Corrosion isn't linked only to pH! It's a subtle process, partially understood, which depends essentially on the properties (adherent, permeable...) of the intermediate products.
For instance, chloride ions are devastating to most stainless steel compositions, even if not under acidic conditions, like in sea water.
Don't forget neither to note the condition of the materials you test. Typically a weld seam, due to its heat treatment history, has a very different resistance to corrosion than the plain alloy.