**4. Morphological and compositional characterization of RCe-HAsc coating**

**Figure 11** presents the SEM image of the film obtained in solution containing 50 mM Ce (NO<sup>3</sup> ) <sup>3</sup> + 6 mM H<sup>2</sup> O2 (**Figure 11A**) or 50 mM Ce(NO<sup>3</sup> ) <sup>3</sup> + 6 mM H<sup>2</sup> O<sup>2</sup> + 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].

In order to determine the surface chemical composition of RCe-HAsc coating, the XPS was

**Figure 12.** SEM micrograph showing the cross-section of RCe-HAsc coating. The coating was electrosynthesized at

) 3 , 6 mM H<sup>2</sup>

O2

, and 5 mM HAsc.

Cerium Oxides for Corrosion Protection of AZ91D Mg Alloy

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

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 **Figure15**, it can be determined that Mg in the coating is present as MgO and Mg(OH)<sup>2</sup>

**Figure 13.** EDS spectrum of RCe-HAsc coating formed on AZ91D alloy. The coating was electrosynthesized at −0.75 V

O2

, and 5 mM HAsc.

)3 , 6 mM H<sup>2</sup> [2].

35

employed. The XPS results are shown in **Figure 15**.

during 30 min in a solution containing 50 mM Ce(NO<sup>3</sup>

−0.75 V during 30 min in a solution containing 50 mM Ce(NO<sup>3</sup>

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.

**Figure 14** presents the XRD patterns of the AZ91D alloy and treated samples. By comparison it can be concluded that the coatings are composed of CeO<sup>2</sup> , Ce<sup>2</sup> O3 , and Mg(OH)<sup>2</sup> .

**Figure 11.** SEM image of the films obtained on AZ91D alloy. The film was formed at - 0.75 V during 30 min in 50 mM Ce(NO<sup>3</sup> )3 and 6 mM H<sup>2</sup> O2 : (A) without HAsc and (B) with 5 mM HAsc.

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.

**4. Morphological and compositional characterization of RCe-HAsc** 

between the alloys and the obtained film, is responsible of the cracked structure [41].

phy, the thickness of the RCe-HAsc coating is approximately 5 μm (**Figure 12**).

it can be concluded that the coatings are composed of CeO<sup>2</sup>

(**Figure 11A**) or 50 mM Ce(NO<sup>3</sup>

**Figure 11** presents the SEM image of the film obtained in solution containing 50 mM Ce

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

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 microgra-

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

**Figure 14** presents the XRD patterns of the AZ91D alloy and treated samples. By comparison

**Figure 11.** SEM image of the films obtained on AZ91D alloy. The film was formed at - 0.75 V during 30 min in 50 mM

: (A) without HAsc and (B) with 5 mM HAsc.

)

<sup>3</sup> + 6 mM H<sup>2</sup>

, Ce<sup>2</sup> O3

, and Mg(OH)<sup>2</sup>

.

O<sup>2</sup> + 5 mM HAsc (**Figure 11B**).

**coating**

(NO<sup>3</sup> )

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

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

O2

<sup>3</sup> + 6 mM H<sup>2</sup>

34 Cerium Oxide - Applications and Attributes

O2

at the interface substrate/solution.

**Figure 12.** SEM micrograph showing the cross-section of RCe-HAsc coating. The coating was electrosynthesized at −0.75 V during 30 min in a solution containing 50 mM Ce(NO<sup>3</sup> ) 3 , 6 mM H<sup>2</sup> O2 , and 5 mM HAsc.

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 **Figure15**, it can be determined that Mg in the coating is present as MgO and Mg(OH)<sup>2</sup> [2].

**Figure 13.** EDS spectrum of RCe-HAsc coating formed on AZ91D alloy. The coating was electrosynthesized at −0.75 V during 30 min in a solution containing 50 mM Ce(NO<sup>3</sup> )3 , 6 mM H<sup>2</sup> O2 , and 5 mM HAsc.

**Figure 14.** XRD spectra for: (a) AZ91D alloy and (b) RCe-HAsc coating.

**Figure 16B** shows the spectrum of O 1 s. The peak at 531.25 eV is attributed to metallic oxides [2]. The Ce 3d5/2 and Ce 3d3/2 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<sup>2</sup> , CeO, and Ce<sup>2</sup> O3 , 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<sup>2</sup> , CeO, Ce<sup>2</sup> O3 , MgO, and Mg(OH)<sup>2</sup> .

**5. Conclusions**

30 min in 50 mM Ce(NO<sup>3</sup>

in solutions containing cerium nitrate, H<sup>2</sup>

) 3 , 6 mM H<sup>2</sup>

the additive through the formation of insoluble chelates.

O2

, and 5 mM HAsc.

Adherent and uniform cerium-based coatings were obtained on AZ91D magnesium alloy

**Figure 16.** XPS intensities of: (A) Mg 2p, (B) O 1 s, and (C) Ce 3d. The coating was electrosynthesized at −0.75 V during

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

, and three different additives (H<sup>3</sup>

Cerium Oxides for Corrosion Protection of AZ91D Mg Alloy

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

37

Cit, HAsc,

O2

**Figure 15.** XPS survey spectrum of RCe-HAsc coating formed on AZ91D alloy. The coating was electrosynthesized at −0.75 V during 30 min in 50 mM Ce(NO<sup>3</sup> )3 , 6 mM H<sup>2</sup> O2 , and 5 mM HAsc.

Cerium Oxides for Corrosion Protection of AZ91D Mg Alloy http://dx.doi.org/10.5772/intechopen.79329 37

**Figure 16.** XPS intensities of: (A) Mg 2p, (B) O 1 s, and (C) Ce 3d. The coating was electrosynthesized at −0.75 V during 30 min in 50 mM Ce(NO<sup>3</sup> )3 , 6 mM H<sup>2</sup> O2 , and 5 mM HAsc.

### **5. Conclusions**

**Figure 15.** XPS survey spectrum of RCe-HAsc coating formed on AZ91D alloy. The coating was electrosynthesized at

**Figure 16B** shows the spectrum of O 1 s. The peak at 531.25 eV is attributed to metallic oxides [2]. The Ce 3d5/2 and Ce 3d3/2 peaks are presented in **Figure 16C**. The analysis established that the

tively. 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

, CeO, Ce<sup>2</sup>

O3

, CeO, and Ce<sup>2</sup>

, MgO, and Mg(OH)<sup>2</sup>

O3

.

, respec-

binding energies at 916.0, 897.89, and 880.90 eV correspond to CeO<sup>2</sup>

that the RCe-HAsc film is mainly composed of CeO<sup>2</sup>

**Figure 14.** XRD spectra for: (a) AZ91D alloy and (b) RCe-HAsc coating.

36 Cerium Oxide - Applications and Attributes

, and 5 mM HAsc.

O2

)3 , 6 mM H<sup>2</sup>

−0.75 V during 30 min in 50 mM Ce(NO<sup>3</sup>

Adherent and uniform cerium-based coatings were obtained on AZ91D magnesium alloy in solutions containing cerium nitrate, H<sup>2</sup> O2 , and three different additives (H<sup>3</sup> 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.
