Stainless steel for fuel cell separators
09871258 ยท 2018-01-16
Assignee
Inventors
- Masayasu Nagoshi (Kawasaki, JP)
- Shinsuke Ide (Ames, IA)
- Shin Ishikawa (Chiba, JP)
- Noriko Makiishi (Chiba, JP)
Cpc classification
G01R27/205
PHYSICS
C22C38/002
CHEMISTRY; METALLURGY
H01M8/0265
ELECTRICITY
Y02E60/50
GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
International classification
G01R27/20
PHYSICS
Abstract
A stainless steel having a low surface contact resistance for fuel cell separators is provided. The stainless steel has a Cr content of 16 to 40 mass % or more. The stainless steel includes a region having a fine textured structure on its surface, and the area percentage of the region is 50% or more, preferably 80% or more. The region having a fine textured structure is a region which has a structure having depressed portions and raised portions at an average interval between depressed portions or raised portions of 20 nm or more and 150 nm or less when observed with a scanning electron microscope.
Claims
1. A stainless steel having a low surface contact resistance for fuel cell separators and a Cr content in the range of 16 to 40 mass %, wherein the stainless steel includes a region having a fine textured structure on its surface, the area percentage of the region being 50% or more, and the region having a fine textured structure is a region which has a structure having depressed portions and raised portions at an average interval between the depressed portions or the raised portions of 20 nm or more and 150 nm or less when observed with a scanning electron microscope, the raised portions having a triangular pyramidal top having an average angle of 80 degrees or more and 100 degrees or less.
2. The stainless steel having a low surface contact resistance for fuel cell separators according to claim 1, wherein the triangular pyramidal tops of the raised portions are disposed at an average interval of 100 nm or less.
3. The stainless steel having a low surface contact resistance for fuel cell separators according to claim 1, wherein the area percentage is 80% or more.
4. The stainless steel having a low surface contact resistance for fuel cell separators according to claim 1, wherein the area percentage is 80% or more, and the triangular pyramidal tops of the raised portions are disposed at an average interval of 100 nm or less.
5. The stainless steel having a low surface contact resistance for fuel cell separators according to claim 1, wherein the stainless steel includes trapezoidal projection structures, the area percentage of the dispersed trapezoidal projection structures being 5% or more and 30% or less.
6. The stainless steel having a low surface contact resistance for fuel cell separators according to claim 5, wherein the trapezoidal projection structures have an average height of 0.15 m or more and 2 m or less.
7. The stainless steel having a low surface contact resistance for fuel cell separators according to claim 5, wherein the trapezoidal projection structures have an average diameter of 3 m or more and 50 m or less.
8. The stainless steel having a low surface contact resistance for fuel cell separators according to claim 5, wherein the trapezoidal projection structures correspond to crystal grains of the stainless steel.
9. The stainless steel having a low surface contact resistance for fuel cell separators according to claim 1, wherein the stainless steel has a surface contact resistance of 10 m.Math.cm.sup.2 or less.
10. The stainless steel having a low surface contact resistance for fuel cell separators according to claim 1, wherein after the stainless steel is immersed in a sulfuric acid solution containing 0.1 ppm fluoride ions at pH 3 and at 80 C. for 24 hours at 0.6 V versus a AgAgCl reference electrode, the stainless steel has a surface contact resistance of 10 m.Math.cm.sup.2 or less.
11. The stainless steel having a low surface contact resistance for fuel cell separators according to claim 1, wherein after the stainless steel is immersed in a sulfuric acid solution containing 0.1 ppm fluoride ions at pH 3 and at 80 C. for 500 hours at 0.6 V versus a AgAgCl reference electrode, the stainless steel has a surface contact resistance of 10 m.Math.cm.sup.2 or less.
Description
BRIEF DESCRIPTION OF DRAWINGS
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DESCRIPTION OF EMBODIMENTS
(14) The present invention will be further described below in detail.
(15) First, a stainless steel directed to in the present invention will be described below.
(16) A stainless steel according to the present invention used as a material is not limited to a particular type of steel, provided that the stainless steel has required corrosion resistance in the operating environment of fuel cells. In order to have essential corrosion resistance, the Cr content must be 16 mass % or more. A Cr content of less than 16 mass % results in a separator having insufficient durability for long-term use. The Cr content is preferably 18 mass % or more. A Cr content of more than 40 mass % results in an excessively increased cost of the stainless steel. Thus, the Cr content is 40 mass % or less.
(17) The other components and their concentrations are not particularly limited. A stainless steel according to the present invention may contain the following elements sufficient for practical use or so as to improve its corrosion resistance.
(18) C: 0.03% or Less
(19) C combines with chromium in the stainless steel and precipitates chromium carbide in grain boundaries, thus C may adversely affect the corrosion resistance of the stainless steel. Thus, the C content is preferably lowered. The corrosion resistance does not significantly deteriorate at a C content of 0.03% or less. Thus, the C content is preferably 0.03% or less, more preferably 0.015% or less.
(20) Si: 1.0% or Less
(21) Si is an effective element for deoxidation and is added to the stainless steel in a smelting process. The addition of an excessive amount of Si, however, causes hardening of a stainless steel sheet and may decrease the ductility. Thus, the Si content is preferably 1.0% or less.
