# Insight into the Role of Cerium (III) Addition to a MgAl-LDH Coating on AA6082
###### Abstract
[[https://doi.org/10.33390/app11178252](https://doi.org/10.33390/app11178252)]([https://doi.org/10.33390/app11178252](https://doi.org/10.33390/app11178252))  
Footnote †: : 2021: 11, 8252. [[https://doi.org/10.3390/app11178252](https://doi.org/10.3390/app11178252)]([https://doi.org/10.3390/app11178252](https://doi.org/10.3390/app11178252))  
Footnote †: 2021: 11, 8252. [[https://doi.org/10.33390/app11178252](https://doi.org/10.33390/app11178252)]([https://doi.org/10.33390/app11178252](https://doi.org/10.33390/app11178252))
LDHs are hydrotalcite-like clays (generally termed as anionic clays) constituted by lamellar hydroxides (Figure 1) typically having two types of metallic cations arranged in octahedral sites forming a layered crystalline structure [31; 32]. The overall positively charged layers are electrically counterbalanced by interlayer water molecules and anionic species [33; 34]. The general formula of an LDH is expressed as \(\left[\text{M}_{1-x}^{2+}\text{M}_{x}^{3+}(\text{OH})_{2}\right]^{q+}\left( \text{X}^{n-}\right)_{q/n}\cdot\text{mH}_{2}\text{O}\) where \(\text{M}^{2+}\) and \(\text{M}^{3+}\) are divalent and trivalent metal cations, respectively, \(x\) stands for the molar ratio \(\text{M}^{2+}/\left(\text{M}^{2+}+\text{M}^{2+}\right)\) and \(\text{X}^{n-}\) represents an anion having charge \(n^{-}\)[35]. Usually, the divalent and trivalent metal cations are Mg\({}^{2+}\), Zn\({}^{2+}\), Cu\({}^{2+}\), Mn\({}^{2+}\), Co\({}^{2+}\), Al\({}^{3+}\), Cr\({}^{3+}\) and Fe\({}^{3+}\) ions but also lanthanides and bismuth, having comparable ionic radii, can isomorphically substitute the trivalent metallic cations [36]. For what concern the inter-domain anions, \(\text{F}^{-}\), \(\text{Cl}^{-}\)\(\text{Br}^{-}\), \(\text{I}^{-}\), oxides as \(\text{NO}_{3}^{-}\), \(\text{CO}_{3}^{2-}\), sulfates and chromates or more complex organic and inorganic compounds can be present as counterbalancing species. Generally, anions having high charge density are preferentially intercalated into the LDH complex [37; 38; 39]. According to the recent reviews on this topic [40; 41], several LDH based coatings were developed by the in-situ method on aluminum alloys and were demonstrated to provide the substrate with improved corrosion protection. The anti-corrosion properties of these compounds are recognized to rely on two factors: (i) the ion-exchange mechanism through which corrosive species as chlorides are entrapped into the LDH structure, being thermodynamically more favored than other species to lay inside the domains [42; 43], and (ii) a physical barrier effect that protects the substrate [44; 45]. Rare earth elements (in particular, lanthanum) have been successfully employed to modify LDH coatings either synthesized by means of the co-precipitation method [46; 47] and synthesized in situ on aluminum alloys [48; 49; 50; 51]. In a previous work devoted to Ce(III)-doped MgAl-LDHs layers developed on anodized AA6082 [52], we observed a beneficial effect in terms of improved durability provided by the presence of cerium ions. However, the role of cerium was not investigated in-depth, and the corrosion protection mechanism was only hypothesized. In this work, we aim at gaining mechanistic insight into the role of Ce(III) as an active corrosion inhibitor embedded in the LDHs layer. For this reason, we performed the deposition of Ce doped MgAl-LDHs layers by the in-situ method on AA6082. The synthesis time was varied to assess the potential of the synthesis route to embed the lanthanide ions in the LDH structure. The structural and chemical properties and the morphological features of the LDHs layers were evaluated prior and after the exposure to a saline containing environment to investigate the role of the cerium on the corrosion processes occurring on the metal surface. The evolution of the surface conditions during the exposure was monitored by means of electrochemical impedance spectroscopy (EIS).