WO2019163674A1 - Stainless steel member having heat-resistant cover layer or coating film, and method for producing same - Google Patents

Abstract

Provided, as a stainless steel member having chromium poisoning prevention performance, which is sufficiently suited to applications such as collector members of solid oxide fuel cells (SOFC) to be exposed to high temperatures, is a stainless steel member which uses a ferritic stainless steel sheet containing 11-40% by mass of Cr as the base material and has a laminate as a coating film that is formed on the base material, said laminate having a thickness of 0.5-50 μm. The laminate comprises a layer that contains an Co oxide in the outermost surface, while having a layer that contains CoWO4 at a position closer to the base material than the layer that contains an Co oxide.

Description

Stainless steel member having heat-resistant coating layer or coating and method for producing the same

The present invention relates to a cobalt-tungsten (Co—W) -coated ferritic stainless steel member that has excellent oxidation resistance at a high temperature range of 600 to 850 ° C. and suppresses elution of chromium in a water vapor environment.

In recent years, there has been a demand for practical use of a new power generation system due to problems such as depletion of fossil fuels represented by petroleum and global warming due to CO ₂ emissions. A fuel cell is a power generation system that can be used as a power source for automobiles and the like. There are several types of fuel cells. Among them, a solid oxide fuel cell (SOFC) is one of power generation systems that have high energy efficiency and are expected to be put to practical use.

The operating temperature of a solid oxide fuel cell (SOFC) has conventionally been as high as about 1000 ° C., and ceramics are mainly used for its constituent members, making it difficult to use metal materials such as stainless steel. However, in recent years, the operating temperature has been lowered to about 600 to 850 ° C. by improving the solid electrolyte membrane. This is a temperature range in which a heat-resistant metal material can be applied to the constituent members.

In such a low-temperature operation type solid oxide fuel cell (SOFC), it is advantageous in terms of cost to use a heat-resistant metal material for a current collecting member (separator, interconnector, current collector plate, etc.). The characteristics required for such a heat-resistant metal material are good electrical conductivity (temperature of 30 mΩ · cm ² or less) in the temperature range of 600 to 850 ° C., steam oxidation resistance, and equivalent to ceramic solid oxide The coefficient of thermal expansion is about 13 × 10 ⁻⁶ (1 / K) from room temperature to 800 ° C. In addition, thermal fatigue resistance is also required when starting and stopping are frequently repeated.

High Cr, high Ni austenitic stainless steel has excellent resistance to steam oxidation, but has a high thermal expansion coefficient. Therefore, in solid oxide fuel cells (SOFCs) such as automobile applications that frequently start and stop, expansion and contraction are required. By repeating the above, the oxide scale is easily peeled off and is difficult to apply. On the other hand, since ferritic stainless steel has a low thermal expansion coefficient equivalent to that of ceramic solid oxide, it is suitable as a constituent member of a solid oxide fuel cell (SOFC).

In addition, when an oxide film with poor conductivity is formed on the surface of stainless steel, the internal resistance of the battery increases when applied to a current collecting member of a solid oxide fuel cell (SOFC), and battery performance is reduced. It becomes a negative factor in improving. For this reason, it has become necessary to study a method for improving the conductivity of the oxide film.

Patent Document 1 discloses a steel for a solid oxide fuel cell separator made of ferritic stainless steel containing Cr: 15 to 30% by mass. This steel forms an oxide film with good electrical conductivity at about 700 to 950 ° C., and has good oxidation resistance and scale peeling resistance over a long period of use, and a difference in thermal expansion from the electrolyte. Is said to be small.

However, since the scale generated on the surface of stainless steel contains a high concentration of chromium derived from the steel components, the separator environment of a solid oxide fuel cell (SOFC) that is exposed to a steam atmosphere at 600 to 850 ° C. Then, there is a problem that this chromium reacts with water vapor and evaporates, poisoning the solid electrolyte and the electrode catalyst material. As a means for solving this problem, there is a method in which expensive silver or the like is applied to the surface layer of stainless steel, and this causes an increase in cost.

As a technique for preventing poisoning by chromium, use of stainless steel containing Al as a base material has been proposed (Patent Documents 2 and 3). However, in a solid oxide fuel cell (SOFC), the current collector is heated to a high temperature when the current collector is joined to the solid electrolyte and the electrode catalyst material in the manufacturing stage, or during actual use. If Al-containing stainless steel is used for the electric member, insulating Al ₂ O ₃ is generated on the surface of the stainless steel during the heating, and the surface conductivity is further lowered. Moreover, even if the surface of the Al-containing stainless steel base material is coated with a conductive film (eg, Ag), an oxide film of Al ₂ O ₃ is generated between the base material and the coating layer during heating. Although it is possible to prevent the evaporation of chromium, the surface conductivity cannot be improved.

As another method for preventing poisoning by chromium, it is also conceivable to use stainless steel having a nickel coating layer as a base material (Patent Document 4). However, since the ferritic stainless steel used as a base material needs to contain 1% or more of Al as an essential element, there is a disadvantage that a ferritic stainless steel defined in JIS such as general-purpose SUS430 stainless steel cannot be used. It was.

JP 2003-105503 A JP 2003-187828 A JP 2006-107936 A JP 2010-2336012 A

In view of the current situation, the present invention is a solid electrolyte that can sufficiently cope with applications exposed to high temperatures, such as current collecting members (separators, interconnectors, current collecting plates) of solid oxide fuel cells (SOFC). And it aims at providing the stainless steel member which can prevent the chromium poisoning of an electrode catalyst material.

