Title of invention: Stainless steel for polymer fuel cell separator with excellent corrosion resistance
technical field
[One]
The present invention relates to a stainless steel for a polymer fuel cell separator having excellent corrosion resistance, and more particularly, to a stainless steel for a polymer fuel cell separator having excellent corrosion resistance in a sulfuric acid environment, which is a fuel cell driving environment.
background
[2]
A polymer electrolyte fuel cell is a fuel cell using a polymer membrane having hydrogen ion exchange characteristics as an electrolyte, and has a low operating temperature of about 80° C. and high efficiency compared to other types of fuel cells. In addition, it can be used for automobiles, households, etc. because the starting is fast, the output density is high, and the structure of the battery body is simple.
[3]
A polymer electrolyte 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) consisting of an electrolyte, an anode and a cathode electrode, and such a unit A fuel cell stack composed of several cells connected in series is called a fuel cell stack.
[4]
The separator supplies fuel (hydrogen or reformed gas) and oxidizer (oxygen and air) to the fuel cell electrodes, respectively, and has a flow path for discharging water, an electrochemical reaction product, and mechanically separates the membrane electrode assembly and the gas diffusion layer. It performs a supporting function and an electrical connection function with an adjacent unit cell.
[5]
Graphite material has been conventionally used as a material for such a separator, but recently, stainless steel has been widely used in consideration of manufacturing cost, weight, and the like. However, if the corrosion resistance of the stainless steel used as the separator is not sufficiently secured, corrosion occurs in the sulfuric acid environment, which is the driving environment of the fuel cell. As a result, there is a problem in that the output of the fuel cell is lowered.
[6]
Accordingly, austenitic stainless steel to which molybdenum (Mo) is added, such as 316L stainless steel, is mainly used as a material for polymer fuel cell separators to secure corrosion resistance and formability. However, 316L stainless steel contains a large amount of molybdenum (Mo) at 2% or more. Accordingly, there is a disadvantage of low price competitiveness due to large fluctuations in manufacturing cost due to an increase in the price of molybdenum (Mo). In addition, the existing 316L stainless steel has insufficient corrosion resistance in a sulfuric acid environment that is a fuel cell driving environment, so there is a problem that corrosion may still occur.
[7]
In order to solve this problem, conventional Patent Documents 1 and 2 secure corrosion resistance by plasma-treating the surface of the stainless steel separator, and Patent Document 3 secures corrosion resistance by coating the surface of the stainless steel separator with gold, platinum, ruthenium, iridium, etc. are doing However, since this method requires an additional surface modification process or a coating process separately, price competitiveness is relatively low, and there is a problem in that productivity is lowered.
[8]
(Patent Document 1) Korean Patent Registration No. 10-1172163
[9]
(Patent Document 2) Korean Patent Publication No. 10-1054760
[10]
(Patent Document 3) Korean Patent Registration No. 10-1165542
DETAILED DESCRIPTION OF THE INVENTION
technical challenge
[11]
In order to solve the above problems, the present invention is to provide a stainless steel for a polymer fuel cell separator capable of securing excellent corrosion resistance in a sulfuric acid environment, which is a fuel cell driving environment.
means of solving the problem
[12]
As a means for achieving the above object, the austenitic stainless steel for a polymer fuel cell separator according to an embodiment of the present invention is, by weight, C: 0.09% or less, Si: 1.0% or more and less than 2.5%, Mn: 1.0% or less (excluding 0), S: 0.003% or less, Cr: 20 to 23%, Ni: 9 to 13%, W: 1.0% or less (excluding 0), N: 0.10 to 0.25%, balance Fe and other unavoidable impurities Including, the corrosion resistance index represented by the following formula (1) may be 7 or more.
[13]
(1) 3*W + 1.5*Si + 0.1*Cr + 20*N - 2*Mn
[14]
In the above formula (1), W, Si, Cr, N, and Mn mean the content (% by weight) of each element.
[15]
In the austenitic stainless steel for each polymer fuel cell separator of the present invention, the value of the following formula (2) may be 2.0 or more.
[16]
(2)
[17]
In the austenitic stainless steel for each polymer fuel cell separator of the present invention, the value of the following formula (3) may be 1.4 to 2.0.
[18]
(3) (Cr + Mo + 1.5Si + 0.75W)/(Ni + 0.5Mn + 20N + 24.5C)
[19]
In the formula (3), Cr, Mo, Si, W, Ni, Mn, N, and C mean the content (% by weight) of each element.
[20]
In the austenitic stainless steel for each polymer fuel cell separator of the present invention, the thickness of the passivation film may be 6 nm or less.
