CORROSION CONTROL

By pH – Alkalanity RelationshipThis graph may be used for estimating the corrosiveness of water. If the total alkalinity and pH values intersect at a point on the graph below the bottom line, the water is corrosive. Raise the alkalinity and pH of the finished water by adding lime or soda ash until the point of intersection falls above the center curve. If the point of intersection falls above the top curve, overtreatment is indicated and undesirable calcium carbonate deposits may be expected.

One of the best tests for determining the corrosivity of water is the Langelier Saturation Index (Ll). The Ll requires the following tests: Calcium hardness, total alkalinity, pH, total dissolved solids (TDS), and water temperature in centigrade. Once the tests have been performed they are assigned a value from the following tables. Notice that hardness and alkalinity (C and D) both use the same table.

After each test has been run and a value assigned, the following formula is used.

1. pHs = A + B – C – D
   then
2. LI = pH actual – pHs