Canadian Journal of Chemistry
The Effect of Static Liquid Galinstan on Common Metals and Non-Metals at Temperatures up to 200 °C
Journal: Canadian Journal of Chemistry
Manuscript ID cjc-2020-0227.R1
Manuscript Type: Article
Date Submitted by the 19-Jul-2020 Author:
Complete List of Authors: Geddis, Philip; Natural Resources Canada, CanmetENERGY-Ottawa Wu, Lijun; Natural Resources Canada, CanmetENERGY-Ottawa McDonald, Andrew; Natural Resources Canada, CanmetENERGY-Ottawa Chen, Steven; Natural Resources Canada, CanmetENERGY-Ottawa Clements, DraftBruce; Natural Resources Canada, CanmetENERGY-Ottawa Is the invited manuscript for consideration in a Special Not applicable (regular submission) Issue? :
Galinstan corrosion, Gallium corrosion, liquid metal corrosion, liquid Keyword: metal embrittlement, magnetohydrodynamics
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The Effect of Static Liquid Galinstan on Common Metals
and Non-Metals at Temperatures up to 200 °C
Philip Geddis*, Lijun Wu, Andrew McDonald, Steven Chen, Bruce Clements
* [email protected], 613-947-3128 ORCID 0000-0002-5610-9748
CanmetENERGY-Ottawa, Natural Resources Canada, Ottawa, Ontario, K1A 1M1, Canada Draft KEYWORDS
Galinstan corrosion, Gallium corrosion, liquid metal embrittlement, liquid metal corrosion,
magnetohydrodynamics
DECLARATIONS
FUNDING This research has been funded by Natural Resources Canada through the Energy Innovation Program (EIP).
CONFLICTS OF The authors declare that they have no conflict of interest. INTEREST/COMPETING INTERESTS: AVAILABILITY OF DATA The datasets generated during and/or analyzed during the current study are AND MATERIALS available from the corresponding author on reasonable request.
AUTHORS’ CONTRIBUTIONS Study conception and design was carried out by Philip Geddis. Material preparation and data collection were carried out by Philip Geddis, Andrew McDonald, and Steven Chen. Analysis and interpretation were carried out by Philip Geddis, Lijun Wu, and Steven Chen. The first draft of the manuscript was written by Philip Geddis and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.
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The Effect of Static Liquid Galinstan on Common Metals
and Non-Metals at Temperatures up to 200 °C
Philip Geddis*, Lijun Wu, Andrew McDonald, Steve Chen, Bruce Clements
CanmetENERGY-Ottawa, Natural Resources Canada, Ottawa, Ontario, K1A 1M1, Canada
KEYWORDS
Galinstan corrosion, Gallium corrosion, liquid metal embrittlement, liquid metal corrosion, magnetohydrodynamics Draft ABSTRACT
Liquid metal Galinstan (GaInSn) is corrosive in nature against other solid metals as its base component is gallium. This study experimentally investigated the compatibility of GaInSn with eight common metals at temperatures up to 200 °C for 2000 hours, including aluminum, copper, brass, ferritic and austenitic stainless steels (E-brite, SS304L, SS316L) and nickel-chromium alloys (Inconel and Hastelloy). This assessment aims to assist in design and material selection of a liquid metal magnetohydrodynamics system that houses Galinstan for power generation by low temperature natural heat sources or industrial waste heat. Design and fabrication of this renewable power system required assurance of material compatibility with common construction and instrumentation materials. The most severe corrosion effects of GaInSn on the metal alloys were observed on aluminum, copper and brass, which confirms the results of previously conducted studies. No obvious corrosion on stainless steel or nickel-chromium alloys were observed by this
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study, which reveals that stainless steel has a good resistance to attack by GaInSn up to 200 °C.
Six non-metals were also evaluated, including acronitrile butadiene styrene (ABS), acrylic, nitrile
rubber (Buna N), nylon, polyvinyl chloride (PVC), and Teflon, which were deemed to be
compatible with GaInSn up to the temperatures tested.
1 INTRODUCTION
Galinstan (GaInSn) is a non-toxic liquid metal, a eutectic alloy of gallium (Ga), indium (In), and
tin (Sn). The composition of GaInSn was patented in 2000 and its melting temperature was claimed
to be about -19.5 °C under normal pressure and atmospheric conditions, and its vaporization point
was reported to be above 1800 °C [1]. With a reported melting temperature above 0 °C [2, 3] many
GaInSn alloys retain their liquid state at Draftroom temperature. GaInSn has a very low vapour pressure
and will not evolve any respirable metal vapour at room conditions, which generally makes GaInSn
safe to use.[4, 5] GaInSn is most commonly used as a replacement for toxic mercury in
thermometers. Table 1 compares the physical properties of gallium and gallium alloys including
GaInSn with reference to mercury.