[21]
In the austenitic stainless steel for each polymer fuel cell separator of the present invention, the elongation may be 40% or more.
[22]
In the austenitic stainless steel for each polymer fuel cell separator of the present invention, in a mixed solution of sulfuric acid (H 2 SO 4 ) at 80 ° C., pH 3 and hydrofluoric acid (HF) at pH 5.3, 0.6V potential compared to the calomel electrode was 24 Corrosion current density measured by applying for time may be less than or equal to 0.05 μA/cm 2 .
[23]
In the austenitic stainless steel for each polymer fuel cell separator of the present invention, in a mixed solution of sulfuric acid (H 2 SO 4 ) at 80 ° C., pH 3 and hydrofluoric acid (HF) at pH 5.3, 0.6V potential compared to the calomel electrode was 24 The amount of dissolved metal applied over time may be 0.7 or less compared to 316L stainless steel.
Effects of the Invention
[24]
According to the present invention, it is possible to provide a stainless steel for a polymer fuel cell separator having better corrosion resistance than 316L stainless steel in a sulfuric acid environment, which is a fuel cell driving environment.
[25]
Specifically, by adding silicon (Si) and tungsten (W) instead of expensive molybdenum (Mo) and controlling the alloy composition according to the corrosion resistance index, it is possible to provide a stainless steel for a polymer fuel cell separator having excellent corrosion resistance. In addition, it is possible to provide a stainless steel for a polymer fuel cell separator having excellent corrosion resistance by increasing the abundance ratio of Si and W oxides having excellent corrosion resistance in the passivation film compared to the base material.
[26]
According to the present invention, it is possible to provide a stainless steel for a polymer fuel cell separator excellent in both corrosion resistance and workability.
[27]
Specifically, it is possible to secure excellent hot workability by controlling the delta ferrite fraction in the ratio of Cr equivalent and Ni equivalent. The elongation of the stainless steel for a polymer fuel cell separator according to an example may be 40% or more.
[28]
In addition, according to the present invention, it is possible to provide a stainless steel for a polymer fuel cell separator having excellent corrosion resistance in a sulfuric acid environment that is a fuel cell driving environment without a separate surface modification process or coating process.
Brief description of the drawing
[29]
1 is a graph showing the correlation between the corrosion resistance index, the corrosion current density, and the relative dissolution amount compared to 316L stainless steel. 1a is a graph showing a correlation between a corrosion resistance index and a corrosion current density, and FIG. 1b is a corrosion resistance index and a relative dissolution amount compared to 316L stainless steel.
[30]
2 is a graph showing the correlation between the value of Equation (2) and the corrosion current density, the relative amount of dissolution compared to 316L stainless steel, and the thickness of the passivation film. Figure 2a is a graph showing the correlation between the value of the formula (2) and the corrosion current density, Figure 2b is the value of the formula (2) and the relative dissolution amount compared to 316L stainless steel, Figure 2c is the value of the formula (2) and the passivation film thickness .
Best mode for carrying out the invention
[31]
Austenitic stainless steel for a polymer fuel cell separator according to an embodiment of the present invention is, by weight, C: 0.09% or less, Si: 1.0% or more and less than 2.5%, Mn: 1.0% or less (excluding 0), S: 0.003 % or less, Cr: 20 to 23%, Ni: 9 to 13%, W: 1.0% or less (excluding 0), N: 0.10 to 0.25%, the remainder including Fe and other unavoidable impurities, and represented by the following formula (1) The expressed corrosion resistance index may be 7 or more.
[32]
(1) 3*W + 1.5*Si + 0.1*Cr + 20*N - 2*Mn
[33]
In the above formula (1), W, Si, Cr, N, and Mn mean the content (% by weight) of each element.
Modes for carrying out the invention
[34]
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The following describes preferred embodiments of the present invention. However, the embodiment of the present invention may be modified in various other forms, and the technical idea of ​​the present invention is not limited to the embodiment described below. In addition, the embodiments of the present invention are provided in order to more completely explain the present invention to those of ordinary skill in the art.
[35]
The terms used in this application are only used to describe specific examples. Therefore, for example, a singular expression includes a plural expression unless the context clearly requires it to be singular. In addition, the terms "comprises" or "includes" as used in the present application are used to clearly indicate that the features, steps, functions, components, or combinations thereof described in the specification exist, and other features It should be noted that it is not intended to be used to preliminarily exclude the existence of elements, steps, functions, components, or combinations thereof.