In order to solve the above problems, the present invention employs the following means.
(1) Based on a ferritic stainless steel containing Cr: 11 to 40% by mass,
As a coating layer formed on the substrate, a Co—W coating layer comprising a thickness of 1 to 100 μm, a W content of 3 to 67.5% by mass (1 to 40 atom%), the remainder Co and unavoidable impurities. A stainless steel member provided.
(2) Cr: Ferritic stainless steel containing 11 to 40% by mass is used as a base material, and the coating formed on the base material has a layer containing Co oxide on the outermost surface, and the Co oxide A stainless steel member comprising a laminate having a thickness of 0.5 to 50 μm and having a layer containing CoWO ₄ on the substrate side of the layer containing.
(3) The coating includes a layer containing Cr ₂ O ₃ and a layer containing (Co, Fe, Cr) ₃ O ₄ in this order from the substrate side to the substrate side of the layer containing CoWO _4. The stainless steel member according to (2), further including: the Co oxide layer including Co ₃ O ₄ .
(4) Ferritic stainless steel of the base material is in mass%, Cr: 11 to 40%, Si: 1.5% or less, Mn: 1.5% or less, C: 0.12% or less, P: The stainless steel member according to any one of (1) to (3) above, having a composition comprising 0.1% or less, S: 0.01% or less, N: 0.1% or less, the balance Fe and inevitable impurities.
(5) The ferritic stainless steel of the base material is mass%, and further Al: 6% or less, Mo: 4% or less, W: 4% or less, Cu: 2% or less, Nb: 0.8% or less, Containing at least one of Ti: 0.5% or less, Zr: 0.5% or less, V: 0.5% or less, Ta: 0.5% or less, Ni: 2% or less Stainless steel member.
(6) Ferritic stainless steel of the base material is mass%, Y: 0.1% or less, other REM (rare earth element): 0.1% or less, Ca: 0.01% or less, B: The stainless steel member according to (4) or (5) above, containing one or more of 0.01% or less and Mg: 0.01% or less.
(7) The Co—W coating layer further contains one or more selected from Fe, Ni, Ti, Nb, Zr, Ta, V, Mo, P, and B within a total range of 10% by mass or less. The stainless steel member according to any one of (1) and (4) to (6), wherein the Co—W coating layer has a Cr content of 0 to 2 mass%.
(8) The stainless steel member according to any one of (1) to (7), wherein the stainless steel member is a current collecting member of a solid oxide fuel cell (SOFC).
(9) A step of preparing ferritic stainless steel containing Cr: 11 to 40% by mass as a substrate, and a thickness of 1 to 100 μm and a W content of 3 to 67.5% by mass on the surface of the substrate ( Forming a Co—W coating layer of 1 to 40 atomic%).
(10) The base material on which the Co—W coating layer is formed is heat-treated in a range of 600 ° C. to 850 ° C., and the surface of the base material has a layer containing a Co oxide on the outermost surface. And a step of forming a laminate film having a thickness of 0.5 to 50 μm having a layer containing CoWO ₄ on the substrate side of the layer containing the Co oxide, 9) A method for producing a stainless steel member.
(11) The laminate film includes a layer containing Cr ₂ O _{3 in this} order from the substrate side and (Co, Fe, Cr) ₃ O ₄ on the substrate side relative to the layer containing CoWO _4. The method for producing a stainless steel member according to (10), further including a layer, wherein the Co oxide layer includes a Co ₃ O ₄ layer.

According to the stainless steel member having the heat-resistant coating layer of the present invention, chromium vaporization (solid electrolyte and electrode catalyst) has good steam oxidation resistance at high temperatures without using an expensive metal such as silver for coating. A stainless steel member having a heat-resistant coating layer that solves the problem of material poisoning can be provided.
In addition, according to the method for producing a stainless steel member having a heat-resistant coating layer of the present invention, the heat-resistant coating layer can be formed simply by performing a heat treatment under a predetermined condition after forming the Co—W coating layer. Even a steel member can easily form a heat-resistant coating layer and can easily upgrade and upgrade existing equipment.

It is a figure which shows the micrograph (A) of the cross section of the test piece electroplated with Co-2.4W (atomic%), and the element distribution (B) of the cross section according to one embodiment of the present invention. FIG. 4 is a diagram showing how an X-ray diffraction spectrum is changed by isothermal oxidation at 800 ° C. for a test piece electroplated with Co-2.4 W (atomic%) according to an embodiment of the present invention. (B) is at the start of the oxidation test (t _ox = 0s), (c) is after 15 minutes, (d) is after 30 minutes, (e) is after 250 hours ( f) shows after 1000 hours, respectively. FIG. 7 is a micrograph of a cross section of a test piece electroplated with Co-2.4 W (atomic%) after isothermal oxidation at 800 ° C. and a diagram showing element distribution in the cross section, where (a) shows the start, (b ) After 15 minutes, (c) after 30 minutes, (d) after 1 hour, (e) after 3 hours, (f) after 25 hours, (g) after 250 hours Later, (h) shows after 1000 hours. FIG. 6 shows an Ellingham-Richardson diagram constructed using NIST standard database number 69. A parabolic plot (a) of mass change and a long-term oxidation rate (b) when SUS430 stainless steel is oxidized with and without a Co—W electroplating layer having a thickness of 10 μm. ). It is a schematic diagram which shows the structure of the coating layer thru | or heat resistant film of each sample. The thickness of each oxide layer formed on SUS430 stainless steel with a Co-2.4W (atomic%) electroplating layer during isothermal oxidation in air at 800 ° C. containing 3% by volume of water. It is a figure which shows the time change of a ratio. FIG. 5 is a diagram showing evaporation index η _W at different oxidation times calculated from the mass balance of W in SUS430 stainless steel with a Co-2.4 W (atomic%) electroplating layer.

