layer capacitance and film

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capacitance, respectively. A CPE is frequently availed as a replacement for a pure capacitor to nullify the non-ideal capacitive and inhomogeneity feature of coated metallic substrates [60-62]. It is clear from Table 3 that Rct value for pure PPy and PPy/Ge composite coatings are higher in comparison with the uncoated AZ31, revealing that the two kinds of polymeric coatings deliver a strong barrier against penetration of aggressive ions from SBF. In addition, the Rct value of PPy/Ge2 (6.74 × 105 Ω cm2) is four times higher than that of pure PPy-A40 (1.91 × 105 Ω cm2) one order of magnitude higher that of pure PPy (2.31 × 104 Ω cm2) respectively, demonstrating that the anodized AZ31 Mg substrate with the composite coating exhibited the best surface

Journal Pre-proof protective performance against corrosion. Rf and CPEf are the most significant factors directly linked with the corrosion protection behavior of coatings on metallic substrates, which reveal the ability of the coatings to perform as a strong barrier against aggressive ions from the solution. From Table 2, it is observed that Rf values increased with increasing the anodization time up to 40 minutes and then slightly decreased. Also, the values of Rf of PPy composite coatings were increased with increasing the gelatin content up to 1 wt% and then slightly decreased. Besides,

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the CPEf values of PPy/Ge composite coatings were lower than those of pure PPy and the change

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in CPEf also follows the similar trend, which further confirms the beneficial role of anodization

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layer and the addition of gelatin into PPy matrix to improve its corrosion resistant performance

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in SBF medium.

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3.6 Hydrogen evolution measurements by SECM analysis

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Compared to conventional electrochemical techniques, SECM possesses few advantages which comprise spatial resolution in micrometer scale, low iR drop and different modes of operation. In

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recent years, the estimation of the hydrogen evolution phenomenon on the Mg surface has been

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performed by many researchers through substrate generation/tip collection mode in which the evolution of H2 at the substrate surface and the consumption of H2 at the UME tip can be recorded [63-65]. The deviation in the amount of current recorded at UME tip is a sign of the different quantity of hydrogen evolution from the Mg surface. SECM mapping images of uncoated and coated Mg substrates for estimating the H2 released in SBF are presented in Figure 11. As seen in Figure 12a, uncoated AZ Mg substrate displayed higher H2 evolution by presenting the higher tip current, which specified that more intense corrosion taking place at the Mg surface.

Journal Pre-proof The zones showing higher tip current are the regions releasing higher H2, indicating strong corrosion in the regions. Furthermore, the magnitude of the tip current is increased with increasing the immersion time, suggesting the harshness of corrosion at the Mg surface. It is clear from Figure 12b that the tip current on the entire scan surface of the uncoated substrate surface presented a significant variation and the maximum value of 150 nA was observed after 24 hr of immersion.

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Compared to that of uncoated, noticeable changes as quantified by the value of the tip current are

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identified for all the coated Mg substrates. The SECM maps of coated substrates seemed to be

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more homogeneous than the uncoated substrates. Furthermore, compared with the uncoated, the

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coated substrates displayed lower values of tip currents (0-20 nA) even after 24 hr of immersion, representing lesser corrosion rates on these surfaces in SBF. In particular, the tip current above

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the Mg surface with PPy/Ge2 coating is observed to be much lesser than that of other samples.

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To corroborate the notable decrease of hydrogen evolution on coated Mg substrates, the values of normalized UME tip current are plotted against the function of the exposure period in Figure

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13. From the comparison of the plots, the main reduction in tip current was detected in the initial

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hours of immersion, and then it further slightly increased as the immersion period extended. The strong barrier performance of PPy coatings on Mg surface was confirmed by the alteration of the profile of SECM mapping and by the reduction in the measured normalized current value. A comparison of all the obtained SECM result scan draws the conclusion that the PPy/Ge composite coating is efficiently obstructed the hydrogen evolution reaction by forming a strong barrier on AZ 31 Mg surface thereby pointedly decreasing the corrosion rate as suggested by the other electrochemical corrosion test results.

Journal Pre-proof Based on the obtained results, it could be understandable that the anodized AZ31 Mg alloy coated with PPy/Ge composite display enhanced corrosion protection behavior in SBF medium. The corrosion protection mechanism of polypyrrole coatings on metallic substrates have been elucidated by many researchers, varying from barrier protection, anodic protection, and controlled inhibitor release [66-69]. In general, Mg undergoes corrosion by producing the Mg2+ ions through anodic dissolution and evolution of H2 gas from cathodic reduction based on the

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following reactions.

