JPS6221871B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a novel method for forming wear-resistant oxide coatings on zirconium alloy shaped articles that exhibit improved wear resistance. Specifically, the present invention provides an improved dark brown oxide on zirconium alloy shaped articles used to manufacture products such as bearings, valves, etc. that are subject to wear under abnormal conditions, including corrosive conditions. A novel method of forming a coating. Zirconium's excellent resistance to corrosion by hot water has long been known and was one of the factors that led to its adoption as a fuel-clad material in boiling water and pressurized water nuclear reactors. . Zirconium also exhibits excellent corrosion resistance in many aqueous and non-aqueous media,
For this reason, their use in the chemical process industry is increasing. Limitations to the wider use of zirconium in this area are due to its relatively low hardness as a result of its relatively low hardness of approximately 190 BHN (200 Kgmm ^-2 ) as commonly used and its tendency to scratch. be. In the prior art, various attempts have been made to form zirconium oxide coatings on zirconium shaped articles in order to increase their wear resistance. One of these methods is that disclosed in Watson U.S. Pat. No. 3,615,885,
The US patent includes Zircaloy 2 and Zircaloy 4.
A method of forming a thick (0.23 mm or less) oxide layer is disclosed. However, this method results in significant dimensional changes, especially for parts with a thickness of about 5 mm or less, and the oxide film formed does not exhibit particularly high wear resistance. Another patent that discloses a method of forming a dark blue oxide coating on a zirconium alloy shaped article to increase its wear resistance is US Pat. No. 2,987,352. Both this patent and the aforementioned patent form zirconium dioxide coatings on zirconium alloys by air oxidation. The first mentioned patent continues the air oxidation long enough to obtain a thicker beige coating than the deep mauve coating of the latter patent. This beige coating does not have the abrasion resistance of dark mauve coatings and is therefore suitable for applications such as bearings, sliding fittings, and valves where two working surfaces are in close proximity. I can't. Because the beige coating wears away more quickly than the dark mauve oxide coating, precise tolerances are lost and the part becomes unusable. A dark blue oxide coating is thinner and has a significantly greater hardness. Coatings of this type are useful for the above-mentioned applications. Although the dark blue coating has better abrasion resistance than the beige coating, it is a relatively thin coating. It would therefore be desirable to obtain a deep maroon coating with increased abrasion resistance without obtaining the beige type coatings of the prior art. In accordance with the present invention, an improved method of forming a wear-resistant zirconium oxide coating on a zirconium alloy is provided, the method being characterized in that the alloy is treated in a molten salt bath containing a small amount of sodium carbonate. . By utilizing this method, zirconium oxide coatings with increased thickness and wear resistance can be obtained on zirconium alloys, which are particularly wear-resistant, scratch-resistant and corrosion-resistant, and dark-brown zirconium oxide coatings. can be obtained on zirconium alloy shaped articles. In a preferred embodiment of the invention, shaped articles obtained from zirconium alloys are obtained by treating these alloys in a molten bath of sodium chloride and potassium chloride, to which sodium carbonate is added; Alternatively, these alloys may be oxidized by treatment in molten sodium cyanide containing a small amount of sodium carbonate to form a deep mauve oxide with improved wear resistance than that obtained by air oxidation. A coating is obtained on the shaped article. When sodium carbonate is present in a sufficient amount in the molten salt, it can oxidize the surface of the zirconium alloy to form zirconium dioxide. The sodium carbonate is usually present in the molten salt in an amount of about 0.1 to 1% by weight, but may be present in a larger amount if desired. The molten salts used to process the alloys may include chlorides, nitrates, cyanides, and the like. For purposes of illustrating the invention, a combination of sodium chloride and potassium chloride or molten sodium cyanide alone will be used. However, as mentioned above, many other salts can also be used in the method of the invention. In the specific examples below, an equimolar mixture of sodium chloride and potassium chloride was used. This is because, at this ratio, the mixture has the lowest melting point of the mixture, approximately 600°C. Under this particular condition, no more than 5% sodium carbonate was added.
This addition also lowers the melting point of the salt. However, if a temperature of 800°C is used, it is possible to use pure sodium chloride and no potassium chloride is required in the molten salt bath. The sodium cyanide is heated until it melts, and the heating is controlled by adding sodium carbonate. However, the melting point is at least about 550°C
It is. The rate of oxidation is directly proportional to the temperature increase. It has also been found that the corrosion resistance of the treated products remains at the high level normally associated with zirconium alloys, even after wear-increasing treatments. As a result of corrosion resistance tests, it has also been found that in some cases this corrosion resistance can also increase as a result of exposure to acids. Tests were carried out for 96 hours using boiling 70% nitric acid, boiling 20% hydrochloric acid and boiling 60% sulfuric acid, resulting in no change in corrosion resistance, with a slight improvement in the case of nitric acid. Specific examples and tests illustrating comparative results of the method of the present invention and prior art methods are described below. Wear resistance tests were conducted on Zircadine 702, Zircaloy 2 and Zircaloy 4 alloys after oxidation treatment. The compositions of these alloys are shown in Table 1 below.