In the present invention, ferritic stainless steel containing 11 to 40% by mass of Cr is used as the base material. A Co—W coating layer is formed on the surface of the substrate. A base material having a Co—W coating layer is heated to a high temperature and subjected to diffusion treatment to form a heat resistant coating. Co and W contained in the Co—W coating layer react with each other during high temperature heating to form an intermediate layer containing CoWO ₄ in the heat resistant coating. A member composed of the ferritic stainless steel substrate and the old Co—W coating layer including the intermediate layer is a stainless steel member having a heat resistant coating. CoWO ₄ in the heat resistant coating is very stable, and by preventing the Cr from diffusing from the base material side to the old Co—W coating layer surface side, the evaporation of Cr from the surface can be suppressed. Further, since CoWO ₄ has conductivity at 600 to 850 ° C., it is expected that the electric conductivity of the heat resistant coating is increased. When producing stainless steel having the above-mentioned heat-resistant coating, a Co—W coating layer can be formed using a continuous line when the base stainless steel is in the state of a steel strip (pre-coating). ). For this reason, productivity improves markedly compared with the case where it coats to each member after forming in a predetermined shape (post coat).

Refractory coating comprising CoWO ₄ is a layer containing a CoWO ₄ formed in film shape. The total thickness of the heat-resistant coating film needs to be 0.5 μm or more, and is preferably 1 μm or more. However, since an excessive thickness leads to hardening and embrittlement of the material, the thickness of the intermediate layer is preferably 50 μm or less, and more preferably 20 μm or less.

In order to produce a heat-resistant film having a sufficient thickness as described above, the average thickness of the Co—W coating layer formed on the substrate must be 1 μm or more, and should be 2 μm or more. Is preferred. In order to obtain a Co—W coating layer with few defects, an average thickness of 5 μm or more is effective. However, when the Co—W coating layer becomes excessively thick, stress due to the difference in thermal expansion occurs between the base material and the Co—W coating layer, and the solid oxide fuel cell (SOFC) is repeatedly started and stopped. In addition, voids are easily generated between the two, which causes the coating layer to peel off. As a result of various studies, the average thickness of the Co—W coating layer is preferably 100 μm or less, and more preferably 50 μm or less.

The Cr content in the Co—W coating layer is preferably 0 to 2% by mass. If the Cr content is higher than that, diffusion or evaporation of Cr into the battery cell becomes a problem. The Co—W coating layer can contain one or more of Fe, Ni, Ti, Nb, Zr, Ta, V, and Mo as other elements. These metal elements have the effect of improving the oxidation resistance and scale peeling resistance of the Co—W coating layer. In addition, these elements diffuse to the base material side, and an intermetallic compound is formed at the interface and grain boundary, so that there is an effect of increasing the electrical conductivity. Further, the Co—W coating layer may contain P and B. P and B are effective additive elements particularly when a Co—W coating layer is formed as electroless cobalt plating or cobalt brazing. That is, it is added as a reducing agent for a cobalt plating bath in the case of electroless plating and as an element for lowering the melting point of the brazing material in the case of brazing. As a result of various studies, the total content of these Fe, Ni, Ti, Nb, Zr, Ta, V, Mo, P, and B contained in the Co—W coating layer is preferably 10% by mass or less.

The W content in the Co—W coating layer needs to be 3 to 67.5 mass% (1 to 40 atomic%). When the W content is less than 3% by mass, CoWO ₄ appearing in the heat-resistant film after heat treatment is reduced, and the effect of preventing Cr from evaporating from the substrate cannot be obtained sufficiently. When the W content exceeds 67.5 mass%, Co is insufficient, FeCo ₂ O ₄ and Co ₃ O ₄ covering CoWO ₄ do not appear, and W may be vaporized from the heat resistant coating. The W content in the Co—W coating layer is preferably 6 to 44 mass% (2 to 20 atomic%), more preferably 7.1 to 25.7 mass% (2.4 to 10 atomic%). It is.

As a method for forming the Co—W coating layer, a method of applying electro-Co—W plating to the surface of a ferritic stainless steel can be adopted. Alternatively, various methods such as a method of coating a foil by a clad method can be applied. As a method for forming the tungsten coating layer, for example, a sputtering deposition method, a plasma spraying method, or the like can be used.

For the heat treatment for forming the heat-resistant film containing CoWO ₄ , for example, heating conditions of holding at 600 to 850 ° C. for 10 to 3600 minutes can be applied, and the holding time is preferably 1 to 3 hours. The heating condition is more preferably a condition of holding at 700 to 800 ° C. for 60 to 540 minutes. This heating may be combined with heat treatment for joining the current collecting member, the solid electrolyte, and the electrode catalyst material in the manufacturing stage of the solid oxide fuel cell (SOFC), or by heating when the fuel cell device is started up. it can.

The component composition of the ferritic stainless steel base material is preferably within the following range, for example. Hereinafter, “%” in the component composition of the substrate means “% by mass” unless otherwise specified.

Cr is a component necessary for imparting corrosion resistance, oxidation resistance, and electrical conductivity necessary for stainless steel. In order to ensure steam oxidation resistance at around 600 ° C. and good electrical conductivity, a Cr content of 11% or more is necessary. In particular, when emphasizing durability when exposed to a water vapor atmosphere, it is preferable to secure a Cr content of 15% or more. However, the Cr content exceeding 40% leads to a decrease in workability of ferritic stainless steel, a decrease in low temperature toughness, and an increase in 475 ° C embrittlement sensitivity. Therefore, the Cr content is 40% or less and preferably 35% or less.

Si has the effect of stabilizing the Cr-based oxide and is effective in improving the steam oxidation resistance. However, excessive Si content becomes a factor for generating SiO ₂ having high electrical resistance in the surface layer. Moreover, it becomes a factor which causes the fall of low-temperature toughness, generation | occurrence | production of surface flaws, and a productivity fall. The Si content is preferably in the range of 1.5% or less.

