Corrosion Control Calculator

This calculator computes three classic water-corrosivity indices from a standard water chemistry profile — pH, alkalinity, calcium hardness, total dissolved solids, and temperature — the same inputs a water treatment operator would pull from a routine lab analysis. The core calculation is the Langelier Saturation Index (LSI), found by first deriving the water's saturation pH (pHs, the pH at which the water is exactly in equilibrium with calcium carbonate) from four empirical terms built on the log of TDS, temperature in Kelvin, calcium concentration, and alkalinity, then subtracting that from the water's actual measured pH. A positive LSI means the water tends to deposit protective (or nuisance) calcium carbonate scale; a negative LSI means it's aggressive enough to dissolve pipe scale and expose bare metal to corrosion. The Ryznar Stability Index (RSI) is derived directly from the same pHs value using a different weighting (2×pHs minus actual pH) and is interpreted on its own numeric scale, so don't expect LSI and RSI to read as the same kind of number — they use opposite sign conventions for corrosive vs. scale-forming water.

The Aggressive Index is a simpler, cruder screening metric based only on pH, alkalinity, and calcium, included mainly for AWWA compliance reporting rather than as a design tool. All three converge on a plain-language treatment recommendation and a Lead and Copper Rule risk flag, since keeping LSI slightly positive (mildly scale-forming) is the standard target for utilities managing pipe corrosion and lead leaching risk — EPA's own Optimal Corrosion Control Treatment Evaluation Technical Recommendations identifies indices like the LSI as one of the tools primacy agencies and water systems use to evaluate and document corrosion control performance under the Lead and Copper Rule. The CaCO₃ precipitation potential is explicitly a simplified approximation, not a rigorous equilibrium calculation, so use it directionally rather than as a precise dosing number.