[36]
Meanwhile, unless otherwise defined, all terms used herein should be regarded as having the same meaning as commonly understood by those of ordinary skill in the art to which the present invention pertains. Accordingly, unless explicitly defined herein, certain terms should not be construed in an unduly idealistic or formal sense. For example, a singular expression herein includes a plural expression unless the context clearly dictates otherwise.
[37]
In addition, in this specification, "about", "substantially", etc. are used in or close to the numerical value when manufacturing and material tolerances inherent in the stated meaning are presented, and are used in a precise sense to aid the understanding of the present invention. or absolute figures are used to prevent unreasonable use of the mentioned disclosure by an unconscionable infringer.
[38]
In addition, "316L stainless steel" in this specification means STS316L stainless steel of KS standard, in weight%, C: 0.03% or less, Si: 1.0% or less, Mn: 2.0% or less, P: 0.045% or less, S : 0.03% or less, Ni: 10.0 to 14.0%. Cr: 16.0 to 18.0%, Mo: interpreted as stainless steel containing 2.0 to 3.0%. However, it is not necessarily interpreted as being limited to stainless steel having the above content range, and it can be interpreted as STS316L stainless steel of KS standard within the range that can be clearly recognized by those skilled in the art.
[39]
In addition, the term "passive film" in the present specification means an oxide layer formed on the surface of stainless steel, and is interpreted as a passivation oxide layer formed on the surface of stainless steel within a range that can be clearly recognized by those skilled in the art. can be
[40]
In addition, the "base material" of the present specification means stainless steel excluding the passivation film formed on the surface of the stainless steel, and a passivation oxide layer formed on the surface of the stainless steel within a range that can be clearly recognized by those skilled in the art. It can be interpreted as stainless steel except for
[41]
The austenitic stainless steel for a polymer fuel cell separator according to an embodiment of the present invention is, by weight, C: more than 0% and less than 0.09%, Si: 1.0% or more and less than 2.5%, Mn: 1.0% or less (excluding 0), S: 0.003% or less, Cr: 20 to 23%, Ni: 9 to 13%, W: 1.0% or less (excluding 0), N: 0.10 to 0.25%, balance Fe and other unavoidable impurities.
[42]
Hereinafter, the reason for limiting the alloy composition will be described in detail.
[43]
Carbon (C): 0.09% by weight or less
[44]
C is an austenite forming element, and is an element that improves high-temperature strength when added. However, when excessively added, it reacts with Cr in the steel to form chromium carbide, and as a result, the corrosion resistance of the Cr-depleted region is lowered. Therefore, in the present invention, it is preferable that the content of C is lower, and in the present invention, the C content is controlled to 0.09% by weight or less.
[45]
Silicon (Si): 1.0 wt% or more and less than 2.5 wt%
[46]
Si is an element that improves the corrosion resistance of stainless steel. In particular, Si is an element with excellent corrosion resistance improvement in a sulfuric acid environment. According to the present invention, in order to secure excellent corrosion resistance in a sulfuric acid environment, which is a fuel cell environment, in the present invention, Si is actively added in an amount of 1.0 wt% or more. When the Si content is less than 1.0% by weight, sufficient corrosion resistance cannot be secured in a sulfuric acid environment, which is a fuel cell environment.
[47]
However, when Si is excessively added, elongation is reduced, and corrosion resistance is lowered by forming SiO 2 oxide, so that the Si content is controlled to be less than 2.5 wt%. As described above, when the Si content is excessive, elongation and corrosion resistance may be lowered. More preferably, the Si content is controlled to 2.0 wt% or less.
[48]
Manganese (Mn): 1.0 wt% or less (excluding 0)
[49]
Mn is an austenite phase stabilizing element, which is an element that can replace expensive Ni, but when excessively added, corrosion resistance is lowered, so the Mn content is controlled to 1.0 wt% or less in the present invention.
[50]
Sulfur (S): 0.003 wt% or less
[51]
S is a trace impurity element, which is segregated at grain boundaries and causes work cracks during hot rolling . Therefore, the upper limit of the S content is controlled to be as low as 0.003% or less.
[52]
Chromium (Cr): 20 to 23 wt%
[53]
Cr is an element that improves corrosion resistance by forming Cr oxide on the steel surface, and should be added in an amount of 20 wt% or more to secure corrosion resistance in a fuel cell driving environment, which is a strong acidic environment. However, in order to stabilize the austenite phase when excessively added, expensive Ni, Mn that reduces corrosion resistance, or N that reduces workability must be additionally added. In consideration of this, the Cr content is controlled to 23 wt% or less in the present invention.