Table 1. Physical properties of mercury, gallium and gallium alloys. [5, 6]
Property Mercury Gallium Ga75.5In24.5 Ga67In20.5Sn12.5 Ga61In25Sn13Zn1 melting point, °C -38.8 29.8 15.5 10.5 7.6 boiling point, °C 357 2204 2000 > 1300 > 900 density, kg/m3 13530 6080 6280 6360 6500 electrical 1.0 x 106 3.7 x 106 3.4 x 106 3.1 x 106 2.8 x 106 conductivity, Ω-1m-1 thermal conductivity 8.5 29.3 26.6 16.5 unk. (W/m/K) kinematic viscosity, 13.5 x 10-7 3.24 x 10-7 2.7 x 10-7 2.98 x 10-7a 7.11 x 10-8 m2/s (a) surface tension, N/m 0.5 0.7 0.624 0.533b 0.5 water compatibility Soluble Insoluble Insoluble Insoluble Insoluble Notes:(a): Measurements with a glass viscometer tube are typically closer to 4.0 x 10-7 m2/s N. (b): Measurements indicate that oxides can significantly reduce surface tension
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Thermal conductivity data from Liu et al. 2018 [6]. Remainder of data reproduced from Morley, N.B., Burris, J.; Cadwallader, L.C.; Nornberg, M.D. GaInSn usage in the research laboratory, Review of scientific instruments, vol. 79, pp. 056107-1-056107-3, 2008, with the permission of AIP Publishing.
Since GaInSn remains in a liquid state over quite a wide temperature range, some researchers
have investigated its properties and explored its industrial uses for a range of applications. It was
found that GaInSn is a very good heat transfer fluid with a high heat transfer coefficient. The most
recent published reports on the use of GaInSn are in the microscopic domain, for miniaturization
and deformability of microelectronic devices at or near room temperature.[7] Some examples include GaInSn magnetohydrodynamics (MHD) micro-pump for micro-cooling[8], electromechanical relays[9] and GaInSn droplet based integrated liquid cooling systems [10]. Other emerging applications such as soft electrodesDraft and sensors, flexible and stretchable electronics, reconfigurable filters and antennae, stretchable conductive fibres, bio-devices, and low
temperature bonding methods in electronic packaging have also been reported.[6,7,11]
In the macroscopic domain, GaInSn was proposed for use as a coolant for high temperature nuclear fusion reactors.[4, 5, 12] It was also proposed to use GaInSn as heat transfer fluid for concentrated solar power that will need to work at higher temperatures (i.e., > 900 °C).[13, 14] In
addition, GaInSn is often used as a substitute for other liquid metals in experimental studies. The
properties of GaInSn also make it an attractive option for a low temperature magnetohydrodynamic
(LTMHD) power generation system to work as a means of low temperature waste heat to power
(LTWHP), which is the study of this research project. In LTMHD systems (or liquid metal MHD
systems) a highly conductive fluid is passed through a permanent magnet generator, and the power
produced is proportional to the fluid’s conductivity, velocity, magnetic field strength, and the
generator geometry. Previous work showed promising results and successfully proved the concept.
Methods to induce fluid flow include gas-lift type devices[15] and more recently wave energy
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systems.[16] This type of LTWHP system features low temperature waste heat recovery and power
generation with zero greenhouse gas emissions. The low temperature waste heat could come from
a variety of sources, including industrial processes, and low temperature solar or geothermal heat.
Many early liquid metal MHD systems used mercury as the conductive fluid but today, the non-
toxic option GaInSn has emerged and could improve the viability of the technology.
CanmetENERGY is currently building a gas-lift type MHD power system, and to do so, better
data concerning the compatibility of GaInSn with other materials is required. As GaInSn is a
relatively new alloy, published reports and articles specifically detailing its compatibility with
common materials is limited. Studies on material interactions of pure gallium Ga or the binary
alloy Gallium–Indium (GaIn) are however available. Gallium, the main element in GaInSn, has
been found to be corrosive and was reportedDraft widely to cause the embrittlement of aluminum.[17] It
was found that gallium attacked 316 L austenitic stainless steel and 1.4914 martensitic steel at a
temperature of 400 °C.[18] A tensile test conducted at 300 °C showed that gallium did not embrittle
type 316 stainless steel, but tests at room temperature showed that gallium did cause a slight
decrease in ductility.[19] The compatibility of gallium with four typical electronic chip metal
substrates (6063 Aluminum-Alloy, T2 Copper-Alloy, Anodic Coloring 6063 Aluminum-Alloy and
1Cr18Ni9 Stainless Steel) at a temperature of 60 °C was investigated.[20] It was found that: a) the
corrosion of the 6063 Aluminum-Alloy was rather evident and serious; b) T2 Copper-Alloy
showed slow and general corrosion; and c) Anodic Coloring 6063 Aluminum-Alloy and 1Cr18Ni9
Stainless Steel were found to have excellent corrosion resistance among these four specimens.
