If dissolved oxygen increases, the corrosion rate changes as well as the effect of increasing pH and alkalinity on corrosion rate.

• A. DO increases; pH/alk increases
• B. DO decreases; pH/alk decreases
• C. DO increases; pH/alk increases
• D. DO increases; pH/alk decreases

Answer: (D)

Dissolved oxygen acts as the main oxidant in the corrosion of metals in water. When oxygen levels rise, the cathodic reaction speeds up, pulling electrons from the metal faster and increasing the rate at which the metal dissolves. So corrosion rate goes up with more DO.

Increasing pH and alkalinity, on the other hand, buffer the water and promote the formation of protective films on the metal surface, such as hydroxide or oxide layers. These films slow down ongoing metal dissolution and overall corrosion. So higher pH or alkalinity tends to decrease corrosion.

Therefore, the scenario described is that corrosion rate increases as DO increases, and corrosion rate decreases as pH/alkalinity increase. The best choice reflects DO increasing while pH/alkalinity decreases.