It could be acid dust deposition. What is commonly called acid rain is actually acid dust that is dragged out of the atmoshere during rainfall. It can also collect on exposed surfaces, especially if those surfaces are electostatically attractive to charged dust particles.
The acid dust will not do any harm as long as the surface is dry. However, if the upper surface is pointed toward the open sky, it will cool off at night due to radiative heat transfer. Then, the surface will condense water vapor, especially in climates like Maine. Now you have a thin layer of condensed water that turns very acidic due to the acid dust that has collected on the upper surfaces of your AS.
After years of this exposure, you will get the type of corrosion you have.
Of course this is just my theory and you should consider other explanations.