Al is an alloy element that forms an Al ₂ O ₃ oxide film on the surface of a stainless steel substrate. This Al ₂ O ₃ coating brings about a significant improvement in high-temperature oxidation resistance, and also evaporates chromium from the steel substrate exposed at the cut end face, particularly in a current collecting member of a solid oxide fuel cell (SOFC). It works effectively in suppressing However, excessive Al content decreases the workability and toughness of the steel, and impairs manufacturability. As a result of various studies, the Al content is preferably 6% or less. A more preferable Al content is less than 1%.

Mn has the effect of improving the scale peel resistance of ferritic stainless steel, but the excessive Mn content hardens the steel and causes a decrease in workability and low temperature toughness. The Mn content is preferably in the range of 1.5% or less.

Mo and W are elements that improve high temperature strength and heat fatigue resistance by solid solution strengthening and Cu by solid solution strengthening or precipitation strengthening, respectively, and one or more of these can be added as necessary. In particular, the addition of these elements is effective in applications where creep strength due to stacking in a stack and thermal fatigue properties due to repeated starting and stopping are problematic. For Mo, W, and Cu, it is more effective to secure a content of 0.1% or more. However, since the steel hardens when the content of these elements increases, when one or more of these elements are contained, the content range of Mo and W is 4% or less and Cu is 2% or less. It is preferable.

Nb, Ti, Zr, V, and Ta are elements that further improve the high temperature strength of the ferritic stainless steel by solid solution strengthening or precipitation strengthening, and can contain one or more of these as required. It is more effective to set these elements to a content of 0.03% or more. However, excessive content hardens the steel, so when one or more of these are included, Nb is 0.8% or less, and Ti, Zr, V, and Ta are all 0.5% or less. It is preferable to be in the range.

C and N are elements that improve the high-temperature strength, especially creep properties, of the base stainless steel, but if added excessively to ferritic stainless steel, the workability and low-temperature toughness are reduced. In addition, carbonitrides are easily generated by reaction with Ti and Nb, and solid solution Ti and solid solution Nb effective in improving high temperature strength are reduced. As a result of the study, the target steel of the present invention preferably has C of 0.12% or less and N of 0.1% or less.

Y, other REMs (rare earth elements), and Ca are elements that are dissolved in the oxide film and are effective in strengthening the oxide film and improving the oxidation resistance. In the present invention, one or more of these elements are required. It can be contained according to. In order to fully exhibit these actions, it is more effective to set the content of 0.0005% or more for Y, other REMs, and Ca. However, these elements harden steel and cause surface flaws. Therefore, when one or more of these elements are contained, Y and other REMs are each 0.1% or less, and Ca is 0.01%. The content range is preferably as follows.

B and Mg are elements that improve the hot workability of stainless steel, and in the present invention, one or more of these may be contained as necessary. It is more effective to secure a content of B of 0.0002% or more and Mg of 0.0005% or more. However, excessive content adversely decreases hot workability. Therefore, when one or more of these are included, it is preferable that both B and Mg have a content range of 0.01% or less.

P is allowed to contain up to 0.1%.
S has an adverse effect on hot workability and weld hot cracking resistance, and also serves as a starting point for abnormal oxidation, so it is preferably made 0.01% or less.

Ni mixed in steel in the steelmaking process is allowed up to 2%, but is preferably 0.6% or less. As other mixed elements, O is 0.02% or less, Re is 2% or less, Sn is 1% or less, Co is 2% or less, Hf is 1% or less, and Sc is 0.1% or less. It is preferable to manage so as to suppress it.

Hereinafter, examples of the present invention will be described, but the present invention is not limited thereto.

A commercially available SUS430 stainless steel was prepared. The chemical composition of the stainless steel was 16.2Cr-1.0Mn-0.7Si-0.12C-0.02S-0.04P (mass%). The stainless steel was cut into small test pieces having dimensions of 20 × 15 × 0.5 mm ³ .

The test piece was wet-polished with SiC paper up to # 2000 grade while adding distilled water, and then ultrasonically cleaned with acetone. The washed test piece was used as a cathode, and thin Co was deposited on the test piece in a strike electroplating solution having the composition shown in Table 1 at a cathode current density _ic of 20 A / dm ² for 60 seconds. Here, the strike plating is a base plating performed for improving the adhesion of the plating after removing and activating the passive film on the substrate. Next, using an acidic bath having the composition shown in Table 1, a Co—W alloy electrodeposition layer (Co—W coating layer) was formed. The cathode current density _ic was 3 A / dm ² , and the electroplating time was adjusted so that a Co—W layer (85 g / m ² ) having a thickness of 10 μm was obtained. All solutions were prepared with analytical grade reagents and deionized water. The electroplating cell is composed of two graphite anode rods (diameter 6 mm) and a cathode disposed between the two anodes. The solution temperatures of the Co strike plating and Co—W alloy electroplating baths were room temperature and 40 ° C., respectively. The composition of Co—W electroplating was measured using an energy dispersive X-ray fluorescence analyzer (XRF).

The test piece after the plating was formed was inserted into a reaction tube maintained at a temperature of 800 ° C. and held for a certain time. Oxidation time t _ox = 0s was defined as when the sample temperature reached 800 ° C. 167 seconds after insertion. The specimens were oxidized at 800 ° C. in air containing 3% by volume of water vapor for various times up to the oxidation time t _ox = 1000 hours. The gas flow rate in the reaction tube was 100 cm ³ / min.

The mass change of the test piece before and after the oxidation treatment was calculated from the mass of the sample before and after the oxidation. The structure of the oxide formed was confirmed by X-ray diffraction (XRD) using 50 kV and 45 mA CuKα radiation and a graphite monochromator. The form of the sample was observed with a scanning electron microscope (SEM), and the element distribution in the depth (thickness) direction was analyzed with an energy dispersive X-ray spectrometer (EDS).

