A METHOD FOR ELECTROPLATING WITH A REFRACTORY METAL

Abstract: The invention relates to electroplating with refractory metal, mainly tantalum and niobium, from molten salts and can be applied in chemical, metallurgical, pharmaceutical, medicinal industries, turbine manufacture, air- and spacecraft, and other areas of engineering, in creation of corrosion-resistant and barrier coatings. The essence of the invention is that when the article to be coated is immersed into a molten electrolyte containing fluorides of both refractory and alkali metal and a eutectic melt of sodium, potassium and cesium chlorides, the article is warmed up to the working temperature of the electrolyte of 700-770 DEG C whereupon direct or reverse electric current is passed through the electrolyte, the current parameters being adjusted so that quantity of electricity in the anodic Qa, and cathodic Qc, parts of the electroplating cycle corresponds to the ratio O</=Qa/Qc < 0.9. To improve the article quality it is desirable that the weight of the electrolyte exceeds that of the article by 5 times or more. The technical result attained is the production of uniform-thickness, high quality tantalum or niobium coatings on articles for industrial applications made of conventional materials. Open porosity of the resulting coatings is not higher than 0.001 %, adhesion to the substrate is as high as 8 kg/mm<2>.

Description: A METHOD FOR ELECTROPLATING WITH A REFRACTORY METAL THE PRESENT INVENTION relates to a method of electroplating with a refractory metal such as tantalum or niobium. Because of the resistance of metals such as tantalum and niobium to corrosion and high temperatures, plating of articles with these metals would be potentially advantageous in many fields, including the chemical, metallurgical, pharmaceutical, medicinal and food industries, in turbine manufacture, aircraft and space industries and in other areas of engineering. Known techniques for plating metal articles with such refractory metals involve electroplating in an electrolyte comprising a mixture of fused salts, the mixture including a salt or salts of the refractory metal with which the article is to be plated. Known problems with electroplating with refractory metals are that it is difficult to make the resulting coatings of refractory metal non-porous and uniform in thickness, and that it is difficult to secure good adhesion of the coating of refractory metal to the substrate. These problems are aggravated where the articles to be electroplated are large and/or of complex configurations, as is often the case with, for example, articles intended for industrial use. The inventors have concluded that this is because of the greater weight of the articles to be coated and their more complicated configuration, (as compared with the small and simple test items used in laboratory trials) , which results in uneven heating of the articles and leads to the appearance of defects in the coatings and poorer adhesion to the substrate. On the inadequately heated portions of such articles, powder-like deposits instead of uniform coatings may form. At the edges and ridges of the articles, dendrites are observed to form and grow, which adversely affect the uniformity of the coating and cause the formation of open pores in the coating. These defects are particularly characteristic of intensive electrolysis regimes conducted at high current densities. A method is known for electroplating with a refractory metal, more particularly niobium, (see U. Cohen, J. Electroche . Soc, v. 128, 1981, p. 731-740), in which the article to be coated is placed inside a hermetic electrolyzer which is filled with argon. The hermetic electrolyzer also contains a molten electrolyte, at a temperature of 740-770°C, consisting of niobium, lithium and potassium fluorides. The starting electrolyte has the following composition, mole % : K 2 NbF 7 - 6, LiF - 47, KF - 37, KHF 2 - 10. The article is immersed in the electrolyte, and the article is plated in a reverse current regime, that is to say, periods in which the article being plated forms the cathode in the electrolytic cell and in which, accordingly, electric current is passed in through the cell in a sense to deposit the refractory metal on the article (the cathodic part of the cycle) , alternate in a repeating cycle with periods in which the article being plated forms the anode in the electrolytic cell and in which, accordingly, electric current is passed in the opposite sense (the anodic part of the cycle) . The density of this current is 0.05-0.3 A/cm 2 and 0.250-0.375 A/cm 2 during the cathodic and anodic part of the cycle respectively, and the durations of the cathodic and anodic parts of the cycle are 2.5-25 seconds and 1-10 seconds, respectively. The article is thus plated with niobium. A drawback to this method is production of porous coatings even with relatively low current densities and with a considerable (685 μm) thickness of the coating deposited. The porosity is mainly caused by oxygen impurity in the electrolyte melt. The principal source of this