Microgalvanic Corrosion Behavior of Cu-Ag Active Braze Alloys Investigated with SKPFM

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Article Microgalvanic Corrosion Behavior of Cu-Ag Active Braze Alloys Investigated with SKPFM

Armen Kvryan, Kari Livingston, Corey M. Efaw, Kyle Knori, Brian J. Jaques, Paul H. Davis, Darryl P. Butt and Michael F. Hurley *

Department of Materials Science and Engineering, College of Engineering, Boise State University, Boise, ID 83725-2090, USA; [email protected] (A.K.); [email protected] (K.L.); [email protected] (C.M.E.); [email protected] (K.K.); [email protected] (B.J.J.); [email protected] (P.H.D.); [email protected] (D.P.B.) * Correspondence: [email protected]; Tel.: +1-208-426-4075

Academic Editors: Vineet V. Joshi and Alan Meier Received: 3 February 2016; Accepted: 6 April 2016; Published: 19 April 2016

Abstract: The nature of microgalvanic couple driven corrosion of brazed joints was investigated. 316L stainless steel samples were joined using Cu-Ag-Ti and Cu-Ag-In-Ti braze alloys. Phase and elemental composition across each braze and parent metal interface was characterized and scanning Kelvin probe force microscopy (SKPFM) was used to map the Volta potential differences. Co-localization of SKPFM with Energy Dispersive Spectroscopy (EDS) measurements enabled spatially resolved correlation of potential differences with composition and subsequent galvanic corrosion behavior. Following exposure to the aggressive solution, corrosion damage morphology was characterized to determine the mode of attack and likely initiation areas. When exposed to 0.6 M NaCl, corrosion occurred at the braze-316L interface preceded by preferential dissolution of the Cu-rich phase within the braze alloy. Braze corrosion was driven by galvanic couples between the braze alloys and stainless steel as well as between different phases within the braze microstructure. Microgalvanic corrosion between phases of the braze alloys was investigated via SKPFM to determine how corrosion of the brazed joints developed.

Keywords: active braze alloy; corrosion; microgalvanic corrosion; electrochemistry; scanning kelvin probe force microscopy; stainless steel; Cu-Ag alloy; joining; scanning electron microscopy; energy dispersive spectroscopy

1. Introduction Failure, whether chemical or mechanical in nature, often occurs where dissimilar materials are joined together. Common joining techniques range from mechanical fastening (e.g., bolt or rivet) to solid state joining (e.g., diffusion bonding). Brazing is an alternative joining technique in which a filler material is heated so as to form metallurgical bonds with the surfaces of the parts (parent materials) being joined. Brazing is a relatively low temperature process wherein the filler (braze material) is heated to a temperature above its melting point but below that of the parent materials. Joints formed by brazing can achieve very tight tolerances and offer desirable mechanical properties, similar to diffusion bonded materials, but have the advantage of being easily disassembled [1]. Many different braze alloys are available—designed for specific applications. A braze that performs well mechanically may have limited utility based on environmental compatibility. For example, silver-based braze alloys typically have a lower corrosion rate in an industrial atmosphere than in a marine environment [2]. When used in settings where contact with certain types of fuel may occur, silver-based brazes have poor corrosion resistance that can ultimately lead to failure [3]. The effect of brazing on the corrosion behavior of the resultant parts (including parent materials) has not

Metals 2016, 6, 91; doi:10.3390/met6040091 www.mdpi.com/journal/metals Metals 2016, 6, 91 2 of 17 been widely investigated. Through gaining a better understanding of this vital relationship between brazing and its inﬂuence on corrosion behavior, it may be possible to optimize processing parameters and develop more effective brazes. Thus, it is important to understand the mechanism(s) that control the corrosion development at and near joints. Consequently, the impact of brazing on the corrosion behavior of the overall system is investigated in this work.

Background The Cu-Ag based systems investigated in this study were chosen because of their ease of application, commercial availability, and favorable compatibility with common engineering metals and technical ceramics. Good materials compatibility is due in part to effective wetting. Wetting, the ability of a liquid to maintain contact with a solid surface, is an important factor in the overall effectiveness of a braze [4]. The wetting parameter, or wettability, is the geometric complement of the contact angle, and thus is inversely related to the contact angle [5]. During brazing, good compatibility is achieved when the braze alloy wets the target material well. This ensures excellent surface coverage and any diffusion that occurs does not result in poor mechanical behavior. To promote wetting and joint compliance, reactive elements are added to brazing alloys [1]. The resultant brazing alloys are known as active brazing alloys (ABA). Titanium is often added to silver based braze alloys in order to enhance their ability to wet the parent material(s) being joined, which is vital when joining dissimilar materials, including ceramics [1]. In addition, an increased amount of titanium in brazes has been found to yield thicker joints [6]. In this study, commercially available Cu-Ag-Ti and Cu-Ag-In-Ti braze alloys were obtained and used to join 316L stainless steel samples. It should be noted that oxide layers on stainless steel, including 316L, can act as a barrier to wetting, resulting in lower degrees of wettability for the braze [7]. The indium additions (~12%) present in the Cu-Ag-In-Ti braze lower the metal surface tension [8]. Additionally, indium additions in a titanium-containing braze alloy increase activity of titanium and wettability [8,9]. In any joining technique, the joint must preserve mechanical integrity and environmental compatibility. Heat input and the introduction of dissimilar materials may provide initiation spots for pitting corrosion at microstructural heterogeneities, galvanic attack at the metal/braze interface, or dealloying within the braze material. Initially, corrosion within the braze alloy may cause loss of joint integrity and sustained braze corrosion may create an aggressive local chemical environment that can lead to depassivation of the parent material. Relatively low temperature brazing alloys typically rely on noble metals to provide good wetting behavior, adhesion, and joint compatibility. However, these alloys generally provide a thermodynamic driving force for galvanic corrosion when joined with the parent metal and are exposed to conditions that support active corrosion. Galvanic corrosion as the mechanism of failure for silver based brazes coupled to stainless steels was first proposed by Takemoto et al. [10]. Current research on the effect of brazing has mainly focused on compatibility and resultant mechanical behavior [11–14]. However, once a candidate braze has been identified, the reliability and resistance to environmental attack must be addressed to determine long-term viability. With the exception of a few isolated studies [15–18], the effect of brazing on corrosion behavior has received little attention in the open literature [18]. Because of the multicomponent aspect of brazes, a multiphase microstructure is usually seen post brazing where the two parent materials are joined [18]. This is mainly due to precipitation of stable or meta-stable phases during the brazing cycle [16]. Once these phases are present, they would be expected to have different electrochemical behavior based on composition. The local potential difference between dissimilar regions in the microstructure will influence corrosion behavior, and is known as microgalvanic corrosion. The term microgalvanic corrosion describes a galvanic corrosion cell occurring on a sub-grain scale. Regions of varying composition result in potential differences amongst the individual phases in the braze, and tend to increase the corrosion rate of the less noble phases [16]. The concept of microgalvanic corrosion is Metals 2016, 6, 91 3 of 17 important to joint reliability, as manipulations of the filler metal in the braze can dramatically influence the overall corrosion behavior of a braze [19]. Confirmation of a microgalvanic corrosion mechanism requires accurate characterization of the surface of the materials, both in terms of composition and electrochemical potential. Scanning Kelvin Probe Force Microscopy (SKPFM) has been shown to be an effective technique to characterize expected electrochemical behavior of surface inhomogeneities within a metal alloy’s microstructure [20]. A Kelvin probe measures the work function difference (Volta potential) between the surface of a sample and the probe itself. Correlation between the Volta potential difference (VPD) obtained via SKPFM and electrode solution potentials, and hence the likely development of galvanic couples during active corrosion conditions has previously been established [21]. However, this relationship should be approached with some caution for unconfirmed systems. SKPFM is a surface technique and sensitive to the formation of surface reaction products such as oxide layers that may influence the measurement, and thus may not always directly correlate with solution potential. However, by using an Atomic Force Microscope (AFM) with surface potential feedback and nullification, SKPFM is able to map Volta potential differences on a surface with extremely fine (sub-micron) resolution. Compared to other local techniques, SKPFM currently provides the highest achievable spatial resolution for studying corrosion initiation driven by microstructure inhomogeneities. This paper utilized SKPFM to analyze and characterize the VPD between the different metallic/intermetallic phases present within the brazes studied. This method was used to investigate and explain corrosion initiation and propagation driven by galvanic corrosion arising from compositional differences within brazing alloys used to join 316L stainless steel.

