METHOD FOR MANUFACTURING STAINLESS STEEL FOR POLYMER FUEL CELL SEPARATOR

Abstract

Disclosed is a method for manufacturing a stainless steel for a polymer electrolyte membrane fuel cell separator, and more particularly, a method for manufacturing a stainless steel for a polymer electrolyte membrane fuel cell separator capable of obtaining low contact resistance and high corrosion resistance by effectively removing a non-conductive coating and forming a new coating. According to an embodiment, the disclosed method for manufacturing a stainless steel for a polymer electrolyte membrane fuel cell separator includes performing alternating current electrolysis by immersing, in a sulfuric acid solution, a stainless steel having a passivation coating formed on a surface thereof by cold rolling and bright annealing, wherein the alternating current electrolysis is performed by applying a current density of 10 to 30 A/dm.sup.2.

Claims

1. A method for manufacturing a stainless steel for a polymer electrolyte membrane fuel cell separator, the method comprising: performing alternating current electrolysis by immersing, in a sulfuric acid solution, a stainless steel having a passivation coating on a surface thereof formed by cold rolling and bright annealing, wherein the alternating current electrolysis is performed by applying a current density of 10 to 30 A/dm.sup.2.

2. The method according to claim 1, wherein the stainless steel comprises, in percent by weight (wt %), more than 0% and not more than 0.1% of C, more than 0% and not more than 0.3% of N, more than 0% and not more than 0.7% of Si, more than 0% and not more than 10% of Mn, more than 0% and not more than 0.04% of P, more than 0% and not more than 0.02% of S, 15 to 34% of Cr, 25% or less of Ni, and the remainder of Fe and other inevitable impurities.

3. The method according to claim 1, wherein a concentration of the sulfuric acid solution is from 50 to 300 g/.

4. The method according to claim 1, wherein a temperature of the sulfuric acid solution is from 40 to 80° C.

5. The method according to claim 1, wherein a time required to apply the current density is within 10 seconds.

6. The method according to claim 1, wherein a contact resistance under a contact pressure of 100 N/cm.sup.2 is 12 mΩ.Math.cm.sup.2 or less.

Description

EXAMPLES

[0072] A stainless steel having the composition of alloying elements shown in Table 1 below was cold-rolled using a Z-mill, as a cold rolling mill, and subjected to bright annealing heat treatment under a reducing atmosphere including 75 vol % of hydrogen and 25 vol % of nitrogen to prepare a cold-rolled steel sheet having a thickness of 0.1 mm.

[0073] In Table 1, steel type A is a ferritic stainless steel.

TABLE-US-00001 TABLE 1 Steel Composition of alloying elements (wt %) type C N Si Mn P S Cr Ni Ti V Nb A 0.008 0.012 0.1 0.15 0.015 0.0015 30 0.15 0.1 0.4 0.15

[0074] The cold-rolled steel sheet prepared according to Table 1 above was subjected to alternating current electrolysis under the conditions shown in Table 2 below. Meanwhile, Comparative Example 5 in Table 2 was subjected to electrolysis using a high electric potential using DC power according to the two-step DC current electrolysis of the prior art, and then electrolysis using a low electric potential.

[0075] In Table 2, the contact resistance values were obtained by cutting the prepared cold-rolled stainless steel sheet having a thickness of 0.1 mm into two pieces each having an area of 25 cm.sup.2, disposing carbon paper (SGL-10BA) having an area of 4 cm.sup.2 as a gas diffusion layer between the cold-rolled steel sheets, measuring contact resistances four times under a contact pressure of 100 N/cm.sup.2, and calculating an average of the measured contact resistances.

[0076] In Table 2, corrosion resistance was evaluated under an operating environment of a fuel cell, i.e., by immersing the prepared cold-rolled stainless steel sheet having a thickness of 0.1 mm in a mixed acid solution containing sulfuric acid (1 M) and hydrofluoric acid (2 ppm) at a temperature of 70° C., and measuring an electric potential relative to a saturated calomel electrode (SCE), as a reference electrode. When the measured electric potential was 0.4 V.sub.SCE or more, corrosion resistance was evaluated as good. When the measured electric potential was less than 0.4 V.sub.SCE, corrosion resistance was evaluated as poor.

