**2. Coating formation**

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

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 Ce3+ and Ce4+ species with OH− to form

the substrate and the electrolyte solution [7, 8]. On the other hand, it has been reported that dissolved oxygen can promote the oxidation of Ce3+ to Ce4+ 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

It has been showed that the addition of additives in a cerium solution improves the corro-

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, Ce3+ ions oxidize to Ce4+. Several studies expose a model of the mechanism by which the cerium-based coatings are

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]

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

sion solution increases, more intense yellow color of the coating is obtained on the magnesium

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

presence of oxygen in the cerium solution promotes the anodic formation of CeO<sup>2</sup>

sion resistance of magnesium alloys [11]. Scholes et al. show that the addition of H<sup>2</sup>

due to the increase of the pH in the interface between

[12–14]. The addition of hydrogen peroxide in the cerium

can accelerate the conversion reaction in the formation process; however,

) to cerium salts solutions leads to the formation of yellow color coatings due

added to the solution has an important role. In adequate

O2

which is

O2 to the

in the cerium conver-

elements acting as active cathode on the microstructure [3].

and Ce(OH)<sup>4</sup>

O2

O2

to the presence of Ce(IV). Dabalà et al. report that as the amount of H<sup>2</sup>

better for the formation of compact ceria films [10].

insoluble salts of Ce(OH)<sup>3</sup>

24 Cerium Oxide - Applications and Attributes

formed in the presence of H<sup>2</sup>

O2

concentrations, H<sup>2</sup>

peroxide (H<sup>2</sup>

alloy surface [6].

expose that the concentration of H<sup>2</sup>

O2

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 cm2 [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<sup>3</sup> ) 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

**Figure 1.** Polarization curves in Ringer solution at 37°C for: (a) AZ91D Mg alloy uncoated and covered with RCe films electrosynthesized in a solution containing 50 mM Ce(NO<sup>3</sup> )3 during 30 min at (b) −0.60 V, (c) −0.75 V, and (d) −0.90 V. The scan rate was 0.001 Vs−<sup>1</sup> .

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<sup>3</sup> ) 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<sup>3</sup> )3 , 6 mM H<sup>2</sup> O2 , and pH 3.6. This additive produces the oxidation of Ce3+ to Ce4+ 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<sup>2</sup> O2 . According to the literature, cerium(IV) is responsible of the appearance of the yellow coating [17], while the presence of Ce<sup>2</sup> O3 or Ce(OH)<sup>3</sup> 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<sup>3</sup> )3 and H<sup>2</sup> O2 . The stable species of Mg in solutions with pH value less than 8.5 is Mg2+ while in the case of aluminum is Al3+ for solutions with pH value less than 4 [18]. Proton, oxygen, and H<sup>2</sup> O2 reductions can occur simultaneously with the oxidation of the substrate. Based on the experimental conditions of our work, H<sup>2</sup> O2 is the main oxidizing agent, in accordance with the proposition of Yu et al. [9]. Moreover, the addition of H<sup>2</sup> O2 to the treatment solution is necessary for the development of a yellowed coating which is associated with the presence Ce4+ species. As was mentioned above, H<sup>2</sup> O2 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<sup>2</sup> O2 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)<sup>3</sup> and Ce(OH)<sup>4</sup> 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<sup>2</sup> O2 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<sup>2</sup> O2 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<sup>2</sup> O2. Thus, the addition of oxidant in the cerium-based bath enhances the corrosion performance of RCe coating.

(citric acid (H<sup>3</sup>

coating. The scan rate was 0.001 Vs−<sup>1</sup>

.

