Contents lists available at ScienceDirect Desalination journal homepage: www.elsevier.com/locate/desal A kinetic approach to desalinated water corrosion control by CaCO 3 flms David Hasson ⁎ , Raphael Semiat, Hilla Shemer GWRI Rabin Desalination Laboratory, Department of Chemical Engineering, Technion-Israel Institute of Technology, Haifa 3200003, Israel ARTICLE INFO Keywords: Post-treatment Supersaturation Induction time Calcium carbonate Re-mineralization ABSTRACT The corrosive tendency of desalinated water is usually controlled by contacting acidifed water with CaCO 3 particles to make it slightly supersaturated with respect to CaCO 3 . The desired result is to deposit on the water pipe surface a corrosion prevention flm of calcium carbonate. However the considerable research eforts aiming to predict water characteristics ensuring formation of adequate protective coatings have so far yielded only rough qualitative guidelines. A major inadequacy of all available water composition criteria for ensuring the formation of adequate corrosion prevention flms is that they are of thermodynamic nature while the precipitation of CaCO 3 flms is governed by process kinetics. The most commonly used criteria, LSI and CCPP, denote the extent of the supersaturation level with no information on the rate of deposit formation. The objective of this paper is to stress the need to extend the current approach by quantitative kinetic criteria. Induction time and CaCO 3 deposition rate are essential kinetic tools for guiding the formation of adequate CaCO 3 protective layers and should be integrated in regulatory specifcations for re-mineralized desalinated water composition. 1. Current approach to desalinated water corrosion control by a protective CaCO 3 layer Since desalinated water is devoid of minerals, it is of corrosive nature and cannot be conveyed through iron pipe systems without appropriate treatment. The current approach to this problem is based on the traditional method of forming a corrosion protective coating of calcium carbonate. It is however generally recognized that the nu- merous attempts to establish criteria ensuring formation of adequate protective coatings yielded only rough qualitative guidelines, with the goal of reliable quantitative criteria remaining elusive [1–8]. The terms “Corrosion Index” and “Calcium Carbonate Saturation Index” are as- sociated with these criteria. The saturation ratio S of water containing dissolved calcium and carbonate ions is defned by: = + K S [Ca ] [CO ] s 2 3 2 (1) where K s ′ is the solubility product of CaCO 3 corrected for the ionic strength efect. Among the numerous indexes that have been proposed for controlling the protective coating, the most widely used parameters are the Langelier Saturation Index (LSI) and the Calcium Carbonate Precipitation Potential (CCPP). LSI is defned as the diference between the measured pH of water and pH s , the pH of the water if it were in equilibrium with CaCO 3 at the prevailing calcium and bicarbonate ions concentrations. For the usual composition of remineralized desalinated waters, LSI can be closely approximated from the value of the satura- tion ratio: = LSI pH pH log S s (2) CCPP denotes the total concentration of CaCO 3 that will precipitate from a supersaturated solution until equilibrium conditions are reached. The CCPP of a solution of initial calcium [Ca 2+ ], alkalinity [T alk ] and hydrogen ion [H + ] concentrations can be calculated by solving for the three equilibrium concentrations [Ca 2+ ] eq , [T alk ] eq , [H + ] eq appearing in Eqs. (3) to (5). Eq. (3) defnes the CCPP, Eq. (4) expresses the equilibrium solubility product of CaCO 3 while Eq. (5) expresses the condition of constant solution acidity during CaCO 3 pre- cipitation: = + + CCPP [Ca ] [Ca ] 2 2 eq (3) = = + + + + + + + K K K {[Ca ] [CO ] } [Ca ] [T ] [T ] 2 [T ] [H ] /[H ] 2 [H ] / sp w 2 eq 3 2 eq 2 alk alk eq alk eq eq eq eq 2 (4) https://doi.org/10.1016/j.desal.2018.10.015 Received 12 September 2018; Received in revised form 15 October 2018; Accepted 15 October 2018 ⁎ Corresponding author. E-mail address: hasson@technion.ac.il (D. Hasson). Desalination 449 (2019) 50–54 Available online 23 October 2018 0011-9164/ © 2018 Elsevier B.V. All rights reserved. T