Finer the grain size better the mechanical properties and lesser the corrosion resistance. Ist normal that when we increase the mechanical properties the corrosion resistance decreases.
The reason for this is, finer the grains the eefctive grain boundary is high and the risk of corrosion is high. The grain boundries act as weak or criticak section sfor various types of corrosions mainly intergranular type. The grain boundries act as preferential anodic Areas (micro galvanic corrosion could occur).