Which chemical is best for corrosion inhibition in low alkalinity and calcium concentration water?

Polyphosphates are particularly effective for corrosion inhibition in water with low alkalinity and calcium concentrations. In such water conditions, the risk of corrosion increases because there are typically fewer minerals to provide natural buffering and protective scaling on the pipes and infrastructure.

Polyphosphates work by forming a protective film on the surfaces of metal pipes, which helps to prevent corrosion. They do this by adsorbing onto the surface of the metal, reducing the rate of corrosion reactions occurring there. Additionally, polyphosphates can help in complexing metal ions, which can further reduce the corrosiveness of the water by minimizing the concentration of free ions that can contribute to corrosion processes.

Other options, while they have their own uses, do not provide the same level of effectiveness for corrosion inhibition in the specific context of low alkalinity and calcium levels. Sodium bicarbonate can raise alkalinity, but it is not primarily a corrosion inhibitor. Calcium sulfate could increase calcium levels but may not effectively inhibit corrosion without adequate alkalinity. Magnesium chloride does provide magnesium ions, which can be beneficial in some cases, but it does not actively inhibit corrosion in the same manner as polyphosphates do.