Why would a metal corrode more in a lower concentration of nitric acid compared to a higher concentration sulfuric acid? Isn't sulfuric acid suppose to be a stronger acid compared to nitric acid?
Sulfuric acid is not as strong of an oxidizer. Nitric acid, particularly when hot, is very strongly oxidizing.
Note that both acids are "strong acids" in that they completely dissociate (at least one proton in the case of sulfuric acid) in water. However, this does not completely determine a compound's ability to oxidize metals. Oxygen and chlorine for example will oxidize many metals very well, and they certainly are not strong acids!