Ferritic Stainless Steel Used for Bipolar Plates of Fuel Cells, Controlling Method of Surface Roughness, Method of Forming Passivation Films, and Use

Abstract

Provided are a ferritic stainless steel used for bipolar plates of fuel cells, a controlling method of surface roughness, a method of forming passivation films, and use. The ferritic stainless steel comprises C of less than or equal to 0.03 wt. %, N of less than or equal to 0.02 wt. %, Si of less than or equal to 0.4 wt. %, Mn of less than or equal to 0.5 wt. %, Cr of 16-23 wt. %, Cu of 0-2.0 wt. %, Mo of 1.8-2.5 wt. %, Ni of 0.2-2.0 wt. %, Ti of 0.1-0.5 wt. %, Nb of 0.005-0.5 wt. %, P of less than or equal to 0.02 wt. %, S of less than or equal to 0.02 wt. %, and a remainder composed of Fe and other unavoidable accompanying elements, and the ferritic stainless steel has a grain size number of 4-9. The ferritic stainless steel has excellent corrosion resistance and electrical conductivity, and good elongation and deformation as well, exhibiting both economy and cost advantages.

Claims

1. A ferritic stainless steel used for bipolar plates of fuel cells, wherein based on that a mass of the ferritic stainless steel is 100%, the ferritic stainless steel comprises: C less than or equal to 0.03 wt. %; N less than or equal to 0.02 wt. %; Si less than or equal to 0.4 wt. %; Mn less than or equal to 0.5 wt. %; Cr 16-23 wt. %; Cu 0-2.0 wt. %; Mo 1.8-2.5 wt. %; Ni 0.2-2.0 wt. %; Ti 0.1-0.5 wt. %; Nb 0.005-0.5 wt. %; P less than or equal to 0.02 wt. %; S less than or equal to 0.02 wt. %; and a remainder composed of Fe and other unavoidable accompanying elements; the ferritic stainless steel has a grain size number of 4-9.

2. The ferritic stainless steel used for bipolar plates of fuel cells according to claim 1, wherein the ferritic stainless steel has a grain size number of 6-8.

3. The ferritic stainless steel used for bipolar plates of fuel cells according to claim 1, wherein the ferritic stainless steel further comprises V of less than or equal to 0-1 wt. % and/or W of 0-1 wt. %.

4. The ferritic stainless steel used for bipolar plates of fuel cells according to claim 1, wherein the ferritic stainless steel further comprises a rare earth metal of 0.0002-1 wt. %.

5. The ferritic stainless steel used for bipolar plates of fuel cells according to claim 1, wherein a surface roughness of the ferritic stainless steel is within 100-700 nm, preferably 100-600 nm, and further preferably 200-500 nm.

6. The ferritic stainless steel used for bipolar plates of fuel cells according to claim 1, wherein the ferritic stainless steel is provided with a passivation film on the surface, and the passivation film comprises a p-type passivation film and an n-type passivation film; preferably, a molar ratio of chromium hydroxide to chromium oxide in the p-type passivation film is represented as Ip[Cr(OH)3/Cr2O3], and Ip[Cr(OH)3/Cr2O3] is at least 10, preferably at least 15; preferably, a molar ratio of chromium hydroxide to chromium oxide in the n-type passivation film is represented as In[Cr(OH)3/Cr2O3], and In[Cr(OH)3/Cr2O3] is at most 10, preferably less than 5; preferably, Ip[Cr(OH)3/Cr2O3]/In[Cr(OH)3/Cr2O3] is more than 3, and preferably, Ip[Cr(OH)3/Cr2O3]/In[Cr(OH)3/Cr2O3] is more than or equal to 4.

7. The ferritic stainless steel used for bipolar plates of fuel cells according to claim 6, wherein a thickness of the passivation film is 5-20 nm, preferably 10-15 nm; preferably, a thickness of the p-type passivation film is represented as tp, a thickness of the n-type passivation film is represented as tn, and tp/tn is more than 0.2 but less than 0.6; preferably, in the passivation film, an inner layer is the n-type passivation film, an outer layer is the p-type passivation film, and tp/tn is more than 0.2 but less than 0.6.

