DE2034863B2 - Process for the surface treatment of zirconium and zirconium alloys for fuel elements or other nuclear reactor components - Google Patents

Description

The invention relates to a method for the surface treatment of zirconium and zirconium alloys for fuel elements or other nuclear reactor components by pickling in a fluorine-containing bath and subsequent production of an oxide layer

Process for cleaning zirconium surfaces by electrolytic polishing in an electrolyte, which contains about ammonium fluoride and ammonium nitrate to in a subsequent oxidation stage a Producing corrosion-resistant oxide surfaces are generally known (SE-PS 1 79 120 and DE-PS 11 98 166). No cleaning of the surface by the electrolytic polishing is applied to this with steam at high pressure and high temperature for the oxidation process

The known methods make electrolytic cleaning unavoidable, which does not result in one The possibility that leads to inconsiderable increase in cost and complication of the process flow is also possible To make fluoride-containing contaminations sufficiently harmless, what is not given enough is of great importance for use in connection with fuel assemblies for nuclear reactors

A method is also known by means of which colored oxide coatings on metals of groups IV, V and VI of the periodic table. It is no longer new in this context that the anodic oxidation of zirconium leads to colored surface coatings (FR-PS 12 41646). Ar.odic processes, which only serve decorative purposes, are not, however, for the production of corrosion-resistant coatings for those of interest here Purpose suitable; and the demands placed on them are not comparable.

Rather, the problem at hand comes from a known radioactive decontamination process contaminated objects and surfaces closer, after which the contaminated objects are subjected to electrolytic polishing, for which they are called The anode is completely immersed in an electrolyte bath, or the electrolyte is set into circulation gradually over the area to be disinfected by means of a movable cathode (DD-PS 24 843). However, fluoride-containing contaminants cannot be removed by this method alone.

It is known that an oxide coating is used in the zirconium plating of fuel assemblies for nuclear reactors produced, for example by treating the surface in an autoclave with water or steam at a high temperature of approx. 400 "C and high pressures of approx 100 at or »I by heating in oxygen.

Before this oxidation is carried out, the Surfaces are thoroughly cleaned or stained,

for example in a mixture of nitric acid and fluorine water chemical acid and water. A suitable bathroom solution for this pickling process is for example

ENT ₃ -HF-HzO

in a weight ratio of 41-4-55.

When the surface of the material has been cleaned by this commonly carried out pickling process is, the surface can still be slightly warped or muddy during the subsequent thorough rinsing in water. be contaminated and here still with fluoride compound dung be applied, which is very disadvantageous affect the corrosion resistance at high temperatures. These fluoride contaminations, which are classified as Any residue left on the surface cannot be satisfactory by any of the known methods removed. There is also a lack of usable bath solutions for zirconium and zirconium alloys Pickling process that does not contain any fluorine compounds. When using fluoride containing Bathroom solutions have so far sought to reduce the amount of Reduce fluoride contamination by leaving the treated surface as early as possible after Pickling was rinsed off with water. In that regard, a procedural rule was that the rinsing process Surface of the material no more than five seconds to be carried out after pickling In practice this is the case however, extremely difficult to perform.

The invention is therefore based on the object of developing a method of the type mentioned at the outset in such a way that that it is possible a longer time for the transmission of the pickled objects from the bath solution in a To have a rinsing bath available, while at the same time the disadvantageous fluoride contamination should be excluded with certainty. The object is achieved according to the invention achieved that is anodically decontaminated after pickling and anodically oxidized, both measures in an agitated 0.01 to 0,5molaren alkali metal hydroxide solution, preferably with a content of 0.5 wt .-% KOH, at a temperature below 10O ⁰ C be made and the Spannw / ig with no more than 10 V per second up to a final voltage of at least 30 V and the final voltage for so long is maintained until about a constant current flows.