oxygen is hygroscopic potassium fluoride. To suppress the negative influence of the latter, potassium bifluoride is added to the known electrolyte. The potassium bifluoride decomposes in the course of heating up the electrolyte to the working temperature, with the formation of hydrogen fluoride which purifies the melt from the oxygen impurity. However, the excess hydrogen fluoride causes corrosion of the equipment, which in turn has a negative effect on the coating quality. As noted above, coating quality may be further reduced, in the case of large, (e.g. industrial) articles due to the non-uniformity of warming up of such large articles and their complex configuration. A further known method of electroplating with a refractory metal is disclosed in RU Patent No. 2062205. This method again involves placing the article to be plated inside a hermetic electrolyzer which is filled with argon. The hermetic electrolyzer also contains a molten electrolyte, heated to a working temperature of 680-740°C, which contains potassium-niobium and sodium fluorides and a eutectic melt of sodium, potassium and cesium chlorides. The starting electrolyte has the following composition, wt % : K 2 NbF 7 - 5.0-20.0, NaF - 7.9-11.0, NaCl- 9.9-11.0, Kcl- 10.4-11.9, CsCl - 51.7 - 61.0. The article is immersed in the electrolyte, a direct current is passed through the electrolyte at a density of 0.1-0.4 A/cm 2 , and a coating of a predetermined thickness is deposited on the article. This known method allows the production of dense non-porous coatings when they are deposited onto articles small in size and of a simple configuration. However, plating larger and intricately shaped articles again faces the problem of non-uniform heating up of the articles, and results in the appearance of defects in the coating and poor adhesion of the coating to the substrate. On sharp edges and ridges of the article dendrites emerge and grow quickly, which disturbs the uniformity of the layer, and is a potential source of open pores. This is most evident under intensive electrolysis regimes conducted at high current densities. An object of the present invention is to provide an improved method of electroplating with a refractory metal. According to this invention, there is provided a method for electroplating with a refractory metal, comprising the steps of: locating the article to be coated inside a hermetic electrolyzer which is filled with inert gas and contains a molten electrolyte consisting of refractory and alkali metal fluorides and a eutectic melt of sodium, potassium and cesium chlorides, said electrolyte being heated up to its working temperature, immersing the article in the electrolyte, passing an electric current through said electrolyte, and depositing a coating of predetermined thickness onto the article surface, characterized in that said article, prior to said step of passing electric current is warmed up in the electrolyte to a temperature substantially equal to the working temperature of the electrolyte, the weight ratio between the electrolyte and the article being not less than 5, and the parameters of the electric current are adjusted so that the quantity of electricity in the anodic Qa and the cathodic Qc parts of the electrodeposition cycle corresponds to the relationship 0<Q a /Q c <0.9 It will be understood that the quantities of electricity Q a and Q c are amounts of electrical charge, measurable in Coulombs, and that, in a single cycle, the quantities Q a and Q c represent the products of current amplitude during the respective phase by the duration of that phase. Preferably the quotient Q a /Q c lies in the range 0.1 to 0.5. Preferably, the temperature of the electrolyte is between 700°C and 770°C. Preferably the heating of the article to a temperature substantially equal to the temperature of the electrolyte is carried out by immersing the article in the electrolyte. Heating the article, in the electrolyte, to the electrolyte temperature is aimed at creating definite conditions for crystallizing the refractory metal deposited on the article in the form of a high-quality coating. This preheating of the article serves to ensure good adhesion of the deposited layer of refractory metal to the surface of the article. It will be understood that the surface of the article to be plated has a lower temperature than the electrolyte at the moment of immersing the unheated article in the electrolyte, so that the article must remain immersed in the electrolyte for some time before it attains the same temperature as the electrolyte. This time is herein referred to as the warming-up time. The warming-up time depends upon the mutual influence of various factors. In general, warming-up time depends on the ratio of the mass of the electrolyte to the mass of the article as well as on the thermophysical properties of the electrolyte and the article, power supply, etc. The ratio of the mass of the electrolyte to mass of the article to be plated is of a determining significance when industrial-purpose articles are coated with a refractory metal. With