JP2010106305A - Stainless steel for cell composing member and method for producing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an inexpensive stainless steel which has corrosion resistance in a cell environment and also does not deteriorate the performance of a cell due to the elution of metal ions, as a metallic material composing a collector and a separator composing a methanol reformed fuel cell or the like, and to provide a method for producing the stainless steel. <P>SOLUTION: The stainless steel for a cell composing member includes, by mass, 16.0 to 32.0% Cr, 0.5 to 2.0% Mo, ≤0.015% C, ≤0.5% Si and ≤2.0% Mn, and the balance Fe with inevitable impurities, and is characterized in that the surface layer of the stainless steel is provided with a passive film of ≤4 nm which satisfies, by atomic%, Cr/(Cr+Fe)>0.2 and also CrOOH>Cr<SB>2</SB>O<SB>3</SB>. The method for producing the stainless steel uses immersion treatment to a solution containing organic acid and nitric acid or electrolytic treatment where potential of 700 to 900 mV, SCE is applied in a solution containing sulfuric acid. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to stainless steel for battery constituent members, and more specifically to stainless steel constituting a current collector and separator for a fuel cell.

  A fuel cell continuously supplies fuel and oxygen or air from the outside and reacts electrochemically to extract electric energy, and is classified according to its operating temperature, type of fuel used, application, and the like. Depending on the type of electrolyte used, it can be classified into five types: solid oxide fuel cells, molten carbonate fuel cells, phosphoric acid fuel cells, polymer electrolyte fuel cells, and alkaline aqueous fuel cells. It is common.

These fuel cells use hydrogen gas generated from methane or the like as a fuel. Recently, a direct methanol fuel cell (hereinafter referred to as DMFC) using an aqueous methanol solution as a fuel is known. Some DFMCs use a solid polymer electrolyte membrane as an electrolyte, as in a polymer electrolyte fuel cell using hydrogen gas as a fuel. In such a DMFC, a stack structure in which a plurality of unit cells each having an air electrode (oxygen electrode) and a fuel electrode (hydrogen electrode) are stacked on both sides of a solid polymer electrolyte membrane to increase the electromotive force according to the purpose. Is common. A reaction such as the following formula (1) occurs in the fuel electrode of the DMFC, and a reaction such as the following formula (2) occurs in the air electrode.
CH ₃ OH + H ₂ O → 6H ⁺ + CO ₂ + 6e ⁻ (1)
3 / 2O ₂ + 6e ⁻ + 6H ⁺ → 3H ₂ O (2)

  In this DMFC, as the methanol is oxidized, carbon monoxide, formaldehyde, formic acid, and the like are by-produced at the anode, resulting in an acidic environment. Further, in the case of DMFC, catalyst particles made of platinum or a platinum alloy are bound to a conductive porous substrate as an electrode catalyst layer using a proton conductive polymer as a binder. When S is present in the binder or the catalyst layer, sulfuric acid is generated. Therefore, when a metal is used for the current collector, excellent acid resistance is required. In addition, since potential is applied to these electrode members, high potential and corrosion resistance in an acidic environment are desired. The elution of metal ions from the metal material is a major obstacle to battery performance.

  When a noble metal having acid resistance, such as Ti, is used for these metal materials, it is disadvantageous in cost, and even Ti may not have acid resistance depending on the use situation.

  The present inventors examined the applicability of stainless steel having corrosion resistance, particularly acid resistance, as these battery members. These problems can be solved by using stainless steel in which the base material is specified and a Cr hydroxide-rich passive film is formed on the surface of the base material.

In order to achieve the object of the present invention, in mass%, Cr: 16.0 to 32.0%, Mo: 0.5 to 2.0%, C: 0.015% or less, Si: 0.5% Hereinafter, Mn: 2.0% or less of stainless steel with the balance being Fe and inevitable impurities, or Ni: 2.0% or less, Cu: 2.0% or less 1 or 2 of Nb: 0.1-0.8%, Ti: 0.05-0.4%, Al: 0.01-0.5%, B: 0.3% or less In stainless steel containing more than seeds, a passive film of 4 nm or less exists on the surface layer, and in the passive film, Cr / (Cr + Fe)> 0.2 and CrOOH> Cr ₂ 0 ₃ in atomic% Battery components with passive film where the equation holds, especially direct methanol fuel Provide a useful cell components in the battery.

  Furthermore, in order to obtain those passive films, it has been found that it is effective to perform an immersion treatment in a solution containing an organic acid or nitric acid or an electrolytic treatment in which a potential of SCE of 700 to 900 mV is applied with a solution containing sulfuric acid. It was.

  ADVANTAGE OF THE INVENTION According to this invention, it is possible to provide the stainless steel for battery members which is cheap and does not impair battery performance by deterioration of corrosion resistance, and its manufacturing method.