A major advantage of this process is that it can be completely removed or rendered harmless of fluoride compounds, the smallest traces of which would cause an extraordinary increase in the rate of corrosion of the surface. As for the In the event that an undesirable fluoride contamination should still be present, the surfaces of the machined Objects would appear blotchy due to color interference, the method also gives a clear indication of the contamination.

The surface treatment of zirconium or zirconium alloys of interest here in connection with the Manufacture of fuel assemblies for nuclear reactors acting as tubes from zirconium material with nuclear

Fuels can be filled and sealed by welding is due to the two-stage nature of the Method particularly simple and effective, especially as both the anodic decontamination and the anodic oxidation can take place in one and the same bath, but optionally also separately in two Baths.

The electrolyte fluid is moving as the is attached The gas layer that otherwise accumulates on the surface of the fuel assembly leads to non-uniform treatment ι ο In addition, the fluoride that is removed from the surface during electrolysis is absorbed by the Movement better distributed.

After the fluoride contamination has been removed from the surface or is harmless the protective oxide layer is applied. Decontamination and oxidation can also be carried out at the same time.

The anodic oxidation of zircon is according to the invention in a moving 0.01 to 0.5 molar Alkali, preferably and a content of 0.5 wt .-% KOH, carried out when using diluted KOH it is possible to obtain a coating of zirconium, which is just as good Provides corrosion protection as it is obtained by autoclaving, provided that the after The conditions prescribed in the present procedures are maintained.

An advantage of the invention also lies in the low expenditure of time for the entire process sequence. A yo Oxide film of 2000 A, as can be achieved by anodic oxidation, is so insensitive to contamination like a film of 10 OQO A Dirke that can be obtained in an autoclave

It is also advantageous that the anodic decontamination and oxidation are caused by the development of Oxygen at the anode does not allow hydrogen to enter the zirconium-containing material.

Another advantage of the anodic decontamination and oxidation used according to the invention is is in contrast to the processing in an autoclave in that it does not increase the Brings corrosion rate with irradiation or radiation, because the defect structure in the oxide layer in is different in both cases.

Surfaces of a number of zirconium alloys have been treated using the "Zircaloy-2" and "Zircaloy-4" currently on the market. It was found that the alloy composition had no different effects on the Has removal of the fluoride from the surface.

On the other hand, however, it has been found that the thickness of the applied oxide coatings for different Alloys slightly different. The following alloys were treated with positive results:

"Zircaloy-2" with 134% Sn, 0.13% Fe.

0.09% Cr, 0.05% Ni, balance Zr. "Zircaloy-4" with 1.52% Sn, 0.20% Fe,

0.08% Cr, 0.006% Ni, balance Zr. 1.0% Cr, 0.16% Fe, balance Zr. 2.5% Nb, 03% Ta, remainder Zr. 0.7% V, 0.75% Fe, balance Zr. 1.2% Cu, 03 Fe, remainder Zr. 0.2% Sn, 0.09% Fe, 0.07% Ni,

0.14% Nb, balance Zr.

The percentages given relate to percentages by weight.

60

65 Comparative tests with different electrolyte solutions are described in more detail below:

Example t

"Zircaloy-2" specimens were surface-contaminated with fluoride in the following way: The specimens were placed in a pickling bath solution containing HNO ₃ -HF-H ₂ O in a weight ratio of 41-4-55 Minutes. The temperature of the pickling liquid was 33 ° C. The samples were then removed from the bath solution and kept in air for 20 minutes without the adhering pickling liquid having been removed. During this time, insoluble oxy- and hydroxy-oxide formed on the surface of the samples. The samples were then thoroughly washed and dried

The samples contaminated with fluoride in this way underwent anodic oxidation at different Subjected to electrolyte solutions. Basic as well as neutral and acidic aqueous electrolyte solutions were used here to use. The different electrical solutions that have been used are in the table 1 summarized As can be seen from this table, the examined electrolytes contain 03 to 3 wt .-% a salt, an acid or a base. The temperature of the electrolyte solution was kept at 20 ° C. during all tests.

