IL29218A - Method of electroplating - Google Patents

Description

29218/2 Method of elec rqstatlng OOHMISSA IAT A L*12?ERGfIE ATOMIQUB C. 27657 This invention is directed to a method for electroplating parts which are formed at least at the surface of the base metal either of uranium, plutonium, a uranium-base alloy or a plutonium-base alloy. The terms "uranium-base alloy" or "plutonium-base alloy" must be understood to mean alloys containing a proportion of these metals which is such that their oxidizability remains virtually the same. The invention is ei ittilaxly for use in plating rods or plates of alloyed uranium which are intended to serve as nuclear fuel, although other applications may be contemplated.

A particular object of the invention is to endow the plated parts with resistance to corrosive attack by hot water and to ensure perfect imperviousness of the plating. Generally speaking, the texture of metallic claddings.' formed by electrodeposition exhibits higher porosity than that of claddings formed with the same metal but by metallurgical process. In point of fact, a porosity of the same order as that which is obtained by the majority of methods employed in the prior art is unacceptable in some applications in which the plated coating has to remain very thin. This is the case in particular with plates which are fabricated from alloys having a natural uranium base and which are employed as nuclear fuel in some types of test. reactors.

In French patents N° 1.260.085 and N* 1.479.389 of the Commissariat a l'Energie Atomique, methods for electroplating uranium parts have been proposed, but these methods do not provide plated coatings which afford perfect resistance to the corrosive action of hot water. In French patent Ne 1.4-79.389, provision is made for a coating which comprises a plurality of superposed layers and impervious-ness to the attacking action of outside agents is ensured by preparation of the surface on which the final layer is deposited ; the coatings obtained by means of this prior art method afford adequate resistance to the corrosive action of hot air but, as has been stated above, they have lower resistance to corrosion caused by hot water. Moreover, such coatings do not usually prove suitable either when the products to be coated are thin plates for nuclear applications in view of the lack of flexibility of these coatings and their relatively high neutron capture cross-section or when the coated products are intended to slide within guides which have poor frictional characteristics „ Furthermore, as a result of differential expansion processes and at relatively high temperatures, coatings of the type employed heretofore were liable to break or to form cracks „ A further object of this invention is to overcome the different disadvantages referred-to above and to produce as a final result a plated coating which affords a high degree of imperviousness and resistance to the corrosive action of hot water, exhibits both flexibility and elasticity, permits of good sliding action between guides and has a low neutron capture cross-section.

With this object in mind, the invention proposes an electroplating process which comprises : the electro-deposition of a first layer of a metal which adheres powerfully to the previously pickled surface of the part to be plated and, after any operations of weighing and correc-tion of local surface irre ularities which may prove necessary, the electrodeposition of at least a second layer of tin, zinc, lead or indium ; and a mechanical flattening operation which is intended to close the pores and/or any surface irregularities which may be present and to provide the plated part with a smooth outer surface as well as to endow said second layer with perfect imperviousness to outside agents, the total thickness of the plating being greater than 55 microns.

When it proves essential to ensure that any increase in the neutron capture, cross-section of the part which may result from the presence of the plating should remain of a low order, the presence of indium and lead is therefore excluded,, In that case, preference will be given to the use of tin in order to form the second coating. In a preferred mode of execution, in which it is assumed that the part to be treated is either a rod or plate formed of a uranium alloy, there are successively deposited on the part a single layer of nickel having a thickness of 30 to 60 microns followed by a single layer of tin having a thickness of 25 to 50 microns..

Although it has hitherto been found possible to protect parts with a nickel-tin coating having an overall thickness of 55 microns (30 microns of nickel and 25 microns of tin), there is nevertheless considerably greater advantage to be gained by providing a thickness of at least 70 microns as satisfactory protective action is thus ensured in the case of the large majority of parts treated.

Each electrolytic treatment and the methods of correction of local surface irregularities resulting from the fabrication process (casting, rolling and so forth), from the forming process (machining), from mechanical accidents (shocks, scoring and so forth) or from the electrodeposition process itself can conform in particular to the methods described in the above-cited French patent N° l A70j ,389 Gontrarily to the method proposed in the patent referred-to, the method according to the present invention ensures imperviousness to outside agents by virtue of the quality of the external surface, this quality being obtained by means of the ductility of the surface metal (which is preferably tin)c One example of execution of the invention will now be described without implied limitation, reference being made to the accompanying drawings, in which : - Fig, 1 is a view in perspective with portions broken away and showing a plate of uranium alloy which has been plated in accordance with the invention ; - Figs. 2 and 3 are transverse sectional views on a larger scale showing a fragment of said plate respectively prior to and after execution of the final surface-flattening operation.

