JP3065771B2 - Metal surface treatment method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a surface treatment method using an aqueous acid solution for obtaining a ferritic stainless steel having excellent oxidation resistance at high temperatures.

[0002]

2. Description of the Related Art Fe-Cr-Al based sintered alloys are known to be applied to electric heaters or catalytic converter carriers because of their excellent oxidation resistance. Alternatively, an improvement in high-temperature oxidation resistance has been required to improve the life. For example, the exhaust system of automobiles and the like is exposed to exhaust gas containing nitrogen oxides, carbon monoxide, hydrocarbons, etc. at high temperatures, while the conventional Fe-Cr-Al-based sintered honeycomb is exposed to such high temperatures. Has insufficient oxidation resistance, and improvement in high-temperature oxidation resistance has been desired.

[0003] Accordingly, the present inventors have conducted research on the high-temperature oxidation resistance of Fe-Cr-Al-based sintered honeycomb, and as a result, the particles adhering to the honeycomb wall surface have become the starting point of local abnormal oxidation. It was found that the oxidation resistance was reduced.
It was considered that the main component of the particles was alumina, and the cutting powder and the like adhering during molding were oxidized during drying or firing.

Although it is desired to remove such adhered particles, pickling is a conventional surface treatment method used for this purpose. As pickling of ferritic stainless steel, pickling for the purpose of descaling rolled stainless steel is known. In this pickling, a high-concentration acid is used to improve descaling.
For example, in a stainless steel handbook (supervised by Masayoshi Hasegawa, Nikkan Kogyo Shimbun, 1973), in a mixed acid obtained by adding hydrochloric acid of 13N or more or nitric acid of 0.8N to 2.6N to another acid.
It describes that pickling of Fe-Cr and Fe-Cr-Ni stainless steels is described.

[0005] Japanese Patent Publication No. 3-50837 discloses pickling of ferritic stainless steel with 6.3 to 14.6 N nitric acid for descaling. Further, Nisshin Steel Technical Report No. 62, P38-52, (1990) discloses a mixed acid of 0.25 to 2.5N hydrofluoric acid and 1.1N nitric acid, 4.7N sulfuric acid, 4.7N sulfuric acid and 0N. Pickling of SUS430 or SUS304 stainless steel with a mixed acid of 2.5N hydrofluoric acid, a mixed acid of 2.7N hydrochloric acid, a mixed acid of 1.6N nitric acid and 8.2N hydrochloric acid, etc. has been reported.

[0006]

The relatively high concentration of acid has hitherto been used for pickling ferritic stainless steels. However, as a result of the inventor's intense research on a surface treatment method for obtaining a metal having excellent resistance to high-temperature oxidation, as a result of applying such treatment to ferritic stainless steel, the resistance to high-temperature oxidation deteriorates conversely. It turned out to be. Therefore, the present invention has been completed to solve this problem that has been overlooked and to provide a surface treatment method for obtaining a metal having excellent oxidation resistance at high temperatures.

[0007]

That is, according to the present invention,
A surface treatment method for obtaining a metal having excellent resistance to high-temperature oxidation, characterized by maintaining a ferritic stainless steel containing Al in an acid aqueous solution comprising hydrochloric acid and / or nitric acid having a concentration of less than 0.01 N, Is provided. Further, according to the present invention, a ferritic stainless steel containing Al
There is provided a surface treatment method for obtaining a metal excellent in high-temperature oxidation resistance, which is maintained in an acid aqueous solution containing sulfuric acid having a concentration of less than 01N.

[0008]

The present inventors held a sintered honeycomb of ferritic stainless steel in an aqueous acid solution consisting of a predetermined concentration of nitric acid, hydrochloric acid or sulfuric acid, as shown in the examples described later, The effect of high temperature on oxidation resistance was investigated. For comparison, the effect on the high-temperature oxidation resistance of a sintered honeycomb of steel not subjected to the acid treatment was also examined.
As a result, it was found that the acid treatment with a dilute acid aqueous solution was more excellent in the high-temperature oxidation resistance of the sample with any of nitric acid, hydrochloric acid and sulfuric acid. That is, for example, acid treatment with a 0.01N aqueous acid solution is more excellent in the high-temperature oxidation resistance of a sample than 0.1N, and acid treatment with a 0.001N aqueous acid solution is more resistant to the high-temperature oxidation of the sample than 0.01N. It was found that the oxidizing property was further excellent. Here, the high-temperature oxidation resistance of the sample not subjected to the acid treatment substantially corresponds to that obtained by the acid treatment of a 0.01 N aqueous solution of nitric acid and hydrochloric acid, and at the same time, 0.001%.
A result was obtained which was almost equivalent to that obtained by acid treatment of a 1N aqueous sulfuric acid solution.