2. Materials and Methods

2.1. Materials and Joining Procedure Commercial 316L stainless steel sheet with a thickness of 3 mm was water-jet cut into disk shaped samples of various sizes to be brazed. The braze alloys used were Cu-Ag-Ti-ABA with a thickness of approximately 50 µm and Cu-Ag-In-Ti-ABA with a thickness of 55 µm. Compositions of all materials are listed in Tables1 and2. Stainless steel disks were polished to a 1 µm ﬁnish with SiC polishing pads. The as-received Cu-Ag-Ti and Cu-Ag-In-Ti foils were lightly polished to an 800 grit ﬁnish and cut to sizes just smaller than the stainless steel disks. After polishing, all samples were ultrasonically cleaned in ethanol for 15 min, washed in deionized water, and dried using compressed air.

Table 1. Compositions in atomic percent of the braze alloys.

Material Ag Cu Ti In Cu-Ag-Ti 63.1 35.1 1.8 – Cu-Ag-In-Ti 59.00 27.25 1.25 12.5

Table 2. Compositions in atomic percent of the material being joined.

Material C P Ni Cr Mn Mo N Si S Fe 316L 0.0002 0.00036 0.101 0.1663 0.0168 0.0203 0.00066 0.00485 0.00026 Balance

Two geometrical configurations were used during brazing: sandwich (stainless steel/braze foil/stainless steel) and coated (braze foil on stainless steel disk). Prior to firing, the cleaned and polished samples were placed in an alumina boat wrapped in niobium foil and inserted into the furnace hot zone. The system was then purged with ultra-high purity argon gas (UHP Ar, 99.999%, Norco, Boise, ID, USA) for 20 min before continuously flowing with oxygen gettered UHP Ar. Stainless steel joining with Cu-Ag-Ti was achieved through the following thermal cycle suggested by the supplier: ramp to 700 ˝C (80 ˝C below the solidus temperature of Cu-Ag-Ti) at 5 ˝C/min and hold for 20 min Metals 2016, 6, 91 4 of 17 before ramping to 830 ˝C (15 ˝C above the liquidus temperature of Cu-Ag-Ti braze alloy) at a rate of 5 ˝C/min and held there for 15 min. The furnace was then cooled to room temperature at a rate of 5 ˝C/min. To create Cu-Ag-In-Ti joints, the furnace was ramped from room temperature to a temperature of 500 ˝C (130 ˝C below the solidus temperature of Cu-Ag-In-Ti) at a rate of 5 ˝C/min and held for 20 min, followed by a ramp to 730 ˝C (15 ˝C above the liquidus temperature of Cu-Ag-In-Ti) at a rate of 5 ˝C/min and held for 15 min, then cooled at a rate of 5 ˝C/min to room temperature, again as per the supplier recommendation.

2.2. Scanning Kelvin Probe Force Microscopy (SKPFM) of Brazed Regions For SKPFM, brazed samples were cross sectioned, cold mounted in epoxy, and polished with progressively finer grit silicon carbide pads and diamond slurries starting with 400 grit and ending with a 1 µm diamond slurry. Next, the samples were cleaned ultrasonically in a bath of non-denatured (200 proof) High Performance Liquid Chromatography (HPLC)/spectrophotometric grade ethanol (Sigma-Aldrich, St. Louis, MO, USA). The final polishing step employed a VibroMet 2 vibratory polisher (Buehler, Lake Bluff, IL, USA) in which samples were polished for 1 h using a 12” Mastertex PSA pad (Buehler) covered with a 0.50 µm diamond slurry (MasterPrep Polishing Solution, Buehler). The polished samples were then rinsed with ethanol and blown dry with compressed air. Prior to imaging, an electrical connection between the sample surface and the AFM stage was established using colloidal silver paint and verified with a voltmeter. Imaging was conducted using a Bruker Dimension Icon AFM operating in frequency modulation Peak Force KPFM (FM PF-KPFM) mode with a PFQNE-AL probe (Bruker, Santa Barbara, CA, USA). PF-KPFM is a dual-pass method wherein the first pass acquires topography via Peak Force tapping (i.e., rapid force curves). The second pass is then used to measure the tip-sample surface potential difference at a user-determined fixed lift height above the sample surface. The SKPFM technique and important experimental considerations have been described in greater detail previously [22], but lift heights of ~100 nm and a frequency modulation based detection scheme were employed. To enhance signal to noise and minimize the effects of residual sample roughness on the surface potential image, a slow scan rate was used (~0.05–0.1 Hz). Because SKPFM measures the difference in work function between the AFM probe and the sample surface (i.e., the relative rather than absolute surface potential), potentials of the different phases are reported relative to each other. Absolute potentials can be determined; e.g., using an inert gold standard as a reference material.

2.3. Braze Phase Composition A Scanning Electron Microscope (SEM) (Hitachi S-3400N-II, Hitachi, Tokyo, Japan) equipped with energy dispersive spectroscopy (EDS) capabilities was used to image and analyze the mounted braze joint cross-sections for both Cu-Ag-Ti and Cu-Ag-In-Ti alloys after SKPFM characterization. Performing SEM after SKPFM was to avoid effects of electron beam irradiation on the surface potential measured. The cross section of each braze alloy was imaged in both secondary electron mode and backscatter electron mode. Subsequently, elemental mapping was performed on the cross sections of each sample, along with multi-point analysis at selected locations.

2.4. Corrosion Testing To assess corrosion behavior electrochemical and exposure testing in 0.6 M NaCl solution at room temperature (22–24 ˝C) was conducted. Potentiodynamic testing was performed on both the braze alloys and stainless steel samples separately following thermal treatment using the prescribed brazing furnace proﬁle. The braze alloy samples were prepared by applying multiple coatings of either the foil or paste braze compound to one face of a stainless steel coupon. Multiple braze coatings ensured that only the braze material was exposed to solution when tested, with no impact from the underlying stainless steel possibly present at the pinholes that may occur with a single coating. A conventional Metals 2016, 6, 91 5 of 17

3-electrode cell with a Pt mesh counter electrode and a saturated calomel (SCE) reference electrode was used for potentiostat controlled polarization measurements. Metals 2016, 6, 91 5 of 16 3. Results 3. Results Metals 2016, 6, 91 5 of 16 3.1. Brazed Stainless Steel Joint Characterization 3.1. Brazed Stainless Steel Joint Characterization 3. Results Metallographic samples of the post-braze joint cross-section were examined via SEM in order to Metallographic samples of the post‐braze joint cross‐section were examined via SEM in order to characterize the resulting joint (Figure1). The joints formed a clean, tight, hermetic seal with the parent characterize3.1. Brazed Stainlessthe resulting Steel Joint joint Characterization (Figure 1). The joints formed a clean, tight, hermetic seal with the 316L samples. Both brazes were approximately 50 µm in width and displayed distinctive eutectic type parentMetallographic 316L samples. samples Both brazes of the werepost‐ brazeapproximately joint cross ‐50section μm inwere width examined and displayed via SEM in distinctive order to phases in the braze region following the prescribed thermal cycles. eutecticcharacterize type phases the resulting in the brazejoint (Figureregion following1). The joints the prescribedformed a clean, thermal tight, cycles. hermetic seal with the parent 316L samples. Both brazes were approximately 50 μm in width and displayed distinctive eutectic type phases in the braze region following the prescribed thermal cycles.

