When present in water, which of the following will increase corrosion rates?

When dissolved oxygen is present in water, it significantly increases corrosion rates. This is primarily because oxygen plays a crucial role in the electrochemical processes that lead to corrosion. In particular, it participates in the cathodic reactions where it can react with either metal ions or metal surfaces, leading to the formation of hydroxide ions.

The presence of dissolved oxygen can exacerbate the corrosion of metals, especially in environments where other corrosive agents are also present. Oxygen facilitates the oxidation of metal surfaces, particularly in aqueous environments, accelerating the corrosion process. This phenomenon is particularly noticeable in systems like pipelines or storage tanks, where the interaction between metals and oxygen in water can cause significant damage over time.

In contrast, while other substances like calcium, chlorine, and sulfate can also influence corrosion, they do so in different ways or are less impactful than dissolved oxygen. Calcium can lead to scale formation, potentially protecting surfaces from corrosion, while chlorine is more associated with pitting corrosion under specific conditions. Sulfates can contribute to corrosion as well but generally do not have the immediate and broad effect on corrosion rates that dissolved oxygen does. Therefore, the presence of dissolved oxygen stands out as the primary factor that leads to an increase in corrosion rates in water.