In other words, focusing only on the high-temperature oxidation resistance, it is understood that the acid aqueous solution used for the acid treatment is desirably diluted. Also, from the comparative example, 0.01N for nitric acid and hydrochloric acid
It was found that the acid treatment with a higher concentration aqueous acid solution reduced the high temperature oxidation resistance of the steel. Similarly, 0.001
Acid treatment with a sulfuric acid aqueous solution having a concentration higher than N, on the contrary, lowers the high-temperature oxidation resistance of the steel. In other words, the acid concentration conventionally used for pickling is much higher than these critical concentrations, and it is from these results that the high temperature oxidation resistance of steel cannot be deteriorated by pickling under such conditions. it is obvious. If the acid concentration of the treatment liquid is too high, it is considered that the high-temperature oxidation resistance is deteriorated due to the roughened surface.

Conversely, if the acid concentration in the acid treatment is too low, the particles attached to the steel surface are not removed, and the effect of improving the high-temperature oxidation resistance is small. Therefore, in the case of hydrochloric acid and nitric acid, a preferable acid concentration is 0.0001N or more and less than 0.01N.

If the temperature of the treatment liquid is too high, the corrosion of the acid is so strong that the surface of the steel becomes rough, making it difficult to selectively reduce only the particles attached to the surface. Deteriorates. Conversely, if the temperature of the treatment liquid is too low, the particles attached to the surface cannot be removed efficiently, and the high-temperature oxidation resistance does not improve much. Empirically, the temperature of the processing solution is preferably from 20C to 40C.

[0012] If the acid treatment time of the stainless steel is too long, not only does the cost increase, but also the high-temperature oxidation resistance deteriorates. Conversely, if the acid treatment time is too short, the particles adhered to the surface will not be removed, and unevenness tends to occur. In consideration of these, the processing time is suitably 30 minutes to 2 hours.

The surface treatment with a low-concentration aqueous solution of nitric acid, hydrochloric acid, or sulfuric acid provides some surface modification effects such as removal of particles adhering to the surface such as Al 2 O 3, reduction of surface precipitates, and passivation. It is presumed that the high temperature oxidation resistance was improved.

The acid treatment method of the present invention is superior to the conventional pickling method not only in terms of resistance to high-temperature oxidation but also in terms of cost and treatment of waste liquid. In other words, the acid treatment method of the present invention uses an acid aqueous solution that is much more dilute than conventional acid washing, so that the cost is reduced, and the acid concentration of the waste liquid is also much dilute like the treatment liquid, so that the treatment of the waste liquid is also easy Because it becomes.

Furthermore, even when the acid treatment method of the present invention is compared with mechanical treatment such as shot blasting, it is excellent in high-temperature oxidation resistance, cost, applicability to a honeycomb shape, and treatment problems such as waste liquid. Shot blasting is known to leave shots on the steel, and it is known that mechanical pretreatment causes work hardening on the steel surface layer.However, such formation of a surface altered layer is not preferable, and the oxidation resistance of the steel is Worsen. However, in the acid treatment method of the present invention, the surface treatment can be performed without such an adverse effect. Also, acid treatment with a dilute aqueous acid solution would be less costly than mechanical treatment such as shot blasting. Further, in view of the applicability of the surface treatment to the honeycomb shape, the acid treatment method of the present invention is more excellent in this point because mechanical treatment such as shot blasting cannot be performed on the cell surface of the honeycomb. Further, it is considered that the treatment with the waste dilute acid aqueous solution is simpler than the treatment with dust by shot blasting or the like.

Finally, when the acid treatment method of the present invention is compared with plating, it is clear that acid treatment with a dilute aqueous acid solution is lower in cost than plating by electrolysis or chemical reduction or the like. It is also easier to treat the waste dilute acid aqueous solution than to treat the electrolytic solution. Therefore, compared to conventional surface treatment methods such as conventional pickling, mechanical treatment such as shot blasting, plating, etc., the acid of the present invention has problems in terms of oxidation resistance, cost, applicability to honeycomb shapes, treatment of waste liquid, and the like. The treatment method is considered to be excellent.