---------- (1)

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Mg → Mg+2 + 2e− (anodic reaction)

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2H2O + 2e− → H2 + 2OH− (cathodic reaction) --------- (2)

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The PPy/Ge composite inhibits corrosion by intervening both the anodic and cathodic reactions with an improved barrier effect and adhesion strength by filling PPy/Ge coatings on the pores

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and cracks of the anodized Mg surface. PPy/Ge composite coatings exhibited the effective

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physical barrier film with higher adhesion strength due to the presence of gelatin molecules in the PPy matrix as it possesses more functional groups which are the anchoring points towards the

ur

pores exist in the anodized Mg surface. It has already been that the PPy coating could perform as

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a physical barrier against a corrosive environment and also provide anodic protection, contributing to the redox reactions on the surface of the metal [70]. Further, PPy chain has the ability to transfer between oxidized (doped form) and reduced states (dedoped form) by accepting the electrons released from the metal dissolution, which further reduces the rate of cathodic reaction. Subsequently, the performance of the PPy coatings improves with increasing the surface area. Incorporating gelatin molecules into the PPy matrix provides a denser and more compact surface with the increased surface area (as in Figure 5), which could improve the capability of the PPy/Ge coating to have more interaction with ions released during the corrosion

Journal Pre-proof process. The corrosion mitigation is continued till PPy exhibits adequate redox capability to experience constant charge transfer process at the metal–electrolyte interface. PPy + n/4O2 + nH+ → PPyn+ + n\2H2O ------------------------ (3) Hence, the redox tendency of PPy is necessary for the polymeric chain to endure as an electroactive film [71]. Moreover, the re-oxidation of PPy film can occur through dissolved oxygen existing in the solution and corroboration for the re-oxidation of PPy through oxygen is

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earlier described [72]. Thus, the addition of gelatin in the PPy coating could improve the

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corrosion protection performance through the strong barrier performance, and also the doping

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and de-doping tendency of the PPy chain.

Conclusions

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Surface characterization results including morphology, topography and wettability revealed the

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favorable role of the anodized layer on AZ31 Mg substrates. Using the electrochemical route, the synthesis of pure PPy and PPy/Ge composite coatings were performed on anodized AZ31 Mg

ur

substrates. Surface and structural characterization results validated the influence of anodization

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time and gelatin addition on the surface features of polymeric coatings on Mg substrates. In vitro corrosion test results confirmed that the Rct value of PPy/Ge2 (6.74 × 105 Ω cm2) is four times higher than that of pure PPy-A40 (1.91 × 105 Ω cm2) one order of magnitude higher that of pure PPy (2.31 × 104 Ω cm2) respectively, demonstrating that the anodized AZ31 Mg substrate with the composite coating exhibited the best surface protective performance against corrosion. Hydrogen evolution measurements using SECM corroborated that the amount of hydrogen evolved is reduced significantly in the presence of anodization layer before coating and gelatin addition into PPy coating. In conclusion from the obtained results, the anodization film before

Journal Pre-proof polymeric coating and the addition of gelatin into polypyrrole matrix noticeably improved the biodegradability and corrosion resistant behavior of AZ31 Mg substrates in SBF medium.

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Journal Pre-proof Figure Captions Figure 1 SEM images of (a) A20 (b) A40, (c) A60 and EDS analysis of (d) A40. Figure 2 Surface topographic images of (a) Bare (b) A20, (c) A40 and (d) A60 Figure 3 WCA images of (a) Bare (b) A20, (c) A40 and (d) A60 Figure 4 CV curves of (a) sodium salicylate (b) pure PPy on bare Mg, (c) pure PPy on anodized Mg and (d) PPy/Ge composite coatings.

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Figure 5 SEM images of (a) PPy on bare Mg, (b) PPy on A40, (c) PPy on A60, (d) PPy/Ge1, (e)

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PPy/Ge2 and (f) PPy/Ge3

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Figure 6 ATR-IR curves of (a) Pure PPy on bare Mg, (b) Pure PPy on A40, (c) PPy/Ge2 and (d)

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PPy/Ge3

Figure 7 Raman spectra of pure PPy and PPy/Ge composite coated AZ31 Mg substrates

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Figure 8 OCP curves of uncoated and coated AZ31 Mg substrates with the function of

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immersion time (Error bars represent the standard deviation of triplicates).

medium

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Figure 9 EFM curves of PPy and PPy/Ge composited coated AZ31 Mg substrates in SBF

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Figure 10 EIS curves of (a) PPy on anodized and PPy/Ge composite coatings on anodized Mg substrates in SBF medium

Figure 11 EIS equivalent circuit models of (a) Uncoated and (b) Coated Mg substrates. Figure 12 SECM mapping images of (a) Bare Mg-1hr, (b) Bare Mg-24hr, (c) PPy on bare-1hr, (d) PPy on bare-24hr, (e) PPy on A40-1hr, (f) PPy on A40-24hr, (g) PPy/Ge2-1hr and (h) PPy/Ge2-24hr. Figure 13 The normalized SECM tip current of PPy and PPy/Ge composite coated AZ31 Mg substrates in SBF medium (Error bars represent the standard deviation of triplicates).

