From my untrained eye....that does not look like corrosion. It appears to be a manufacturing defect....called occlusions or porosity formed when pouring the billet.
I've not looked at enough of these to know if this is normal or allowable. It could very well be common and not an issue. A prop...
If you want a nice plane....pay the money and buy it. You will not save money buy doing it yourself. Even as an A&P I/A....I won't make much doing it myself.
scotchbrite the spot till it cleans up....then hit it with zinc chromate. That's very common in some areas of the US....especially Florida.
Note the pulled rivets....that should be a clue.