Stainless steel for separation plate of polymer electrolyte membrane fuel cell having improved hydrophilic property and corrosion resistance, and manufacturing method therefor
Provided are stainless steel for a separator of a polymer electrolyte membrane fuel cell, which exhibits enhanced hydrophilicity and enhanced corrosion resistance, and a method of manufacturing the same. In the stainless steel for a separator of a polymer electrolyte membrane fuel cell, which exhibits enhanced hydrophilicity and enhanced corrosion resistance, according to an embodiment of the present invention, a ratio of Cr hydroxide/Cr oxide included in a passivation film of the stainless steel ranges from 0.5 to 1.7, and the passivation film has a contact angle (θ) of 70° or less. Thus, not only corrosion resistance may be enhanced by removing a non-conductive film formed on a surface of the stainless steel and forming a new conductive film thereon, but hydrophilicity may also be secured without additional surface treatment such as a separate coating or the like, and thus manufacturing costs may be reduced and productivity may be increased.
1. A stainless steel for a separator of a polymer electrolyte membrane fuel cell, the stainless steel comprising, based on wt %, greater than 0% to 0.02% of C, greater than 0% to 0.02% of N, greater than 0% to 0.25% of Si, greater than 0% to 0.2% of Mn, greater than 0% to 0.04% of P, greater than 0% to 0.02% of S, 20% to 34% of Cr, greater than 0% to 0.6% of V, greater than 0% to 0.5% of Ti, greater than 0% to 0.5% of Nb, and the remainder, Fe and other unavoidable impurities,
wherein a ratio of Cr hydroxide/Cr oxide included in a passivation film of the stainless steel is more than 0.7 to 1.7, and the passivation film has a contact angle (θ) of 70° or less,
wherein the stainless steel has a surface roughness (Ra) of 0.02 μm to 0.5 μm.
2. The stainless steel of claim 1 , wherein the surface roughness (Ra) is an average value of surface roughness in a rolling longitudinal direction and surface roughness in a rolling transverse direction.
3. The stainless steel of claim 1 , wherein the stainless steel further comprises 0.05% to 2.5% of Mo.
4. The stainless steel of claim 1 , wherein the passivation film has a thickness of 3.5 nm or less (excluding 0).
5. The stainless steel of claim 1 , wherein the passivation film has a corrosion potential of 0.3 V (SCE) or more.
6. A method of manufacturing a stainless steel for a separator of a polymer electrolyte membrane fuel cell, the method comprising: manufacturing a stainless steel sheet by performing cold rolling on stainless steel, the stainless steel comprising, based on wt %, greater than 0% to 0.02% of C, greater than 0% to 0.02% of N, greater than 0% to 0.25% of Si, greater than 0% to 0.2% of Mn, greater than 0% to 0.04% of P, greater than 0% to 0.02% of S, 20% to 34% of Cr, greater than 0% to 0.6% of V, greater than 0% to 0.5% of Ti, greater than 0% to 0.5% of Nb, and the remainder, Fe and other unavoidable impurities;
a heat treatment process of forming a first passivation film on a surface of the stainless steel sheet by performing bright annealing on the stainless steel sheet; and a film reforming process of forming a second passivation film on the surface of the stainless steel sheet by reforming the first passivation film, wherein a ratio of Cr hydroxide/Cr oxide included in the second passivation film is more than 0.7 to 1.7, and the second passivation film has a contact angle (θ) of 70° or less,
wherein the film reforming process comprises:
a first film reforming process of performing electrolytic treatment in a sulfuric acid solution at a first current density;
a second film reforming process of performing electrolytic treatment in the sulfuric acid solution at a second current density, the second current density being equal to or less than the first current density; and
a third film reforming process performed by immersion in a mixed acid solution comprising nitric acid and hydrofluoric acid.
7. The method of claim 6 , wherein the first film reforming process and the second film reforming process are consecutively performed.
8. The method of claim 6 , wherein in the first film reforming process, a potential of the stainless steel sheet, corresponding to the first current density, satisfies Equations 1 and 2 below:
E cathode ≥1.0 (1)
| E cathode |+|E anode |≥2.0 (2)
wherein the unit of the potential of the stainless steel sheet is volt (V).
9. The method of claim 6 , wherein in the first film reforming process and the second film reforming process, a concentration of the sulfuric acid solution ranges from 50 g/l to 300 g/l, and a temperature of the sulfuric acid solution ranges from 40° C. to 80° C.
10. The method of claim 6 , wherein in the third film reforming process, a concentration of the nitric acid in the mixed acid solution ranges from 100 g/l to 200 g/l, a concentration of the hydrofluoric acid is 70 g/l or less, and a temperature of the mixed acid solution ranges from 40° C. to 60° C.