Which of the following is NOT commonly used as an additive for corrosion control in storage tanks?

Aluminum sulfate (alum) is primarily used as a coagulant in water treatment processes to remove suspended solids and clarify water. It is effective in particle aggregation and sedimentation but does not function as a corrosion inhibitor in storage tanks.

In contrast, the other additives listed—orthophosphate, polyphosphate, and calcium carbonate—are recognized for their roles in corrosion control. Orthophosphate helps form a protective film on the metal surfaces, thereby reducing corrosion rates, while polyphosphates can inhibit corrosion through similar mechanisms, as well as by sequestering metals that might otherwise promote corrosion. Calcium carbonate can also contribute to protective scaling on the interior surfaces of tanks, acting as a barrier against corrosive elements.

Understanding the specific purposes of these additives is key in determining their role in water systems and the prevention of corrosion in storage facilities.