(a) (b)

Figure 1. Scanning Electron Microscopy (SEM) images of cross sections of the resultant braze joint Figure 1. Scanning Electron(a) Microscopy (SEM) images of cross sections of(b) the resultant braze joint between two stainless steel 316L samples joined using: Cu‐Ag‐Ti (a) and Cu‐Ag‐In‐Ti (b) braze alloys. betweenFigure two 1. stainlessScanning steelElectron 316L Microscopy samples joined (SEM) using: images Cu-Ag-Ti of cross sections (a) and of Cu-Ag-In-Ti the resultant (b braze) braze joint alloys. EDSbetween line scanstwo stainless are presented steel 316L in samples Figure joined 2 (Cu using:‐Ag‐ Ti)Cu‐ Agand‐Ti Figure (a) and 3Cu (Cu‐Ag‐Ag‐In‐‐TiIn (‐bTi)) braze below. alloys. The Cu‐ AgEDS‐Ti braze line had scans a Ag are‐rich presented matrix with in numerous Figure2 (Cu-Ag-Ti)distributed regions, and Figure generally3 (Cu-Ag-In-Ti) ~1–10 μm across, below. of EDS line scans are presented in Figure 2 (Cu‐Ag‐Ti) and Figure 3 (Cu‐Ag‐In‐Ti) below. The Cu‐ Thea Cu-Ag-Tiprecipitated braze Cu‐rich had phase. a Ag-rich Generally, matrix the with EDS numerous results did distributed not indicate regions, noticeable generally diffusion ~1–10 of theµ m Ag‐Ti braze had a Ag‐rich matrix with numerous distributed regions, generally ~1–10 μm across, of across,braze of filler a precipitated materials into Cu-rich the stainless phase. Generally, steel. However, the EDS in a results few areas did notthe EDS indicate scans noticeable indicated diffusion some a precipitated Cu‐rich phase. Generally, the EDS results did not indicate noticeable diffusion of the of thetransport braze of ﬁller the braze materials alloy into elements the stainless into the stainless steel. However, steel, resulting in a few in a areas Ti and the Ag EDS‐containing scans indicatedregion braze filler materials into the stainless steel. However, in a few areas the EDS scans indicated some adjacent to the braze for the Cu‐Ag‐Ti braze sample. It was also observed that that the bulk of the sometransport transport of ofthe the braze braze alloy alloy elements elements into the into stainless the stainless steel, resulting steel, resulting in a Ti and in Ag a Ti‐containing and Ag-containing region titanium in the braze segregated to one side of the braze/stainless steel interface (Figure 2). The Cu‐ regionadjacent adjacent to the to braze the braze for the for Cu the‐Ag Cu-Ag-Ti‐Ti braze sample. braze sample. It was also It was observed also observed that that the that bulk that of the the bulk Ag‐In‐Ti braze phase structure consisted of an Ag‐In‐rich matrix phase with numerous regions of a of thetitanium titanium in the in thebraze braze segregated segregated to one toside one of sidethe braze/stainless of the braze/stainless steel interface steel (Figure interface 2). The (Figure Cu‐ 2). Theprecipitated Cu-Ag-In-TiAg‐In‐Ti braze Cu braze‐ richphase phase. phase structure structureTi was consisted also consisted found of an to Ag of be‐ an Inpresent‐ Ag-In-richrich matrix in some matrixphase of the with phase Cu numerous‐rich with regions. numerous regions of regions a of a precipitatedprecipitated Cu Cu-rich‐rich phase. phase. Ti Tiwas was also also found found to be to present be present in some in someof the ofCu the‐rich Cu-rich regions. regions.

(a) (a) Figure 2. Cont. FigureFigure 2. ContCont..

Metals 2016, 6, 91 6 of 17 Metals 2016, 6, 91 6 of 16 Metals 2016, 6, 91 6 of 16

(b) (b) Figure 2. (a) SEM micrograph and (b) energy dispersive spectroscopy (EDS) line scan results from the Figure 2. (a) SEM micrograph and (b) energy dispersive spectroscopy (EDS) line scan results from the Figurecross section 2. (a) SEM of a micrograph316L stainless and steel/Cu (b) energy‐Ag‐ dispersiveTi brazed joint. spectroscopy (EDS) line scan results from the cross section of a 316L stainless steel/Cu-Ag-Ti brazed joint. cross section of a 316L stainless steel/Cu‐Ag‐Ti brazed joint.

(a) (a)

(b) (b) Figure 3. (a) SEM micrograph and (b) EDS line scans of a cross section of a 316L stainless steel/Cu‐ FigureAg‐In‐ Ti3. 3. brazed(a)( aSEM) SEM joint. micrograph micrograph and and (b) EDS (b) EDSline scans line scansof a cross of a section cross sectionof a 316L of stainless a 316L steel/Cu stainless‐ Agsteel/Cu-Ag-In-Ti‐In‐Ti brazed joint. brazed joint. Phase separation within the braze regions was further characterized via optical microscopy to aid withPhase identification separation within of the thevarious braze phases regions present was further when characterizedinitiating SKPFM via opticalscans. Opticalmicroscopy images to aidobtained with identification of the Cu‐Ag ‐ofTi theand various Cu‐Ag ‐phasesIn‐Ti braze present areas when showed initiating clear variationSKPFM scans. of phase Optical composition images obtained of the Cu‐Ag‐Ti and Cu‐Ag‐In‐Ti braze areas showed clear variation of phase composition

Metals 2016, 6, 91 7 of 17

Phase separation within the braze regions was further characterized via optical microscopy to aid with identiﬁcation of the various phases present when initiating SKPFM scans. Optical images Metals 2016, 6, 91 7 of 16 obtained of the Cu-Ag-Ti and Cu-Ag-In-Ti braze areas showed clear variation of phase composition withinwithin the the braze, braze, with with uniform uniform 316L 316L stainless stainless steel steel on on either either side side (Figure (Figure4). 4). The The identical identical sample sample regionsregions seen seen in in Figure Figure4 were4 were also also characterized characterized and and discussed discussed later later in in this this paper. paper.

Phase Cu Ag Ti Cu Rich 94.65 5.10 0.50 Ag Rich 10.01 89.98 0.01

(a)

Phase Cu Ag Ti In Cu Rich 90.18 5.86 2.09 1.88 Ag Rich 13.29 75.26 0 11.45 Cu‐Ti Rich 77.27 0.84 21.21 0.67

(b)

Figure 4. Optical image of a Cu‐Ag‐Ti braze (a) and Cu‐Ag‐In‐Ti braze (b) with labeled markers Figure 4. Optical image of a Cu-Ag-Ti braze (a) and Cu-Ag-In-Ti braze (b) with labeled markers indicating the microconstituent phases present. Tables to the right of each image present average indicating the microconstituent phases present. Tables to the right of each image present average atomic % for each phase in (a) and (b), respectively, calculated from EDS data. atomic % for each phase in (a) and (b), respectively, calculated from EDS data.