(22) Mn: 1.0% or Less
(23) Combining with incidental sulfur, Mn is an effective element for reducing sulfur dissolved as a solid-solute in the stainless steel, suppressing segregation of sulfur at grain boundaries, and preventing cracking of a steel sheet during hot rolling. However, the addition of more than 1.0% Mn does not significantly improve these effects. Such an excessive addition increases the cost of the stainless steel. Thus, the Mn content is preferably 1.0% or less.
(24) S: 0.01% or Less
(25) S combines with manganese and forms manganese sulfide, thereby deteriorating the corrosion resistance. Thus, the S content is preferably lowered. A S content of 0.01% or less does not significantly deteriorate the corrosion resistance. Thus, the S content is preferably 0.01% or less.
(26) P: 0.05% or Less
(27) Since P deteriorates the ductility of the stainless steel, it is desirable to reduce P. However, a P content of 0.05% or less does not significantly deteriorate the ductility of the stainless steel. Thus, the P content is preferably 0.05% or less.
(28) Al: 0.20% or Less
(29) Al is used as a deoxidation element. However, an excessively high Al content results in low ductility of the stainless steel. Thus, the Al content is preferably 0.20% or less.
(30) N: 0.03% or Less
(31) N is an effective element for suppressing the local corrosion, such as crevice corrosion, of the stainless steel. However, the addition of more than 0.03% N to the stainless steel in a smelting process takes a long time and reduces the productivity of the stainless steel and may deteriorate steel formability. Thus, the N content is preferably 0.03% or less.
(32) At Least One of Ni: 20% or Less, Cu: 0.6% or Less, and Mo: 2.5% or Less
(33) Ni: 20% or Less
(34) Ni is an element that stabilizes an austenite phase and is added in the manufacture of an austenitic stainless steel. A Ni content of more than 20% results in an increased cost of the stainless steel due to excessive consumption of Ni. Thus, the Ni content is preferably 20% or less.
(35) Cu: 0.6% or Less
(36) Cu is an effective element for improving the corrosion resistance of the stainless steel. However, a Cu content of more than 0.6% may result in poor hot workability and may cause low productivity. Further, the addition of an excessive amount of Cu increases the cost of the stainless steel. Thus, the Cu content is preferably 0.6% or less.
(37) Mo: 2.5% or Less
(38) Mo is an effective element for suppressing the local corrosion, such as crevice corrosion, of the stainless steel. Thus, the addition of Mo is effective in the case where the stainless steel is used in a severe environment. However, a Mo content of more than 2.5% may result in embrittlement of the stainless steel, low productivity, and an increased cost of the stainless steel due to excessive consumption of Mo. Thus, the Mo content is preferably 2.5% or less.
(39) Total Nb, Ti, and/or Zr Content of 1.0% or Less
(40) In addition to above described elements, a stainless steel according to the present invention may contain at least one of Nb, Ti, and Zr to improve the intergranular corrosion resistance. However, the total Nb, Ti, and Zr content of more than 1.0% may result in low ductility. Further, in order to avoid an increase in cost due to the addition of these elements, the total Ti, Nb, and Zr content is preferably 1.0% or less, when these elements are added.
(41) The remainder are Fe and incidental impurities.
(42) Next, the characteristics necessary for a stainless steel for separators according to the present invention will be described below. A stainless steel according to the present invention includes a region having a fine textured structure that has an average interval between depressed portions or raised portions (average distance between fine asperities) of 20 nm or more and 150 nm or less (hereinafter also referred to simply as a region having a fine textured structure) on its surface when observed with a scanning electron microscope (hereinafter also referred to as SEM). The area percentage of the region having a fine textured structure is 50% or more, preferably 80% or more. In the textured structure, the height difference between the bottom of a depressed portion and the top of a raised portion adjacent to the depressed portion is 15 nm or more. An above defined average interval above its upper limit or an above defined area percentage below its lower limit results in an insufficient decrease in contact resistance of the stainless steel. This is probably because of a decreasing number of contact points. An average interval of less than 20 nm is undesirable in terms of durability. In a durability test for estimating durability conducted in a simulated operating environment of fuel cells, it was found that foreign matter was formed over time on the surface of stainless steel and increased the contact resistance of the stainless steel. This is probably because when a textured structure is excessively fine the contact resistance is susceptible to foreign matter.
(43) The durability of the stainless steel is further improved when the raised portions of the fine textured structure have a triangular pyramidal top having an average angle of 80 degrees or more and 100 degrees or less. The average angle refers to the average of the angles at the tops of three planes forming the top of the triangular pyramid. In the present invention, the three planes are generally corresponding to cubic crystal 110 planes. In such a case, the angles in the three planes are approximately 90 degrees, and the average angle is approximately 90 degrees (=(90+90+90)/3). The top of the triangular pyramid shape is not necessarily to be sharpen at the atomic level.
(44) The advantage of the triangular pyramid structure at the top of each raised portion is probably as follows, although the present invention is not limited to the following mechanism.
(45) (1) An oxidized layer having a thickness of several nanometers is formed on a stainless steel. Although being thin, the oxidized layer can be a factor which increases the contact resistance of the stainless steel. Thus, it is desirable that such an oxidized layer be broken when surface of the stainless steel comes into contact with, for example, an opposing carbon paper in a fuel cell. In particular, after a durability test, the thickness or composition of the oxidized layer may be changed, and the resistance may be increased. The triangular pyramid structure at the top of the projection has a small curvature radius. This increases the contact pressure and facilitates the breakage of the oxide film. Thus, it is considered that after the durability test the contact resistance of the stainless steel is lowered.
