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(11) | EP 1 236 813 A2 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Members coated with composite oxide coatings for preventing the permeation of hydrogen isotopes and a process for producing such members |
| (57) The surface of a chemical densified coating formed on the surface of a stainless
steel substrate is coated or sprayed with an aqueous solution based on chromic acid
and a material capable of forming an amorphous inorganic substance upon sintering
or the chemical densified coating is dipped in the aqueous solution and recovered;
the chemical densified coating is then sintered by heating at 250 - 750°C so that
the pores and cracks in the chemical densified coating are filled with the fine particles
of a composite of chromium oxide and an amorphous inorganic material and that the
surface of the chemical densified coating is covered with a layer of such fine particles. |
BACKGROUND OF THE INVENTION
[0001] This invention relates to members coated with composite oxide coatings having high ability to prevent the permeation of hydrogen isotopes and a process for producing such members.
[0002] Continuous coatings of a SiO2-Cr2O3 system that are chemically densified by chromium oxide have very high hardness, so they have been and are still used as wear-resistant coatings on various mechanical structural members.
[0003] Techniques of forming hard coatings of fine Cr2O3 particles on stainless steel substrates by chemical conversion from CrO3 are described in Japanese Patent Laid-Open Nos. 59-9171, 61-52374, 63-126682 and 63-317680. These chemical densified coatings, however, are shown to have tiny pores and cracks in their interior upon examination by optical microscopy and it is by no means rare that most of those pores and cracks are deep enough to reach the surfaces of stainless steel substrates. If the chemical densified coatings are used in environments where hydrogen isotopes exist, those isotopes penetrate the coatings into the stainless steel substrate, through which they will pass to leak into the external environment. Hence, the coatings are unsuitable for use in applications where the permeation of hydrogen isotopes has to be prevented.
SUMMARY OF THE INVENTION
[0004] An object, therefore, of the invention is to establish a technology that can eliminate the aforementioned defect of the prior art SiO2-Cr2O3 based chemical densified coatings by transforming them to composite oxide coatings having high ability to prevent the permeation of hydrogen isotopes.
[0005] The following are the basic approaches taken by the present inventors to attain the stated object of the invention.
(1) Tiny pores and cracks in the chemical densified coating are filled with the fine particles of a chromium oxide-amorphous inorganic material having high ability to prevent the permeation of hydrogen isotopes, thereby suppressing the permeation of hydrogen isotopes through the coating.
(2) In addition to filling the tiny pores and cracks in the chemical densified coating with the fine particles of a chromium oxide-amorphous inorganic material, a thin coating of the inorganic composite material is formed on the surface of the coating, thereby ensuring that the coating is protected against encroaching hydrogen isotopes more positively.
(3) An aqueous solution of chromic acid and an aqueous solution containing a material which forms an amorphous inorganic substance upon sintering are applied to the chemical densified coating and the resulting coating is heated to produce a solidified precipitate that fills the tiny pores and cracks in the coating; this process has been found to be capable of preventing the permeation of hydrogen isotopes.
[0006] In order to suppress the encroaching of hydrogen isotopes into the SiO2-Cr2O3 based chemical densified coatings and thereby attain the stated object of the invention, the present inventors treated the tiny pores and cracks in the coatings by one of the following methods.
(1) The surface of the chemical densified coating is directly coated or sprayed with an aqueous solution containing a material capable of forming an amorphous inorganic substance upon sintering or the coating is dipped in the same aqueous solution and recovered therefrom, followed by sintering in either case.
(2) Process (1) is repeated several times so that the tiny pores and cracks in the chemical modified coating are filled with the amorphous inorganic substance and that a thin coating of surplus amorphous inorganic substance is also formed on the outermost surface of the chemical densified coating.
[0007] Various aspects of the invention are described below.
(1) The invention first relates to a composite oxide coating coated member which is formed not only by providing a chemical densified coating on the surface of a metallic substrate such as a stainless steel substrate but also by filling the pores and cracks in the coating with an amorphous inorganic substance.
(2) The invention also relates to a composite oxide coating coated member which is formed not only by providing a chemical densified coating on the surface of a metallic substrate such as a stainless steel substrate but also by filling the pores and cracks in the coating with an amorphous inorganic substance; in addition, a layer of an amorphous inorganic substance is formed on the surface of the chemical densified coating.