In order to obtain semi‐quantitative compositions of the phases, EDS multi point scans were acquiredIn order for both to obtain Cu‐Ag semi-quantitative‐Ti and Cu‐Ag‐In compositions‐Ti samples. Multiple of the phases, spectra EDS were multi taken point at representative scans were acquiredpoints within for both the Cu-Ag-Ti individual and phase Cu-Ag-In-Ti regions of samples. each sample. Multiple The average spectra werecomposition taken at values representative in atomic pointspercent within obtained the individual for each phase phase present regions are of listed each sample. in the tables The average adjacent composition to the respective values images in atomic (see percentFigure obtained4). The Cu for‐Ag each‐Ti sample phase present had a standard are listed deviation in the tables below adjacent 3, whilst to the respectiveCu‐Ag‐In‐Ti images had a (seestandard Figure deviation4). The Cu-Ag-Ti below 5. sample had a standard deviation below 3, whilst the Cu-Ag-In-Ti had a standardFor deviationthe Cu‐Ag below‐Ti braze 5. alloy, two primary phases, composed primarily of either copper or silver, dominateFor the the Cu-Ag-Ti microstructure braze alloy, of the two braze primary in agreement phases, composed with previous primarily findings of either [8]. copper This orbead silver,‐like dominatemicrostructure the microstructure develops with of overall the braze low solubility in agreement of titanium with previous within the ﬁndings Cu‐Ag [‐8Ti]. braze. This bead-like Titanium microstructurewas not detected develops in the eutectic with overall phases, low but solubility was instead of titanium found withinalong the the interface Cu-Ag-Ti between braze. Titanium the braze wasand not parent detected stainless in the steel eutectic metal phases, [8]. Additionally, but was instead some found titanium along the diffused interface into between the neighboring the braze andstainless parent steel. stainless This steel was metal also [observed8]. Additionally, in previous some titaniumresearch, diffused and is attributed into the neighboring to the lack stainless of both steel.titanium This solubility was also within observed the braze in previous and miscibility research, within and is the attributed adjacent stainless to the lack steel, of leading both titanium to near‐ solubilitycomplete within segregation the braze [8,23]. and In miscibility contrast, within three thedistinct adjacent phases stainless were steel, observed leading in to the near-complete Cu‐Ag‐In‐Ti segregationbrazed joint. [8 Also,,23]. In segregation contrast, three of the distinct titanium phases to the were interface observed was insuppressed. the Cu-Ag-In-Ti Ti content brazed varied joint. in Also,and near segregation regions of where the titanium a distinct to theCu interface‐Ti phase was was suppressed. observed (Figure Ti content 4b) variedand suggested in and near incomplete regions where a distinct Cu-Ti phase was observed (Figure4b) and suggested incomplete phase transformation. phase transformation. The Cu‐Ti rich phase present was likely the Cu4Ti intermetallic phase [23,24]. The Cu-Ti rich phase present was likely the Cu4Ti intermetallic phase [23,24]. 3.2. Corrosion Behavior of Brazed Joints

3.2.1. Electrochemical Testing of Braze Materials Representative polarization curves from Cu‐Ag‐Ti (blue trace) and Cu‐Ag‐In‐Ti (green trace) braze alloys are presented in Figure 5. Polarization data obtained from a bare 316L coupon (red trace) is also included in Figure 5 for comparison. The two different braze alloys displayed nearly identical

Metals 2016, 6, 91 8 of 17

3.2. Corrosion Behavior of Brazed Joints

3.2.1. Electrochemical Testing of Braze Materials Representative polarization curves from Cu-Ag-Ti (blue trace) and Cu-Ag-In-Ti (green trace) braze alloys are presented in Figure5. Polarization data obtained from a bare 316L coupon (red trace)

Metalsis also 2016 included, 6, 91 in Figure5 for comparison. The two different braze alloys displayed nearly identical8 of 16 polarization behavior. The braze alloys exhibited normal Tafel behavior indicative of activation ´5 2 polarization controlbehavior. with The an corrosionbraze alloys rate, exhibitedicorr (at open normal circuit) Tafel of approximately behavior indicative 1 ˆ 10 ofAmps/cm activation, polarizationapproximately control two with orders an of corrosion magnitude rate, greater icorr (at thanopen the circuit)icorr of approximately passive 316L. Moreover, 1 × 10−5 Amps/cm the Ecorr2, approximately(corrosion potential) two orders of the of braze magnitude alloys was greater approximately than the icorr 50 of mV passive less than 316L. that Moreover, of 316L, ´the0.16 Ecorr V (corrosionvs. SCE. potential) of the braze alloys was approximately 50 mV less than that of 316L, −0.16 V vs. SCE.

Figure 5. Potentiodynamic polarization scans conducted on brazes (blue and green traces) and a 316L Figure 5. Potentiodynamic polarization scans conducted on brazes (blue and green traces) and a 316L stainless steel sample (red trace) in 0.6 M NaCl. The scan rate for all testing was 0.166 mV/s. stainless steel sample (red trace) in 0.6 M NaCl. The scan rate for all testing was 0.166 mV/s. To determine any effect the thermal braze cycle might have on the inherent corrosion behavior of 316L,To determinepolarization any curves effect were the thermal conducted braze on cycle bare might316L samples have on that the inherenthad been corrosion fired according behavior to theof 316L, braze polarization cycle mentioned curves in werethe experimental conducted on section. bare 316L When samples compared that to had the been unfired fired stainless according steel, to thethe brazefired stainless cycle mentioned steel sample in the displayed experimental a much section. smaller When passive compared region to with the unfired pitting stainless occurring steel, at approximatelythe fired stainless 0.05 steel V vs. sample SCE (Figure displayed 6). The a muchpitting smaller potential, passive or potential region at with which pitting pitting occurring corrosion at occurs,approximately was seen 0.05 when V vs. a SCEsharp (Figure increase6). The in current pitting potential,occurred upon or potential increasing at which potential. pitting The corrosion pitting potentialoccurs, was of the seen fired when sample a sharp was increase significantly in current lower occurredthan the unfired upon increasing 316L sample, potential. likely due The to pitting grain boundarypotential of sensitization the fired sample [25]. wasThe significantlyeffect on localized lower thancorrosion the unfired behavior 316L could sample, be expected likely due since to grain the sampleboundary was sensitization held in the sensitization [25]. The effect range on for localized approximately corrosion 35 behaviormin. Optical could microscopy be expected of the since pitted the surfacesample (not was shown held in here) the sensitization revealed grain range boundary for approximately attack had 35occurred min. Optical at areas microscopy adjacent to of pits the on pitted the firedsurface sample. (not shown In contrast, here) revealedpits on the grain unfired boundary sample attack did hadnot show occurred evidence at areas of adjacentpreferential to pits grain on boundarythe fired sample. attack near In contrast, the pit openings, pits on the confirming unfired sample some diddegree not showof sensitization evidence ofon preferential the 316L sample grain asboundary a result attackof the thermal near the braze pit openings, cycle. confirming some degree of sensitization on the 316L sample as a result of the thermal braze cycle. 0.5 3.2.2. Exposure Testing at Open Circuit Potential 0.4