(46) (2) As described above, foreign matter is formed over time on the stainless steel in the operating environment and increases the contact resistance of the stainless steel. When the top of the projection has the triangular pyramid structure, the contact resistance is insusceptible to the foreign matter. Even in the presence of some amount of foreign matter, the increase in contact resistance of the stainless steel is suppressed as there is the triangular pyramid portion which locally maintains the contact pressure. When the average interval between the tops of the triangular pyramid portions is 150 nm or less, preferably 100 nm or less, this results in further improved performance of the stainless steel.
(47) The fine textured structure as described above is observed with a scanning electron microscope (SEM). The SEM used to observe the fine textured structure is, but not limited to, an apparatus that can provide clear secondary electron images at an incident electron accelerating voltage of 5 kV or less, desirably 1 kV or less, and at a magnification of tens of thousands of times. The average spacing of fine asperities can be determined from such a secondary electron image. For example, the number of fine projections or depressed portions crossing a line of a given length is counted with such a SEM, and the length is divided by the number. In the case of triangular pyramidal tops, the number of triangular pyramidal tops in a given area is counted. The average interval between projections is simply calculated by 1000/(N.sup.0.5) nm, wherein N denotes the number of tops per square micrometer. Alternatively, the distance between raised portions and the peak-to-peak distance may be determined from a Fourier-transformed image. The average of the angles at the tops of three planes forming the top of the triangular pyramid is 80 degrees or more and 100 degrees or less. Here, the angles at the tops of three planes are angles viewed in a direction perpendicular to the corresponding plane. Examples of SEM observation results are shown as follows.
(48)
(49) The average interval between depressed portions or raised portions is required to be 20 nm or more and 150 nm or less, preferably 120 nm or less, more preferably 100 nm or less. The relationship among the average spacing of fine asperities, the area percentage of a region having a fine textured structure, surface contact resistance, and surface contact resistance after a durability test was studied. The area percentage can be determined through SEM observation and was determined in a 100 m square. A region having a fine textured structure is shown as a light part in contrast because of increased secondary electron emission. The area percentage of a region having a fine textured structure was determined through the binary conversion of a secondary electron image using commercially available software.
(50) The surface contact resistance and the surface contact resistance after a durability test were measured while a carbon paper CP120 manufactured by Toray Industries, Inc. was in contact with a steel at a load of 20 kgf/cm.sup.2.
(51) In a durability test, a sample was immersed in a sulfuric acid solution at pH 3 at 0.6 V versus a AgAgCl reference electrode at room temperature for 24 hours.
(52) Thus, in order to achieve a sufficiently low surface contact resistance (10 m.Math.cm.sup.2 or less) of a stainless steel, it is necessary to provide 50% or more of the stainless steel surface with a region having a fine textured structure that includes fine asperities at an average interval of 20 nm or more and 150 nm or less. When the area percentage of the region having a fine textured structure is less than 50%, this results in an insufficient effect of increasing the number of contact points with an electrode by fine asperities of the stainless steel, and the stainless steel does not have a sufficiently low surface contact resistance (10 m.Math.cm.sup.2 or less).
(53) Furthermore, in order to maintain the surface contact resistance for a long time in the operating environment of fuel cells, that is, in order to suppress an increase in surface contact resistance in the operating environment of fuel cells, the area percentage of the region having a fine textured structure is 50% or more as described below. Although the reason for this is not clear, the following is a possible reason for this. In the operating environment, the electrical conductivity of a thin oxidized layer on a steel surface decreases because of an increase in the thickness of the oxidized layer or variations in the composition of the oxidized layer. The influence of this effect on surface contact resistance is probably greater in a smooth surface without fine asperities than in a surface having fine asperities. Thus, in order to maintain a low surface contact resistance in the operating environment, the smooth surface must be decreased as far as possible, and the area percentage of the surface having fine asperities must be increased.
(54) In the examples described above, the fine textured structure has a fine granular or gently sloping shape ([
(55) The present inventors also studied the relationship between the surface profile observed on the order of micrometers, contact resistance, contact resistance after a durability test, and contact resistance after a sliding test, on a surface having a fine textured structure described above. Such a surface profile can be easily measured with a confocal laser microscope or an optical interferotype profilometer. [
(56) It was found that although examples of the present invention have trapezoidal projection structures in [
(57) The contact resistance and the contact resistance after a durability test were measured while a carbon paper CP120 manufactured by Toray Industries, Inc. was in contact with a steel at a load of 20 kgf/cm.sup.2. The contact resistance was also measured using the same method after a sliding test.
(58) The following findings were obtained from these results. The formation of a fine structure in a predetermined range on a surface results in a low contact resistance of 10 m.Math.cm.sup.2 or less, as described above. When the trapezoidal projection structures have an average height of 0.15 m or more and 2 m or less and an average diameter of 3 m or more and 50 m or less when viewed from the top, and the area percentage of the trapezoidal projection structures is 5% or more and 30% or less, the contact resistance of the stainless steel is as low as 10 m.Math.cm.sup.2 or less even after the sliding test.