8. A controlling method of surface roughness for a stainless steel, comprising: taking a main material of stainless steel, and subjecting the main material of stainless steel to electrolysis in an acid solution, wherein during the electrolysis, a polarization voltage satisfies the following formula (I): $\begin{array}{cc}E?\lg D+12+\mathrm{pH}& \left(I\right)\end{array}$ wherein E is the polarization voltage with a unit of V, D is a grain size of the main material of stainless steel with a unit of micron, and pH is a pH value of the initial acid solution.

9. The controlling method of surface roughness for a stainless steel according to claim 8, wherein the polarization voltage is 5-15 V.

10. The controlling method of surface roughness for a stainless steel according to claim 8, wherein a time of the electrolysis is 10-300 s, preferably 20-120 s.

11. The controlling method of surface roughness for a stainless steel according to claim 8, wherein a temperature of the electrolysis is 25-70? C., preferably 25-40? C.; preferably, the acid solution used during the electrolysis is sulfuric acid, or a mixed acid solution of sulfuric acid and hydrohalic acid; preferably, the hydrohalic acid is at least one of hydrofluoric acid, hydrochloric acid, hydrobromic acid and hydroiodic acid, preferably hydrochloric acid and/or hydrofluoric acid; preferably, a concentration of the sulfate acid is 0.1-14 mol/L, preferably 0.1-7 mol/L; preferably, in the mixed acid solution of sulfuric acid and hydrohalic acid, a concentration of the hydrohalic acid is 0-3 mol/L but not including 0, preferably at most 0.5 mol/L.

12. A method of forming passivation films on stainless steel surface, wherein an electrochemical passivation method is used to prepare a passivation film, comprising the following steps: providing a main material of stainless steel, using a three-electrode system, placing the main material of stainless steel, a counter electrode and a reference electrode into an electrochemical passivation solution, and performing potentiostatic polarization, forming a passivation film on the surface of the main material of stainless steel.

13. The method according to claim 12, wherein the electrochemical passivation solution is a nitric acid solution with a concentration of 0.05-10 mol/L, and preferably, the concentration of the nitric acid solution is 1.5-5 mol/L.

14. The method according to claim 12, wherein a temperature of the electrochemical passivation is 20-85? C., preferably 35-65? C.

15. The method according to claim 12, wherein an anode potential of the electrochemical passivation is at least 0.45 V, preferably 0.8-1.2 V; preferably, a time of the electrochemical passivation is 5-120 min, preferably 50-90 min.

16. A method of preparing bipolar plates of fuel cells, comprising: using of the ferritic stainless steel according to claim 1.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0074] The drawings are used to provide a further understanding of the technical solutions of the present disclosure, constitute a part of the description, explain the technical solutions of the present disclosure in conjunction with the embodiments of the present application, and do not limit the technical solutions of the present disclosure.

[0075] FIG. 1 is a graph showing the relationship between stainless steel roughness and interface contact resistance in an example of the present application, in which Samples 1-6 correspond to Examples 16-21 in sequence respectively.

[0076] FIG. 2 and FIG. 3 are XPS spectra of passivation films of a stainless steel in an example of the present application.

[0077] FIG. 4 is a diagram showing M-S curves of passivation films of a stainless steel in an example of the present application.

[0078] FIG. 5 is a graph showing carrier concentrations of p-type and n-type passivation films of a stainless steel in an example of the present application.

[0079] FIG. 6 is an image showing the cross section morphology of a stainless steel in an example of the present application.

DETAILED DESCRIPTION

[0080] Technical solutions of the present disclosure are further described below with reference to the accompanying drawings and through specific embodiments.

[0081] Table 1 shows the chemical composition of materials (mass percentage, wt. %) used in steelmaking.

TABLE-US-00002 TABLE 1 Steel Chemical Composition of Materials (wt. %) No. C Mn Si Ni Cr Mo Ti Nb Cu Ce V W 1 0.0017 0.21 0.032 0.29 17.05 1.94 0.235 0.117 0.44 2 0.002 0.28 0.091 1.05 17.27 1.99 0.211 0.168 0.81 3 0.003 0.27 0.127 1.02 17.87 1.86 0.156 0.131 0.66 0.001 4 0.021 0.13 0.107 1.04 17.85 1.88 0.152 0.108 0.63 5 0.011 0.082 0.134 0.56 17.58 1.86 0.059 0.005 0.78 0.11 0.14 6 0.03 0.42 0.069 0.501 17.48 1.03 0.16 0.105 0.32 7 0.02 0.306 0.079 1.93 17.09 2.51 0.16 0.112 0.33 8 0.008 0.228 0.070 0.272 22.72 1.98 0.24 0.128 0.74 9 0.007 0.049 0.011 1.02 19.14 1.54 0.218 0.127 0.49 10 0.008 0.051 0.012 1.03 21.16 1.48 0.224 0.125 0.39