The anodization was carried out in the same way in all electrolyte solutions. For this purpose, the samples were connected to the anode of a direct current source, the connecting material consisting of "Zircaloy-2". The cathode was made of platinum. To carry out the electrolysis, the current was switched on and the voltage increased from 0 to 125 V at a rate of 5 V / per second was constant. Subsequently, the sample pieces were taken out, washed in water and dried Subsequently, these test pieces were subjected together with reference samples of a corrosion treatment with superheated steam over a period of 4 days, wherein the temperature of the vapor at 400 ⁰ C was and the expended pressure was 50 kgf / cm ² . The appearance of the test pieces after the anodization or electrolysis treatment and the corrosion test as well as the weight increase is summarized in Table 1 and can be taken from this.

According to the American Society for Testing Materials, Specification 353-64T for zirconium alloys, "Zircaloy-2" and "Zircaloy-4", the corrosion ^{test was carried out in a steam stream at 400 °} C. After the material had been subjected to this test, it should be one Have a uniform, shiny black oxide layer, free of white and brown corrosion products. The weight gain should be less than 22 mg / dm ^2. According to this specification, the fluoride-contaminated, non-anodized test pieces were unsatisfactory in the corrosion test, as were some of the anodized test pieces.

As can be seen from the above table, a satisfactory appearance was obtained in connection with Lowest signs of corrosion can only be achieved when using the following alkaline solutions as electrolyte, which all have a sufficient concentration of

Contained OH ions. _{NH 4} OH is also included in these alkaline solutions.

0.5% NH ₄ OH 0.5% UOH 0.5% NaOH KOH

Stronger corrosive effects were found during anodization in NaNO2, while L1NO3 and KNO3 oxide layers or films without satisfactory

10 Appearance revealed, which, however, is proportionate produced low weight gains on exposure to superheated steam.

Borate or boric acid electrolytes seem to be of little advantage in this context, while sulfates, sulfites, carbonates and phosphates, for example, also turned out to be unsuitable for removing fluoride contamination. In this context it should be mentioned that the solubility of the fluoride Ioiien in solutions of Ba ⁺ + and Ca ++ at low temperatures is very low

Table 1 pH Look after the Total weight Look after the Electrolyte specification in% by weight Anodizing increase n.d. Autoclave test Autoclave test in mg / dm ² 7th gray-white 16.6 grayish, dull LiNO ₃ 1% 13th appealing 17.9 pnspr. black NH ₄ OH 0.5% 13th appealing 17.9 claim black LiOH 0.5% 7th gray-white 18.4 grayish, thick KNO ₃ 1% 13th claim green 18.8 claim black NaOH 0.5% 13th claim green 19.0 claim black KOH 0.5% 8th claim red-pink 22.1 claim black NaNO ₂ 1% 6th claim light green 24.9 some white spots NH ₄ tartrate 3% 10 claim red-pink 25.4 claim black NaB ₄ O ₇ 1% 1 anipr. light green 26.4 claim black citric acid 1 claim light green 27.5 some white spots Tartaric acid 10 claim green 29.7 some white spots K ₂ CO ₃ 1% 13th claim green 31.6 appealing Ca (OH) ₂ 1 claim green 31.8 appealing Ba (OH) ₂ 7th claim Red Green 33.7 white-spotted Na ₂ HPO ₄ 1% 9 claim Red Green 44.0 gray-white Na ₃ PO ₄ 1% 8th claim light green 46.5 White K ₂ SO ₃ 1% 7th claim light green 49.2 White K ₂ SO ₄ 1% 1 claim light green 52.2 white8 H ₂ SO ₄ 1% 7th claim light green 53.3 White Na ₂ SO ₄ 1% 20.0 black Not anodized, immediately after normal stain washed resp. rinsed off 42.1 White Not anodized, fluoride contaminated

Regarding the weight gain and appearance of the anodized, fluoride-contaminated specimens: Dk weight gain ΪΛ determined as the total weight gain after anodizing and treatment in the autoclave and represents the mean value of two parallel tests.

