Larson-Skold Corrosivity Index Calculator
Chlorides and sulfates pit steel; alkalinity protects it. Enter three lab values to get the Larson–Skold ratio and a plain-English corrosivity read for mild steel.
Larson-Skold Index
| Item | Value |
|---|
How the calculation works
Formulas (all terms in meq/L)
Index = (Cl / 35.45 + SO4 / 48.03) / (alkalinity as CaCO3 / 50)
Equivalent weights: chloride 35.45, sulfate 96.06/2 = 48.03, CaCO3 100.09/2 ≈ 50.0.
No pH-based carbonate split is needed: on a charge (meq/L) basis, [HCO3−] + [CO32−] equals total alkalinity in meq/L.
Index = (Cl / 35.45 + SO4 / 48.03) / (alkalinity as CaCO3 / 50)
Equivalent weights: chloride 35.45, sulfate 96.06/2 = 48.03, CaCO3 100.09/2 ≈ 50.0.
No pH-based carbonate split is needed: on a charge (meq/L) basis, [HCO3−] + [CO32−] equals total alkalinity in meq/L.
| Index value | Corrosivity toward mild steel |
|---|---|
| < 0.8 | Low |
| 0.8 – 1.2 | Moderate (gray zone) |
| > 1.2 | High |
Notes & limitations
Larson & Skold (1958), Illinois State Water Survey — developed from in-situ corrosion of mild steel lines in Great Lakes waters. A screening tool, not a corrosion-rate prediction.
- The index was calibrated on mild steel; it does not directly predict copper or lead corrosion.
- Chloride and sulfate must be entered as the ion (mg/L as Cl, mg/L as SO4), alkalinity as CaCO3 — mixing bases invalidates the ratio.
- An index above 1.2 warrants corrosion-rate monitoring (coupons) and a review of inhibitor or pH/alkalinity adjustment — confirm with field data.
- Road salt and anion-exchange or RO concentrate can swing this index seasonally; trend it, do not read a single value in isolation.
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