[54]
Nickel (Ni): 9 to 13% by weight
[55]
Since Ni is an austenite phase stabilizing element or an expensive element, in the present invention, the upper limit of the Ni content is controlled to 13% by weight or less in consideration of economic feasibility. However, excessive reduction of the Ni content requires the addition of Mn that reduces corrosion resistance or N that reduces workability for stabilization of the austenite phase. In consideration of this, in the present invention, the lower limit of the Ni content is controlled to 9 wt% or more.
[56]
Tungsten (W): 1.0 wt% or less (excluding 0)
[57]
W is an element that improves the corrosion resistance of stainless steel, and is an element with excellent price competitiveness because it has a large corrosion resistance improvement effect even in a small amount compared to Mo. However, since excessive addition promotes the formation of a sigma phase that deteriorates the mechanical properties of steel, in the present invention, the upper limit of the W content is controlled to 1.0 wt% or less.
[58]
Nitrogen (N): 0.10 to 0.25 wt%
[59]
N is an austenite phase stabilizing element, and is an element that can replace expensive Ni as an austenite phase stabilizing element. In addition, since N is an element that improves strength and pitting resistance when added, the N content is controlled to 0.10 wt% or more in the present invention. In order to secure better corrosion resistance, the N content is preferably controlled to 0.15% by weight or more.
[60]
However, since there is a disadvantage in that workability such as elongation is reduced when excessively added, the upper limit of the N content is controlled to 0.25 wt% or less in the present invention.
[61]
The remaining component of the present invention is iron (Fe). However, since unintended impurities from raw materials or the surrounding environment may inevitably be mixed in the normal manufacturing process, this cannot be excluded. All of the impurities are not specifically mentioned in the present specification, since any person skilled in the art can know the impurities.
[62]
The austenitic stainless steel for a polymer fuel cell separator of the present invention having the above-described alloy composition may have a corrosion resistance index of 7 or more in Equation (1) below.
[63]
(1) 3*W + 1.5*Si + 0.1*Cr + 20*N - 2*Mn
[64]
In Formula (1), W, Si, Cr, N, and Mn mean the content (wt%) of each element.
[65]
Equation (1) above was designed by the inventors of the present invention to secure stainless steel having excellent corrosion resistance in a fuel cell driving environment, which is a strong acid environment. The coefficient multiplied by the content of each element in Equation (1) means the weight of the alloying element controlled in the stainless steel of the present invention to secure corrosion resistance.
[66]
For example, W, Si, Cr, and N are elements that improve corrosion resistance, and the coefficient multiplied by the content of each element in Equation (1) is a positive number. On the other hand, although Mn is an austenite phase stabilizing element, the coefficient multiplied by the Mn content in Equation (1) is negative because it reduces corrosion resistance.
[67]
The higher the corrosion resistance index according to the formula (1), the better the corrosion resistance can be secured, but it should be noted that the contents of W, Si, Cr, N, and Mn in the formula (1) are controlled within the above-described content range.
[68]
In the present invention, by controlling the corrosion resistance index of Equation (1) to be 7 or more, it is possible to provide an austenitic stainless steel for a polymer fuel cell separator having excellent corrosion resistance in a fuel cell driving environment, which is a strong acidic environment.
[69]
The passivation film of stainless steel is formed on the surface of stainless steel, and serves to prevent exposure of the base material to a corrosive environment. Accordingly, the corrosion resistance of stainless steel is determined by how high the corrosion stability of the passivation film formed on the surface of the stainless steel under a corrosive environment.
[70]
In addition, in order to secure excellent corrosion resistance in a sulfuric acid environment, which is a fuel cell environment, it is preferable that Si, which is an element having excellent corrosion resistance improvement in a sulfuric acid environment, is contained in a large amount in the passivation film.
[71]
In consideration of the above, the inventors of the present invention derived the following Equation (2) to control Si, W having excellent corrosion resistance in the passivation film to be contained in a large amount in the passivation film.
[72]
(2)
[73]
The larger the value of Equation (2) above, the higher the ratio of Si and W in the form of oxide having excellent corrosion resistance in the passivation film is higher than the content of the base material.
[74]
According to an example of the present invention, in the austenitic stainless steel for a polymer fuel cell separator, the value of Equation (2) may be 2.0 or more. If the value of Equation (2) is less than 2.0, sufficient corrosion resistance cannot be secured in the fuel cell environment.
