**3. 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). It can also be noted that the addition of H<sup>3</sup> Cit or Na citrate in the treatment solution does not improve the anticorrosive performance of RCe-H<sup>2</sup> O2 coatings.

Based on the above results, we decided to only study the anticorrosive performance of RCe-H2 O2 and RCe-HAsc coatings. First, it is important to mention that the RCe coating was very adherent and could be removed only by mechanical polishing. Adherence force of RCe-H<sup>2</sup> O2 and RCe-HAsc was 34.3 and 43.4 N, respectively. Thus, adhesion of the films increases when the additive is added to the treatment solution.

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

and kept the same value during 18 h. The OCP was approximately −1.350 V after 72 h of

**Figure 8** shows the Tafel polarization plots for uncoated and coated AZ91D alloy. Estimation

measured for the coatings are lower than that for the bare AZ91D alloy. In the case of treated samples, the icorr value is one order of magnitude lower than that of the uncoated substrate. However, higher displacement of Ecorr to more noble potentials was obtained for the RCe-

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 relax-

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 (Rct) and therefore with the corrosion resistance. The Rct values

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

or Mg(OH)<sup>2</sup>

O2

O2

) and anodic (Ba

) Tafel slopes and corrosion

O2

film, and (c)

31

and RCe-HASc coatings. The icorr values

Cerium Oxides for Corrosion Protection of AZ91D Mg Alloy

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

O2

) on the surface of untreated alloys/

film indicating a better corrosion

and RCe-HAsc

immersion. This potential value was still nobler than that for the uncoated electrode.

**Figure 7.** Time dependence of the OCP in Ringer solution for: (a) AZ91D magnesium alloy, (b) RCe-H<sup>2</sup>

coating (**Table 1**).

of the corrosion parameters (Ecorr, cathodic (Bc

ation processes of adsorbed species (Mg(OH)<sup>+</sup>

for RCe-HAsc coating are much larger than that of RCe-H<sup>2</sup>

HAsc film compared to RCe-H<sup>2</sup>

RCe-HAsc coating.

protection.

current (icorr)) is reported in **Table 1** for RCe-H<sup>2</sup>

O2

To obtain more information about the anticorrosion protection of RCe-H<sup>2</sup>

**Figure 6.** Polarization behavior in Ringer solution at 37°C for: (a) uncoated AZ91D alloy, (b) RCe-H<sup>2</sup> O2 coating, (c) RCe-H3 Cit coating, (d) RCe-HAsc coating, and (e) RCe-citrate coating. The scan rate was 0.001 Vs−<sup>1</sup> .

In all cases, a golden-yellow-colored coating was observed with the naked eyes when the

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). It can also be noted that the

Based on the above results, we decided to only study the anticorrosive performance of RCe-

adherent and could be removed only by mechanical polishing. Adherence force of RCe-H<sup>2</sup>

and RCe-HAsc was 34.3 and 43.4 N, respectively. Thus, adhesion of the films increases when

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

**Figure 6.** Polarization behavior in Ringer solution at 37°C for: (a) uncoated AZ91D alloy, (b) RCe-H<sup>2</sup>

Cit coating, (d) RCe-HAsc coating, and (e) RCe-citrate coating. The scan rate was 0.001 Vs−<sup>1</sup>

and RCe-HAsc coatings. First, it is important to mention that the RCe coating was very

Cit or Na citrate in the treatment solution does not improve the anticorrosive

O2

O2

.

coating, (c) RCe-

substrate was polarized at −0.75 V during 30 min in the treatment solution at 50°C.

**3. Evaluation of the anticorrosive properties of the coatings**

addition of H<sup>3</sup>

H2 O2

H3

performance of RCe-H<sup>2</sup>

30 Cerium Oxide - Applications and Attributes

O2

the additive is added to the treatment solution.

coatings.

**Figure 7.** Time dependence of the OCP in Ringer solution for: (a) AZ91D magnesium alloy, (b) RCe-H<sup>2</sup> O2 film, and (c) RCe-HAsc coating.

and kept the same value during 18 h. The OCP was approximately −1.350 V after 72 h of immersion. This potential value was still nobler than that for the uncoated electrode.

**Figure 8** shows the Tafel polarization plots for uncoated and coated AZ91D alloy. Estimation of the corrosion parameters (Ecorr, cathodic (Bc ) and anodic (Ba ) Tafel slopes and corrosion current (icorr)) is reported in **Table 1** for RCe-H<sup>2</sup> O2 and RCe-HASc coatings. The icorr values measured for the coatings are lower than that for the bare AZ91D alloy. In the case of treated samples, the icorr value is one order of magnitude lower than that of the uncoated substrate. However, higher displacement of Ecorr to more noble potentials was obtained for the RCe-HAsc film compared to RCe-H<sup>2</sup> O2 coating (**Table 1**).