the value of this ratio less than 5, the coating uniformity is impaired, the rate of dendrite growth affecting the coating quality becomes higher and the process of electrodeposition becomes less intensive. The selection of the ratio Q a /Q c of the quantity Q a of electricity in the anodic phase to the quantity Q c in the cathodic phase of the cycle of electrodeposition within the range defined by the relationship 0<Q a /Q 0 <0.9 is influenced by the fact that as the ratio exceeds 0.9, the deposition of the coating in the cathodic part of the cycle will be swiftly approaching its dissolution in the anodic part, thus making the electrodeposition process extremely ineffective, whilst at Q a /Q c = 0, the process will be carried out using direct current. The process carried out using pure direct current is highly effective when depositing thin coatings. However, increasing the coating thickness while depositing at a constant direct current results in dendrite growth and deterioration of the coating quality. Reversing or alternating the current at the proposed ratio of the electricity quantity Q a and Q c in the anodic and cathodic parts of the cycle respectively has been found to eliminate the aforementioned drawbacks. Current density in both the cathodic and anodic parts of the cycle depends on a number of factors, the most important of them being the working temperature of the electrolyte, the level of impurities in the electrolyte, particularly oxygen, and the required thickness of the coating. The duration of current impulses in the cathodic and anodic parts of the cycle can be different or the same. Under the conditions of the proposed method, the most effective results have been obtained by conducting the process at values of the given ratio Q a /Q c of 0.1 - 0.5. Preferably, the duration of current impulses in the cathodic and anodic parts of the cycle are 50-100 sec and 5-20 sec, respectively. However, it is possible to operate the process with current impulses of equal duration in the cathodic and anodic parts of the cycle. (This may be convenient where, for example, the current used is derived substantially directly from the sinusoidal alternating current main supply) . Where current impulses of equal duration are used in the cathodic and anodic parts of the cycle, it is desirable that the ratio of the amplitude of the current in the cathodic part of the cycle to the amplitude in the anodic part should be in the range 3 : 1 to 8:1. At a working temperature of the electrolyte of less than 700°C it has been found that the conditions of the refractory metal electrocrystallization deteriorate, together with the quality of the coating. A metal powder forms the proportion of which increases with decrease in the temperature. At the same time, the adhesion of the coating to the base decreases. At a temperature higher than 770°C dendrites are observed to appear and the size of grains in the coating have been found to increase noticeably, thus worsening the properties of the coating. In addition, power consumption increases, the electrolyte losses increase through evaporation, and corrosion of the equipment increases. The working temperature in the process of electro-deposition is preferably measured and maintained with an accuracy of not less than ±5°C. In order that the invention may be more readily understood, and so that further features thereof may be appreciated, embodiments of the invention will now be described by way of example, with reference to the accompanying drawings in which : Figure 1 shows a plot of temperature of a 3.1 kg steel article against the time of its heating up to 720°C prior to tantalum plating in 16 kg by weight of a chloride-fluoride electrolyte, the power supplied for heating being 6.8 Kw, Figure 2 shows the refractory metal electrodeposition time dependence on the ratio of the quantity of electricity in the anodic and cathodic parts of the electrodeposition cycle - Q a /Q c , and Figure 3 is a Table of results. As may be seen from Figure 1, when an article to be plated, initially at ambient temperature, is immersed in a molten electrolyte, the temperature of the article changes according to two laws. In an initial brief period of time the article immersed into the melt is rapidly heated up while the temperature of the electrolyte drops. This is followed by a longer period, during which the entire system, including both the article and the electrolyte, is heated to the working temperature due to externally supplied power. The method for electroplating with a refractory metal is carried out in the following way: The electrolyzer is charged with a eutectic melt of sodium, potassium and cesium chlorides taken in a weight ratio of 15.6:16.3:68.1, respectively, and sodium fluoride in an amount of 5-11 wt% of the total weight of the electrolyte. The electrolyzer, containing these salts, is then hermetically sealed and evacuated at room temperature. It is then vacuum-thermally treated, by stepwise heating up to 400°C. The electrolyzer is then filled with an inert gas such as argon, purified from moisture and oxygen, and the salt mixture is fused. This is followed by cooling to room temperature, whereupon potassium heptafluometallate is added in amounts of 5-20 wt.