Calcium Saturation and Corrosion Control

Alkalinity (carbonates CO3 and bicarbonates HCO3) and the free carbon dioxide CO2 in water influence corrosion and scale formation. Scaling typically refers to hardness scale and its degree of scale potential (normally calcium hardness). Sulfate and chloride ions also can increase the corrosion potential of water. Above 250 ppm silica can act as a corrosion inhibitor.

Historically, the Langelier Calcium Carbonate Saturation Index, and the Ryznar Index have been used to approximate calcium saturation in water systems. In recent years computer models predict water chemistry and take into account all of the ions present in a water sample to allow corrosion control utilizing existing water ions.

Scale modification polymers, chelants, phosphate and phosphonate threshold inhibitors are commonly used to prevent scale formation in water systems. These molecules interfere with crystal formation kinetics or the crystal lattice structure which allows them to be in solution and provide corrosion resistance.

Langelier developed an index based on solubility products and dissociation constants which ingeniously related alkalinity, pH, and calcium hardness to determine the pH at which saturation should occur at various temperatures.