Materials Performance

JUN 2016

Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.

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46 JUNE 2016 MATERIALS PERFORMANCE NACE INTERNATIONAL: VOL. 55, NO. 6 CHEMICAL TREATMENT of this deposit. A particular and often com- plicating feature in the CaCO 3 system is the polymorphism. Some of the various CaCO 3 p o ly m o r p h s e n c o u n t e re d i n a q u e o u s media, in order of decreasing solubility, are calcium carbonate monohydrate, vaterite, aragonite, and calcite. Calcite is the most stable phase thermodynamically, although it is frequently encountered and identified as the sole component of CaCO 3 scale for- mations and may result from the formation of less stable vaterite. Commonly used inhibitors to control CaCO 3 scale formation may be classified in the following broad categories: polyphos- phates, polyphosphonates, synthetic poly- mers, natural polymers, and proprietar y formulations. Polyphosphates have been known as efficient threshold inhibitors in the sense that they can inhibit CaCO 3 scale formation at substoichiom etric le vel s. They have been widely used despite their main disadvantage, which is hydrolysis of the esoteric P-O bond that yields ortho- phosphate and, upon further reaction with calcium ions, results in the formation of tenacious Ca 3 (PO 4 ) 2 deposits. Organo phos- phonates have been shown to be excellent CaCO 3 inhibitors. These compounds, how- ever, lead to th e formation of calcium phosphonate salts under high hardness conditions. It is interesting to note that PAPEMP, compared to AMP, HEDP, PBTC, and HPA, has been reported to be more tol- erant to Ca ions. 8 As previously discussed, many systems are operating under stressed conditions (i.e., alkaline pH, high temperature, and high hardness levels) due to water short- ages. To study the performance of phospho- nates under stressed conditions, several experiments were carried out. The precipi- tation of CaCO 3 was investigated under two different scaling conditions (SI-1 and SI-2). Data presented in Figure 2 show that the performance of phosphonates strongly depends on solution supersaturation. At low CaCO 3 supersaturation (i.e., saturation index [SI] = 1.67), 4 AMP and HEDP outper- form PBTC, HPA, and PAPEMP. However, the performance of these phosphonates is quite different under stressed conditions (SI = 2.2). Results presented in Figure 2 show that all phosphonates provide good to mediocre performance at 10 ppm as FIGURE 1 Gypsum inhibition by phosphonates. FIGURE 2 CaCO 3 inhibition by phosphonates as a function of supersaturation. noting: a) the g ypsum inhibition value increases with increasing inhibitor dosage and b) the inhibition value depends upon the inhibitor architecture. Phosphonates (i.e., HPA, HEDP) containing one (–OH) and one (–PO 3 H 2 ) group, as shown in Table 1, exhibit poor performance as gypsum inhibi- tors when compared to AMP (containing three [–PO 3 H 2 ] groups. For example, %I val- ues obtained for HPA and HEDP at 3.0 ppm dosage are 8 and 12%, respectively, com- pared to 89% obtained for AMP. Results on the performance of PBTC and PAPEMP are also presented in Figure 1. When compared to PBTC, PAPEMP exhib- its excellent performance as a g ypsum inhibitor. For example, %I values obtained in the presence of 1.5 ppm PBTC are 18% compared to 95% obtained for PAPEMP. Based on the data presented in Figure 1, phosphonate performance can be ranked as follows: PAPEMP > AMP > PBTC > HEDP > HPA. Calcium Carbonate CaC O 3 i s ver y oft en found in scale deposits, both in colloidal and amorphous states and in the form of polymorphs. The presence of relatively high concentrations of calcium and carbonate ions in water, in combination with the inverse solubility of CaCO 3 , is the main cause for the formation

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