[75]
The passivation film of stainless steel is an oxide layer formed by oxidizing metal elements such as Fe, Cr, Si, and W in the base material when the base material is exposed to a corrosive environment. Fe oxide has many defects in the oxide and is not dense, so it cannot block oxygen penetrating into the base material, and the passivation film continues to grow. On the other hand, metal oxides such as Cr, Si, and W are denser than Fe oxides and prevent oxygen penetrating into the base material, thereby inhibiting the growth of the passivation film.
[76]
When the value of Equation (2) according to the present invention satisfies 2.0 or more, a large number of Si and W oxides, which are denser than Fe oxides, are formed in the passivation film, thereby preventing oxygen penetrating into the base material, thereby inhibiting the growth of the passivation film. . According to an example, the growth of the passivation film is suppressed and the thickness of the thin and dense passivation film may be 6 nm or less.
[77]
As described above, the austenitic stainless steel for polymer fuel cell separator satisfying the alloy composition and the values ​​of Equation (1) and Equation (2) defined in the present invention has excellent corrosion resistance.
[78]
According to an embodiment, the fuel cell driving environment is 80° C., in a mixed solution of sulfuric acid (H 2 SO 4 ) of pH 3 and hydrofluoric acid (HF) of pH 5.3, compared to the calomel electrode, and 0.6V potential is applied for 24 hours. can be According to an example of the present invention, the corrosion current density measured in the above composition environment may be 0.05 μA/cm 2 or less, and the amount of dissolved metal may be 0.7 or less compared to 316L stainless steel.
[79]
Even if the corrosion resistance of stainless steel is excellent, if the workability is poor, if a number of surface defects such as cracks occur on the edge or surface of the stainless steel during the process for manufacturing a thin fuel cell separator, it is not preferable because the error rate is lowered. Such cracks are highly likely to occur during the hot rolling process during the manufacturing process, and in order to be applied as a thin fuel cell separator material, it is necessary to improve the hot workability.
[80]
It was found that the occurrence frequency of surface defects such as edge or surface cracks in austenitic stainless steels is determined by the fraction of delta ferrite present in the microstructure of the stainless steel. Specifically, if the delta ferrite fraction is too large, there is a high possibility that the surface defects of the edge or the surface will occur due to rolling for the two-phase region of austenite and delta ferrite. On the other hand, if the delta ferrite fraction is too small, the possibility of surface defects is high due to coarsening of the austenite grains. Therefore, it is necessary to appropriately control the fraction of delta ferrite formed during solidification.
[81]
The inventors of the present invention found that the delta ferrite fraction was particularly greatly affected by the Cr equivalent and the Ni equivalent.
[82]
(3) (Cr + Mo + 1.5Si + 0.75W)/(Ni + 0.5Mn + 20N + 24.5C)
[83]
In the formula (3), Cr, Mo, Si, W, Ni, Mn, N, and C mean the content (% by weight) of each element.
[84]
In formula (3), the molecular part '(Cr + Mo + 1.5Si + 0.75W)' means the Cr equivalent (Cr eq ), and the denominator part '(Ni + 0.5Mn + 20N + 24.5C)' is the Ni equivalent ( Ni eq ). The Cr equivalent is an index in which the influence of alloying elements causing ferrite formation is converted, and the Ni equivalent is an index in which the influence of the alloying elements causing austenite formation is converted. When the value of Equation (3) is large, the delta ferrite fraction becomes large, and when the value of Equation (3) is small, the delta ferrite fraction becomes small.
[85]
According to an example of the present invention, the value of Equation (3) may be 1.4 to 2.0. When the value of Equation (3) is less than 1.4, the proper delta ferrite fraction is too small, and the possibility of surface defects is high due to poor hot workability due to coarsening of austenite grains. On the other hand, when the value of Equation (3) exceeds 2.0, the delta ferrite fraction becomes excessively high, and surface defects are highly likely to occur during hot rolling.
[86]
As described above, the austenitic stainless steel for polymer fuel cell separator satisfying the range of the value of Equation (3) limited by the present invention has excellent hot workability.
[87]
According to one embodiment, the austenitic stainless steel for a polymer fuel cell separator of the present invention may have an elongation of 40% or more. If the elongation is less than 40%, the processability cannot be sufficiently secured and it cannot be processed into a thin polymer fuel cell separator, so it is not suitable as a stainless steel for a polymer fuel cell separator.
[88]
Hereinafter, the present invention will be described in more detail through examples. However, it is necessary to note that the following examples are only intended to illustrate the present invention in more detail and are not intended to limit the scope of the present invention. This is because the scope of the present invention is determined by the matters described in the claims and matters reasonably inferred therefrom.
