Mitigation of Galvanic Corrosion in Bolted Joint of AZ31B and Carbon Fiber-Reinforced Composite Using Polymer Insulation

materials

Article Mitigation of Galvanic Corrosion in Bolted Joint of AZ31B and Carbon Fiber-Reinforced Composite Using Polymer Insulation

Jiheon Jun 1,*, Yong Chae Lim 1,* , Yuan Li 1 , Charles David Warren 2,† and Zhili Feng 1

1 Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA; [email protected] (Y.L.); [email protected] (Z.F.) 2 Energy and Transportation Science Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA; [email protected] * Correspondence: [email protected] (J.J.); [email protected] (Y.C.L.) † Author note: Charles David Warren retired since February 2020.

Abstract: The use of polymer insulation to mitigate galvanic corrosion was examined for bolted joints of AZ31B Mg alloy and carbon ﬁber-reinforced composite. To assess the corrosion behaviors of bolted joints with and without polymer insulation, solution immersion and salt spray exposure (ASTM B117) tests were conducted, and the corrosion depths and volumes were determined for the joint specimens after the tests. The polymer-insulated bolted joints exhibited much lower corrosion depths and volumes, highlighting the effective mitigation of galvanic corrosion. The reductions of joint strength in the post-corrosion joint specimens were relatively small (up to ~10%) in the polymer-insulated group but greater (up to 90%) in the group with no insulation. Cross-sectional characterization of post-corrosion joints with polymer insulation revealed local pits developed on

AZ31B under galvanic inﬂuence, indicating that limited galvanic attack (that did not decrease the joining integrity signiﬁcantly) could still occur during a long salt spray exposure (~1264 h) owing to

Citation: Jun, J.; Lim, Y.C.; Li, Y.; the permeation of an aqueous corrosive medium. Warren, C.D.; Feng, Z. Mitigation of Galvanic Corrosion in Bolted Joint of Keywords: dissimilar material joint; carbon fiber reinforced composite; magnesium alloy; galvanic AZ31B and Carbon Fiber-Reinforced corrosion; mechanical joint integrity Composite Using Polymer Insulation. Materials 2021, 14, 1670. https:// doi.org/10.3390/ma14071670 1. Introduction Academic Editor: Mirco Peron Multi-materials consisting of polymer composites (e.g., carbon fiber- or glass fiber- reinforced polymers) to lightweight metals (magnesium alloys, high-strength aluminum Received: 26 January 2021 alloys, and advanced/ultra-high-strength steels) structures can effectively improve vehicle Accepted: 17 March 2021 fuel efficiency to enable compliance with government regulations (i.e., for greenhouse Published: 29 March 2021 gas emissions) [1]. Among lightweight metals, magnesium (Mg) alloys are the lightest structural materials [2] and can lead to weight reductions of up to 55% when used in- Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in stead of conventional steels [1]. Highly engineered polymer composites such as carbon published maps and institutional affil- fiber-reinforced composites (CFRC) are another low-density material option that provides iations. superior mechanical strength, corrosion resistance, and design flexibility [3]. Conceptu- ally, noble hybrid autobody structures composed of Mg alloys and CFRC can result in significant weight reduction for improved fuel efficiency. However, joining of lightweight metals to polymer composites is one of the key technical obstacles in such multi-material autobody structures. Copyright: © 2021 by the authors. Extensive research and development efforts have been made to advance multi-material Licensee MDPI, Basel, Switzerland. This article is an open access article joining techniques, including fusion welding [4,5], ultrasonic welding [6], friction-based distributed under the terms and joining [7–12], mechanical fastening [13–15], adhesive bonding [11,16] and weld bonding conditions of the Creative Commons (use of adhesive combined with other joining techniques) [11,17]. However, these tech- Attribution (CC BY) license (https:// niques are not fully developed to ensure a reliable joining of Mg alloy and CFRC. For creativecommons.org/licenses/by/ example, fusion welding, ultrasonic welding, and some friction-based joining techniques 4.0/).

Materials 2021, 14, 1670. https://doi.org/10.3390/ma14071670 https://www.mdpi.com/journal/materials Materials 2021, 14, 1670 2 of 15

are only used to join thermoplastic polymer to metals joints, while the joining of thermoset polymer composites to metals is not feasible. Conventional mechanical fastening such as self-piercing riveting, may induce cracking in low ductility materials such as Mg alloys at room temperature [18]. Although adhesive bonding has been widely used for automotive and aerospace applications with good bonding strength, notable drawbacks for overall joining reliability include low peel strength, poor impact performance, and environment degradation. Bolting is one of the most well-established joining techniques and is widely used in transportation sectors because it provides for relatively easy assembly of similar or dissimilar materials, as well as and disassembly of joints for repair and recycle. Bolting also provides good mechanical joint performance [13]. However, bolting requires a pre-drilled hole to assemble the materials, which can lead to damage on polymer composites [19]. To minimize the mechanical damage from hole drilling, selection of an optimum machining technique is critical. Another major concern in any dissimilar material joint with Mg alloys is galvanic attack, in which Mg suffers from accelerated corrosion [12]. This occurs because Mg is one of the most anodically active metals and will readily dissolve to form metal ions. To mitigate galvanic corrosion, electrical insulation can be placed between dissimilar materials to increase resistance at the contact interface of anodic and cathodic materials. For a bolted joint of AZ31B Mg alloy and CFRC, electrical insulation can be applied at the contact interfaces of Mg with a steel bolt or Mg with a carbon ﬁber bundle of CFRC to mitigate galvanic corrosion. In this work, polymeric materials, including curable epoxy resins and polytetraﬂuoroethylene (PTFE), were selected to electrically insulate AZ31B and other components of the bolted joints. To perform a quantitative evaluation of Mg galvanic corrosion in dissimilar material joints, a method was implemented to selectively expose Mg anode and coupled cathode surfaces by masking other areas of the joint with polymer tapes [12]. The selectively exposed Mg anode developed a corrosion volume which allowed quantitative evaluation and visualization of galvanic attack in a dissimilar joint. This research also adopted a similar selective corrosion exposure method for the bolted joints, along with the conventional salt spray exposure test that has previously been used by others for dissimilar material joints [20–23]. In the present work, bolted joints of AZ31B Mg alloy and CFRC were prepared with and without polymer resin with and without PTFE tape insulation, and the joint specimens underwent corrosion testing by 0.1 M NaCl solution immersion and ASTM B117 salt spray exposure. The corrosion volumes that developed on the exposed Mg areas were then compared for the polymer-insulated and bare (i.e., without polymer insulation) joints to evaluate the effectiveness of polymer-based insulation on galvanic corrosion mitigation. The strength of bare and polymer-insulated joints was also measured before and after the salt spray exposure tests. Finally, cross-sectional characterization of polymer-insulated joints before and after corrosion was conducted to investigate the corrosion attack that occurred at the joints’ inner dissimilar material interfaces.

