All water, whether it's distilled or tap or RO will promote corrosion on some level. The purer the water (distilled/rain) the lower the pH will be and the less buffering capability it has. Conversely, the more buffering ability a water has (tap water) due to higher mineral content, the less corrosive it will be because the minerals act like a buffer. So while using tap water can introduce impurities like calcium and lime into the system that causes scale, it actually is the least corrosive of all due to it's higher/more neutral pH and greater ability to buffer.
You can alter a waters corrosion properties with additives. Coolant is an additive and most coolant mfgs add buffering agents and corrosion inhibitors to their product to improve a water+coolant solutions ability to resist corrosion and scale buildup. You can further improve upon these properties by adding aftermarket products like Lubegards Kool It, which has additional buffering agents and pH neutralizers. I personally have used Lubegard products in both my power steering and transmission systems with stellar results. In my opinion, Lubegard makes some real deal stuff. That said, it's totally NOT necessary to add an aftermarket corrosion inhibitor. Most of the additives in coolant will keep your system safe whether you're running RO (deionized water) or Distilled. In practice, you could run straight tap and probably do just as good (or better) as far as corrosion is concerned, so long as you were changing out the coolant regularly. The problem your going to run into with tap is the mineral content. Depending on where you live it can be quite high and even with the inhibitors, scale buildup can be a problem. This is why coolant mfgs recommend using distilled, it's pure H2O and nothing more - so the coolant mfg knows EXACTLY how their additive pkg works (best) on water that starts out at a ZERO mineral content. The additive pkg in the coolant (should) then balances the pH, add buffering agents to protect against oxidation and inhibitors to limit scale buildup.
Fwiw, when aluminum oxidizes it actually forms a protective layer that inhibits further oxidation. And unlike rust, which flakes off, the oxidative layer formed on aluminum is quite resilient and not so easily removed.