Langelier saturation index (LI)

The magnitude and sign of the LI value show water’s tendency to form or dissolve scale and thus to inhibit or encourage corrosion.

In developing the LI, Langelier derived an equation for the pH at which water is saturated withcalcium carbonate (PHs).

The LI is a gauge of whether a water will precipitate or dissolve calcium carbonate. If the PHs is equal to the actual pH, the water is considered “balanced”. This means that calcium carbonate will not be dissolved or precipitated. If the PHs is less than the actual pH (the LI is a positive number), the water will tend to deposit calcium carbonate and is scale-forming (nonaggressive). If the PHs is greater than the actual pH (the LI is a negative number), the water is not saturated and will dissolve calcium carbonate (aggressive). In summary:

PHS = PHactual, Water is balancedPHS < PHactual, LI = positive number, water is scale forming (non-aggressive)PHS > PHactual, LI = negative number, water is not scale forming (aggressive)

It is important to remember that the LI value is not a quantitative measure of calcium carbonate saturation or corrosion.Because the protective scale formation is dependent on pH, bicarbonate ion, calcium carbonate,dissolved solids and temperature; each may affect the water’s corrosive tendencies independently.Soft, low-alkalinity waters with either low or excessively high pH are corrosive, even though this may not be predicted by the LI. This is because insufficient amounts of calcium carbonate and alkalinity are available to form a protective scale.Water with high PH values and sufficient hardness and alkalinity may also be corrosive, even if the LI predicts the opposite. This is the result of calcium and magnesium complexes that cannot actively participate in the scale forming process. Analytical procedures do not distinguish between these complexes and available calcium and magnesium; therefore, the LI value is not accurate in such situations.Corrosive tendencies may also be exhibited by water containing high concentrations of sulfate,chloride and other ions which interfere with uniform carbonate film formation.

As a result of these and other problems, the LI is useful only for determining the corrosivity of waters containing more than 40 mg/L of alkalinity, sufficient calcium ion concentration and ranging between pH 6.5 and 9.5.

Corrosive characteristics Langelier index Aggressive index

Highly aggressive < –2.0Moderately aggressive = –2.0 to 0.0Nonaggressive >0.0

CAUSES OF CORROSION

Alkalinity, hardness, and pH interact to determine whether the water will produce scale or corrosion or will be stable.  The table below summarizes characteristics of corrosive water and of scale-forming water.

Corrosive Water Scale-forming Water

low pH soft or with primarily

noncarbonate hardness low alkalinity

high pH hard with primarily

carbonate hardness high alkalinity

1. Low PH reflects acidic nature. Acidic water have lot of H+ Ion to react with electron at the cathode.

2. Low alkalinity water lack the buffering capacity to deal with acid so become easily acidic and corrosive.

The graph above is known as the Baylis Curve.  It shows the relationship between pH, alkalinity, and water stability.  Water above the lines is 

scale-forming while water below the lines is corrosive.  Stable water is found in the white area between the lines.

3. Oxygen react with hydrogen gas at the cathode causing depolarization and speeding up the corrosion. As a result water with high dissolve oxygen will tend to be corrosive.

4. CO2 combine with water to form carbonic acid which lower the PH.5. High temperature reduce the solubility of CaCo3 in water which promotes scale

forming and slow the process of corrosion.