2. Materials and Methods 2.1. Materials A commercially available 3.29-mm-thick AZ31B Mg alloy was used for the top sheet material. Table1 summarizes the chemical composition of AZ31B from the manufacturer’s report. For the bottom sheet, a 4 mm thick thermoset carbon ﬁber reinforced composite was fabricated with the G-83 prepreg (T700, Toray) in a +45◦/−45◦ stacking sequence with 20-ply layup (Clearwater Composites, Duluth, MN, USA). The mechanical properties of AZ31B and CFRC used in this work are summarized in Table2. Materials 2021, 14, 1670 3 of 15 Materials 2021, 14, x FOR PEER REVIEW 3 of 15

Table 1. Summary of chemical compositions for AZ31B from material specification sheet. Both AZ31B and CFRC were machined by waterjet cutting for the assembled parts of Element Al Cu Mn Zn Ca Ni Be Si Fe Other lap and cross-tension type joints. The appearance and dimensions of machined AZ31B andWt% CFRC 3.03parts are 0.001 shown 0.42 in Figure 1.08 1a,b. 0.001For bolting, 0.001 commercially<0.001 0.015 available0.0025 mechanical<0.3 fasteners, steel bolts and nuts with yellow zinc plating finish, and type 316 stainless steel (ss316) washers were purchased from an external vendor (Mcmaster carr, Elmhurst, IL, Table 2. Summary of mechanical properties for AZ31B and CFRC (for CFRC longitudinal, carbon fibers are aligned in 45◦ USA). A bolt and a nut with two ss316 washers, which were used in this work, are shown angle to the tensile direction, and for CFRC 45◦, carbon fibers are parallel to the tensile direction). in Figure 1c. Two commercial epoxy resins were purchased to prepare the polymer-insu- Propertieslated Yield bolt joints: Strength (1) (MPa) a clear adhesive Ultimate type Tensile resin (Master Strength Bond (MPa) EP33CLV) Elongation for coating (%) on the AZ31BAZ31B surface, 220.5 and (2) a low-viscosity type 299.5 resin (DAP fast cure epoxy-acrylate) 13.05 for (a) CFRC (Longitudinal direction) healing of microcracks96.3 that would potentiall 194y be produced during a predrilled 22.7 hole pro- CFRC (45◦ direction)cess and (b) N/Afor electrical insulation around 907.7 a pilot hole on CFRC. A thin 0.23film PTFE tape was also purchased for the threads on bolts. Both AZ31B and CFRC were machined by waterjet cutting for the assembled parts Table 1. Summary of chemical compositions for AZ31B from material specification sheet. of lap and cross-tension type joints. The appearance and dimensions of machined AZ31B andElement CFRC partsAl are shownCu inMn Figure Zn1 a,b.Ca For bolting, Ni commercially Be Si available Fe mechanical Other fasteners,Wt% steel 3.03 bolts 0.001 and nuts0.42 with 1.08 yellow 0.001 zinc plating0.001 finish,<0.001 and0.015 type 3160.0025 stainless <0.3 steel (ss316) washers were purchased from an external vendor (Mcmaster carr, Elmhurst, IL, Table 2. Summary of mechanicalUSA). properties A bolt and for a AZ31B nut with and two CFRC ss316 (for washers, CFRC longitudinal, which were carbon used fibers in this are work, aligned are in shown 45° in angle to the tensile direction,Figure and for1c. CFRC Two commercial45°, carbon fibers epoxy are resins parallel were to the purchased tensile direction). to prepare the polymer-insulated bolt joints: (1) a clear adhesive type resin (Master Bond EP33CLV) for coating on the AZ31B Ultimate Tensile Strength Properties surface, and Yield (2) a Strength low-viscosity (MPa) type resin (DAP fast cure epoxy-acrylate)Elongation for (a) (%) healing (MPa) of microcracks that would potentially be produced during a predrilled hole process and AZ31B 220.5 299.5 13.05 (b) for electrical insulation around a pilot hole on CFRC. A thin film PTFE tape was also CFRC (Longitudinal direction) 96.3 194 22.7 purchased for the threads on bolts. CFRC (45° direction) N/A 907.7 0.23

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FigureFigure 1.1. MachinedMachined AZ31B AZ31B and and CFRC CFRC sheets sheets with with 12.5 12.5 mm mm pilot pilot holes holes for for (a) lap(a) lap shear shear and and (b) cross-tension(b) cross-tension joint joint specimens. speci- Formens. bolting For bolting in a pilot in hole, a pilot a steel hole, bolt a steel and bolt a nut and with a yellownut with Zn yellow plating, Zn as plating, well as twoas well ss316 as washerstwo ss316 shown washers in (c shown) were used.in (c) were used. 2.2. Assembly of Bolted CFRC-AZ31B Joints 2.2. Assembly of Bolted CFRC-AZ31B Joints The lap and cross-tension bolted joints are schematically illustrated in Figure2a. Cross- The lap and cross-tension bolted joints are schematically illustrated in Figure 2a. sectional views of bare bolts, without polymer insulation, and polymer-insulated bolt joints Cross-sectional views of bare bolts, without polymer insulation, and polymer-insulated are depicted in Figure2b,c, respectively. The most probable galvanic paths (red and green bolt joints are depicted in Figure 2b,c, respectively. The most probable galvanic paths (red dot arrows) in a bare bolted joint are described in Figure2b. The primary purpose of and green dot arrows) in a bare bolted joint are described in Figure 2b. The primary pur- polymer insulation is to prevent/inhibit the formation of such galvanic paths, as shown pose of polymer insulation is to prevent/inhibit the formation of such galvanic paths, as in Figure2c. For the assembly of polymer-insulated joints, the following treatments were conductedshown in Figure for each 2c. part:For the (1) assembly an adhesive-type of polymer-insulated epoxy was painted joints, ontothe following the top and treatments bottom AZ31Bwere conducted surfaces aroundfor each the part: pilot (1) hole, an adhesive-type (2) a low-viscosity epoxy epoxy was painted was applied onto onthe the top pilot and holebottom surfaces AZ31B of surfaces CFRC to around ﬁll in the the exposed pilot hole microcracks, (2) a low-viscosity formed in epoxy the composite was applied during on thethe holepilot drilling,hole surfaces and (3) of aCFRC thin PTFEto fill in tape the was exposed wrapped microcracks around theformed threaded in the sections composite of during the hole drilling, and (3) a thin PTFE tape was wrapped around the threaded sections of the steel bolts. As illustrated in Figure 2c, the combination of the polymer resin

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the steel bolts. As illustrated in Figure2c, the combination of the polymer resin and PTFE tapeand applicationPTFE tape application is designed foris designed electrical for insulation electrical between insulation AZ31B, between CFRC, AZ31B, and the CFRC, bolt setup.and the To bolt avoid setup. any To potential avoid any damage potential on precoateddamage on epoxy precoated resin epoxy on the resin AZ31B on the surface, AZ31B a moderatesurface, a clampingmoderate torqueclamping of 5.65 torque N·mm of 5.65 was N·mm applied was to applied tighten theto tighten mechanical the mechanical fasteners (i.e.,fasteners bolt and (i.e., nuts). bolt and nuts).