(59) Sliding of an indenter (die) on a surface of a steel sheet causes damage to fine textured structures on the surface of the steel sheet. In the presence of trapezoidal projection structures on the surface, the projections come into contact with the indenter (die) after the sliding test, and the damaged region is limited to the trapezoidal projection structures. Thus, it seems that most of the fine textured structures remain after the sliding test and maintain the low contact resistance. This effect requires the height of 0.15 m or more. Excessively high trapezoidal projection structures take a longer time and a higher cost to form. Thus, the height of the trapezoidal projection structures is preferably 2 m or less. The area percentage of the trapezoidal projection structures is 5% or more and 30% or less. When the area percentage of the trapezoidal projection structures is outside this range, the contact resistance is unfavorably increased after processing. When the area percentage is less than 5%, the trapezoidal projection structures are easily scraped by sliding, and the area having no fine textured structure increases. When the area percentage is more than 30%, fine textured structures on the surface of the trapezoidal projection structures are easily scraped by sliding, and the contact resistance increases. Localized trapezoidal projection structures have little effect. It is desirable that trapezoidal projection structures be uniformly dispersed on the surface.
(60) When the average diameter of the trapezoidal projection structures is less than 3 m, the trapezoidal projection structures collapse easily by contact with a die. When the average diameter of the trapezoidal projection structures is more than 50 m, the contact area increases unfavorably. Thus, it is desirable that the average diameter of the trapezoidal projection structures be 3 m or more and 50 m or less. The term trapezoidal projection structures, as used herein, refers to regions that are higher than their surroundings and have a certain area. The trapezoidal projection structures can be quantitatively observed using the profile measurement method as described above. The trapezoidal projection structures can be qualitatively observed with SEM while a sample is tilted. [
(61) The following is a specific example. The surface profile is measured in a field of 0.35 mm0.26 mm with an optical interferotype profilometer. In this field, a trapezoidal projection structure is a flat portion having a length of 2 m or more on at least one of five straight lines being set for measurement parallel to the 0.35-mm long side and having a height at least 0.05 m higher than left and right portions adjacent to the trapezoidal projection structure. The top of the trapezoidal projection structure is not necessarily parallel to the surface of the sample and is not necessarily a flat surface but may be a gently curved surface. Further, a fine textured structure may be situated on the top of the trapezoidal projection structure. The height of an individual trapezoidal projection structure is a difference between the average height of 10 points on a flat portion of the trapezoidal projection structure disposed on straight lines for measurement and the average height of 5 points on straight lines for measurement in left and right portions adjacent to the trapezoidal projection structure that have no trapezoidal projection structure. The average height is the average of the heights of all the trapezoidal projection structures disposed on the straight lines for measurement. The diameter of an individual trapezoidal projection structure is the length of a straight line projected on the surface of the sample corresponding to a flat portion of the trapezoidal projection structure disposed on the straight line. The average diameter of trapezoidal projection structures is the average of the diameters of all the trapezoidal projection structures disposed on the straight lines used for measurement. The area percentage is the ratio of the total diameter of trapezoidal projection structures (the total of the lengths of the tops of the flat portions of the trapezoidal projection structures) disposed on the straight lines for measurement to the length of the lines for measurement (0.35 mm5). If three trapezoidal projection structures in the observation field are disposed on five straight lines each having a length of 0.35 mm (350 m) and have diameters (the lengths of the tops of the flat portions) of 20, 30, and 10 m, the average diameter is 20 m ((20+30+10)/3)), and the area percentage is 3.4% ((20+30+10)/(3505)).
(62) A method for manufacturing a stainless steel according to the present invention having a low surface contact resistance for fuel cell separators will be described below. The following are non-limiting suitable manufacturing conditions.
(63) A slab having an appropriate composition is heated to a temperature of 1100 C. or more and is hot-rolled. The hot-rolled slab is annealed at a temperature in the range of 800 C. to 1100 C. and is repeatedly cold-rolled and annealed to form a stainless steel. The stainless steel sheet preferably has a thickness in the range of approximately 0.02 to 0.8 mm. The stainless steel sheet is then subjected to final annealing, and preferably further, electrolytic treatment, and acidizing. Aqueous sulfuric acid can be used in the electrolytic treatment. Immersion in a hydrofluoric acid solution can be used in the acidizing, for example. The formation and the adjustment of the area percentage of a region having a fine textured structure can be performed by changing the conditions (the concentration and type of the solution, temperature, and immersion time) for these treatments, in particular acidizing.
(64) Raised portions of a triangular pyramid shape may be formed by any method. It is desirable to use the crystal orientation dependence of etching with an acidic solution because it can treat a wide area without using a complicated process (such as ion irradiation). The present inventors found that a triangular pyramid shape can be formed in a wide area on a surface by controlling the crystal orientation and the etching conditions. In a ferritic stainless steel, raised portions of a triangular pyramid shape composed of (001) micro facets can be formed of crystal grains, in which the orientation perpendicular to the surface is close to ND <111>, at a high density. Although the raised portions of a triangular pyramid shape may be formed of crystal facets having a different orientation, the number of raised portions is smaller in this case than in the case of crystal grains having the orientation close to ND <111>. Thus, rolling is performed such that the texture contains many crystal grains in which the orientation perpendicular to the surface is close to ND <111>. Such a texture can be easily observed by texture observation and by taking electron backscatter diffraction scattering (EBSD) images. It is desirable that the raised portions of a triangular pyramid shape composed of (001) micro facets be formed by reducing Fe in a pretreatment electrolyte solution or by immersion in a solution containing minimized amounts of nitric acid and Fe in hydrofluoric acid used for immersion for a predetermined period of time (for 5 mass % aqueous HF at 55 C., the time preferably ranges from 80 to 600 seconds).