[0082] The functions of each element in ferritic stainless steel are described below. [0083] Cr is a fundamental element that determines the corrosion resistance of ferritic stainless steel. Chromium reacts with oxygen in the corrosive medium and forms a thin oxide film on the steel surface, which can prevent the steel substrate from being corroded further. However, the increase of chromium content will accelerate the formation and precipitation of ? and ? phases, thus reducing the toughness and significantly increasing the brittle transition temperature, which is not conducive to the processing for manufacturing stainless steel. [0084] Mo is another major element for improving the corrosion resistance of stainless steel. Mo facilitates the passivation of FeCr alloy, and improves the corrosion resistance of steel in reducing medium, especially the resistance to localized corrosion in chloride solution, such as pitting corrosion and crevice corrosion. However, when Mo has a high content, the ferrite ? phase and other brittle phases are prone to appear, resulting in a reduced toughness and increased strength for the steel, which is not conducive to processing materials. [0085] C has the effect of solid solution strengthening. Its solubility in ferrite is very low, and the excess carbon is precipitated in the form of carbides, which will also cause chromium depletion at grain boundaries and intergranular corrosion for the ferritic stainless steel, affecting the mechanical and welding properties of the material. [0086] Si is an element useful for deoxidation. However, as the content increases, the processability of the material decreases. [0087] Mn is an unavoidably accompanying element in steel, which has some deoxidation effect and also improves the strength of steel. However, the impurity MnS is prone to becoming a corrosion starting point and reduces the corrosion resistance. [0088] N and C will form Cr carbonitrides with Cr, resulting in a Cr depletion region and reducing the corrosion resistance of stainless steel. [0089] Ti and Nb both preferentially combine with C and N to form carbonitrides, thereby suppressing corrosion resistance reduction due to precipitation of Cr carbonitrides. However, if the content is too high, the processability will decrease. [0090] Cu is an element that improves the corrosion resistance of stainless steel, and also improves the cold workability of the material. [0091] Ni is an element that improves the corrosion resistance of stainless steel, and meanwhile, plays a role in reducing the contact resistance.

[0092] A remainder is composed of Fe and the unavoidable impurities.

[0093] In addition to the above, for the purpose of improving corrosion resistance, V of 0-1 wt. % and/or W of 0-1 wt. % may be each included. In order to realize the effect, the two elements both preferably have a content of more than or equal to 0.1 wt. %.

[0094] For the purpose of improving hot workability, a rare earth metal of 0.0002-1 wt. % may also be included, preferably Ce or Y. In order to realize the effect, the rare earth metal of more than or equal to 0.0005 wt. % is preferably included.

Examples 1-21

[0095] Ingots were prepared according to the ferritic stainless steels of different steel numbers shown in Table 1, in which the corresponding relationship in each example between the ingots and the ferritic stainless steels of different steel numbers is shown in Table 2. The ingots were subjected to cogging for 100 mm to obtain a stainless steel plate, and then were hot-rolled, in which the heating and holding temperature was 1200? C., the holding time was 2 h, the primary rolling temperature was controlled at 1100? C., eight subsequent rolling processes were carried out for 3 mm, and the ultimate rolling temperature was controlled at 800? C. An annealing treatment was carried out after hot rolling, in which the annealing temperature was 1050? C., and the holding time depended on the size of hot rolled coil, and a pickling treatment was carried out after hot rolling. Then, the steel was taken out and cooled in the air, then subjected to cold rolling to give a foil with the required thickness, and then subjected to an annealing treatment at 950? C. for 2 min, so as to obtain the final foil sample, namely, the stainless steel material.

[0096] For the stainless steel material in Example 1, the elongation after fracture at room temperature was tested according to GB/T 228.1-2010. The sample was prepared into a sheet shape according to the tensile standard and tested, and it was obtained that the sample has an elongation after fracture of 33.5%.