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FigureFigure 2.2. Schematics of of ( (aa)) lap lap and and cross-tension cross-tension joints joints of ofAZ31B AZ31B and and CFRC CFRC.. The The cross cross sections sections of bolted of bolted AZ31B AZ31B and CFRC and are also schematically described for (b) bare joint with predicted galvanic paths (red and green dot arrows) and (c) poly- CFRC are also schematically described for (b) bare joint with predicted galvanic paths (red and green dot arrows) and (c) mer-insulated joints. polymer-insulated joints. 2.3. Corrosion Testing 2.3. Corrosion Testing Solution immersion tests were performed for the polymer tape-masked cross-tension jointsSolution illustrated immersion in Figure tests 3a. wereThese performed masked joint for thesamples polymer were tape-masked partially immersed cross-tension in 0.1 jointsM NaCl illustrated solution, in as Figure depicted3a. Thesein Figure masked 3b, to joint allow samples corrosion were of partially AZ31B surfaces immersed at inthe 0.1bottom M NaCl and solution,inside the as joint depicted gap section. in Figure With3b, this to allow specific corrosion immersion of AZ31B test, the surfaces degree at of the bottom and inside the joint gap section. With this specific immersion test, the degree Mg corrosion can be visually quantified by analyzing the corrosion volume formed after of Mg corrosion can be visually quantified by analyzing the corrosion volume formed the immersion test. A photo of solution immersion test and another photo of a post-im- after the immersion test. A photo of solution immersion test and another photo of a post- mersion bolt joint that formed corrosion volume at the bottom are shown in Figure S1 in immersion bolt joint that formed corrosion volume at the bottom are shown in Figure S1 the Supplementary Materials. Several tape-masked cross-tension specimens—bare and in the Supplementary Materials. Several tape-masked cross-tension specimens—bare polymer-insulated—were used with increasing solution immersion time. For one selected and polymer-insulated—were used with increasing solution immersion time. For one bare sample and one selected polymer-insulated sample, a reference saturated calomel selected bare sample and one selected polymer-insulated sample, a reference saturated electrode (SCE) was placed between the bolt head and the AZ31B bottom surface to meas- calomel electrode (SCE) was placed between the bolt head and the AZ31B bottom surface ure the corrosion potential at the end of the immersion test. to measure the corrosion potential at the end of the immersion test. A salt spray corrosion test according to ASTM B117 was also conducted for several A salt spray corrosion test according to ASTM B117 was also conducted for several bare bare and polymer-insulated joints with increasing exposure time. Both lap and cross-ten- and polymer-insulated joints with increasing exposure time. Both lap and cross-tension typesion jointstype joints were used,were withused, tape with masking tape maskin appliedg applied in the end in the section(s) end section(s) of AZ31B of toAZ31B expose to onlyexpose the only central the section central of section joints of where joints galvanic where galvanic corrosion corrosion would be would most significant.be most signifi- The exposedcant. The areas exposed of AZ31B areas inof theAZ31B central in the sections central of sections the lap and of the cross-tension lap and cross-tension joints were 38joints× 2 2 43were mm 382 and × 43 50 mm× 60 and mm 502 ,× respectively, 60 mm , respectively, excluding excluding the bolted the centers. bolted The centers. photos The of boltedphotos jointsof bolted with joints tape maskingwith tape for masking salt spray for testssalt spray are shown tests are in Figure shown S2 in in Figure the Supplementary S2 in the Sup- Materials.plementary The Materials. tape-masked The tape-masked joints were then joints loaded were ontothen theloaded plastic onto racks the plastic in a salt racks spray in chamber,a salt spray which chamber, is also which shown is in also Figure shown S2. The in Figure salt spray S2. The corrosion salt spray test experiencedcorrosion test several expe- offrienced times several during whichoff times no during salt spray which was no generated salt spray due was to angenerated instrumental due to limitation. an instrumental These offlimitation. times mostly These lasted off times for 48 mostly to 72 h lasted during for weekends, 48 to 72 buth during longer weekends, in certain circumstances. but longer in certain circumstances. The off times were not considered as part of the total exposure time. Another note is that tap water was used for pressurized steam and 3.5 wt.% NaCl solution

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The off times were not considered as part of the total exposure time. Another note is that tap water was used for pressurized steam and 3.5 wt.% NaCl solution rather than 4 ratherMohm–grade than 4 Mohm–grade distilled water. distilled The tap wa waterter. exhibitedThe tap water3.7 × exhibited10−3 Mohm 3.7 and× 10 pH−3 Mohm 7.9, and and it pHcontained 7.9, and some it contained ionic species some ationic relatively species low at relatively concentrations, low concentrations, as presented inas Tablepresented S1 in inthe Table Supplementary S1 in the Supplementary Materials. Materials.

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Figure 3. Schematics ofof ((aa)) exposed exposed AZ31B AZ31B surfaces surfaces without without tape tape masking masking in ain cross-tension a cross-tension joint joint and and (b) an(b immersed) an immersed joint jointspecimen specimen with with tape maskingtape masking from from the side the view.side view.

ToTo remove corrosion products without using chemical agents, all post-corrosion post-corrosion joint samplessamples were were cleaned cleaned by ultrasonication in distilled water and mechanical rubbing with hardhard polymer polymer brushes. brushes. This This cleaning cleaning method method was was repeated repeated at least at least three three times times until until no furtherno further removal removal of corrosion of corrosion products products was wasobse observed.rved. Overall, Overall, this physical this physical cleaning cleaning suc- cessfullysuccessfully removed removed most most visible visible corrosion corrosion products products from from the thejoint joint specimens. specimens.

2.4.2.4. Static Static Lap Lap Shear Shear Tensile Tensile Testing TheThe mechanical mechanical joint joint integrity integrity before before and and after after the thesalt saltspray spray corrosion corrosion test was test eval- was uatedevaluated by static by static lap lapshear shear tensile tensile testing testing us usinging an an MTS MTS tensile tensile machine machine with with a a constant −1 crossheadcrosshead speed of 10 mmmm·min·min−1 atat room room temperature. temperature. To To avoid avoid a a bending bending effect during tensiletensile shear shear testing, testing, spacers spacers were were used used to toclamp clamp the the lap lapshear shear coupons coupons to align to align them them ver- ticallyvertically between between the thegrips. grips. 2.5. Characterizations (Optical and Electron Microscopy) 2.5. Characterizations (Optical and Electron Microscopy) To quantify the corrosion volume after the salt spray exposure test, an optical pro- To quantify the corrosion volume after the salt spray exposure test, an optical profilometry instrument (Keyence, Osaka, Japan) was used. The baseline and post-corrosion filometry instrument (Keyence, Osaka, Japan) was used. The baseline and post-corrosion samples were mounted in epoxy and cut into halves using a diamond saw to obtain the samples were mounted in epoxy and cut into halves using a diamond saw to obtain the cross sections. The cross-sectioned samples were then ground using silicon carbide (SiC) cross sections. The cross-sectioned samples were then ground using silicon carbide (SiC) papers with 600, 800 and 1200 grits, followed by fine-polishing with diamond progressively papers with 600, 800 and 1200 grits, followed by fine-polishing with diamond progres- finer suspensions 3, 1, and 0.5 µm using a Struers auto-polisher machine. A Zeiss Axio sively finer suspensions 3, 1, and 0.5 µm using a Struers auto-polisher machine. A Zeiss microscope was used for optical characterization. The images were stitched using the Mo- Axio microscope was used for optical characterization. The images were stitched using saiX mode. Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy the(EDS) MosaiX characterizations mode. Scanning were electron carried microscopy out using (SEM) TESCAN and MIRA3energy dispersive with an acceleration x-ray spec- troscopyvoltage of (EDS) 20 kV. characterizations were carried out using TESCAN MIRA3 with an acceleration voltage of 20 kV. 3. Results 3.3.1. Results 0.1 M NaCl Immersion Test 3.1. 0.1The M lowerNaCl Immersion center sections Test of AZ31B after 0.1 M NaCl immersion tests are shown in FigureThe4 for lower the barecenter and sections polymer-insulated of AZ31B after cross-tension 0.1 M NaCl joints. immersion The bare tests joints are underwent shown in Figure 4 for the bare and polymer-insulated cross-tension joints. The bare joints underwent up to 314 h immersion (Figure 4a), and the material loss by galvanic corrosion

