The coating with optimum quality is nowadays obtained by deposition in an electrochemical cell. The sheet, connected to the negative pole of a direct current generator, faces an anode (connected to the generator positive pole) and passes with a predetermined speed through the cell which contains a solution of a zinc salt (for example 300 grams/litre of zinc sulphate heptahydrate) and optionally of a nickel or iron salt. The sheet works as cathode and zinc is therefore deposited thereupon, optionally alongside nickel and iron, together with an usually limited amount of hydrogen evolution that represents a loss of electric efficiency. The quantity Q (grams) of deposited metal is governed by Faraday's law according to the following equation: Q lxtxPAxE/M wherein I is the current produced by the generator (amps), t the electrolysis time (hours), PA is the atomic weight of the deposited metal, M is the amount of electricity (Amperehour) required for depositing one mole of metal and E is the yield of deposition (fraction of electric current effectively used for depositing the metal).

The average quantity of metal D deposited per unit surface, from which the average obtained thickness depends directly, is given by the equation: D (g/m2) = I xtx PAxE/lMxLxVl wherein L is the length of the sheet (metres) and V is the velocity of passage of the sheet in the electrolytic solution (metres/hour).

The latter equation clearly indicates that if an increase in the productive capacity of a given plant is desired, in other words if high values of V are required, it is also necessary to increase the electric current intensity accordingly to maintain the amount of metal deposited per unit surface of sheet unvaried.

As at any given instant a certain number of square metres of sheet are immersed in the electrolytic solution, one could also say that the requested high productive capacities require high current densities (A/m2 of sheet). In particular, with a velocity of passage of the sheet of 100-300 m/min, current densities around 10,000-20, 000 A/m2 are generally needed: the experience has shown that in these extreme conditions, a coating with uniform thickness and good adherence is obtained when the electrolytic solution is fed inside the gap between sheet and anode with high flow-rates, indicatively from 1 to 10 m3/min, with a sheet to anode gap comprised in the range of 10 to 30 mm.

In these conditions the voltage established between sheet and anode usually ranges from 10 to 30 Volts.

Similar considerations apply for the processes of application of tin coatings to sheets employed for the production of cans for the food industry and in general for all those processes involving coating sheets, wires or other moving objects through the electrolysis cell with a metal or metal alloy.

The anode and the sheet, which are kept mutually parallel, can be disposed in a horizontal or vertical position or as coaxial cylinders. The former position allows using a simpler cell design, even though the tendency of the sheet to assess along a catenary under the effect of its own weight must be counteracted with suitable internal devices. The vertical position eliminates this inconvenience and allows using more compact cells, however with the disadvantage of a greater complexity of construction.

The anode facing the moving sheet was originally of the soluble type, made of the same metal contained in the electrolytic solution : the advantage of this option, consisting in the capability of the process of keeping the metal concentration in the electrolytic solution virtually constant, could't compensate however the big problem of the anode's own dimensional change and of the associated changes of the gap thereof with the membrane. These changes in their turn would cause an inhomogeneous distribution of the current density with consequent local variations in the thickness of deposited metal. Moreover, particles coming from the disintegration of the soluble anode could remain embedded in the deposit hampering its quality.

The above mentioned problems were solved with the insoluble anodes made of a substrate, usually of titanium, coated with a catalytic layer for the evolution of oxygen and/or chlorine, comprising at least one metal of the group of platinum or oxides thereof, optionally mixed with at least one oxide of a metal selected between titanium, niobium, zirconium, tantalum, tin, lead, manganese.

These anodes are dimensionally stable and solve therefore the problem of the required constancy of the sheet-anode gap: nevertheless, they evolve gas, and to avoid that the bubbles accumulate and adhere to the surface of the anode itself or to the one of the sheet causing irregularities in the coating growth it was necessary to adopt the aforementioned circulation flow-rates of the electrolytic solution. With such flow-rates, the bubbles are dispersed within the bulk of the solution and withdrawn effectively from the cells. A limitation still characterising the commercial insoluble anodes is given by the relatively reduced lifetime, approximately 1-2 years in the above indicated conditions of high current density, and substantially arising from dissolution mechanisms of one or more components of the catalytic layer or of passivation induced by the growth of insulating layers at the interface between the substrate and the catalytic layer.