In a study on the compatibility of GaIn with T2 copper, 304 stainless steel and anodized 6061
aluminum alloy in the temperature range of 100–400 °C, it was found that at 400 °C only a slight
sign of corrosion was observed on the surface of the anodized 6061 aluminum alloy, surface
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corrosion was observed on 304 stainless steel, and penetration corrosion was observed on T2 copper.[21] A study on the compatibility of ternary gallium alloys (Ga–Sn–Zn) with austenitic stainless steel 316L at high temperature (500 °C) for up to 700 hours revealed that the weight change and metal loss of specimens were generally reduced by the effect of adding alloying elements tin and zinc compared to those in pure gallium at a high temperature.[22]
The objective of this study is to determine appropriate materials for the construction of a
LTMHD waste heat-to-power system housing GaInSn. The system will have a maximum operating temperature of 200 °C, experience high fluid velocities, and will include a set of electrodes directly wetted by GaInSn. Through a series of static exposure experiments, this study primarily aims to build on available data on the compatibility of GaInSn with common materials. This will assist in design decision making and material selection.Draft A secondary aim of the work is to better understand the attack mechanisms of GaInSn, based on the information identified in previous literature on pure gallium and other gallium alloys. Results of corrosion tests with eight metal alloys are presented in this paper: a common set of aluminum, copper, brass, ferritic and austenitic stainless steels (E-brite, SS304L, SS316L) and nickel-chromium alloys (Inconel, Hastelloy) were assessed at temperatures up to 200 °C for 2000 h. A number of non-metals were also evaluated, including acronitrile butadiene styrene (ABS), acrylic, nitrile rubber (Buna N), nylon, polyvinyl chloride
(PVC), and Teflon.
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2 EXPERIMENTAL STUDY
2.1 MATERIALS AND METHODS
The GaInSn used in these experiments was purchased from Neo Rare Metals LLC with a
composition of 67 wt.% Ga, 20.5 wt.% In, and 12.5 wt.% Sn. Its properties are listed in Table 1.
The plastic and metal alloy coupons were fabricated by Alabama Specialty Products Incorporated.
Table 2 presents the composition of the metal coupons, as per certifications on material test reports
provided by the supplier. A 120-grit sanding process was applied to all metals. This results in a
linearly directional finish and is often used for corrosion tests. Non-metals included in this study
included acrylic, ABS, Buna N, Nylon, PVC, and Teflon. Table 2. Metal composition by mass percentage.Draft Common AL1100 CDA CDA 230 I-725 C2000 E26-1 SS304L SS316L 122 UNS A91100 C12200 C23000 N07719 N06200 S44627 S30403 S31603 Trade Aluminum Copper Brass Inconel Hastelloy E-brite Austenitic Austenitic Ferritic Stainless Stainless Stainless Si 0.16 0.04 0.02 0.023 0.57 0.45 C < 0.01 0.002 0.002 .0026 0.024 Mn 0.01 0.0033 0.21 0.17 1.81 1.55 Mg 0.00 0.08 Al 99.28 0.21 0.28 Cu 0.08 99.90 85.51 0.09 1.47 0.02 0.43 0.37 Zn 0.01 - 14.48 Pb - 0.004 Fe 0.37 - 0.002 7.69 1.27 Bal. Bal. Bal. P 0.024 - 0.002 0.004 0.01 0.031 0.030 Cr 0.00 20.86 22.74 26.8 18.19 16.77 Ni 0.01 57.92 58.281 0.12 8.01 10.07 Ti 0.01 1.58 S 0.0007 0.003 0.009 0.0110 0.0010 Mo 7.95 15.65 0.9 0.30 2.03 N 0.005 0.07 0.035 Nb 3.49 Co 0.08 0.07
The experimental procedure was adapted from the work carried out by Narh et al. for the flat
strip coupons[19] as well as ASTM standards.[23-26] The coupons and the trays used to hold them in
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the oven were first cleaned using Type IV grade reagent water. The dry coupons were weighed by
an analytical balance. Four GaInSn droplets with diameters of approximately 6-7 mm were applied
to each metal coupon, as shown in Figure 1.
Figure 1. Coupon with guide marks marking GaInSn bead locations and image of static corrosion coupons set in temperature-controlled oven
From there, the test pieces and controlsDraft were exposed in air at several temperatures (room temperature or approximately 25 °C, and 100 °C and 200 °C by means of a ventilated oven), and durations of 100, 500, and 2000 h. The test matrix is summarized in
Table 3. The exposed materials were tested alongside control material samples. Therefore, for a given unique material, up to eight unique exposure conditions were produced. After an exposure test, samples and their controls were removed from the oven and weighed once more. Galinstan was removed by washing with deionized water and wiping with soft tissues when possible.
Observations on the adherence to the surface were noted. Note that the upper test temperature for the non-metals was selected based on their typical operation limits.