Cit), ascorbic acid (Hasc), and sodium citrate (Na citrate)) on the electrochemi-

.

cal 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

**Figure 3.** Polarization behavior for AZ91D alloy at 37°C in Ringer solution containing different H<sup>3</sup>

0 mM, (b) 5 mM, (c) 10 mM, and (d) 15 mM. The scan rate was 0.001 Vs−<sup>1</sup>

**Figure 2.** Polarization behavior in Ringer solution at 37°C for: (a) AZ91D Mg alloy, (b) RCe coating, and (c) RCe-H<sup>2</sup>

O2

27

Cerium Oxides for Corrosion Protection of AZ91D Mg Alloy

http://dx.doi.org/10.5772/intechopen.79329

Cit concentrations: (a)

In order to improve the corrosion resistance of the cerium-based coating obtained in the presence of H<sup>2</sup> O2 , different additives were evaluated. Thus, the inhibition effect of three additives

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

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

additive produces the oxidation of Ce3+ to Ce4+ favoring the incorporation of cerium(IV) in the film [15]. A yellow coating is observed on the substrate after the potentiostatic formation.

It is known that both Mg and Al are immediately oxidized during immersing of AZ91D alloy

value less than 8.5 is Mg2+ while in the case of aluminum is Al3+ for solutions with pH value

oxidizing agent, in accordance with the proposition of Yu et al. [9]. Moreover, the addition

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,

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

salts occurs, and in effect, a porous coating is formed on the substrate surface [15]. Thus,

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 posi-

In order to improve the corrosion resistance of the cerium-based coating obtained in the pres-

O2

to the treatment solution is necessary for the development of a yellowed coating

can produce the oxidation of Ce(III) to Ce(IV). Ce(III) and Ce(IV) species

and Ce(OH)<sup>4</sup>

concentration is around 80 mg/L, an increase in the deposition rate of insoluble

, different additives were evaluated. Thus, the inhibition effect of three additives

O2

O2

dation of the substrate. Based on the experimental conditions of our work, H<sup>2</sup>

which is associated with the presence Ce4+ species. As was mentioned above, H<sup>2</sup>

)3

, 6 mM H<sup>2</sup>

O3

. The stable species of Mg in solutions with pH

in the treatment bath improves the corrosion

O2. Thus, the addition of oxidant in the cerium-based bath

reductions can occur simultaneously with the oxi-

. According to the literature, cerium(IV) is responsible of

O2

or Ce(OH)<sup>3</sup>

O2

O2

as film components. As the pH

) 3

, and pH 3.6. This

solution

is related

is the main

is a strong

Uniform white coatings were obtained on the AZ91D alloy in a 50 mM Ce(NO<sup>3</sup>

film was formed from the solution containing 50 mM Ce(NO<sup>3</sup>

O2

the appearance of the yellow coating [17], while the presence of Ce<sup>2</sup>

)3 and H<sup>2</sup>

This coating will be called RCe-H<sup>2</sup>

26 Cerium Oxide - Applications and Attributes

in a solution containing Ce(NO<sup>3</sup>

less than 4 [18]. Proton, oxygen, and H<sup>2</sup>

O2

react with OH− to form insoluble salts of Ce(OH)<sup>3</sup>

the incorporation of a proper amount of H<sup>2</sup>

tive values is observed for RCe-H<sup>2</sup>

performance of Ce-based film on AZ91D alloys.

enhances the corrosion performance of RCe coating.

to the white color.

the presence of H<sup>2</sup>

O2

of H<sup>2</sup> O2

when H<sup>2</sup>

ence of H<sup>2</sup>

O2

experiments.

**Figure 2.** Polarization behavior in Ringer solution at 37°C for: (a) AZ91D Mg alloy, (b) RCe coating, and (c) RCe-H<sup>2</sup> O2 coating. The scan rate was 0.001 Vs−<sup>1</sup> .

**Figure 3.** Polarization behavior for AZ91D alloy at 37°C in Ringer solution containing different H<sup>3</sup> Cit concentrations: (a) 0 mM, (b) 5 mM, (c) 10 mM, and (d) 15 mM. The scan rate was 0.001 Vs−<sup>1</sup> .