Journal Pre-proof Table Captions Table 1 Surface roughness parameter of anodized AZ31 Mg substrates Table 2 Coatings thickness and EFM parameters of uncoated and coated substrates in SBF medium (The mentioned values are the mean of triplicates and (±) represents to the standard deviations). Table 3 EIS parameter for uncoated and coated Mg substrates in SBF medium (The mentioned

-p

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values are the mean of triplicates and (±) represents to the standard deviations).

Ra (µm)

Rp (µm)

Rq (µm)

Rz (µm)

Rv (µm)

Bare

0.289

4.493

lP

0.385

13.541

-9.074

A20

2.793

75.279

4.431

85.722

-10.443

A40

4.225

36.818

5.513

50.683

-13.866

47.602

5.974

69.447

-21.845

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Substrates

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Table 1 Surface roughness parameters for bare and anodized Mg substrates

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Journal Pre-proof

Table 2 Coatings thickness and EFM parameters of uncoated and coated substrates in SBF medium (The mentioned values are the mean of triplicates and (±) represents to the standard deviations). Substrates Average Coating

Ecorr

icorr

CF2

V

µA cm-2

-1.702±0.02

44.523±2.58

CF3

of

2.89±0.12

1.91±0.38

2.94±0.24

0.254±0.02

1.94±0.29

2.91±0.19

re

thickness

1.93±0.37

2.93±0.15

0.189±0.01

1.94±0.34

2.94±0.21

0.092±0.02

1.90±0.28

2.96±0.32

0.014±0.01

1.93±0.33

2.94±0.34

0.054±0.01

1.94±0.36

2.95±0.27

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PPy-Bare

10.65±0.28 -1.480±0.04

8.251±0.54

PPy-A20

10.70±0.24 -1.412±0.04

PPy-A40

10.95±0.31 -1.394±0.03

PPy-A60

10.80±0.29 -1.403±0.02

PPy-Ge1

11.10±0.27 -1.325±0.01

PPy-Ge2

11.25±0.19 -1.309±0.02

PPy-Ge3

11.20±0.25 -1.315±0.02

-p

0.124±0.01

lP

na

ur

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1.85±0.41

ro

Uncoated

Journal Pre-proof Table 3 EIS parameter for uncoated and coated Mg substrates in SBF medium (The

Rct

Qdl

ndl

Ω cm2

kΩ cm2

µF cm-2

Rf

Qf

nf

kΩ cm2

µF cm-2

----

----

----

121±5.21

1.18±0.21

97.34±18.87

0.90±0.03

PPy-Bare

118±8.21

21.31±1.54

8.76±0.51

0.94±0.02 8.77±1.89

19.54±2.14

0.93±0.04

PPy-A20

136±9.25

66.59±3.57

0.95±0.05

0.95±0.01 12.46±3.24

6.59±0.98

0.95±0.04

PPy-A40

98±6.54

191.91±14.27

0.12±0.02

0.95±0.01 30.84±4.89

1.92±0.21

0.96±0.02

PPy-A60

105±4.56

141.54±18.24

0.43±0.03

0.95±0.02 15.79±1.25

0.83±0.03

0.96±0.03

PPy-Ge1

113±8.21

359.04±35.14

0.96±0.01 34.99±3.24

0.71±0.01

0.97±0.01

PPy-Ge2

103±7.21

674.31±29.87

0.01±0.001

0.97±0.03 109.42±9.54

0.09±0.01

0.98±0.02

PPy-Ge3

127±6.98

583.12±44.21

0.03±0.001

0.96±0.02 47.80±2.14

0.12±0.02

0.97±0.01

-p

ro

Uncoated

of

Rs

re

Sample

na

lP

0.07±0.01

Jo

ur

mentioned values are the mean of triplicates and (±) represents to the standard deviations).

1. Conflict of Interest No conflict of interest exists. 2. Funding No funding was received for this work.

Journal Pre-proof

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Graphical Abstract

Journal Pre-proof Highlights

 The effect of anodisation on the surface characteristics of subsequent polypyrrole coatings was evaluated on Mg alloy  Surface analysis findings indicated that the performance of the polypyrrole coating was intensely altered

of

 The corrosion studies implied the improved barrier protection performance of polypyrrole

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composite coatings with surface treatment

 Hydrogen evolution rate is controlled by the addition of gelatin

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 Conclusion, the processed anodized layer before coating facilitated the overall

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performance of polypyrrole composite coatings

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

Figure 11

Figure 12

Figure 13