<Co-W coating layer>
The content of W in the Co—W electroplating (coating layer) was estimated to be 2.4 atomic% (7.1% by mass) by XRF. Henceforth, this coating layer was referred to as a Co-2.4W coating layer. I will call it. A micrograph of a cross section of a test piece having a Co-2.4W coating layer is shown in FIG. The Co-2.4W coating layer is firmly adhered to the substrate surface and is a dense layer. Moreover, it is confirmed from the element distribution of this cross section shown as (B) in FIG. 1 that W is uniformly distributed throughout the coating layer. The layer thickness is about 10 μm as designed. The X-ray diffraction spectrum of the Co-2.4W coating layer is shown as (a) in FIG. In the coating layer, only the Co peak is confirmed.

<Oxide structure>
FIG. 3 shows a micrograph of a cross section of a test piece having a heat resistant film after isothermal oxidation at 800 ° C. in an oxidation time t _ox of 0 minutes to 1000 hours and element distribution in the cross section. In the figure, (a) shows an oxidation time t _ox of 0 minutes (hereinafter expressed as “t _ox = 0 minutes”), (b) shows t _ox = 15 minutes, and (c) shows t _ox = 30 minutes. , (D) is t _ox = 1 hour, (e) is t _ox = 3 hours, (f) is t _ox = 25 hours, (g) is t _ox = 250 hours, (h) is t _ox = 1000 hours. Is shown. In FIG. 3, a region a is a Co diffusion region in the substrate, a region b is Cr ₂ O ₃ , a region c is (Co, Fe, Cr) ₃ O ₄ , a region d is a coating layer, and a region e is also a coating layer. , Region f is a W-enriched region, region g is an iron-containing oxide region, and region h is a Co oxide region.

In the oxidation time t _ox = 0s, as seen in (a) of FIG. 3, a region h made of Co oxide is formed above the regions d and e which are Co-2.4W coating layers. As shown in FIG. 2 as (b), Co ₃ O ₄ and CoO are assigned to this Co oxide by XRD. Tungsten accumulates in a region f located below the Co oxide, the region is likely to contain CoWO _4. A small amount of oxygen is contained outside the region e which is the coating layer. The interface between the coating layer and the substrate was smeared compared to that observed before oxidation as shown in FIG. 1 due to interdiffusion of the main components of the two layers.

In the oxidation time t _ox = 15 minutes, the diffusion of iron to the outside proceeds, and as shown in (b) of FIG. 3, the region g that is an iron-containing oxide is changed to the region h that is a Co oxide. It is observed between the region f which is a W-enriched region. The oxidation time _t ox = 30 min, as seen in (c) in FIG. 3, the thickness of the region h which is _Co 3 _{O 4} layer of the uppermost layer is increased, under the _Co 3 _{O 4,} Fe _A region g which is an oxide of ₂ CoO ₄ is observed. As shown in FIG. 2 as (d), this oxide was found to be CoFe ₂ O ₄ from the XRD analysis results. Further, from (c) in FIG. 3, it is confirmed that a small amount of the region d which is the coating layer remains at t _ox = 30 minutes. Inside the remaining coating layer, enrichment of O and Cr is observed, which means that Cr is oxidized at the interface between the substrate region a and the coating layer region d.

At the oxidation time t _ox = 1 hour, the coating layer is completely converted to oxide as seen in (d) of FIG. The amount of the region f which is located under the region g which is CoFe ₂ O _{4 and} which is a W-enriched region including CoWO ₄ is increased, and the region b and the region c which are formed Cr oxides form CoWO ₄ . Facing the region f which is a W-enriched region.
At oxidation time t _ox = 3, 25 hours, the structure of the oxide layer is similar to the structure after oxidation for 1 hour as seen in (e) and (f) of FIG. The interface becomes clear.

The amount of Co in the region a, which is a Co diffusion region in the substrate, increases at the oxidation time t _ox = 30 minutes and decreases at t _ox = 25 hours, which is from 30 minutes to 1 hour when the coating layer disappears. This means that the interdiffusion between the coating layer and the substrate ends. At t _ox = 250 hours, Cr accumulates at the substrate / oxide interface as seen in (g) in FIG. 3, but this Cr appears to form Cr ₂ O ₃ in region b. Accumulation of Cr is also observed in the region c located between Cr ₂ O ₃ and CoWO ₄ . This feature becomes apparent at t _ox = 1000 hours, as can be seen in (h) in FIG. The outer Cr-enriched layer contains Cr, Co, Fe and O, possibly in the form of (Co, Fe, Cr) ₃ O ₄ in region c. After 1000 hours of oxidation, the oxide (heat resistant coating) is Cr ₂ O ₃ , (Co, Fe, Cr) ₃ O ₄ , CoWO ₄ , FeCo ₂ O ₄ , and Co ₃ O ₄ from the substrate side to the outside. It consists of five layers. It is important that Cr is not found on the top side of the CoWO ₄ layer (opposite the substrate), which means that CoWO ₄ acts as a barrier to Cr ion outdiffusion.

FIG. 4 shows an Ellingham-Richardson diagram constructed using NIST standard database number 69 (NIST Chemistry webbook). The kinks in some curves reflect discontinuities in the heat capacity used to calculate the Gibbs energy produced. The formation and location of the Cr ₂ O ₃ , CoWO ₄ , FeCo ₂ O ₄ and Co ₃ O ₄ layers is reasonable. Although the Gibbs generation energy of (Co, Fe, Cr) ₃ O ₄ itself could not be used, when (Co, Fe, Cr) ₃ O ₄ is recognized as CoCr ₂ O ₄ containing a small amount of dissolved Fe, Cr ₂ Generation of (Co, Fe, Cr) ₃ O ₄ is expected between O ₃ and CoWO ₄ . The formation of CoO, FeO, and WO ₃ is predicted from FIG. 4, but these oxides were not detected in FIG. This seems to be due to the reaction of these three compounds.

