Very pure water is corrosive. I built a CVD lab that had aluminum deposition chambers plumbed into a DI water cooling system. The water was fed from a purifier

Very pure water is corrosive. I built a CVD lab that had aluminum deposition chambers plumbed into a DI water cooling system. The water was fed from a purifier that provided 18MΩ-cm water. The water corroded the aluminum chamber cooling passages, leaving aluminum hydroxide behind. This conversion of metallic aluminum to a gelatinous, clogging hydroxide was what alerted us to the problem. Feed pressure (regulated) was fine, but flow rates fell and chambers heated up. We had to wire brush all cooling channels, a total pain. We ran on tap water for a couple of weeks while we researched the issue, and had very little corrosion but had other issues (mineral deposits). Finally, we reverted to 18MΩ water and added ~10% volume of ethylene glycol (antifreeze, basically). Once we "degraded" the water purity with glycol, our corrosion problems stopped. It was very counterintuitive. Here's an industry source that has a little bit to say about the corrosive properties of pure water: http://www.lytron.com/Tools-and-Technical-Reference/Application-Notes/Deionized-Water http://www.lytron.com/Tools-and-Technical-Reference/Applicat.... Given the industrial importance of the problem, I'm sure there are in-depth chemistry explanations. Haven't pursued them.