Table: The alloy was obtained in the form of rods approximately 0.74 cm (0.29 inch) in diameter and annealed at 705°C for 2 hours.
Grind the rod without a center to a diameter of 0.640+0.000-
0.01 cm (0.249 + 0.000 - 0.001 inch) and a 20 micro inch RMS finish. Cut the rod into 5 cm (2.0 inch) lengths and shave the diameter of each piece to 0.32 cm (0.125 inch) over a length of 0.32 cm (0.125 inch) from each end to create a stub. ) and the stub was used to hold the rod in an abrasion test frame. The rod thus has a cylindrical surface with a 20 microinch finish and a length of 4.35 cm (1.75 cm).
inch) and its diameter is 0.63~0.64cm (0.248~
0.249 inch). The rod was degreased with trichloroethane and dried. The rod is immersed in an acid mixture consisting of 37% by volume of 70% nitric acid, 3% by volume of 49% hydrofluoric acid, and the balance water at 38° to 46°C with stirring for 1 to 2 minutes, then drained. The rods were washed for 5 minutes with tap water, then with distilled water, then with ethyl alcohol, then air dried, and only cotton gloves were used thereafter to handle the rods. These rods were air oxidized in a horizontal tubular furnace with a diameter of 7.5 cm (3 inches) and a heating length of 61 cm (24 inches) at temperatures of 600°C to 800°C. Air was passed through the tube at a flow rate of 2 standard cc/sec. Oxidation times ranged from 10 minutes to 50 hours. The rods were oxidized in molten salt at temperatures between 600°C and 800°C. The chloride melt was placed in an Inconel 600 can heated in a pot furnace. In all treatments, sample temperature was adjusted to ±10°C. Wear resistance was determined by “W. Tsai, JACH Humphrey, I.
Cornet and AVLevy, Wear, 68 (1981) 289”
The measurements were carried out using a slurry pot tester with a modified design. In this case, two cylindrical specimens are
Each is mounted on opposite sides of the frame through which the spindle passes. The axes of both specimens are
parallel to and equidistant from the spindle axis. The entire piece is immersed in an abrasive slurry in a container with a cutting board. Rotation of the spindle causes movement of the sample relative to the slurry, and wear due to the abrasive agent is measured by weight loss of the sample. In this operation, the sample was mounted at a radius of 52.4 mm (2.063 inches) from the spindle axis, and the rotational speed was 29.17 ± 0.17 revolutions per second (1750 ± 10 rpm). The abrasive is
As stated in ASTM G65−80,
Box 577, Ottawa, I11.61350, USA
Made by Ottawa Silica Co. Particle size 0.300mm to 0.212mm (50
-70 mesh) AFS50-70 test sand (test sand). Mix the abrasive with water to make 30% by weight
(13.9% by volume) slurry. Approximately 2.9 volumes of fresh slurry were used for each set of specimens.
To avoid errors resulting from excessive wear near the ends of the specimen, possibly caused by turbulence caused by the frame, the ends were cut from thin rubber 6.35 mm (0.25 in.) long from surgical tubing. Protected from the frame by a sleeve. Therefore, the length of the exposed specimen was 32.0 mm (1.25 inches). The specimens were subjected to abrasion for a total time ranging from 120 seconds to 9000 seconds. At regular time intervals, specimens were removed from the slurry pot, washed with water, dried, and weighed to an accuracy of ±0.1 mg. Volume loss was calculated by dividing weight loss by density. 7880, 6510 and
A density of 5741 Kg m ^-3 was used for steel, zirconium alloy and ZrO ₂ respectively. For oxidized specimens, the removed volume is:
Calculated as _ZrO2 , independent of the degree of wear. Scratch resistance was estimated using the Schumatschier method. Now, the end of the perfect circular cylinder is rotated 360° while applying a known load to the flat surface. The contact surface was then examined for scratches. Corrosion resistance is 70% boiling and 60% H ₂ SO ₄ with addition of 100ppm or 500ppm of Fe ⁺³ , 70% boiling
The weight loss of the specimens after exposure to HNO ₃ and boiling 20% HCl for 96 hours was measured and investigated. Example 1 A rod of Zircadine 702 was treated in a melt consisting of an equimolar mixture of sodium chloride and potassium chloride to which 5% by weight of anhydrous sodium carbonate was added. The processing is
The test was carried out at 800°C for 3 hours and 6 hours, and the weight gain is shown in the table.