Exposure testing was conducted0.3 to observe macroscopic corrosion propagation behavior of the braze alloy and stainless0.2 steel at the free corrosion potential under natural, galvanic coupling conditions. Circular braze foil0.1 coupons were fabricated such that the foil occupied approximately 80% of the exposed surface area of0 the polished face of the 316L disk. Following ﬁring, the coated samples and an untreated 316L control sample were immersed in 0.6 M NaCl solution and the samples were -0.1 E(V) vs. SCE vs. E(V) observed periodically to monitor corrosion progression,SS as 316L shown in Figure7. -0.2 SS 316L (Braze Cycle) -0.3

-0.4

-0.5 1x10-9 1x10-8 1x10-7 1x10-6 1x10-5 1x10-4 1x10-3 1x10-2 i (Amps/cm2)

Figure 6. Potentiodynamic polarization scans conducted on 316L stainless steel subjected to a thermal brazing cycle (blue curve) compared to unfired 316L (red curve). Testing was conducted in 0.6 M NaCl with a scan rate 0.166 mV/s.

Metals 2016, 6, 91 8 of 16 polarization behavior. The braze alloys exhibited normal Tafel behavior indicative of activation polarization control with an corrosion rate, icorr (at open circuit) of approximately 1 × 10−5 Amps/cm2, approximately two orders of magnitude greater than the icorr of passive 316L. Moreover, the Ecorr (corrosion potential) of the braze alloys was approximately 50 mV less than that of 316L, −0.16 V vs. SCE.

Figure 5. Potentiodynamic polarization scans conducted on brazes (blue and green traces) and a 316L stainless steel sample (red trace) in 0.6 M NaCl. The scan rate for all testing was 0.166 mV/s.

To determine any effect the thermal braze cycle might have on the inherent corrosion behavior of 316L, polarization curves were conducted on bare 316L samples that had been fired according to the braze cycle mentioned in the experimental section. When compared to the unfired stainless steel, the fired stainless steel sample displayed a much smaller passive region with pitting occurring at approximately 0.05 V vs. SCE (Figure 6). The pitting potential, or potential at which pitting corrosion occurs, was seen when a sharp increase in current occurred upon increasing potential. The pitting potential of the fired sample was significantly lower than the unfired 316L sample, likely due to grain boundary sensitization [25]. The effect on localized corrosion behavior could be expected since the sample was held in the sensitization range for approximately 35 min. Optical microscopy of the pitted surface (not shown here) revealed grain boundary attack had occurred at areas adjacent to pits on the fired sample. In contrast, pits on the unfired sample did not show evidence of preferential grain boundaryMetals 2016, 6attack, 91 near the pit openings, confirming some degree of sensitization on the 316L sample9 of 17 as a result of the thermal braze cycle.

0.5

0.4

0.3

0.2

0.1

-0.1 E(V) vs. SCE vs. E(V) SS 316L -0.2 SS 316L (Braze Cycle) Metals 2016, 6, 91 9 of 16 -0.3 3.2.2. Exposure Testing at Open-0.4 Circuit Potential -0.5 Exposure testing was conducted1x10-9 1x10 to-8 observe1x10-7 1x10 macroscopic-6 1x10-5 1x10 -4corrosion1x10-3 1x10 propagation-2 behavior of the i (Amps/cm2) braze alloy and stainless steel at the free corrosion potential under natural, galvanic coupling conditions. Circular braze foil coupons were fabricated such that the foil occupied approximately FigureFigure 6. 6. PotentiodynamicPotentiodynamic polarization polarization scans scans conducted conducted on 316L 316L stainless stainless steel steel subjected subjected to a a thermal thermal 80% of the exposed surface area of the polished face of the 316L disk. Following firing, the coated brazingbrazing cycle (blue(blue curve) curve) compared compared to to unﬁred unfired 316L 316L (red (red curve). curve). Testing Testing was was conducted conducted in 0.6 in M 0.6 NaCl M samples and an untreated 316L control sample were immersed in 0.6 M NaCl solution and the NaClwith awith scan a rate scan 0.166 rate mV/s.0.166 mV/s. samples were observed periodically to monitor corrosion progression, as shown in Figure 7.

FigureFigure 7. TimeTime lapse lapse photographs photographs of of corrosion corrosion propagation propagation during during long long term term exposure exposure testing testing in 0.6 in 0.6M NaCl. M NaCl. Cu Cu-Ag-Ti‐Ag‐Ti (top (top) and) and Cu Cu-Ag-In-Ti‐Ag‐In‐Ti (bottom (bottom) braze) braze alloy alloy foil foil disks disks were were used to coat ~80%~80% ofof thethe exposedexposed faceface areaarea onon 316L316L stainlessstainless steelsteel samples.samples. TheThe stainlessstainless steelsteel diskdisk diameterdiameter waswas 1616 mmmm forfor bothboth samples.samples.

TheThe 316L316L controlcontrol samplesample (not(not shown)shown) diddid notnot displaydisplay evidenceevidence ofof depassivationdepassivation oror productionproduction ofof anyany signiﬁcantsignificant corrosioncorrosion productsproducts forfor thethe durationduration ofof testingtesting (166(166 days,days, oror ~24~24 weeks). However, afterafter onlyonly 2222 daysdays ofof immersion bothboth ofof the braze foil coated samples showed evidence of macroscale corrosioncorrosion damagedamage (Figure(Figure7 7).). Visual Visual observation observation revealed revealed that that corrosion corrosion was was limited limited to to the the braze braze foil foil surface,surface, whilewhile thethe remainingremaining uncoateduncoated areaarea ofof thethe 316L316L samplesample appearedappeared unaffected.unaffected. WhileWhile thethe extentextent ofof thethe corrosioncorrosion damagedamage waswas notnot quantiﬁed,quantified, thethe presencepresence ofof blue-greenblue‐green corrosioncorrosion reactionreaction productsproducts coveringcovering both the the Cu Cu-Ag-Ti‐Ag‐Ti and and Cu Cu-Ag-In-Ti‐Ag‐In‐Ti foil foil surfaces surfaces suggested suggested preferential preferential corrosion corrosion of the of Cu the‐ Cu-richrich phase phase present present in each in each braze braze alloy alloy [14]. [14 Moreover,]. Moreover, both both samples samples also also exhibited exhibited regions regions of dark of dark red redcorrosion corrosion products products deposited deposited within within the foil the‐ foil-coatedcoated region region toward toward the end the endof the of test the duration. test duration. It is Itsuspected is suspected these these corrosion corrosion products products were were iron iron oxide oxide-hydroxides‐hydroxides generated generated from from the underlyingunderlying stainlessstainless steelsteel whichwhich hadhad becomebecome exposedexposed followingfollowing corrosioncorrosion perforation perforation of of the the braze braze foil foil coating. coating.

3.3. SKPFM Measurements SKPFM was used to measure Volta potential differences (VPD) among the various phases present on the surface of the brazed region of the stainless steel joint. SKPFM measurements were obtained prior to SEM/EDS characterization of the same regions on both braze alloy joints to avoid any effect of electron irradiation on surface potential measurements [22,26,27]. Potential maps acquired from SKPFM coupled with composition maps obtained from SEM/EDS clearly showed that the observed variations in potential correlated with changes in composition within the braze regions (Figures 8 and 9).

