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

摘要

A number of calcium carbonate (CaCO 3 ) saturation indices are being used for many years to evaluate thescale forming or scale dissolving tendencies of water. Assessment of these tendencies is useful in corrosioncontrol program of water transmission lines and preventing CaCO 3 scaling inside plumbing and equipment,such as industrial heat exchangers or even domestic water heaters. The most commonly used indices areLangelier Saturation Index (LSI or SI), Ryznar Stability Index (RSI), Relative Saturation Index (RS) - alsoknown as Driving Force Index (DFI), which was proposed by McCauley in 1960. However, there are somemore 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 (β), the Momentary ExcessIndex (MEI), which was proposed by Dye in 1952, etc. There is one more index for CaCO 3 scale predictionin cooling tower systems, known as Practical Scale Index (PSI). Many indices reported in the literature areessentially the same but expressed differently. Some of these indices are either absolutely empirical ordefined in such a way that during computation the thermodynamic properties or the parameters of waterchemistry that influence the corrosion or scale formation are often neglected. Each of these indices haveserved some useful purposes, but many have been controversial, incompletely understood and misappliedfor a number of decades, particularly when they are used without recognition of their specific applicationsand limitations. Many of the indices are more useful in helping to understand possible causes of the problemrather than being useful in a predictive sense. In general, indices based on CaCO 3 saturation have not beenshown to have any significant predictive value for the corrosion and/or leaching of lead, zinc, and copperfrom brass, bronze, soldered joints, and their respective pipe materials. In this paper a subset of variousindices, which have appeared in the literature and their known limitations to indicate the inhibition property ofwater, will be addressed. A better understanding and proper application of these indices may prove helpful indeciding or designing the post-treatment strategy of highly aggressive desalinated waters for safe transmissionand inhibit corrosion of metal surfaces or dissolution of cement-mortar lining of the transport pipelines.