It goes from this table that only the 0.5% alkaline solutions give satisfactory results in the subsequent corrosion test.

Example 2

The surface of "ZircaIoy-2" samples is treated with fluoride as described above. the The temperature of the pickling bath was 35 ° C. and the residence time h: air was 20 minutes. The test pieces were washed particularly thoroughly during this time, so that all of the fluoride that is not chemically attached the surface was bound, has been removed.

The samples were then anodized in a basic electrolyte solution (0.5% KOH) in the same manner as reported above. The reference coupons were not anodized. The electrolyte used, a fresh solution, was tested for fluoride content. Here, were found in the electrolyte solution 2.0 x 10 ^"s moles of F, the Ftuoridkotizentration whereas in the fresh solution below the detection limit of the analysis method was that 3.5 x 10 ^-6 mole F- weight. The nachgewiesere fluoride content corresponding to the distance of 14 μg F per cm ^{2 of} the surface.

All test pieces were subjected to a corrosion treatment in 400 ⁰ C hot steam, and it

it was found that the anodized samples had a uniform black oxide layer, while the non-anodized specimens were spotty white.

About the ratio between the amount of fluoride on the surface of the zirconium alloys and the degree of white oxide that forms in standard corrosion tests (400 ° C steam) carried out the following experiments:

On the surface of "Zircaloy-2" drops of a water-soluble compound of HF were distributed over 1 cm ^{2 of} surface area. The drops had a volume of 0.05 ml and a fluoride content that produced a fluoride contamination of 0.5 to 200 μg / cm ² on the surface.

A number of test pieces were subjected to a treatment in an autoclave with steam of 400 ^° C. and a pressure of 50 kp / cm ² for two days. It was found that traces of white or colorless oxide (the rejection limit according to ASTM Specification 353-64T) ^{amounted to approx. 5 μg / cm 2} , in accordance with Rynasiewiez (USA EC Report KAPL-2000-19, Reactor Technology , Report No. 22 - Chemistry, J. R y η asie -wie ζ, 1962), which reports that 8.5 to 17 μg F / cm ² are required for an approximately uniform white oxide layer.

It was also found that, with a corresponding comparison, the standard test method, which was carried out using appropriately prepared, fluoride-contaminated surfaces in Example 1 , led to an areal density of 10 to 20 μg F / cm ² .

Comparing these patterns with the amount of fluoride found in the electrolyte, the conclusion can be drawn that the fluoride is removed from the surface during anodization and that the amount removed is of the same order of magnitude as that Total amount of fluoride produced by the white oxide on the non-anodized surface of the reference specimens.

example

To determine the voltage required to remove all of the fluoride from a specimen. in which the pickling solution had been dried from the surface, the following tests were carried out: The pickling liquid used was the same as described in Example I, and the specimens were fluoride-contaminated in the same way. The amount of fluoride on the surfaces of the specimens was accordingly prior to anodization approx. 10 to 20 μg F / cm ² .

The anodizing solution consisted of 0.5% KOH. and the test material was "Zircaloy-4". The samples were thus anodized. that the voltage for all probers was increased at the same rate, the final voltage being varied between 6.4 to 100 volts. After reaching the desired final voltage, this was kept constant for two minutes before the current was switched off again. The test pieces were then subjected to a treatment in an autoclave, specifically at a steam of 400 ^° C. and 50 kp / cm ² pressure over a period of three days.

The anodizing stresses, the weight gain during anodizing and treatment in the Autoclave, as well as the appearance after treatment in the autoclave, are summarized in Table 2.