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formed relatively uniform depths from the initial exposed area (bottom surface, see Figure 3). For the longer immersion times (Figure 4b), the bare joints showed greater corrosion depths, whereas the polymer-insulated joints only formed “crevices”, without uniform material loss. The corrosion potential measured after 614 h immersion were −1.26 and −1.48 VSCE for the bare and polymer-insulated joints, respectively, indicating a higher anodic polarization in the bare case due to greater galvanic impact. Note that AZ31B in 0.1 Materials 2021, 14, 1670 M NaCl at room temperature commonly exhibits −1.55 ~ −1.6 VSCE, with no galvanic6 cou- of 15 pling [12,24,25]. Apparently, the polymer-insulation method was effective in mitigating galvanic corrosion of AZ31B in the bolted joint configuration used in this work. For a quick quantification of Mg galvanic corrosion, the depths of corroded volumes upwere to measured 314 h immersion for AZ31B (Figure in the4 barea), and and the polyme materialr-insulated loss by bolted galvanic joints, corrosion as designated formed by relativelyyellow arrows uniform in Figure depths 4b. fromThe measured the initial corrosi exposedon depths area (bottom are plotted surface, as a function see Figure of im-3). Formersion the longertime in immersionFigure 5. The times depth (Figure formed4b), fr theom barethe bare joints joints showed increased greater with corrosion time but depths,seemingly whereas at a slower the polymer-insulated rate after 314 h. This joints slow-down only formed is likely “crevices”, associated without with the uniform lateral- materialdirection loss.corrosion The corrosionloss, which potential was noticed measured in the samples after 614 immersed h immersion for 314, were 404− and1.26 623 and h −(see1.48 Figure VSCE 4).for Meanwhile, the bare and there polymer-insulated was only one polymer-insulated joints, respectively, joint indicating sample that a higherdevel- anodicoped a measurable polarization corrosion in the bare depth case after due 623 to greaterh. The corrosion galvanic depth impact. was Note about that 1.7 AZ31B mm and in − − 0.1was M much NaCl smaller at room than temperature the depth commonly measured exhibitsfrom the bare1.55 joints ~ 1.6 (≥5 V mm).SCE, with In light no galvanic of these couplingresults, together [12,24,25 with]. Apparently, the corrosion the potential polymer-insulation data, it can methodbe stated was with effective confidence in mitigating that Mg galvanic corrosion of AZ31B in the bolted joint conﬁguration used in this work. galvanic corrosion was mitigated by adopting a polymer insulation method.

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FigureFigure 4. Post-immersionPost-immersion AZ31B AZ31B sheets sheets that that developed developed corrosion corrosion volumes volumes from from (a) bare (a) bare bolt boltjoints joints with with immersion immersion time timeup to up 314 to h, 314 and h, (b and) bare (b) and bare polymer-insulated and polymer-insulated joints with joints immersion with immersion times of times 404 and of 404 623 and h. The 623 triangle h. The symbols triangle sym-indicate corrosion depth measurement, and star symbols indicate corrosion potential measurements in which the values at the bols indicate corrosion depth measurement, and star symbols indicate corrosion potential measurements in which the completion of the immersion tests were −1.26 and −1.48 VSCE for the bare and polymer-insulated joint specimens, respec- values at the completion of the immersion tests were −1.26 and −1.48 V for the bare and polymer-insulated joint tively. SCE specimens, respectively.

For a quick quantiﬁcation of Mg galvanic corrosion, the depths of corroded volumes were measured for AZ31B in the bare and polymer-insulated bolted joints, as designated by yellow arrows in Figure4b. The measured corrosion depths are plotted as a function of immersion time in Figure5. The depth formed from the bare joints increased with time but seemingly at a slower rate after 314 h. This slow-down is likely associated with the lateral-direction corrosion loss, which was noticed in the samples immersed for 314, 404 and 623 h (see Figure4). Meanwhile, there was only one polymer-insulated joint sample that developed a measurable corrosion depth after 623 h. The corrosion depth was about 1.7 mm and was much smaller than the depth measured from the bare joints

(≥5 mm). In light of these results, together with the corrosion potential data, it can be stated with conﬁdence that Mg galvanic corrosion was mitigated by adopting a polymer insulation method. MaterialsMaterials 20212021, 14,,14 x, FOR 1670 PEER REVIEW 7 of 15 7 of 15