(3) The amorphous inorganic substance with which the pores and cracks in the chemical densified coating are to be filled is formed by applying or spraying an aqueous solution containing a material capable of forming said amorphous inorganic substance upon sintering or dipping the chemical densified coating in the same aqueous solution, and heating and sintering the coating; preferably, the amorphous inorganic substance as the pore or crack filler is based on silicate glass, borosilicate glass or phosphate glass.
(4) To make the coated member of the invention, the chemical densified coating is first provided on the surface of a metallic substrate such as a stainless steel substrate and the surface of said coating is coated or sprayed with an aqueous solution containing a material capable of forming an amorphous inorganic substance upon sintering or the chemical densified coating is dipped in the same aqueous solution; in either case, the coating is heated and sintered so that the pores and cracks in the coating are filled with the amorphous inorganic substance to form a composite oxide coating for preventing the permeation of hydrogen isotopes.
(5) The coated member of the invention can also be produced by a process comprising the steps of providing the chemical densified coating on the surface of a metallic substrate such as a stainless steel substrate, coating or spraying the surface of said coating with an aqueous solution containing a material capable of forming an amorphous inorganic substance upon sintering or, alternatively, dipping the chemical densified coating in the same aqueous solution, and heating and sintering the coating so that the pores and cracks in the coating are filled with the amorphous inorganic substance to form a composite oxide coating for preventing the permeation of hydrogen isotopes and that a layer of said amorphous inorganic substance is also formed to cover the surface of the chemical densified coating.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008]
Fig. 1 is a flowsheet for forming a chemical densified coating on the surface of a stainless steel substrate;
Fig. 2 is a partial section of a composite oxide coating coated member that is protected against the permeation of hydrogen isotopes and which comprises a stainless steel substrate, a chemical densified coating formed on the surface of the substrate, and a chromium oxide-amorphous inorganic composite coating formed on the chemical densified coating using a processing fluid based on an aqueous solution containing chromic acid and a material capable of forming an amorphous inorganic substance upon sintering;
Fig. 3 is a partial schematic view of a deuterium permeation test apparatus comprising a lined SUS 316L tube through which argon gas containing a specified concentration of deuterium is flowed such that the deuterium permeating the tube wall is directed to a mass spectrometer together with pure argon gas flowing outside the tube for measurement of its concentration; and
Fig. 4 is a graph showing the results of deuterium permeation tests at 600°C by time profiles of the change in the volume of deuterium permeating the tube wall after argon gas containing a specified concentration of deuterium was flowed through the tube.
DETAILED DESCRIPTION OF THE INVENTION
[0009] We now describe a specific mechanism of action by which the composite oxide coating of the invention exhibits high ability to prevent the permeation of hydrogen isotopes.
(1) A SiO2-Cr2O3 based chemical densified coating that covers the surface of a stainless steel substrate is formed by the process flow shown in Fig. 1. The chemical densified coating formed by this process is very hard and highly wear-resistant; on the other hand, the coating contains tiny pores and cracks, some of which are deep enough to reach the substrate.
[0010] In the presence of these tiny pores and cracks, hydrogen isotopes will permeate the chemical densified coating to reach the stainless steel substrate. They may even permeate the stainless steel substrate to flow out into the external environment. Having this defect, the chemical densified coating is not suitable for the purpose of preventing the permeation of hydrogen isotopes.
[0011] In the present invention, the surface of the chemical densified coating is directly coated or sprayed with a mixture of chromic acid in aqueous solution and an aqueous solution containing a material capable of forming an amorphous inorganic substance upon sintering, or alternatively, the chemical densified coating is dipped in the mixture of such aqueous solutions; the coating is then heated and sintered so that the amorphous inorganic substance gets into the pores and cracks in the coating to fill them up. The aqueous solution containing a material capable of forming the amorphous inorganic substance upon sintering is one that contains a borate compound, a silicate compound or a phosphate compound.
[0012] According to the experiments conducted by the present inventors, the temperature of heating the chemical densified coating depends on what mixture of aqueous solutions it is treated with. If it is treated by coating with a mixture of chromic acid in aqueous solution and an aqueous solution containing chromic acid and a borate compound, the chemical densified coating is heated at 550 - 720°C for 0.5 - 2 hours. If the treatment is by coating with a mixture of chromic acid in aqueous solution and an aqueous solution of a phosphate compound, the heating temperature is at 250 - 750°C. In either case, the water in the aqueous solutions evaporates whereas the fine particles of chromium oxide which remain as heating residue not only fill up the pores and cracks in the chemical densified coating but they are also deposited on the surface of the coating.
