Cerium Oxides for Corrosion Protection of AZ91D Mg Alloy Cerium Oxides for Corrosion Protection of AZ91D Mg Alloy

Die-cast AZ91D magnesium alloy (8.9 wt.% Al, 0.6 wt.% Zn, 0.2 wt.% Mn, and balance Mg), as novel alternative biodegradable material, has received great attention due to their potential use in biomedical implants. However, their poor corrosion resistance in physiological fluids restricts practical applications. Cerium-based coatings have been studied as an environmental friendly option to enhance the corrosion resistance of magnesium alloys. In order to control the biodegradation rate of AZ91D magnesium alloy in simulated physiological solution, the formation of a coating from a solution containing cerium nitrate (Ce(NO 3 ) 3 ) was studied. The effect of different additives in the treatment solution (ascorbic acid, citric acid, and sodium citrate) on the anticorrosive properties of the coatings was evaluated. The characterization of the coatings was done using electrochemical techniques and SEM/EDS, XRD, and XPS analyses. The corrosion properties were examined in Ringer solution by polarization studies, open circuit measurements, and faradaic impedance spectroscopy. Results showed that the incorporation of additives improves the anticorrosive properties of the Ce-based film. The coating modified with ascorbic acid provides the best corrosion resistance. According to XPS results, the film is mainly composed by Mg oxides or hydroxides and Ce oxides.


Introduction
Magnesium and its alloys are biodegradable and biocompatible materials which exhibit an attractive combination of low density and high strength/weight ratio making them ideal candidates for biomedical applications like substitution and generation of tissues [1]. Temporary implants of biodegradable materials eliminate the need of a second surgery for implant removal since they are destined to corrode and dissolve postoperatively [2]. However, magnesium-based materials present poor corrosion resistance in physiological environments limiting their applications in the biomedical field. AZ91D magnesium alloy is one of the most commonly used materials, and its corrosion resistance depends on the presence of impure elements acting as active cathode on the microstructure [3].
In order to improve the corrosion resistance of magnesium alloys, some environmentally friendly chemical treatments has been developed [4]. Among them, coatings based on rare earth elements appear as a promising system. Cerium has been studied as alternative to generate protective films. Several researches demonstrated that treatments with cerium salts solutions inhibited the metal corrosion. The formation of cerium oxides and hydroxides on the metal surfaces is generally the reason of this inhibition process because it gives rise to a blocking effect and reduces the rate of reduction reactions [1,5,6]. It is known that magnesium alloys oxidation is accompanied with the reduction of hydrogen ions as cathodic reaction. This hydrogen discharge promotes the reaction of Ce 3+ and Ce 4+ species with OH − to form insoluble salts of Ce(OH) 3 and Ce(OH) 4 due to the increase of the pH in the interface between the substrate and the electrolyte solution [7,8]. On the other hand, it has been reported that dissolved oxygen can promote the oxidation of Ce 3+ to Ce 4+ species. Yu et al. have reported that the cerium precipitation reaction could be affected by the presence of oxygen when the pH solution is in the proper range (pH 4-6) [9]. In addition, Yang et al. demonstrated that the presence of oxygen in the cerium solution promotes the anodic formation of CeO 2 which is better for the formation of compact ceria films [10].
It has been showed that the addition of additives in a cerium solution improves the corrosion resistance of magnesium alloys [11]. Scholes et al. show that the addition of H 2 O 2 to the cerium solution is intimated and involved in the deposition process. Hydrogen peroxide acts as oxidant agent and when it is added to the conversion solution, Ce 3+ ions oxidize to Ce 4+ . Several studies expose a model of the mechanism by which the cerium-based coatings are formed in the presence of H 2 O 2 [12][13][14]. The addition of hydrogen peroxide in the cerium conversion solution promotes the formation of a cerium hydroxide/oxide coating containing mainly Ce(IV) species which are associated with higher degrees of protection. Chen et al. [15] expose that the concentration of H 2 O 2 added to the solution has an important role. In adequate concentrations, H 2 O 2 can accelerate the conversion reaction in the formation process; however, if the content of the oxidant exceeds some break value, the coating formed will not be protective for the substrate [6]. In addition, it has been demonstrated that the addition of hydrogen peroxide (H 2 O 2 ) to cerium salts solutions leads to the formation of yellow color coatings due to the presence of Ce(IV). Dabalà et al. report that as the amount of H 2 O 2 in the cerium conversion solution increases, more intense yellow color of the coating is obtained on the magnesium alloy surface [6].
