materials Article Mapping the Galvanic Corrosion of Three Metals Coupled with a Wire Beam Electrode: The Influence of Temperature and Relative Geometrical Position Hong Ju 1,* ID , Yuan-Feng Yang 2 ID , Yun-Fei Liu 1, Shu-Fa Liu 1, Jin-Zhuo Duan 1 and Yan Li 1 ID 1 College of Mechanical and Electronic Engineering, China University of Petroleum, Qingdao 266580, China; [email protected] (Y.-F.L.); [email protected] (S.-F.L.); [email protected] (J.-Z.D.); [email protected] (Y.L.) 2 Corrosion and Protection Centre, The University of Manchester, Manchester M13 9PL, UK; [email protected] * Correspondence: [email protected]; Tel.: +86-187-2473-3900 Received: 19 January 2018; Accepted: 26 February 2018; Published: 28 February 2018 Abstract: The local electrochemical properties of galvanic corrosion for three coupled metals in a desalination plant were investigated with three wire-beam electrodes as wire sensors: aluminum brass (HAl77-2), titanium (TA2), and 316L stainless steel (316L SS). These electrodes were used with artificial seawater at different temperatures. The potential and current–density distributions of the three-metal coupled system are inhomogeneous. The HAl77-2 wire anodes were corroded in the three-metal coupled system. The TA2 wires acted as cathodes and were protected; the 316L SS wires acted as secondary cathodes. The temperature and electrode arrangement have important effects on the galvanic corrosion of the three-metal coupled system. The corrosion current of the HAl77-2 increased with temperature indicating enhanced anode corrosion at higher temperature. In addition, the corrosion of HAl77-2 was more significant when the HAl77-2 wires were located in the middle of the coupled system than with the other two metal arrangement styles. Keywords: desalination; galvanic corrosion; wire-beam electrode; heterogeneous electrochemistry; temperature; relative geometrical position 1. Introduction Water and energy shortages plague many communities around the world [1–5]. More than 1.2 billion people lack access to clean and safe drinking water [1,4]. This water shortage will likely increase in the coming decades. Therefore, much effort has been focused on desalination. Various desalination technologies—both thermally-driven and membrane-based—have been increasingly used to convert seawater to fresh water. Multi-effect desalination (MED) is one of the most common techniques [6]. A growing number of industrial plants have performed this type of thermal desalination in recent years [6–8]. It offers lower capital requirements and costs, simple operation/maintenance, higher thermal efficiency, higher heat-transfer coefficients, lower energy consumption, and higher performance ratios [7,8]. The key evaporators of MED technologies are made of aluminum brass, titanium, and stainless steel. They must offer long-term stability even with corrosive seawater. Indeed, distillation equipment is susceptible to salt corrosion at high temperature [9]; galvanic corrosion is also inevitable in these metals and manifests gradually over the lifetime of a desalination plant. Galvanic corrosion easily induces and accelerates other types of localized corrosion and can pose a serious threat to the safe operation of the desalination plants. Somewhat surprisingly, there is little documentation on multi-metal galvanic corrosion—especially in desalination plants. Therefore, a comprehensive study into the behavior and Materials 2018, 11, 357; doi:10.3390/ma11030357 www.mdpi.com/journal/materials Materials 2018, 11, 357 2 of 23 mechanism of multi-metal galvanic corrosion—particularly the smallest variations in electrochemical signals—remains an important study area. Galvanic corrosion is an enhanced corrosion between two or more kinds of electrically connected metals [10] in which the more active metal acts as the anode, and the less active metal is the cathode [11]. Galvanic corrosion is common in municipal infrastructure and industry [12] and is a common research topic. The galvanic corrosion rate and the potential distribution over a galvanic couple depend on the electrochemicalMaterials 2018 properties, 11, x FOR PEER of REVIEW the metals; environmental variables such as temperature,2 salinity,of 24 oxygen content, and solution flow; and the geometry of the corroding system [13]. Therefore, determining behavior and mechanism of multi-metal galvanic corrosion—particularly the smallest variations in the correlationelectrochemical between signals—remains electrochemical an important parameters study area. underlying galvanic corrosion and the corrosion factors is importantGalvanic corrosion to clarifying is an enhanced the behavior corrosion and between mechanism two or more of kinds the galvanic of electrically corrosion. connected To date,metals several[10] in which established the more active electrochemical metal acts as the techniques anode, and the have less beenactive usedmetal is for the this cathode purpose. [11]. However, the electrochemicalGalvanic corrosion processes is common of galvanicin municipal corrosion infrastructure are and usually industry inhomogeneous. [12] and is a common Time-honored research topic. The galvanic corrosion rate and the potential distribution over a galvanic couple electrochemicaldepend on techniques the electrochemical that useprop aerties single of the conventional metals; environmenta electrodel variables are limitedsuch as temperature, to determining these localizedsalinity, corrosion oxygen processes. content, and To solution overcome flow; and this the problem, geometry of multi-electrodes the corroding system (also [13]. known Therefore, as wire-beam electrodedetermining (WBE)) werethe correlation first developed between electrochemica in 1991 byl Tanparameters [14,15 underlying]. The WBE galvanic has beencorrosion used and in corrosion researchthe for corrosion several factors years is important [16–22]. to The clarifying remarkable the behavior feature and mechanism of this method of the galvanic is that corrosion. potential/current To date, several established electrochemical techniques have been used for this purpose. However, distributionsthe electrochemical can be measured processes under of galvanic complex corrosion surface are usually conditions inhomogeneous. [23]. Each Time-honored wire in a WBE is an individualelectrochemical electrochemical techniques sensor. that use This a single enables conven ational WBE electrode to measure are limited electrochemical to determining parameters these from local areaslocalized of the corrosion electrode processes. surface To via overcome wires this located problem, in thesemulti-electrodes areas. Therefore, (also known the as electrodewire-beam array could be usedelectrode equally (WBE)) wellto were investigate first develo theped in galvanic 1991 by Tan corrosion [14,15]. The of couplesWBE has been composed used in corrosion of different metals research for several years [16–22]. The remarkable feature of this method is that potential/current or alloys.distributions can be measured under complex surface conditions [23]. Each wire in a WBE is an Thisindividual work will electrochemical investigate sensor the influence. This enables of temperaturea WBE to measure and electrochemical geometrical parameters arrangement from for mapping the galvaniclocal areas corrosion of the ofelectrode three coupledsurface via metals wires locate in desalinationd in these areas. plant Therefore, using the an electrode advanced array WBE method. The distributioncould be used of both equally the well potential to investigate and thethe galvanic current corrosion density of with couples different composed temperatures of different and metal metals or alloys. arrangementsThis of work electrode will investigate arrays were the influence obtained. of Thistemperature provides and valuable geometrical information arrangement on for the localized corrosionmapping process. the galvanic This work corrosion studied of three multiple coupled metals metals in desalination and their plant galvanic using an corrosion advanced WBE in desalination plants. Itmethod. also provides The distribution important of both theoretical the potential values and the with current practical density significance with different for temperatures the structural design of safe futureand metal desalination arrangements devices. of electrode arrays were obtained. This provides valuable information on the localized corrosion process. This work studied multiple metals and their galvanic corrosion in desalination plants. It also provides important theoretical values with practical significance for the 2. Materialsstructural and design Methods of safe future desalination devices. 2.1. WBE2. FabricationMaterials and Methods The2.1. WBE WBE was Fabrication fabricated from 96 wires arranged in an 8 × 12 matrix (1-mm diameter). These were embedded inThe an WBE epoxy was resinfabricated at 1-mm from 96 intervals wires arranged (Figure in an1). 8 The× 12 matrix 96 microelectrodes (1-mm diameter). in These the WBE were fabricatedwere from embedded aluminum in an epoxy brass resin (HAl77-2), at 1-mm 316Lintervals stainless (Figure steel1). The (316L 96 microelectrodes SS), and titanium in the WBE (TA2). The area of the 96were microelectrodes fabricated from wasaluminum approximately brass (HAl77-2), 0.75 316L cm stainless2. The chemicalsteel (316L compositionSS), and titanium of (TA2). the real samples (HAl77-2,The 316L area of SS, the and 96 microelectrodes TA2) are given was in approximately Table1. 0.75