The models which are available are used primarily by research laboratories and the water treatment service companies as a service to their customers. Models which predict the performance of a corrosion inhibitor versus inhibitor dosage can be used to establish the economics for treatment to a given corrosion rate target. The wide spread use of personal computers and the availability of PC based software have provided the cooling water chemist with a method for in-depth water chemistry evaluation, and for the application of statistical methods to modeling historical data bases of treatment results versus water chemistry, operating parameters, and treatment levels. Model development requires a baseline method for predicting the untreated corrosion rate as well as models for predicting the corrosion rates in the presence of inhibitors. The Ryznar Stability index combines empirical data with the theory of calcium carbonate saturation to predict the scale forming tendency and mild steel corrosivity of a water.
Calcium carbonate is the sole parameter for determining the corrosivity of a water.
Larson and Skold used a data base of corrosion rate and type observations to formulate an index for the corrosivity of water towards mild steel. They found that alkalinity tended to reduce the corrosion rates of mild steel and postulated that it is a natural inhibitor that participates in the formation of an inhibitor film. Chloride and sulfate were found to increase the corrosivity of a water. This effect is explained by the interference of these anions in the formation of a natural inhibitor film. Larson and Skold proposed the use of the ratio of sulfate and chloride concentration to alkalinity as an indicator of the corrosivity of a water towards mild steel.