To obtain more information about the anticorrosion protection of RCe-H<sup>2</sup> O2 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)<sup>+</sup> or Mg(OH)<sup>2</sup> ) 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 (Rct) and therefore with the corrosion resistance. The Rct values for RCe-HAsc coating are much larger than that of RCe-H<sup>2</sup> O2 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

**Figure 8.** Tafel curves obtained in Ringer solution at 37°C for: (a) uncoated AZ91D alloy, (b) RCe-H<sup>2</sup> O2 , and (c) RCe-HAsc. The scan rate was 0.001 Vs−<sup>1</sup> .


**Table 1.** Corrosion parameters estimated from Tafel polarization plots for uncoated AZ91D, RCe-H<sup>2</sup> O2 , and RCe-HAsc formed on AZ91D alloy.

exhibit a capacitive loop in the high- and medium-frequency ranges. As can be observed, the diameter of capacitive loop increases gradually with increasing time until 36 h, indicating an improvement in the anticorrosion performance of the coating with time. This result corroborates that the RCe-HAsc can effectively improve the corrosion resistance of the alloy.

In order to confirm the improvement in the corrosion protection of the AZ91D alloy imparted by the RCe-HAsc film, the quantity of Mg released in Ringer solutions after 5 h of immersion under open circuit potential conditions was determined. When the substrate was covered with the RCe-HAsc coating, the amount of Mg released was 2.01 mg/L and for the uncoated sample the value was 3.90 mg/L. So the corrosion rate is nearly twice less for the sample coated by the RCe-HAsc film. This result confirms a good performance of the RCe-HAsc coating even after a long exposure time.

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 Mg2+ with simultaneous water reduction. Cathodic reactions

**Figure 10.** Nyquist plots of the impedance spectra obtained at OCP conditions in Ringer solutions at 37°C for RCe-HAsc

coating, after different immersion times: (a) 5 min, (b) 6 h, (c) 12 h, (d) 24 h, (e) 36 h, and (f) 48 h.

**Figure 9.** Nyquist plots of the impedance spectra obtained at OCP conditions after 5 min of immersion in Ringer solution

film, and (c) RCe-HAsc film.

O2

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

precipitation [36, 37]. The β-phase acts as

Cerium Oxides for Corrosion Protection of AZ91D Mg Alloy

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

33

lead to a local alkalization which produces Mg(OH)<sup>2</sup>

at 37°C for: (a) AZ91D Mg alloy, (b) RCe-H<sup>2</sup>

**Figure 9.** Nyquist plots of the impedance spectra obtained at OCP conditions after 5 min of immersion in Ringer solution at 37°C for: (a) AZ91D Mg alloy, (b) RCe-H<sup>2</sup> O2 film, and (c) RCe-HAsc film.

**Figure 10.** Nyquist plots of the impedance spectra obtained at OCP conditions in Ringer solutions at 37°C for RCe-HAsc coating, after different immersion times: (a) 5 min, (b) 6 h, (c) 12 h, (d) 24 h, (e) 36 h, and (f) 48 h.

exhibit a capacitive loop in the high- and medium-frequency ranges. As can be observed, the diameter of capacitive loop increases gradually with increasing time until 36 h, indicating an improvement in the anticorrosion performance of the coating with time. This result corrobo-

**corr/mAcm−<sup>2</sup> Ba**

**/V Bc**

O2

, and RCe-HAsc

**/V**

, and (c) RCe-

O2

In order to confirm the improvement in the corrosion protection of the AZ91D alloy imparted by the RCe-HAsc film, the quantity of Mg released in Ringer solutions after 5 h of immersion under open circuit potential conditions was determined. When the substrate was covered with the RCe-HAsc coating, the amount of Mg released was 2.01 mg/L and for the uncoated sample the value was 3.90 mg/L. So the corrosion rate is nearly twice less for the sample coated by the RCe-HAsc film. This result confirms a good performance of the RCe-HAsc coat-

rates that the RCe-HAsc can effectively improve the corrosion resistance of the alloy.

**Table 1.** Corrosion parameters estimated from Tafel polarization plots for uncoated AZ91D, RCe-H<sup>2</sup>

AZ91D −1.501 ± 0.050 0.1050 ± 0.0050 0.045 −0.293

**Figure 8.** Tafel curves obtained in Ringer solution at 37°C for: (a) uncoated AZ91D alloy, (b) RCe-H<sup>2</sup>

O<sup>2</sup> −1.002 ± 0.020 0.0057 ± 0.0002 0.034 −0.122 RCe-HAsc −0.952 ± 0.015 0.0054 ± 0.0002 0.032 −0.126

**Ecorr/V i**

.

The mean values and their standard deviation are presented.

ing even after a long exposure time.

RCe-H<sup>2</sup>

formed on AZ91D alloy.

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

32 Cerium Oxide - Applications and Attributes

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 Mg2+ with simultaneous water reduction. Cathodic reactions lead to a local alkalization which produces Mg(OH)<sup>2</sup> 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.