[0013] To be more specific, chromic acid turns into Cr2O3 (chromium oxide) via an intermediate and the concomitant aqueous solution of a borate compound, a silicate compound or a phosphate compound releases water to become an amorphous inorganic substance. The Cr2O3 precipitated from this aqueous solution comprises very small particles that are hard and have high resistance to wear and corrosion. Each of the phosphate, borate and silicate compounds is amorphous and partly glassy; they are precipitated in the pores and cracks in the chemical densified coating to seal them up. They are also effective in enhancing the adhesion between the constituent particles in the coating.
[0014] In the invention, the pathways for the encroaching hydrogen isotopes are effectively blocked by the precipitation of fine Cr2O3 particles and enhancement of the strength of their binding by the amorphous inorganic substance resulting from the borate, silicate or phosphate compound.
[0015] As already mentioned, the chemical densified coating is coated or sprayed with a processing fluid based on chromic acid and an aqueous solution of the borate, silicate or phosphate compound or, alternatively, the coating is dipped in the processing fluid and the thus treated coating is then heated. If this process is repeated several times, the pores and cracks in the coating are not only filled with Cr2O3 and the amorphous inorganic substance, it is also covered with a layer of excess composite of chromium oxide and the amorphous inorganic compound. The coating effectively maintains the ability to prevent permeation of hydrogen isotopes. The following Table 1 shows the conditions for applying the composite oxide coating capable of preventing the permeation of hydrogen isotopes.
| Composite oxide coating for preventing the permeation of hydrogen isotopes | ||
| Major ingredients | Sinter | |
| Slurry | slurry of SiO2 particles plus CrO3 in aqueous solution | yes |
| Impregnant | CrO3 in aqueous solution | |
| CrO3 in aqueous solution plus aqueous solution of borate, silicate or phosphate compound | yes | |
[0016] Fig. 2 is a partial section of a member coated with a composite oxide coating to prevent the permeation of hydrogen isotopes in accordance with the invention. In Fig. 2, the numeral 1 designates a stainless steel substrate, 2 is a chemical densified coating, 3 is a pore (or crack) that is open through the chemical densified coating, 4 refers to fine particles of chromium oxide and amorphous inorganic substance in composite form that fill up the pore or crack 3, and 5 is a layer of such fine particles of chromium oxide and amorphous inorganic substance in composite form that covers the surface of the coating 2. As is clear from Fig. 2, the fine particles which comprise the layer 5 of chromium oxide and amorphous inorganic substance in composite form completely fill up the pores and cracks in the chemical densified coating 2, so the layer 5 exhibits very strong adhesion.
Example 1
[0017] In Example 1, a chemical densified coating was applied to the surface of an SS 400 steel substrate; the pores and cracks in the coating were filled and covered with a layer of the fine particles of chromium oxide and amorphous inorganic substance in composite form to produce a composite oxide coating capable of preventing the permeation of hydrogen isotopes. The corrosion resistance of the resulting coating coated member was evaluated by the CASS test according to JIS H8502.
(1) Invention composite oxide coating capable of preventing the permeation of hydrogen isotopes: The surface of the substrate (SS 400 steel) was coated with a chemical densified coating, dipped in a 1:1 mixture of aqueous solutions of 55% chromic acid and 15% boric acid, recovered and heated at 750°C for 1 hour. This process was conducted six times to produce a composite oxide coating 50 µm thick.
(2) Chemical densified coating as a comparison: The chemical densified coating (50 µm) was used as such.
(3) The CASS test: The results of the test are shown in Table 2. Obviously, the chemical densified coating which received no treatment according to the invention was sensitive to moisture from the environment, which permeated through cracks in the coating to corrode the substrate carbon steel, forming an extensive layer of red rust on the coating.
[0018] In contrast, there was no visible formation of red rust on the composite oxide coating of the invention which was protected against the permeation of hydrogen isotopes. The pores and cracks in the chemical densified coating may have been sufficiently filled up with the layer of fine particles of chromium oxide and borosilicate glass in composite form which effectively suppressed the encroaching of moisture, rendering the coating highly resistant to corrosion.
| Type of coating | Thickness | CASS test |
| Chemical densified coating | 50 µm | 24 hours later, red rust due to the substrate corrosion became exposed on the coating surface |
| Composite oxide coating capable of preventing the permeation of isotopes substrate | 50 µm | even 240 hours later, no red rust due to the substrate corrosion became exposed on the coating surface |
Example 2
[0019] In this example, the composite oxide coating capable of preventing the permeation of hydrogen isotopes which was produced by the treatment of the invention was evaluated for permeability of deuterium.