FigureFigure 5. Corrosion5. Corrosion depths depths measured measured from post-immersion from post-immersio AZ31B shownn AZ31B in Figure shown4 as a functionin Figure 4 as a function ofof immersion immersion time. time. 3.2. ASTM B117 Salt Spray Exposure 3.2. ASTM B117 Salt Spray Exposure The bare and polymer-insulated joints were also exposed to an ASTM B117 salt spray environmentThe bare with and the specific polymer-insulated masking applied joints on both were lap and also cross-section exposed configurations,to an ASTM B117 salt spray asenvironment described in the with experimental the specific section. masking The post-corrosion applied on lapboth sheer lap joints and arecross-section shown configura- intions, Figure as6 fordescribed the bare andin the polymer-insulated experimental conditions. section. The The post-corrosion bare joints developed lap sheer joints are corrosion ditches on Mg (designated as p1-3 in the top row of Figure6) around the ss316 washers,shown clearlyin Figure indicating 6 for severethe bare Mg galvanicand polymer- corrosioninsulated proximity conditions. to the noble metallicThe bare joints devel- componentsoped corrosion of the boltingditches setup on Mg (which (designated were galvanically as p1-3 protected, in the top and row therefore of Figure not 6) around the corroded).ss316 washers, It was visually clearly noted indicating that the severe corrosion Mg ditch galvanic fully penetrated corrosion the proximity AZ31B sheet to the noble me- aftertallic 438 components h exposure. Inof polymer-insulatedthe bolting setup joints (which exposed were longer galvanically than 438 h, protected, however, and therefore no distinct groove attack was found around the ss316 washers, and both the steel bolts andnot AZ31B corroded). corroded It was in a relatively visually uniform noted manner.that the This corrosion observation ditch indicates fully penetrated that the the AZ31B polymersheet after insulation 438 h was exposure. effective inIn impedingpolymer-insula galvanicted coupling joints so exposed that AZ31B longer did not than 438 h, how- sufferever, fromno distinct severe corrosion groove ditchattack attack, was whereas found thearound steel bolt,the ss316 which waswashers, no longer and both the steel galvanicallybolts and AZ31B protected, corroded corroded simultaneously.in a relatively uniform manner. This observation indicates that Figure7 shows the cross-tension joints after salt spray exposure for one bare case the polymer insulation was effective in impeding galvanic coupling so that AZ31B did not (238 h) and three polymer-insulated cases (238, 438, 732 h). Again, the formation of a severesuffer corrosion from severe ditch withcorrosion an uncorroded ditch attack, steel bolt wh wasereas observed the steel in thebolt, bare which joint. was In no longer gal- thevanically polymer-insulated protected, joints, corroded no corrosion simultaneously. ditch was visually detected, and the steel bolts formedFigure rust layers 7 shows that covered the cross-tensio an increasinglyn joints large areaafter over salt time. spray Overall, exposure the corrosion for one bare case (238 attacksh) and and three morphologies polymer-insulated observed after cases the salt (238, spray 438, exposure 732 wereh). Again, very similar the between formation of a severe the lap and cross-tension joints prepared with the bare and polymer-insulated conditions. corrosionThe corrosion ditch volumeswith an of uncorroded the bare and polymer-insulatedsteel bolt was observed joints were in quantitatively the bare joint. In the poly- analyzedmer-insulated using an joints, optical profilometryno corrosion technique, ditch was with visually the uncorroded detected, (i.e., tape-maskedand the steel bolts formed duringrust layers the salt that spray covered exposure) an AZ31Bincreasingly surface servinglarge area as the over reference time. plane.Overall, The the post- corrosion attacks corrosionand morphologies joints were disassembled, observed after and the only salt the sp AZ31Bray exposure plates were were used very for optical similar between the profilometry measurements, as described further in Figure S3 in the Supplementary Ma- lap and cross-tension joints prepared with the bare and polymer-insulated conditions. terials. The quantified corrosion volumes are plotted as a function of salt spray exposure time inThe Figure corrosion8. In both volumes the lap and of cross-tension the bare and joints, polymer-insulated the bare condition showedjoints were the quantitatively corrosionanalyzed volumes using increasingan optical with profilometry time, with atec muchhnique, greater with increase the uncorroded than seen in the (i.e., tape-masked polymer-insulatedduring the salt conditionspray exposure) due to the prevailingAZ31B surfac and aggregatinge serving corrosion as the reference ditch attack. plane. The post- Incorrosion contrast, the joints corrosion were volumes disassembled, of polymer-insulated and only the joints AZ31B were relatively plates were low and used did for optical pro- not increase with time, implying that Mg corrosion was saturated without being alleviated byfilometry the galvanic measurements, impact that was as prominent described in the further bare joints. in Figure S3 in the Supplementary Materi- als. The quantified corrosion volumes are plotted as a function of salt spray exposure time in Figure 8. In both the lap and cross-tension joints, the bare condition showed the corrosion volumes increasing with time, with a much greater increase than seen in the polymer- insulated condition due to the prevailing and aggregating corrosion ditch attack. In contrast, the corrosion volumes of polymer-insulated joints were relatively low and did not increase with time, implying that Mg corrosion was saturated without being alleviated by the galvanic impact that was prominent in the bare joints.

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Figure 6. Lap shear bolted joints after salt spray exposure presented for (a) bare and (b) polymer-insulated conditions. The Figure 6. Lap shear bolted joints after salt spray exposure presented for (a) bare and (b) polymer-insulated conditions. The Figureexposure 6. Lap time shear is designatedbolted joints for after each salt specimen. spray exposure presented for (a) bare and (b) polymer-insulated conditions. The exposure time is designated for each specimen. exposure time is designated for each specimen. Figure 6. Lap shear bolted joints after salt spray exposure presented for (a) bare and (b) polymer-insulated conditions. The exposure time is designated for each specimen.

(a) (b) (c) (d) (a) (b) (c) (d) Figure 7. Cross-tension bolted joints after salt spray exposure presented for (a) bare and (b–d) polymer-insulated conditions. The(a) exposure time is designated (forb) each specimen. (c) (d) FigureFigure 7. Cross-tension 7. Cross-tension bolted bolted joints joints after after salt salt spray spray exposure exposure presented presented for for (a )(a bare) bare and and (b –(bd–)d polymer-insulated) polymer-insulated conditions. conditions. The exposure time is designated for each specimen. TheFigure exposure 7. Cross-tension time is designated bolted joints for each after specimen.salt spray exposure presented for (a) bare and (b–d) polymer-insulated conditions. The exposure time is designated for each specimen.

Figure 8. Corrosion volumes of lap and cross-tension joints plotted as a function of salt spray exposure time for the bare and polymer-insulated conditions. The corrosion volumes were determined by optical proﬁlometry, as exempliﬁed in Figure S3.

Materials 2021, 14, x FOR PEER REVIEW 9 of 15

Materials 2021, 14, 1670 9 of 15 Figure 8. Corrosion volumes of lap and cross-tension joints plotted as a function of salt spray exposure time for the bare and polymer-insulated conditions. The corrosion volumes were determined by optical profilometry, as exemplified in Figure S3. 3.3. Mehanial Joint Performance and Fractograph 3.3. Mehanial Joint Performance and Fractograph Figure9a,b depicts representative load and displacement curves from lap shear test- Figure 9a,b depicts representative load and displacement curves from lap shear testing for bare and polymer-insulated bolted CFRC-AZ31B joints with different corrosion ing for bare and polymer-insulated bolted CFRC-AZ31B joints with different corrosion exposure time. The bare joints showed greater reduction of failure load and elongation at exposure time. The bare joints showed greater reduction of failure load and elongation at failure with increasing exposure time. In contrast, the failure loads and displacements for failure with increasing exposure time. In contrast, the failure loads and displacements for polymer-insulated joints were not decreased signiﬁcantly in any of the cases, even after polymer-insulated joints were not decreased significantly in any of the cases, even after 1264 h exposure. Figure9c,d present the averaged lap shear peak failure load for the bare 1264 h exposure. Figure 9c,d present the averaged lap shear peak failure load for the bare andand polymer-insulated polymer-insulated bolted bolted joints joints with with increasing increasing ASTM ASTM B117 B117 salt salt spray spray exposure exposure times. ± Fortimes. the bare For the condition, bare condition, the averaged the averaged failure load failure for load uncorroded for uncorroded joints was joints15.96 was 15.960.32 ± kN . This0.32 is kN. higher This or is comparablehigher or comparable to the metal-to-composite to the metal-to-composite joints fabricated joints fabricated by other by technolo- other giestechnologies and summarized and summarized in the open in literaturethe open liter [11,ature26]. As [11,26]. corrosion As corrosion exposure exposure time increased, time theincreased, averaged the peak averaged failure peak load failure was greatly load was decreased, greatly decreased, as shown inas Figureshown9 ina. Figure Finally, 9a. the retainedFinally, failure the retained load was failure only load 1.64 was± 0.33 only kN 1.64 at 438± 0.33 h duekN at to 438 signiﬁcant h due to galvanic significant corrosion gal- onvanic AZ31B. corrosion Figure 9onb showsAZ31B. the Figure averaged 9b shows peak th sheare averaged failure peak load shear for the failure polymer-insulated load for the boltedpolymer-insulated joints at up to bolted 1264 hjoints corrosion at up to exposure 1264 h corrosion time. The exposure averaged time. failure The loadaveraged for un- corrodedfailure load joints for was uncorroded 15.51 ± 0.07joints kN, was which 15.51 is± 0.07 similar kN, towhich the valueis similar of the to the bare value uncorroded of the joints.bare Althoughuncorroded some joints. deviations Although ofsome the deviations average failure of the loads average were failure observed, loads were 80~90% ob- of theserved, original 80~90% joint failureof the original load was joint generally failure obtainedload was generally in the polymer-insulated obtained in the polymer- joints after theinsulated salt spray joints exposures. after the salt spray exposures.

