Durability of metallic interconnects and protective coatings.pdf

Durability of metallic interconnects and protective coatings Z. G. Yang and J. W. Stevenson Pacific Northwest National Laboratory, Richland, WA, USA 1 INTRODUCTION 1.1 Interconnects To build up a useful voltage, a number of planar solid oxide fuel cells (SOFCs) are usually electrically con- nected in series to form a stack, as shown in Figure 1. To accomplish this, interconnects are placed between adjacent cells. In addition to functioning as a bipolar electrical connector, the interconnect also acts as a sep- arator plate that separates the fuel at the anode side of one cell from the air at the cathode side on an adjacent cell. In addition, interconnects typically provide mechanical support to the stack (see Interconnects,Vol- ume 4). For many years, the leading candidate material for inter- connect was lanthanum chromite (LaCrO 3 ) doped with alkaline earths (Mg, Ca, or Sr). This ceramic mate- rial could easily withstand the traditional 900–1000 ? C operating temperatures, but there were significant chal- lenges associated with this material, [1–3] including high cost of raw materials and fabrication, difficulty in fabri- cating high-density parts at reasonable sintering tempera- tures, and a tendency to partially reduce in the fuel gas environment, causing the component to warp or crack. The recent trend towards lower SOFC operating tem- peratures (7.0) and found pit formation and metal dusting in the tested alloys. In comparison, Ni-based alloys, which developed scales with less spinel content, performed better than the Fe-based alloys, which formed scales with more spinel content. When exposed to a car- bonaceous environment with a carbon activity less than unity, oxidation-resistant alloys are much less likely to 6 Materials for high temperature fuel cells suffer metal dusting or carbon-induced corrosion. Hortia et al. [32] examined both Fe–Cr- and Ni–Cr-based alloys at 800 ? CinaCH 4 –H 2 O atmosphere with an equilibrium car- bon activity 0.8 and found no carbide formation after nearly 300 h. In addition to bulk alloys, a clad structure that had Ni layers on both sides of a ferritic stainless steel substrate was also investigated. [42] After testing at 750 ? C for 1000 h in 53.1%N 2 –25.2%H 2 –18.3%CO–3.3%CO 2 –0.17%CH 4 , the clad structure exhibited severe structural degradation due to carbon penetration through the Ni layers and forma- tion of carbides in the ferritic stainless steel substrate along grain boundaries. 2.1.3 Oxidation/corrosion under air/fuel dual exposure conditions During SOFC operation, interconnects are simultaneously exposed to air at the cathode side and fuel at the anode side, and therefore experience a hydrogen partial pressure gradient from the fuel side to the air side. Investigations of Fe–Cr-based stainless steels, Ni–Cr-based alloys, and ele- mental metals under hydrogen/air dual exposure conditions have indicated an anomalous oxidation/corrosion behavior of the metals or alloys under the simultaneous dual expo- sures. In particular, the composition and microstructure of the scale grown on the air side differed, sometimes sig- nificantly, from the behavior when exposed to air on both sides, while the oxidation/corrosion behavior at the hydro- gen fuel side was comparable to that when exposed to the hydrogen fuel at both sides. Fe–Cr-based ferritic stainless steels, in particular those with a relatively low Cr%, were reported to be suscepti- ble to hematite (Fe 2 O 3 ) phase nodular growth in the scale grown on the air side of the air/hydrogen sample. For example, AISI 430, with 17%Cr, exhibited formation of hematite nodules (see Figure 3a and b) in the scale grown on the air side of an air/hydrogen sample during isother- mal heating at 800 ? C after 300 h. [43] In comparison, there was no hematite phase formation on the air/air sample, on the hydrogen side of the air/hydrogen sample, nor on the hydrogen/hydrogen sample. The oxidation behavior on the hydrogen side of the air/hydrogen sample was simi- lar to that on the hydrogen/hydrogen sample. The potential detrimental effects of the dual exposures appeared to be dependent on the alloy composition, in particular Cr% in the Fe–Cr substrate. For Crofer 22APU, with 22–23%Cr, no hematite phase formation or nodule growth was observed under the same test conditions as for AISI 430. Instead, it was found that the spinel top layer of the scale on the air side of a hydrogen/air sample was enriched in iron and grew into a different morphology from that on an air/air sample under the same conditions. [19] However, when ambi- ent air (～1%H 2 O) was replaced by moist air (～3%H 2 O), the hematite phase was observed in the scale on the air Mn Cr Fe SiO 2 α-Fe 2 O 3 10 μm Electron image 1 20 kV × 5000 5 μm 03s030c (a) (b) Figure 3. (a) Surface and (b) cross-section SEM images of the air side of an AISI 430 coupon after 300 h of oxidation at 800 ? C, with one side exposed to air and the other