A lot of city water companies use copper sulplate to control algae in water impoundments in warmer months. I have noticed that a small amount of copper in my pickling acid that I use to clean rusty parts and old tools will actually leave a visible plating without electric current. I have read warnings about using city water with trace copper sulphate in the hot bluing process, and that is essentially a much accelerated formation of black oxides. So if it causes spotting or other problems in the fast process, it's likely to affect the slow rust process. I have well water but use rain water. Midway through a long rain, after most dust and bird poop is rinsed off the roof, divert a downspout into a large container.