Materials 2021, 14, x FOR PEER REVIEW 10 of 15

(a) (b)

FigureFigure 9. Representative 9. Representative load load and and displacement displacement curves curves from from laplap shearshear tensile testing testing for for (a (a) )bare bare and and (b ()b polymer-insulated) polymer-insulated boltedbolted CFRC-AZ31B CFRC-AZ31B joints joints with with different different ASTM ASTM B117 B117 exposure exposure times. times. The The average average values values of of lap lap shear shear peak peak failure failure load load are presentedare presented for (c) barefor (c and) bare (d and) polymer-insulated (d) polymer-insulated bolted bolted joints joints with with the differentthe different exposure exposure times. times.

Tables 3 and 4 summarize the averaged lap shear failure load, elongation at failure, and retained joint strength for both conditions at different corrosion exposure times. In the comparison at 438 h of exposure time, only 10.3% of the original strength was retained for the bare joints, whereas 78% of the original joint strength was retained for the polymer- insulated joints. The average elongation for the bare joints significantly reduced as the corrosion exposure time increased due to the galvanic corrosion of Mg alloy in the joint. However, the polymer-insulated joints showed relatively small reduction of the averaged elongation at failure because galvanic corrosion of AZ31B was mitigated by electrical insulation between the metal interfaces. Overall, the polymer-insulation method used in this work was effective in reducing galvanic corrosion of Mg alloy, thereby mitigating the degradation of joint integrity.

Table 3. Summary of lap shear tensile testing for the as-bolted CFRC-AZ31B joints.

ASTM B117 Exposure Time Peak Fracture Load Elongation at Failure Retained Strength (h) (kN) (mm) (%) 0 15.96 ± 0.32 7.16 ± 1.25 100 100 13.85 ± 1.81 4.84 ± 1.20 86.8 226 11.59 ± 1.93 3.58 ± 1.15 72.7 358 5.18 ± 1.19 2.02 ± 1.11 32.5 438 1.64 ± 0.33 0.45 ± 0.34 10.3

Table 4. Summary of lap shear tensile testing for three-step insulated bolted CFRC-AZ31B joints.

ASTM B117 Exposure Time Peak Fracture Load Elongation at Failure Retained Strength (h) (kN) (mm) (%) 0 15.51 ± 0.07 6.45 ± 0.31 100 238 13.55 ± 0.53 7.42 ± 0.59 88.3 438 12.09 ± 1.48 5.81 ± 0.49 78.0 705 12.33 ± 2.43 6.19 ± 1.64 79.5 829 13.57 ± 0.64 7.03 ± 0.20 87.5 936 14.22 ± 0.47 7.10 ± 1.12 91.7 1264 13.06 ± 1.61 5.79 ± 0.49 84.2

Materials 2021, 14, 1670 10 of 15

Tables3 and4 summarize the averaged lap shear failure load, elongation at failure, and retained joint strength for both conditions at different corrosion exposure times. In the comparison at 438 h of exposure time, only 10.3% of the original strength was retained for the bare joints, whereas 78% of the original joint strength was retained for the polymer- insulated joints. The average elongation for the bare joints signiﬁcantly reduced as the corrosion exposure time increased due to the galvanic corrosion of Mg alloy in the joint. However, the polymer-insulated joints showed relatively small reduction of the averaged elongation at failure because galvanic corrosion of AZ31B was mitigated by electrical insulation between the metal interfaces. Overall, the polymer-insulation method used in this work was effective in reducing galvanic corrosion of Mg alloy, thereby mitigating the degradation of joint integrity.

Table 3. Summary of lap shear tensile testing for the as-bolted CFRC-AZ31B joints.

ASTM B117 Peak Fracture Load Elongation at Failure Retained Strength Exposure Time (h) (kN) (mm) (%) 0 15.96 ± 0.32 7.16 ± 1.25 100 100 13.85 ± 1.81 4.84 ± 1.20 86.8 226 11.59 ± 1.93 3.58 ± 1.15 72.7 358 5.18 ± 1.19 2.02 ± 1.11 32.5 438 1.64 ± 0.33 0.45 ± 0.34 10.3

Table 4. Summary of lap shear tensile testing for three-step insulated bolted CFRC-AZ31B joints.

ASTM B117 Peak Fracture Load Elongation at Failure Retained Strength Exposure Time (h) (kN) (mm) (%) 0 15.51 ± 0.07 6.45 ± 0.31 100 238 13.55 ± 0.53 7.42 ± 0.59 88.3 438 12.09 ± 1.48 5.81 ± 0.49 78.0 705 12.33 ± 2.43 6.19 ± 1.64 79.5 829 13.57 ± 0.64 7.03 ± 0.20 87.5 936 14.22 ± 0.47 7.10 ± 1.12 91.7 1264 13.06 ± 1.61 5.79 ± 0.49 84.2

Figure 10 shows fractography for the bare (Figure 10a–d) and polymer-insulated bolted joints (Figure 10e–h) with increasing corrosion exposure times. For the bare case, the failure mode for uncorroded joints was mixed cleavage-tension (red arrow #1) and net-tension (red arrow #2) failures, as shown in Figure 10a. As corrosion exposure time increased, galvanic corrosion of AZ31B formed a corrosion ditch on the periphery of the bolted center, as previously shown in Figure6. This ditch corrosion attack on AZ31B resulted in the crack initiation, as designated by red arrows in Figure 10b,c, and it subsequently led to ﬁnal failure during the tensile shear testing. For the polymer-insulated joints, mixed cleavage tension and net tension failure modes were commonly found in uncorroded and salt spray- exposed conditions, indicating that the primary failure mode was not changed due to the mitigation of AZ31B galvanic corrosion. Materials 2021, 14, x FOR PEER REVIEW 11 of 15

Figure 10 shows fractography for the bare (Figure 10a–d) and polymer-insulated bolted joints (Figure 10e–h) with increasing corrosion exposure times. For the bare case, the failure mode for uncorroded joints was mixed cleavage-tension (red arrow #1) and net-tension (red arrow #2) failures, as shown in Figure 10a. As corrosion exposure time increased, galvanic corrosion of AZ31B formed a corrosion ditch on the periphery of the bolted center, as previously shown in Figure 6. This ditch corrosion attack on AZ31B resulted in the crack initiation, as designated by red arrows in Figure 10b,c, and it subsequently led to final failure during the tensile shear testing. For the polymer-insulated Materials 2021, 14, 1670 joints, mixed cleavage tension and net tension failure modes were commonly found11 of 15in uncorroded and salt spray-exposed conditions, indicating that the primary failure mode was not changed due to the mitigation of AZ31B galvanic corrosion.