Metals 2016, 6, 91 10 of 17

Figure 8. Secondary electron SEM image of Cu Cu-Ag-Ti‐Ag‐Ti sample (a) followed by corresponding Scanning Kelvin ProbeProbe ForceForce MicroscopyMicroscopy (SKPFM)(SKPFM) surfacesurface potentialpotential imageimage ((bb).). EDS elemental maps of thethe identicalidentical regionregion for:for: Titanium ((cc);); CopperCopper ((dd);); andand SilverSilver ((ee)) areare shown.shown.

From the SKPFM VPD data, there is a clear difference in potential values between the two primary phases for the Cu-Ag-Ti braze sample, with the Ag-rich phase as the brighter or more noble area, and the Cu-rich phase darker (more active potential). The Volta potential observed for the stainless steel outside the braze region was more negative than either of the braze phases (Figure8b). These differences are expected based solely on composition, and the difference promotes dissimilar metal, galvanic couple driven corrosion within the braze alloy. Similar results were obtained on the Cu-Ag-In-Ti sample, with the most noble phase being the Ag-rich phase. The most active braze phase is the Cu-Ti rich phase, while the Volta potential of the Cu-rich phase falls in between the two. The surrounding stainless steel was similar to the Ti-Cu rich phase (Figure9).

Figure 9. Secondary electron SEM image of Cu‐Ag‐In‐Ti sample with the red box indicating where SKPFM was performed (a). SKPFM surface potential image (b) and EDS elemental maps of the identical region for: Copper (c); Silver (d); Indium (e); and Titanium (f) are shown.

Metals 2016, 6, 91 10 of 16

Figure 8. Secondary electron SEM image of Cu‐Ag‐Ti sample (a) followed by corresponding Scanning MetalsKelvin2016, 6, Probe 91 Force Microscopy (SKPFM) surface potential image (b). EDS elemental maps of the11 of 17 identical region for: Titanium (c); Copper (d); and Silver (e) are shown.

FigureFigure 9.9. SecondarySecondary electron SEM image of Cu-Ag-In-TiCu‐Ag‐In‐Ti samplesample with thethe redred boxbox indicatingindicating wherewhere SKPFMSKPFM waswas performed performed (a ).(a SKPFM). SKPFM surface surface potential potential image image (b) and (b EDS) and elemental EDS elemental maps of maps the identical of the regionidentical for: region Copper for: (c Copper); Silver ( (cd);); Silver Indium (d); (e Indium); and Titanium (e); and Titanium (f) are shown. (f) are shown.

4. Discussion Research on braze reliability has largely focused on evaluating mechanical integrity of the joint [11–14]. Accordingly, corrosion behavior of brazes has received little attention in the open literature. The limited studies available focusing on stainless steel have considered different Ag-based braze compositions than studied here, but the systems behaved similarly and it was proposed that galvanic cells developed at the braze-stainless steel interface promoting corrosion driven by the electrically connected dissimilar metals [10,15–18,28]. However, in these studies, local galvanic couple corrosion was only postulated from bulk corrosion observations, it was not directly confirmed as in the present work with SKPFM. Other studies observed that partitioning of more noble alloying elements (preferred cathode sites) within the braze alloy occurred, causing depletion in the surrounding matrix and likely lead to preferential corrosion in those regions. In the systems investigated herein, similar corrosion behavior was confirmed [15–17]. Visual observation of corrosion propagation on braze alloy-stainless steel couples (Figure7) showed that active corrosion rapidly initiated on the braze alloy first, with the surrounding uncoated stainless steel unaffected. The polarization curves presented in Figure5 also suggest that the more anodic braze alloy will be preferentially attacked at an increased corrosion rate when galvanically connected to the more noble stainless steel cathode. Stainless steel had an open circuit potential (OCP) approximately 50 mV more noble that either of the braze alloys alone. On a macroscale, the stainless steel acts as cathode when galvanically coupled to the more anodic braze alloys, driving preferential corrosion of the braze alloy. This galvanic couple situation is particularly detrimental for joint integrity because of the typically large cathode (stainless steel) to anode (braze alloy) area ratio. At free corrosion conditions the anodic reaction occurring on the braze alloy is polarized towards the stainless steel, further accelerating the anodic reaction rate of the braze alloy. Because of the large cathode to anode area ratio there is ample cathode area available to support the increased dissolution rate of the braze, leading to eventual loss of joint integrity. Following initiation, during corrosion propagation, the stainless steel also becomes susceptible to corrosion degradation near the braze interface, which is a further detriment to joint integrity. Corrosion attack on a 316L coupon with a foil braze coating on the surface showed that the copper phase was Metals 2016, 6, 91 12 of 17 preferentially attacked, and the presence of the braze alloy served as a crevice former under which localized corrosion was able to propagate into the stainless steel (Figure 10). In Figure 10, the samples were exposed for 7 days to 0.6 M NaCl at OCP followed by a potentiodynamic scan from open circuit to 0.5 V vs. SCE, prior to cleaning and cross sectioning. The left image (Figure 10) was obtained from a cross section that bisected a corrosion pit which grew and propagated underneath the braze alloy coating. Eventual corrosion attack of the stainless steel in a brazed sample was also observed during free corrosion conditions when immersed in 0.6 M NaCl solution. As seen in Figure7, the generation of voluminous dark red corrosion products indicated that braze alloy corrosion likely generated a sufficiently aggressive local chemical environment to depassivate the underlying stainless steel. While the uncoated regions of the stainless steel coupon did not display evidence of corrosion damage, it is evident that the galvanically driven braze alloy corrosion led to subsequent corrosion attack of the joined stainless steel at the interface for both braze alloys considered. Metals 2016, 6, 91 12 of 16

Figure 10. Cross sectioned 316L stainless steel sample with a Cu‐Ag‐Ti braze alloy coating following Figure 10. Cross sectioned 316L stainless steel sample with a Cu-Ag-Ti braze alloy coating following exposure for seven days in 0.6 M NaCl followed by a potentiodynamic scan. exposure for seven days in 0.6 M NaCl followed by a potentiodynamic scan.

