The numeric difference ...
Published by Bob Foerster, Executive Director at Nevada Rural Water Association
The numeric difference between pH and pHs provides an indication of corrosivity, but only in terms of whether the water will dissolve or precipitate calcium carbonate. The pHs is a calculated value, inputs being the TDS, temperature, alkalinity and hardness. If the (pH -pHs) is negative by more than approximately 0.5 unit, the water tends corrosive to calcium carbonate; if the result of the calculation is a positive number calcium carbonate scale will tend to form. Good practice is to adjust the pH and/or alkalinity to maintain a +0.5 to +1 index, in other words maintain non-corrosive conditions. Please keep in mind that steel coupon gravimetric testing provides certainty and the saturation index is just an indicator.