With the objective to reduce the velocity of corrosion of AZ91D Mg alloy in physiological simulated solution, the generation of cerium-based coatings on AZ91D Mg alloy was studied in this work. The influence of both the presence of additives in the treatment solution and employed technique on the properties of the coatings was evaluated. Electrochemical and surface analysis techniques such as SEM, EDS, X-ray diffraction, and X-ray photoelectron spectroscopy (XPS) were employed for characterization of the coatings. The anticorrosive performance of the films was investigated in Ringer solution at 37°C by polarization studies, open circuit measurements, and faradaic impedance spectroscopy.

Coating formation
All measurements presented in this chapter were obtained using working electrodes prepared from AZ91D magnesium alloys rods embedded in a Teflon holder with an exposed area of 0.070 cm 2 [16]. The potentials were measured against a saturated Ag/AgCl and a platinum sheet was used as a counter electrode. All chemicals were reagent grade and solutions were made with twice distilled water.
Optimal conversion parameters such as applied potential, pH of the solution, and additives concentrations were determined in order to obtain protective cerium coatings on AZ91D Mg alloy. Potentiodynamic polarization tests in Ringer solution at 37°C were performed to evaluate the corrosion behavior of the electrodes treated in different cerium-based solutions. Thus, the best formation conditions were established.
First, the formation of coating by immersion of AZ91D alloy in a 50 mM Ce(NO 3 ) 3 solution of pH 4.7 at 50°C under open circuit potential conditions was investigated. After 30 min, a discontinuous and not adherent white coating was obtained on the substrate. Different potentials were applied to the working electrode employing the same electrolyte solution in order to check the effects of polarization. Figure 1 presents the polarization curves in Ringer solution obtained for cerium coatings synthesized at different potentials on AZ91D alloy. The curve for the bare alloy shows an active dissolution process which starts at −1.478 V (Figure 1, curve a). When the substrate is covered by the coating, this process is retarded. The major improvement in corrosion resistance is observed for the film formed at −0.750 V (Figure 1, curve c). The corresponding curve exhibits the lowest current densities in the applied potential range. A shift to more positive potentials was also observed indicating that the corrosion reaction of AZ91D alloy is retarded by the presence of the coating. Thus, this potential was selected for further experiments.
Uniform white coatings were obtained on the AZ91D alloy in a 50 mM Ce(NO 3 ) 3 solution at −0.75 V during 30 min. For simplicity purposes this film will be called RCe. It has been demonstrated that the use of additives in the treatment solution could improve the anticorrosive performance of the RCe films [11]. Thus, the effect of the addition of different hydrogen peroxide concentrations (1-20 mM) in the cerium-based baths was evaluated. A more uniform film was formed from the solution containing 50 mM Ce(NO 3 ) 3 , 6 mM H 2 O 2 , and pH 3.6. This additive produces the oxidation of Ce 3+ to Ce 4+ favoring the incorporation of cerium(IV) in the film [15]. A yellow coating is observed on the substrate after the potentiostatic formation. This coating will be called RCe-H 2 O 2 . According to the literature, cerium(IV) is responsible of the appearance of the yellow coating [17], while the presence of Ce 2 O 3 or Ce(OH) 3 is related to the white color.