For the Cu‐Ag‐Ti braze alloy, corrosion caused by segregation of Ti to the braze‐stainless steel interfaceFor the(Figure Cu-Ag-Ti 8) could braze also alloy, promote corrosion loss of caused joint coherency. by segregation The ofimage Ti to on the the braze-stainless right in Figure steel 10 interfaceshows preferential (Figure8) couldcorrosion also attack promote on the loss Cu of‐ jointrich phase coherency. of the The braze image alloy on and the an right area in of Figure corrosion 10 showsdamage preferential along the corrosion316L and attack Cu‐Ag on‐Ti the coating Cu-rich interface. phase of The the brazebehavior alloy of and Ti anwas area similar of corrosion to that damageobserved along in literature the 316L [2] and and Cu-Ag-Ti can be attributed coating interface. to the strong The behavior interfacial of Tireactions was similar between to that the observed Cu and inTi literaturefound in the [2] braze and can [5]. be The attributed addition to of the In strongin the Cu interfacial‐Ag‐In‐Ti reactions samples between did not support the Cu and Ti segregation Ti found in theto the braze joint [5 interface,]. The addition and hence of In inwould the Cu-Ag-In-Ti be expected samples to be beneficial did not supportto joint integrity. Ti segregation to the joint interface,In addition and hence to the would macroscale be expected galvanic to becouple beneﬁcial between to joint braze integrity. alloys and stainless steel, both braze alloysIn formed addition multiple to the distinct macroscale phases galvanic following couple thermal between treatment, braze alloysresulting and in stainless microgalvanic steel, bothcells brazebetween alloys phases formed during multiple active distinct corrosion phases followingconditions. thermal Open treatment, circuit potentials resulting inobtained microgalvanic from cellspotentiodynamic between phases polarization during activetesting corrosion of both braze conditions. alloys were Open intermediate circuit potentials to those obtained listed in from the potentiodynamicgalvanic series for polarization seawater for testing pure copper of both and braze pure alloys silver were [15,29]. intermediate Based on the to galvanic those listed series, in the galvanicmeasured series OCP for of seawaterthe bulk fortwo pure phase copper (copper and‐rich pure and silver silver [15‐,rich)29]. Basedbraze onstructure the galvanic is in agreement series, the measuredwith what OCP would of the be bulk expected two phase from (copper-rich mixed potential and silver-rich) theory. Within braze structurethe braze is alloy in agreement the expected with whatelectrode would potential be expected difference from implies mixed that potential when theory. corrosion Within occurs, the the braze Cu‐ alloyrich phase the expected of the braze electrode alloy potentialwill be anodic difference to the implies more noble that when Ag‐rich corrosion phase occurs,and suffer the Cu-richfrom preferential phase of the microgalvanic braze alloy willattack, be anodiceven in to the the absence more noble of a stainless Ag-rich phasesteel couple. and suffer from preferential microgalvanic attack, even in the absenceThe of nature a stainless of microgalvanic steel couple. corrosion within the braze alloys was further investigated with SKPFM.The The nature goal of of microgalvanic using SKPFM was corrosion to measure within Volta the brazepotential alloys differences was further between investigated phases within with SKPFM.the microstructure The goal of to using determine SKPFM relative was to measurenobility Voltaof the potential individual differences phases. betweenSchmutz phases and Frankel, within thealong microstructure with Leblanc, to have determine established relative a direct nobility correlation of the individual between Volta phases. potentials Schmutz measured and Frankel, in air alongand respective with Leblanc, solution have establishedpotentials of a direct metals correlation [26,27,30]. between Importantly, Volta potentials the VPD measured observed in was air and an effective indicator of how corrosion developed due to microstructural features. Subsequent to these findings, several research groups [31–39] have been able to verify that SKPFM is both reliable and effective in characterizing various alloy systems to accurately explain corrosion initiation behavior. Galvanic corrosion on the microscale strongly influenced corrosion within the braze alloys. As determined via SKPFM, within the Cu‐Ag‐Ti braze, the Ag‐rich phase was the most noble, followed by the Cu‐rich phase, and finally stainless steel had the lowest overall potential (see Figure 11a, Table 2). For the Cu‐Ag‐In‐Ti braze, the Ag‐rich phase is the most noble, followed by the Cu‐rich phase, then the Ti‐Cu rich phase, as in Figure 12a,b and Table 3. The approximate average Volta potentials differences between phases in both braze alloys are presented in Tables 2 and 3 with corresponding images of the SKPFM images of the brazes in Figures 11 and 12, respectively. The SKPFM data presented in Figures 11 and 12 are from identical areas characterized with SEM/EDS (Figures 8 and 9). The co‐localization of SKPFM and SEM/EDS techniques at the identical area provides direct evidence of the influence of local composition on Volta potential. Hence, in addition to the galvanic couple between the braze and stainless steel, variations in composition between phases provided the basis for microgalvanic corrosion within the braze alloys studied.

Metals 2016, 6, 91 13 of 17

respective solution potentials of metals [26,27,30]. Importantly, the VPD observed was an effective Metalsindicator 2016, 6 of, 91 how corrosion developed due to microstructural features. Subsequent to these findings,13 of 16 several research groups [31–39] have been able to verify that SKPFM is both reliable and effective in characterizingTable 2. Relative various Volta alloy potential systems difference to accurately (VPD) explainvalues in corrosion mV of phases initiation within behavior. the Cu‐Ag‐Ti brazed joint.Galvanic The first corrosion phase/metal on theis the microscale more positive strongly of the influencedtwo. corrosion within the braze alloys. As determined viaMicrogalvanic SKPFM, within theCouple Cu-Ag-Ti (Cathode braze,– theAnode) Ag-rich phaseΔVPD was (±30 the mV) most noble, followed by the Cu-rich phase, and finallyAg stainless Rich–Cu steel Rich had the lowest overall potential59 (see Figure 11a, Table3). For the Cu-Ag-In-Ti braze, the Ag-rich phase is the most noble, followed by the Cu-rich phase, then the Cu Rich–Stainless steel 94 Ti-Cu rich phase, as in Figure 12a,b and Table4. The approximate average Volta potentials differences between phases in both braze alloys are presented in Tables3 and4 with corresponding images of the Table 3. Relative VPD in mV of the Cu‐Ag‐In‐Ti sample. The first phase/metal is the more positive of SKPFM images of the brazes in Figures 11 and 12 respectively. The SKPFM data presented in Figures 11 the two. and 12 are from identical areas characterized with SEM/EDS (Figures8 and9). The co-localization of SKPFM and SEM/EDSMicrogalvanic techniques Couple at the (Cathode identical–Anode) area providesΔ directVPD (±30 evidence mV) of the influence of local composition on Volta potential.Ag Rich–Cu Hence, Rich in addition to the galvanic52 couple between the braze and stainless steel, variationsCu in Rich–(Ti composition‐Cu) betweenRich phases provided the97 basis for microgalvanic corrosion within the braze(Ti alloys‐Cu) studied.Rich–Stainless Steel 53

(a) (b)

(c)

FigureFigure 11. 11. (a)(a Three) Three dimensional dimensional (3 (3-D)‐D) SKPFM SKPFM Volta Volta potential potential image image of of Cu Cu-Ag-Ti‐Ag‐Ti sample sample with with a a box indicatingbox indicating area of area scan; of scan; (b) Zoomed (b) Zoomed in image in image with with arrow arrow indicating indicating where where scan scan was was taken; taken; and (c) Graph(c) Graph of potential of potential values values obtained obtained from from scan scan as as a function a function of of distance distance with with indicating indicating arrow.

(a) (b) Figure 12. Cont.

Metals 2016, 6, 91 13 of 16

Table 2. Relative Volta potential difference (VPD) values in mV of phases within the Cu‐Ag‐Ti brazed joint. The first phase/metal is the more positive of the two.

Microgalvanic Couple (Cathode–Anode) ΔVPD (±30 mV) Ag Rich–Cu Rich 59 Cu Rich–Stainless steel 94

Table 3. Relative VPD in mV of the Cu‐Ag‐In‐Ti sample. The first phase/metal is the more positive of the two.

Microgalvanic Couple (Cathode–Anode) ΔVPD (±30 mV) Ag Rich–Cu Rich 52 Cu Rich–(Ti‐Cu) Rich 97 (Ti‐Cu) Rich–Stainless Steel 53

(a) (b)

(c)

Figure 11. (a) Three dimensional (3‐D) SKPFM Volta potential image of Cu‐Ag‐Ti sample with a box Metals 2016indicating, 6, 91 area of scan; (b) Zoomed in image with arrow indicating where scan was taken; and14 of (c 17) Graph of potential values obtained from scan as a function of distance with indicating arrow.

Metals 2016, 6, 91 (a) (b) 14 of 16 Figure 12. Cont.

