The gases frequently found in water that encourage corrosion are

The presence of oxygen and carbon dioxide in water can significantly contribute to corrosion processes. Oxygen is a vital factor in oxidative corrosion, particularly in metals, as it reacts with metal surfaces leading to oxidation. When iron, for example, is exposed to oxygen in an aqueous environment, it can form iron oxides, commonly known as rust.

Carbon dioxide, on the other hand, can dissolve in water to form carbonic acid, which can lower the pH level of the water. This acidic environment can enhance the corrosiveness of the water, particularly affecting metal pipes and structures by promoting further oxidation reactions. The combination of both oxygen and carbon dioxide thus creates conditions that are favorable for corrosion, especially in environments where metals are present.

The other gases mentioned do not engage in corrosion processes to the same extent. Methane, while it can be present in water, is generally not corrosive. Chlorine is a strong oxidizer but is not typically found at levels in water that would encourage corrosion as readily as oxygen and carbon dioxide. Hydrogen sulfide, while corrosive, depends on specific conditions and generally would not be considered a primary contributor in the same way. Hence, oxygen and carbon dioxide are correctly identified as the key gases that promote corrosion in water.