[89]
{Example}
[90]
A stainless steel hot-rolled steel sheet was manufactured by using a rough rolling mill and a continuous finishing mill for steel having the composition shown in Table 1 below, followed by annealing and pickling. In addition, the corrosion resistance index defined by the following formula (1) of each invention example and comparative example is also shown in Table 1.
[91]
(1) 3*W + 1.5*Si + 0.1*Cr + 20*N - 2*Mn
[92]
[Table 1]
steel grade C Si Mn Cr Ni W N Mo Corrosion resistance index
Invention Example 1 0.015 1.5 0.8 20 10 0.4 0.16 0 7.05
Invention Example 2 0.017 1.1 0.9 23 12 0.5 0.21 0 7.85
Invention example 3 0.017 1.5 0.8 23 12 0.1 0.23 0 7.85
Invention Example 4 0.018 1.5 0.5 23 12 0.2 0.23 0 8.75
Invention Example 5 0.060 1.0 0.5 22 11 0.5 0.15 0 7.2
Invention example 6 0.060 2.0 0.5 22 11 0.5 0.15 0 8.7
Invention Example 7 0.019 1.4 1.0 22 12 0.8 0.21 0 8.9
Comparative Example 1 0.050 0.4 1.2 18 8 0 0.03 0 0.6
Comparative Example 2 0.013 0.5 1.3 20.5 10 0 0.17 0 3.6
Comparative Example 3 0.025 0.4 0.9 21.3 10.3 0 0.21 0.6 5.13
Comparative Example 4 0.020 1.5 0.5 26 15 0 0.25 0 8.85
Comparative Example 5 0.021 1.5 0.5 28 16 0 0.32 0 10.45
316L 0.015 0.6 0.7 17.5 10 0 0.04 2.0 2.05
[93]
The physical properties of each invention example and comparative example steel having the alloy composition of Table 1 are evaluated below.
[94]
(1) Corrosion resistance evaluation
[95]
In order to evaluate the corrosion resistance of each invention example and comparative example steel having the alloy composition shown in Table 1, an environment similar to the fuel cell driving environment was created. Specifically, the prepared environment is 80° C., in a mixed solution of sulfuric acid (H 2 SO 4 ) at pH 3 and hydrofluoric acid (HF) at pH 5.3 by applying a 0.6V potential compared to the calomel electrode for 24 hours.
[96]
Equation (2) of Table 2 is a value derived by substituting the sum of Si and W contents (wt%) in the passivation film and the sum of Si and W contents (wt%) in the base material into the following formula (2).
[97]
(2)
[98]
In addition, in order to evaluate the corrosion resistance, the corrosion current density and the amount of dissolved metal measured in the above composition environment are shown in Table 2 below. 'Relative dissolved amount compared to 316L' in Table 2 is based on the amount of dissolved metal in the environment in which 316L stainless steel is composed above, the relative amount of dissolved metal of each invention example and comparative example compared to the amount of dissolved metal of 316L stainless steel represents the ratio.
[99]
[Table 2]
division Equation (2) Corrosion current density (㎂/cm 2 ) Relative dissolution volume compared to 316L Passivation film thickness (nm)
Invention Example 1 2.6 0.044 0.70 4.8
Invention Example 2 2.1 0.026 0.52 5.5
Invention example 3 2.2 0.037 0.66 5.1
Invention Example 4 2.2 0.035 0.63 5.0
Invention Example 5 2.0 0.026 0.59 5.7
Invention example 6 3.1 0.022 0.45 4.1
Invention Example 7 2.6 0.017 0.39 4.6
Comparative Example 1 1.3 0.186 2.45 7.3
Comparative Example 2 1.5 0.075 1.04 6.6
Comparative Example 3 1.3 0.071 0.92 6.2
Comparative Example 4 2.2 0.036 0.54 3.8
Comparative Example 5 2.3 0.037 0.56 3.4
316L 1.5 0.107 1.00 6.7
[100]
Referring to Table 2, it can be seen that the corrosion current density of Inventive Examples 1 to 7 compared to the corrosion current density of the 316L stainless steel was reduced by 50% or more, and the relative dissolution amount compared to the 316L stainless steel was 0.7 or less. From these results, it can be seen that the stainless steel satisfying the alloy composition of the present invention, formulas (1) and (2), can secure superior corrosion resistance than 316L stainless steel even in a fuel cell driving environment, which is a strong acidic environment.
[101]
Meanwhile, referring to Table 2, in Comparative Examples 1 to 3, the Si content was less than 1.0% by weight or W was not added, unlike the Si and W content ranges limited by the present invention. In addition, in Comparative Examples 1 to 3, the corrosion resistance index derived from the formula (1) was also less than 7, and the value of the formula (2) was less than 2.0, so the corrosion resistance was inferior.