% of the total electrolyte weight. The metallic component of the potassium fluometallate comprises a refractory metal such as tantalum or niobium. The electrolyzer is evacuated during heating up to 100°C, filled with argon and further heated to the working temperature of 700-770°C. Then, preliminary electrolysis at a current density 0.005A/cm 2 is conducted for fine purification of the electrolyte from the residual impurities. The degree of readiness of the electrolyte for working electrodeposition depends upon the quality of the deposit at the cathode after the preliminary electrolysis. With a satisfactory quality of the deposit, the article to be coated, whose weight should be one fifth or less of the weight of the electrolyte, is introduced into the electrolyte through a sluice chamber of the electrolyzer, and is immersed in the electrolyte. The article is kept in the electrolyte melt until the temperature of the article attains that of the electrolyte, i,e. 700-77O°C. Then, direct or reverse electric current is passed through the electrolyte and the process of electro-deposition is conducte . The electric current parameters are such that the quantity of electricity in the anodic Q a and the cathodic Q c parts of the electro-deposition cycle correspond to the relationship 0<Q a /Q c <0.9 (see Fig.2). As seen from Fig. 2, the duration of the process increases practically linearly with the growth of the Q a /Q c ratio from 0 to 0.5. Increasing this ratio further results in a more considerable increase of the time of deposition, and at values greater than 0.9 it tends to approach infinity. Therefore, carrying out the process in the interval of the 0.9-1.0 ratio values is ineffective. Electrolysis is conducted using a soluble anode of either niobium or tantalum depending on the refractory metal deposited. After a refractory metal coating of the required thickness has been formed on the surface of the article, the article is lifted to the sluice chamber and cooled, under argon, to room temperature. Residual electrolyte is then washed off the article which is dried before coating quality control is performed. The essence and advantages of the proposed method will become more fully apparent from the following examples of specific implementations: Example 1 A tantalum coating was deposited onto a stainless steel impeller blade weighing 3.1 kg. The article to be coated was immersed in an electrolyte having the composition: potassium heptafluotantalate - 10 wt.%, sodium fluoride - 7 wt.%, the balance being a eutectic melt of sodium, potassium and cesium chlorides taken in the ratio 15.6 : 16.3 : 68.1. Total weight of the electrolyte was 16 kg, and the ratio of the electrolyte weight to that of the article was 5.16. The article was held in the melt for 65 minutes (see Fig. 1) to allow it to become heated to the working temperature of the electrolyte, i.e. to 720°C. Then, electrodeposition was carried out over 60 minutes, at a cathodic current density of 0.15 A/cm 2 , and an anodic current density of 0.4A/cm 2 . The time of the cathodic part of the cycle was 100 seconds and the time of the anodic part of the cycle was 10 seconds. The Q a /Q c ratio was 0.27. The resulting 48μm thick tantalum coating on the impeller blade was uniform, and had no dendrites on the surface or open pores. Iron content in the coating was 0.001 % wt and adhesion to the substrate was 7kg/mm 2- The coating porosity did not exceed 0.001%. The principal conditions of the electroplating with a refractory metal and data obtained for Example 1 and Examples 2-13 described below are summarized in the accompanying Table, which forms Figure 3. Example 2 A tantalum coating was deposited onto a stainless steel cut-off valve weighing 2.2 kg. The article to be coated was immersed in an electrolyte having the following composition: potassium heptafluotantalate - 10 wt.%, sodium fluoride - 7 wt.%, the balance being the eutectic mixture of sodium, potassium and cesium chlorides with the ratio 15.6 : 16.3 : 68.1. Total weight of the electrolyte was 16 kg, and the electrolyte to article weight ratio was 7.27. The article was held in the melt for 45 minutes while it attained the working temperature of the electrolyte, equal to 700°C, whereupon the electrodeposition was conducted over 51 minutes at a cathodic current density of 0.15 A/cm 2 and an anodic current density of 0.4 A/cm 2 . Duration of the cathodic part of the cycle was 100 seconds, and for the anodic part of the cycle the duration was 10 seconds. The Q a /Q c ratio was 0.27. The resulting 40 μm thick tantalum coating was uniform, and had no dendrites on the surface or open pores. Iron content in the coating was 0.001 wt.