Question
.....I understand that acidic RO water is not harmful to the human body. However, what about RO water in metal piping instead of the human body? People have found that RO water corroded their metal piping. What would be your recommendation to prevent such corrosion?

Answer
.....Reverse Osmosis-treated water is aggressive to metallic pipe or to substances that can ionize and dissolve in water—not because of low pH (acidity), but because it is highly pure and has few dissolved substances in it.
.....Water is called the universal solvent; it always "wants" to have substances dissolved in it. The less it has, the more aggressive it becomes in attacking things that can dissolve. This doesn’t hurt the human body, of course, because our physiology quickly obtains homeostasis using saliva, stomach fluids, etc. to equilibrate all bodily fluids.
.....However, with plumbing materials, inert plastic piping—not metal piping—is recommended for contacting all very high purity waters. That being said, you should know that low-pressure, household RO treatment removes only about 90%+ of dissolved constituents from drinking water supplies (most natural water supplies start with multiple hundreds of milligrams per liter of total dissolved solids).
.....Typically then, household point-of-use, RO-treated water will still have more than 10 mg/L of TDS, not overly aggressive water— especially compared to high-purity industrial and even distilled water that can usually have less than 1mg/L of total dissolved solids.
.....Compare this 10 mg/L of TDS with some municipal waters, such as those of Portland, Oregon; Seattle, Washington; Lake Tahoe, California—and even parts of New York City and San Francisco—which deliver their regular tap water well below 50 mg/L of TDS.
 
Joseph F. Harrison, P.E., CWS-VI