JPH08188853A - Ferritic stainless steel excellent in fused carbonate corrosion resistance - Google Patents

Abstract

PURPOSE: To produce an inexpensive ferritic stainless steel excellent in fused carbonate corrosion resistance. CONSTITUTION: This steel is a ferritic stainless steel excellent in fused carbonate corrosion resistance, which has a composition consisting of, by weight, <=0.08% C, 0.01-2% Si, <=1% Mn, B-22% Cr, <=2% Mo, 0.1-3% Ti, and the balance essentially Fe with inevitable impurities and further containing, if necessary, 0.1-2% Cu.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a ferritic stainless steel having excellent resistance to molten carbonate corrosion, which is used in molten carbonate fuel cells and the like.

[0002]

2. Description of the Related Art Due to environmental problems such as exhaustion of petroleum resources and air pollution in the 21st century, fuel cells that can use coal reformed gas as a next-generation power supply source have come into the spotlight. There are various types of fuel cells, such as phosphoric acid type, molten carbonate type, and solid electrolyte type, depending on the electrolyte that generates electromotive force, and the operating temperature and power generation efficiency are different. Of these, L
A molten carbonate fuel cell using NG and coal reformed gas has been attracting attention as a large-scale centralized power source by a combined power generation with a distributed power source and a gas turbine. Currently, development of 100kW class stack has been completed, and 1MW class plant has been started.

However, in order to realize such an increase in size and put it into practical use as a commercial plant, it is important that the device be stable and reliable for a long time, and that the cost be reduced.

One of the major problems at present is the corrosion of metallic materials by molten carbonate which is an electrolyte. In particular,
Separator materials and collector plates that come in contact with molten carbonate at a high temperature of 600 to 700 ℃ are exposed to severe corrosive environments, which is a factor in the deterioration of battery life.

Conventional separator material is SUS316L (Fe-17Cr-
12Ni-2.5Mo) and SUS310S (Fe-25Cr-20Ni) have been used, but their corrosion resistance is not sufficient.

As a measure for improving the corrosion resistance to molten carbonate, Japanese Patent Publication No. 4-37154 discloses adding 1 to 2% of Al, and Japanese Patent Laid-Open Publication No. 63-190143 discloses adding 0.1 to 0.9% of Al.
It is disclosed that Y is added in combination.
In 252750 and Japanese Patent Laid-Open No. 1-252757, the Si content is 0.2%.
Stainless steel and Ni with Al added and regulated below
Base alloys are disclosed.

Japanese Unexamined Patent Publication (Kokai) No. 64-68449 discloses stainless steel in which the amounts of Fe, Cr, and Ni are specified, and Japanese Unexamined Patent Publication (Kokai) No. 4-247852 discloses the amounts of Cr, Ni, and Mn.

However, these stainless steels and N
Although the i-based alloy has improved corrosion resistance to molten carbonate as compared with conventional materials such as SUS316L, it cannot be said to be sufficient in consideration of long-term durability.

Further, the above steel is Fe-C containing Ni.
Most of them are r-Ni type stainless steels, and they cannot be said to be satisfactory in consideration of cost and adverse effects of Ni on molten carbonate corrosion as described later.

[0010]

As described above, the metal material used in the molten carbonate fuel cell is not a decisive corrosion resistant material because the corrosion mechanism in the molten carbonate is not fully understood. The current situation is that no development has been carried out.

An object of the present invention is to provide a stainless steel which is inexpensive and has excellent resistance to molten carbonate corrosion.

[0012]

[Means for Solving the Problems] In developing the material of stainless steel which has excellent corrosion resistance in a molten carbonate environment and is inexpensive, the present inventors It is cheap because it is not contained in the steel, and Ni in the steel dissolves in the atmosphere on the cathode side, which can be a factor for deterioration of corrosion resistance for a long time. From the viewpoint that it is promising as a constituent member of a carbonate fuel cell,
Fe-Cr system stainless steel was examined.

[0013] As a result of systematic investigations and researches focusing on the structure of the scales of the materials produced by changing the chemical composition of inexpensive Fe-Cr stainless steel in a wide range, the following findings were obtained. I arrived.

A) When the scale formed on the material surface is an oxide mainly composed of Cr, it easily reacts with Li ₂ CO ₃ which is an electrolyte to form LiCrO ₂ , so that not only the corrosion resistance is deteriorated but also the loss of the electrolyte. To invite.

B) By adding an appropriate amount of Ti to an alloy that produces a Cr-based oxide, an oxide is formed on the outer layer side of the Cr-based oxide. Inside is TiO _{2 as an} internal oxide layer Cr ₂ O ₃
/ It exists at the bare metal interface and enhances scale adhesion.

C) An outer layer scale made of Fe-Cr oxide and Ti oxide is formed on the outside.

