# Corrosion Susceptibility and Microhardness of Al-Ni Alloys with Different Grain Structures
## 4 Conclusions  
From the results and discussion in the previous sections, the main conclusions of this investigation on the directional solidification of Al-Ni alloys are as follows.  
1. The CET occurs in a zone where columnar and equiaxed grains coexist, and the position of the zone in the sample decreases as the Ni content in the alloy increases.
2. The values of critical gradients (at the CET) were determined, and the values are 1.3 to 2.9 \({}^{\circ}\)C/cm.
3. The eutectic region has greater microhardness than the \(\alpha\)-phase, reasonably associated with the higher Ni-content.
4. The microhardness values are smaller in the areas furthest from the base because, in further areas, the heat extraction rate is lower, and the grains are less refined.
5. The highest cooling rate generates refinement in the secondary dendritic arm spacing and lamellar spacing, obtaining greater microstructural refinement in the zones of the samples close to the base.
6. A small increase in the microhardness was observed as a function of the Ni content. For each concentration, as the cooling rate increases, the values of microhardness increase linearly.
7. The Al-5.7 wt. %Ni alloy, which corresponds to the eutectic composition, presents the noblest pitting potential values, followed by the Al-8 wt. %Ni alloy, with a hypereutectic composition. Hypoetectic alloys have the least noble potential values that are very close to each other.  
Figure 19: Polarization resistance, R\({}_{\rm p}\), versus microhardness, HV, for both types of grain structures (columnar and equiaxed) of Al-Ni alloys. (**a**) Al-1 wt. %Ni and Al-3 wt. %Ni. (**b**) Al-5.7 wt. %Ni and Al-8 wt. %Ni.  
For the same alloy composition, the zone of columnar grains presents nobler pitting potential values than the zone of equiaxed grains. However, these differences between values are less than 5 mV, so they are not considered significant differences.  
When the Ni content increases, the resistance to polarization decreases for samples with equiaxed grain structure throughout the range of compositions studied. Furthermore, the equiaxed grain structure presents higher resistance to polarization values than the columnar grain zone for alloys with a composition equal to or lower than the eutectic composition. For the hypereuetectic alloy, Al-8 wt. % Ni, the trend is modified, with the area of columnar grains being the one with the highest resistance to polarization.  
With respect to the correlation between resistance to polarization and microhardness for Al-Ni alloys studied, when microhardness increases, the resistance to polarization decreases for hypoeutectic (Al-1 wt. %Ni and Al-3 wt. %Ni) and eutectic (Al-5.7 wt. %Ni) alloys. The trend for the hypereuetectic (Al-8 wt. %Ni) alloy changes. The different behavior can be justified because the Al\({}_{3}\)Ni intermetallic could play a dual role in the corrosion of an Al-Ni alloy. It can act as a barrier against corrosion or as a galvanic cathode that accelerates the corrosion process.  
Conceptualization, A.E.A.; methodology, A.E.A.; formal analysis, A.E.A.; investigation, A.S.R., E.R.I, N.S.Z. and C.M.M.; resources, A.E.A.; data curation, A.E.A. and C.M.M.; writing--original draft preparation, A.E.A., A.S.R. and E.R.I; writing--review and editing, A.E.A.; visualization, A.E.A.; supervision, A.E.A.; project administration, A.E.A. and C.M.M.; funding acquisition, A.E.A. All authors have read and agreed to the published version of the manuscript.  
Consejo Nacional de Investigaciones Cientificas y Tecnicas (CONICET): RES.2019-574-APN-DIR#CONICET; Agencia Nacional de Promocion Cientifica y Tecnologica: PICT-2017-0079.  
The data used in this study will be provided on request to the corresponding author.  
All authors gratefully thank the National Scientific CONICET of Argentina (Consejo Nacional de Investigaciones Cientificas y Tecnicas (CONICET), Argentina). We also appreciate the support provided by the Universidad Nacional de Misiones (UNaM).  
The authors declare no conflicts of interest.