Heat exchanger failure analysis in the simulated marine environment: Prediction of the fouling removal temperature

Highlights

• Analysis of temperature effect on electrochemical behavior of Cu-Ni 90/10 immersed in synthetic seawater.

• Investigation of temperature influence on fouling composition and morphology.

• Prediction of fouling removal temperature in heat exchanger in desalination plant.

Abstract

The present work aims to analyze the failure resulting from temperature change and its influence on the corrosion and fouling resistance of the cupronickel Cu-Ni 90/10 in simulated marine solution to predict the removal fouling temperature in the desalination heat exchanger and how affects fouling composition and morphology. The Cu-Ni 90/10 electrochemical behavior has been studied through: polarization tests, following the evolution of open circuit potential (OCP) and electrochemical impedance spectra (EIS) with immersion time under thermal and hydrodynamic conditions close to those prevailing in the heat exchanger of desalination industry. Electrochemical measurements show that the corrosion rate of Cu-Ni 90/10 alloy change with temperature, due to a competition between the formation of stable complexes on the metal surface and the dissolution of salts to generate aggressive ions in the corrosion medium. 60 °C ensures an equilibrium between the complexation reactions that promotes fouling stability, and fouling mitigation that leads to metal oxidation. In contrast, at high temperatures (80 °C), the dissolution of salts predominates over the complexation process. It can be deduced that 60 °C is a critical point for heat exchanger that promotes the formation of thin layer with low thermal fouling resistance that protects the installation against damage, maintain a good heat transfer and extend the cleaning scheduling of the machine. Surface analyses were performed by means Raman spectroscopy, SEM and EDS quantification, reveals the reinforcement of fouling by iron compound which make deposit more resistant at 60 °C.

Introduction

Cupronickel alloys Cu-Ni 90/10 are largely adopted in the heat exchanger materials selection notably in desalination industries, due to its good oxidation resistance associated with the development of fouling film in the marine environment [1]. In spite of its excellent attitude against corrosion, this alloy suffers from a severe failure that limits its performance and heat exchanger lifetime, especially at high temperature. Temperature affects the stability of copper-nickel alloys when they are used in seawater. It favors its degradation at hydrodynamic conditions. On the one hand, it reduces the stability of materials and accelerates the kinetics of transfer and transport reactions. On the other hand, its increase reduces the solubility of dissolved oxygen and therefore limits the corrosion rate [2].

A large number of research documents have been published on this topic [3], [4]: Ali et al. (2020) [5] involves useful guidelines to prevent such failures. He proposed that the main causes of fatigue are thermal overloads resulting from an elevation in temperature or localized overheating and led to fracturing in the machine. The author suggests that careful control of operating conditions such as temperature can prolong the service life of heat exchangers.

IJsseling (1983) [6] treated the temperature impact on the corrosion of 90–10 in seawater and highlighted this influence in aerated water. The author mentioned that a fluctuating effect can be seen in aerated water due to the combined influences of flow and oxygen content (temperature rise limits solubility of oxygen). This effect was greatest and important at low temperatures. He also observed a very rapid decrease of corrosion rate after only a few days of exposure at 40 °C. He concluded that at 40 °C, a protective and stable corrosion products are produced rapidly [7]. Besides, Numerous authors like Srinivasan and Watkinson (2005) have been focused on the parameters influencing the fouling deposition, they have shown that the rate of fouling increases with decreasing fluid temperature [8]. On the other hand, A range of fouling investigative experiments at different temperatures has been reported by Behbahani et al. (2008) [9] to determine the mechanisms that control the deposition process.

Furthermore, Nadjet Fadel (2010) [10] studied the influence of the fluid temperature on the performance of a heat exchanger at the Algiers refinery. He mentioned that the temperature of the heat exchanger surface is an important parameter regardless of the type of fouling, the deposition of solid particles on the metal surface is favored when its temperature is high. Thus the formation of fouling is accelerated under these conditions. The author found as a result: the induction fouling time decreases as the surface temperature increases.

Varbanov et al. (2017) [9] said that the fouling growth on the heat transfer surfaces is dependent on a thermal factor, where the temperature has a major impact on the rate of fouling deposition, as reported by several scientists, see for example Pääkkönen et al. (2015) [11].

Recent works have investigated the temperature impact on fouling formation in marine medium, Dadić et al. (2019) [12] was studied the corrosion behavior of the CuNi10Fe1Mn alloy condenser tubes in seawater, the author found that the corrosion resistance of a sample with low Fe content is considerably affected by increasing the temperature, the corrosion current icorr increase and resistance to polarization Rp drops with the temperature. The author confirms also the beneficial impact of temperature on the protective qualities and proprieties of the oxide layer and fouling developed on the metallic surface in the temperature range between 20 and 40 °C [12]. Turnbull and Parvizi et al. [6], examine the temperature effect in the range from 40 to 80 °C, they found that the corrosion rate of Cu-Ni 90/10 increased from 34.4 to 85.5 mdd. However, Fink and Parvizi et al. [6] found that in the range temperature from 55 to 120 °C the oxidation rate decreased from 4 to 0.5 mpy after 156 exposing days in sea water. It seems that the influence of temperature on fouling and corrosion is properly appreciated. However, limited study has been done to predict the re-entrainment fouling temperature and the composition of the fouled surface affected by temperature change [13].

The present work presents a continuity of our previous works [14], it aims to study the influence of temperature on the corrosion and fouling behavior of the Cu-Ni 90/10 alloy in synthetic solution simulating seawater, by means the recording of anodic and cathodic linear polarization curves and electrochemical impedance spectra (EIS). The flow velocity is set at 0.127 m.s⁻¹. Three temperature values, 40 °C, 60 °C and 80 °C, were selected in order to be closer to the industrial condition of a double-effect heat exchanger in thermal desalination plant whose operating temperature range is between 40 °C and 59 °C, and 80 °C was chosen to better appreciate the impact of thermal factor on the fouling development at high temperature.

The originality of this paper is reflected in the prediction of fouling removal temperature based on surface analysis and visual inspection of the heat exchanger of an existing desalination plant in the south of Tunisia. This work presents an experimental and fundamental approach that can be used by industrialists in desalination units to optimize the operating temperature of heat exchangers by means electrochemical tests. This temperature is a critical point that guarantees the equilibrium between the formation of a thin layer of deposit with low thermal resistance to fouling that limits the metal degradation, and fouling mitigation that brings the tube surface into an active state. This approach allows us to extend the exchanger cleaning schedule and subsequently increase its performance and lifetime.