(citric acid (H<sup>3</sup> 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

the corrosion of metals and may act as either an inhibitor or corrosive [20]. With respect to the

formance [17]. The authors postulated that Mg ions originated from the dissolution of the alloy during immersion in the treatment bath reacted with Cit<sup>3</sup>− and partly deposited on the alloy

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 [21–23]. 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−<sup>3</sup> 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−<sup>3</sup> M. It has been informed that HAsc presents a dual role, in some conditions it can act as corrosion inhibitor,

[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 [26–29]. 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 Mg2+. Although the chelate is soluble, it can be absorbed on the surface of

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

Cit.

) 3

Cit, Wang reported that a cerium conversion coating on AZ91 magnesium alloy

that is insoluble in ethanol. According to the theory of chemical

and citric acid showed good corrosion per-

Cerium Oxides for Corrosion Protection of AZ91D Mg Alloy

http://dx.doi.org/10.5772/intechopen.79329

29

<sup>−</sup> [33].

Cr13 in HCl solutions

effects of H<sup>3</sup>

Ce(NO<sup>3</sup> )3

• 10 mM H<sup>3</sup>

, 6 mM H<sup>2</sup>

O2 , and:

surface in the form of Mg3

formed from ethanol solution containing Ce(NO<sup>3</sup>

Cit<sup>2</sup>

and in other cases, it can increase the corrosion rate of stainless steel (SS)X4

the substrate by the oxygen atom, hindering the adsorption of Cl

Cit. This film will be called RCe-H<sup>3</sup>

• 15 mM Na citrate. This film will be called RCe-citrate.

• 5 mM HAsc. This film will be called RCe-HAsc.

**Figure 4.** Polarization behavior for AZ91D alloy at 37°C in Ringer solution containing different HAsc concentrations: (a) 0 mM, (b) 1 mM, (c) 5 mM, and (d) 10 mM. The scan rate was 0.001 Vs−<sup>1</sup> .

**Figure 5.** Polarization behavior for AZ91D alloy at 37°C in Ringer solution containing different Na citrate concentrations: (a) 0 mM, (b) 1 mM, (c) 5 mM, and (d) 15 mM. The scan rate was 0.001 Vs−<sup>1</sup> .

rate of AZ91D alloy is retarded by the presence of the H<sup>3</sup> 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<sup>3</sup> 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<sup>3</sup> Cit, Wang reported that a cerium conversion coating on AZ91 magnesium alloy formed from ethanol solution containing Ce(NO<sup>3</sup> ) 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<sup>3</sup>− and partly deposited on the alloy surface in the form of Mg3 Cit<sup>2</sup> 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 [21–23]. 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−<sup>3</sup> 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−<sup>3</sup> 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)X4 Cr13 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 [26–29]. 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 Mg2+. Although the chelate is soluble, it can be absorbed on the surface of the substrate by the oxygen atom, hindering the adsorption of Cl <sup>−</sup> [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<sup>3</sup> )3 , 6 mM H<sup>2</sup> O2 , and:

• 10 mM H<sup>3</sup> Cit. This film will be called RCe-H<sup>3</sup> Cit.

rate of AZ91D alloy is retarded by the presence of the H<sup>3</sup>

(a) 0 mM, (b) 1 mM, (c) 5 mM, and (d) 15 mM. The scan rate was 0.001 Vs−<sup>1</sup>

0 mM, (b) 1 mM, (c) 5 mM, and (d) 10 mM. The scan rate was 0.001 Vs−<sup>1</sup>

28 Cerium Oxide - Applications and Attributes

inhibitor effect was obtained for 15 mM (**Figure 5**, curve c).

Thus, the best inhibitive performance was obtained for 10 mM H<sup>3</sup>

(**Figure 1**, curve c), while the effect is opposite for higher concentrations (**Figure 3**, curve d).

**Figure 5.** Polarization behavior for AZ91D alloy at 37°C in Ringer solution containing different Na citrate concentrations:

**Figure 4.** Polarization behavior for AZ91D alloy at 37°C in Ringer solution containing different HAsc concentrations: (a)

.

.

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

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

Cit at concentrations up to 10 mM

Cit. A similar procedure was


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.