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Table 3: Material Test Matrix
Test Temperature Test Duration 25 °C 100 °C 200 °C All metals 100 h All materials - Acrylic, Nylon, Teflon All metals 500 h All materials All metals Acrylic, Nylon, Teflon All metals 2000 h All materials All metals Acrylic, Nylon, Teflon
2.2 MEASUREMENT AND CHARACTERIZATION The average rate of corrosion can be calculated as:
퐾 ∙ 푊 Equation 1 퐶표푟푟표푠푖표푛 푟푎푡푒 = 퐴 ∙ 푇 ∙ 퐷 where: K = 8.76 x 104,[25] T = time Draftof exposure in hours, A = area in cm2, W = mass loss in grams, and D = density in g/cm3.
A stereomicroscope (Olympus research stereo microscope, SZH10) was then used to observe
the surface of each material sample and compare it to controls for signs of attack (i.e.,
discolouration, etching, pitting). If there was a notable difference between the control and the test
coupons, more detailed characterization was initiated.
In order to be observed by a Hitachi S‐3400N scanning electron microscope (SEM) system, the
surface of the coupons tested at 100 °C and 200 °C were carbon-coated. The elemental composition
of corrosion areas was analyzed by semi-quantitative energy dispersive X-ray spectroscopy
(EDX/EDS) with the same instrument. Afterwards, coupons were cut with a Buehler low-speed
saw so that cross-sections of the corrosion areas could also be analyzed by the same method.
Samples were mounted in an epoxy puck, polished, and again coated with carbon to reduce
charging. The general arrangement of the epoxy samples is shown in Figure 2.
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Figure 2. Epoxy puck preparation
3 RESULTS 3.1 VISUAL OBSERVATIONS All non-metal surfaces appeared unaltered after exposure (see Figure 3). Only some superficial smearing of GaInSn was seen after cleaningDraft samples of PVC, ABS, and Buna N. At 100 °C, a colour change in the Nylon sample from its original alabaster to a galliano colour was observed in both the control and the exposed specimens.
Figure 3. GaInSn droplets and exposure areas of non-metals after testing 2000 h. Top: before cleaning; bottom: after cleaning. (A) ABS, 25 °C; (B) Buna N, 25 °C; (C) PVC, 25 °C; (D) Acrylic, 100 °C; (E) Nylon, 100 °C; (F) Teflon; 100 °C. Figures 3 and 4 depict the metal coupons after testing and the corrosion areas left behind after cleaning. The most severe corrosion effects of GaInSn on the metal alloys were observed on
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AL1100 aluminum, CDA122 copper, and CDA230 brass. In each of these cases, the surface of the
GaInSn bead appeared to harden and form an oxidized skin. This layer was strong enough to hold
the beads in place even if tilting the samples to angles exceeding 45°. The beads were also seen to
shrink in size, especially for the aluminum case at 200 °C, and the copper and brass cases above
100 °C. At higher temperatures, the extent of corrosion was more pronounced and considerable
discolouration of the metal coupons occurred.
For AL1100 (Figure 4A), corrosion could visually be seen on the surfaces for temperatures at
25 °C and 200 °C. At 100 °C, the AL1100 sample (Figure 4A2) only developed the dark
discolouration during the cleaning process. Otherwise, the top surface of that coupon would have
appeared similar to the control piece that was not exposed to GaInSn. At 200 °C, significant
corrosion of AL1100 prevented tests to beDraft completed for the 2000 hour duration. Instead, an image
of the test duration of 500 hours is presented (Figure 4A3).
For CDA122 (Figure 4B), GaInSn corrosion could visually be seen on the surfaces and appeared
to intensify with rising temperature. At 100 °C the GaInSn droplet appeared to have shrunk
(Figure 4B2). At 200 °C, CDA122 (Figure 4B3) turned dark grey on both sides of the coupon.
For CDA230 brass (Figure 4C), GaInSn corrosion could also visually be seen on the surfaces
and intensified with rising temperature.
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Draft
Figure 4. GaInSn droplets and corrosion areas after test. Pre-test photos are shown in (A) AL1100 (B) CDA122, and (C) CDA230. Smaller images are from after testing. The top row for each metal shows the sample before cleaning, and the lower row shows the sample afterwards. Exposure at 25 °C is shown in A1, B1, and C1. Exposure at 100 °C is shown in A2, B2, and C2. Exposure at 200 °C is shown in A3, B3, and C3. Post-test images are all from a duration of 2000 h except for aluminum at 200 °C, where the image from 500 h is shown due to severe degradation of the 2000 h sample.
No obvious differences between the control specimen and the test specimen were observed visually or at high magnification for the nickel-alloy and stainless steel samples (Figure 5). The
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remaining GaInSn material was most easily removed from the I-725 sample, followed by C2000
Hastelloy. More effort was required to remove it from the E-brite and stainless steels, where dark
streaks remained after wiping.