After the samples were autoclaved, they showed a noticeable difference in their appearance. The samples that undergo anodization or electrolysis at 30 V and below all showed a similar appearance and were uniformly gray-white while all specimens where the voltage during electrolysis was 50 V and above, a black one, showed a bright shiny appearance. There was no smooth transition from the grayish to the

JO black appearance, but a very noticeable one Difference between samples treated at 30 and those treated at 50 volts. At the total weight gain it can be seen that those specimens which were anodized at 30 volts and below did not are acceptable, while the specimens whose anodizing voltage was 50 V and above fail quite move in the ASTM regulations.

Table 2

Appearance and weight gain after treatment in the autoclave of fluoride-contaminated "Zircaloy-4" specimens after anodizing at various voltages

Final voltage in volts

Weight gain in, izg / dm during the total Look after during the treatment the treatment Anodizing in the autoclave in the autoclave 34.3 34.5 0.2 33.3 33.6 gray-white 0.3 32.3 32.8 gray-white 0.5 29.8 30.6 gr35! - ™ C5w 0.8 29.8 30.8 gray-white 1.0 gray-white

6.4

8th 10 15th 20th

ίο

continuation Weight increase in μ ^ / άτη ² Example 4 total Look after E.id voltage in volts during the during the the treatment Anodizing treatment in the autoclave in the autoclave 24.7 1.6 23.1 20.7 gray-white 30th 2.7 18.0 21.0 black 50 4.2 16.8 18.8 black 75 5.7 13.1 17.7 black 100 7 10.7 black Not with fluoride contaminated - 125 - gray-white Contaminated with fluoride not anodized

Samples of "Zircaloy-2" were fluoride-contaminated by drops, as described in Example 2. The concentration on the surface was 10 μg F / cm ² .

A number of specimens were made in 0.5% strength NaOH anodized at different final voltages (30.50, 70 and 90 V), namely at 0 ° C, while a The first series of test pieces were anodized in the same electrolyte at the same voltages at 100 ° C.

The anodized at equal voltages Probestükke showed different interference colors at 0 ⁰ C and 10O ⁰ C, which indicated that at 10O ⁰ C, a thicker oxide layer is formed as at 0 ° C.

After the test in the autoclave (3 days at a steam of 400 ^° C. and a pressure of 50 kg / cm ² ), no significant differences could be found in the two test series with regard to the removal of fluoride. The samples anodized at the three highest voltages were black and completely similar. In the case of the anodizing at low voltages, the samples whose anodizing temperature was 100 ° C. turned out somewhat better than those which had been anodized at 0 ° C. Accordingly, the temperature of the anodizing bath should have so little influence on the removal of the fluoride that any temperature at which the electrolyte is liquid can be used. In practice it is advantageous to work at room temperature.

example

“Zicaloy ^ w coupons were surface treated as described in Example 1. The duration of these test pieces in air before the subsequent washing process was between 5 and 15 minutes. Reference specimens were washed off immediately after pickling. Some of the test pieces were anodized in a basic electrolyte. The anodizing process or the electrolysis was carried out in the same way as previously described. The electrolyte contained an aqueous solution of KOH (0.5%), and it was carried out at a final voltage of 125 V All specimens were then tested in water and steam at 230 ⁰ C. The results obtained are summarized in Table 3.

From this table it can be seen that an enlarged Degree of fluoride contamination increases the corrosion rate for non-anodized samples, whereby the corrosion rate increases The oxide on the test pieces was significantly higher than for an anodized sample gray-white and spotty. All anodized coupons showed the same low weight gain during treatment in the autoclave, regardless of the degree of original fluoride contamination.

Those samples which were subjected to the shortest exposure time in air (5 minutes) were "received a completely uniform red-green color coating after anodization and were after the autoclave treatment evenly black with a shade or tint of red-green. the Samples which had been kept in the air longer (30 minutes), with a higher one Fluoride contamination, were spotty after anodization and showed areas with a pure green color and other areas with a pure red tint. The stained surface was also in the black oxide layer visible, although it was not white or shaded gray, which was attributed to abnormal oxide formation can be.

