*3.2. Tensile Properties*

Figure 4 shows the stress–strain curves of the AZ31B magnesium alloy in the as received and anodized conditions. The average values of yield strength, ultimate strength,and elongation at break of the alloy in the as received and anodized conditions are displayed in Table 2. It is clear that the ultimate tensile strength and yield strength decreased after anodization. According to the literature [34], this effect is due to the presence of pores and defects in the oxide layer that act as crack nucleation sites, reducing the mechanical strength of the anodized substrate.

**Figure 4.** Stress-strain curves in uniaxial tensile tests at room temperature.

**Table 2.** Stress-strain curves in uniaxial tension tests at room temperature for anodized and as received specimens.


#### *3.3. Fatigue and Corrosion Fatigue Behavior*

#### 3.3.1. Influence of the Corrosive Environment

Figure 5 presents the S-*N* curves for the polished AZ31B alloy exposed to the air and to the PBS solution. Comparatively, a decrease on the maximum applied stress is noticed when the tests were performed in the PBS solution. Considering 10<sup>6</sup> cycles to failure, according to ASTM F1801, the fatigue limit was reduced from 142.5 MPa when the tests were conducted in the air, to 137.5 MPa when the tests were conducted in PBS. However, at an intermediate number of cycles to failure, the influence of the testing environment is even more evident. It can be seen that an applied stress of 140 MPa led to the fatigue life of approximately 69,000 cycles, while the fatigue life was extended to 10<sup>6</sup> cycles at a compatible applied stress when the tests were conducted in the air. These results indicate that there is an abrupt change in the fatigue life for a relatively low variation of the applied stress in the corrosion-fatigue condition. A similar trend was observed by Nan et al. during fatigue tests of the AZ31 alloy [35]. A slight loading variation was sufficient to drastically reduce the fatigue life of this material. In another work, Nan et al. have also investigated the influence of the 3.5 wt.% NaCl on the corrosion fatigue behavior of the AZ31 alloy [36]. A remarkable decrease in fatigue resistance was noticed when the tests were performed in the chloride solution. Authors have pointed out the combination of cyclic stress and pit growth as responsible for this behavior [37,38].

**Figure 5.** S-*N* curves for the polished AZ31B alloy specimens in air and in PBS solution at room temperature.

The S-*N* curves for the anodized specimens exposed to air and PBS solution are shown in Figure 6. Corrosion was less detrimental for the surface treated samples than the polished ones. As displayed in Figure 5, the fatigue life was little affected by the corrosive environment for stress levels lower than 130 MPa.

In order to give a more quantitative interpretation on the influence of corrosion on the fatigue resistance of the AZ31B alloy, we employed Equations (7) and (8) to calculate the reduction rate ( *R <sup>σ</sup>N*) of the maximum fatigue strength at 10<sup>6</sup> cycles, as proposed by He et al. [35]. Based on this procedure, fitting equations shown in Table 3 were obtained by a logarithmic transformation of the experimental data points shown in Figures 4 and 5.

 in air

*=*

*k*

*+*

*bx*

$$\text{Read AZ31B alloy specifies in air and in PBS solution at room temperature.}$$

$$y = k + b\mathbf{x} \tag{7}$$

$$R\sigma\_N = \left(\frac{\sigma\_{air} - \sigma\_{PBS}}{\sigma\_{air}}\right) \times 100 \tag{8}$$

PBS solution at room

**Table 3.** Fitting equations for each experimental condition.

**Figure 6.** S-*N* curves for the anodized AZ31B


In Equation (2), *y* and *x* are log (<sup>σ</sup>*max*) and log *N*, respectively. The maximum applied stress is represented by *σmax*, while *N* is the number of cycles to failure at a certain stress level. The parameters *k* and *b* are coefficients. In Equation (3), *RσN* indicates the reduction rate of the maximum applied stress at specific fatigue cycles in air and in PBS solution. The maximum applied stress that specimens can resist at specific fatigue cycles are denoted by *σair* and *σPBS*. Table 4 displays the values of these parameters, along with the corresponding *RσN* at 10<sup>6</sup> cycles.

**Table 4.** Reduction rate of the maximum applied stress for the AZ31B alloy in the polished and anodized conditions.


As seen in Table 4, the reduction rate of the polished AZ31B alloy was 5.6% when immersed in the PBS solution, while the anodized condition, in turn, showed an increment. Therefore, the effect of the PBS solution was less harmful to the fatigue strength of the anodized samples. This behavior is probably associated with the protection ability promoted

by the anodization treatment. Generally, the main mechanism related to the corrosion fatigue failure of magnesium and its alloys is pit nucleation and growth [19,32,39]. In this case, the presence of the oxide layer possibly provided a higher local dissolution resistance for the AZ31B alloy.

#### 3.3.2. Influence of Anodization

Figures 7 and 8 show the S-*N* curves for the polished and anodized samples exposed to air and PBS solution, respectively. As seen in these figures, there was a remarkable reduction of the fatigue resistance for the anodized specimens compared to the polished condition regardless of the environment. In both scenarios, the reduction of the fatigue limit at 10<sup>6</sup> cycles was approximately 60% for the anodized samples.

**Figure 7.** S-*N* curves for the polished and anodized AZ31B alloy specimens in air.

The reduction of the fatigue strength for the anodized specimens was much more intense at low stress levels, independently of the testing environment. This behavior has been associated to a significant influence of the crack nucleation than the crack propagation, which causes the premature failure of the material [40]. In other words, the anodic layer facilitates fatigue crack nucleation. Khan et al. [41] observed a distinguished effect of the crack nucleation of anodized magnesium alloy when subjected to fatigue tests. The authors concluded that the crack nucleation step, which is defined by the material surface conditions in a crucial manner, exerts more influence than the crack growth and propagation stages. As the anodized layer greatly affects the surface characteristics, this means that fatigue failure would easily happen when a certain number of cycles are able to nucleate the crack. Besides that, other adverse effects concerned with the fatigue behavior of anodized magnesium alloys have been reported in the current literature. Yerochin et al. [42] mentioned that the plasma micro-discharges which occur during the oxidation process led to strain distortions of the metal subsurface layers. Some other works have pointed out the presence of a microcrack network [39] and disordered porous structure [24,32] of the coating layer. Furthermore, the compactness and uniformity of the oxide layer, its adhesion to the substrate, and relevant aspects of roughness on the interface of coating and the material may also affect the fatigue properties of anodized alloys [43,44].

The results obtained in the present work highlighted that the presence of the oxide layer caused a hostile effect to the AZ31B magnesium alloy regardless of the environment conditions.

**Figure 8.** S-*N* curves for the polished and anodized AZ31B alloy specimens in the PBS solution at room temperature.
