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An Overview of Calcium Carbonate Saturation Indices as a Criterion to Protect Desalinated Water Transmission Lines From Deterioration

A number of calcium carbonate (CaCO3) saturation indices are being used for many years to evaluate the scale forming or scale dissolving tendencies of water. Assessment of these tendencies is useful in corrosion control program of water transmission lines and preventing CaCO3 scaling inside plumbing and equipment, such as industrial heat exchangers or even domestic water heaters. The most commonly used indices are Langelier Saturation Index (LSI or SI), Ryznar Stability Index (RSI), Relative Saturation Index (RS) - also known as Driving Force Index (DFI), which was proposed by McCauley in 1960. However, there are some more indices which are listed and cited in the literature, viz., Aggressive Index (AI), Larson Ratio (LR), Saturation Disequilibrium Index (SDI), Riddick Index (RI), Buffer Intensity Index (b), the Momentary Excess Index (MEI), which was proposed by Dye in 1952, etc. There is one more index for CaCO3 scale prediction in cooling tower systems, known as Practical Scale Index (PSI). Many indices reported in the literature are essentially the same but expressed differently. Some of these indices are either absolutely empirical or defined in such a way that during computation the thermodynamic properties or the parameters of water chemistry that influence the corrosion or scale formation are often neglected. Each of these indices have served some useful purposes, but many have been controversial, incompletely understood and misapplied for a number of decades, particularly when they are used without recognition of their specific applications and limitations. Many of the indices are more useful in helping to understand possible causes of the problem rather than being useful in a predictive sense. In general, indices based on CaCO3 saturation have not been shown to have any significant predictive value for the corrosion and/or leaching of lead, zinc, and copper from brass, bronze, soldered joints, and their respective pipe materials. In this paper a subset of various indices, which have appeared in the literature and their known limitations to indicate the inhibition property of water, will be addressed. A better understanding and proper application of these indices may prove helpful in deciding or designing the post-treatment strategy of highly aggressive desalinated waters for safe transmission and inhibit corrosion of metal surfaces or dissolution of cement-mortar lining of the transport pipelines.

Ghazi Ozair

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Ms. Apurva Goel