Draft
Figure 5. GaInSn droplets and exposure areas after testing at 200˚C for 2000 h. Top: before cleaning; bottom: after cleaning. (A) I-725 Inconel; (B) C2000 Hastelloy; (C) E-26-1 E-brite; (D) SS304L; and (E) SS316L.
3.2 CORROSION RATE The results of calculated corrosion rates for the 2000 h duration tests are shown in Table 4. For
AL1100, essentially no change in weight was observed for the 25 and 100 °C tests and no physical
damage was observed as previously illustrated visually. At 200 °C for 2000 h, significant
deterioration of AL1100 was observed early in the test and the GaInSn fully seeped into the
sample. The reported corrosion rate for AL1100 in Table 4 assumes 2000 h. The actual rate at
which this damage occurred is likely much faster (for example, the 500 h specimen shown in
Figure 4A3 had a corrosion rate of approx. -22 mm/y). More detailed tests would be required to
quantify the effects of time and temperature on the corrosion rate for the aluminum samples.
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Build-up of possible corrosion products was observed on CDA122 and CDA230 samples which
is why negative values for corrosion rate were obtained, with the most severe case being CDA122
at 200 °C and 2000 h. For the other metals the measured corrosion rate was about the same as the
control value and corrosion was not therefore not observed by this method. It was expected that
no obvious corrosion would occur to stainless steel samples since the temperature in the test was
limited to 200 °C, where stainless steel has a good resistance to attack by pure gallium.[27] With corrosion rates below 0.02 mm/y, stainless steel also has a good resistance to attack by ternary
GaInSn. Nickel alloys also performed well with GaInSn at temperature up to 200 °C. Some weight change in acrylic and PVC was observed at 25 °C however these results are likely due to experimental error. Mass loss in nylon and acrylic at 100 °C was likely due to thermal degradation as the materials were above or at their softeningDraft points of ~50 °C and 100 °C, respectively. [28] This is consistent with visual observations showing yellowing of nylon at 100 °C.
Table 4. Corrosion Rate for 2000 h GaInSn-exposed and control test pieces (mm/y)
25 °C 100 °C 200 °C Material exposed control exposed control exposed control AL 1100 0.00 0.00 0.00 0.00 -7.10 -0.01 CDA122 -0.02 0.00 -0.79 -0.01 -3.18 -0.02 CDA230 0.00 0.00 -1.78 -0.01 -0.35 -0.01 E26-1 - - 0.00 0.00 -0.01 -0.01 SS304L 0.00 0.00 -0.01 -0.01 -0.01 -0.01 SS316L 0.00 0.00 -0.01 -0.01 -0.01 -0.01 C2000 - - 0.00 0.04 -0.01 -0.01 I725 - - 0.00 0.00 -0.01 0.00 Acrylic 0.23 0.03 0.48 0.72 - - Nylon -0.04 -0.02 1.45 1.46 - - Teflon -0.03 0.09 -0.01 -0.03 - - PVC 0.18 -0.12 - - - - ABS 0.10 0.07 - - - - Buna N 0.10 0.10 - - - - - = not tested; n.m. = not measurable
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3.3 SEM AND EDX Further analysis of the elemental composition of corrosion areas was performed with SEM and
EDX. Figure 6 shows the SEM and EDX images of the tested coupons of AL1100. The black
surface material was found to be composed of aluminum and oxygen. Composition measurements
by EDX for cross-section samples detected low concentrations of Ga (0.5-1.1 wt.%) and In (0.38
wt.%) 7 µm into the surface for the 25 °C sample; below this depth only the substrate components
were detected. No Ga, In, or Sn were registered in the 100 °C sample. This may be due to
imprecision in cutting the cross-section through the affected area on the surface or removal of
surface material during polishing.
Figure 7 shows the SEM and EDX images of the tested coupons of CDA122 copper. The SEM images show the forming compound ofDraft GaInSn with CDA122 copper and EDX cross-section images show the penetration depth of GaInSn into CDA122 copper.
After cleaning, surfaces of the 100 and 200 °C test coupons showed regions with Ga and Cu (in
proportions of about 2.2:1 by weight). Some trace In and Sn remained, but in small amounts. The
cross section revealed the depth of penetration into the surface of 0, 100, and 250 µm for the 25,
100, and 200 °C samples of 2000 h exposure, respectively. This is consistent with the weight gain
data presented in Table 4. The layer boundary was very physically distinct and EDX imaging
confirmed a Ga-Cu mixture sitting above an unaffected substrate. The molar ratio of this layer is
CuGa2.
Figure 8 shows the SEM and EDX images of the tested coupons of CDA230 brass. SEM images
show the forming compound of GaInSn with CDA230 and EDX cross-section images show the
penetration depth of GaInSn into CDA230.
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Draft
Figure 6. SEM and EDX results for AL1100 samples. Image (A) surface SEM: control surface; (B) corrosion area EDX cross-section: test at 25 °C; (C) corrosion area SEM: test at 100 °C which clearly shows the edge of corrosion area for test at 100 °C (discoloured section, towards left side of image; (D) corrosion area EDX cross-section: test at 100 °C; All samples 2000 h except (E) corrosion area SEM: test at 200 °C for 500 h.
