JPS604906B2 - How to reduce hydrogen diffusion through metals - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Technical Field of the Invention The present invention relates to coatings for metals, particularly coatings for reducing the diffusion of hydrogen (hydrogen being used broadly herein and also including deuterium and tritium isotopes). Regarding.

Although there are many operations in which it is desirable to minimize the diffusion of hydrogen through metals (e.g., to reduce hydrogen sulfidation or to reduce leakage in hydrogen-oxygen fuel cells), the most critical problem is probably in the reactor. Diffusion of radioactive tritium through metal walls in a primary fluid-containing device.

This is particularly a problem in fast neutron breeder reactors and fusion reactors. PRIOR ART One method of reducing hydrogen diffusion consists in the use of piping containing significant amounts of metals or with layers of metals that have a high resistance to hydrogen diffusion.

For example, aluminum was found to have a diffusion rate three orders of magnitude lower. However, such arrangements have problems with manufacturing, hot sodium compatibility, and/or cost. Although in some cases organic coatings can be used to slightly reduce hydrogen diffusion, in many cases (including nuclear reactors) such organic coatings are generally limited by the operating conditions of the equipment (e.g. metal temperature or radiation). ) cannot be tolerated.

Another method of limiting hydrogen diffusion is to apply a sulfuric material on the metal, dry it, and then heat the applied metal in a furnace for about 700 m
Calcinate at ~100,000 °C to remove temporary binder;
It's about melting pongee medicine.

Such methods are relatively expensive, and large objects such as nuclear reactor piping are extremely difficult to fire in a furnace. Additionally, handling and temperature cycling tend to cause such coatings to crack, rendering them useless. Accordingly, it is a principal object of the present invention to provide a coating for metal surfaces that safely inhibits the spread of metal.

SUMMARY OF THE INVENTION With these objects in mind, the present invention provides for applying a coating of a phosphate plate-containing phosphate glass-forming material to at least one surface of a metal, heating said coating to at least 35,000 ℃ to convert said coating to phosphoric acid. A method of reducing the diffusion of hydrogen through a metal is characterized by converting it into a salt glass.

It has been found that helicates containing P04 phosphate groups can be applied to metal surfaces and converted to phosphate glasses that inhibit hydrogen expansion at moderate temperatures.

Although these coatings can withstand heating to slightly higher temperatures, the glasses are produced in the range of 350 qo to 600 qo. In the case of nuclear reactor piping, coatings can be applied by applying a phosphoric acid solution onto the reactor metal piping, equipping it with insulation, and using normal heat from reactor operations to form a glass. . The phosphate glass coating is preferably operated in such a range that it does not melt, but softens and glass cracking from temperature cycling is generally avoided. If the phosphate glass has to be operated at temperatures above 60,000 ℃, it is preferable to use additives to increase its melting point. Embodiments of the Invention Preferred embodiments of the invention will be described below.

Although most gases do not diffuse through metals to any significant extent, hydrogen diffusion is extremely rapid.

Diffusion increases with temperature and decreases with atomic size (ie tritium diffusion is slightly slower than deuterium). Because tritium is radioactive, even the relative leakage rate of tritium is important, and it was thus decided to test various coatings on stainless steel commonly used in such reactor piping. Ta. Early experiments were conducted at operating temperatures (the design operating temperature for fast neutron breeder reactor piping in this particular case was analyzed to be approximately 550 pm).
was carried out using coatings of chromium and manganese (both separately and together) to obtain a material that creates a protective oxide coating on the surface.

Packcoating has been considered, but pack coating of these chromium and manganese cometal layers requires special furnaces.
Various methods of applying manganese and chromium combination coatings have been tested to avoid the use of special furnaces.

Tests using mixtures of Na2Cr207, MnS04 and ratio P04 have shown to be surprisingly effective. The hydrogen permeation rate at 550 oo decreased by almost a factor of 20 within 6 hours during testing using the mixture described above.

Post-test inspection of the tube revealed that the metal surface was covered with a thin, almost transparent, glassy layer. The next test was done with MnS04 and melon P04 (without chromium) and then finally with melon P04 alone. The last coating was found to be the most effective hydrogen barrier of all. That is, tests using manganese, chromium and phosphoric acid gave good results, but it is clear that it is not because of the manganese or chromium, but because of the phosphoric acid. It is believed that the phosphate glasses were formed by reaction of the phosphate radicals with metal oxides or metals from the surface to be coated.

