Mapping the Galvanic Corrosion of Three Metals Coupled with a Wire Beam Electrode: the Inﬂuence of Temperature and Relative Geometrical Position

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 cm2. The chemical composition of the real samples (HAl77-2, 316L SS, and TA2) are given in Table 1.

Figure 1. Schematic of a wire-beam electrode. Figure 1. Schematic of a wire-beam electrode.

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Materials 2018, 11, x FOR PEER REVIEW 3 of 24 Table 1. Composition of HAl77-2, 316L SS, and TA2. Table 1. Composition of HAl77-2, 316L SS, and TA2.

CompositionComposition CuCu C C Zn Zn HAl77-2HAl77-2 wt %% 59.3359.33 0.930.93 39.7439.74 CompositionComposition CC Si Si Mn Mn S S P P Ni Ni Mo Mo Cr Cr CuCu Fe Fe 316L316L SS SS wt %% 0.020.02 0.330.33 1.51.5 0.020.02 0.0290.029 10.110.1 2.12.1 16.716.7 0.40.4 68.80 68.80 CompositionComposition CC Si Si Ti Ti TA2TA2 wt %% 0.260.26 1.061.06 98.6898.68

Three different WBEs were used (Figure 2): WBE1 (HAl77-2/316L SS/TA2); WBE2 (HAl77- Three different WBEs were used (Figure2): WBE1 (HAl77-2/316L SS/TA2); WBE2 (HAl77-2/ 2/TA2/316L SS); and WBE3 (TA2/HAl77-2/316L SS). The surface of the WBE was polished with TA2/316L SS); and WBE3 (TA2/HAl77-2/316L SS). The surface of the WBE was polished with increasingly fine sandpaper (500- to 1200-grit). It was then cleaned with acetone and deionized water. increasingly ﬁne sandpaper (500- to 1200-grit). It was then cleaned with acetone and deionized water.

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Figure 2. Schematic of the three WBEs with different metal arrangement sequences: (a) WBE1, Figure 2. Schematic of the three WBEs with different metal arrangement sequences: (a) WBE1, HAl77-2/316L SS/TA2; (b) WBE2, HAl-77-2/TA2/316L SS; and (c) WBE3, TA2/HAl77-2/316L SS HAl77-2/316L SS/TA2; (b) WBE2, HAl-77-2/TA2/316L SS; and (c) WBE3, TA2/HAl77-2/316L SS (● Aluminum-brass (HAl77-2); ○Titanium (TA2); Ø Stainless steel (316L SS)). ( Aluminum-brass (HAl77-2); Titanium (TA2); Ø Stainless steel (316L SS)). 2.2. Experimental Conditions # 2.2. Experimental Conditions WBE experiments used artificial seawater at varying temperatures. The concentrations of Cl− − wereWBE 1.5 experiments wt % or 2.7 used wt %. artificial The constituents seawater at were varying NaCl temperatures. (19.46 g/L, Shanghai, The concentrations China), MgCl of Cl2·6Hwere2O 1.5 wt(8.80 % g/L, or 2.7 Shanghai, wt %. The China), constituents Na2SO4 (3.25 were g/L, NaCl Shanghai, (19.46 g/L, China), Shanghai, and CaCl China),2 (0.92 MgCl g/L, Shanghai,2·6H2O(8.80 China) g/L , Shanghai,with a China),Cl− concentration Na2SO4 (3.25 of 1.5 g/L, wt Shanghai,%. The second China), formulation and CaCl 2was(0.92 NaCl g/L, (35.26 Shanghai, g/L), China)MgCl2·6H with2O a − Cl (16.03concentration g/L), Na2 ofSO 1.54 (5.80 wt %.g/L), The and second CaCl2 formulation (1.67 g/L) for was a Cl NaCl− concentration (35.26 g/L), of MgCl 2.7 wt2· 6H%. 2TheO (16.03 artificial g/L), − Na2seawaterSO4 (5.80 was g/L), prepared and CaCl with2 (1.67 ultrapure g/L) forwater a Cl andconcentration A.R. reagents of in 2.7 accordance wt %. The with artificial the national seawater wasstandard. prepared The with experiments ultrapure were water conducted and A.R. at 30, reagents 40, 50, in60, accordanceand 70 °C. with the national standard. The experimentsOpen-circuit were potential conducted (OCP), at 30, local 40, 50, potential, 60, and 70and◦C. local current were measured by WBE technologyOpen-circuit [12,17,22]. potential The (OCP),equipment local consisted potential, of a and chassis local (PXI-1033, current wereNI™, measuredAustin, TX, by USA), WBE technologya 7.5-digit [ 12digital,17,22 multimeter]. The equipment (PXI-4071, consisted NI™, Austin, of a TX, chassis USA), (PXI-1033, an effective NI guard/current™, Austin, amplifier TX, USA), a 7.5-digit(PXI-4022, digital NI™, multimeter Austin, TX, (PXI-4071, USA), and NI a™ high-densi, Austin, TX,ty FET USA), switch an effectivematrix module guard/current (PXI-2535, amplifier NI™, (PXI-4022,Austin, NITX,™ USA)., Austin, The TX, performance USA), and metrics a high-density of these FETcomponents switch matrix have been module reported (PXI-2535, in detail NI™ , Austin,[12,17,22]. TX, USA). The performance metrics of these components have been reported in detail [12,17,22]. All wire sensors were disconnected for approximately 30 min to monitor their open circuit All wire sensors were disconnected for approximately 30 min to monitor their open circuit potential (OCP) values. After reaching a stable OCP, the WBEs were short-circuited for 12 h, and the potential (OCP) values. After reaching a stable OCP, the WBEs were short-circuited for 12 h, and the local potential and current measurements were collected separately. Each WBE electrode wire was local potential and current measurements were collected separately. Each WBE electrode wire was temporarily separated from the other connected electrodes to record the local potential in the temporarily separated from the other connected electrodes to record the local potential in the sequence sequence against a saturated calomel electrode (SCE). These were then reconnected to the other againstelectrodes. a saturated Individual calomel wire electrode sensors short-circuited (SCE). These the were electrodes then reconnected described above. to the These other were electrodes. first Individualseparated wire and sensors connected short-circuited to a device to the record electrodes the local described current; above.all other These wires were were firstshorted separated together. and connectedThese processes to a device were to record controlled the local by current;a computer all other and wiresvia self-designed were shorted software, together. in These a LabVIEW processes were controlled by a computer and via self-designed software, in a LabVIEW environment. Finally,

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Materials 2018, 11, x FOR PEER REVIEW 4 of 24 the potential and local current distributions were mapped with Surfer 8.0 software (Golden, CO, USA)environment. [22]. Each Finally, experiment the potential was repeated and local over current three distributions parallel runs were with mapped unique with samples Surfer 8.0 to verifysoftware the (Golden, CO, USA) [22]. Each experiment was repeated over three parallel runs with unique samples accuracy of the experiment. to verify the accuracy of the experiment. 3. Results and Discussion 3. Results and Discussion 3.1. Effect of Temperature on the Corrosion Behavior of Three-Metal Coupled Systems 3.1. Effect of Temperature on the Corrosion Behavior of Three-Metal Coupled Systems The temperature-effect investigation on the corrosion behavior of the three-metal coupled systems The temperature-effect investigation on the corrosion behavior of the three-metal coupled systems was performed in artificial seawater with a chloride ion concentration of 2.7 wt %. was performed in artificial seawater with a chloride ion concentration of 2.7 wt %. 3.1.1. Corrosion Behavior of WBE1 in Artificial Seawater at Different Temperatures 3.1.1. Corrosion Behavior of WBE1 in Artificial Seawater at Different Temperatures OCP distribution maps of the WBE1 after immersion in artificial seawater at different temperatures OCP distribution maps of the WBE1 after immersion in artificial seawater at different temperatures are shown in Figure3 (the 2D maps also are shown in Figure S1 of the Supplementary Materials). are shown in Figure 3 (the 2D maps also are shown in Figure S1 of the Supplementary Material). The OCP value of HAl77-2 was the most negative of the three metals. The OCP values of 316L SS and The OCP value of HAl77-2 was the most negative of the three metals. The OCP values of 316L SS and TA2 were similar and were both higher than HAl77-2. The HAl77-2 was an anode, and 316L SS and TA2 were similar and were both higher than HAl77-2. The HAl77-2 was an anode, and 316L SS and TA2TA2 were were cathodes cathodes in thein the three-metal three-metal coupled coupled system. system.

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Figure 3. Open circuit potential OCP (vs. saturated calomel electrode (SCE)/V) distribution maps of Figure 3. Open circuit potential OCP (vs. saturated calomel electrode (SCE)/V) distribution maps of the WBE1 (HAl77-2/316L SS/TA2) after immersion in artificial seawater at different temperatures: the WBE1 (HAl77-2/316L SS/TA2) after immersion in artiﬁcial seawater at different temperatures: (a) 30 °C; (b) 40 °C; (c) 50 °C; (d) 60 °C; and (e) 70 °C. (a) 30 ◦C; (b) 40 ◦C; (c) 50 ◦C; (d) 60 ◦C; and (e) 70 ◦C. The average OCP distribution of each microelectrode line for WBE1 after immersion in artificial seawaterThe average at different OCP distributiontemperatures ofis eachpresented microelectrode in Figure 4. lineFigure for 4 WBE1shows afterthe OCP immersion values of in HAl77-2, artiﬁcial seawater316L SS, at differentand TA2 temperatureshad constant isnegative presented shifts in Figurewith increasing4. Figure 4temperature. shows the OCP This values suggests of HAl77-2,that the

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tendencyMaterials of 2018 corrosion, 11, x FOR PEERin metals REVIEW increases with temperature because the mass-transfer rate5 increasedof 24 316Lwith SS, temperature; and TA2 had the constantdiffusion negative of dissolve shiftsd oxygen with increasing also accelerated temperature. with temperature. This suggests that the tendencytendency of corrosion of corrosion in metalsin metals increases increases with with temperaturetemperature because because the the mass mass-transfer-transfer rate rateincreased increased with temperature;with temperature; the the diffusion diffusion of of dissolved dissolved oxygen oxygen alsoalso accelerated with with temperature. temperature.

Figure 4. Average OCP distribution of each microelectrode line for WBE1 after immersion in artificial Figure 4. Average OCP distribution of each microelectrode line for WBE1 after immersion in artiﬁcial Figureseawater 4. Average at different OCP temperatures. distribution of each microelectrode line for WBE1 after immersion in artificial seawater at different temperatures. seawater at different temperatures. Figure 5 shows the spatial potential and current–density distribution maps of WBE1 after being Figureshort-circuitedFigure5 shows5 shows for the the12 spatial hspatial in artificial potential potential seawater and and current–density at current– differentdensity temperatures distribution distribution (the 2D maps maps maps of ofalso WBE1 WBE1 are aftershown after being being short-circuitedshort-circuitedin Figure S2 for offor the 12 12 Supplementary h h in in artiﬁcial artificial seawaterMaterials). seawater at Theat different different data indicate temperatures temperatures that the potential (the (the 2D 2D ofmaps HAl77-2maps also also is lowest. are are shown shown Its corresponding local current density is the most positive of the three metals. Therefore, HAl77-2 is inin Figure Figure S2S2 of the the Supplementary Supplementary Materials). Materials). The The data data indicate indicate that that the potential the potential of HAl77-2 of HAl77-2 is lowest. is a corrosion anode in WBE1. The potential of TA2 is the most positive of the three metals. Its lowest.Its corresponding Its corresponding local current local density current is density the most is thepositive most of positive the three of themetals three. Therefore, metals. Therefore,HAl77-2 is corresponding current density is negative. Thus, TA2 is the cathode in the coupling system. The local HAl77-2 is a corrosion anode in WBE1. The potential of TA2 is the most positive of the three metals. a corrosionpotential anodevalue of in 316L WBE1. SS is The between potential HAl77-2 of andTA2 TA2. is the Most most 316L positive SS wires of had the negative three metals.local Its Itscorresponding correspondingcurrents; only current a current few 316Ldensity density SS wires is negative. is had negative. positive Thus, loca Thus, TA2l currents. is TA2 the cathode is Therefore, the cathode in thesome coupling in of thethe 316L coupling system. SS wires The system. local Thepotential localwere potential cathodes,value of and value316L others ofSS 316L wereis between SSlocal is betweenanodes. HAl77-2 HAl77-2 and TA2. and Most TA2. Most316L 316LSS wires SS wires had hadnegative negative local localcurrents; currents; only onlya few a few316L 316L SS wires SS wires had hadpositive positive local local currents. currents. Therefore, Therefore, some some of the of 316L the 316LSS wires SS wireswere werecathodes, cathodes, and others and others were were local localanodes. anodes.

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Figure 5. Cont.

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(b)

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(d)

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Figure 5. Spatial potential (left, E vs. SCE/V) and current–density (right, I/A) distribution maps of Figure 5. Spatial potential (left, E vs. SCE/V) and current–density (right, I/A) distribution maps of WBE1 after being short-circuited for 12 h in artiﬁcialartificial seawater at different temperatures: ( aa)) 30 30 °C;◦C; (b) 40 ◦°C;C; ( c) 50 °C;◦C; ( d)) 60 60 °C;◦C; and and ( (ee)) 70 70 °C.◦C.

Figure 6 shows the average potential and current–density distributions of each WBE1 microelectrode Figure6 shows the average potential and current–density distributions of each WBE1 line after being short-circuited for 12 h in artificial seawater at different temperatures. The local microelectrode line after being short-circuited for 12 h in artiﬁcial seawater at different temperatures. potential of the three metals drops with increasing temperature. The local current of HAl77-2 The local potential of the three metals drops with increasing temperature. The local current of HAl77-2 increases with temperature indicating that the anode corrosion is of greater significance when the increases with temperature indicating that the anode corrosion is of greater signiﬁcance when the temperature of the solution increases. temperature of the solution increases.

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Figure 6. (a) Average potential; and (b) current–density distributions of each WBE1 microelectrode Figure 6. (a) Average potential; and (b) current–density distributions of each WBE1 microelectrode line after being short-circuited for 12 h in artificial seawater at different temperatures. line after being short-circuited for 12 h in artiﬁcial seawater at different temperatures.

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3.1.2. Corrosion Behavior of WBE2 in ArtificialArtificial Seawater at Different TemperaturesTemperatures Figure 77 shows shows OCPOCP distributiondistribution mapsmaps ofof thethe WBE2WBE2 afterafter immersionimmersion inin artificialartificial seawaterseawater atat different temperatures. The OCP valuevalue of HAl77-2 is the most negative of the three metals. The OCP values of 316L316L SSSS andand TA2TA2 areare similar, similar, and and both both are are higher higher than than HAl77-2. HAl77-2. Therefore, Therefore, HAl77-2 HAl77-2 results results in anodicin anodic corrosion, corrosion, and and 316L 316L SS SS and and TA2 TA2 are are cathodes cathodes in in the the three-metal three-metalcoupled coupled systemsystem inin artificialartificial seawater. In addition, the OCP distribution of HAl77-2HAl77-2 is homogeneous, and the OCP distribution of 316L SS and TA2 fluctuantfluctuant because the passive filmfilm on the metal surface is damaged by the chloride ions. These results are similar to WBE1 findings.findings.

(a) (b)

(e)

Figure 7. OCP (vs. SCE/V) distribution maps of the WBE2 after immersion in artificial seawater at Figure 7. OCP (vs. SCE/V) distribution maps of the WBE2 after immersion in artiﬁcial seawater at different temperatures: (a) 30 °C; (b) 40 °C; (c) 50 °C; (d) 60 °C; and (e) 70 °C. different temperatures: (a) 30 ◦C; (b) 40 ◦C; (c) 50 ◦C; (d) 60 ◦C; and (e) 70 ◦C.

Figure 8 shows the average OCP distribution of each microelectrode line for WBE2 after immersionFigure 8in shows artificial the averageseawater OCP at different distribution temperatures. of each microelectrode Figure 8 also line shows for WBE2 the after OCP immersion values of inHAl77-2; artiﬁcial both seawater 316L SS at and different TA2 shift temperatures. toward a negative Figure potential.8 also shows the OCP values of HAl77-2; both 316L SS and TA2 shift toward a negative potential.

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Figure 8. Average OCP distribution of each microelectrode line for WBE2 after immersion in artificial Figure 8. Average OCP distribution of each microelectrode line for WBE2 after immersion in artiﬁcial seawaterseawater atat differentdifferent temperatures.temperatures. Figure 8. Average OCP distribution of each microelectrode line for WBE2 after immersion in artificial Figureseawater 9 shows at different spatial temperatures. potential and current–density distribution maps of WBE2 after being Figure9 shows spatial potential and current–density distribution maps of WBE2 after being short-circuited for 12 h in artificial seawater at different temperatures. The potential of HAl77-2 is the short-circuitedFigure for9 shows 12 h in spatial artiﬁcial potential seawater and atcurrent–den differentsity temperatures. distribution Themaps potential of WBE2 of after HAl77-2 being is the most negative of the three metals. Its current density is positive indicating that HAl77-2 has anode mostshort-circuited negative of the for three12 h in metals. artificial Its seawater current at densitydifferent istemperatures. positive indicating The potential that of HAl77-2 HAl77-2 hasis the anode corrosionmost negativein WBE2. of TA2the three is the metals. cathode Its currentbecause density of its ispositive positive potential indicating and that negative HAl77-2 currenthas anode values. corrosion in WBE2. TA2 is the cathode because of its positive potential and negative current values. The potentialcorrosion inof WBE2. 316L SS TA2 is isbetween the cathode HAl77-2 because and of TAits positive2, but the potential current and perfor negativemance current of most values. 316L SS The potential of 316L SS is between HAl77-2 and TA2, but the current performance of most 316L SS wiresThe is negative.potential of This 316L reveals SS is between that 316L HAl77-2 SS is andthe secondaryTA2, but the cathode current perforin WBE2.mance of most 316L SS wireswires is negative. is negative. This This reveals reveals that that 316L 316L SS SS is is thethe secondarysecondary cathode cathode in in WBE2. WBE2.

(a) (a)

Figure 9. Cont.

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Figure 9. Spatial potential (left, E vs. SCE/V) and current–density (right, I/A) distribution maps of Figure 9. Spatial potential (left, E vs. SCE/V) and current–density (right, I/A) distribution maps of WBE2 after being short-circuited for 12 h in artificial seawater at different temperatures: (a) 30 °C; ◦ WBE2(b after) 40 °C; being (c) 50 short-circuited°C; (d) 60 °C; and for (e) 1270 h°C. in artiﬁcial seawater at different temperatures: (a) 30 C; (b) 40 ◦C; (c) 50 ◦C; (d) 60 ◦C; and (e) 70 ◦C. Figure 10 shows the average potential and current–density distributions of each WBE2 Figuremicroelectrode 10 shows line after the being average short-circuited potential for and 12 h current–densityin artificial seawater distributions at different temperatures. of each WBE2 microelectrodeThe potentials line of after the beingthree metals short-circuited obviously for have 12 ha innegative artiﬁcial shift seawater with increasing at different temperature. temperatures. The positive local current of HAl77-2 increases with temperature, whereas the negative local current The potentials of the three metals obviously have a negative shift with increasing temperature. of 316L SS and TA2 decreases with temperature. There is more anode and galvanic corrosion at higher The positivetemperature. local current of HAl77-2 increases with temperature, whereas the negative local current of 316L SS and TA2 decreases with temperature. There is more anode and galvanic corrosion at higher temperature. Materials 2018, 11, 357 11 of 23 Materials 2018, 11, x FOR PEER REVIEW 11 of 24

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FigureFigure 10.10. ((aa)) AverageAverage potentialpotential andand ((bb)) current–densitycurrent–density distributionsdistributions ofof eacheach WBE2WBE2 microelectrodemicroelectrode lineline afterafter beingbeing short-circuitedshort-circuited forfor 1212 hh inin artiﬁcialartificial seawaterseawater atat differentdifferent temperatures. temperatures.

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3.1.3. Corrosion Behavior ofof WBE3WBE3 inin ArtiﬁcialArtificial SeawaterSeawater atat DifferentDifferent TemperaturesTemperatures Figure 1111 showsshows OCPOCP distributiondistribution maps of thethe WBE3WBE3 afterafter immersionimmersion inin artiﬁcialartificial seawaterseawater atat different temperatures.temperatures. Similar Similar to to WBE2 WBE2 and and WBE3 WBE3,, HAl77-2 has the most negative OCP value; thethe OCPOCP valuesvalues ofof 316L316L SSSS andand TA2TA2 areare higherhigher thanthan HAl77-2.HAl77-2.

(a) (b)

(e)

Figure 11. OCP (vs. SCE/V) distribution maps of the WBE3 after immersion in artificial seawater at Figure 11. OCP (vs. SCE/V) distribution maps of the WBE3 after immersion in artiﬁcial seawater at different temperatures: (a) 30 °C; (b) 40 °C; (c) 50 °C; (d) 60 °C; and (e) 70 °C. different temperatures: (a) 30 ◦C; (b) 40 ◦C; (c) 50 ◦C; (d) 60 ◦C; and (e) 70 ◦C.

Figure 12 shows the average local potential and current–density distribution of each WBE3 microelectrodeFigure 12 showsline after the being average short-circuited local potential for 12 and h in current–density artificial seawater distribution at different of temperatures. each WBE3 microelectrodeThe OCPs of HAl77-2, line after 316L being SS, short-circuitedand TA2 shifted for negatively 12 h in artiﬁcial with increases seawater in at temperature different temperatures. because the Thereaction OCPs activity of HAl77-2, enhances 316L with SS, andincreasing TA2 shifted temperature. negatively with increases in temperature because the reaction activity enhances with increasing temperature.

Materials 2018, 11, 357 13 of 23 Materials 2018, 11, x FOR PEER REVIEW 13 of 24 Materials 2018, 11, x FOR PEER REVIEW 13 of 24

FigureFigure 12. 12.Average Average OCP OCP distribution distribution of of each each microelectrodemicroelectrode line for for WBE3 WBE3 after after immersion immersion inin artificial artificial seawaterFigureseawater 12. at atdifferent Average different OCP temperatures. temperatures. distribution of each microelectrode line for WBE3 after immersion in artificial seawater at different temperatures. Figure 13 shows spatial potential and current–density distribution maps of WBE3 after being Figure 13 shows spatial potential and current–density distribution maps of WBE3 after being short-circuitedFigure 13 showsfor 12 hspatial in artificial potential seawater and atcurrent–density different temperatures. distribution HAl77-2 maps hasof WBE3the most after negative being short-circuited for 12 h in artificial seawater at different temperatures. HAl77-2 has the most negative short-circuitedlocal potential infor WBE3; 12 h in itsartificial corresponding seawater localat different current temperatures. is positive indicating HAl77-2 thathas theHAl77-2 most negative behaves local potential in WBE3; its corresponding local current is positive indicating that HAl77-2 behaves localas the potential anode in in WBE3. WBE3; The its TA2corresponding has the highest local localcurrent potential is positive in WBE3, indicating but its that corresponding HAl77-2 behaves local asas thecurrent the anode anode is negative in in WBE3. WBE3. suggesting The The TA2 TA2 that has has TA2 the the highest ishighest a cathode locallocal in potential WBE3. The in in WBE3,local WBE3, potential but but its its corresponding of corresponding 316L SS is higher local local currentcurrentthan that is negativeis ofnegative HAl77-2 suggesting suggesting but is slightly that that TA2 TA2 lower is isa thana cathodecathode TA2 indicating in WBE3. The Thethat local local316L potential SS potential is the ofsecondary of 316L 316L SS SS iscathode. ishigher higher thanthanThese that that results of of HAl77-2 HAl77-2 are similar but but is isto slightly slightlythe findings lowerlower for thanthan WBE1 TA2 and indicating WBE2. that that 316L 316L SS SS is isthe the secondary secondary cathode. cathode. TheseThese results results are are similar similar to to the the findings findings for for WBE1 WBE1 andand WBE2.WBE2.

(a) (a)

Figure 13. Cont.

Materials 2018, 11, 357 14 of 23 Materials 2018, 11, x FOR PEER REVIEW 14 of 24

(b)

(c)

(d)

(e)

Figure 13. Spatial local potential (left, E vs. SCE/V) and current–density (right, I/A) distribution maps Figure 13. Spatial local potential (left, E vs. SCE/V) and current–density (right, I/A) distribution maps of WBE3 after being short-circuited for 12 h in artificial seawater at different temperatures: (a) 30 °C; of WBE3 after being short-circuited for 12 h in artiﬁcial seawater at different temperatures: (a) 30 ◦C; (b) 40 °C; (c) 50 °C; (d) 60 °C; and (e) 70 °C. (b) 40 ◦C; (c) 50 ◦C; (d) 60 ◦C; and (e) 70 ◦C. Figure 14 shows the average local potential and current–density distributions of each WBE3 microelectrodeFigure 14 shows line after the being average short-circuited local potential for 12 and h in current–density artificial seawater distributions at different temperatures. of each WBE3 microelectrodeThe local potential line after of the being three short-circuited metals shift negati for 12ve hwith in artiﬁcial increasing seawater temperature. at different The local temperatures. current Theof localHAl77-2 potential increases of with the temperature three metals indicating shift negative that corrosion with increasingbecomes more temperature. prominent at The higher local currenttemperatures. of HAl77-2 increases with temperature indicating that corrosion becomes more prominent at higher temperatures.

MaterialsMaterials2018 2018, 11, ,11 357, x FOR PEER REVIEW 15 15of 24 of 23

(a)

(b)

Figure 14. (a) Average local potential; and (b) current–density distributions of each WBE3 microelectrode Figure 14. (a) Average local potential; and (b) current–density distributions of each WBE3 line after being short-circuited for 12 h in artificial seawater at different temperatures. microelectrode line after being short-circuited for 12 h in artificial seawater at different temperatures. 3.2. Effect of the Relative Position of Metal on the Corrosion Behavior of Three Metal-Coupled Systems 3.2. Effect of the Relative Position of Metal on the Corrosion Behavior of Three Metal-Coupled Systems 3.2.1. Corrosion Behavior of Three Coupled Systems in Artificial Seawater: Cl− 1.5 wt % and 30 °C 3.2.1. Corrosion Behavior of Three Coupled Systems in Artificial Seawater: Cl− 1.5 wt % and 30 ◦C The OCP distributions of three coupled electrodes after immersion in artificial seawater with 1.5 wtThe % Cl OCP− at 30 distributions °C are shown of in three Figure coupled 15. Regardless electrodes of the after arrangement immersion of the inartificial three metals, seawater the OCP with − ◦ 1.5of wt HAl77-2 % Cl isat always 30 C arethe most shown nega intive, Figure followed 15. Regardless by 316L SS; of the the OC arrangementP of TA2 is always of the the three highest. metals, the OCP of HAl77-2 is always the most negative, followed by 316L SS; the OCP of TA2 is always the highest.

Materials 2018, 11, 357 16 of 23 Materials 2018, 11, x FOR PEER REVIEW 16 of 24

(a) WBE1 (b) WBE2

Figure 15. OCP (vs. SCE/V) distribution maps of three coupled electrodes after immersion in artificial Figure 15. OCP (vs. SCE/V) distribution maps of three coupled electrodes after immersion in seawater: 1.5% (Cl−, wt %) and 30 °C: (a) WBE1, HAl77-2/316L SS/TA2; (b) WBE2, HAl-77-2/TA2/316L artificial seawater: 1.5% (Cl−, wt %) and 30 ◦C: (a) WBE1, HAl77-2/316L SS/TA2; (b) WBE2, SS; and (c) WBE3, TA2/HAl77-2/316L SS. HAl-77-2/TA2/316L SS; and (c) WBE3, TA2/HAl77-2/316L SS. Figure 16 shows the average OCP distribution of each microelectrode line for the three coupled Figureelectrodes 16 showsafter immersion the average in artificial OCP distributionseawater with of1.5 each wt % microelectrode Cl− at 30 °C. The lineOCP forvalues the of three HAl77-2 coupled electrodesin WBE3 after are immersion lower than in those artificial in WBE1 seawater and WBE2. with 1.5The wt potential % Cl− ofat 316L 30 ◦C. SS The was OCP between values the ofvalues HAl77-2 of the HAl77-2 and the TA2, accompanied by numerical fluctuations. In addition, the OCP in WBE3 are lower than those in WBE1 and WBE2. The potential of 316L SS was between the values of distribution maps of 316L SS and TA2 were more inhomogeneous than those of HAl77-2, possibly the HAl77-2 and the TA2, accompanied by numerical fluctuations. In addition, the OCP distribution because of the effect of chloride ions on the passivation films of 316L SS and TA2 in artificial seawater. maps of 316L SS and TA2 were more inhomogeneous than those of HAl77-2, possibly because of the effect of chlorideMaterials 2018 ions, 11, onx FOR the PEER passivation REVIEW films of 316L SS and TA2 in artificial seawater.17 of 24

Figure 16.FigureAverage 16. Average OCP distribution OCP distribution of eachof each microelectrode microelectrode line line for forthe three the threecoupled coupled electrodes electrodes after after immersion in artificial seawater with 1.5 wt % Cl− at 30 °C. immersion in artiﬁcial seawater with 1.5 wt % Cl− at 30 ◦C. Figure 17 shows the spatial potential and current–density distribution maps of the three coupled electrodes after being short-circuited for 12 h in artificial seawater with 1.5 wt % Cl− at 30 °C. The findings in Figure 17 suggest that the local potential of HAl77-2 is always the most negative regardless of the sequence of the coupled systems. The local current remains positive indicating that HAl77-2 is the anode in all three coupled systems. Figure 18 presents the average potential and current–density distributions of each microelectrode line after being short-circuited for 12 h in artificial seawater with 1.5 wt % Cl− at 30 °C. The potential of HAl77-2 is the most negative when HAl77-2 is between TA2 and 316L SS in the WBE3. The potential of HAl77-2 is more positive when HAl77-2 is at the edge and next to the TA2 in WBE2. The average current of HAl77-2 in WBE3 behaves at the maximum value, and anode corrosion is most significant when HAl77-2 is in the middle of the coupled system. Moreover, the current values of the fifth and eighth lines of WBE3 were significantly higher than those of the sixth and seventh lines, which reveals that the corrosion at the interface between the anode and the cathode is more serious than that away from the interface.

(a) WBE1

Materials 2018, 11, x FOR PEER REVIEW 17 of 24

Materials 2018, 11, 357 17 of 23 Figure 16. Average OCP distribution of each microelectrode line for the three coupled electrodes after immersion in artificial seawater with 1.5 wt % Cl− at 30 °C. Figure 17 shows the spatial potential and current–density distribution maps of the three coupled − ◦ electrodesFigure after 17 beingshows short-circuitedthe spatial potential for and 12 current–d h in artificialensity seawaterdistribution with maps 1.5 of the wt three % Cl coupledat 30 C. Theelectrodes findings inafter Figure being 17 short-circuited suggest that for the 12 local h in potential artificial ofseawater HAl77-2 with is always1.5 wt % the Cl most− at 30 negative °C. regardlessThe findings of the in sequence Figure 17 ofsuggest the coupled that the systems.local potential The localof HAl77-2 current is always remains the positive most negative indicating thatregardless HAl77-2 isof thethe anodesequence in of all the three coupled coupled systems. systems. The local Figure current 18 presents remains thepositive average indicating potential that and current–densityHAl77-2 is the distributions anode in all of three each coupled microelectrode systems. line Figure after 18 being presents short-circuited the average for potential 12 h in artificialand seawatercurrent–density with 1.5 wt distributions % Cl− at 30 of◦ C.each The microelectro potential ofde HAl77-2 line after is being the most short-circuited negative when for 12 HAl77-2 h in is betweenartificial TA2 seawater and 316L with SS 1.5 in wt the % WBE3. Cl− at The30 °C. potential The potential of HAl77-2 of HAl77-2 is more is the positive most negative when HAl77-2 when is at theHAl77-2 edge andis between next to TA2 the and TA2 316L in WBE2. SS in the The WBE3. average The potential current ofof HAl77-2 is in more WBE3 positive behaves when at the HAl77-2 is at the edge and next to the TA2 in WBE2. The average current of HAl77-2 in WBE3 behaves maximum value, and anode corrosion is most significant when HAl77-2 is in the middle of the coupled at the maximum value, and anode corrosion is most significant when HAl77-2 is in the middle of the system. Moreover, the current values of the fifth and eighth lines of WBE3 were significantly higher coupled system. Moreover, the current values of the fifth and eighth lines of WBE3 were significantly thanhigher those than of the those sixth of and the seventhsixth and lines, seventh which lines, reveals which that reveals the corrosion that the corrosion at the interface at the interface between the anodebetween and the the cathodeanode and is morethe cathode serious is thanmore thatserious away than from that theaway interface. from the interface.

Materials 2018, 11, x FOR PEER REVIEW 18 of 24 (a) WBE1

(b) WBE2

Figure 17. Spatial potential (left, E vs. SCE/V) and current–density (right, I/A) distribution maps of Figure 17. Spatial potential (left, E vs. SCE/V) and current–density (right, I/A) distribution maps three coupled electrodes after being short-circuited for 12 h in artificial seawater with 1.5 wt % Cl− at of three coupled electrodes after being short-circuited for 12 h in artiﬁcial seawater with 1.5 wt % 30 °C: (a) WBE1, HAl77-2/316L SS/TA2; (b) WBE2, HAl-77-2/TA2/316L SS; and (c) WBE3, TA2/HAl77- − ◦ a b c Cl 2/316Lat 30 SS.C: ( ) WBE1, HAl77-2/316L SS/TA2; ( ) WBE2, HAl-77-2/TA2/316L SS; and ( ) WBE3, TA2/HAl77-2/316L SS.

(a)

Materials 2018, 11, x FOR PEER REVIEW 18 of 24

(b) WBE2

Figure 17. Spatial potential (left, E vs. SCE/V) and current–density (right, I/A) distribution maps of three coupled electrodes after being short-circuited for 12 h in artificial seawater with 1.5 wt % Cl− at 30 °C: (a) WBE1, HAl77-2/316L SS/TA2; (b) WBE2, HAl-77-2/TA2/316L SS; and (c) WBE3, TA2/HAl77- Materials2/316L2018 ,SS.11, 357 18 of 23

Materials 2018, 11, x FOR PEER REVIEW 19 of 24 (a)

(b)

FigureFigure 18. 18. ((a)) Average potential;potential; and and ( b()b current–density) current–density distributions distributions of eachof each microelectrode microelectrode line afterline − afterbeing being short-circuited short-circuited for 12 for h 12 in artiﬁcialh in artificial seawater seawater with with 1.5 wt 1.5 % wt Cl −% atCl 30 at◦ C.30 °C.

3.2.2. Corrosion Behavior of Three Coupled Systems in Artificial Seawater: Cl− 2.7 wt % and 70 °C 3.2.2. Corrosion Behavior of Three Coupled Systems in Artiﬁcial Seawater: Cl− 2.7 wt % and 70 ◦C Figure 19 shows the OCP distribution maps of the three electrodes (WBE1, WBE2, and WBE3) in Figure 19 shows the OCP distribution maps of the three electrodes (WBE1, WBE2, and WBE3) in artificial seawater with 2.7 wt % Cl− at 70 °C. The OCP value of HAl77-2 is obviously always the most artiﬁcial seawater with 2.7 wt % Cl− at 70 ◦C. The OCP value of HAl77-2 is obviously always the most negative in WBE1, WBE2, and WBE3. negative in WBE1, WBE2, and WBE3.

(a) WBE1 (b) WBE2

Figure 19. The OCP distribution maps of the three electrodes after immersion in artificial seawater with 2.7 wt % Cl− at 70 °C: (a) WBE1, HAl77-2/316L SS/TA2; (b) WBE2, HAl-77-2/TA2/316L SS; and (c) WBE3, TA2/HAl77-2/316L SS.

Materials 2018, 11, x FOR PEER REVIEW 19 of 24

(b)

Figure 18. (a) Average potential; and (b) current–density distributions of each microelectrode line after being short-circuited for 12 h in artificial seawater with 1.5 wt % Cl− at 30 °C.

3.2.2. Corrosion Behavior of Three Coupled Systems in Artificial Seawater: Cl− 2.7 wt % and 70 °C Figure 19 shows the OCP distribution maps of the three electrodes (WBE1, WBE2, and WBE3) in − Materialsartificial2018 seawater, 11, 357 with 2.7 wt % Cl at 70 °C. The OCP value of HAl77-2 is obviously always the19 most of 23 negative in WBE1, WBE2, and WBE3.

(a) WBE1 (b) WBE2

Figure 19. The OCP distribution maps of the three electrodes after immersion in artificial seawater Figure 19. The OCP distribution maps of the three electrodes after immersion in artiﬁcial seawater − withwith 2.72.7 wtwt %% Cl− atat 70 70 °C:◦C: (a (a) )WBE1, WBE1, HAl77-2/316L HAl77-2/316L SS/TA2; SS/TA2; (b) (bWBE2,) WBE2, HAl-77-2/TA2/316L HAl-77-2/TA2/316L SS; and SS; Materialsand(c) 2018WBE3, (c), WBE3, 11 , TA2/HAl77-2/316Lx FOR TA2/HAl77-2/316L PEER REVIEW SS. SS. 20 of 24

Figure 2020 shows the average OCP distribution of each microelectrode line for the three electrodes − after immersionimmersion inin artiﬁcialartificial seawaterseawater withwith 2.72.7 wtwt %% ClCl− at 70 °C.◦C. The OCP value of HAl77-2 in WBE3 isis lowerlower thanthan thatthat inin WBE1WBE1 andand WBE2.WBE2.

Figure 20. Average OCP distribution of each microelectrode line for the three electrodes after immersion inin artiﬁcialartificial seawaterseawater withwith 2.72.7 wtwt %% ClCl−− atat 70 70 °C.◦C.

Figure 21 shows the spatial potential and current–density distribution maps of three electrodes after being short-circuited for 12 h in artificial seawater with 2.7 wt % Cl− at 70 °C. Regardless of changes in the sequence, the local potential of HAl77-2 is always the most negative, and its corresponding current is positive, indicating that HAl77-2 is a corrosion anode relative to the other two metals in the three coupled systems.

(a) WBE1

(b) WBE2

Materials 2018, 11, x FOR PEER REVIEW 20 of 24

Figure 20 shows the average OCP distribution of each microelectrode line for the three electrodes after immersion in artificial seawater with 2.7 wt % Cl− at 70 °C. The OCP value of HAl77-2 in WBE3 is lower than that in WBE1 and WBE2.

MaterialsFigure2018 20., 11 Average, 357 OCP distribution of each microelectrode line for the three electrodes after immersion20 of 23 in artificial seawater with 2.7 wt % Cl− at 70 °C.

FigureFigure 21 shows the spatial potential and current–d current–densityensity distribution distribution maps maps of of three three electrodes electrodes − ◦ afterafter being being short-circuited short-circuited for for 12 12 h h in in artificial artiﬁcial seawater seawater with with 2.7 2.7 wt wt % % Cl Cl− at at70 70°C. C.Regardless Regardless of changesof changes in inthe the sequence, sequence, the the local local potential potential of ofHAl77-2 HAl77-2 is is always always the the most most negative, negative, and and its correspondingcorresponding current current is is positive, positive, indicating indicating that that HAl77-2 HAl77-2 is is a a corrosion corrosion anode anode relative relative to to the the other twotwo metals metals in in the the three three coupled systems.

(a) WBE1

Materials 2018, 11, x FOR PEER REVIEW 21 of 24 (b) WBE2

FigureFigure 21. 21. SpatialSpatial potential potential (left, (left E ,vs. E SCE/V) vs. SCE/V) and curre andnt–density current–density (right, I/A) (right distribution, I/A) distribution maps of − threemaps electrodes of three electrodesafter being aftershort-circuited being short-circuited for 12 h in artificial for 12 hseawater in artificial with seawater2.7 wt % Cl with at 70 2.7 °C: wt (a %) WBE1,Cl− at HAl77-2/316L 70 ◦C: (a) WBE1, SS/TA2; HAl77-2/316L (b) WBE2, HAl-77-2/TA2/316L SS/TA2; (b) WBE2, SS; HAl-77-2/TA2/316L and (c) WBE3, TA2/HAl77-2/316L SS; and (c) WBE3, SS. TA2/HAl77-2/316L SS. Figure 22 shows the average potential and the current–density distributions of each microelectrode line after being short-circuited for 12 h in artificial seawater with 2.7 wt % Cl− at 70 °C. Figure 22 shows the average potential and the current–density distributions of each microelectrode In the coupling WBE1 system, when the anodic HAl77-2 wires are located at the edge and are line after being short-circuited for 12 h in artificial seawater with 2.7 wt % Cl− at 70 ◦C. In the coupling connected to the 316L SS wires, the current value of the HAl77-2 wires decreases from the first-line WBE1 system, when the anodic HAl77-2 wires are located at the edge and are connected to the 316L to the fourth-line wires, and the fifth-line wires (316L SS) display anodic potential and current. As a SS wires, the current value of the HAl77-2 wires decreases from the first-line to the fourth-line wires, result, the main anodic area enlarges and is even adjacent to the 316L SS wires near the HAl77-2. In and the fifth-line wires (316L SS) display anodic potential and current. As a result, the main anodic the coupling WBE2 system, when the anodic HAl77-2 wires are also located at the edge and are area enlarges and is even adjacent to the 316L SS wires near the HAl77-2. In the coupling WBE2 system, connected with cathodic TA2 directly, the fourth-line (HAl77-2) higher current values are obtained, when the anodic HAl77-2 wires are also located at the edge and are connected with cathodic TA2 indicating a character of local anodic corrosion. The corrosion potential of HAl77-2 in WBE3 is the lowest, and the corrosion current density is the highest. Therefore, the corrosion of HAl77-2 is the most serious when the HAl77-2 is between 316L SS and TA2. In the corrosion cell, the electronic cathode can receive electrons from the anode and form a loop. When the anodic HAl77-2 is in the middle, the two cathode metals (TA2 and 316L SS) are evenly distributed around the anode. It has a shorter distance and less resistance between the cathode and the anode. Thus, the corrosion of HAl77-2 in WBE3 is more significant than in WBE1 and WBE2. Moreover, the current values of the fifth and eighth lines in WBE3 are significantly higher than those in the sixth and seventh lines. This shows that corrosion at the TA2/HAl77-2 and HAl77-2/316L interfaces are more serious than if they were away from the interface.

Materials 2018, 11, 357 21 of 23 directly, the fourth-line (HAl77-2) higher current values are obtained, indicating a character of local anodic corrosion. The corrosion potential of HAl77-2 in WBE3 is the lowest, and the corrosion current density is the highest. Therefore, the corrosion of HAl77-2 is the most serious when the HAl77-2 is between 316L SS and TA2. In the corrosion cell, the electronic cathode can receive electrons from the anode and form a loop. When the anodic HAl77-2 is in the middle, the two cathode metals (TA2 and 316L SS) are evenly distributed around the anode. It has a shorter distance and less resistance between the cathode and the anode. Thus, the corrosion of HAl77-2 in WBE3 is more significant than in WBE1 and WBE2. Moreover, the current values of the fifth and eighth lines in WBE3 are significantly higher than those in the sixth and seventh lines. This shows that corrosion at the TA2/HAl77-2 and HAl77-2/316L interfaces are more serious than if they were away from the interface. Materials 2018, 11, x FOR PEER REVIEW 22 of 24

(a)

(b)

Figure 22. (a) Average potential; and (b) current–density distribution of each microelectrode line after Figure 22. a b being( )coupled Average for potential;12 h in artificial and seawater ( ) current–density with 2.7 wt % distributionCl− at 70 °C. of each microelectrode line after being coupled for 12 h in artiﬁcial seawater with 2.7 wt % Cl− at 70 ◦C. 4. Conclusions Since the surface area of each wire in the WBE is much smaller compared to the total electrode working area, each wire surface can be assumed to be electrochemically uniform even if the whole WBE surface is electrochemically non-uniform. This assumption allows electrochemical theories and methods of describing uniform electroplating and electrodissolution processes to apply to each wire in a WBE. It is possible to obtain not only the average electrochemical signal but also the localized electrochemical signals from the WBEs. In addition, WBEs should equally consist of an electrode

Materials 2018, 11, 357 22 of 23

4. Conclusions Since the surface area of each wire in the WBE is much smaller compared to the total electrode working area, each wire surface can be assumed to be electrochemically uniform even if the whole WBE surface is electrochemically non-uniform. This assumption allows electrochemical theories and methods of describing uniform electroplating and electrodissolution processes to apply to each wire in a WBE. It is possible to obtain not only the average electrochemical signal but also the localized electrochemical signals from the WBEs. In addition, WBEs should equally consist of an electrode matrix of different metal or alloy wires. Clearly, WBEs could be ideally suited for investigating the galvanic corrosion and localized corrosion of different metals or alloys couples. Our observations above led to several conclusions:

(1) The potential of HAl77-2 is lowest. Its corresponding local current density is the most positive of the three metals. The potential and current distribution of 316L SS was between the values of the HAl77-2 and the TA2, accompanied by numerical ﬂuctuations. HAl77-2 is always the anode in these three-metal coupled systems. The TA2 is the cathode. Some 316L SS wires are cathodes, and the others are local anodes. (2) The potential of the three metals decreases with increasing temperature. The current of HAl77-2 increases with temperature indicating enhanced anode corrosion at elevated temperature. (3) Geometrical arrangement is an impact factor for the corrosion of the three-material coupled system. When the anodic HAl77-2 wires are located at the edge and are connected to the 316L SS wires, the main anodic area enlarges and is even adjacent to the 316L SS wires near the HAl77-2. When the anodic material HAl77-2 wires are located between the other two materials, the most severe corrosion occurs compared with the other two geometrical arrangements. Corrosion of the anode is the most signiﬁcant when the HAl77-2 is in the middle of the coupled system. Corrosion at the TA2/HAl77-2 and HAl77-2/316L interface become more serious father from the interface.

Supplementary Materials: The following are available online at http://www.mdpi.com/1996-1944/11/3/357/s1, Figure S1: OCP (vs. SCE/V) distribution maps of the WBE1 after immersion in artificial seawater at different temperatures: (a) 30 ◦C; (b) 40 ◦C; (c) 50 ◦C; (d) 60 ◦C; and (e) 70 ◦C, Figure S2: Spatial potential (left, E vs. SCE/V) and current–density (right, I/A) distribution maps of WBE1 after being short-circuited for 12 h in artificial seawater at different temperatures: (a) 30 ◦C; (b) 40 ◦C; (c) 50 ◦C; (d) 60 ◦C; and (e) 70 ◦C. Acknowledgments: This work was supported by grant from National Natural Science Foundation of China (NSFC, 41206063), Shandong Provincial Natural Science Foundation, China (ZR2017MEM015), and the Fundamental Research Funds for the Central Universities of China (14CX02201A and 15CX05023A). The authors also acknowledge the support from the China Scholarship Council (CSC). We thank Accdon (www.accdon.com) for language editing. Author Contributions: Hong Ju conceived and designed the experiments; Yunfei Liu, Shufa Liu, and Jinzhuo Duan performed the experiments; Yan Li analyzed the data; Hong Ju wrote the paper; and Yuanfeng Yang checked and revised the paper. Conflicts of Interest: The authors declare no conflict of interest.

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