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Draft
Figure 7. SEM and EDX results for CDA122 for 2000 h tests, and elemental composition from surface of coupon corrosion area to substrate. Image (A) surface SEM for control sample; (B) corrosion area EDX cross-section: test at 25 °C; (C) corrosion area SEM: test at 100 °C; (D) corrosion area EDX cross-section: test at 100 °C. The Ga-Cu layer is approximately 235 µm thick. Image (E) corrosion area SEM: test at 200 °C; (F) corrosion area EDX cross-section: test at 200 °C. The Ga-Cu layers are approximately 600 µm thick. Elemental composition from surface of coupon corrosion area into substrate for (G) 100 °C and (H) 200 °C samples.
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Draft
Figure 8. SEM and EDX results for CDA230 for 2000 h tests, and elemental composition from surface of coupon corrosion area to substrate. Image (A) surface SEM: control surface; (B) corrosion area EDX cross-section: test at 25˚C; (C) corrosion area SEM: test at 100 °C; (D) corrosion area EDX cross-section: test at 100 °C. The Ga-Cu layers are approximately 200 µm thick. Image (E) corrosion area SEM: test at 200 °C; (F) corrosion area EDX cross-section: test at 200 °C. The Ga-Cu layers are approximately 120 µm thick. Elemental composition from surface of coupon corrosion area into substrate for (G) 100 °C and (H) 200 °C samples.
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No obvious physical changes were observed by SEM or EDX for CDA230 at 25 °C at the surface
or cross-sections. As with the copper coupon, mixtures of Ga-Cu were observed on the surface
surrounded by valleys of GaInSn. At the boundaries of the affected areas on the surface, the solid
particles of In-Sn were observed. At elemental concentrations less than 2 %, very little Zn was
observed on the surface. The cross section revealed the depth of penetration into the surface of 0,
55, and 70 µm for the 25, 100, and 200 °C samples of 2000 h exposure, respectively. The layer
boundary was very physically distinct and EDX imaging confirmed a Ga-Cu mixture sitting above
an unaffected substrate for the 200 °C case, but a transition zone was observed for the 100 °C case.
Very little Zn was observed in the remaining material above the substrate.
Figure 9 shows the SEM and EDX images of the tested coupons of I-725. SEM images show
no forming compound of GaInSn withDraft I-725 Inconel. The EDX cross-section images show no
penetration of GaInSn into I-725. Similar results were obtained for SS316L (Figure 10). Sample
surfaces and cross-sections appeared unaffected by exposure to GaInSn. No penetration of GaInSn
was observed into the stainless steel surface.
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Figure 9. SEM and EDX results of exposure areas for I-725 for 2000 h tests. Image (A) surface SEM: control surface; (B) exposure areaDraft surface SEM: test at 100 °C; (C) exposure area EDX cross-section: test at 100 °C; (D) exposure area EDX cross-section: test at 200 °C.
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Draft
Figure 10. SEM and EDX results for SS 316L for 2000 h tests. Image (A) surface SEM: control surface; (B) exposure area EDX cross-section: test at 25 °C; (C) exposure area SEM: test at 100 °C; (D) exposure area EDX cross-section: test at 100 °C; (E) exposure area SEM: test at 200 °C; (F) exposure area EDX cross-section: test at 200 °C.
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4 DISCUSSION 4.1 AL1100 ALUMINUM The destructive effect of gallium on aluminum has been well-documented and is a prime
example of liquid metal embrittlement. Hugo et al. provided an excellent study of the intergranular
movement of gallium into pure aluminum.[29] The authors observed this phenomenon in real-time
with a transmission electron microscope and CCD camera: after oxide barriers of each material
(Al2O3 and Ga2O3) were broken or depassivated through physical etching, gallium was seen to
sweep through aluminum grain boundaries. The authors provided the following explanation:
“For spontaneous grain boundary penetration to occur, the interaction force between an
advancing Ga atom and an Al atom on the grain boundary must be strongly attractive. In fact, a first-principles study by Strumpf and FeibelmanDraft found that the adsorption energy of Ga on Al surfaces was greater than the α-Ga cohesive energy, suggesting that Ga atoms are more strongly
bound to Al surface atoms than to Ga neighbors.”[29]
Observations of surface morphology and penetration into the surface were also observed by
Deng with SEM and EDS technique.[20] Anodizing appeared to successfully reduce the corrosion penetration of the aluminum surface over the periods tested in the study. Deng presented that the penetration of Ga into Al was due to the removal of the natural oxidized Al2O3 layer which acted as an initial barrier to corrosion (as depicted in Figure 4). Once this barrier is breached however, the corrosion occurs swiftly. Therefore, anodizing the surface would certainly serve to delay corrosion – but if etched or scratched there is usually deep pitting of the underlying material.[30] A
similar result was observed by others, who noted that the aluminum changed colour to black
following exposure.[3] This black material was identified using EDS by Ahlberg et al.[31] as
aluminum oxides with a high electrical resistance; Lindersson[32] observed qualitatively that this
reaction occurred faster in the presence of water vapour.
