The oxidation stops and that single layer is 'permanent' ONLY IF the insulating layer of oxide isn't disturbed.
I just had to purchase a new wheel for my bicycle because the aluminum rim was getting eaten through by the oxidation.
Keep in mind I live in Canada and ride straight through the winters and all kinds of inclement weather. My suspicion is that the water was seeping through the spoke holes and therefore continually washing away the insulating layer of aluminum oxide and upon inspection of the process it seemed to me that somehow the salt was assisting to accelerate the process.
I bought the bike brand new in 2007 and visually confirmed the severe deterioration of the aluminum myself a couple weeks ago when the wheel was replaced and it was replaced for that reason: it was so bad it became a safety concern.
Up until then I - like you - assumed such a thing could not happen due to the nature of aluminum oxidation. But you must remember aluminum oxidizes EVEN MORE READILY than steel it's just that the insulating layer USUALLY protects it. You get something constantly washing away that insulating layer and you're getting a disintegrating piece of aluminum!