[102]
Specifically, in Comparative Examples 1 to 3, it can be seen that the corrosion current density is rather increased or decreased by 30% or less compared to the 316L stainless steel, so that the degree of decrease is small. In addition, in Comparative Examples 1 to 3, referring to Table 2, it can be seen that the relative amount of dissolution compared to 316L stainless steel is rather increased or similar. In addition, in Comparative Examples 1 to 3, the value of Formula (2) is less than 2.0, as a result, the oxide of the passivation film is not dense, it can be seen that the thickness of the passivation film is grown to exceed 6nm.
[103]
From these results, it can be seen that when the steel of Comparative Examples 1 to 3 is used as a separator in a fuel cell driving environment, which is a strong acidic environment, corrosion occurs, and thus it is not suitable for a fuel cell separator.
[104]
In particular, Comparative Example 3 contains Mo like 316L stainless steel, but the corrosion current density and the relative dissolution amount compared to the 316L stainless steel were higher than those of the present invention. From this, it can be seen that it is difficult to secure corrosion resistance by the addition of Mo.
[105]
The above result can be visually confirmed from FIGS. 1 and 2 .
[106]
1 is a graph showing the correlation between the corrosion resistance index, the corrosion current density, and the relative dissolution amount compared to 316L stainless steel. Figure 1a is a corrosion resistance index and corrosion current density, Figure 1b shows the correlation between the corrosion resistance index and the relative dissolution amount compared to 316L stainless steel.
[107]
Referring to the dotted line region of FIG. 1A , when the corrosion resistance index is 7 or more, it can be seen that the corrosion current density is 0.05 μA/cm 2 or less. In addition, referring to the dotted line region of FIG. 1B , if the corrosion resistance index is 7 or more, it can be seen that the relative dissolution amount is 0.7 or less compared to the 316L stainless steel. Referring to FIGS. 1A and 1B , it can be seen that it is preferable to control the corrosion resistance index value of Equation (1) to 7 or more in order to secure excellent corrosion resistance.
[108]
2 is a graph showing the correlation between the value of Equation (2) and the corrosion current density, the relative amount of dissolution compared to 316L stainless steel, and the thickness of the passivation film. Figure 2a shows the value of Equation (2) and the corrosion current density, Figure 2b shows the value of Equation (2) and the relative amount of dissolution compared to 316L stainless steel, and Figure 2c shows the correlation between the value of Equation (2) and the passivation film thickness.
[109]
Referring to the dotted line region of FIG. 2A , when the value of Equation (2) is 2 or more, it can be seen that the corrosion current density is 0.05 μA/cm 2 or less. In addition, referring to the dotted line region of FIG. 2B , it can be seen that when the value of Equation (2) is 2 or more, the relative dissolution amount compared to 316L stainless steel is 0.7 or less. In addition, referring to the dotted line region in FIG. 2C , it can be seen that the passivation film thickness is 6 nm or less when the value of Equation (2) is 2 or more. Referring to FIGS. 2A, 2B, and 2C , it can be seen that the value of Equation (2) is preferably controlled to be 2 or more in order to secure excellent corrosion resistance.
[110]
On the other hand, referring to Table 2, in Comparative Examples 4 and 5, the corrosion current density was decreased compared to 316L stainless steel, and it can be confirmed that the relative dissolved amount was decreased compared to 316L stainless steel.
[111]
However, in order to be used as a material for a thin fuel cell separator, processability must be sufficiently secured. From this point of view, it can be seen that Comparative Examples 4 and 5, despite having excellent corrosion resistance, did not sufficiently secure processability as shown in the results of '(2) Processability evaluation' below, and thus were not suitable as fuel cell separator materials. Hereinafter, the related content will be described in detail.
[112]
(2) Processability evaluation
[113]
After hot rolling to evaluate the hot workability of each invention example and example, whether or not surface defects occurred is shown in Table 3 below.
[114]
The value of Equation (3) of Table 3 was derived by substituting the content (wt%) of the alloying element of Table 1 in Equation (3) below.
[115]
(3) (Cr + Mo + 1.5Si + 0.75W)/(Ni + 0.5Mn + 20N + 24.5C)
[116]
Surface defects were determined according to whether surface defects such as cracks occurred on the edge or surface of the hot-rolled stainless steel.
