**1. Introduction**

The interest in magnesium alloys for applications demanding high strength-to-weight ratio is growing rapidly, mainly driven by the inherent low density of these materials. Additional attributes, such as high damping capacity, good castability, and machinability, are also attractive for industrial purposes [1,2]. One further, but no less important, aspect of magnesium alloys is their well-known low corrosion resistance in aqueous environments [3]. The naturally formed magnesium hydroxide layer is non-protective in chloride-containing electrolytes, making the material susceptible to pitting corrosion [4]. Corrosion control is, therefore, a serious concern for magnesium alloys, as its high chemical reactivity limits a widespread use in several applications [5].

In fact, the low corrosion resistance of magnesium alloys is undesirable for most commercial applications. Nevertheless, it may be advantageous if one considers biodegradable implant materials. Due to its intrinsic biocompatibility, magnesium can be employed in temporary implants for fracture fixation [6,7]. Research on magnesium alloys as temporary orthopedic devices has gained huge interest in the past few years. Recently, Sezer et al. [8] reviewed the main aspects of biodegradable Mg-based implants. The most important feature of temporary fixation devices is to withstand the mechanical loads to which the implant is subject during its use while the fracture heals. In order to meet this goal, the inherent high chemical reactivity of magnesium alloys must be controlled to avoid premature failure of the fixation device [9].

Several methods for improving the corrosion resistance of magnesium have been reported in the literature, such as alloying and surface treatments. In the case of alloying,

**Citation:** de Oliveira, L.A.; dos Santos, S.L.; de Oliveira, V.A.; Antunes, R.A. Influence of Anodization on the Fatigue and Corrosion-Fatigue Behaviors of the AZ31B Magnesium Alloy. *Metals* **2021**, *11*, 1573. https://doi.org/10.3390/ met11101573

Academic Editor: Sebastian Feliú, Jr.

Received: 3 September 2021 Accepted: 27 September 2021 Published: 1 October 2021

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microstructural control is pursued to promote the formation of precipitates and/or grain refining, thus improving the corrosion properties of the alloy by reducing the propensity to the formation of local microgalvanic cells [10]. Surface modification methods for the corrosion control of magnesium alloys include, but are not limited to, chemical conversion coatings, ion implantation, microarc oxidation, physical vapor deposition, plasma spraying, and anodization [11].

Anodization is a traditional way of improving the corrosion resistance of magnesium alloys. Many studies are devoted to investigating the effect of electrical parameters on the corrosion protection ability of the anodic film or the electrolyte type and concentration of chemical species in the formation of a compact and protective anodized layer [12–15]. The most recent trends in this research field are focused on anodizing treatments based on environmentally friendly electrolytes. Silicate-containing alkaline baths play a prominent role in this scenario. Salami et al. [16] have shown that dense and uniform anodic films could be produced on the AZ31B alloy by controlling the concentration of sodium silicate in the electrolyte, favoring the formation of Mg2SiO4 in the coating layer. Due to its non-toxic character, silicate-based electrolytes are also envisaged as good options for the surface treatment of magnesium implants [17].

Fatigue resistance plays a central role in the service life of biomedical alloys. Corrosion fatigue is recognized as a serious problem for different metallic biomaterials, being associated with a grea<sup>t</sup> part of the mechanical failures of implantable devices [18]. Raman et al. [19] highlighted the critical aspects of corrosion fatigue of magnesium implants, emphasizing that it is not frequently addressed in the current literature. Nonetheless, despite the scarcity of studies in this area, some reports can be found. Liu et al. [20] studied the corrosion fatigue behavior of a biocompatible Mg-Zn-Y-Nd alloy in simulated body fluid (SBF). The fatigue resistance decreased in SBF in comparison with the fatigue tests conducted in air. Another interesting aspect was related to the source of fatigue cracks. In SBF, multiple cracks were generated, while only one crack source was found in the absence of the corrosion medium. Gu et al. [21] have also reported a deleterious effect of the corrosive physiological environment in the fatigue resistance of the AZ91D alloy. Corrosion pits were associated with the preferential sites for fatigue crack nucleation. Bian et al. [22] studied the corrosion fatigue of Mg-Ca and Mg-Zn-Ca alloys in SBF. A significant decrease of the fatigue properties was reported when compared to the tests conducted in air.

In spite of the relevant findings accumulated so far on the corrosion fatigue behavior of magnesium-based biomaterials, the concomitant effect of the presence of an anodized layer and a corrosive environment on the fatigue response is not currently found in the literature. In one hand, anodization can be an interesting method for the corrosion control of temporary fixation devices, as it allows one to tailor the morphology and composition of the oxide layer to produce a dense, uniform, and biocompatible anodic film. On the other hand, the presence of the anodic film may affect the fatigue properties of the anodized alloy. According to Eifert et al. [23], anodization influences both the crack initiation and propagation processes during cyclic loading of magnesium alloys. Khan et al. [24] reported that the fatigue resistance of anodized AM60 alloy (Mg-Zn-Mn) decreased due to the porous nature of the anodic film. Hence, the morphology of the anodized layer influences the fatigue behavior of the material. Nemcova et al. [25] reported a reduction of 56% for the fatigue limit of a microarc oxidized AZ61 alloy in NaCl solution due to the presence of the oxide layer. Although these reports provide relevant information on the interplay between anodization and corrosion fatigue of structural magnesium alloys, similar information is not found if one envisages their application as temporary fixation devices for biomedical purposes.

In light of this scenario, the present work aims to fill this gap by investigating the effect of anodization on the corrosion fatigue of the AZ31B alloy in phosphate buffered solution. Anodization was carried out in an environmentally friendly sodium silicatebased electrolyte. The fatigue and corrosion fatigue tests were conducted in the tensiontension mode.

#### **2. Materials and Methods**

#### *2.1. Material and Specimen Preparation*

The material was a hot rolled AZ31B magnesium alloy sheet (composition in wt.% Al 2.54%, Zn 1.08%, Mn 0.38% and Mg balance), supplied by Xi'an Yuechen Metal Products Co. Ltd. (Xi'an, China). The tensile and fatigue samples were machined by laser-cutting along the rolling direction following ASTM E8M-16a [26] and ASTM E466-15 [27], respectively. Figure 1 shows the shape and size of the specimens employed for tensile and fatigue tests. The dog-bone shaped specimens were sequentially ground using waterproof silicon carbide paper (from #220 to #4000), and polished using diamond paste slurry (diameter 3 μm and 1 μm). Right after, they were cleaned using deionized water and ethanol, being dried in a warm air stream provided by a conventional heat gun.

**Figure 1.** Shape and dimensions (in mm) of (**a**) tensile and (**b**) fatigue specimens.

For the corrosion test, the AZ31B magnesium alloy was cut using a cut-off saw in a square section with area of 100 mm<sup>2</sup> and thickness of 3.5 mm. The AZ31B alloy pieces were connected to a copper wire at the rear side using a conductive colloidal silver paste, being subsequently embedded in epoxy resin. Next, the surfaces were ground using waterproof silicon carbide paper (from #220 to #2400 grit size), polished using diamond paste (diameter 1 μm), washed using deionized water, and dried in a warm air stream provided by a conventional heat gun.