These oxides have low solubility in molten carbonate and exhibit excellent corrosion resistance against molten carbonate corrosion.

D) The Ti-based oxide in the outer layer scale increases,
When the Ti-based oxide covering the outermost surface of the scale increases, the ductility of molten carbonate is suppressed. Here, the ductility is
When molten carbonate adheres to the alloy surface, it refers to the phenomenon that the molten carbonate spreads to the peripheral area while eroding the surface.

E) Corrosion resistance is further improved by adding Cu in addition to Ti.

The present invention has been completed on the basis of these findings, and the gist of the invention is "in weight%,
C: 0.08% or less, Si: 0.01 to 2%, Mn: 1% or less, Cr:
8 to 22%, Mo: 2% or less, Ti: 0.1 to 3%, optionally Cu: 0.1 to 2%, and the balance substantially consisting of Fe and inevitable impurities. Ferritic stainless steel with excellent carbonate corrosion ”.

[0021]

The function and the function of limiting the component composition in the present invention will be described below.

C: If the content of C in steel exceeds 0.08% and becomes large, it adversely affects the corrosion resistance of the weld and the toughness of the weld and the base metal, so the upper limit is made 0.08%.

Si: Si is an effective deoxidizing element during melting. Further, in a molten carbonate atmosphere, SiO ₂ is formed at the Cr ₂ O ₃ / metal interface to suppress corrosion. However, if its content is less than 0.01%, it has no effect. On the other hand, if it exceeds 2%, the toughness and formability at room temperature are significantly reduced, so the upper limit was made 2%.

Like Si, Mn: Mn is a useful deoxidizing component, but if its content is large, it forms S and MnS in steel and causes deterioration of corrosion resistance, so the upper limit is made 1%.

Cr: Cr is one of the important elements in the present invention. In the cathode side environment, it has the effect of improving the molten carbonate corrosion resistance, and its effect is exhibited at 8% or more. However, if the Cr content is too high, the surface layer becomes a single oxide scale mainly composed of Cr, and the L
It reacts with i ₂ CO ₃ to form LiCrO _2, which easily elutes in the electrolyte, deteriorates the stability of the film, and causes electrolyte loss, so Cr is limited to 22% or less. Preferably 9
~ 20%.

Mo: Mo is a solid solution strengthening element, especially at 600 ° C.
There is an effect of increasing the strength.

However, addition of a large amount has an adverse effect on molten carbonate corrosion and further deteriorates workability, so the upper limit is made 2%.

Ti: Ti is the most important element in the present invention. If 0.1% or more is contained, Ti-based oxide is generated on the outer layer side of the Cr ₂ O ₃ scale, and Fe-based oxide (Fe ₃ O ₄ or
It becomes a mixed form with LiFeO ₂ ). These oxides have low solubility in molten carbonate and improve the corrosion resistance of steel. Further, when the content of Ti is 0.1% or more, a TiO ₂ internal oxide is formed at the bare metal interface on the inner layer side of Cr ₂ O ₃ . Cr ₂ together with the internal oxide to suppress diffusion of oxygen to the bare metal side
It has the effect of significantly improving the adhesion at the O ₃ / metal interface.
Furthermore, when the Ti content is increased,
When the amount of Ti-based oxides produced increases and the Ti content exceeds 0.5%, the effect of preventing the spreadability of molten carbonate on the steel surface becomes remarkable, which suppresses corrosion and prevents the formation of molten carbonate as an electrolyte. Control loss. However, Ti content is 3%
Even if it exceeds the above, not only further effect on molten carbonate corrosion is not recognized, but also the flaws on the steel surface become noticeable and the workability deteriorates significantly, so the upper limit is made 3%. It is preferably 0.5 to 2.5%.

By limiting the chemical components as described above, it is possible to obtain a stainless steel having excellent corrosion resistance in molten carbonate, but the effect is further enhanced by adding the appropriate amount of the following elements. .

Cu: Cu is an electrochemically noble element and improves corrosion resistance, so it is preferable to contain Cu if necessary. Particularly in a molten carbonate atmosphere, the addition of Ti in combination makes the corrosion resistance noticeably improved. This effect is exhibited when 0.1% or more is added. However, addition of a large amount forms an intermetallic compound in the steel and conversely deteriorates corrosion resistance.
2% It is preferably 0.3 to 1.5%.

[0031]

EXAMPLES Next, the present invention will be described in detail with reference to examples.

Chemical composition shown in Table 1 (% by weight, balance Fe)
Inventive steels (No. 1 to 12) and comparative steels (No. 13 to 20) were melted in a high frequency electric furnace (vacuum melting). The molten 25 kg steel ingot was forged and hot rolled to obtain a hot rolled steel sheet with a thickness of 5 mm.

