Corrosion indices stability

When the number of concentrations of the circulating water is in the order of 3-7, some of the salts dissolved can exceed their solubility limits and precipitate, causing scale formation in pipes and coolers. The purpose of the treatment of the cooling water is to avoid scale formation. This is achieved by the injection of sulfuric acid to convert Ca and Mg carbonates (carbonate hardness) into more soluble sulfates. The amount of acid used must be limited to maintain some residual alkalinity in the system. If the system pH is reduced to far below 7.0, it would result in an accelerated corrosion within the system. As stated earlier, scale formation and/or corrosion tendency is defined by the Saturation Index (Langelier Index) and Stability Index (Ryznar equation). 

Over the years, the use and application of several different saturation indices that aid the prediction of scaling and corrosion potential in different waters have evolved to become an integral part of the business of treating and managing cooling water systems. The two most popular are the Lange-lier Saturation Index (LSI) and the Ryznar Stability Index (SI), which were both introduced more than 50 years ago. 

The corrosivity of hard waters is defined by the stability index or Ryznar index (RI) defined as ... 

John Ryznar of NALCO developed a useful modification called the Stability Index, based on empirical data from field locations. The Saturation pH is determined exactly the same way, but the Index is obtained differently. The Stability Index is calculated from (2 pH - pH). It always will be a positive whole number. stability Index numbers around 7 show the water is quite stable, neither corrosive nor scaling. As the Stability Index drops below 6, water becomes progressively more scaling. On the other hand, as the Stability Index rises above 7, water becomes progressively more corrosive. A cooling water with a Stability Index of 10 might produce severe to intolerable corrosion conditions. 

RYZNAR STABILITY INDEX - An index based on calcium carbonate pH of saturation vs. actual pH to determine scaling or corrosion tendencies of a water (R.I. = 2 pHs). 

STABILITY INDEX - An imperical modification of the saturation index used to predict scaling or corrosive tendencies in water systems. 

The primary factor affecting long-term corrosion is whether a water does or does not form scales, which in turn depends on hardness and pH. This property is determined by calculating the Langelier index or the Ryzner stability index. The simpler form of Langelier s equation is ... 

Hycite EXM and Sorbacid EXM (Sud-Chemie) are extremely fine magnesium/ aluminium hydrotalcites, for use as co-stabilizers for polyolefins and PVC, respectively. They effectively neutralize acids from catalyst residues or decomposition products of primary and secondary stabilizers. Corrosiveness of polymers is decisively reduced by absorptive immobilization of anions, especially chloride. Sorbacid is mainly used in environmentally friendly and non-toxic formulations free of heavy metals, together with Ca/Zn systems. It has excellent dispersion in small particles, which, with a special surface modification and a refractive index close to that of PVC, gives formulations for transparent applications without any restrictions. 

Equilibrium curves can be determined and, on the basis of a given water composition, used to establish whether a water is scaling (not corrosive) or non-scaling (corrosive). Many different methods have been proposed for prediction of the formation of CaCOa. Commonly used equations or indices are the Langelier Saturation Index, LSI (Langelier, 1936) and the Stability Index, RSI, a modification of the LSI proposed by Ryznar (1944). In the German literature the LSI is known simply as the Saturation Index, Is (Morbe et al., 1987). These indices describe the algebraic difference between the actual pH of the water and the calculated pH at which it would be saturated with calcium carbonate ... 

Langelier index A calculated number used to predict the calcium carbonate (CaCOs) stability of a water that is, whether a water will precipitate, dissolve, or be in equilibrium with calcium carbonate. It is sometimes erroneously assumed that any water that tends to dissolve calcium carbonate will always be corrosive. 

Features When used with anionics such as corrosion inhibitors and coemulsifiers, provides emulsion stability with low foaming and good hard water stability Properties Amber cl. oily liq. forms cl. sol n. in min. oils disp. in water sp.gr. 0.96 pH 6 (1% aq.) HLB 10 ref. index 1.44 100% cone. 

The tendency of water to form scale or cause corrosion is measured by either the Langelier Scaling Index (LSI), which is also called the Saturation Index, or the Ryznar Stability Index (RSI), which is also called the Stability Index (Table 4.2). 

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