US-12618128-B2 - Stainless steel for separator of polymer fuel cell having excellent corrosion resistance

Abstract

Disclosed is a stainless steel for a separator of a polymer fuel cell having excellent corrosion resistance. More particularly, disclosed is a stainless steel for a separator of a polymer fuel cell having excellent corrosion resistance in a sulfuric acid environment which is a fuel cell operating environment. According to an embodiment, the stainless steel for a separator of a polymer fuel cell includes, in percent by weight (wt %), 0.09% or less of C, 1.0% or more and less than 2.5% of Si, 1.0% or less (excluding 0) of Mn, 0.003% or less of S, 20 to 23% of Cr, 9 to 13% of Ni, 1.0% or less (excluding 0) of W, 0.10 to 0.25% of N, and the remainder of Fe and other inevitable impurities, wherein a corrosion resistance index represented by Formula (1) below is 7 or more. 3*W+1.5*Si+0.1*Cr+20*N−2*Mn  (1) In Formula (1), W, Si, Cr, N, and Mn represent the content (wt %) of each element.

Inventors

• Kwang Min Kim
• Bong-Wn Kim
• Bo-Sung Seo
• Dong-Hoon Kim
• Mun-Soo LEE

Assignees

• POSCO

Dates

Publication Date

20260505

Application Date

20200210

Priority Date

20191219

Claims (4)

1. 1 . An austenitic stainless steel for a separator of a polymer fuel cell comprising, a matrix comprising, in percent by weight (wt %), 0.09% or less of C, 1.0% or more and less than 2.5% of Si, 1.0% or less (excluding 0) of Mn, 0.003% or less of S, 20 to 23% of Cr, 9 to 13% of Ni, 1.0% or less (excluding 0) of W, 0.10 to 0.25% of N, less than 0.6% of Mo and the remainder of Fe and other inevitable impurities; and a passivated layer formed on a surface of the matrix in a form of oxide layer, wherein a corrosion resistance index represented by Formula (1) below is 7 or more, and wherein a value of Formula (2) below is 2.0 or more, and wherein a value of Formula (3) below is from 1.4 to 2.0, and wherein a thickness of the passivated layer is 6 nm or less: 3*W+1.5*Si+0.1*Cr+20*N−2*Mn Formula (1) (wherein in Formula (1), W, Si, Cr, N, and Mn represent the content (wt %) of each element), sum ⁢ of ⁢ contents ⁢ ( wt ⁢ % ) ⁢ of ⁢ Si ⁢ and ⁢ W ⁢ contained in ⁢ passivated ⁢ layer sum ⁢ of ⁢ contents ⁢ ( wt ⁢ % ) ⁢ of ⁢ Si ⁢ and ⁢ W ⁢ contained ⁢ in ⁢ matrix , Formula ⁢ ( 2 ) (Cr+Mo+1.5Si+0.75 W)/(Ni+0.5Mn+20N+24.5 C) Formula (3) (wherein in Formula (3), Cr, Mo, Si, W, Ni, Mn, N, and C represent the content (wt %) of each element).
2. 2 . The austenitic stainless steel according to claim 1 , wherein an elongation is 40% or more.
3. 3 . The austenitic stainless steel according to claim 1 , wherein a corrosion current density measured by applying a potential of 0.6 V relative to a calomel electrode for 24 hours in a mixed solution of sulfuric acid (H 2 SO 4 ) having a pH of 3 and hydrofluoric acid (HF) having a pH of 5.3 at 80° C. is 0.05 μA/cm 2 or less.
4. 4 . The austenitic stainless steel according to claim 1 , wherein an amount of metal melted by applying a potential of 0.6 V relative to a calomel electrode for 24 hours in a mixed solution of sulfuric acid (H 2 SO 4 ) having a pH of 3 and hydrofluoric acid (HF) having a pH of 5.3 at 80° C. is 0.7 or less relative to that of the stainless steel 316L.