<Oxidation kinetics>
FIG. 5A shows a parabola plot when SUS430 stainless steel having a Co—W coating layer having a thickness of 10 μm and SUS430 stainless steel not having the coating layer are oxidized. The parabola plot shows the relationship between the mass increase ΔW and the parabolic law (ΔW ² ∝t _ox ) with respect to the oxidation time t _ox .
For specimens without a coating layer, the mass increase ΔW increases rapidly with increasing square root of the oxidation time t _ox and then the slope of the curve decreases after 10 minutes. The parabolic rate constant (straight line) up to 10 minutes is 2 × 10 ⁻³ g ² m ⁻⁴ s ⁻¹ , which is two orders of magnitude greater than that of the Fe-25Cr alloy at 800 ° C. At the start of the oxidation of the FeCr alloy, both Fe and Cr are oxidized. Then, after forming a uniform Cr ₂ O ₃ oxide on the surface, preferential oxidation of Cr is performed. Oxidation prior to 10 minutes in specimens without a coating layer appears to be a temporary step prior to diffusion controlled growth of Cr ₂ O ₃ .

On the other hand, in the test piece having the Co-2.4W coating layer, when the oxidation time t _ox = 180 minutes (t _ox ^0.5 = 13.4 min ^1/2 ) and ΔW reaches about 30 g / m ² , The curve is almost flat. The estimated parabolic rate constant before 180 minutes is 4 × 10 ⁻² g ² m ⁻⁴ s ⁻¹ , comparable to that of Co oxidation in air at 800 ° C.

(B) in FIG. 5 shows the long-term oxidation reaction rate after the oxidation time t _ox = 160 hours. The increase in mass of the test piece having the coating layer was 29 to 38 g / m ² . When 85 g / m ² of the Co-2.4W coating layer is fully oxidized, the mass gain of the specimen can be calculated as follows: According to the Gibbs free energy Δ _γ G ⁰ _{800 °} C. of the reaction at 800 ° C. that can be read from FIG. 4, Co ionizes to Co ²⁺ or Co ³⁺ and W ionizes to W ⁴⁺ or W ⁶⁺ . At this time, assuming that Co ³⁺ and W ⁶⁺ are preferentially formed, the maximum mass increase ΔW _{calc, max} can be estimated by Equation (1).

Here, m _plating is the mass of the coating layer, f _W is the atomic fraction of W in the coating layer (0.024), and M _O , M _Co and _MW are the molar masses of O, Co and W, respectively. . The calculated ΔW _{calc, max} is 28.4 g / m ² , and since the actual mass increase shown in FIG. 5B is larger, the electroplating layer has an oxidation time t _ox = 160. It can be said that it has been converted to oxide by time. The difference between ΔW _{calc, max} and the actual mass increase may correspond to the mass of oxygen used to oxidize Fe and Cr in the SUS430 stainless steel substrate. The mass change after the oxidation time t _ox = 160 hours is small for all experiments up to t _ox = 1000 hours, corresponding to the prevention of Cr outdiffusion by the CoWO ₄ layer, as shown in FIG. It seems to be. On the other hand, in the specimen without the coating layer, the mass began to decrease after the oxidation time t _ox = 160 hours, possibly due to the evaporation of Cr from Cr ₂ O ₃ . The mass loss rate is estimated to be 8 × 10 ⁻⁸ g / m ² s, which is slightly higher than the evaporation rate of Cr ₂ O ₃ in 760 ° C. air + H ₂ O atmosphere.

<Multilayer oxide formation mechanism>
As described above, the Co-2.4W coating layer first reacts with oxygen in the atmosphere to generate several types of oxides (step 1). Next, a multilayer oxide is formed after the coating layer is consumed (step 2). Below, the formation mechanism of the multilayer oxide based on both steps is demonstrated based on a thermodynamic viewpoint.

Step 1 is a transient oxidation stage. It proceeds by three small sub-stages with oxidation times t _ox = 0, 15 minutes and 30 minutes as shown in FIGS. 6 (a) to (c). As shown in FIG. _6A , the external Co ₃ O ₄ layer and the internal CoO layer are formed in the oxidation time t _ox = 0 minutes. According to FIG. 4, both Co and W may be oxidized until the temperature of the sample reaches 800 ° C., and the Gibbs formation free energy at 800 ° C. of the W oxide Δ _γ G ⁰ _{800 ° C.} Is more negative than that of Co oxide, but it is believed that Co forms an outer oxide layer due to the lower W content of the plating. This phenomenon is similar to the phenomenon that Cr forms an internal oxide under the continuous Fe oxide layer during the oxidation of the Fe—Cr alloy having a low Cr content. From the viewpoint of free energy, WO ₃ (s) should also be formed in the coating, but its formation could not be confirmed from the XRD results. The reason is understood that the formed WO ₃ reacted immediately with CoO according to the formula (2) to produce CoWO ₄ .

Incidentally, formula (2) Gibbs energy _Δ γ G ⁰ _{800 ℃} in is a value obtained from Figure 4.

It was also confirmed by XRD that WO ₂ (s) was not formed. Oxide ions (O ²⁻ ) migrate inward through CoWO ₄ and oxidized W metal in the coating layer, and then the generated W oxide is continuously converted to CoWO ₄ , It is understood that CoWO ₄ has grown towards the substrate. The increase in the amount of CoWO ₄ at the oxidation time t _ox = 15 minutes shown in (b) of FIG. 6 represents this. In (a) to (c) in FIG. 3 which is the transient oxidation stage, it is understood that the oxygen observed in the region e which is the coating layer is caused by this CoWO ₄ internal oxide. In addition, as can be seen from (a) to (c) in FIG. 3, in the transient oxidation stage, it was confirmed that outward diffusion of Fe occurred from the base material and the diffusion front surface reached the center of the coating layer. It was. It is understood that the diffused Fe reaches the coating layer / oxide interface and reacts with CoO to form FeCo ₂ O ₄ according to the equation (3) having negative Gibbs formation free energy.

