**3. Results**

As mentioned before, the weight changes for each sample were calculated according to Equation (1) and the obtained data are plotted in Figures 1 and 2. Reference Zn samples showed a gradual weight loss for both as-cast and annealed conditions as expected. It can be observed that for several as-cast samples, a weight gain rather than a weight loss was recorded. The annealed samples showed the weight loss for all conditions as expected. MZ + 3.0Sn showed the best results at even 40% lower values compared to MZ + 0.0Sn.

**Figure 1.** Weight change after corrosion measured on as-cast samples.

**Figure 2.** Weight change after corrosion measured on annealed samples.

Since the weight gain instead of the weight loss was recorded for several as-cast conditions, it was decided to prepare longitudinal cuts of the samples and investigate potential reasons of this phenomena. The intergranular corrosion was present in most samples to a significant extent. Most phases present in the interdendritic spaces were corroded. Such corrosion products could not be cleaned by CrO3 acid solution [24]. These corrosion products, anchored among the still mainly intact η(Zn) dendrites, were increasing the total weight of the samples even after the cleaning process (Figure 3a). Their presence is visualised by chemical element distribution maps in Figure 3b.

Backscattered-electron scanning electron microscopy (BSEM) images of the longitudinal sections for representative as-cast samples with 0.0, 0.5 and 3.0 wt.% of Sn after 1000 h of NSST are given in Figure 4. Corresponding quantitative analysis results of the intergranular corrosion penetration depth are summarized in Figure 5. The same is available for the annealed samples in Figures 6 and 7.

**Figure 3.** Anchoring effect of η(Zn) dendrites with corroded interdendritic spaces (MZ + 2.0Sn, as-cast): (**a**) overview BSEM image; (**b**) chemical element distribution maps of Zn, Sn, O, Mg, Al and Cl.

**Figure 4.** BSEM images indicating the extend of IG corrosion observed for the as-cast samples after 1000 h of NSST: (**a**) MZ + 0.0Sn (**b**) MZ + 0.5Sn (**c**) MZ + 3.0Sn.

**Figure 5.** Depth of IG corrosion—as-cast samples.

**Figure 6.** BSEM images indicating the extent of IG corrosion observed for the annealed samples: (**a**) MZ + 0.0Sn (**b**) MZ + 0.5Sn (**c**) MZ + 3.0Sn.

**Figure 7.** Depth of IG corrosion—annealed samples.

SEM investigation of as-cast MZ + 0.0Sn samples after 250 h of NSST revealed a significant portion of the microstructure being affected by the intergranular corrosion reaching as deep as ~50 μm (Figure 5). This effect is even more pronounced on the as-cast samples with 1–3 wt.% of Sn. IG corrosion can be formed as deep as ~150 μm for the as-cast MZ + 3.0Sn samples (Figure 5). This effect is further emphasised during longer exposures in the salt spray chamber. The IG corrosion can reach depths of over 370 μm on average for the as-cast MZ + 2.0Sn and MZ + 3.0Sn samples exposed for 1000 h (Figures 4c and 5). Complex ZnAlMg interdendritic areas were affected preferentially by the IG corrosion (Figure 8).

**Figure 8.** MZ + 2.0Sn as-cast microstructure after 1000 h in NSST affected by IG corrosion: (**a**) overview, (**b**) detail.

The depth of IG corrosion is significantly lower for the annealed samples with maximums reaching only about 70 μm even after 1000 h of NSST. For 250 and 500 h, all alloys behaved rather similar with IG corrosion depths of 10 and 22 μm, respectively. MZ + 1.0Sn and MZ + 2.0Sn seem to be more susceptible to the IG corrosion when comparing the samples after the full 1000 h test. On the contrary, annealed MZ + 3.0Sn samples showed values comparable even to MZ + 0.0Sn, or MZ + 0.5Sn reaching a maximum depth of about 45 μm.

