One of the reasons aluminum oxide is protective is because it is an adherent oxide (the other reason being that aluminum oxide is such a stable compound). It is adherent because when it is converted to oxide, the volume doesn't change. Other oxides, such as the common hydrated iron oxide, grows. This is why you get rust scale on automotive iron that spalls off and isn't considered protective.

Strider, I still don't see where you disagree with me. It is commonly held that absolutely pure water will strip ions from unpassivated metal surfaces. Just the fact that you control the pH slightly acidic tells me you don't run pure water in your system. The water in itself does not passivate the metal. I believe you missed a step in your explanation. In fact, that is how one of my clients checks his product for proper passivation. After the passivation process and rinse, he rinses it in ultra high purity water (I believe he said one-millionth ohm water) and checks that the conductivity doesn't significantly increase.

__________________
95 E320 Cabriolet, 169K