It is known that both Mg and Al are immediately oxidized during immersing of AZ91D alloy in a solution containing Ce(NO 3 ) 3 and H 2 O 2 . The stable species of Mg in solutions with pH value less than 8.5 is Mg 2+ while in the case of aluminum is Al 3+ for solutions with pH value less than 4 [18]. Proton, oxygen, and H 2 O 2 reductions can occur simultaneously with the oxidation of the substrate. Based on the experimental conditions of our work, H 2 O 2 is the main oxidizing agent, in accordance with the proposition of Yu et al. [9]. Moreover, the addition of H 2 O 2 to the treatment solution is necessary for the development of a yellowed coating which is associated with the presence Ce 4+ species. As was mentioned above, H 2 O 2 is a strong oxidizing agent, and its presence in the cerium solution can promote the oxidation which accelerates the precipitation of the conversion coating. On the other hand and, as was stated, the presence of H 2 O 2 can produce the oxidation of Ce(III) to Ce(IV). Ce(III) and Ce(IV) species react with OH − to form insoluble salts of Ce(OH) 3 and Ce(OH) 4 as film components. As the pH in the interface between the substrate and the solution increases as a result of the hydrogen discharge, the precipitation reaction is favored. On the other hand, it has been informed that when H 2 O 2 concentration is around 80 mg/L, an increase in the deposition rate of insoluble salts occurs, and in effect, a porous coating is formed on the substrate surface [15]. Thus, the incorporation of a proper amount of H 2 O 2 in the treatment bath improves the corrosion performance of Ce-based film on AZ91D alloys.
Polarization curves for the bare alloy and alloy covered with cerium-based coatings are presented in Figure 2. In comparison with RCe, a considerable potential shift to more positive values is observed for RCe-H 2 O 2. Thus, the addition of oxidant in the cerium-based bath enhances the corrosion performance of RCe coating.
In order to improve the corrosion resistance of the cerium-based coating obtained in the presence of H 2 O 2 , different additives were evaluated. Thus, the inhibition effect of three additives (citric acid (H 3 Cit), ascorbic acid (Hasc), and sodium citrate (Na citrate)) on the electrochemical response of bare AZ91D alloy in Ringer solution was studied. For each additive a range of concentrations was evaluated in order to establish the best conditions to be used in Ce film formation. Figures 3-5 show the polarization curves of AZ91D magnesium alloy in Ringer solutions containing different additive concentrations. It can be observed that the degradation  rate of AZ91D alloy is retarded by the presence of the H 3 Cit at concentrations up to 10 mM (Figure 1, curve c), while the effect is opposite for higher concentrations (Figure 3, curve d).
Thus, the best inhibitive performance was obtained for 10 mM H 3 Cit. A similar procedure was carried out in order to analyze HAsc (Figure 4) and Na citrate (Figure 5), respectively. The optimum concentration of HAsc was 5 mM (Figure 4, curve b), while for Na citrate the best inhibitor effect was obtained for 15 mM (Figure 5, curve c).
It is known that compounds with functional groups containing oxygen act as effective corrosion inhibitors for metallic materials in aqueous chloride solution by a surface complex formation [19]. Also, it is known that a chelating agent should have two opposite effects on  the corrosion of metals and may act as either an inhibitor or corrosive [20]. With respect to the effects of H 3 Cit, Wang reported that a cerium conversion coating on AZ91 magnesium alloy formed from ethanol solution containing Ce(NO 3 ) 3 and citric acid showed good corrosion performance [17]. The authors postulated that Mg ions originated from the dissolution of the alloy during immersion in the treatment bath reacted with Cit 3− and partly deposited on the alloy surface in the form of Mg 3 Cit 2 that is insoluble in ethanol. According to the theory of chemical adsorption of organic inhibitors, the chelating agent which forms a stable and insoluble chelate with a metal in certain medium can inhibit corrosion. HAsc is a well-known inhibitor for several metallic materials. The inhibitor character of HAsc has been extensively studied for steel in acid and neutral media [21][22][23].
Valek et al. reported that the generation of a protective oxide film on steel is associated with the formation of an insoluble surface chelate at an optimal concentration of 10 −3 M [23]. However, these authors also informed that the formation of a soluble chelate has a stimulatory action in Fe dissolution at concentrations above 5 × 10 −3 M. It has been informed that HAsc presents a dual role, in some conditions it can act as corrosion inhibitor, and in other cases, it can increase the corrosion rate of stainless steel (SS)X 4 Cr 13 in HCl solutions [24]. At the present time the inhibition effect of HAsc on the corrosion of magnesium alloys has not been reported. The results obtained here are in accordance with the tendency informed in the bibliography for other metallic materials. So, for an optimal HAsc concentration (5 mM), the precipitation of an insoluble surface chelate confers protection to the magnesium alloy through the formation of a physical barrier. On the other hand, for a HAsc concentration above 5 mM, the degradation rate of AZ91D alloy increases due to the formation of soluble chelates.