Figure 12. (a) 3‐D SKPFM potential image of Cu‐Ag‐In‐Ti sample with a box indicating area of scan. Figure 12. (a) 3-D SKPFM potential image of Cu-Ag-In-Ti sample with a box indicating area of scan. (b) Zoomed in image with arrows indicating where scan was taken. Graph of potential values (b) Zoomed in image with arrows indicating where scan was taken. Graph of potential values obtained obtained from scan as a function of distance with indicating arrow: (c) Blue arrow (Ag‐rich phase to from scan as a function of distance with indicating arrow: (c) Blue arrow (Ag-rich phase to Ti-Cu-rich Ti‐Cu‐rich phase); and (d) Red arrow (Ag‐rich phase to Cu‐rich phase). phase); and (d) Red arrow (Ag-rich phase to Cu-rich phase).

Interestingly, SKPFM results showed the stainless steel surface had an average Volta potential Table 3. Relative Volta potential difference (VPD) values in mV of phases within the Cu-Ag-Ti brazed that was less noble than any of the braze phases measured (Figures 11 and 12). This is contrary to the joint. The first phase/metal is the more positive of the two. known and observed corrosion behavior, in that corrosion preferentially occurs within the braze alloy, with theMicrogalvanic coupled stainless Couple steel (Cathode–Anode) acting as a cathode in the∆ galvanicVPD (˘30 couple mV) (Figures 5 and 7). This finding is important because it highlights an instance where caution must be exercised when inferring Ag Rich–Cu Rich 59 electrochemical behavior Cufrom Rich–Stainless Volta potential steel measurements. In the case of stainless 94 steel, the presence of a protective passive oxide layer makes it an ineffective anode, and the resulting solution potential is more Tablenoble 4.thanRelative the braze VPD in material. mV of the Hence, Cu-Ag-In-Ti it is essential sample. to The verify first phase/metal expected relative is the more nobility positive obtained of from SKPFMthe two. measurements with the observed corrosion and electrochemical behavior in solution, as done here. Microgalvanic Couple (Cathode–Anode) ∆VPD (˘30 mV) 5. Conclusions Ag Rich–Cu Rich 52 The composition, phaseCu Rich–(Ti-Cu) separation, Rich surface potential differences, and 97 corrosion behavior of Cu‐ (Ti-Cu) Rich–Stainless Steel 53 Ag braze alloys (Cu‐Ag‐Ti and Cu‐Ag‐In‐Ti) and the joined material (316L) were investigated and correlated. SKPFM measurements provided new insight to the origins of microgalvanic corrosion withinInterestingly, the brazed region SKPFM and results confirmed showed the the manner stainless that steel corrosion surface develops, had an averagewhich previously Volta potential had onlythat been was less postulated noble than to anyexplain of the bulk braze corrosion phases measuredobservations (Figures of brazed 11 and joints. 12). ThisSignificant is contrary findings to the include:known and observed corrosion behavior, in that corrosion preferentially occurs within the braze alloy, with(1) the Co coupled‐localized stainless SKPFM steel and acting SEM/EDS as a cathode provided in the evidence galvanic of couplephase separation (Figures5 and within7). This the braze finding regionsis important that becauseresulted it highlightsin surface an potential instance wheredifferences. caution Moreover, must be exercised the measured when inferring surface electrochemical potentials correlatedbehavior from with Volta the potentialobserved measurements microgalvanic. In corrosion the case ofbehavior, stainless thereby steel, the highlighting presence of the a protectiveutility of combiningpassive oxide these layer methods makes for it the an future ineffective study, anode, prediction, and the and resulting prevention solution of microgalvanic potential is morecorrosion. noble (2) Microgalvanic cells were confirmed via SKPFM VPD values. The two phases present in the Cu‐Ag‐Ti braze alloy samples differed by ~60 mV in surface potential, while the Cu‐Ag‐In‐Ti samples exhibited a range in surface potential differences up to ~250 mV across three phases. (3) Electrochemical testing on the individual materials was used to verify expected galvanic behavior. Stainless steel exhibited passive behavior and had an OCP that was noble to either of the braze alloys, which exhibited active corrosion behavior and icorr approximately two orders of magnitude larger than stainless steel. The role of stainless steel in the direction of the galvanic couple was in conflict with that expected from SKPFM measurements and provided an exceptional case where SKPFM Volta potential did not correlate with solution potential. (4) Exposure testing of stainless steel samples coated with each braze alloy were conducted at OCP in 0.6 M NaCl. It was found the braze alloy undergoes active corrosion that induces accelerated attack on the underlying 316L material. Additionally, it was seen that the Cu‐rich phase of the braze

Metals 2016, 6, 91 15 of 17 than the braze material. Hence, it is essential to verify expected relative nobility obtained from SKPFM measurements with the observed corrosion and electrochemical behavior in solution, as done here.

5. Conclusions The composition, phase separation, surface potential differences, and corrosion behavior of Cu-Ag braze alloys (Cu-Ag-Ti and Cu-Ag-In-Ti) and the joined material (316L) were investigated and correlated. SKPFM measurements provided new insight to the origins of microgalvanic corrosion within the brazed region and confirmed the manner that corrosion develops, which previously had only been postulated to explain bulk corrosion observations of brazed joints. Significant findings include: (1) Co-localized SKPFM and SEM/EDS provided evidence of phase separation within the braze regions that resulted in surface potential differences. Moreover, the measured surface potentials correlated with the observed microgalvanic corrosion behavior, thereby highlighting the utility of combining these methods for the future study, prediction, and prevention of microgalvanic corrosion. (2) Microgalvanic cells were confirmed via SKPFM VPD values. The two phases present in the Cu-Ag-Ti braze alloy samples differed by ~60 mV in surface potential, while the Cu-Ag-In-Ti samples exhibited a range in surface potential differences up to ~250 mV across three phases. (3) Electrochemical testing on the individual materials was used to verify expected galvanic behavior. Stainless steel exhibited passive behavior and had an OCP that was noble to either of the braze alloys, which exhibited active corrosion behavior and icorr approximately two orders of magnitude larger than stainless steel. The role of stainless steel in the direction of the galvanic couple was in conflict with that expected from SKPFM measurements and provided an exceptional case where SKPFM Volta potential did not correlate with solution potential. (4) Exposure testing of stainless steel samples coated with each braze alloy were conducted at OCP in 0.6 M NaCl. It was found the braze alloy undergoes active corrosion that induces accelerated attack on the underlying 316L material. Additionally, it was seen that the Cu-rich phase of the braze alloy underwent preferential attack compared to the Ag-rich phase and corrosion propagation in the braze alloy was aggressive enough to depassivate the adjacent 316L stainless steel. (5) The thermal brazing cycle caused sensitization of the 316L and resulted grain boundary attack and a pitting potential that was more than 200 mV more active than the unfired 316L.

Acknowledgments: Fabrication of samples was supported by National Science Foundation contract no. OISE-0709664. The generous support of the Boise State Surface Science Laboratory (SSL), the Center for Advanced Energy Studies (CAES), and Central Metallurgical Research & Development Institute, Egypt are also acknowledged. The authors thank Jasen B. Nielsen for assistance with SKPFM sample preparation. Author Contributions: Michael F. Hurley, Paul H. Davis and Darryl P. Butt conceived and designed the experiments; Corey M. Efaw, Armen Kvryan and Michael F. Hurley performed the experiments; Kari Livingston, Armen Kvryan, Paul H. Davis and Corey M. Efaw analyzed the data; Kyle Knori, Brian J. Jaques, Darryl P. Butt and Paul H. Davis contributed reagents, materials and analysis tools. All authors contributed to writing the paper. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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