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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45 NACE INTERNATIONAL: VOL. 55, NO. 6 MATERIALS PERFORMANCE JUNE 2016 acid, aspartic acid, and copolymers containing monomers of dif ferent functional groups) Although there are many phosphonates available, three of the most commonly used phosphonates in water treatment formula- tions are aminotrismethylene phosphonic a c i d ( AM P) ; 1 - hyd r ox y e t hy l i d i n e , 1 , - 1 diphosphonic acid (HEDP); and 2-phos- p h o n o - b u t a n e 1 , 2 , 4 - t r i c a rb oxy l i c a c i d (PBTC). However, under certain pH, concen- tration, and temperature conditions, phos- phonates have been shown to precipitate in the presence of calcium ions. The precipita- tion of calcium phosphonate salts not only creat e s foulin g of h eat exchan ger and reverse osmosis (RO) membrane surfaces, it also decreases the solution concentration of a phosphonate to such an extent that severe calcium carbonate (CaCO 3 ) scaling can occur. 1-2 The focus of this study is to evalu- ate the performance of polyamino polyether methylene phosphonic acid (PAPEMP) as an inhibitor for various scales (e.g., CaCO 3 , cal- cium sulfate dihydrate [CaSO 4 ·2H 2 O], and calcium phosphate [Ca 3 (PO 4 ) 2 ]) and a stabi- lization agent for Fe(III) or Fe 3+ ions. Experimental Protocols All chemicals were obtained from com- mercial sources. They include AMP, HEDP, PBTC, 2-hydroxyphosphono acetic acid (HPA), PAPEMP, and polyacrylic acid (PAA). Detailed procedures for reagents solution preparation; percent inhibition (%I) calcu- l a t i o n f o r c a l c i u m s u l f a t e d i hy d ra t e (CaSO 4 ·2H 2 O), CaCO 3 , Ca 3 (PO 4 ) 2 , and Fe 3+ stabilization ; and instruments used are reported elsew here. 3-6 Table 1 lists the inhibitors tested. Prevention of Mineral Scales Calcium Sulfate Dihydrate Considerable attention has been given to the various forms of CaSO 4 crystallizing from aqueous solutions as affected by tem- perature, pH, solution stoichiometric ratio of lattice ions, and impurity level. These and other important factors involved in the nucleation and growth of CaSO 4 ·2H 2 O (gyp- sum), hemihydrate (CaSO 4 ·0.5 H 2 O) (plaster of Paris), and anhydrite (CaSO 4 ) should have direct application to the control and inhibition of scale formation in industrial water systems. To preclude excessive costs, scale formation has to be prevented. The effect of low concentrations (few ppm) of polymeric inhibitors on both the growth rate and cr ystal modification of gypsum has been investigated by several researchers. Amjad 3 showed that polymers containing carboxyl groups (i .e., poly- acrylic acid, polymaleic acid, polyitaconic acid) are particularly effective as gypsum growth inhibitors. Dogan, et al. 7 arrived at similar conclusions aft er studying th e effect of various acrylic acid-based copoly- mers as gypsum scale inhibitors. Figure 1 presents %I data for various phosphonates at 1.5 and 3.0 mg/L dosages, respectively. There are two points worth TABLE 1. PHOSPHONATES TESTED Phosphonate Acronym Structure Functional Group Ionic Charge (A) Molecular Weight 2-hydroxyphosphono acetic acid HPA H | HOOC—C—PO 3 H 2 | OH –COOH –OH –PO 3 H 2 negative 156 Aminotris (methylene phosphonic acid) AMP CH 2 PO 3 H 2 ∕ N—CH 2 PO 3 H 2 \ CH 2 PO 3 H 2 –PO 3 H 2 negative 299 1-hydroxyethylidine 1,1-diphosphonic acid HEDP OH | H 2 PO 3 —C—PO 3 H 2 | CH 3 –OH –PO 3 H 2 neutral negative 206 2-phosphono 1,2,4 butane tricarboxylic acid PBTC PO 3 H 2 | CH 2 — CH — CH 2 — CH 2 | | | COOH COOH COOH –COOH –PO 3 H 2 negative 270 Polyamino polyether methylene phosphonic acid PAPEMP (H 2 O 3 P-CH 2 ) 2 -N-CH(CH 3 )CH 2 -(OCH 2 CH(CH 3 ) n- N-(CH 2 -PO 3 H 2 ) 2 –PO 3 H 2 –(O-CH 2 -CH 2 )n- negative neutral 600 to 630 (A) At the working solution pH.

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