[Table] Volume loss after 10 minutes, 30 minutes and 60 minutes of wear for rods treated as described in Comparative Examples 1 to 5 below, untreated Zircadine 702, 1018 steel and heat-treated Rockwell-C60 hardness rods. AISI
The values are shown in the table along with the values for the 01 tool steel rods. Comparative Example 1 A rod of Zircadine 702 was heated at 800℃ for 3 hours.
Heated in air for 6 and 10 hours and at 600° C. for 50 hours. The weight increase is as shown in the table.

[Table] Comparative Example 2 Zircaloy 2 rods were heated at 700℃ for 10 hours and
Heat treatment was performed at 800°C for 2 hours in air. The weight increase is as shown in the table.

[Table] Comparative Example 3 A rod of Zircaloy 4 was heat treated in air at 800°C for 2 hours. The weight increase was 104.2 mg, and the specific weight increase was 10.27 mg/cm ² . Comparative Example 4 Zircadine 702 rods were heated at 700°C and 800°C.
Oxidized in sodium cyanide exposed to air for 3 and 6 hours at .degree. The weight increase is as shown in the table.

[Table] Comparative Example 5 Zircadine 702 rods were heated at 700℃ and 800℃.
C. for 3 and 6 hours in molten sodium cyanide under an oxygen-free argon atmosphere. The weight increase is as shown in the table.

【table】

[Table] As can be seen from the table, the oxide film obtained by molten salt treatment has less volume loss, and the test results show that the film obtained by air oxidation is It can be seen that the lifetime is longer than that of a coating containing both a dark blue oxide coating and a beige oxide coating. In summary of the present disclosure, the present invention provides a novel method of forming wear-resistant oxide coatings on zirconium alloys. Various modifications are possible within the scope of the invention.

Claims (1)

[Scope of Claims] 1. A method for forming a wear-resistant zirconium oxide coating on a zirconium alloy, characterized in that the alloy is treated in a molten salt bath containing a small amount of sodium carbonate. . 2. The method of claim 1, wherein the molten salt bath comprises molten sodium cyanide, molten sodium chloride or a mixture of sodium chloride and potassium chloride. 3. Claim 2, wherein the molten salt is a mixture of equimolar amounts of potassium chloride and sodium chloride.
The method described in section. 4. A method according to any one of claims 1 to 3, characterized in that the sodium carbonate is present in the molten salt in an amount sufficient to oxidize the zirconium alloy. 5. A method according to any one of claims 1 to 4, characterized in that the molten salt bath is heated to a temperature between 550°C and 800°C. 6. A method according to claim 4, characterized in that the sodium carbonate is present in the molten salt in an amount of 0.1 to 5% by weight.