%. The coating porosity did not exceed 0.001%, and adhesion to the substrate was 5.5 kg/mm 2 . In Examples 3-4 set out in the Table which forms Figure 3 , the process was conducted according to the conditions of Example 2 , except for the working temperatures of the electrolyte. Accordingly no detailed description of the method of Examples 3 and 4 is given below. Example 5 A tantalum coating was deposited onto a stainless steel cut-off valve weighing 3.2 kg. The article to be coated was immersed in an electrolyte having the composition: potassium heptafluotantalate - 10 wt.%, sodium fluoride - 7 wt%., the balance being a eutectic melt of sodium, potassium and cesium chlorides with the ratio 15.6 : 16.3 : 68.1. Total weight of the electrolyte was 16 kg, and the electrolyte to article weight ratio was 5.0. The article was held in the melt for 65 minutes, in the course of which it attained the working temperature of the electrolyte, equal to 730°C. The electrodeposition was then conducted over 103 minutes, at a cathodic current density of 0.15 A/cm 2 and an anodic current density of 0.4 A/cm 2 . Duration of the cathodic part of the cycle was 100 seconds, and the duration of the anodic point of the cycle was 10 seconds. The Q a /Q c ratio was 0.27. The resulting 77 μm thick tantalum coating was uniform and did not have dendrites on the surface or pores. Iron content in the coating was 0.001 wt.%. The coating porosity did not exceed 0.001%, and adhesion to the substrate was 7.5 kg/mm 2 . Example 6 A niobium coating was deposited onto a copper electrode weighing 2.2 kg. The article to be coated was immersed in an electrolyte having the composition : potassium heptafluoniobate - 10 wt%., sodium fluoride - 7 wt.%, the balance being a eutectic melt of sodium, potassium and cesium chlorides taken in the ratio 15.6: 16.3 : 68.1. Total weight of the electrolyte was 16 kg, and the electrolyte to article weight ratio was 7.27. The article was held in the melt for 40 minutes, in the course of which it attained the working temperature of the electrolyte equal to 720°C. This was followed by electrodeposition over 36 minutes at a cathodic current density 0.15 A/cm 2 , and an anodic current density 0.3A/cm 2 , and duration of the cycle parts 100 and 5 seconds, respectively. The Q a /Q c ratio was 0.1. The resulting 46 μm thick niobium coating was uniform, and had no dendrites on the surface or open pores. Iron content in the coating was 0.001 wt.%. The coating porosity did not exceed 0.001%, and adhesion to the base was 5.5 kg/mm 2 . Example 7 A niobium coating was deposited onto a copper electrode weighing 3.2 kg. by the following process. The article to be coated was immersed in an electrolyte having the following composition: potassium heptafluoniobate - 10 wt.%, sodium fluoride - 7 wt.%, the balance being a eutectic melt of sodium, potassium and cesium chlorides with the ratio 15.6 : 16.3 : 68.1. Total weight of the electrolyte was 16 kg, and the electrolyte to article weight ratio was 5.0. The article was held in the melt for 60 minutes until it attained the working temperature of the electrolyte equal to 760°C. This was followed by electrodeposition over 73 minutes at a cathodic current density of 0.15A/cm 2 , and an anodic current density of 0.3A/cm 2 - Duration of the cycle parts was 100 and 5 seconds respectively. The Q a /Q c ratio was 0.1. The resulting 95 μm thick niobium coating was uniform, rough, and did not have dendrites on the surface or open pores. Iron content in the coating was 0.001 wt.%. The coating porosity did not exceed 0.001%, and adhesion to the base was 6.5 kg/mm 2 . Example 8 A tantalum coating was deposited onto a stainless steel cut-off valve weighing 3.0kg. The article to be coated was immersed in an electrolyte having the following composition : potassium heptafluotantalate - 10 wt.%, sodium fluoride - 7 wt.%, the balance being a eutectic mixture of sodium, potassium and cesium chlorides taken in the ratio 15.6 : 16.3 : 68.1. Total weight of the electrolyte was 16 kg, and the electrolyte to article weight ratio was 5.33. The article was held in the melt for 65 minutes, in the course of which it attained the working temperature of the electrolyte equal to 730°C. The electrodeposition was conducted for 176 minutes at a cathodic current density of 0.1 A/cm 2 and an anodic current density of 0.25A/cm 2 . Duration of the cathodic part of the cycle was 100 seconds, and that of the anodic part was 20 seconds. The Q a /Q c ratio was 0.5. The resulting 79μm thick tantalum coating was uniform, and had no dendrites on the surface, or open pores. Iron content in the coating was 0.001 wt.%. The coating porosity did not exceed 0.001%wt, and adhesion to the substrate was 7.5 kg/mm 2 . Example 9 A tantalum coating was deposited on a stainless impeller blade weighing 3.1 kg. The article to be coated was immersed in an electrolyte of the following composition: potassium heptafluotantalate - 10 wt.%, sodium fluoride - 7 wt.