Larson-Skold Index (LS)

Estimates the corrosivity of water toward mild steel by quantifying the ratio of aggressive anions (chloride and sulfate) to protective inhibitors (carbonate and bicarbonate). Below 0.8 is lower risk and the passive film is likely stable, 0.8 to 1.2 is potential risk with likely film interference, above 1.2 is high risk with localized corrosion and pitting likely. All concentrations must be in milliequivalents per liter.

$$\text{LS}=\frac{[{\text{Cl}}^{-}]+[{\text{SO}}_4^{2-}]}{[{\text{HCO}}_3^{-}]+[{\text{CO}}_3^{2-}]}$$

Variables

Symbol	Meaning	Unit
$[C{l}^{-}]$	Chloride concentration	meq/L
$[S{O}_4^{2-}]$	Sulfate concentration	meq/L
$[HC{O}_3^{-}]$	Bicarbonate concentration	meq/L
$[C{O}_3^{2-}]$	Carbonate concentration	meq/L

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Part of The Blueprint of Water