(65) The trapezoidal projection structures may be formed by any method. It is desirable to form the trapezoidal projection structures utilizing a difference in crystal orientation of crystal grains because this can obviate the need for an additional process, such as masking and etching. The fine textured structure on the surface described above can be formed in a relatively short period of time. The height difference in different crystal orientations increases with the treatment time because of a difference in etch rate in the different crystal orientations. Since the ND <001> plane is difficult to etch, grains having this crystal orientation form the trapezoidal projection structure. These can be controlled by changing acidizing conditions (the concentration and type of the solution, temperature, and immersion time). Under a certain solution condition, there is a minimum treatment time for the formation of the fine textured structure. There is also a minimum treatment time, which is longer than the former minimum treatment time, for the formation of the trapezoidal projection structure. There is also a maximum treatment time, after which the trapezoidal projection structure disappears. For 5 mass % aqueous HF at 55 C. described above, the treatment time preferably ranges from 80 to 450 seconds in terms of the height of the trapezoidal projection structure. The trapezoidal projection structure disappeared when the treatment time was 600 seconds. Since the surface profile is easy to be measured as described above, on the basis of the indications according to the present invention, the treatment conditions can be determined by measuring the surface profile without undue consideration.
Example 1
(66) A commercially available austenitic stainless steel SUS 304L having a Cr content of 18.1 mass % and a commercially available ferritic stainless steel 443CT having a Cr content of 21.1 mass % were repeatedly subjected to cold rolling, annealing, and pickling, thus yielding stainless steel sheets having thickness of 0.2 mm.
(67) The stainless steel sheet was then annealed and was subjected to electrolytic treatment and acidizing, which was immersion in a pickling solution, under the conditions shown in Table 1. The electrolytic treatment was conducted using an electrolyte solution containing iron (II) sulfate in an amount corresponding to 1 g/L of iron ions dissolved in 3 mass % aqueous sulfuric acid with alternating current at an electric current density of 5 A/dm.sup.2 for 4.5 seconds. For comparison purposes, some samples were not subjected to electrolytic treatment and acidizing, which was immersion in a pickling solution, and some samples were subjected only to the electrolytic treatment. The stainless steel thus obtained was subjected to the measurement of surface contact resistance and surface observation using SEM (SUPRA 55VP manufactured by Carl Zeiss).
(68) The surface profile of a region having a fine textured structure was determined from a secondary electron image taken with a chamber detector at an accelerating voltage of 0.5 kV and at a magnification in the range of 20,000 to 50,000. More specifically, five separated straight lines each having a length of 1 m were drawn in a certain direction at five positions in the visual field. The average spacing of fine asperities was calculated by dividing 5 m by the total number of fine projections crossing the straight lines. The area percentage of the region having a fine textured structure was calculated from an image taken with an inlens detector at an accelerating voltage of 0.5 kV. The area of a bright region having fine asperities was calculated through the binary conversion of a secondary electron image using commercially available software (Photoshop).
(69) The surface contact resistance was measured while a carbon paper CP120 manufactured by Toray Industries, Inc. was in contact with the steel at a load of 20 kgf/cm.sup.2. After the measurement of the surface contact resistance, samples having a low surface contact resistance were further subjected to the measurement of surface contact resistance after a durability test. In the durability test, a sample was immersed in a sulfuric acid solution at pH 3, to which sodium fluoride was added such that the concentration of fluoride ions was 0.1 ppm, at 0.6 V versus a AgAgCl reference electrode at 80 C. for 24 hours. The surface contact resistance was measured using the method described above. An increase in surface contact resistance due to the durability test (surface contact resistance after the durability testsurface contact resistance before the durability test) was calculated. Table 1 lists the average spacing of fine asperities, the area percentage of the region having a fine textured structure, the surface contact resistance before and after the durability test, and an increase in surface contact resistance due to the durability test.
(70) [Table 1]
(71) TABLE-US-00001 TABLE 1 Increase Area Contact in contact percentage resistance resistance of region after due to Electrolysis having Average Contact durability durability in fine spacing resistance test test Type sulfuric Immersion in acid textured of fine (m .Math. cm.sup.2) (m .Math. cm.sup.2) (m .Math. cm.sup.2) of acid Time structure asperities 20 kgf 20 kgf 20 kgf No. steel Solution Solution (sec) (%) (nm) per side per side per side Note 1 304L 3% H.sub.2SO.sub.4 5% HF-2% 150 98 15 5.2 36.0 30.8 Comparative 1 g/L Fe HNO.sub.3 55 C. example 2 304L 5% HF-1% 150 91 34 4.3 7.5 3.2 Example HNO.sub.3 55 C. 3 304L 5% HF 55 C. 120 81 48 4.1 7.3 3.2 Example 4 304L 5% HF 55 C. 60 63 49 5.3 9.8 4.5 Example 5 443CT 3% H.sub.2SO.sub.4 5% HF- 40 40 80 7.1 18.0 10.9 Comparative 1 g/L Fe 2% HNO.sub.3 example 6 443CT 55 C. 90 71 114 5.0 9.2 4.2 Example 7 443CT 120 74 120 5.2 9.5 4.3 Example 8 443CT 5% HF-4% HNO.sub.3 200 84 28 4.0 5.5 1.5 Example 9 443CT 5% HF-1% HNO.sub.3 40 80 79 4.3 7.4 3.1 Example 10 443CT 55 C. 90 86 110 4.1 7.0 2.9 Example 11 443CT 120 96 120 4.0 6.9 2.9 Example 12 443CT 200 93 148 4.3 8.1 4.2 Example 13 443CT 500 85 230 5.7 16.0 10.3 Comparative example 14 443CT 5% HF 55 C. 10 40 120 7.8 42.0 34.2 Comparative example 15 444CT 20 46 150 10.5 48.0 37.5 Comparative example
(72) Table 1 shows that the examples in which the area percentage of the region having a fine textured structure was 50% or more and the average spacing of fine asperities was 20 nm or more and 150 nm or less had a surface contact resistance of 10 m.Math.cm.sup.2 or less. The surface contact resistance after the durability test in the examples was also 10 m.Math.cm.sup.2 or less. Thus, it is confirmed that the examples maintained a low surface contact resistance even in the long-term operating environment and had good surface contact resistance maintenance characteristics.