%, the balance being a eutectic melt of sodium, potassium and cesium chlorides taken in the ratio 15.6 : 16.3 : 68.1. Total weight of the electrolyte was 16 kg, and the electrolyte to article weight ratio was 5.16. The article was held in the melt for 65 minutes, in the course of which it attained the working temperature of the electrolyte (730°C) . This was followed by electrodeposition over 1000 minutes at a cathodic current density of 0.2 A/cm 2 and an anodic current density of 0.45 A/cm 2 . Duration of the cathodic part of the cycle was 50 seconds and that of the anodic part was 20 seconds. The Q a /Q c , ratio was 0.9. The resulting 81 μm thick tantalum coating was uniform, and had no dendrites on the surface or open pores. Iron content in the coating was 0.001 wt.%. The coating porosity did not exceed 0.001 wt.% and, adhesion to the substrate was 8 kg/mm 2 . Example 10 A niobium coating was deposited onto a stainless steel thermocouple shell weighing 2.3 kg. The article to be coated was immersed in an electrolyte having the composition: potassium heptafluoniobate - 10 wt %, sodium fluoride - 7 wt %, the balance being a eutectic melt of sodium, potassium and cesium chlorides in the ratio 15.6 : 16.3 : 68.1. Total weight of the electrolyte was 16 kg, and the electrolyte to article weight ratio was 6.96 kg. The article was held in the melt for 45 minutes in the course of which it attained the working temperature of the electrolyte, equal to 730°C. This was followed by electrodeposition over 60 minutes at a direct current density 0.05A/cm 2 . The Q a /Q c ratio was 0. The resulting 48μm thick niobium coating was uniform, and had no dendrites on the surface or open pores. Iron content in the coating was 0.001 wt.%. The coating porosity did not exceed 0.001%, and adhesion to the substrate was 6.5kg/mm 2 . Example 11 A niobium coating was deposited on a thermocouple shell from stainless steel 2.3 kg by weight. The article to be coated was immersed in an electrolyte of the following composition : potassium heptafluoniobate - 10 wt.%, sodium fluoride - 7 wt.%, the balance being the eutectic melt of sodium, potassium and cesium chlorides taken in the ratio 15.6 : 16.3 : 68.1. Total weight of the electrolyte was 16 kg, the electrolyte to article weight ratio was 6.96. The article was held in the melt for 45 in. in the course of which it attained the working temperature of the electrolyte equal to 720°C. This was followed by electrodeposition during 50 minutes using sinusoidal periodic current of 50 Hz frequency with the amplitude in the cathodic part equal to 0.1 A/cm 2 and that in the anodic part being 0.033 A/cm 2 . The Q a /Q c ratio was 0.33. The resulting 54 μm thick niobium coating was uniform and had no dendrites or open pores on the surface. Iron content in the coating was 0.001 wt.% The coating porosity did not exceed 0.001%, adhesion to the substrate was 6.0 kg/mm 2 . Example 12 A tantalum coating was deposited on a copper electrode 2.2kg by weight. The article to be coated was immersed in an electrolyte of the following composition : potassium heptafluotantalate - 10 wt.%, sodium fluoride - 7 wt.%, the balance being the eutectic melt of sodium, potassium and cesium chlorides taken in the ratio 15.6 : 16.3 : 68.1. Total weight of the electrolyte was 16 kg, the electrolyte to article weight ratio was 7.27. The article was held in the melt during 40 minutes, in the course of which it attained the working temperature of the electrolyte equal to 720°C. This was followed by electrodeposition during 40 minutes using sinusoidal alternating current of 50 Hz frequency with the amplitude in the cathodic part equal to 0.13 A/cm 2 and that in the anodic part being 0.022 A/cm 2 . The Q a /Q c ratio was 0.17. The resulting 58 μm thick tantalum coating was uniform, had no dendrites or open pores on the surface. Iron content in the coating was 0.001 wt.%. The coating porosity did not exceed 0.001%, adhesion to the substrate was 6.0 kg/mm 2 . Example 13 A tantalum coating was deposited on a copper electrode 2.2kg by weight. The article to be coated was immersed in an electrolyte of the following composition : potassium heptafluotantalate - 10 wt.%, sodium fluoride - 7 wt.%, the balance being the eutectic melt of sodium, potassium and cesium chlorides taken in the ratio 15.6 : 16.3 : 68.1. Total weight of the electrolyte was 16 kg and the electrolyte to article weight ratio was 7.27. The article was held in the melt for 40 minutes, in the course of which it attained the working temperature of the electrolyte (720°C) . This was followed by electrodeposition for 50 minutes using sinusoidal alternating current of 50 Hz frequency with the amplitude in the cathodic part equal to 0.112 A/cm 2 and that in the anodic part being 0.014 A/cm 2 . The Q a /Q c ratio was 0.13. The resulting tantalum coating, 65 μm thick, was uniform, rough and had no dendrites or open pores on the surface. Iron content in the coating was 0.001 wt.%. The coating porosity did not exceed 0.001%, adhesion to the substrate was 6.5 kg/mm 2 . Analysis of the data summarized in the Table shows that the proposed method makes it possible to obtain uniform, high-quality tantalum and niobium coatings on industrial articles produced from conventional materials. Open porosity of the resulting coatings does not exceed 0.001%, and adhesion to the base may be as great as 8kg/mm 2 .