The industrial experience has demonstrated that the lifetime is a function of the current density, which however cannot be decreased, as already said, without inducing a loss in the productive capacity of the cell : for this reason, in the attempt of prolonging the lifetime, many efforts have been directed at proposing new formulations of the catalytic layer which however have not substantially changed the situation up to now. An alternative way to achieve the scope is disclosed in US 4,936, 971, wherein it has been proposed to make use of an anode with a ribbed surface in order to increase the effective surface thereof : nevertheless, the ratio of the width of the"crests"the width of the"valleys"is specified as at least 1, with a particular preference for values of 3-5. With such values of the ratio it is apparent that the increase in the effective surface is relatively modest.

An alternative proposal, based on a design recalling the geometry of the surface of US 4,936, 971, is relative to the anodes known in the art as"blade"anodes : the latter are made of parallel strips fixed by welding to cross-bars which, besides sustaining the strips, have the purpose of distributing the electric current thereto in a uniform fashion. These anodes, which were primarily developed for the mercury cathode chlor-alkali electrolysis, are effective as regards the gas bubble release when they are disposed horizontally, while in the vertical position they show some tendency to entrap the gas in the upper part with consequent alteration of the current distribution. In fact, in order to increase the effective surface it is necessary that the gap between strips be rather reduced, which explains the difficulty in releasing the gas bubbles into the bulk of the electrolytic solution.

A further constructive design is disclosed in US 4,828, 653, wherein it is described the use of an anode consisting in a package of meshes, provided with a catalytic layer for oxygen evolution and characterised by a regularly variable geometry from the outer to the inner surface so as to obtain a uniform current partitioning on all the meshes and a substantial decrease of the local current density.

For this reason, this anode shows a prolonged lifetime, but on the other hand it is subjected to some inconveniences, such as the oxygen bubble entrapment within the package, the perturbation of the solution motion with possible negative repercussions on the quality of the coating deposited on the sheet and a very unsatisfactory behaviour when damages caused by the contact with the sheet during the electrolysis and consequent short-circuiting occur.

The latter problem, particularly serious with the insoluble anodes, is generated by the oscillations that the sheet experiences in its motion across the cell : these oscillations in some cases, such as in the presence of irregularities in the profile of the sheet or when resonance effects arise, can assume those amplitudes that bring to the contact with the anode. When the anode and the sheet detach, the high electric power involved establish an arc which burns the material sometimes destroying wide areas of the catalytic layer : in these areas, the current can't be supplied anymore, with an immediate negative impact on the quality of the metal coating. It should be noted that with the conventional soluble anodes, the loss of material by vaporisation under the effect of the electric arc would not hamper the functioning of the anode itself, that would keep on dissolving with the residual material of the damaged zone. A sharp profile variation would evidently occur with a local increase in the gap with the sheet: this situation would cause in its turn the local current density to be somewhat decreased, but not to be cancelled as it happens with the insoluble anodes instead.

To overcome this problem, spacers made of insulating materials are known in the art, in particular with reference to the above cited US 4,936, 971 wherein fluorinated polymeric materials of low friction coefficient are disclosed, shaped as inserts protruding from the anode surface: this solution is scarcely efficient as the elevated energy associated to the possible smashing of the moving sheet against the anode can certainly damage the inserts to a substantial extent, so that they quickly lose their effectiveness. Once this situation is established, the anodes of-US 4,936, 971 inevitably undergo the above cited irreversible damage and need therefore to be replaced. The repairing of these anodes is cumbersome and entails a certain number of steps: the whole anode is first deprived of the catalytic layer through sandblasting and/or etching, the damaged zone is then machined to remove the residues of catalytic layer and of solidified fused metal, the cleaned zone is resurfaced with the base material, for instance titanium, by welding, and then milled to obtained the required planarity and to restore the ribbed pattern. After recovering the structural integrity of the anode, a new catalytic layer is applied, characterised however by a decreased lifetime in correspondence of the repaired zones. The previous analysis of the prior art and the operative experience of the plants for metal coating deposition on sheets or wires or other moving objects show that a really satisfying solution for the two needs of prolonging the lifetime of the anode catalytic layers and of improving the resistance to short-circuiting is not presently available.

