How to monitor and control Corrosion/Scaling in Cooling Water System


Corrosion and scaling in cooling water system can be monitor and control by following methods.

1. Corrosion Coupon 

2. ORP (Oxidation Reduction Potential)

3. The Langlier Saturation Index (LSI) and the Ryznar Stability Index (RSI)

Corrosion Coupon:

It is installed to check the cooling water tendency of corrosion. A weighted metal piece is used in coupons. After definite period the metal strip is taken out, washed and weighted again. The loss in weight indicates the corrosive tendency of water. It is measured in mils per year (mpy).1 mpy = 0.025mm

Corrosion Rate = Weight loss(mg) * 365 * 39.4 / Density of metal (mg/mm3) * Surface Area (mm2)* No of Days

Surface Area (A) = 2*[(L*B) + (B*T) + (L*T)] + 3.14 D * T - (3.14 D2/2 * )

Where, L = Length of the coupon.

B = Width of the coupon.

T = Thickness of the coupon.    

D = Diameter of hole in coupon. 

ORP (Oxidation Reduction Potential):

ORP is a differential measurement of the mV potentials built up when electrodes are exposed to solution containing oxidants and reductants.

In a redox pair, one chemical loses electron through the process of oxidation i.e. they become ionize and is called reductant (Li+, Mg2+, Fe2+, Cr3+ etc). Other chemical acquires (gain) electron through the process of reduction i.e. they become reduce and is called oxidant (HOCl, OCl-, ClO2 etc.).

When the solution being measured has a high concentration of oxidizers, it will accept more electrons than it loses, the measuring electrode (pure platinum) develops a higher electrode potential than the reference electrode (silver/silver chloride). Once the entire system reaches equilibrium, the resulting net potential difference represents the ORP either positive reading (oxidizing solution) or negative reading (reducing solution).

The more oxidizer in the cooling tower water, the higher the mV output of the ORP electrode. At a given chlorine concentration, the ORP increases as the pH decreases. The higher the chlorine concentration, the smaller the change in ORP at a given pH.


Langlier Saturation Index (LSI)

LSI deals with the conditions at which a water is in equilibrium with calcium carbonate and make it possible to predict the tendency of calcium carbonate either to precipitate or to dissolve under varying conditions.

A positive LSI number (RSI less than 5.0) indicates a scale forming water while a negative LSI number (RSI greater than 7.0) indicates a scale dissolving, or corrosive, water. Normal practice is to maintain a slightly positive LSI number, +.2 to +.5, (RSI between 5.0 and 6.0) when utilizing pH adjustment by acid addition and add some chemical scale inhibitor to cope with the resultant slight tendency to scale

Ryznar Stability Index (RSI)

Two different waters, one of low hardness (corrosive) and the other of high hardness (scale-forming), can have the same Saturation Index. Stability Index developed by Ryzner makes it possible to distinguish between two such waters. Waters have a Stability Index of 6.0 or less, scaling increases and the tendency to corrode decreases. Where the Stability Index exceeds 7.0, scaling may not occur at all. As the Stability Index rises above 7.5 or 8.0, the probability of corrosion increases.

LSI Value

2.0 = Scale Forming but non corrosive

0.5 = Slightly scale forming and corrosive

0.02 = Balanced but pitting corrosion possible 

-0.5 = Slightly corrosive but non-scaling forming

-2.0 = Serious corrosion 

RSI Value 

4.0 -5.0 = Heavy Scale

5.0 -6.0 = Light Scale

6.0 - 7.0 = Little scale or corrosion 

7.0 - 7.5 = Corrosion Significant

7.5 - 9.0 = Heavy Corrosion

>9.0 = Corrosion intolerable 

Calcium carbonate scaling can be predicted qualitatively by the Langelier Saturation Index (LSI) and Ryznar Stability Index (RSI). Use of the LSI together with the Stability Index contributes to more accurate prediction of the scaling or corrosive tendencies of a water. The indixe are determined as follows:

Langelier Saturation Index = pHa - pHs
Ryznar Stability Index = 2(pHs) - pHa

pHa = Actual pH of the water.

pHs = (pH of saturation) is a function of total solids, temperature, calcium, and alkalinity

 The pH at which water with a given calcium content and alkalinity is in equilibrium with calcium carbonate

pHs can be determine by:





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