If the fluoride contamination exceeds a certain limit, then by the Anodization can be directly demonstrated that the object is fluoride-contaminated The weight gain during the subsequent corrosion treatment

60. lung showed that the originally different fluoride-contaminated specimens exhibited the same corrosion rate after anodization. When a If fluoride residue was present on the test pieces, this amount was the same on the test pieces after anodization two rows of specimens, and this amount was almost as little or less than on a specimen, which was subjected to a washing process immediately after the pickling process.

Table 3

Weight gain of »Zircaloy-2« during the corrosion test in water + steam below

elevated pressure and 230 'C

Residential anodization atmosphere Weight increase duration Li air in the autoclave in mg / dm ²

after the pickling process

10 days 30 days

2 see. no water 5.5 5 min. Yes steam 7 *) 5 min. Yes water 7 *) 30 min. Yes steam 7 *) 30 min. Yes water 7 *) 5 min. no steam 5.8 5 min. no water 5.2 3 min. no steam 7, i 30 min. no water 9.5

Appearance

6.6 black

7 + 0.1 black

7+ 0.1 black

7+ 0.1 black, blotchy

7+ 0.1 black, blotchy

7.8 flecked gray-white

10.4 flecked gray-white

11.7 mottled gray-white

21.4 flecked gray-white

*) The anodized test pieces show no weight gain during the first test period. The weight gain of 7 mg / dm ² was obtained during the anodization.

Example 6

The determination of the detection limit for fluoride was carried out in such a way that test pieces were contaminated with fluoride, namely with fluoride amounts of 0.5; 10; 25; 50; 100; 200 mg / cm ² using the same technique as described in Example 2. The specimens were then anodized in NaOH at a pH of 14 and a final voltage of 125 V and then treated in an autoclave under steam at 400 ° C. and at a pressure of 50 kp / cm ² . Corresponding reference specimens were treated directly in the autoclave without prior anodization. A contamination of approx. 5 μg F / cm ² and higher results in the color shades during the anodization were found. The exact detection limit could not be determined; however, it is presumably lower than it could be of importance for the traces of fluoride detected here.

After the treatment in the autoclave, the non-anodized samples with a contamination of 5 μg F / cin ^{2 were} slightly gray, while the samples with 0.5 μ # F / cm ² that had been anodized showed a uniform black color.

It was also found that the fluoride contamination can be removed from the surface with concentrations of up to 50 μg F / cm ^2. At higher concentrations, the specimens cannot be subjected to anodization, by means of which the fluoride contamination is present as a residue on the surface, which showed rapid corrosion during the autoclaving. However, the limit is higher than the contamination that is present if the surface has been dried off immediately after pickling

Example 7

In order to check further oxidation processes which could be suitable for removing the fluoride contamination, oxygen was used. The test pieces were ^{oxidized for 24 hours at 350 °} C. under the influence of oxygen

35

40

45

50

55

60 specimens an oxide layer, which was of the same thickness as that obtained in the anodic oxidation of the samples in Example 1, about 5000 A.

The thus treated test pieces were subjected to the corrosion test at 400 ⁰ C. The corrosion test showed that the heat oxidation has no effect on the corrosion effect of the samples, since these test pieces have a higher weight gain than the untreated fluoride-contaminated samples and the surface of the samples was white and stained. However, thermal oxidation cannot replace anodization to remove fluoride contamination.