BRIEF DESCRIPTION OF THE INVENTION The present invention is directed to an anode for a cell suitable for processes of deposition of a metallic coating from a solution containing the ions of the metal or of the metals to be deposited on a sheet or wire or other object in motion across said cell, wherein said anode, of the insoluble type and provided with a catalytic layer for the evolution of oxygen or other gas, is placed in parallel position with respect to the sheet or wire or other object in motion thereby defining a gap. In the foregoing description, the invention will be illustrated, for the sake of simplicity, making reference to processes of coating of sheets in motion across the cell.

The assembly consisting in the moving sheet and the anode of the invention, disposed horizontally, vertically or according to a coaxial cylindrical geometry, is characterised by a prolonged lifetime and by the capability of effectively withstanding the collision with said moving sheet and the consequent short- circuiting followed by an electric arc.

Under a first aspect, the anode of the present invention comprises a generally planar substrate, made of a conducting material which is resistant to corrosion in the cell operating conditions, wherein at least one three-dimensional structure, preferably made of the same material of the planar substrate, is secured to the surface of the planar substrate facing the moving sheet during operation, and wherein said structure is positioned in parallel to the axis of the planar substrate oriented in the direction of the motion, protrudes from the surface of the planar substrate and has a length substantially equal to the one of the planar substrate measured along the axis thereof oriented in the direction of the motion.

Under a second aspect, the structure secured to the planar substrate comprises a U-shaped strip with the base of said U in contact with the planar substrate and the two lateral surfaces oriented toward the moving sheet. In a different embodiment, the structure secured to the planar substrate comprises at least one strip with a C-, Z-or L-shaped cross section, the minor surface thereof being fixed to the planar substrate, and positioned such that it results generally parallel to the axis of the planar substrate oriented in the direction of the motion of the sheet. Such U-, C-, Z-or L-shaped strip has a length which is substantially equal to that of the planar substrate measured along the axis oriented in the direction of the motion. Finally, suitable results in terms of lifetime and resistance to short-circuits are obtained also with structures consisting in strips with circular cross section.

Under a third aspect, the structure-planar substrate assembly is produced by means of a spot or continuous welding with Tungsten Inert Gas (TIG) or equivalent arc-welding, or more preferably by laser welding.

In a further aspect of the invention, the anode comprises a multiplicity of said structures, parallel to each other, wherein the surfaces of said structures directly facing the moving sheet are preferably equally spaced in order to avoid affecting significantly the flow of the solution containing the gas bubbled formed in operation, to facilitate a homogeneous distribution of the electric current and to offer a uniform mechanical resistance toward the collisions with the moving sheet on the whole anode surface. In the case of U-shaped strips, the after may also be placed in mutual contact so that the coupling of the lateral walls of the different strips can oppose an increased resistance to the collisions with the moving sheet.

Under a further aspect, the anode of the invention is provided with a catalytic layer for gas evolution, preferably oxygen and/or chlorine evolution, on the whole surface exposed to the solution during operation which comprises the surface of the planar substrate that is not occupied by the structures of the invention and the portion of surface of the structures which is not in direct contact with the surface of the planar substrate. In a preferred embodiment, the surface of the structures directly facing the moving sheet during operation, consisting in the set of minor surfaces of the strips, is free of catalytic layer so as to increase the contact electric resistance between anode and sheet in case of collisions.

Under a further aspect of the invention, the anode is constructed through the steps of degreasing of the planar substrate, preparation of the strips, for example U-shaped strips, and degreasing of the same, insertion of the planar substrate and of the strips in a suitable mould, spot or preferably continuous laser welding, sandblasting and/or etching and application of the catalytic layer for oxygen and/or chlorine evolution as known in the art, preferably with the addition of a mill finishing step that allows to simultaneously achieve the two objectives of obtaining a perfectly planar surface, important for a best quality of the coating, and of removing the catalytic layer from the surfaces of the structures directly facing the moving sheet, with the aforementioned advantages.

Under a further aspect of the invention, the above indicated construction procedure can also me employed for the conversion of anodes of the prior art, extracted from the cells after short-circuit induced damages, to anodes of the invention: in this case the above listed steps are preferably preceded by a total removal of the damaged catalytic layer, for instance by sandblasting and/or etching, and by an optional resurfacing of the planar substrate in case the short- circuiting has provoked a consistent loss of material. In this occurrence it is necessary to carry out a grinding of the planar substrate to ensure an acceptable starting planarity.