[0017]

EXAMPLES Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited to these Examples. (Example) Pure Fe powder, Pure Cr powder, Fe-50% by weight
Al alloy powder, Fe-20 wt% B alloy powder, Fe-7
5 wt% Si alloy powder was mixed with Fe-20Cr-6Al-2.
5Si-0.05B was prepared and mixed with 5% by weight of methyl cellulose as an organic binder, and 2% by weight of oleic acid was added as a lubricant and an antioxidant for the metal powder. After that, 20% by weight of water
The addition was mixed. After the obtained mixture was degassed by a clay kneader, a honeycomb structure having a diameter of 100 mm, a rib thickness of 200 μm, and a cell density of 300 cpi ² was formed through an extrusion mold. Next, after drying this honeycomb structure, it was sintered at 1300 ° C. for 4 hours in a hydrogen atmosphere in a molybdenum case to obtain a sintered honeycomb body. The sintering shrinkage was 17% and the open porosity was 5%. Also, as a result of chemical analysis of the sintered body, Fe: 72.5 wt%, Cr: 19.4 wt%, Al: 5.2 wt%, Si: 2.
3 wt%, B: 0.04%, C: 0.08 wt%, O: 0.12 wt%.

A 1-cm-sided cube cut out from the sintered honeycomb body thus obtained was used as a sample for acid treatment. The aqueous acid solution was prepared from reagent-grade hydrochloric acid, nitric acid, or sulfuric acid and ion-exchanged water. The acid treatment was performed as follows. After holding each sample in an acid aqueous solution at the acid type, concentration, solution temperature and holding time shown in Table 1, washing with ethanol,
Dry at 110 ° C. for 3-12 hours. In addition, the weight loss (wt%) in Table 1 means a decrease in the sample weight due to the acid treatment. In each condition, three samples were used, and the characteristic value indicates the average value. Next, the dried sample was held in an electric furnace at 1190 ° C. for 52 hours in an air atmosphere to increase the weight per unit surface area (mg / c before and after heating).
m ² ) and the dimensional increase (%) were measured. The results are shown in Tables 1, 2, 3 and 4.

[0019]

[Table 1]

[0020]

[Table 2]

[0021]

[Table 3]

[0022]

[Table 4]

As is clear from Tables 1, 2, 3, and 4, the method of the present invention using an acid of a predetermined type and concentration has a higher concentration than an acid of a concentration outside the range. It can be seen that the oxidation increase and dimensional increase rate of the sample are extremely low.
FIG. 18 and 19 are scanning electron microscope (SEM) photographs showing the particle structure of the surface of the sintered body of Example 18 (Example 18). 7 (Comparative Example 7), 15 is a scanning electron microscope (SEM) photograph showing the particle structure of the sintered body surface of Comparative Example 15 (Comparative Example 15).

[0024]

As described above, according to the metal surface treatment method of the present invention, the ferritic stainless steel containing Al is held in a low-concentration aqueous acid solution, so that the pickling with a high-concentration acid can be performed. Thus, a metal excellent in high-temperature oxidation resistance can be obtained without suffering deterioration of high-temperature oxidation resistance in the conventional method. Further, as compared with the conventional high-concentration aqueous acid solution, the dilute aqueous acid solution used in the present invention facilitates the treatment of waste liquid and increases the degree of freedom of the material of the treatment tank.

[Brief description of the drawings]

FIG. 18 is a scanning electron microscope (SEM) photograph showing the particle structure of the surface of a sintered body of Example 18 (Example 18).

FIG. 7 is a scanning electron microscope (SEM) photograph showing the particle structure of the sintered body surface of Comparative Example 7 (Comparative Example 7).

FIG. 15 is a scanning electron microscope (SEM) photograph showing the particle structure of the sintered body surface of Comparative Example 15 (Comparative Example 15).

──────────────────────────────────────────────────続 き Continuation of front page (56) References JP-A-3-193885 (JP, A) JP-A-56-33482 (JP, A) JP-A-59-83783 (JP, A) (58) Field (Int. Cl. ⁷ , DB name) C23C 22/00-22/86

Claims (4)

(57) [Claims] 1. A ferritic stainless steel containing Al,
A method for treating a metal surface, wherein the metal is kept in an aqueous acid solution comprising hydrochloric acid and / or nitric acid having a concentration of less than 0.01 N. 2. The surface treatment method according to claim 1, wherein the kind of the acid is hydrochloric acid. 3. The method according to claim 1, wherein the acid is nitric acid. 4. A ferritic stainless steel containing Al,
A method for treating a metal surface, wherein the method is maintained in an aqueous acid solution containing sulfuric acid having a concentration of less than 0.001 N.