Figure 10.10. Comparison of fractography from lap shear tensile testingtesting with different ASTM B117 exposure times for (a–d) bare andand (e(e––hh)) polymer-insulated polymer-insulated CFRC-AZ31B CFRC-AZ31B bolted bolted joints. joints. Red Red arrows arrows indicate indicate the failurethe failure locations locations on AZ31B, on AZ31B, and black and black arrows show the epoxy resin overfill layer on the AZ31B surface. arrows show the epoxy resin overfill layer on the AZ31B surface. 3.4. Cross-Sectional Characterization 3.4. Cross-Sectional Characterization The cross-sectioned lap joints with polymer insulation are compared for uncorroded The cross-sectioned lap joints with polymer insulation are compared for uncorroded and corroded (by salt spray exposure for 1264 h) conditions in Figure 11. The comparison and corroded (by salt spray exposure for 1264 h) conditions in Figure 11. The comparison of low-magnification optical images (Figure 11a,b) reveals corrosion pits formed in AZ31B of low-magnification optical images (Figure 11a,b) reveals corrosion pits formed in AZ31B around the ss316 washer (denoted as pitting), and it also shows corrosion loss of AZ31B around the ss316 washer (denoted as pitting), and it also shows corrosion loss of AZ31B underneath the washer and bolt (A and B vs. C, D and E). The corrosion pits near the underneath the washer and bolt (A and B vs. C, D and E). The corrosion pits near the washer that were not identified by visual inspection indicate that the galvanic effect was washer that were not identified by visual inspection indicate that the galvanic effect was mitigated but was not completely removed by applying polymer insulationinsulation on the AZ31B surface. A notablenotable corrosioncorrosion loss loss of of steel steel bolt bolt (F) (F) is is also also observed observed at aat contact a contact point point with with the ss316the ss316 washer, washer, indicating indicating possible possible local local galvanic galvanic corrosion corrosion of steel of steel by ss316. by ss316. A gap between the washer and AZ31B of the uncorroded joint (A in FigureFigure 1111a)a) isis shown at at high high magnification magnification in in Figure Figure 11c, 11 c,revealing revealing that that the the pre-coated, pre-coated, porous, porous, upto upto 200 200µm µthickm thick epoxy epoxy layer layer adhered adhered firmly firmly on AZ31B. on AZ31B. A similar A similar gap location gap location of a corroded of a corroded joint joint(C in (C Figure in Figure 11b) 11 isb) presented is presented in inFigure Figure 11d 11 dthat that exhibited exhibited shallow shallow and and deep deep pits onon AZ31B. ThisThis observationobservation indicates indicates that that the the pre-coated pre-coated epoxy epoxy layer layer was was not not fully fully protective, protec- presumablytive, presumably due to due the to permeation the permeation of the of aqueous the aqueous corrosive corrosive medium. medium. In the In corroded the corroded joint, anjoint, EDS an mapping EDS mapping analysis analysis was conducted was conducted for the area for designatedthe area designated by C and Dby in C Figure and D 11 inb, asFigure presented 11b, as in presented Figure 11 e.in TheFigure overlap 11e. The of O overlap and the of relatively O and the low relatively Mg signals low indicate Mg signals the corrodedindicate the area corroded in AZ31B. area It in is AZ31B. also noted It is that also the noted ss316 that washer the ss316 exhibited washer strong exhibited Fe without strong anyFe without overlap any with overlap O, indicating with O, that indicating the washer that remainedthe washer uncorroded. remained uncorroded. More EDS mapping mapping analyses analyses were were conduct conducteded for for the the areas areas designated designated by by B and B and E in E inFigure Figure 11a,b, 11a,b, respectively, respectively, as aspresented presented in Figure in Figure 12. 12In. the In theuncorroded uncorroded joint, joint, Fe and Fe andMg Mgdid not did overlap not overlap with withO, and O, a and strong a strong F intensity F intensity was detected was detected from the from PTFE the tape PTFE (Figure tape (Figure 12a). In contrast, extensive corrosion of AZ31B was indicated by the overlap of Mg and O, as well as corrosion of steel bolt threads (overlap of Fe and O) underneath the PTFE tape as designated by F intensity) (Figure 12b). Despite the corrosion mitigation effect, the application of polymer insulation could not completely stop corrosion of AZ13B and the steel bolt in the inner area of the joint. Materials 2021, 14, x FOR PEER REVIEW 12 of 15

12a). In contrast, extensive corrosion of AZ31B was indicated by the overlap of Mg and O, as well as corrosion of steel bolt threads (overlap of Fe and O) underneath the PTFE tape Materials 2021, 14, 1670 as designated by F intensity) (Figure 12b). Despite the corrosion mitigation effect, the ap-12 of 15 plication of polymer insulation could not completely stop corrosion of AZ13B and the steel bolt in the inner area of the joint.

(a) (b)

(e)

FigureFigure 11. Light 11. Light microscopy microscopy images images of cross-sectionedof cross-sectioned polymer-insulated polymer-insulated bolted bolted joints joints ( a(,ac,)c) in in as-joined as-joined conditioncondition andand (b,d) (b,d) after salt spray exposure for 1264 h. The local areas designated A in (a) and C in (b) are shown in a higher magnifi- after salt spray exposure for 1264 h. The local areas designated A in (a) and C in (b) are shown in a higher magniﬁcation in cation in (c,d), respectively. An SEM image is presented in (e), and an EDS mapping analysis for a local area designated (c,d),by respectively. C and D is presented An SEM imagein (b). isSE presented indicates the in ( esecondary), and an electron EDS mapping image mo analysisde. All for EDS a localelemental area designatedmaps were recorded by C and D is Materials 2021, 14, x FOR PEER REVIEW 13 of 15 presentedusing K-alpha in (b). SE characteristicindicates the X-ray secondary of each element. electron image mode. All EDS elemental maps were recorded using K-alpha characteristic X-ray of each element.

(a) (b)

FigureFigure 12. SEM12. SEM images images with with corresponding corresponding EDSEDS mapsmaps for ( (aa)) the the area area designated designated by byB in B Figure in Figure 11a 11anda and(b) the (b) area the area designateddesignated by E by in E Figure in Figure 11 b.11b. All All EDS EDS elemental elemental mapsmaps were re recordedcorded using using K-alpha K-alpha characteristic characteristic X-ray X-ray of each of each element. element.