  Stainless steel can maintain excellent corrosion resistance due to the passive film on the surface. However, when exposed to an acid environment, the passive film on the surface of the stainless steel cannot be passivated and causes active dissolution. Their passivating ability is influenced by the constituents and varies depending on the composition of the surface passivation film, its ratio and thickness. Those having the composition and passive film of the present invention can have appropriate corrosion resistance as a battery component.

To obtain elution resistance in acid environment, than the thickness 4nm the surface layer of stainless steel, Cr / (Cr + Fe) composition in atomic%> 0.2, and CrOOH> _Cr 2 _{0 3} relationship holds passivating It is necessary to produce a film.
The passive film on the stainless steel surface consists of amorphous Cr oxide and hydrate. Since the higher the Cr composition ratio, the less likely the corrosion due to elution of Fe is, and the better the corrosion resistance, Cr / (Cr + Fe)> 0.2 is set as the composition for maintaining the corrosion resistance in the battery environment. In addition, Cr in the passive film exists in the form of an oxide or hydrate, but CrOOH has a stronger binding force between Cr and oxygen and water, and the higher the composition ratio of CrOOH, the better the elution resistance. To CrOOH> Cr ₂ O ₃ . Furthermore, since the corrosion resistance of the passive film is thinner and finer, the thickness of the passive film is set to 4 nm or less.

  The passive film is naturally formed on the surface of the stainless steel even in a normal corrosive environment, but it is thick and porous film is easily formed when oxygen in the air is taken in. Therefore, a dense film having a high Cr composition ratio can be obtained by pickling in an oxidizing environment where the acid or stainless steel that can selectively dissolve Fe in the passive film can be maintained in a passive state. In annealing under a reducing atmosphere such as bright annealing, although the Cr composition ratio is high, the film is rough and has many defects, and sufficient corrosion resistance cannot be obtained in an acid environment such as the inside of the battery. In addition, when the acid used for pickling is a non-oxidizing acid such as hydrochloric acid and has no repassivation property, the corrosion resistance is inferior because of a low Cr film.

The operation of the present invention will be described in detail below.
Cr is the most important element for maintaining the corrosion resistance as stainless steel in the battery environment. In order to provide corrosion resistance, the content of 16.0% is necessary. However, if the Cr content is increased, the toughness and workability are reduced, so the upper limit of the Cr content is 32.0%.

  C forms carbides, which serve as recrystallization nuclei for the randomization of recrystallized ferrite in the final annealing. However, C is an element that increases the strength after cold rolling annealing, and if it is too high, the ductility is lowered.

  Si is usually used for the purpose of deoxidation, but its solid solution strengthening ability is high, and if its content is too large, the material is hardened and the ductility is lowered, so the content was made 0.5% or less.

  Mn is an austenite-forming element, has a low solid solution strengthening ability and has little adverse effect on the material. However, if the content is large, manufacturability deteriorates, such as generation of Mn fumes during melting, so the component range was set to 2.0% or less.

  Mo is an element effective for improving the corrosion resistance. In particular, it is effective for improving acid resistance, and its action is recognized at 0.5% or more. However, excessive addition causes a reduction in hot workability and an increase in cost due to solid solution strengthening at a high temperature and delay of dynamic recrystallization, so the content is set to 0.5 to 2.0%.

  Ni is an austenite-forming element and is an element effective for improving the corrosion resistance in an acid environment. However, addition over 2.0% causes hardening and cost increase, so the upper limit is set to 2.0%. It was.

  Cu is effective for improving the corrosion resistance of stainless steel, and is also an effective element for reducing surface contact resistance as a battery material. However, excessive addition reduces hot workability and corrosion resistance, so it was made 2.0% or less.

  Nb is an element that fixes C and N and improves impact resistance and secondary workability, and has an effect of improving workability for processing battery cases and the like. However, if it is added too much, the material hardens and adversely affects the workability. Further, since the recrystallization temperature is raised, the content is made 0.10 to 0.80%.

  Ti is an element that fixes C and N and improves workability and corrosion resistance. However, if it is added too much, surface defects such as Ti inclusions that cause cracks in drawing work exist, so 0.05% to 0.40% when added.

  Al is an effective element for deoxidation and oxidation resistance, but excessive addition causes surface defects, so when added, the content was made 0.01 to 0.50%.

  B is an alloy component that has the effect of fixing N and improving the corrosion resistance and workability, and is added as necessary. In order to exert the above action, it is desirable to add 0.0005% or more. However, if added in excess, it causes a decrease in maturing workability and weldability, so 0.3% or less.

  Although the following elements are not described in the claims, they may contain the following elements in addition to P and S which are inevitably included.