It has been demonstrated that sodium citrate can act as brightening, leveling, and buffering agent in electrodeposition electrolytes and, thus, eliminates the need for other additives [25]. Moreover, it is mainly known as a complexing agent. Organic compounds with carboxylate group have been presented as promising corrosion inhibitors of carbon steel in high-alkalinemedia-containing chloride ions [26][27][28][29]. In addition, citrate ions were presented as good pitting inhibitors, as they could adsorb on the carbon steel (without a passive layer), avoiding chloride ions adsorption due to a steric effect [28]. It has been reported that citrate ions present a chelating effect, forming soluble complexes with Fe(II) and Fe(III) [30]. Bahramian et al.
show that sodium citrate proved to be an effective and economical additive to improve the properties of Ni-P coatings obtained on Cu substrate; its impact depended only slightly on its concentration [31]. The effect of Na citrate as corrosion inhibitor in chloride solution was studied for AZ31D and AM60 Mg alloys [32,33]. It has been demonstrated that Na citrate forms chelates with Mg 2+ . Although the chelate is soluble, it can be absorbed on the surface of the substrate by the oxygen atom, hindering the adsorption of Cl − [33].
From the corrosion inhibition behavior shown above, the optimal concentration of each additive for coating formation was selected. Thus, the treatment solutions contain 50 mM Ce(NO 3 ) 3 , 6 mM H 2 O 2 , and: • 10 mM H 3 Cit. This film will be called RCe-H 3 Cit.
• 15 mM Na citrate. This film will be called RCe-citrate. In all cases, a golden-yellow-colored coating was observed with the naked eyes when the substrate was polarized at −0.75 V during 30 min in the treatment solution at 50°C.

Evaluation of the anticorrosive properties of the coatings
Polarization measurements in Ringer solution were carried out in order to study the corrosion behavior of the coating formed in the presence of additives (Figure 6). A positive potential shift and the low current density measured indicate a better corrosion resistance of the coated samples. In the case of the RCe-HAsc film, a significative increase in the anodic current can be observed at more anodic potential values (Figure 6, curve d To evaluate the degree of corrosion protection attained after covering the substrate, the variation of the open circuit potential (OCP) as a function of time was carried out in Ringer solution (Figure 7). Uncoated sample reaches the pitting potential (−1.503 V) after approximately 5 h of immersion (Figure 7, curve a). The electrode covered with the coating obtained without the additive reached the corrosion potential of the uncoated AZ91D alloy after 36 h of immersion (Figure 7, curve b). In the case of the coating generated in the presence of the HAsc, at the beginning, the OCP value was −1.320 V (Figure 7, curve c). Then, the OCP reached −1.150 V   Table 1 for RCe-H 2 O 2 and RCe-HASc coatings. The i corr values measured for the coatings are lower than that for the bare AZ91D alloy. In the case of treated samples, the i corr value is one order of magnitude lower than that of the uncoated substrate. However, higher displacement of E corr to more noble potentials was obtained for the RCe-HAsc film compared to RCe-H 2 O 2 coating ( Table 1).
To obtain more information about the anticorrosion protection of RCe-H 2 O 2 and RCe-HAsc film, EIS measurements were conducted in Ringer solution (Figure 9). For comparison, the impedance response of uncoated substrate was also presented in Figure 9, curve a. Two capacitive loops and one inductive loop were observed in the Nyquist diagram of the AZ91D Mg alloy as was observed for other magnesium alloys [34]. It has been postulated that relaxation processes of adsorbed species (Mg(OH) + or Mg(OH) 2 ) on the surface of untreated alloys/ pure Mg leads to an inductive response at low frequencies [35]. After 5 min of immersion, the impedance response for the coated electrodes exhibit two capacitive loops in the high-and low-frequency ranges. It is known that the diameter of the capacitive loops is associated with the charge-transfer resistance (R ct ) and therefore with the corrosion resistance. The R ct values for RCe-HAsc coating are much larger than that of RCe-H 2 O 2 film indicating a better corrosion protection.