[0033]

[Table 1]

The hot-rolled sheet thus obtained was annealed and pickled, and then cold-rolled to 1 mm to obtain 970-1050 ° C (SUS310
S, SUS316L was soft-annealed and pickled at 1150 ℃. A test piece having a thickness of 1 mm, a width of 20 mm and a length of 80 mm was cut out from the obtained cold-rolled sheet and subjected to a corrosion resistance test.

The corrosion resistance test was carried out by applying Li ₂ CO ₃ : K ₂ CO ₃ = 6 to the test piece.
Apply a mixed salt of 2:38 (molar ratio) and gas composition C at 650 ℃
It was kept for a predetermined time in an atmosphere gas of O ₂ : air = 30: 70.

The corrosion resistance was evaluated by the weight reduction amount obtained by subtracting the weight of the test piece after descaling from the weight of the test piece before heating.

Further, in order to investigate the ductility of molten carbonate,
The test piece was hung with a wire and held in the mixed salt for a predetermined period of time while being semi-immersed. The test was performed under the same conditions as the corrosion resistance test in both temperature and atmosphere. The spread of salt is evaluated by visual observation of the state of the salt rising from the gas-liquid interface of the test piece to the non-immersed part after the test, and the smaller the rise of salt to the non-immersed part is, the more the ductility is suppressed, and the longer the time is. It was judged to be excellent in corrosion of molten carbonate and loss of electrolyte.

FIG. 1 shows that after the molten carbonate was applied to the test piece,
This figure shows the relationship between the weight reduction and Ti content of each test material held at 50 ° C for 200 hours.

It can be seen that the corrosion weight loss is reduced and the corrosion resistance is excellent by adding 0.1% or more of Ti as in the case of steel Nos. 1 to 6 of the present invention. In addition, No. 7 ~ 9 with the composite addition of Cu
Has a tendency to have a smaller corrosion weight loss.

On the other hand, the commercially available SUS316L (No. 14) and the comparative steel No. 19 have a very large corrosion weight loss. Comparative steel No.
No. 17 has a small amount of corrosion loss due to the addition of Cu, but its corrosion resistance is not satisfactory as compared with the steel of the present invention containing 0.1% or more of Ti. In addition, Comparative Steel No. 20 has a large amount of Mo added despite the addition of Ti, and thus has a large corrosion weight loss.

From the above, it can be said that the addition of Ti is effective against the corrosion of molten carbonate, and the addition of Cu has little effect by itself, and the effect can be exhibited by adding it in combination with Ti.

Table 2 shows the results of evaluating the corrosion weight loss of the steels of the present invention and the comparative steels by the corrosion resistance test and the state of rise of the molten carbonate to the non-immersed part in the 650 ° C. × 200 hours half immersion test.

[0043]

[Table 2]

The steel of the present invention has a low ductility of molten carbonate,
In particular, when Ti exceeds 0.5%, almost no increase in the non-immersed part is observed, and it can be seen that the addition of Ti has a large effect of preventing the salt infiltration. Thus, the low ductility of the molten carbonate not only prevents corrosion, but also suppresses the loss of the electrolyte, which can be expected to significantly improve the battery life.

[0045]

As described above, the steel of the present invention is SUS310S used as a metal material for conventional molten carbonate fuel cells.
Compared to steel and SUS316L steel, it is a stainless steel that has excellent resistance to molten carbonate corrosion. In addition, the ductility of molten carbonate is small and the loss of electrolyte is suppressed.

Further, since the steel of the present invention is a ferritic stainless steel composed of Fe-Cr, it has been conventionally used Fe.
-It is cheaper than Cr-Ni stainless steel. When the steel of the present invention is used as a collector plate or a separator plate of a molten carbonate fuel cell, the life of the cell can be significantly improved.

[Brief description of drawings]

FIG. 1 is a diagram showing the relationship between the corrosion weight loss and the Ti content in steel after the molten carbonate was applied to a test piece and held at 650 ° C. for 200 hours.

Claims (2)

[Claims] 1. By weight%, C: 0.08% or less, Si: 0.01 to 2
%, Mn: 1% or less, Cr: 8 to 22%, Mo: 2% or less, Ti:
A ferritic stainless steel with excellent resistance to molten carbonate corrosion, characterized by containing 0.1 to 3% and the balance consisting essentially of Fe and inevitable impurities. 2. By weight%, C: 0.08% or less, Si: 0.01 to 2
%, Mn: 1% or less, Cr: 8 to 22%, Mo: 2% or less, Ti:
A ferritic stainless steel having excellent resistance to molten carbonate corrosion, containing 0.1 to 3% and further containing Cu: 0.1 to 2%, and the balance being substantially Fe and unavoidable impurities.