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According to Kovaleva et al.[33], gallium metal can be formed on the surface of aluminum
through the conventional ion-exchange reaction:
Al + Ga3+ = Al3+ + Ga Equation 2
Kovaleva also studied the aforementioned penetration behaviour, through exposure of
polycrystalline aluminum to solutions of aqueous gallium and NaOH, which served to actively
[33] remove the Al2O3 surface layers. Samples became extremely brittle after limited exposure. It is
interesting to note that carbon was observed on the surfaces at the brittle and ductile fracture points:
the authors attributed its presence to contamination with the air. [33] A similar investigation was
carried out by Pereiro-López et al.[34] From this study, Figure 4 (A1 to A3) shows that the colour of the aluminum coupon became darkerDraft as the temperature increased to 200 °C.
4.2 CDA122 COPPER AND CDA230 BRASS
Burton explored the use of different compositions of GaInSn alloys for use as a lubricant with
brush-type current collectors within motors, which would be exposed to low-voltage high-current,
highly abrasive conditions.[3] There was previous evidence that when copper was in cathodic
contact with binary GaIn eutectic, copper would dissolve resulting in embrittlement and the
formation of a pulpy compound due to attack by indium.[3] The authors hypothesized that the
dissolution of Cu was controlled by the oxide layer, and therefore carried out an examination over
several years with three separate types of long-term tests. It was confirmed by another investigator
that the main product of Ga and Cu was found to be CuGa2, produced through a diffusion process
[35] likely through a product layer. The rate of production of CuGa2 was found to be dependent on
the grain size of the Cu. Lin et al. examined the interfacial reaction between the molten Ga and Cu
substrate at 160 °C, 200 °C, 220 °C, 240 °C, 280°C, and 300 °C and elaborated reaction
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mechanisms.[36] Micrographs of the microstructural evolution of the interfacial reaction at 200 °C
revealed that a thick layer of CuGa2 phase formed on the corroded surface and the Cu9Ga4 phase
formed between the CuGa2 phase and the Cu substrate. As discussed, with a mass ratio of 2.2:1
for Ga:Cu as measured by EDX (Figure 7) the results confirm production of a CuGa2 compound.
Van Droffelaar & Atkinson[30] wrote that “virtually all known instances of metallic corrosion
are electrochemical in nature”, thus an understanding of electrochemistry and the electromotive
force (EMF) series can serve to explain corrosion that may occur in the materials (Table 5).
Oxidation will occur in the metal with a more negative EMF, and reduction will occur in the metal
with a more positive EMF.
Table 5. Standard reduction potentials against hydrogen electrode in water at 298.15 K. Adapted from [37]. Draft Reaction E° (V) Al3+ + 3 e ⇋ Al -1.677 Zn2+ + 2 e ⇋ Zn -0.762 Cr3+ + 3 e ⇋ Cr -0.74 Ga3+ + 3 e ⇋ Ga -0.549 Fe2+ + 2 e ⇋ Fe -0.44 In3+ + 3 e ⇋ In -0.338 Co2+ + 2 e ⇋ Co -0.282 Ni2+ + 2 e ⇋ Ni -0.236 Sn2+ + 2 e ⇋ Sn -0.141 Cu2+ + 2 e ⇋ Cu +0.339
In Figure 4 (B1 to B3) of this study, the colour of the CDA122 copper coupon became darker as the temperature increased due to oxidation. The reaction between copper and gallium was likely electrochemical in nature due to the difference in their half cell potential. Gallium would have left the liquid metal eutectic to react with copper at the surface. The penetration of the forming Ga-Cu layer went deeper with higher temperature as shown in Figure 7B, D, and E.
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For brass, which is approximately 70 % Cu and 30 % Zn, “the net effect [of corrosion] is layer-
type dezincification, in which the redeposited copper mimics the physical form of the original
brass anode. The redeposited copper is spongy and devoid of mechanical strength”. [30] Burton
also reported “fierce” intergranular attack of gallium onto zinc alloys and the formation of a black
product.[3] This is certainly explained by the high solubility of zinc into gallium.[38] In fact, zinc is
often alloyed along with gallium and indium to form GaInZn and GaInSnZn.