TABLE-US-00002 TABLE 2 Alternating current electrolysis Sulfuric acid solution Current Time for Contact Steel Concentration Temperature density process resistance Corrosion Example type (g/l) (° C.) (A/dm.sup.2) (s) (mΩ .Math. cm.sup.2) resistance Inventive A 200 60 10 7 8.4 good Example 1 Inventive A 200 60 15 4 9.7 good Example 2 Inventive A 200 60 15 7 5.7 good Example 3 Inventive A 200 60 20 4 6.2 good Example 4 Inventive A 200 60 30 7 10.9 good Example 5 Inventive A 200 80 10 7 11.6 good Example6 Inventive A 200 80 15 4 9.5 good Example 7 Inventive A 200 80 20 4 9 good Example 8 Comparative A 200 30 20 4 14.6 good Example 1 Comparative A 200 60 5 7 23.8 poor Example 2 Comparative A 200 60 5 20 25.3 poor Example 3 Comparative A 200 60 40 7 15.8 good Example 4 Comparative A 200 60 DC 39 5.1 good Example 5

[0077] Referring to Table 1, because Inventive Examples 1 to 8 satisfied the ranges of the concentration of the sulfuric acid solution, the temperature, and the current density suggested by the present disclosure, an application time of the current density was within 10 seconds, the contact resistance under the contact pressure of 100 N/cm.sup.2 was 12 mΩ.Math.cm.sup.2 or less, and good corrosion resistance was obtained.

[0078] On the contrary, in Comparative Example 1, the temperature of the sulfuric acid solution was 30° C., which is out of the temperature range of the sulfuric acid solution suggested by the present disclosure, and thus the contact resistance under the contact pressure of 100 N/cm.sup.2 exceeded 12 mΩ.Math.cm.sup.2.

[0079] In Comparative Example 2, the current density was 5 A/dm.sup.2, which is out of the current density range suggested by the present disclosure, and thus the contact resistance measured by applying the current density for 7 seconds under the contact pressure of 100 N/cm.sup.2 exceeded 12 mΩ.Math.cm.sup.2, and poor corrosion resistance was obtained. Therefore, it was confirmed that the surface of the stainless steel was not modified.

[0080] In Comparative Example 3, although the same current density as that of Comparative Example 2 was applied for 20 seconds, the contact resistance under the contact pressure of 100 N/cm.sup.2 exceeded 12 mΩ.Math.cm.sup.2, and poor corrosion resistance was obtained. Therefore, it was confirmed that the surface of the stainless steel was not modified.

[0081] In Comparative Example 4, the current density was 40 A/dm.sup.2, which is out of the current density range suggested by the present disclosure, and thus the surface modification effect rather deteriorated and the contact resistance under the contact pressure of 100 N/cm.sup.2 exceeded 12 mΩ.Math.cm.sup.2.

[0082] In Comparative Example 5, although surface modification was completed by applying the conventional two-step direct current electrolysis, the time required for the surface modification process was 39 seconds which was slower than that of the alternating current electrolysis.

[0083] Based on the results of Inventive Examples and Comparative Examples, it was confirmed that the conventional two-step direct current electrolysis may be simplified to the one-step alternating current electrolysis, and the process of immersing in a mixed acid solution after the electrolysis may be omitted.

[0084] Particularly, upon comparison of Inventive Examples with Comparative Example 5, the time required for the alternating current electrolysis was significantly reduced compared to the time required for the conventional direct current electrolysis. According to the present disclosure, productivity may be improved by reducing manufacturing time and costs by increasing the efficiency of a post-treatment process to reduce interfacial contact resistance of the stainless steel for fuel cell separators.

[0085] While the present disclosure has been particularly described with reference to exemplary embodiments, it should be understood by those of skilled in the art that various changes in form and details may be made without departing from the spirit and scope of the present disclosure.

INDUSTRIAL APPLICABILITY

[0086] According to the present disclosure, a method for manufacturing a stainless steel for a polymer electrolyte membrane fuel cell separator may be provided.