Example 2
(73) A commercially available austenitic stainless steel SUS 304L having a Cr content of 18.1 mass % and a commercially available ferritic stainless steel SUS 443CT having a Cr content of 21.1 mass % were repeatedly subjected to cold rolling, annealing, and pickling, thus yielding stainless steel sheets having thickness of 0.2 mm. In some samples, a sheet having a thickness of 2 mm was used. The sheet having a thickness of 2 mm was subjected to mirror polishing using alumina buffing before the treatment. The samples were treated under conditions shown in Table 2. An electrolytic treatment was performed in 3 mass % aqueous sulfuric acid. An acid immersion treatment was performed in 5 mass % hydrofluoric acid, to which, depending on the condition, iron (II) sulfate in an amount corresponding to 3 g/L of iron ions was added. Further, depending on the condition, the acid immersion treatment was performed in 10 mass % hydrochloric acid.
(74) The stainless steel thus obtained was subjected to the measurement of contact resistance and the observation of a surface fine structure using SEM (SUPRA 55VP manufactured by Carl Zeiss). The profile of projection structures and the average interval between projections were determined from a secondary electron image taken with a chamber detector at an accelerating voltage of 0.5 kV and at a magnification in the range of 20,000 to 100,000. Whether a raised portion had a triangular pyramid shape was judged according to the presence of three straight lines extending from the center of the raised portion, which correspond to the sides of the triangular pyramid shape (see [
(75) The contact resistance was measured while a carbon paper CP120 manufactured by Toray Industries, Inc. was in contact with the steel at a load of 20 kgf/cm.sup.2. After the measurement of the contact resistance, samples having a low contact resistance were further subjected to the measurement of contact resistance after a durability test. In the durability test, a sample was immersed in a sulfuric acid solution at pH 3, to which sodium fluoride was added such that the concentration of fluoride ions was 0.1 ppm, at 0.6 V versus a AgAgCl reference electrode at 80 C. for 500 hours. The contact resistance was measured using the method described above. Table 2 shows the results together with the sample preparation conditions.
(76) [Table 2]
(77) TABLE-US-00002 TABLE 2 Surface texture Average interval between Area raised Performance evaluation percentage portions Increase Increase of region (average Contact in contact Contact in contact having interval resistance resistance resistance resistance fine between Contact after 24 h due to 24 h after 500 h due to 500 h Type Immersion in acid textured Profile of triangular resistance durability test durability test durability test durability test of Electrolysis in sulfuric acid Time structure fine textured pyramidal (m .Math. cm.sup.2) (m .Math. cm.sup.2) (m .Math. cm.sup.2) (m .Math. cm.sup.2) (m .Math. cm.sup.2) No. steel Solution Electric current Solution (sec) (%) structure tops) (nm) 20 kgf per side 20 kgf per side 20 kgf per side 20 kgf per side 20 kgf per side Note 16 443CT No 514 Comparative projection example 17 443CT 3% H.sub.2SO.sub.4 Alternating No 35.8 Comparative current, electric projection example current density 5 A/dm.sup.2, 4.5 s 18 443CT 3% H.sub.2SO.sub.4 Alternating 5% HF 55 C. 60 83 Triangular 25 4.05 4.31 0.26 7.66 3.61 Example current, electric pyramidal current density 5 A/dm.sup.2, 4.5 s 19 443CT 3% H.sub.2SO.sub.4 Alternating 5% HF 55 C. 120 83 Triangular 50 4.28 4.42 0.14 6.36 2.08 Example current, electric pyramidal current density 5 A/dm.sup.2, 4.5 s 20 443CT 3% H.sub.2SO.sub.4 Alternating 5% HF 55 C. 240 86 Triangular 65 4.53 4.68 0.15 5.36 0.83 Example current, electric pyramidal current density 5 A/dm.sup.2, 4.5 s 21 443CT 3% H.sub.2SO.sub.4 Alternating 5% HF 55 C. 600 94 Triangular 145 5.45 5.80 0.35 9.02 3.57 Example current, electric pyramidal current density 5 A/dm.sup.2, 4.5 s 22 443CT* 3% H.sub.2SO.sub.4 Alternating 5% HF 55 C. 1800 98 Triangular 410 12.40 14.10 1.70 25.6 13.2 Comparative current, electric pyramidal example current density 5 A/dm.sup.2, 4.5 s 23 304L* 3% H.sub.2SO.sub.4 Alternating 5% HF 55 C. 1800 99 Granular 120 6.20 9.26 3.06 36.7 30.5 Example current, electric projection current density 5 A/dm.sup.2, 4.5 s 24 304L 3% H.sub.2SO.sub.4 Alternating 5% HF 55 C. 240 87 Granular 30 5.91 9.85 3.94 45.1 39.2 Example current, electric projection current density 5 A/dm.sup.2, 4.5 s 25 443CT 3% H.sub.2SO.sub.4 Direct current, 5% HF 55 C. 240 91 Triangular 90 4.75 4.83 0.08 5.12 0.37 Example electric current pyramidal density 0.5 A/dm.sup.2, 80 s 26 443CT 3% H.sub.2SO.sub.4 Direct current, 5% HF 55 C. 240 78 Granular 250 8.92 23.76 14.84 30.1 21.2 Comparative electric current 3 g/L Fe projection example density 0.5 A/dm.sup.2, 80 s 27 443CT 10% 240 97 Formation 145 10.3 21.1 10.80 186 175.7 Comparative hydrochloric of pit example acid 50 C. 28 Au sheet 4.00 Reference example *: sheet having a thickness of 2 mm