In this study, no zinc was detected in corroded area of CDA230 sample by EDX (Figure 8C
and E) even though the brass originally had 14.48 % zinc. This suggested that dezincification did
occur by dissolution into the GaInSn droplet. The result of dezincification is that the zinc-depleted
surface contains openings where GaInSn can interact with Cu in a similar manner as described
above for CDA122. Besides zinc’s highDraft solubility in gallium, there is also a small difference in
electrochemical potentials between Ga and Zn causing an electrochemical reaction. Further
evidence of dezincification can be seen in Figure 4 (C1 to C3), where the colour of the brass
coupon becomes closer to pure copper at higher test temperatures, as the rate of the reaction would
have increased. The behaviour of the CDA230 brass coupon, after zinc was dissolved into gallium,
had some similarities to the CDA122 copper coupon. The penetration of the forming Ga-Cu layer
in CDA230 is less than in CDA122 potentially because CDA230 contains a lower Cu
concentration than CDA122. The penetration of the forming Ga-Cu layer in CDA230 went deeper
as the temperature increased. The total layer thickness was however largest for the 100 °C case,
even though surface material could have been removed during cleaning, cutting, and polishing.
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4.3 NICKEL ALLOYS AND STAINLESS STEELS
For reference the composition of the tested steels is provided in Table 2. I-725 and C2000 mainly
consist of Ni, Cr and Mo and Fe. E-brite, SS304L and SS316L mainly consist of Fe, Cr and Ni.
An earlier investigation of gallium attack on materials indicated the temperatures of Ga attacking
the main components of nickel alloys and stainless steel: iron at all temperatures, Ni at around 480
°C, and Cr at around 600 °C. The other two elements (In and Sn) attack Fe, Ni and Cr at
temperatures above 200 °C.[27] It also indicated that Ga may be compatible with stainless steels
and nickel-base alloys up to 200 °C.[27] Figure 5 of this study shows that the results from GaInSn
test agrees with the earlier indication of good resistance of nickel alloys and stainless steel for
temperatures up to 200 °C. Deng also exposed gallium beads to a 1Cr18Ni9 stainless steel for 30
days at 60 °C and found minimal penetrationDraft of gallium into the steel surface which had “integral
corrosion resistance” and high surface strength which would weaken the effect of a flowing or
scouring liquid metal.[20] A study carried out by Narh et al. had both flat and tensile test specimens of 316L stainless steel exposed to beads of Ga at 300 °C and room temperature.[19] A corrosion
depth of about 12 µm was observed at 300 °C after three months of exposure. “No evidence of Ga
penetration into the steel along a preferred path was observed”.[19] These samples remained ductile and yield strength tests provided no quantifiable difference between samples. A very slight reduction in ductility was observed for the lower temperature samples however. A more detailed study found evidence of mild liquid metal embrittlement of 316L stainless steel by Ga, and through comprehensive fracture analysis concluded that the LME was caused by both adsorption-induced decohesion and adsorption-enhanced plasticity mechanisms. No effect of temperature between 35 and 75 °C was observed.[39]
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Other studies found that Ga attack of nickel alloys and stainless steel at higher temperature is
possible. There is some indication that Gallium will corrode stainless steels at 300-400 °C.[4]
Significant corrosion and ‘formation of a thick porous reaction layer’ has been observed at 400
[38] °C, which consisted of FeGa3 and some CrGa4. Use of iron, nickel, and chromium vessels to
hold gallium at high temperatures (above 400 °C) is not recommended, based on results reported
by Barbier et al. [18], Shin et al. [22], and Kolman et al. [39]
5 CONCLUSIONS Through a series of GaInSn static exposure experiments up to 2000 h and review of available
literature on gallium corrosion, this study investigated the compatibility of GaInSn with eight common metals and six non-metals. It confirmedDraft that: 1. ABS, Buna N, and PVC appeared to be unchanged after GaInSn exposure at 25 °C, and
acrylic, nylon, and Teflon appeared to be unchanged after exposure at 100 °C.
2. Aluminum, copper and brass were corroded by GaInSn from room temperature to 200 °C.
Aluminum experienced severe liquid metal embrittlement, copper and brass experienced
general corrosion and the depth of penetration increased with temperature.
3. Alloys I-725, C2000, E26-1, SS304L, and SS316L show a good resistance to GaInSn attack
for temperatures up to 200 °C and therefore could be considered to construct the LTMHD
system.
ACKNOWLEDGEMENTS
This research has been funded by Natural Resources Canada through the Energy Innovation
Program (EIP). The authors also appreciate the work of Silvia Sant’Anna and Valerie Omatsu-
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Baas for their valuable assistance with the SEM and EDX analysis, and students Alexander Wilson and Dana Li for their assistance with the experiments and data collection.
CONFLICT OF INTEREST
The authors declare no competing financial interest.
ABBREVIATIONS ABS acronitrile butadiene styrene, ASTM American Society for Testing and Materials, Buna N nitrile rubber, CDA copper development association, EDX/EDS energy dispersive X-ray spectroscopy, EMF electromotive force, PVC polyvinyl chloride, SEM scanning electron microscope, SS stainless steel, AL aluminum, GaInSn Galinstan, LTMHD low temperature magnetohydrodynamics, LTWHP low temperatureDraft waste heat to power.
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