[0097] The above-mentioned stainless steel materials were subjected to surface roughness treatment sequentially, and the materials were prepared into diverse surface roughness. The specific treatment method included that: the concentrated sulfuric acid and deionized water were used to prepare a sulfuric acid solution, and the above-mentioned stainless steel materials were finally processed into samples whose length and width were both 20 mm and put into sulfuric acid solution for surface roughness treatment with different parameters. The roughness preparation conditions are shown in Table 2.

TABLE-US-00003 TABLE 2 Roughness Preparation Conditions Hydro- Sulfuric fluoric Acid Acid Concen- Concen- Temper- Volt- Steel tration tration ature age Time Example No. mol/L mol/L ? C. V s 1 1 3 0 25 10 60 2 2 3 0.5 25 10 60 3 2 5 0 25 10 60 4 2 0.05 0 25 10 60 5 2 15 0 25 10 60 6 3 3 0 60 10 60 7 3 3 0.2 40 10 60 8 3 3 0 80 10 60 9 3 3 0 10 10 60 10 4 3 0 25 15 60 11 4 3 0 25 20 60 12 4 3 0 25 15 200 13 4 3 0 25 10 400 14 5 3 0 25 5 100 15 5 3 0 25 3 100 16 1 5 0 65 10 300 17 1 5 0 65 10 200 18 1 5 0 65 8 120 19 1 15 0 65 6 40 20 1 15 0 65 6 120 21 1 15 0 65 6 240

[0098] After the stainless steel materials in Examples 16-21 were subjected to roughness treatment, the samples were taken, the material surface was cleaned with acetone and dried by nitrogen blowing, the stainless steel was subjected to surface roughness test using a surface profiler and recorded, and an interface contact resistance value of the material at 150 N/cm.sup.2 was tested and recorded using an interfacial contact resistance measuring instrument. The results are shown in Table 3 and FIG. 1.

TABLE-US-00004 TABLE 3 Example 16 17 18 19 20 21 Roughness (Ra/nm) 460.9 425.3 234.6 81 66.5 10.9 Interface Contact 24.1 23.7 26.75 112.1 198.8 786.7 Resistance (ICR/m? .Math. cm.sup.2)

[0099] It is found in Table 3 and FIG. 1 that within a certain range of surface roughness, the stainless steel material had low interfacial contact resistance, and the material could be used in fuel cells after proper optimization. The preferred roughness was within between 100-700 nm, further preferably 200-500 nm. Materials outside the preferred range had higher interface contact resistance (Examples 19, 20 and 21), and had more difficulty in applying to fuel cells. When the roughness was too small, the interface contact resistance would increase significantly, the internal resistance of the fuel cell would increase, and additionally, a good adherence could not be realized between the material and a gas diffusion layer, so the material was not suitable for application; when the roughness was too large, although the contact resistance was low, the corrosion resistant of the material would reduce significantly, failing to satisfy the requirements of the bipolar plate inside the fuel cell or in the acidic environment. After the roughness treatment described above (Examples 16-21 had been tested for surface roughness and interfacial contact resistance), the steel sheets in Examples 1-21 were subsequently subjected to electrochemical passivation treatment, and the specific treatment method was that: [0100] the obtained steel sheets were subjected to electrochemical passivation with an anode potential of 1.1 V for 1 h in a 1.6 mol/L HNO; solution at 40? C.