(1) Invention composite oxide coating capable of preventing the permeation of hydrogen isotopes: The inner surface of a substrate (SS 316L steel) in tubular form (30.0 mm in outside diameter by 0.9 mm in thickness) was coated with a chemical densified coating, dipped in an aqueous solution based on CrO3 and phosphoric acid, recovered and heated at 450°C for 1 hour. This process was conducted six times to prepare a test tube having a composite oxide coating formed in a thickness of 50 µm on the inner surface.
(2) Chemical densified coating as a comparison: The chemical densified coating (50 µm) was used as such.
(3) Deuterium permeation test: Using a test apparatus of the type shown in Fig. 3, the permeation of deuterium through the coating was measured at 600°C and the reduction in permeation was determined as the ratio of the deuterium permeation after application of the composite oxide coating to the deuterium permeation before application of the composite oxide coating. The test apparatus comprised the sample tube 1, a measuring Hastelloy X tube 2 (50.0 mm in inside diameter by 3.0 mm in thickness), a mass spectrometer 3 for measuring the permeation of deuterium through the test tube wall, a line 6 for supplying deuterium-containing argon gas into the test tube, a line 7 for discharging the deuterium-containing argon gas out of the test tube, a line 8 for supplying pure argon gas into the measuring tube so that the deuterium permeating the test tube wall can be directed to the mass spectrometer, and a line 9 for discharging the pure argon out of the measuring tube. In Fig. 3, the deuterium in deuterium-containing argon gas is indicated by dots 4 and the permeation of deuterium through the test tube wall is indicated by solid arrows 5.
(4) Results of the deuterium permeation test: The test results are shown in Fig. 4 and Table 3. The chemical densified coating as a comparison achieved a reduction in deuterium permeation of about 1:50 at 600°C whereas the composite oxide coating of the invention achieved a reduction in deuterium permeation of only about 1:1000 at 600°C.
| Coating type (with thickness) | Reduction in deuterium permeation at 600°C |
| Chemical densified coating (50 µm) | ca. 1/50 |
| Composite oxide coating capable of preventing the permeation of hydrogen isotopes (50 µm) | ca. 1/1000 |
[0020] According to the invention, members having a chemical densified coating involving pores and cracks are coated or otherwise treated with chromic acid in aqueous solution and an aqueous solution containing a material capable of forming an amorphous inorganic substance and subsequently heated to generate the fine particles of chromium oxide and amorphous inorganic substance in composite form, which are filled into the pores and cracks in the chemical densified coating and optionaly applied to cover its surface in a specified thickness. The chemical densified coating has heretofore been found unsatisfactory in its ability to prevent the permeation of hydrogen isotopes but this problem is solved by the invention and the composite oxide coating of the invention finds a wider scope of applications in mechanical structures and members that need to be protected against gas permeation.
coating or spraying the surface of the chemical densified coating with an aqueous solution containing chromic acid and a material capable of forming an amorphous inorganic substance by sintering or dipping the chemical densified coating in said aqueous solution; and
sintering the chemical densified coating by heating at 250 - 750°C.
applying a chemical densified coating to the surface of a stainless steel substrate;
coating or spraying the surface of the chemical densified coating with an aqueous solution containing chromic acid and a material capable of forming an amorphous inorganic substance by sintering or dipping the chemical densified coating in said aqueous solution; and
holding the chemical densified coating at 250 - 750°C so that the pores and cracks in the chemical densified coating are filled with the fine particles of a composite of chromium oxide and an amorphous inorganic material to form a composite oxide coating.
applying a chemical densified coating to the surface of a stainless steel substrate;
coating or spraying the surface of the chemical densified coating with an aqueous solution containing chromic acid and a material capable of forming an amorphous inorganic substance by sintering or dipping the chemical densified coating in said aqueous solution; and
sintering the chemical densified coating by heating at 250 - 750°C so that the pores and cracks in the chemical densified coating are filled with the fine particles of a composite of chromium oxide and an amorphous inorganic material and that at the same time, the surface of the chemical densified coating is coated with a layer of said composite of chromium oxide and an amorphous inorganic material.