(78) Table 2 shows that the initial contact resistance and the contact resistance after the durability test for 500 hours in the examples in which projections of a triangular pyramid shape were disposed on the surface at an average interval of 150 nm or less were 10 m.Math.cm.sup.2 or less. Thus, the examples maintained a low contact resistance even in the long-term operating environment. In particular, the contact resistance after the durability test in the examples in which the average interval between raised portions was 100 nm or less was 8 m.Math.cm.sup.2 or less. Thus, the examples had further improved durability.
(79) The increase in contact resistance due to the durability test of a granular fine textured structure having substantially the same average interval between raised portions was greater than that of the examples having the fine textured structure of a triangular pyramid shape. Samples not having the fine textured structure but having a small pit structure outside the scope of the present invention had low durability and could not be used.
(80) In this test, a stainless steel evaporated with Au was also tested for comparison, and its result is shown. The results for the examples are no different from the results for the stainless steel evaporated with Au. Thus, the examples are judged to have a very low contact resistance. The relationship between surface crystal orientation and the triangular pyramid shape was studied using the EBSD method in the present example. It was confirmed that the raised portions of a triangular pyramid shape were formed of three (100) facets of a bcc structure. Thus, the average angle at the top of the three facets is approximately 90 degrees.
Example 3
(81) A ferritic stainless steel SUS 443CT of the same lot (lot A) as in the examples was repeatedly subjected to cold rolling, annealing, and pickling to produce a stainless steel sheet having a thickness of 0.2 mm. The rolling reduction or the annealing temperature was altered to produce foils of different textures (lots B to E). In order to prepare a sample having trapezoidal projection structures on its polished surface, a JFE 443CT cold-rolled steel sheet having a thickness of 1 mm was subjected to mirror polishing using alumina buffing to prepare a lot F. The samples were then treated under conditions shown in Table 3. An electrolytic treatment was performed in 3 mass % aqueous sulfuric acid. An acid immersion treatment was performed in 5 to 10 mass % hydrofluoric acid, to which, depending on the condition, iron (II) sulfate in an amount corresponding to 3 g/L of iron ions was added.
(82) The stainless steel thus obtained was subjected to the measurement of contact resistance and the observation of a surface fine structure using SEM (SUPRA 55VP manufactured by Carl Zeiss) in the same manner as in Example 2. The surface profile was measured with an optical interferotype profilometer Zygo (Canon Marketing Japan Inc.). The observation field was 0.35 mm0.26 mm. The presence or absence of trapezoidal projection structures was determined. If present, the average height and average diameter of the trapezoidal projection structures were measured using the method described above.
(83) The area percentage of the region of the trapezoidal projection structures was determined in each visual field. Table 3 shows the average of the area percentages in two visual fields. In foil samples, streaks of raised portions or oil pits in the rolling direction were regarded to be flat and were included in the measured area.
(84) The contact resistance was measured while a carbon paper CP120 manufactured by Toray Industries, Inc. was in contact with the steel at a load of 20 kgf/cm.sup.2. The samples were subjected to a sliding test. The contact resistance of a portion rubbed in the sliding test was measured in the same manner as described above. As the sliding test, a plate sliding test was adopted. A bead having a contact area 10 mm in width and 3 mm in length in the slide direction (the curvature at both ends in the slide direction was 4.5 mmR) was pressed against a steel sheet at a vertical load of 100 kgf. The steel sheet was drawn by 30 mm at a velocity of 100 cm/min without an oil. The samples were rated : double circle, : circle, : triangle, and X: cross in the case that the increase in contact resistance due to the sliding test was 1 m.Math.cm.sup.2 or less, more than 1 m.Math.cm.sup.2 and 3 m.Math.cm.sup.2 or less, more than 3 m.Math.cm.sup.2 and 10 m.Math.cm.sup.2 or less, and more than 10 m.Math.cm.sup.2, respectively. Table 3 shows the results together with the sample preparation conditions.