Example 8

In this exemplary embodiment, three rows of tube-like test pieces of "Zircaloy-2" were pretreated in the pickling bath already described above. After pickling, the specimens were kept in the air for different time intervals (1 sec to 10 min) before they were thoroughly washed and then dried. A number of the specimens were subjected to anodic oxidation. The electrolyte used for this was basic (0.5% KOH), and the final voltage that was used in the electrolysis was 125 V. The remaining two rows of test pieces were heated by superheated steam at 400 ^° C. and a pressure of 100 kp / cm ² or 25 kp / cm ² oxidized over a period of 3 days. All samples were subsequently subjected to a corrosion test in a steam atmosphere of 400 ⁰ C for 30 days. According to this, the non-anodized samples showed a white speckled appearance and a high increase in weight, while the anodized sample pieces were uniformly coated in black with an oxide layer and had a very low corrosion rate. This result shows that the fluoride contamination had been removed or at least rendered harmless before the specimens were exposed to the steam. The anodized specimens also showed

a longer test period (150 days) an even low corrosion rate, while the autoclaved specimens at increased speed corroded.

Example 9

To a 0.5% NaOH solution, fluoride was added in the following amounts were added: 4, 8, 21, 42, 210 and 420 ppm. The specimens were in the solutions anodized and treated in an autoclave. It was found that the anodizing process or the Electrolysis procedures in solutions containing up to 210 ppm F proceeded normally, while one With 420 ppm F in the anodizing solution the process was affected and an oxide layer formed which formed a high rate of corrosion during the subsequent autoclave treatment process produced.

U ·. . ι j η f " ■ γί ¹ Ji.

Mination of the order of 10 to 20 μg F / cm ² per liter of electrolyte, the anodization can be carried out without any disturbances and the fluoride contamination is absorbed by the solution.

Example 10

To provide further clarification for the procedure, an example of editing a given fluoride-contaminated object, the object initially at a certain Voltage is anodized to remove the fluoride bound on the surface, d. H. so one Is subjected to electrolysis, and the object subsequently continues on a higher voltage is anodized to thereby create a protective coating against corrosion and various mechanical Get influences.

For this purpose, a pipe made of »Zircaloy-2«, welded at both ends, is used in the example used.

This tube is placed in a pickling bath solution which contains HF-HNO3-H2O in a weight ratio of 4 - 41 - 55. The temperature of the pickling bath was 34 ° C. and the pickling time was 3 minutes. After the pickling, the tube was held in the air for 10 minutes, during which the liquid dried up. The tube was then thoroughly washed and dried. A bath solution into which 5 g of NaOH per liter had been introduced was used for anodization. The temperature of the bath was 19 ° C and the liquid was kept in constant motion with a motorized stirrer.

The length of the tube was 60 cm and the effective surface 290 cm ² . The cathode consisted of platinum, and that of a wire of 2 mm 0, which was arranged in the ^r> vicinity of the anode over the entire length and an effective surface area of 36 cm ² had

The electrodes were connected to a direct current source and the voltage increased at a rate γηΜ c \ / «j * q second up to sirjer Erids ^ snniin" of GO V. This voltage was kept constant for 2 minutes. The anode then showed a red- purple appearance with light pink spots indicating that the anode was fluoride contaminated.

The voltage was then slowly increased from 60V to 125V at a rate of 2V per second. This final voltage was kept constant over a period of 2 minutes, the direct current having a flux density of about 0.3 A / dm ² . The current was then switched off and the

jo object removed from the bathroom to put in cold water to be washed off.

The thickness of the oxide layer obtained in this way could be increased according to this method, from 2000 A to 4400 A. An object treated in this way was covered with a better oxide layer so that it was insensitive to any disturbances, and at the same time an oxidation process was initiated which is a normal low corrosion ^r ate ensured when the object in water and steam has been exposed to a high temperature.

Claims (1)

Claim: Process for the surface treatment of zirconium and zirconium alloys for fuel elements or other nuclear reactor components by pickling in a fluorine-containing bath and subsequent generation of an oxide layer, characterized in that after pickling, anodic decontamination and anodic oxidation, both measures in a moving 0.01 to 0.5 molar alkali lye, preferably with a content of 0.5 wt .-% KOH, are made at a temperature below 100 ⁰ C and the voltage is increased with not more than 10 V per second up to a final voltage of at least 30 V and the End voltage is maintained until about a constant current flows