The examples of areas affected by the intergranular corrosion are given in Figures 8 and 9 for the as-cast and annealed samples, respectively. The EDS chemical analysis of the microstructure in Table 3 confirms the intergranular corrosion attack. Figure 8a shows the η(Zn) dendritic microstructure affected by the corrosion along the interdendritic areas. η(Zn) primary dendrites also showed the signs of corrosion in the form of fine cracks. These can be attributed to the presence of fine, sub-micron Al-rich particles observed within the η(Zn) primary dendrites. In a more detailed image (Figure 8b) it can be seen that α(Al) and MgxZny particles were corroded. Mg2Sn particles were subject to the process of dealloying [28–31], leaving thus pure metallic Sn particles behind.

**Figure 9.** MZ + 2.0Sn annealed microstructure after 1000 h in NSST affected by IG corrosion: (**a**) overview, (**b**) detail.


**Table 3.** EDS chemical composition of selected sites (at.%).

As reported in the first part of this research [22], the basic dendritic character of the microstructure was still rather well maintained for the MZ + 0.0Sn and MZ + 0.5Sn alloys even after annealing. Hence, the IG corrosion is observed to propagate preferably along the interdendritic areas. For the annealed samples with 1 and more wt.% of Sn the microstructure is more spheroidized. The individual intermetallic phases were coalesced into coarse, discrete particles, while η(Zn) dendrites were reshaped and new grains are formed within the microstructure. The boundaries of these grains contained a significant portion of intermetallic phase particles. As such, they were more susceptible to the IG corrosion. The propagation of the IG corrosion is documented in Figure 9a and the grain boundaries decorated by intermetallic particles are shown in closer detail in Figure 9b.

The XRD analysis was performed on all samples after NSST. As described, loose corrosion products were gathered and investigated. The XRD was used to determine the phase composition of the corrosion products formed on the samples during NSST. The XRD patterns for all corrosion products retrieved from the as-cast and annealed samples are summarized in Figures 10 and 11, respectively. The presence of the identified phases was also confirmed using the Rietveld method (Table 4). Despite the differences between the microstructure of the as-cast and annealed samples, their corrosion products showed an identical qualitative phase composition. The semi-quantitative results from these calculations indicate that the majority of the corrosion products were formed by a hydrozincite for all samples. About 20 vol.% of simonkolleite was measured for all pure

Zn samples (Figures 10a and 11a). The corrosion products of MZ-based samples contained only about 10 vol.% of simonkolleite on average (Figures 10b–d and 11b–d).

ZnO was identified solely in the corrosion products of the pure as-cast Zn sample (Figure 10a), representing only about 2 vol.%.

NaCl was identified in randomly varying amounts in the corrosion products as a remainder of the corrosion environment.

Next to hydrozincite and simonkolleite, the sources indicated that other phases might also be formed [17–20,32–39]. Therefore, the corroded surfaces of the bulk metallic samples were investigated after the loose corrosion products were removed. The measurement in grazing incident diffraction mode with 0.5◦ incident angle was chosen to limit the signal from the substrate (mainly Zn) as much as possible. Additionally, to previously identified phases, zincite (ZnO) and aluminium (II) oxide (AlO) were identified as present directly attached to the sample surface. An example of such a pattern is given in Figure 12 for the MZ + 3.0Sn annealed sample surface after 1000 h of NSST. However, only about 2 and 5 vol.% of ZnO and AlO, respectively, were identified using the Rietveld method.

**Table 4.** Phases identified during XRD analysis.


**Figure 10.** XRD patterns of corrosion product powders retrieved from the as-cast samples after 1000 h of NSST: (**a**) pure Zn (**b**) MZ + 0.0Sn (**c**) MZ + 0.5Sn (**d**) MZ + 3.0Sn.

**Figure 11.** XRD patterns of corrosion product powders retrieved from the annealed samples after 1000 h of NSST. (**a**) pure Zn (**b**) MZ + 0.0Sn (**c**) MZ + 0.5Sn (**d**) MZ + 3.0Sn.

**Figure 12.** XRD pattern on MZ + 3.0Sn annealed sample surface after 1000 h NSST, measured in grazing incident geometry.