That is, since a large amount of Co—W coating layer remains in this transient oxidation stage, Co is first oxidized to generate CoO at the remaining coating layer / Co oxide region interface. Next, Co ions in CoO move outward through the p-type CoO and Co ₃ O ₄ layers, forming oxide nuclei at the Co oxide / gas interface. Further, as the CoWO ₄ internal oxide grows in the direction of the base material, some oxide finally reaches the base material / residual coating layer interface. Oxidation of Cr at the interface is initiated at t _ox = 30 minutes by consumption of oxygen ions transported through CoWO ₄ as shown in (c) of FIG. In this system, as shown in FIG. 4, since Cr ₂ O ₃ is the most stable oxide, preferential oxidation of Cr at this interface is appropriate. As Cr is preferentially oxidized, the Co content in the substrate increases as seen in (c)-(d) in FIG. Both Co and Fe diffuse outward through the coating layer. At this time, since the amount of Co in the remaining coating layer is small, Fe diffusion becomes faster than Co diffusion. As a result, Fe reaches the CoO layer and is oxidized to form a continuous FeCo ₂ O ₄ layer. Thus, the change in oxide structure during the transient oxidation stage can be reasonably explained. If stainless steel containing approximately 22% by mass or more of Cr is used as the base material, Fe diffusion from the base material does not occur and FeCo ₂ O ₄ is not generated.

Step 2 is characterized as an oxidation process in steady state. Since the entire coating layer was consumed in the oxidation time t _ox = 1 hour, diffusion of the metal species to the outside became impossible. As a result, the thicknesses of the FeCo ₂ O ₄ and Co ₃ O ₄ layers hardly change from the thickness of the layer formed at t _ox = 30 minutes. In the vicinity of the substrate / oxide coating (heat resistant coating) interface, as seen in (d) of FIG. 3, the Cr content in the substrate is too low to form a continuous Cr ₂ O ₃ layer. As shown in (d) of FIG. 6, a part of Cr forms (Co, Fe, Cr) ₃ O ₄ together with Fe and Co oxidized on the substrate surface. However, as can be seen from (e) in FIG. 3, the Cr content in the vicinity of the substrate surface increases with the lapse of the oxidation time, so as shown in (e) in FIG. A continuous Cr ₂ O ₃ layer can be formed in t _ox = 3 hours. Thereafter, as can be seen in (g) of FIG. 3, the interface between (Co, Fe, Cr) ₃ O ₄ and Cr ₂ O ₃ becomes clear at t _ox = 250 hours. The order of the layers along the depth direction described above is thermodynamically valid as seen from FIG.

FIG. 7 shows the ratio of the thickness of the formed oxide layer (heat resistant coating) when SUS430 stainless steel with a Co-2.4W coating layer is isothermally oxidized at 800 ° C. After the oxidation time t _ox = 1 hour, the ratio of the layers was almost the same as that up to t _ox = 1000 hours with some variation. Since the diffusion of Cr ions is prevented by CoWO _4-layer, oxidation may progressed under CoWO ₄ layers. At this time, since oxygen ions need to be supplied by solid phase diffusion in the heat resistant coating for the oxidation of Cr, the oxidation rate of Cr is expected to be slower than the oxidation rate in the atmosphere. Accordingly, it is understood that the five-layer oxide shown in FIG. 7 remains until the Cr content in the base metal is lower than the critical level necessary to maintain Cr ₂ O ₃ .

<Mechanism and stability of Cr diffusion block by CoWO ₄ >
In the present invention, it is important that Cr ions do not penetrate the CoWO ₄ layer. It is understood that the reason for preventing diffusion is based on the stability and crystal structure of the oxide.
CoWO ₄ has a monoclinic structure having lattice constants of a = 0.4670 nm, b = 0.55687 nm, and c = 0.54951 nm, and includes two Co ²⁺ ions and four W ^{6+ in} a unit cell. Contains ions.
Since Cr ^{3+ is the} only stable Cr ion at 800 ° C., it is impossible to replace Co ²⁺ ions in CoWO ₄ with Cr ³⁺ ions or to accommodate Cr ³⁺ ions in Co ion vacancies. This is because the Co ion vacancy has two negative charges and can accommodate only divalent ions. CoWO there is no Cr in _4, by the fact that there are Fe, which can be a divalent ion _four layers in CoWO, immiscible with trivalent Cr ions on CoWO _4, and the possible substitution of divalent ions described Is done.

As is clear from the above discussion, CoWO ₄ effectively blocked the diffusion of trivalent Cr ions, and the effect persisted at 800 ° C. for an oxidation time of 1000 hours. However, since the estimated lifetime of a solid oxide fuel cell (SOFC) is 10,000 hours or more, it is necessary to block for 10 times longer than that shown in the experimental results. Then, based on thermodynamics, the reactivity with the adjacent material of CoWO ₄ was investigated. Specifically, the Gibbs free energy for the reaction between Cr ₂ O ₃ and CoWO ₄ was calculated according to the following equation: Possible products of CoWO ₄ and Cr ₂ O ₃ which must be stable compounds were investigated in the Cr—Co—O and Cr—W—O systems.

Since both delta _gamma G ⁰ _{1073 K} value of these reactions are positive, it is impossible to reaction between CoWO ₄ and _Cr 2 _{O 3} at 800 ° C., which CoWO ₄ is stable, long at 800 ° C. It means effectively blocking the transport of Cr ions into the gas phase over time.
From the above discussion, the Co-2.4W coating layer before the start of service can cause poisoning of the solid electrolyte and the electrocatalyst material accompanying the diffusion and evaporation of Cr species in the H ₂ O-containing atmosphere. It can be said that it can be prevented throughout the service period of the battery (SOFC).