[0101] After the electrochemical passivation treatment described above, the samples were rinsed with deionized water and dried by cold nitrogen blowing, and placed in a dry environment (air) at room temperature for 24 h. Then the samples were tested, specifically including the following steps. [0102] (1) The t.sub.p/t.sub.n and I.sub.p/I.sub.n were ascertained according to the following methods, and the results are shown in Table 4. [0103] (I) The passivation film was subjected to depth profiling and narrow energy scanning with X-ray photoelectron spectroscopy (XPS), in which the X-ray source was Al K? micro-focused monochromator, CAE scanning mode was used for scanning, and the narrow energy scanning had a pass energy of 30-50 eV and a step size of 0.05-0.1 eV; argon ion etching was used for depth profiling, and each etching depth was 1 nm, 1 nm, 1 nm, 1 nm, 2 nm, 2 nm, 2 nm, 2 nm, 5 nm and 5 nm, respectively. [0104] (II) The measured results were processed using software, and the peak area corresponding to each phase was used to express its content. Due to that the stainless steel passivation film was basically composed of hydroxides and oxides of Fe and Cr, the XPS test was mainly used to analyze the contents of Fe phase, Cr phase, and the hydroxides and oxides thereof in the passivation film. When the passivation film had a higher content of hydroxides of Fe and Cr than a content of oxides of Fe and Cr, the passivation film was determined to be a p-type semiconductor passivation film; when the passivation film had a lower content of hydroxides of Fe and Cr than a content of oxides of Fe and Cr, the passivation film was determined to be an n-type semiconductor passivation film. Based on the above, the thickness t.sub.p of the p-type semiconductor and the thickness t.sub.n of the n-type semiconductor in the passivation film, and the ratio of chromium hydroxide to chromium oxide in the p-type passivation film and the n-type passivation film can be determined, so as to obtain t.sub.p/t.sub.n and I.sub.p/I.sub.n.

[0105] FIG. 2 and FIG. 3 are XPS spectra of passivation films of the stainless steel in Example 1. FIG. 2 shows the content of hydroxides and oxides of Fe and Cr in the passivation film. As can be seen from the figure, the thickness of the p-type semiconductor passivation film was about 5 nm, while the thickness of the n-type semiconductor passivation film was within 10-15 nm. The ratio t.sub.p/t.sub.n of the two passivation film thicknesses was in the range of 0.33-0.56, and the corrected t.sub.p/t.sub.n was 0.55. In FIG. 3, the bars represent the content of hydroxides and oxides of Cr in different thickness regions of the passivation film, respectively, and the curve represents the corresponding ratio of hydroxides and oxides. In conjunction with FIG. 2, it can be seen that in the p-type passivation film, the ratio of hydroxides to oxides was more than or equal to 10, mostly more than or equal to 15 (taking 15 as the average value); the ratio of hydroxides to oxides of Cr in the n-type passivation film was about 2-5, mostly 2-3 (taking 2.5 as the average value). Therefore, it could be judged that I.sub.p/I.sub.n was 7. [0106] (2) Performing service performance test under simulated working environment of the fuel cell: a 300 h endurance test was carried out, in which the temperature was 80? C., pH of the sulfuric acid solution was 3, and the potential was 0.84 V (vs. SHE), and the corrosion current density value was recorded and the interface contact resistance value at 150 N/cm.sup.2 was measured on the material surface, and the results are shown in Table 4.

TABLE-US-00005 TABLE 4 Interface Contact Corrosion Resistance Current Example t.sub.p/t.sub.n I.sub.p/I.sub.n m? .Math. cm.sup.2 Density ?A .Math. cm.sup.?2 1 0.55 7 4.5 0.3 2 0.45 4.5 3 1.0 3 0.56 6.2 5 2.1 4 0.77 2.2 78 25 5 0.67 2.3 55 7.7 6 0.55 4.7 8 1.5 7 0.44 4.5 5 0.5 8 0.35 2.2 50 12.0 9 0.62 2.2 77 11 10 0.33 6.5 7.2 1.6 11 0.72 1.2 45 17 12 0.60 5.1 6.5 1.1 13 0.88 2.1 89 6 14 0.56 6.6 7.8 0.7 15 0.61 2.2 98 4.5 16 0.45 7.5 7.0 2.1 17 0.32 6.2 5.8 1.3 18 0.49 5.6 6.7 1.3 19 0.12 3.4 162.8 1.1 20 0.11 4.5 276.2 0.6 21 0.14 3.7 867.6 0.5

[0107] It can be found form Table 4 that by performing roughness treatment and electrochemical passivation treatment simultaneously, the surface passivation film, which had a contact resistance of less than or equal to 8 m?.Math.cm.sup.2, a thickness ratio of the p-type and n-type semiconductors within 0.2-0.6, and I.sub.p[Cr(OH).sub.3/Cr.sub.2O.sub.3]/I.sub.n[Cr(OH).sub.3/Cr.sub.2O.sub.3] of more than 4, was obtained, in which the current density was less than 3 ?A.Math.cm.sup.?2, and the interface contact resistance was less than 8 m?.Math.cm.sup.2, indicating that the passivation film had excellent protection capability and conductivity.

