# Effect of Ti-B Grain Refiners on Wear and Corrosion of the A332 Alloy with Sr Modification
## 4 Discussion
### Corrosion Analysis  
For the A332 alloy tested in this case, the phases may cause changes in isolated characteristics, resulting in global modifications. Such is the case of phase Al5 FeSi, which can act as a cathodic function for corrosion. As mentioned by [PERSON] et al. [46], in aluminum alloys, the pitting and initial corrosion are generally related to intermetallic phases. A majority of them have Fe (Al3 Fe, AlFeSi) present in the alloy, which is more cathodic to matrix primary aluminum. Therefore, in these cases the adjacent matrix primary aluminum dissolves around them. The pit propagation in proximity of Fe-phase is one of the corrosion mechanisms observed [35]. According to [PERSON] and [PERSON], [18], the grain refining process itself may disrupt local chemical microstructure through second phases and thus the chemical and ensuing electrochemical heterogeneity of the material.  
The Al5 FeSi phase, at some point meets the grain boundaries, as shown in Figure 16, supporting the intergranular corrosion in direction to primary aluminum. This mechanism is as abundantly found in the literature. [PERSON] et al. [47] analyzed this effect in Al-2.5% Mg cast alloy, which was modified by adding 0.5 Ti and 0.1 B wt.%. Their results showed intergranular corrosion that started in tiny pits and continued to expand across the grain boundaries. The refined alloy exhibited improved corrosion resistance owing to the refined and minimized grain structure.  
According to the microstructural analysis mentioned above, the eutectic phase present more lamellar in the alloys with grain refiner due to the effect of the Sr. Therefore, in comparison with acicular-shaped formations of the as-cast conditions, modified alloys present lamellar formations which has more area and more galvanic couplings may occur, due to the fact that it acts as a local cathode to primary aluminum, by dissolving it [48]. Similar results were observed by [PERSON] et al. [49] when the microstructures and corrosion resistances of hypoeutectic Al-6.5 Si-0.45 Mg casting alloys with the addition of both Sc and Zr were researched. Corrosion tests were made in 3.5 wt.% NaCl with an increase in exposure time. The author comments that the tested alloys with fine eutectic Si exhibit poorer corrosion resistance than those with coarse eutectic and dissolving primary aluminum phase. They attribute these findings to the galvanic couplings which can be formed between the eutectic (as a cathode) and primary aluminum (as an anode). More galvanic couplings can be formed in the test alloys with a fine eutectic.  
There is no consensus about the corrosion rate of alloys with and without a grain refiner. However, [PERSON] et al. [50] achieved a dramatically improved corrosion resistance with grain refiners. They evaluated the corrosion behavior of A356 aluminum alloy refined with Al5 Ti1B and Al3B and modified with Sr. Electrochemical measurements were realized in 3.5 wt.% NaCl solution. The authors show a clear improvement in the corrosion resistance compared to the unmodified A356 alloy in the sequence as follows: 170 ppm Sr-modified alloy, B grain-refined, and Ti-B grain-refined A356. The main reason is associated with microstructural formation, with different situations for B and Ti. The authors explain that for B grain-refined alloy, the whole surface of the corroded region seems to be dissolved; in contrast, in Ti grain-refined castings, local dendrites remain present with eutectic regions that were dissolved. Therefore, based on this, higher amounts of B in the grain refiner Al5 Ti2B probably mean significant corrosion to as-cast and Al5 Ti1B grain refiner. Overall, based on the revised literature, and the review of [PERSON] et al. [9], there is a lack of consensus on how grain refinement affects the corrosion behavior of materials. Both the improvement and deterioration of corrosion resistance have been observed with grain refinement.