[117]
[Table 3]
division Equation (3) Whether or not there is a surface defect (○/ X)
Invention Example 1 1.61 X
Invention Example 2 1.47 X
Invention example 3 1.45 X
Invention Example 4 1.47 X
Invention Example 5 1.52 X
Invention example 6 1.61 X
Invention Example 7 1.44 X
Comparative Example 1 1.78 X
Comparative Example 2 1.48 X
Comparative Example 3 1.45 X
Comparative Example 4 1.36 ○
Comparative Example 5 1.31 ○
316L 1.77 X
[118]
Referring to Table 3, it can be seen that, as a result of the value of Equation (3) of 1.4 or more, surface defects do not occur after hot rolling, and thus, it can be seen that the hot workability is excellent.
[119]
On the other hand, Comparative Examples 1 to 3 and STS 316L steel had excellent hot workability because surface defects did not occur after hot rolling with a value of 1.4 or higher in Equation (3), but as evaluated in '(1) Corrosion resistance evaluation', fuel cell Corrosion resistance as a separator was not sufficiently secured.
[120]
In Comparative Examples 4 and 5, excessive Cr content was added to secure sufficient corrosion resistance as described in '(1) Corrosion resistance evaluation', but as a result of excessive addition of Cr, excessive amounts of Ni and N were added to stabilize the austenite phase. . As a result, the value of Equation (3) according to the increase of the Ni equivalent was 1.4 or less, and the delta ferrite fraction was too small, so that the hot workability was inferior, and thus surface defects occurred.
[121]
From these results, it can be seen that in order to sufficiently secure not only corrosion resistance but also hot workability, the addition of excessive Cr and N should be suppressed, and it is desirable to control the alloy composition range and formula (3) value limited by the present invention. .
[122]
From the above examples and their evaluation results, stainless steel controlled to satisfy the alloy composition and values ​​of Equation (1), Equation (2) and Equation (3) defined in the present invention can secure both excellent corrosion resistance and workability. Therefore, it can be seen that it is suitable for a polymer fuel cell separator.
[123]
In the foregoing, exemplary embodiments of the present invention have been described, but the present invention is not limited thereto, and those of ordinary skill in the art will not depart from the concept and scope of the following claims. It will be appreciated that various modifications and variations are possible.
Industrial Applicability
[124]
The stainless steel for a polymer fuel cell separator according to the present invention can be applied to a separator of a polymer electrolyte fuel cell, and the like.
Claims
[Claim 1]
By weight%, C: 0.09% or less, Si: 1.0% or more and less than 2.5%, Mn: 1.0% or less (excluding 0), S: 0.003% or less, Cr: 20 to 23%, Ni: 9 to 13%, W : 1.0% or less (excluding 0), N: 0.10 to 0.25%, remainder Fe and other unavoidable impurities, and the corrosion resistance index of the following formula (1) is 7 or more austenitic stainless steel for polymer fuel cell separator: (1) ) 3*W + 1.5*Si + 0.1*Cr + 20*N - 2*Mn (in formula (1), W, Si, Cr, N, and Mn mean the content (wt%) of each element) .
[Claim 2]
The austenitic stainless steel for a polymer fuel cell separator according to claim 1, wherein the value of the following formula (2) is 2.0 or more. (2)
[Claim 3]
The austenitic stainless steel for a polymer fuel cell separator according to claim 1, wherein the value of the following formula (3) is 1.4 to 2.0: (3) (Cr + Mo + 1.5Si + 0.75W) / (Ni + 0.5Mn + 20N + 24.5C) (In the above formula (3), Cr, Mo, Si, W, Ni, Mn, N, and C mean the content (% by weight) of each element).
[Claim 4]
The austenitic stainless steel for a polymer fuel cell separator according to claim 1, wherein the passivation film has a thickness of 6 nm or less.
[Claim 5]
The austenitic stainless steel for a polymer fuel cell separator according to claim 1, wherein the elongation is 40% or more.
[Claim 6]
According to claim 1, 80 ℃, in a mixed solution of sulfuric acid (H 2 SO 4 ) of pH 3 and hydrofluoric acid (HF) of pH 5.3, the corrosion current density measured by applying a 0.6V potential compared to the calomel electrode for 24 hours is 0.05 Austenitic stainless steel for polymer fuel cell separators of ㎂/cm 2 or less.
[Claim 7]
According to claim 1, 80 ℃, in a mixed solution of sulfuric acid (H 2 SO 4 ) of pH 3 and hydrofluoric acid (HF) of pH 5.3, 0.6V potential compared to the calomel electrode was applied for 24 hours, and the amount of dissolved metal was 316L stainless steel Austenitic stainless steel for polymer fuel cell separator with a contrast of 0.7 or less.