[0108] At the same time, it can be seen from Table 4 that on the basis of the same electrochemical passivation treatment conditions, the surface state of stainless steel could be changed by changing the roughness preparation conditions, thereby finally improving the performance of the prepared passivation film.

[0109] It can be found from Examples 2-3 that for the roughness preparation conditions, the concentration of hydrohalic acid in the range of 0-3 mol/L was conducive to increasing I.sub.p/I.sub.n, keeping t.sub.p/t.sub.n in a suitable range, and reducing the interface contact resistance and corrosion current density.

[0110] It can be found from Examples 2-5 that for the roughness preparation conditions, the concentration of sulfuric acid in the range of 0.1-7 mol/L was conducive to increasing I.sub.p/I.sub.n, keeping t.sub.p/t.sub.n in a suitable range, and reducing the interface contact resistance and corrosion current density.

[0111] It can be found from Examples 6-9 that for the roughness preparation conditions, the temperature of electrolysis in the range of 25-70? C. was conducive to increasing I.sub.p/I.sub.n, and reducing the interface contact resistance and corrosion current density.

[0112] By comparing Example 10 with Example 11 and comparing Example 14 and Example 15, it can be found that for the roughness preparation conditions, the polarization voltage in the range of 5-15 V was conducive to increasing I.sub.p/I.sub.n, keeping t.sub.p/t.sub.n in a suitable range, and reducing the interface contact resistance and corrosion current density.

[0113] By comprehensively analyzing Examples 10-13, the reason for performance degradation of the passivation film in Example 13 was possibly that the electrolysis time was too long for the roughness preparation conditions, resulting in the poor thickness ratio and composition of p-type and n-type passivation films.

Examples 22-30

[0114] These examples differ from Example 1 in that the electrochemical passivation treatment was performed with different parameters. The same method of Example 1 was used for analyzing and evaluating. The electrochemical passivation conditions and the test results of Example 1 and Examples 22-30 are shown in Table 5.

TABLE-US-00006 TABLE 5 Electrochemical Passivation Conditions Interface Corrosion Nitric Acid Contact Current Concentration Voltage Time Resistance Density Example mol/L Temperature ? C. V min t.sub.p/t.sub.n I.sub.p/I.sub.n m? .Math. cm.sup.2 ?A .Math. cm.sup.?2 1 1.6 40 1.1 60 0.55 7 4.5 0.3 22 15 40 1.1 60 0.1 2.2 78 25 23 4.6 40 1.1 60 0.32 4.3 7 1.5 24 1.6 90 1.1 60 0.77 2.1 150 4.3 25 1.6 40 0.8 60 0.45 4.4 7.7 1.2 26 1.6 40 0.3 60 0.13 1.5 123 7 27 1.6 40 1.1 30 0.34 5.2 8 2.1 28 1.6 40 1.1 240 0.78 1.2 150 4.5 29 1.6 40 0.6 60 0.33 3.5 7.9 3.0 30 / / / / / / 178.4 37.2 Note: Example 30 in Table 5 was a bare sample, and that is, no electrochemical passivation is performed on the basis of Example 1.

[0115] It can be found from Table 4 that by adjusting the condition parameter of electrochemical passivation, such as nitric acid concentration, temperature, potential and passivation time, to the prescribed range, the performance of the passivation film could be further improved. And it can be found from the test results of the stainless steel sample of Example 1 after serving in the fuel cell environment for a period of time that the passivation film could not only reduce the interface contact resistance of the stainless steel to less than or equal to 8 m?.Math.cm.sup.2, but also keep the corrosion current density of the stainless steel at a low level, and the stainless steel exhibited good corrosion resistance and electrical conductivity.

[0116] The Mott-Schottky (M-S) curve test was performed on Example 1, Example 25, Example 29 and Example 30 to determine the carrier concentration of the passivation film, estimating the effect of electrochemical passivation treatment and voltage on the passivation film performance.

[0117] The specific test method of the M-S curve is described below.

[0118] Samples were subjected to the M-S curve test in a sulfuric acid solution with pH=3 at 80? C. In order to increase the conductivity of the sulfuric acid solution, 0.1 mol/L of Na.sub.2SO.sub.4 was added to the solution. An electrochemical workstation was used in the M-S curve test, the test range was ?1-1 V, and the test step size was 25 mV/step. The slope of straight line segment of the p-type passivation film and the n-type passivation film were obtained by fitting, and then the carrier concentration corresponding to the p-type or n-type was calculated according to the M-S model, and the passivation film performance was determined by the carrier concentration.

[0119] FIG. 4 is a diagram showing M-S curves of passivation films of the stainless steels in Example 1 (electrochemical passivation potential of 1.1 V), Example 25 (electrochemical passivation potential of 0.8 V). Example 29 (electrochemical passivation potential of 0.6 V) and Example 30 (no electrochemical passivation performed). It can be found from FIG. 4 that the p-type semiconductor region was not obvious for the sample without electrochemical passivation, and the passivation film mainly exhibited the characteristics of n-type semiconductor region; after the potentiostatic polarization of 0.6 V, the slopes of straight line segment of both the p-type and n-type semiconductor regions increased, but the increase in the p-type region was not obvious; after the potentiostatic polarizations of 0.8 V and 1.1 V, the slopes of onset stage were significantly increased in the p-type and n-type semiconductor regions. Overall, with the potentiostatic polarization voltage increasing, the slope of straight line segment of the p-type semiconductor increased significantly and gradually. The slope of the straight line segment of the n-type semiconductor region also increased gradually.

[0120] FIG. 5 is a graph showing M-S carrier concentrations of passivation films of the stainless steel in Example 1 (electrochemical passivation potential of 1.1 V), Example 25 (electrochemical passivation potential of 0.8 V), Example 29 (electrochemical passivation potential of 0.6 V) and Example 30 (no electrochemical passivation performed). It can be found from FIG. 5 that the passivation film formed after the electrochemical passivation on the sample surface had a gradually reduced carrier concentration in the p-type semiconductor region (nearly reducing by 4 times at 1.1 V compared with the sample without electrochemical passivation), and the reduction in carrier concentration showed an increase in the compactness of the passivation film, indicating that its protective performance became better. After electrochemical passivation, the carrier concentration in the n-type semiconductor region was reduced (reducing by less than 2 times at 1.1 V), which, on the one hand, indicated that the protection capability of the n-type semiconductor passivation film region was increased, and on the other hand, its conductivity was not significantly reduced. Therefore, overall, the passivation film could ensure the improvement of corrosion resistance without significantly increasing the contact resistance.

[0121] Meanwhile, it can be found from Table 4, FIG. 4 and FIG. 5 that the composition of the passivation film could be changed by adjusting the parameters of the electrochemical passivation treatment, and the performance of the passivation film could be optimized. For example, the characteristics of the passivation film could be controlled by applying different potentials in the electrochemical passivation treatment (see FIG. 4 and FIG. 5), in which as the potential increased, the thickness ratio t.sub.p/t.sub.n of p-type semiconductor and n-type semiconductor in the passivation film gradually increased, thereby improving the service performance of stainless steel in fuel cell stacks. Accordingly, the composition of the passivation film could be changed by adjusting the above parameters of the electrochemical passivation treatment, and the performance of the passivation film could be optimized, finally improving the applicability of stainless steel bipolar plates applied to fuel cells.

[0122] The cross section morphology of the passivation film of the stainless steel in Example 1 was characterized. The specific characterization method included that: a transmission electron microscopy (TEM) was used to characterize the cross section of the sample cut by focused ion beam, so as to obtain the cross section morphology image of the passivation film (in order to protect the passivation film from being damaged during the sample preparation by focused ion beam sample, a carbon film was first deposited on the outermost layer), and the result is shown in FIG. 6. The highest part was the deposited C layer, the middle part was the passivation film, and the lowest part was the stainless steel substrate. It can be found from the image that the whole passivation film was continuous, compact and uniform, without obvious defects, and the thickness was about 12-20 nm. The outer layer of the passivation film was uneven, because the outermost passivation film underwent a dynamic growth-dissolution process in the acid solution, and the surface of the passivation film in the acid solution was usually uneven.