to moist hydrogen (97H 2 + 3%H 2 O). side of an air/hydrogen sample. This anomalous oxida- tion therefore appears to be the result of combined effects from both the hydrogen flux from the fuel side to the air side and increased water vapor partial pressure on the air side. E-brite, with 27%Cr, appeared to be more resistant to the formation of hematite nodules at 800 ? Cinthescale grown on the air side of an air/hydrogen sample, though the surface microstructure of the scale was different from an air-only sample. Similar anomalous oxidation behavior was also observed by others. [44] In addition to the hydro- gen/air dual exposures, early publications [45, 46] reported the oxidation behavior of ferritic stainless steels in (argon + hydrogen)/air dual environments as well as a steam/air dual environment that simulated the boiler tube exposure con- ditions in steam turbines. Anomalous oxidation was found on the air side of the dual exposure sample, which exhib- ited a significantly increased oxidation rate on the air side due to formation of hematite phase, which was attributed to hydrogen permeation from the (argon + hydrogen) side or the steam side to the air side of the stainless steels. The anomalous oxidation/corrosion behavior of oxidation- resistant alloys observed under the dual exposure conditions appears to be similar to that found in a high partial pressure Durability of metallic interconnects and protective coatings 7 water vapor single environment. Thus, it appears likely that the anomalous oxidation under the two different environ- ments occurs via a similar mechanism. Further studies are required to identify and understand the specific processes responsible for the observed behavior. Overall, the anomalous oxidation/corrosion observed under dual atmosphere exposure appears to be sensitive to surrounding environments, alloy composition, preexisting conditions, and other factors. For example, AISI 430 was tested by flowing 80%Ar–20%H 2 (saturated with water at room temperature) on one side and air on the other, and was found to have minimal effect of the hydrogen potential gradient on the stainless steel oxidation behavior after up to 300 h at 800 ? C. [47] It is not clear whether this was due to a lower hydrogen gradient in the aforementioned study. Dual atmosphere exposure effects have also been obs- erved on Ni–Cr base alloys. However, unlike the fer- ritic chromia-forming alloys, Ni and Ni–Cr-based alloys formed a uniform, well adherent scale on the air side of air/hydrogen samples that was free from any nodule growth. Also, the dual atmosphere exposure tended to eliminate the porosity that was often observed along the scale/metal interface after air only exposure, likely resulting in improved scale adherence. The absence of detrimental effects of the air/hydrogen dual exposure on the scale stabil- ity of Ni–Cr-based alloys in comparison with Fe–Cr-based alloys is consistent with results reported previously in water vapour. In an effort to gain mechanistic understanding, ele- mental metals have also been studied. The destructive effects of air/hydrogen dual exposures were reported on silver at elevated temperatures. [48] Simultaneous exposure to fuel and oxidizing environments led to extensive poros- ity development in bulk silver at elevated temperatures. The porosity formation took place predominantly along the grain boundaries, and was attributed to the nucleation and growth of high-pressure steam bubbles that combined to form the observed porosity and fissures in the solid sil- ver. The water formation was a consequence of the high solubility and fast diffusivity of H and O in the bulk metal; thermodynamic modeling indicates that the reaction 2[H] Ag + [O] Ag = H 2 O(g) is highly favorable. In compar- ison, minimal effects of air/hydrogen dual exposure were observed on the scale growth on Ni metal. The anomalous oxidation behavior observed on the air side of alloys subjected to air/hydrogen dual atmosphere exposure is currently attributed to the transport of hydrogen through the metal substrate from the fuel side to the air side, and its subsequent presence at the oxide scale/metal interface and/or in the scale. Hydrogen permeation tests on ferritic stainless steels demonstrated that hydrogen can diffuse through the alloys, although the permeation was drastically decreased by formation of a chromia scale on the alloys. The mechanisms by which the presence of hydrogen or protons at the air side affects the oxide scale structure and growth are not clearly understood at this time. Several mechanisms have been proposed to tentatively explain the observed anomalous oxidation behavior. [19, 44] 2.2 Degradation at interfaces with adjacent components In addition to the oxidation and corrosion at the intercon- nect/gas interfaces, interactions can occur at the interfaces between the interconnect and its adjacent components. These components may include seals, electrical contacts, etc., depending on the SOFC stack design (see Durable sealing concepts with glass sealants or compression seals; Application of secondary ion mass spectrometry (SIMS) technique on the durability of solid oxide fuel cell (SOFC) materials, Volume 5). For example, metal- lic interconnects interact with glass seals, including those made from devitrifying barium–calcium–aluminosilicate (BCAS)-based glasses. Previous work [49, 50] found that reac- tions between ferritic stainless steels and the BCAS glass produced tended to promote interfacial defects. For tra- ditional chromia-forming stainless steels, the extent and nature of their interactions with the glass seals depend on the exposure conditions and/or proximity of the glass/steel interface to the ambient air. At or near the edges, where oxygen from the air is accessible, the chromia scale grown on the steel and Cr-containing vapor species reacted with Ba in the glass, forming BaCrO 4 , presumably via the following reactions: 2Cr 2 O 3 (s) + 4BaO(s) + 3O 2 (g) = 4BaCrO 4 (s)(2) CrO 2 (OH) 2 (g) + BaO(s) = BaCrO 4 (s) + H 2 O(g)(3) Owing to the large thermal expansion mismatch with BCAS glass or ferritic stainless steel (e.g., AISI 446), the extensive formation of barium chromate resulted in crack initiation and growth between the sealing glass and alloy coupons, as shown in Figure 4. In the interior seal regions, where access of oxygen from the air was blocked, chromium dissolved into the BCAS glass to form chromium-rich solid solutions. The stainless steel also reacted with residual species in the seal to gen- erate porosity in the partially devitrified glass along the interior regions of the interface. Recent studies [51, 52] fur- ther investigated the compatibility of ferritic stainless steels and sealing glasses under air/hydrogen dual exposures. It was observed that the corrosion at the interface of the sealing glass and the chromia-forming steel was substan- tially different from that when exposed to hydrogen or air only. At the air side, iron oxide nodules formed on the ferritic stainless steel near or at the triple-phase boundary of air/glass/metal, causing short-circuiting of the seals. In contrast, no iron oxide formation was found at the interface 8 Materials for high temperature fuel cells I I YSZ G18 446 A B 446 G-18 446 A–A C 446 446 G-18 B–B 446 Glass ceramics BaCrO 4 C–C 20 kV × 100 100 μm 01s483d 20 kV × 200 100 μm 01s533a 20 kV × 2000 10 μm 01s483e (d) (a) (b) (c) Figure 4. Interfacial reactions between G18 sealing glass and 446 stainless steel: (a) a schematic of the joined coupons (446/G18/446), and SEM images of the interfacial cross section at (b) the edge area A, (c) the interior region, and (d) from the region marked as “C” in (b). The 446 coupons (12.7mm× 12.7mm× 0.5 mm) were joined to the G18 through heat treatment at 850 ? C for 1 h in air, followed by 750 ? Cfor4hinair. between the glass and the ferritic steel on the hydrogen side. However, it was not clear how the dual exposures led to the formation of iron oxide and the subsequent seal degra- dation. Another recent study reported reaction between alkali-containing glass seals and steel interconnects, result- ing in extensive growth of an iron-rich oxide scale as well as the formation and volatilization of alkali chromates. [53] Interactions have also been reported at the interfaces between metallic interconnects and electrical contact layers that are inserted between the cathode and the interconnect to minimize interfacial electrical resistance and facilitate stack assembly. For example, some perovskite cathode materials that are being considered as potential contact materials have been reported to react with interconnect alloys. Reaction between lanthanum manganite and chromia-forming alloys led to formation of a manganese-containing spinel interlayer that actually appeared to help minimize the contact area specific resistance (ASR). [54–56] On the other hand, Sr in the perovskite conductive oxides can react with the chromia scale on alloys to form the undesirable SrCrO 4 . 3 PROTECTIVE COATINGS FOR IMPROVED STABILITY AND PERFORMANCE As previously discussed, metallic interconnects are subject to oxidation and corrosion at both the cathode and anode side, and may chemically interact with adjacent components such as sealing glasses, resulting in high electrical resis- tance, metal loss, and reduction in interconnect and stack stability. In addition, during high-temperature exposures the chromia or Cr-rich scale grown on metallic interconnect is volatile via the following reactions: [13] Cr 2 O 3 (s) + 3 2 O 2 (g) = 2CrO 3 (g)(4) 1 2 Cr 2 O 3 (s) + 1 2 O 2 (g) + H 2 O(g) = CrO 2 (OH) 2 (g)(5) The chromia vapor species can migrate into cathodes and deposit in the electrode and/or at the electrode/electrolyte interface, thereby increasing polarization and causing a rapid degradation in SOFC performance. Newly developed alloys offer a reduction in Cr volatility rate, but that reduction is unlikely to be sufficient to avoid unacceptable degradati