V, Zr: Workability improvement by the effect of precipitating solute C as carbide, Zr is a useful element from the viewpoint of workability and toughness improvement by capturing oxygen in steel as an oxide. However, since the manufacturability decreases when added in a large amount, the appropriate content of V and Zr is 0.01 to 0.30%.
In addition to these, Ca, Mg, Co, REM, and the like may be contained in the scrap, which is a raw material, during melting, but do not affect the corrosion resistance and workability unless they are contained in large amounts.

The passive film formed on the surface of stainless steel is mainly composed of Fe and Cr oxides and hydroxides, and in the surface layer area at a depth of about 10 mm from the outermost surface of the film, there are many Fe oxides and hydroxides. On the deeper substrate side, there are more oxides and hydroxides of Cr. The passive film produced under normal conditions has a very large proportion of Fe oxide and hydroxide in the surface layer region, and the Fe concentration exceeds 70 atomic%. If the Fe concentration is high, the trace of the eluted Fe becomes a film defect. As a result, the dissolution of metals and sulfide inclusions with high melting ability proceeds from the base stainless steel. In order to prevent the elution of Fe and the occurrence of corrosion defects, in the present invention, the composition ratio of the passive film is Cr / (Cr + Fe)> 0.2 and the thickness of CrOOH> Cr ₂ O ₃ in the passive film is 4 nm. It was as follows. The following methods are effective for generating the film.

Pickling treatment dipping treatment using organic acid-containing solution: The organic acid preferentially dissolves the oxides and hydroxides of Fe contained in the passive film, and the composition ratio of the passive film is Cr / It can be modified to a passive film where (Cr + Fe)> 0.2 and CrOOH> Cr ₂ O ₃ Fe. Usable organic acids are citric acid and maleic acid. There are tartaric acid, lactic acid, malic acid, succinic acid, etc. The target passive film can be obtained by immersing stainless steel in a solution added with one or more organic acids at a total concentration of 100 ppm or more for 30 minutes or more. . The treatment temperature is preferably 60 ° C. or higher, which promotes the reaction.

  Nitric acid immersion, sulfuric acid electrolysis: Pickling stainless steel in a solution containing nitric acid with excellent ability to oxidize Cr retains the stainless steel in the passive region, increases the Cr concentration in the passive film, Dissolution of Fe from inclusions such as CaS and MnS and a passive film is promoted. The concentration of nitric acid is preferably 10% or higher, and an immersion treatment of 60 ° C. or higher is desirable to increase the reaction rate. The reaction time must be 30 seconds or longer, and heating for 10 minutes or longer is desirable to obtain a more stable film.

In sulfuric acid electrolysis, it is necessary to apply SCE at a potential of 700 to 900 mV so that the stainless steel can be maintained in the passive region. If the potential is lower than that, the Cr composition ratio cannot be increased, and if the potential is higher than that, Cr will cause active dissolution, and a passive film having the desired composition and thickness cannot be obtained. An acid such as nitric acid may be mixed in the sulfuric acid solution. It is preferable to electrolyze stainless steel immersed in a sulfuric acid solution for 2 seconds or more.
The concentration and temperature of the sulfuric acid solution are the same as in the nitric acid immersion.

A stainless steel plate having the composition shown in Table 1 was vacuum-melted in a laboratory, and a 1.0 mmt cold-rolled annealed plate was produced by rolling and annealing. Steel No. in Table 1 A to I are steels of the present invention whose chemical component values are within the scope of the present invention.
These stainless steels were subjected to organic pickling, nitric acid pickling and sulfuric acid electrolytic pickling to control the passive film.
In the organic pickling, 200 ppm citric acid was used for immersion treatment at 60 ° C. for 1 hour. In the nitric acid pickling, 50% nitric acid was used for immersion for 1 hour at 60 ° C. In the sulfuric acid pickling, electrolytic treatment was performed using 10% sulfuric acid at 60 ° C. and 800 mV for 1 minute under SCE. As a comparison, a pickling treatment with hydrochloric acid, a sulfuric acid electrolytic treatment at 0 mV, SCE and a treatment material subjected to annealing at 400 ° C. in the atmosphere were prepared.

The composition analysis of the passive film was performed by ESCA. The ratio of CrOOH> Cr ₂ O ₃ was calculated from the metal concentration of the outermost layer by Cr / (Fe + Cr) and AES-EELS analysis. The thickness of the passive film depends on ESCA. Calculated from the half value of the peak.

These test materials were immersed in methanol with 1% sulfuric acid at 60 ° C. for one month, and the concentrations of Fe and Cr as the eluted ions in the methanol after the test were measured by the ICP method. Furthermore, the current value after 3 days from the initial current value when a simple DMFC battery stack using two test materials and coated with a catalyst for DMFC was produced and the battery was formed in a 60 ° C. environment for 3 days. Was calculated as efficiency (ratio) to evaluate battery performance.
The evaluation results are shown in Table 2.