In the example which is contemplated, the part to be treated is a metallic plate formed of an alloy having a base of natural uranium ; this part is obtained by rolling, its sharp edges and corners being then rounded off.

After a thickness and weight test, the part is subjected to preparation treatments (degreasing and pickl-~ ing) for the purpose of removing the surface oxide layer which always appears on the part during fabrication and also for the purpose of ensuring that the subsequent coating of electrodeposited metal ¾vill adhere to the base metal. This surface preparation treatment comprises, for example, the following operations : degreasing by means of tri-chloroethylene ; electrolytic degreasing in an alkaline solution (to which a wetting agent may or may not he added) containing 40 g per liter of sodium hydroxide NaOH for a period of three minutes at a current density of 5 "bo 10 A/dm ; pickling with 10 N nitric acid at ambient temperature ; washing ; pickling with 10 N hydrochloric acid at ambient temperature ; washing 5 further pickling with 10 Ή nitric acid and final rinsing. This cycle of operations can be repeated if the degree of pickling appears to be insufficient upon visual inspection. The part which has thus been prepared has a more or less rough state of surface as shown at 2 in Figs. 2 and 3 ; this surface roughness may be well-defined if necessary.

The eiectrodeposition of a first metallic layer (preferably of nickel) having a thickness within the range of 40 to 50 microns is then carried out, for example, in the manner which is described in the French patents cited earlier.

The nickel-plating operation is performed by connecting the part to the negative pole of a low-voltage current generator followed by dipping in a bath containing 265 g/1 of nickel sulphate, 35 g 1 of nickel chloride and 100 g/ 1 of alum KAI(S04 )2 .12 HpO which is employed at 5 °C and at a pH of 3 + 0 .25 o As soon as the part is placed in the bath, the current intensity is progressively increased in order to attain after approximately 1 .5 min. a current density of 2.5 A/dm calculated on the total area of the part.

The electrolytic process is continued for a period of approximately 100 mins, whereupon the layer 3 of deposited nickel has reached the desired thiclaiess of the order of 40 to 50 microns.

The electrolytic process is then discontinued, the part is washed and dried, then weighed : the weight of nickel deposited is computed on the basis of the number of ampere-hours of direct current used during the nickel-plating operation, taking into account the cathode current efficiency of the plating bath which is measured from time to time for checking purposes and which remains in the vicinity of 100 %0 By subtracting from the weight of the nickel-plated part the weight of nickel obtained by calculation, the weight of uranium remaining beneath the layer 3 of nickel is thus determined : the difference between the initial weight of the uranium plate and the weight of the remaining uranium as obtained by calculation gives the quantity of uranium which is removed at the time of picklin Q The part is then subjected to a thickness test in order to ensure that the dimensions obtained after deposition of the initial layer permit the deposition of the next layer while remaining within established dimensional tolerances „ A mechanical operation may be carried out if necessary for the purpose of correcting the dimensions of the nickel layer by removal of metal at any points at which the thickness is found to be excessive. If this course is adopted, it is advisable to perform a further thickness test after completion of the operation and possibly a further weight test in order to determine with precision the weight of nickel v/hich remains.

The thickness indicated for the deposition of nickel (40 to 50 microns) is sufficiently great to prevent breakdown of the plating and to provide effective protection of the underlying uranium during the different operations which are performed between the two electro-deposition operations despite any slight bending stresses to .which the part may be subjected. At the same time, the thickness stated is sufficiently small to ensure that the neutron capture cross-section which results from this layer is not excessive.

At this stage of the process, a heat treatment may be carried out in order to enhance the adhesion of the nickel to the uranium base metal, should this operation be found desirable for a specific purpose „ In the case in which surface flaws (such as a crack 4) remain in the nickel layer, local retouching may be performed, before proceeding with the treatment, by electrolytic "spoti! deposition of nickel (or alternatively of tin).

The nickel-plated part is then subjected to a surface preparation treatment prior to tin-plating by electrolytic degreasing in an alkaline bath which is similar to the solution mentioned earlier at a current density of 5 to 10 A/dm for a period of 2 to minutes followed by rinsing. This method of activation of the surface of the nickel does not produce any attacking action on the metal. Consequently, the method does not cause any damage to the initial layer and in no way affects the quality of this latter or modifies either the dimensions or the weight of the nickel-plated part. Deposition of the second layer which consists of tin in this example is then carried out.

