wiednhft

What gas would cause ABS to dissolve? I investigated a simple home water toilet shutoff valve that failed, causing significant water damage. When I inspected the 14 year old valve I found that the plastic shaft (that you turn to shut off the water to the toilet) had been entirely dissolved away, allowing significant water to be released into the room. The same type of valve in another bathroom was also partially dissolved. But similar valves for the sinks in the house showed no such degradation. The difference between the sink valves and toilet valves? The toilet valves were chrome plated brass. The sink valves were brass, unplated. The plastic exposed to the water (inside the valve) was not attacked, only the plastic outside the valve body that was exposed to air. There was no evidence of significant water leaking out of the valve onto the plastic shaft. The (Gilbert, AZ) water system contains the following: Chloride 170 ppm, CAC03 17.5 gr/gal, Iron 0.15 ppm, Langlier index -2.21, pH 8.5, Sodium 370 ppm, sulfate 110 ppm, Alkalinity 170 ppm. I speculate that impurities in the water are reacting with the chrome, releasing a gas that attacks the plastic. Any ideas as to what reaction that is causing this?