<Vaporization of tungsten species>
From FIG. 3, knowing the thickness δ _CoWO4 of the CoWO ₄ layer, the evaporation index η _W is obtained by equation (6).

In formula (6), S is the surface area of the sample, the density of CoWO ₄ ρ _{CoWO 4} = 8.42 g / cm ³ , and the molar mass of CoWO ₄ M _{CoWO 4} = 306.77 g / mol.

A plot of the calculated evaporation index η _W against the oxidation time is shown in FIG. The evaporation index η _W was almost constant at 1 regardless of the oxidation time. This indicates that the evaporation index η _W is independent of the oxidation time and W vaporization does not occur. At the start of high temperature oxidation, WO ₃ (g) may evaporate from the sample before the continuous Co ₃ O ₄ layer covers the surface, but if the W metal is not exposed to the atmosphere, It is understood that there is no fear. Therefore, as an aspect capable of further suppressing the evaporation of W, a two-layer coating comprising a Co—W inner layer and a pure Co outer layer is applied to the base material.

<Summary of Examples>
Co-W alloy electroplating was applied to SUS430 type stainless steel. The alloy coated steel was then oxidized in air + 3% by volume H ₂ O at 800 ° C. for up to 1000 hours. From the analysis and observation of oxidized samples, the following conclusions can be drawn:
(1) Using an electroplating bath containing citric acid, a coating layer formed of a Co—W alloy having a W content of 3 to 67.5 mass% (1 to 40 atomic%) was obtained.
(2) The oxidation treatment of the coating layer formed of the Co—W alloy effectively forms a continuous CoWO ₄ layer that effectively blocks the diffusion of Cr ions and does not cause Cr vaporization from the interconnects. .
(3) The oxide produced | generated on stainless steel by the oxidation process for 3 hours or more was comprised by five layers. The stacking order of these oxides is thermodynamically valid. Further oxidation after the formation of the five-layer oxide proceeds slowly, showing the diffusion barrier properties of Cr from stainless steel over time.

The ferritic stainless steel having the Co—W coating layer or laminate film of the present invention is applied to, for example, a current collecting member of a solid oxide fuel cell (SOFC), thereby improving the durability of the current collecting member, Expected to improve performance and environmental problems, and lead to the spread of fuel cells.

Claims (11)

1. Based on a ferritic stainless steel containing Cr: 11 to 40% by mass,
   As a coating layer formed on the substrate, a Co—W coating layer comprising a thickness of 1 to 100 μm, a W content of 3 to 67.5% by mass (1 to 40 atom%), the remainder Co and unavoidable impurities. A stainless steel member provided.
2. Based on a ferritic stainless steel containing Cr: 11 to 40% by mass,
   The coating formed on the substrate has a layer containing Co oxide on the outermost surface, and has a layer containing CoWO ₄ on the substrate side than the layer containing Co oxide. A stainless steel member comprising a laminate of 5 to 50 μm.
3. The coating further includes a layer containing Cr ₂ O ₃ and a layer containing (Co, Fe, Cr) ₃ O _{4 in} that order from the base material side on the base material side relative to the layer containing CoWO _4. The stainless steel member according to claim 2, wherein the layer containing Co oxide contains Co ₃ O ₄ .
4. The ferritic stainless steel of the base material is, in mass%, Cr: 11 to 40%, Si: 1.5% or less, Mn: 1.5% or less, C: 0.12% or less, P: 0.1 The stainless steel member according to any one of claims 1 to 3, having a composition comprising:% or less, S: 0.01% or less, N: 0.1% or less, balance Fe and inevitable impurities.
5. Ferritic stainless steel of the base material is in mass%, further Al: 6% or less, Mo: 4% or less, W: 4% or less, Cu: 2% or less, Nb: 0.8% or less, Ti: 0 The stainless steel according to claim 4, comprising at least one of 0.5% or less, Zr: 0.5% or less, V: 0.5% or less, Ta: 0.5% or less, Ni: 2% or less. Element.
6. The ferritic stainless steel of the base material is mass%, further Y: 0.1% or less, other REM (rare earth element): 0.1% or less, Ca: 0.01% or less, B: 0.01 The stainless steel member according to claim 4 or 5, which contains at least one element of not more than% and Mg: not more than 0.01%.
7. The Co—W coating layer further contains one or more selected from Fe, Ni, Ti, Nb, Zr, Ta, V, Mo, P, and B in a total amount of 10% by mass or less, The stainless steel member according to any one of claims 1 and 4 to 6, wherein the Co-W coating layer has a Cr content of 0 to 2 mass%.
8. The stainless steel member according to any one of claims 1 to 7, wherein the stainless steel member is a current collecting member of a solid oxide fuel cell (SOFC).
9. Preparing a ferritic stainless steel containing Cr: 11 to 40% by mass as a base material;
   Forming a Co—W coating layer having a thickness of 1 to 100 μm and a W content of 3 to 67.5 mass% (1 to 40 atom%) on the surface of the substrate;
   A method for producing a stainless steel member, comprising:
10. The substrate on which the Co—W coating layer is formed is heat-treated at a temperature in the range of 600 ° C. to 850 ° C., and has a layer containing a Co oxide on the outermost surface. 10. The method according to claim 9, further comprising a step of forming a laminate film having a thickness of 0.5 to 50 μm, which has a layer containing CoWO ₄ on the base side of a layer containing Co oxide. Manufacturing method of stainless steel member.
11. The laminate film further includes a layer containing Cr ₂ O ₃ and a layer containing (Co, Fe, Cr) ₃ O ₄ in this order from the substrate side to the substrate side of the layer containing CoWO _4. The method for producing a stainless steel member according to claim 10, wherein the layer containing Co oxide is a laminate including Co ₃ O ₄ .
    

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