Examples 31-37

[0123] These examples differ from Example 1 in that: steel number, whether the steel was subjected to the surface roughness treatment and whether the steel was subjected to electrochemical passivation treatment, the results are shown in Table 6. Specifically, the surface roughness treatment may be directly performed after the step (1) without the electrochemical passivation treatment; or the electrochemical passivation treatment may also be directly performed after the step (1) without the surface roughness treatment; or the surface roughness treatment and the electrochemical passivation treatment may be performed sequentially after the step (1).

Examples 38-45

[0124] These examples differ from Example 1 in that: steel number, whether the steel was subjected to the surface roughness treatment and whether the steel was subjected to electrochemical passivation treatment, the results are shown in Table 6. Specifically, the surface roughness treatment may be directly performed after the step (1) without the electrochemical passivation treatment; or the electrochemical passivation treatment may also be directly performed after the step (1) without the surface roughness treatment; or the surface roughness treatment and the electrochemical passivation treatment may be performed sequentially after the step (1).

[0125] In these examples, the roughness treatment conditions were that: under the room temperature (25? C.), the sample was polarized at 10 V for 50 s in a H.sub.2SO.sub.4 solution of 3 mol/L.

[0126] The chemical passivation treatment conditions were that: the steel sheet obtained after the roughness treatment described above was electrochemically passivated with an anode potential of 1.1 V for 1 h in a HNO.sub.3 solution of 1.6 mol/L at 40? C.

TABLE-US-00007 TABLE 6 Interface Interface Interface Corrosion Contact Electrochemical Contact Contact Current Steel Roughness Resistance Passivation Resistance Resistance Density Example No. Treatment (m? .Math. cm.sup.2) Treatment (m? .Math. cm.sup.2) t.sub.p/t.sub.n I.sub.p/I.sub.n (m? .Math. cm.sup.2) (?A .Math. cm.sup.?2) 31 6 Yes 20 Yes 5 0.5 5.7 7.1 1.2 32 6 Yes 20 No 20 0.11 2.1 85 5.6 33 6 No 178 No 178 0.17 1.2 340 112.5 34 7 Yes 18 Yes 8 0.58 7 5.9 0.6 35 7 Yes 18 No 18 0.07 2.5 75 6.4 36 7 No 257 No 257 0.1 1.3 342 145.8 37 8 Yes 28 Yes 7 0.35 6.5 5.7 1.5 38 8 Yes 28 No 28 0.09 1.5 150 13.2 39 8 No 187 No 187 0.08 1.1 256 242.3 40 9 Yes 23 Yes 5 0.22 4.2 7.6 0.6 41 9 Yes 23 No 23 0.15 2.3 118 7.8 42 9 No 176 No 176 0.77 2.2 234 134.3 43 10 Yes 22 Yes 8 0.48 4.8 6.4 1.3 44 10 Yes 22 No 22 0.17 2.4 99 7.7 45 10 No 188 No 188 0.78 1.5 232 245.1

[0127] It can be found from Table 6 that the arrangement of the passivation film after the roughness treatment could further enhance the protection effect, increase the electrical conductivity, and further improve the performance.

[0128] Meanwhile, by sequentially subjecting the stainless steel to a roughness treatment and then an arrangement of passivation film, the passivation film on the stainless steel surface had good performance. The thickness ratio of p-type to n-type semiconductors was within 0.2-0.6, and I.sub.p[Cr(OH).sub.3/Cr.sub.2O.sub.3]/I.sub.n[Cr(OH).sub.3/Cr.sub.2O.sub.3] was more than 4; and after running for a certain period of time, the interface contact resistance was less than 8 m?.Math.cm.sup.2, and the current density was less than 3 ?A/cm.sup.2, showing good performance. Additionally, for either the passivation film was not arranged after the roughness treatment or the passivation film was directly arranged without the roughness treatment, the performances of the surface passivation film were all greatly improved compared with the samples without the roughness treatment and the passivation film arrangement. The subsequent long-term service test also showed that the interface contact resistance and the current density were reduced for the passivation film.

[0129] The applicant has stated that although the detailed method of the present application is described through the examples described above, the present application is not limited to the detailed method described above, which means that the implementation of the present application does not necessarily depend on the detailed method described above. It should be apparent to those skilled in the art that any improvements made to the present application, equivalent substitutions of various raw materials of the product, the addition of adjuvant ingredients, and the selection of specific manners, etc. in the present application all fall within the protection scope and the disclosed scope of the present application.