Description

CROSS-REFERENCE OF RELATED APPLICATIONS This application is the U.S. National Phase under 35 U.S.C. § 371 of International Patent Application No. PCT/KR2020/001844, filed on Feb. 10, 2020 which claims priority to and the benefit of Korean Application No. 10-2019-0170586 filed on Dec. 19, 2019, the entire contents of which are incorporated herein by reference. TECHNICAL FIELD The present disclosure relates to a stainless steel for a separator of a polymer fuel cell having excellent corrosion resistance, and more particularly, to a stainless steel for a separator of a polymer fuel cell having excellent corrosion resistance in a sulfuric acid environment which is a fuel cell operating environment. BACKGROUND ART Polymer electrolyte membrane fuel cells, which are fuel cells using a polymer membrane having hydrogen ion exchange characteristics as an electrolyte, have a low operating temperature of approximately 80° C. and high efficiency compared to other types of fuel cells. Also, polymer electrolyte membrane fuel cells may be used for vehicles, home appliances, and the like due to quick start, high output density, and simple body structure. A polymer electrolyte membrane fuel cell has a unit cell structure in which a gas diffusion layer and a separator are stacked on both sides of a membrane electrode assembly (MEA) including an electrolyte, an anode, and a cathode, and a structure in which a plurality of unit cells are connected in series is called a fuel cell stack. A separator are provided with flow paths to supply a fuel (hydrogen or reformed gas) and an oxidant (oxygen and air) respectively to electrodes and to discharge water that is a product of electrochemical reaction. The separator performs a function of mechanically supporting the MEA and the gas diffusion layer and a function of electrical connection to adjacent unit cells. Although graphite has been conventionally used as a material for the separators, stainless steels has been widely used in recent years in consideration of manufacturing cost and weight. However, unless a stainless steel used as a separator has sufficient corrosion resistance, corrosion may occur in a sulfuric acid environment which is a fuel cell operating environment. As a result, a problem of a decrease in output of a fuel cell occurs. Accordingly, austenitic stainless steels to which molybdenum (Mo) is added in the same manner used for stainless steel 316 L, is mainly used as a material of a separator of a polymer electrolyte membrane fuel cell to obtain corrosion resistance and formability. However, the stainless steel 316 L contains molybdenum (Mo) in a large amount of 2% or more. Accordingly, an increase in price of molybdenum (Mo) causes a wide range of fluctuation in price of raw materials, and thus unstable price may result in low price competitiveness. In addition, the conventional stainless steel 316 L has insufficient corrosion resistance in a sulfuric acid environment which is a fuel cell operating environment, so there is a problem that corrosion may occur. In order to solve these problems, Patent Documents 1 and 2 disclose methods of obtaining corrosion resistance by plasma-treating the surface of a stainless steel separator, and Patent Document 3 discloses a method of obtaining corrosion resistance by coating the surface of a stainless steel separator with gold, platinum, ruthenium, iridium, or the like. However, since these methods require additional surface reforming process or coating process, there are problems of relatively low price competitiveness and a decrease in productivity. (Patent Document 1) Korean Patent Publication No. 10-1172163(Patent Document 2) Korean Patent Publication No. 10-1054760(Patent Document 3) Korean Patent Publication No. 10-1165542 DISCLOSURE Technical Problem To solve the above-described problems, provided is a stainless steel for a separator of a polymer fuel cell having excellent corrosion resistance in a sulfuric acid environment which is a fuel cell operating environment. Technical Solution In accordance with an aspect of the present disclosure to achieve the above-described objects, provided is an austenitic stainless steel for a separator of a polymer fuel cell including, in percent by weight (wt %), 0.09% or less of C, 1.0% or more and less than 2.5% of Si, 1.0% or less (excluding 0) of Mn, 0.003% or less of S, 20 to 23% of Cr, 9 to 13% of Ni, 1.0% or less (excluding 0) of W, 0.10 to 0.25% of N, and the remainder of Fe and other inevitable impurities, wherein a corrosion resistance index represented by Formula (1) below is 7 or more. 3*W+1.5*Si+0.1*Cr+20*N−2*Mn  (1) In Formula (1), W, Si, Cr, N, and Mn represent the content (wt %) of each element. In the austenitic stainless steel for a separator of a polymer fuel cell of the present disclosure, a value of Formula (2) below may be 2.0 or more. sum⁢ of⁢ contents⁢ (wt⁢ %)⁢ of⁢ Si⁢ and⁢ W⁢ containedin⁢ passivated⁢ layersum⁢ of⁢ contents⁢ (wt⁢ %)⁢