Under a last aspect when the anode of the invention is extracted from the cell upon natural exhaustion of the catalytic layer, after removing the catalytic layer residues a planarity control is effected on the surface which is likely to have undergone a number of short-circuiting events in its operating life : should any zones of the surface be considered inadequate for a quality production, such zones are easily removed by milling and replaced with new chunks again secured by laser welding or, in case of minor substitutions, also by TIG or equivalent welding. The machining then proceeds with the above described steps.

The foregoing description of the present invention makes reference to a particular embodiment: nevertheless, the invention must not be regarded as limited to such a particular embodiment, but conversely as encompassing all the modifications that one skilled in the art could identify and which are the object of the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS - Fig. 1 is a three-dimensional view of a first embodiment of the anode of the invention.

- Fig. 2 is a cross section of the anode of fig. 1 - Fig. 3 is a sketch of the cell in which the anode of the invention is installed.

- Fig. 4 is a three-dimensional view of a second embodiment of the anode of the invention.

- Fig. 5 is a cross section of the anode of fig. 4.

DETAILED DESCRIPTION OF THE INVENTION Fig. 1 represents a three-dimensional view of an anode (1) according to the invention, characterised in that it is made by a substrate (2) whose essentially planar surface (3) comprises a multiplicity of structures (4) in the form of as strips with U-shaped cross section. The strips have substantially the same length as the substrate as measured in the parallel direction with respect to the direction of the motion of the sheet to be coated across the cell (direction represented by the arrow (5) ). The strips are disposed on the surface (3) in a reciprocally parallel orientation with the base (6) of the U-shaped section in direct contact with the surface (3) of the planar substrate (2): the securing may be effected with any known method capable of ensuring a good electrical continuity between strips and planar substrate. A suitable procedure is the one based on spot or preferably continuous welding : one technology that can be employed is the TIG type or equivalent, or preferably the laser type, particularly the automated laser type of high productive capacity in which the single pieces are first mechanically pre-set in the desired positions by means of a suitable mould, then subjected to the action of the laser beam and finally set free from the mould as a final product characterised by a low grade of warping caused by the small thermal effect of the welding.

Fig. 2 shows a cross section along the line A-A of figure 1: the U-shaped profile of the strips (4), the weld (7) and the catalytic layer (8) for oxygen and/or chlorine evolution are evidenced. As regards the dimensions of the strips (4), these are characterised by the thickness t, the height of the lateral surfaces h, the base width w and the spacing s.

Particularly satisfying operating results have been obtained with anodes of the type illustrated in figs. 1 and 2 installed in industrial cells, when the thickness t, the height h, the width w and the spacing s are respectively 1,5, 7 and 5 to 7 mm. However, satisfactory results can still be obtained with several combinations of different values of t, h, w and s.

The industrial tests have demonstrated that the optimum value of h can be correlated to the value of the gap established between moving sheet and anode surface and that more particularly such value of h is preferably not higher than 50% of such gap and that more preferably such value h should not exceed 33% of the gap.

The anode of figs. 1 and 2 can be installed alone or together with other equivalent ones, all facing the same surface of the moving sheet to be coated when the coating is applied to that surface only (case sketched in fig. 3) or as an alternative the anodes can be split in two rows placed symmetrically with respect to the moving sheet when both surfaces thereof are to be coated.

Figure 3 shows the sketch of a vertical type cell : (9) indicates the cell body, (10) the sheet in motion (guided from the cell inside by means of at least one <BR> <BR> admission roller (11), one extraction roller (12) and one inversion roller (13) ), (14) the row of anodes placed ni parallel with respect to the only surface (15) to be coated of the sheet (10), (16) and (17) respectively the inlet and the outlet of the solution containing the metal to be deposited in ionic form. The inner design of these cells, like the one of the equivalent horizontal cells, is rather complex as it is necessary to ensure a flow of solution with high linear velocity to guarantee both an effective withdrawal of the gas bubbles from the cell as homogeneous dispersion in the liquid phase, and an adequate mass transport of the ion to be deposited, two conditions that are essential for obtaining good quality deposits. The details of this inner design are not shown in figure 3, and likewise the devices for fastening the anodes within the cells are not shown in the same figure 3 nor in figures 1 and 2.

The experience deriving from the operation of industrial cells has demonstrated that the anodes of the invention are able to withstand the contact with the moving sheets and the relative short-circuit and are able to guarantee a reliable operation continuity to a much greater extent than the equivalent anodes of the prior art simply built out of a substrate with essentially planar surface.