(85) [Table 3]
(86) TABLE-US-00003 TABLE 3 Surface texture Average interval between Performance Evaluation Area raised Contact Increase percent- portions resistance in contact Contact Increase age of (average after resistance resistance in contact region interval Average Area 500 h due to 500 h after resistance having between Average diameter percentage Contact durability durability sliding due to Electrolysis in Immersion fine Profile triangular height of of of resistance test test test sliding test Type sulfuric acid in acid textured of fine pyramidal trapezoidal trapezoidal trapezoidal (m .Math. cm.sup.2) (m .Math. cm.sup.2) (m .Math. cm.sup.2) (m .Math. cm.sup.2) (m .Math. cm.sup.2) of Electric Time structure textured tops) projection projection projection 20 kgf 20 kgf 20 kgf 20 kgf 20 kgf No. steel Lot. Solution current Solution (sec) (%) structure (nm) (m) (m) (%) per side per side per side per side per side Note 18 443CT A 3% H.sub.2SO.sub.4 Alternating 5% HF 60 83 Triangular 25 0.08 10.2 9.2 4.05 7.66 3.61 7.68 3.63 Example current, 55 C. pyramidal electric current density 5 A/dm.sup.2, 4.5 s 19 443CT A 3% H.sub.2SO.sub.4 Alternating 5% HF 120 83 Triangular 50 0.36 11.3 10.9 4.28 6.36 2.08 4.87 0.59 Example current, 55 C. pyramidal electric current density 5 A/dm.sup.2, 4.5 s 20 443CT A 3% H.sub.2SO.sub.4 Alternating 5% HF 240 86 Triangular 65 0.76 12.6 18.3 4.53 5.36 0.83 4.92 0.39 Example current, 55 C. pyramidal electric current density 5 A/dm.sup.2, 4.5 s 21 443CT A 3% H.sub.2SO.sub.4 Alternating 5% HF 600 94 Triangular 145 0 0 0 5.45 9.02 3.57 15.40 9.95 Example current, 55 C. pyramidal electric current density 5 A/dm.sup.2, 4.5 s 26 443CT A 3% H.sub.2SO.sub.4 Direct current, 5% HF 240 78 Granular 250 0 0 0 8.92 30.10 21.18 23.40 14.48 X Comparative electric current 55 C. projection example density 3 g/L Fe 0.5 A/dm.sup.2, 80 s 29 443CT A 3% H.sub.2SO.sub.4 Alternating 5% HF 80 83 Triangular 32 0.15 15.6 13.5 4.11 6.32 2.21 6.53 2.42 Example current, 55 C. pyramidal electric current density 5 A/dm.sup.2, 4.5 s 30 443CT A 3% H.sub.2SO.sub.4 Alternating 5% HF 300 87 Triangular 87 1.24 9.6 23.4 4.32 5.36 1.04 4.65 0.33 Example current, 55 C. pyramidal electric current density 5 A/dm.sup.2, 4.5 s 31 443CT A 3% H.sub.2SO.sub.4 Alternating 5% HF 450 90 Triangular 132 0.74 9.9 16.8 5.19 6.87 1.68 6.08 0.89 Example current, 55 C. pyramidal electric current density 5 A/dm.sup.2, 4.5 s 32 443CT B 3% H.sub.2SO.sub.4 Alternating 5% HF 240 93 Triangular 61 0.74 11.1 5.6 4.11 5.03 0.92 6.02 1.91 Example current, 55 C. pyramidal electric current density 5 A/dm.sup.2, 4.5 s 33 443CT C 3% H.sub.2SO.sub.4 Alternating 5% HF 240 84 Triangular 58 0.65 4.5 9.5 4.56 5.31 0.75 6.89 2.33 Example current, 55 C. pyramidal electric current density 5 A/dm.sup.2, 4.5 s 34 443CT D 3% H.sub.2SO.sub.4 Alternating 5% HF 240 94 Triangular 71 0.57 8.7 3.4 4.23 5.16 0.93 8.93 4.70 Example current, 55 C. pyramidal electric current density 5 A/dm.sup.2, 4.5 s 35 443CT E 3% H.sub.2SO.sub.4 Alternating 5% HF 240 65 Triangular 67 0.63 10.1 33.2 5.37 7.25 1.88 11.30 5.93 Example current, 55 C. pyramidal . electric current density 5 A/dm.sup.2, 4.5 s 36 443CT F 3% H.sub.2SO.sub.4 Alternating 10% HF 600 97 Triangular 93 0.26 26.4 28.3 4.61 5.97 1.36 7.12 2.51 Example current, 50 C. pyramidal electric current density 5 A/dm.sup.2, 4.5 s
(87) Table 3 shows that the examples in which a fine textured structure having a triangular pyramidal top was disposed on the surface thereof and the area percentage of dispersed trapezoidal projection structures having an average height of 0.15 m or more and 2 m or less was 5% or more and 30% or less had a small increase in contact resistance due to the sliding test. Thus, the examples maintain a low contact resistance after contact of the materials with each other or contact of the material with another component or the like, or after processing into a separator component, and are therefore advantageous from a practical standpoint. After the profile measurement, the SEM observation of the surface in the same visual field showed that the trapezoidal projection structures corresponded to crystal grains on the stainless steel surface. An examination of crystal orientation using the EBSD method showed that the (100) plane of the trapezoidal projection structures was almost perpendicular to the surface of the sample. Although a comparative example having an average diameter out of the range of the present invention when viewed from the top is not listed, the examples had satisfactory results when the average diameter ranged from at least 4.5 to 15 m.
REFERENCE SIGNS LIST
(88) 1 membrane-electrode assembly 2 gas diffusion layer 3 gas diffusion layer 4 separator 5 separator 6 air channel 7 hydrogen channel