A common index used for iron and steel corrosion is the Larson-Skold Index (LSI):

LSI = ([HCO₃^-]+[CO₃^2-])/([Cl^-]+[SO₄^2-] where [i] indicates equivalents/liter of component i.

Rating system:

  0 - 0.8     mildly corrosive:accelerated corrosion unlikely

  0.8 - 1.2  corrosive: elevated corrosion rates possible

  > 1.2     very corrosive: accelerated corrosion likely

Another anion ratio index using chloride and sulfate is the CSMR.

Source: Roberge, P., 2006. Corrosion Inspection and Monitoring. Wiley Online, Appendix B

https://onlinelibrary.wiley.com/doi/book/10.1002/0470099763