For this purpose, the nickel-plated part which has been degreased and washed is accordingly connected to the negative pole of an electric current generator, then dipped at a low voltage in a batb containing 155 g l of sodium stannate (Na2SnC>5. 3H≥0) and 16 g l of sodium hydroxide (NaOH), the bath being heated to 70", continuously stirred and filtered.

As soon as the part is placed in the bath, the current intensity is increased to a density of 2.5 A/dm which is maintained at this value for a period of 70 minutes The electrolytic operation is then stopped (the thickness of the tin deposit 5 being of the order of 40 microns), the part is washed with water and then dried.

The removal of any local surface irregularities 6 is then carried out by electrolytic spot deposition of tin followed by surface flattening (for example by means of a buffing or polishing machine) ; if necessary, a dimensional test is carried out and tin may be removed locally in zones of excessive thickness „ Finally, the part which is thus prepared is subjected to a mechanical surface-flattening operation which is intended to provide the tin coating with an external surface which is both smooth and crack-free (as shown at 7 in Fig. 3)« Unless this operation were performed, said external surface would have the irregular appearance which is shown at 8 in Fig. 2 and which would be more highly conducive to incipient local breaks in the plating.

The surface-flattening operation referred-to can advantageously be carried out by brushing, for example by means of a rotary brush of tampico fiber, the work condi-tions of which are regulated so as to ensure that the surface temperature of the brushed part does not rise above approximately 4-0° , these conditions being as follows in the case of a tampico-fiber brush having a width of 80 mm and a radius of 100 mm : peripheral velocity of rotation of I7OO to 2000 in mm, application pressure of the order of 2 kgs.

Brushing can be performed in two passes : the pass defined above is accordingly preceded by another pass performed, for example, with a rotary fiber-brush provided with a fine-grit abrasive (such as number 320) ; the conditions of this pass are as follows in the case of a brush having a width of 75 mm and a radius of 75 mm : peripheral velocity of the order of 2000 mmm, application pressure of the order of 0-5 kg.

The edges of the part can be finished by means of an abrasive paper having a fine grit size (number 320 , for example).

It will be readily understood that the tin-plating operation can be performed if necessary in a number of successive stages separated by intermediate mechanical surface-flattening operations.

Local retouching with tin followed by surface-flattening can also be carried out, the finished part being subjected to the requisite dimension and weight tests.

The plated part thus obtained offers a number of advantages over those which have been known up to the present time, especially in regard to the resistance of the part to the corrosive action of hot water, its low neutron capture cross-section, the flexibility of the cladding without any danger of failure and the ease with which the part is capable of sliding within its guides without any danger of seizure.

This invention is obviously not limited to the applications and forms of execution which have been primarily contemplated in the foregoing but extends, on the contrary, to all alternative forms, and especially to the case in which the initial layer consists of copper instead of nickel (this choice being necessary in practice only if the part is formed either of plutonium or of a plutonium-base alloy).

Claims (7)

HAVING- NOW particularly described and ascertained the nature of our said invention and in what manner the same is* to be performed, we declare that what we claim is: What we claim is :

1. A method for electroplating objects whose outer portion at least consists of uranium, plutonium, a uranium-base alloy or a plutonium-base alloy, comprising the successive steps of : electrodepositing a first layer of a metal which adheres to the surface of the object ; if necessary, weighing the object and removing local surface irregularities ; electrode- positing at least one second layer of tin, ¾inc, lead or indium ; mechanically •fey* surface—flattening the surface whereby the pores and or surface irregularities are closed, a smooth outer surface is obtained and said second layer is rendered impervious to outside agents ; the total thickness of the plating being greater than 55 microns .

2. · A method in accordance with Claim 1, wherein the second layer is formed of tin and its thickness is within the range of 25 to 50 microns.

3. » A method in accordance with Claim 1 or 2, wherein the first electrodeposited layer is formed of nickel and its thickness is within the range of 30 to 60 microns.

4. · A method in accordance with Claim 1, 2 or 3> wherein the surface-* flattening operation is carried out by brushing with rotary brushes.

5. · A coated object consisting of an inner part of uranium, plutonium, a uranium-base alloy or a plutonium-base alloy, coated v/ith an inner layer of nickel or copper having a thickness of 30 to 60 microns, and an outers layer of tin or zinc having a thickness of 25 to 50 microns by means of the method defined in Claim 1, Claim 2, Claim 3 or Claim 4-,

6. A method of electroplating as defined in Claim 1, substantially as hereinbefore described,

7. A plated product obtained by means of said method as defined in Claim l, substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings. B 2580-3