Furthermore it has been observed that the loss in the catalytic activity of the layer applied to the anode surface is substantially slowed down and the lifetime of the anodes of the invention is thereby increased with respect to the anodes of the prior art. Making clear that the validity of the present invention is not bound to any particular theory, it can be assumed that the enhanced resistance to short-circuits is to be attributed to the presence of surfaces which protrude from the planar surface of the substrate (2): when the sheet, due to the oscillations promoted by its high speed motion, smashes on the anode, the collision does not occur anymore between the two planar surfaces of the moving sheet and of the prior art anode with an extended contact area, but rather between the sheet and the crests of the structures (4) in figs. 1 and 2 with a very limited contact area. Moreover the passage of the short-circuit electric current is further reduced by the height h of the U-shaped structure (4) sides: in this regard it would be ideally desirable that the thickness t of the sidewalls of the U-shaped structures be as thin as possible so as to minimise the short-circuit current, nevertheless the sidewalls of the U-shaped structures have also the additional function of opposing a mechanical resistance to the impact with the sheet in order to reduce the damage to the catalytic layer (8) to a minimum, and for this reason it would be apparently advantageous to increase the value of t. As a conclusion, the industrial experience has shown that the optimum values of t come from a trade-off between the two mentioned needs and are affected by the values of h, w and s: in particular, the optimum value of t, at a given value of h, increases when w and s increase. In the specific case of h, w and s respectively equal to 5 0.2, 7 0.2 e 5-7 millimetres, the optimum value of t is in the range of 1 millimetre, being anyway comprised between 0.8 and 1.2 millimetres.

The increased operating life of the anodes of the invention should be also probably attributable to the bigger effective working surface: for example, in the above mentioned case of the U-shaped structures characterised by t, h, w and s respectively equal to 1 0.2, 5 0.2, 7 0.2 and 5-7 millimetres, the effective working area results to be about 2.5 times bigger than that of the corresponding prior art anode having the same projected surface. It has to be noted that in the above discussed specific case, the ratio of the surface of the crests to the surface comprised between two adjacent lateral walls of the same U-shaped structure or between two adjacent U-shaped structures results to be 1: 5-1: 7, substantially lower that the disclosure of US 4,936, 971.

In general the effective area of the anode of the invention mainly depends from the values assumed by h, w and s: in particular, for a given value of h, it is evident that the effective area is increased when the value of w and s is decreased, that is when the lateral walls of the U-shaped strips are closer. This situation is however negative from the standpoint of resistance to short-circuits provoked by the collisions with the moving sheet : in fact, the increase in the population of crests of the U-shapes strips determines also an increase of the contact surface with the sheet and thus of the electric current associated to the short-circuit with a consequent aggravation of the damage. It has been observed that an optimal placement of the U-shaped strips is the one giving substantially similar values of w and s, comprised between 3 and 10 millimetres, when h is 3 to 10 millimetres in its turn. The reason why substantially similar values of w and s are particularly advantageous is probably the homogeneity of access of the current in the areas provided with the catalytic layer and defined by the sidewalls of the U-shaped strips, within each strip and between adjacent pairs of strips. Other combinations of values of h, w, s capable of giving satisfactory results are of course possible. Fig. 2 shows the distribution of the catalytic layer (8) which results to be placed in the area defined within each strip and between each pair of adjacent strips: it can also be observed that in a preferred embodiment the catalytic layer is not present on the crests of the U- shaped strips. As the current procedures do not allow applying the catalytic layer on predetermined zones, the fraction of catalytic layer present on the crests can be removed with a milling or grinding procedure that also allows obtaining anodes with optimum planarity eliminating the profile irregularities generated by small distorsions. The latter are inevitably caused by the welding procedures even though the single components are pre-assembled and fixed in an appropriate mould prior to the beginning of the welding operation. Thus the crests of the three-dimensional structures of the present invention, besides presenting a reduced surface with respect to the indications of the prior art as previously shown, are preferably free from catalytic layer and therefore, as a further difference from the prior art, are absolutely not involved in the evolution of gas (oxygen and/or chlorine), which remains confined in the region of anode delimited by the sidewalls of the U-shaped structures and of the exposed portion of the planar substrate (2). In a preferred embodiment, the ratio of the surface of the crests of the catalytic layer-free U-shaped structures to the total activated surface, on the anodic planar substrate and on the sidewalls of the U- shaped structures, is equal to or lower than 0.2.

