How to determine the water corrosive index . whether any thumb rule or formula availble ?

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1. Of course, it depends upon what metals one is most concerned about. A general formula, analogous to the Langelier Index, is what can be called a saturation index (SI), or
   SI = log10(Q/Ksp) in which
   Q = the ion activity productKsp = the solubility constant for the given temperature
   The interpretation is as follows, again similar to Langelier Index for CaCO3 saturation:
   SI> 0 = supersaturation
   SI = 0 = saturation equilibrium
   SI < 0 = undersaturation.
   Concentrations, in most aqueous solutions with reasonable ionic strength, can be substituted for activities.
   Hope this helps!