What causes hydrogen evolution in a water heater tank?

Hydrogen evolution in a water heater tank is primarily caused by highly acidic conductive water. When the water in the tank has a low pH, or is acidic, it can contribute to the corrosion of the metal surfaces within the tank. This corrosion process often leads to the electrolysis of water, producing hydrogen gas. The presence of conductive ions in the water enhances the process of corrosion and electrolysis, further increasing the rate at which hydrogen gas is evolved.

In various contexts, factors such as high temperature or water pressure could affect a water heater's operation or efficiency, but they do not directly lead to hydrogen gas evolution in the same manner. Furthermore, insufficient ventilation pertains to the safe operation of combustion appliances rather than the chemical processes occurring within the water heater tank itself. Thus, highly acidic conductive water stands out as the primary catalyst for hydrogen evolution in this scenario.