Contents lists available at ScienceDirect Desalination journal homepage: www.elsevier.com/locate/desal Efect of pH on the efciency of sodium hexametaphosphate as calcium carbonate scale inhibitor at high temperature and high pressure Stéphanie C. de Morais a , Djalan F. de Lima a , Thuany M. Ferreira b , Josiel B. Domingos b , Miguel Angelo F. de Souza a , Bruno B. Castro c , Rosangela de C. Balaban a, ⁎ a Universidade Federal do Rio Grande do Norte, UFRN, 59078-970 Natal, RN, Brazil b Universidade Federal de Santa Catarina, UFSC, 88040-900 Florianópolis, SC, Brazil c Centro de Pesquisa e Desenvolvimento Leopoldo Américo Miguêz de Mello, – CENPES/PETROBRAS, – Rio de Janeiro, RJ, Brazil GRAPHICAL ABSTRACT ARTICLE INFO Keywords: Calcium carbonate Sodium hexametaphosphate Scale inhibition pH Dynamic tube blocking test ABSTRACT Calcium carbonate (CaCO 3 ) scale deposition causes serious issues in industries which deal with water, and its formation reaction is pH-depending. This study investigated the efciency of sodium hexametaphosphate (SHMP) as scale inhibitor for calcium carbonate, as a function of pH and SHMP concentration, under drastic conditions of temperature, scaling ions concentration and pressure. The relationship between pH and CaCO 3 formation parameters was evaluated by static and dynamic tube blocking tests and scanning electron microscopy (SEM) images. Quantum chemistry calculations were performed to evaluate thermodynamic values related to interactions between the inhibitor and CaCO 3 . At pH 6.5, excellent results were found with complete inhibition of CaCO 3 deposition on capillary with just 2.5 mg L −1 of SHMP and deformed structures for CaCO 3 crystals were observed by SEM images. Results also showed a reversal behavior of SHMP as inhibitor at pH neutral and slightly alkaline, with a decrease on inhibition efciency even raising SHMP concentration, related to the fast pre- cipitation of CaCO 3 in the drastic conditions evaluated. https://doi.org/10.1016/j.desal.2020.114548 Received 18 April 2020; Received in revised form 21 May 2020; Accepted 24 May 2020 ⁎ Corresponding author. E-mail address: rosangelabalaban@hotmail.com (R.d.C. Balaban). Desalination 491 (2020) 114548 0011-9164/ © 2020 Elsevier B.V. All rights reserved. T