Electrolytic corrosion occurs in plumbing systems that contain dissimilar metals. The process of corrosion is rapid when the water is present in the system, since it acts as a medium between the two metals. The three components: water and two metals, form a "battery", where electric current carries ions from one metal to another, causing it to dissolve. The washer inside the union prevents such current from flowing, thus preventing corrosion itself.
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