The performance of the RCe-HAsc at different immersion times in Ringer solution was analyzed by Nyquist plots (Figure 10). After 6 h of immersion, all the impedance diagrams The mean values and their standard deviation are presented.  When the uncoated AZ91D alloy is immersed in simulated physiological solution, it suffers significant degradation. As mentioned above, it is proposed that the general corrosion mechanism of Mg alloys implies Mg oxidation to Mg 2+ with simultaneous water reduction. Cathodic reactions lead to a local alkalization which produces Mg(OH) 2 precipitation [36,37]. The β-phase acts as the cathode and α-phase acts as the anode. The active surface area is reduced by RCe-HAsc film, and in consequence, less area of the substrate is available to be corroded. The potential difference  between α-and β-phases becomes smaller when the AZ91D alloy is coated by an adherent and stable film. Thus, the micro-galvanic couple effect is reduced [38,39]. In summary, the coating confers a physical barrier between the substrate and the corrosive medium. In addition, the improvement in the corrosion resistance is associated with the presence of insoluble precipitates of cerium and the inhibitor effect of ascorbic acid. As mentioned previously, the HAsc has inhibition ability by insoluble chelates formation and it is responsible for the increased corrosion protection of RCe films. The presence of HAsc decreases the dissolution rate of the substrate during the coating formation allowing the formation of a more compact and protective film. Figure 11 presents the SEM image of the film obtained in solution containing 50 mM Ce (NO 3 ) 3 + 6 mM H 2 O 2 ( Figure 11A) or 50 mM Ce(NO 3 ) 3 + 6 mM H 2 O 2 + 5 mM HAsc ( Figure 11B). A cracked mud morphology is observed of RCe film. The dehydration of the surface film after the deposition leads to crack formation [40]. It has been suggested that the formation of gas bubbles on the substrate, combined with a dehydration process or also with shearing stresses between the alloys and the obtained film, is responsible of the cracked structure [41].

Morphological and compositional characterization of RCe-HAsc coating
A more uniform and compact film with the presence of only some microcracks was obtained by the addition of HAsc in the conversion solution. From the SEM cross-sectional micrography, the thickness of the RCe-HAsc coating is approximately 5 μm (Figure 12).
The presence of cerium in the coating was confirmed by EDX analysis (Figure 13). It is known that RCe coatings are obtained from the precipitation of oxides, due to an increase in local pH at the interface substrate/solution.  In order to determine the surface chemical composition of RCe-HAsc coating, the XPS was employed. The XPS results are shown in Figure 15.
The main components are magnesium, oxygen, and cerium. Figure 16 shows the XPS analysis of the specific electron binding energies of Mg, O, and Ce elements. From the Mg 2p spectrum, presented in Figure 15, it can be determined that Mg in the coating is present as MgO and Mg(OH) 2 [2].    Figure 16B shows the spectrum of O 1 s. The peak at 531.25 eV is attributed to metallic oxides [2]. The Ce 3d 5/2 and Ce 3d 3/2 peaks are presented in Figure 16C. The analysis established that the binding energies at 916.0, 897.89, and 880.90 eV correspond to CeO 2 , CeO, and Ce 2 O 3 , respectively. The satellite peak around 916.0 eV confirms the presence of Ce(IV) ions in the coating. The ratio between Ce(IV) and Ce(III) was 1.503. In summary, from the XPS results, it was concluded that the RCe-HAsc film is mainly composed of CeO 2 , CeO, Ce 2 O 3 , MgO, and Mg(OH) 2 .

Conclusions
Adherent and uniform cerium-based coatings were obtained on AZ91D magnesium alloy in solutions containing cerium nitrate, H 2 O 2 , and three different additives (H 3 Cit, HAsc, and Na-citrate). The most adherent films were obtained by a potentiostatic polarization at −0.75 V. The RCe-HAsc-coated AZ91D alloy exhibited better corrosion resistance in Ringer solution. Magnesium oxides or hydroxides and cerium oxides are the main components of the film. The anticorrosive properties of RCe-HAsc film in simulated body fluid solution is superior to those of films formed in the absence of HAsc. The improvement in the corrosion resistance is associated with the presence of insoluble precipitates of cerium and the effect of the additive through the formation of insoluble chelates.