Figures 4 and 5 (the latter showing a cross section of the former along the line B-B) illustrate a different arrangement of the U-shaped strips, which are positioned in parallel to each other as in the anode of figures 1 and 2, however without being mutually spaced: this configuration has the merit of maintaining the advantages of the anode of figures 1 and 2, with an equivalent increase of active surface but with a higher mechanical resistance to the collision with the oscillating sheet in motion imparted by the bigger population of coupled U- shaped structure sidewalls. Conversely the solution of figs. 4 and 5 entails higher production costs associated to the greater amount of material employed, to the bigger extent of the welds (6) and to the longer pre-assembling times of the moulds.

Thanks to the longitudinal arrangement of the structures (4) of the invention and to their substantially free cross section, the anode surface profile according to the two embodiments of the invention, respectively of figs. 1 and 2 and of figs. 4 and 5, presents the additional advantage of not interfering in an appreciable way with the hydrodynamic regime of the solution inside the cell and in particular within the anode-moving sheet gap, said regime having been studied since a long time in the prior art and optimised for the cells equipped with conventional planar anodes: the prior art teachings, an example among the many references being US 4,584, 066, indicate in fact that the optimum hydrodynamic regime is the result of a delicate balance of many factors whose slightest perturbation may result in a decay of the coating quality.

The above considerations have been developed making reference to a preferred embodiment of the anode of the invention comprising a planar substrate to which U-shaped structures positioned in parallel to each other in the direction of the solution flow are secured. It is clear, nevertheless, that similar results can be obtained with differently shaped structures, for instance C-, Z-, L-shaped structures or by means of simple strips with quadrangular cross section, when the previously disclosed criteria of mechanical resistance and effective area, which evidently have a general validity, are observed.

In a less preferred embodiment, the structures parallel one to the other may be not parallel to the flow of the solution (and therefore to the main dimension of the planar subtrate) but rather form an angle comprised between 0 and 45° with the latter, however, it is preferable that said angle, in this case, be by far lower than 45° not to face the risk of perturbing the hydrodynamic regime of the process; the structures may be provided with suitable slots parallel to the flow of the solution to avoid the incurrence of undesired turbulence.

As far as the fabrication procedures are concerned no difficulties ad thus no particular additional costs are present. In the case of anodes of new production, the procedure provides the following steps: degreasing of the substantially planar substrate, preparation of the structures of the present invention by cutting and/or bending of metallic band and degreasing of the same, insertion and pre-setting of the structures and of the planar substrate in an appropriate mould, securing by means of a known technique and preferably by TIG welding or equivalent and even more preferably by laser welding, the latter preferably being a continuous welding for a better current distribution, extraction from the mould, sandblasting and/or etching with various reactants and application of the catalytic layer comprising at least one metal of the platinum group or oxides thereof, optionally in admixture with oxides of metals selected from the group of tin, antimony, lead, manganese, titanium, niobium, zirconium, tantalum as known to those skilled in the art, and optional final machining such as milling or grinding to obtain the double advantage of an optimum planarity of the surface formed by the crests of the structures of the invention and of the elimination of the catalytic layer from the same. This procedure also allows the retrofitting of conventional anodes made out of a simple planar substrate to anodes of the invention according to one of the following two preferred embodiments: in the former alternative, the substrate with the residues of the spent catalytic layer is used as such after washing to remove the traces of the solution employed in the coating cell, in the latter alternative the residues of the spent catalytic layer are previously eliminated, for instance by sandblasting and/or etching according to the teachings of the prior art. The latter alternative is generally preferred as it permits to obtain a better fixing of the structures of the invention to the planar substrate, both from the mechanical and from the electrical standpoint.

In the case of anodes of the invention removed from industrial cells at the end of their lifetime, the restoring procedure for putting the same once more in operation comprises, prior to the steps mentioned before for the new anodes, a pre-treatment including the measurement the residual planarity after the several short-circuits that are likely to have occurred during the previous operation and the removal of the parts deemed unacceptable in terms of uniformity of electric current distribution, wherein such removal preferably consists in the elimination by milling of the damaged zones, in the placement of new chunks in the milled zones and in the welding of the chunks to the underlying substrate.