4. Discussion The corrosion tests performed in this work—NaCl solution immersion and salt spray exposure—clearly showed the mitigation of galvanic corrosion attack by implementing a polymer-insulation method. However, galvanic corrosion attack at the local spots was still observed in a polymer-insulated joint specimen with a relatively long salt spray exposure (1264 h), implying that the polymer-insulation method cannot completely prevent the formation of galvanic coupling. It is considered that the main cathodic surface(s) for galvanic corrosion are the steel bolt head and ss316 washer for the bare joints, but only the ss316 washer for the polymer-insulated joints. This is because the steel bolts were galvanically protected in the bare joints but not in the polymer-insulated joints. To improve the protection against galvanic corrosion, an insulation treatment on the ss316 washer should be implemented so that the washer no longer functions as a cathode in a polymer-insulated joint. Future work can focus on insulation treatment(s) for the washer and verification of galvanic corrosion prevention in a bolted joint configuration. Regarding mechanical joint integrity, the polymer-insulation method was effective in minimizing joint strength reduction by preventing corrosion ditches that induced crack initiation and propagation under tensile loading. The polymer-insulated joints in uncorroded and corroded conditions shared the same primary failure mode, but the joint strength was reduced by up to 20% after corrosion. One potential cause of this reduction is presumably associated with the local corrosion pits found around the washer (see Fig- ure 11b); the corrosion pits in the path of the shared failure mode likely facilitated crack propagation to the final failure. Meanwhile, the reduction of joint strength in the bare bolted joint of AZ31B and CFRC was very significant after a relatively short salt spray exposure (≤438 h) due to the formation of corrosion diches. In contrast, another joint configuration in which carbon fiber epoxy composites were rivet-joined to Al alloy 6060 showed much lower joint degradation by retaining 77% of the original strength after salt spray exposure for 1176 h [23]. This highlights the greater susceptibility of Mg alloys to galvanic corrosion and subsequent mechanical degradation when joined with a noble metal fastener. To avoid any early corrosion failure of the joints, it is also crucial to determine effective corrosion barrier(s) for Mg alloys. The results of the current work can be applied to the joint design for galvanic corrosion mitigation in lightweight multi-material vehicles and other transportation industries requiring fuel-efficient improvement.

5. Conclusions

Materials 2021, 14, 1670 13 of 15

4. Discussion The corrosion tests performed in this work—NaCl solution immersion and salt spray exposure—clearly showed the mitigation of galvanic corrosion attack by implementing a polymer-insulation method. However, galvanic corrosion attack at the local spots was still observed in a polymer-insulated joint specimen with a relatively long salt spray exposure (1264 h), implying that the polymer-insulation method cannot completely prevent the formation of galvanic coupling. It is considered that the main cathodic surface(s) for galvanic corrosion are the steel bolt head and ss316 washer for the bare joints, but only the ss316 washer for the polymer-insulated joints. This is because the steel bolts were galvanically protected in the bare joints but not in the polymer-insulated joints. To improve the protection against galvanic corrosion, an insulation treatment on the ss316 washer should be implemented so that the washer no longer functions as a cathode in a polymer- insulated joint. Future work can focus on insulation treatment(s) for the washer and verification of galvanic corrosion prevention in a bolted joint configuration. Regarding mechanical joint integrity, the polymer-insulation method was effective in minimizing joint strength reduction by preventing corrosion ditches that induced crack initiation and propagation under tensile loading. The polymer-insulated joints in uncorroded and corroded conditions shared the same primary failure mode, but the joint strength was reduced by up to 20% after corrosion. One potential cause of this reduction is presumably associated with the local corrosion pits found around the washer (see Figure 11b); the corrosion pits in the path of the shared failure mode likely facilitated crack propagation to the final failure. Meanwhile, the reduction of joint strength in the bare bolted joint of AZ31B and CFRC was very significant after a relatively short salt spray exposure (≤438 h) due to the formation of corrosion diches. In contrast, another joint configuration in which carbon fiber epoxy composites were rivet-joined to Al alloy 6060 showed much lower joint degradation by retaining 77% of the original strength after salt spray exposure for 1176 h [23]. This highlights the greater susceptibility of Mg alloys to galvanic corrosion and subsequent mechanical degradation when joined with a noble metal fastener. To avoid any early corrosion failure of the joints, it is also crucial to determine effective corrosion barrier(s) for Mg alloys. The results of the current work can be applied to the joint design for galvanic corrosion mitigation in lightweight multi-material vehicles and other transportation industries requiring fuel-efficient improvement.

5. Conclusions In this work, polymer insulation of bolted AZ31B Mg alloy and CFRC joints was applied to remove the galvanic circuit formation which would cause accelerated anodic dissolution of Mg by cathodic hydrogen reduction on a steel bolt and a nut, ss316 washers, and, to lesser degree, carbon fiber bundles in the composite. The joint specimens were prepared in lap and cross-tension joint configurations in bare and polymer-insulated conditions. After application of different tape masking, the joint specimens underwent corrosion tests by immersion in 0.1 M NaCl and salt spray exposure. The results are summarized below. 1. The corrosion depths of AZ31B measured after the immersion tests were much greater in the bare (i.e., no insulation as the control case) joints than in the polymer-insulated bolted joints, indicating that polymer insulation applied on bolted joints effectively reduced galvanic corrosion. 2. After the salt spray exposure tests, the bare joint developed corrosion ditches around the washers, whereas the polymer-insulated joints did not have any severe attack in the same location. The corrosion volume determined by optical profilometry was greater in the bare joints than in the polymer-insulated joints. 3. Only about 10% of joint strength remained in the bare joints after 438 h salt spray exposure, with the failure initiated at a corrosion ditch of the AZ31B surface. In contrast, 80~90% of joint strength remained in the polymer-insulated joints after 1264 h in the failure mode, a strength similar to the uncorroded specimens. Materials 2021, 14, 1670 14 of 15

4. Post-corrosion polymer-insulated joints (1264 h salt spray exposure) revealed local corrosion pits on the surface of the AZ31B adjacent to the washer, as seen in a cross- sectional characterization, indicating that polymer insulation did not completely remove the galvanic corrosion, but it functioned as a mitigation method.

Supplementary Materials: The following are available online at https://www.mdpi.com/article/10 .3390/ma14071670/s1, Figure S1: Photos of bolted joint, Figure S2: Photos of tape-masked lap and cross-tension joints under different conditions, Figure S3: Example optical profilometry measurement, Table S1: Chemical analysis of the tap water. Author Contributions: Conceptualization, C.D.W., J.J. and Y.C.L.; methodology, J.J. and Y.C.L.; investigation, J.J., Y.C.L. and Y.L.; writing—original draft preparation, J.J., Y.C.L. and Y.L.; writing— review and editing, J.J. and Y.C.L. and Z.F.; supervision, C.D.W. and Z.F.; All authors have read and agreed to the published version of the manuscript. Funding: This research was financially sponsored by the US Department of Energy Vehicle Technolo- gies Office, as part of the Joining Core Program. Oak Ridge National Laboratory (ORNL) is managed by UT-Battelle, LLC, for the US Department of Energy under Contract DE-AC05-00OR22725. Data Availability Statement: The data presented in this study are available on request from the corresponding author. Acknowledgments: The authors would like to acknowledge Donald Erdman III and Rick R. Lowden at ORNL for mechanical testing assistance. John Wade at ORNL conducted optical profilometry measurements. Conflicts of Interest: The authors declare no conflict of interest.

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