Journal of Petroleum Science and Engineering 199 (2021) 108347 Available online 5 January 2021 0920-4105/© 2021 Elsevier B.V. All rights reserved. The electrolyte renewal effect on the corrosion mechanisms of API X65 carbon steel under sweet and sour environments B.A.F. Santos a, * , M.E.D. Serenario a , R.C. Souza a , J.R. Oliveira b , G.L. Vaz b , J.A.C.P. Gomes c , A. H.S. Bueno a a Mechanical Engineering Department, Universidade Federal de S˜ ao Jo˜ ao Del Rei (UFSJ), 170 Praça Frei Orlando, 36307-352, S˜ ao Jo˜ ao Del Rei, MG, Brazil b PETROBRAS, CENPES/PDP/TMEC, Av. Hor´ acio Macedo 950, Cidade Universit´ aria, 21941-915, Rio de Janeiro, Brazil c Metallurgical and Materials Engineering Department, Universidade Federal do Rio de Janeiro (UFRJ), Ilha do Fund˜ ao, Bloco F, Rio de Janeiro, RJ, Brazil A R T I C L E INFO Keywords: Carbon steel CO 2 EIS SEM XRD high temperature corrosion ABSTRACT Sweet and sour services are the most common media found inside the pipelines of petroleum industry. They are one of the main causes of degradation by corrosion that lead to several fnancial and environmental losses worldwide. They also are responsible for the formation of corrosion products that might play a protective feature on the metal surface diminishing the corrosive processes. However, conditions in the feld are not necessarily favourable for the precipitation of protective structures given the physicochemical dynamism inside the pipes. Then, the need to address this issue that occurs inside the tubes becomes crucial for the correct assessment of the development of protective corrosion products hence to give a more realistic approach to the lab conditions. Therefore, this paper presents a methodology to investigate the electrolyte renewal effects on API X65 carbon steel corrosion under CO 2 and H 2 S (thiosulfate) environments at 4,5 bar of pressure, temperatures of 90 ◦ C and 120 ◦ C over 150 h. Electrochemical techniques (LPR and EIS), weight loss and characterizations were conducted in two different average pH, 3,3 and 4,5. The proposed methodology was capable to delay the development of protective scales in renewed media. The corrosion rates were higher for conditions with electrolyte renewal and higher temperature. FeCO 3 formation was favoured at high pH and 120 ◦ C whilst FeS structures were dominant at 90 ◦ C. Sulphides were more cracked and uneven in renewed media whereas carbonate crystals presented higher anchorage to the substrate and were able to develop in sizeable diameters. Unrenewed media presented the best electrochemical responses. 1. Introduction By the years, the petroleum industry has been facing huge challenges in the control and management of the corrosion mechanisms that take place in its structures. The pipes are one of the most affected by corro- sion, becoming the study object of many works (De Motte et al., 2018; Giarola et al., 2017; Hua et al., 2015; Obot et al., 2020; Santos et al., 2020; Souza et al., 2019; Wang et al., 2020). The main reason is due to the large number of contaminants present in the produced and condensed water found into the pipes, such as CO 2 , H 2 S, organic acids and salts (Kermani and Morshed, 2003; Santos et al., 2019; Shamsa et al., 2019; Singer et al., 2011). According to the literature, carbon dioxide, often present in this environment by its use in the oil extraction process, expose the carbon steels in aqueous media to the sweet corrosion mechanism (Barker et al., 2019; De Motte et al., 2018; Drexler et al., 2020). It takes place after the hydration of the CO 2 that forms the H 2 CO 3 acid. Then, species of CO 3 2 and HCO 3  are released from the dissociation of the acid, as well as an extra amount of H + that acidify the medium (Barker et al., 2019). Ac- cording to some authors (Barker et al., 2018; Elgaddaf et al., 2021a, 2021b; Santos et al., 2020; Souza et al., 2019), the possible reactions involving CO 2 in water are: CO 2(g) → CO 2(aq) (1a) CO 2(aq) + H 2 O (l) →H 2 CO 3(aq) (2a) H 2 CO 3(aq) →H + (aq) + HCO 3  (aq) (3a) * Corresponding author. Mechanical Engineering, Centre of Surface Engineering, Tribology and Electrochemistry – CESTEq, Federal University of S˜ ao Jo˜ ao del Rei – UFSJ, Praça Frei Orlando, 170 – Centro, 36.307-352, S˜ ao Jo˜ ao del-Rei, MG, Brazil. E-mail address: bernardo.a.fs@hotmail.com (B.A.F. Santos). Contents lists available at ScienceDirect Journal of Petroleum Science and Engineering journal homepage: http://www.elsevier.com/locate/petrol https://doi.org/10.1016/j.petrol.2021.108347 Received 21 October 2020; Received in revised form 8 December 2020; Accepted 1 January 2021