This exhibits a self-cleaning action and is probably responsible for the extremely good adhesion of phosphate glasses to metal surfaces. It was also found that the glass surface softens slightly at operating temperatures.

The thin layer of glass and its softening appears to help avoid cracking of the phosphate glass during temperature cycling operations. Tests have shown that phosphoric acid (chromium or magnesium) not only achieves a low diffusion rate, but also reaches this low rate almost immediately when heated. Compared to untreated tubes, phosphoric acid treatment reduced diffusion by a factor of 50-100. For these tests, 3.17 inch (1/8 inch) stainless steel tubing was spirally wound to provide a convenient test volume of relatively 4 mm size.

The test solution was applied to the outside of the tube. The interior of the tube was first evacuated and then filled with hydrogen. 550q0 coated pipe
The hydrogen pressure was monitored to determine the diffusion rate. The hydrogen diffusion rate in untreated stainless steel is approximately 1 x 10-6 cc per second per tube thickness per 1 cc surface area at 550 qo for a hydrogen partial pressure difference of 10 torr. Met. The diffusion rate of stainless steel coated according to this invention was reduced to 1 x 10-8 in the same unit. Concentrated phosphoric acid solution (e.g. 7
Various concentrations of phosphoric acid from 5% solution to about 20% solution all appeared to be equally effective.

A single coating of 3P04 solution at a concentration of less than 5% also reduces hydrogen diffusion, but a single coating of 3P04 solution at a concentration of about 5% or higher (
or at least other solutions with that phosphate concentration) are strongly recommended.

Small amounts of water-soluble wetting agents can be added, especially in the case of the thinner solutions described above. Other coatings in which phosphate groups are effective at these temperatures can also be used. Thus, for example, ammonium phosphate can be used, but calcium phosphate cannot be used because the phosphate radicals in calcium phosphate are not effective at these temperatures. Of course, the parts of the solution that volatilize at these temperatures (eg ammonia, water or C02) generally do not affect the glass composition. As previously stated, it is desirable that the phosphate glass soften but not melt at the metal's normal operating temperatures.

The melting point of the phosphate glass coating can be adjusted by adding one or more metal oxide compounds to the phosphate radical-containing solution. MnS04, Si02 and Na2Cr20
7 is an example of a metal oxide compound that can be added. The thickness of the final phosphate glass layer can be adjusted by adjusting the concentration of phosphoric acid and the thickness of the coating of phosphate-containing material applied to the metal surface.

The thickness of the coating can be adjusted by varying the viscosity by adding small amounts (eg 1%) of volatile thickeners such as hydroxycellulose. However, this is generally unnecessary since even a very thin layer of phosphate glass will sufficiently reduce the hydrogen oxidation rate. The specific forms described herein are to be considered illustrative of the invention, not limiting, and the invention should therefore be construed as limited to these specific forms. isn't it.

Advantages of the Invention As described in detail above, the effects of the invention include applying a coating of a phosphate plate-containing phosphate glass-forming material to at least one surface of a metal, heating the coating to at least 35,000 ℃ to convert the coating into a phosphate glass-forming material. The idea is to reduce the diffusion of hydrogen through the metal instead of glass.

Claims (1)

[Scope of Claims] 1. Applying a coating of a phosphate glass-forming substance containing phosphate groups to at least one surface of a metal and heating the coating to at least 350° C. to convert the coating into a phosphate glass. A method of reducing hydrogen diffusion through metals, characterized in that: 2. The method of claim 1, wherein the coating consists essentially of a H_3PO_4 solution. 3 The coating consists essentially of at least 1% of the H_3PO_4 solution.
2. A method according to claim 1, comprising a metal oxide compound of a species. 4. The metal is at least a part of the reactor primary fluid containing system, the hydrogen to be reduced diffusion is hydrogen in the form of tritium, the metal is stainless steel, and the metal is stainless steel with essentially 20% to 75% H_3PO_4 as a coating. 2. The method of claim 1, wherein a single coating of solution is applied. 5. By operating a nuclear reactor heating to at least 350°C, the metal is heated to a temperature between 350°C and 600°C and the phosphate glass is brought to a temperature above its softening point but below its melting point. 5. The method of claim 4, provided by: