Ryznar Index
CORROSION INDEX : Ryznar Stability Index (RSI)

The Ryznar stability index (RSI) attempts to correlate an empirical database of scale thickness observed in municipal water systems to the water chemistry. Like the Langelier Saturation Index (LSI), the RSI has its basis in the concept of saturation level. Ryznar attempted to quantify the relationship between calcium carbonate saturation state and scale formation. The Ryznar index takes the form:

RSI = 2 pHs – pH


So, it gives an indication of the aggressive or encrusting trend (scale-forming) water. And also an indication about the scale and corrosion potential of the water.
The empirical correlation of the Ryznar stability index can be summarized as follows:

Attached table given by the author (in French):

Perconnal indication for aggressive and corrosive tendency:
Ryznar index
<= 6.5
No aggressive trend
> 6.5 and <= 7.2
Slight aggressive
> 7.2 and <= 7.8
Slight corrosion (strong with T> 60 °C)
> 7.8 and <= 8.5
Corosion so T>15°C
> 8.5
Strong corrosion