Steel No. which is an example of the present invention. In the test materials in which A to I were subjected to organic acid immersion, nitric acid immersion and sulfuric acid electrolytic treatment at 800 mV, SCE, the surface layer was 4 nm or less, Cr / (Cr + Fe)> 0.2 in atomic%, and CrOOH> Cr ₂ A passive film satisfying the relationship of O ₃ was formed, and elution of both Fe and Cr was not observed in the methanol immersion test.

  Further, the current value after the simple battery test was 90%, and the battery performance was obtained. However, the steel no. J did not obtain the target film even after the nitric acid immersion treatment. As a result, Fe and Cr were eluted after the methanol immersion test, and the battery efficiency was only 30%. Moreover, steel No. which removes the Mo component of the present invention. Although the intended passive film was obtained for H, Fe and Cr were eluted after the methanol immersion test, and the battery efficiency was 40%. This suggests that Mo improves dissolution resistance in this environment. Furthermore, steel No. which is a steel component of the present invention. When H was subjected to pickling treatment with hydrochloric acid, sulfuric acid electrolysis treatment at 0 mV, SCE, and annealing treatment at 400 ° C. in the atmosphere, the intended passive film was not formed, and as a result, after the methanol immersion test Fe and Cr were eluted, and the battery efficiency was only 50% or less. It was suggested that the treatment of the present invention is effective for maintaining the corrosion resistance in this environment.

  By using the stainless steel of the present invention for a current collector or separator for a fuel cell such as DMFC, it is possible to provide a fuel cell that is inexpensive and does not impair cell performance due to deterioration of corrosion resistance.

Claims (7)

In mass%, Cr: 16.0-32.0%, Mo: 0.5-2.0%, C: 0.015% or less, Si: 0.5% or less, Mn: 2.0% or less Stainless steel containing the balance Fe and inevitable impurities, the surface layer has a passive film of 4 nm or less, Cr / (Cr + Fe)> 0.2 in atomic% in the passive film, and Stainless steel for battery constituent members having a passive film satisfying CrOOH> Cr ₂ 0 ₃ .   Furthermore, it contains 1 type or 2 types of Ni: 2.0% or less and Cu: 2.0% or less, The stainless steel for battery components of Claim 1 characterized by the above-mentioned.   Furthermore, Nb: 0.10 to 0.80%, Ti: 0.05 to 0.40%, Al: 0.01 to 0.50%, or B: one selected from 0.3% or less The stainless steel for battery constituent members according to claim 1, comprising two or more kinds.   The stainless steel for battery components according to any one of claims 1 to 3, wherein the battery is a direct methanol fuel cell.   In mass%, Cr: 16.0-32.0%, Mo: 0.5-2.0%, C: 0.015% or less, Si: 0.5% or less, Mn: 2.0% or less Optionally, Ni: 2.0% or less, Cu: 2.0% or less, Nb: 0.10-0.80%, Ti: 0.05-0.40%, Al: 0.01-0 50%, or B: Stainless steel containing one or more selected from 0.3% or less, the balance being Fe and unavoidable impurities is not treated by dipping in a solution containing an organic acid. The method for producing stainless steel for battery constituent members according to any one of claims 1 to 4, wherein Cr is concentrated in the dynamic film.   In mass%, Cr: 16.0-32.0%, Mo: 0.5-2.0%, C: 0.015% or less, Si: 0.5% or less, Mn: 2.0% or less Optionally, Ni: 2.0% or less, Cu: 2.0% or less, Nb: 0.10-0.80%, Ti: 0.05-0.40%, Al: 0.01-0 .50%, or B: Passive by immersion treatment of a stainless steel containing one or more selected from 0.3% or less, with the balance being Fe and unavoidable impurities in a solution containing nitric acid The method for producing stainless steel for battery constituent members according to any one of claims 1 to 4, wherein Cr is concentrated in the film.   In mass%, Cr: 16.0-32.0%, Mo: 0.5-2.0%, C: 0.015% or less, Si: 0.5% or less, Mn: 2.0% or less Optionally, Ni: 2.0% or less, Cu: 2.0% or less, Nb: 0.10-0.80%, Ti: 0.05-0.40%, Al: 0.01-0 50%, or B: Stainless steel containing one or more selected from 0.3% or less, the balance being Fe and inevitable impurities, 700 to 900 mV, SCE in a solution containing sulfuric acid The method for producing stainless steel for battery constituent members according to any one of claims 1 to 4, wherein Cr is concentrated in the passive film by electrolytic treatment to which a potential is applied.