Claims: CLAIMS 1. A method for electroplating with a refractory metal, comprising the steps of: locating the article to be coated inside a hermetic electrolyzer which is filled with inert gas and contains a molten electrolyte consisting of refractory and alkali metal fluorides and a eutectic melt of sodium, potassium and cesium chlorides, said electrolyte being heated up to its working temperature, immersing the article in the electrolyte, passing an electric current through said electrolyte, and depositing a coating of predetermined thickness onto the article surface, characterized in that said article, prior to said step of passing electric current is warmed up in the electrolyte to a temperature substantially equal to the working temperature of the electrolyte, the weight ratio between the electrolyte and the article being not less than 5, and the parameters of the electric current are adjusted so that the quantity of electricity in the anodic Qa and the cathodic Qc parts of the electrodeposition cycle corresponds to the relationship 0<Q a /Q c <0.9 2. The method of claim 1, wherein the working temperature of the electrolyte is between 700°-770°C. AMENDED CLAIMS [received by the International Bureau on 21 July 1998 (21.07.98); original claims 1-2 replaced by new claims 1-3 (ipage)] 1. A method for electroplating with a refractory metal , comprising the steps of: locating the article to be coated inside a hermetic electrolyzer which is filled with inert gas and contains a molten electrolyte consisting of refractory and alkali metal fluorides and alkali metal chlorides , said electrolyte being heated up to the working temperature, immersing the article into the electrolyte, passing an electric current through said electrolyte, and depositing a coating of predetermined thickness onto the article surface, characterized in that said article, prior to said step of passing electric current, is warmed up in the electrolyte to the working temperature of the electrolyte of 700°-770°C, the weight ratio between the electrolyte and the article being not less than 5 , and the parameters of the electric current are adjusted so that the quantity of electricity in the anodic Qa and the cathodic Qc parts of the electrodeposition cycle corresponds to the relationship 0< Qa/Qc <0.9 2. The method of claim 1 , wherein a refractory metal is tantalum or niobium. 3. The method of claim 2 , wherein said molten electrolyte consists of tantalum or niobium, potassium and sodium fluorides and a eutectic melt of sodium, potassium and cesium chlorides .

Inventors: KOVALEVSKI VLADIMIR PAVLOVICH, MASLOV VALERI PETROVICH, POLIAKOV EVGUENI GEORGIEVICH, POLIAKOVA LARISSA PETROVNA, KOVALEVSKI, VLADIMIR, PAVLOVICH, MASLOV, VALERI, PETROVICH, POLIAKOV, EVGUENI GEORGIEVICH, POLIAKOVA, LARISSA, PETROVNA
Applicants: INST OF CHEMISTRY AND TECHNOLO, KOVALEVSKI VLADIMIR PAVLOVICH, MASLOV VALERI PETROVICH, POLIAKOV EVGUENI GEORGIEVICH, POLIAKOVA LARISSA PETROVNA, INSTITUTE OF CHEMISTRY AND TECHNOLOGY OF RARE ELEMENTS AND MINERAL RAW MATERIALS OF THE RUSSIAN ACADEMY OF SCIENCES KOLA SCIENCE CENTER, KOVALEVSKI, VLADIMIR, PAVLOVICH, MASLOV, VALERI, PETROVICH, POLIAKOV, EVGUENI GEORGIEVICH, POLIAKOVA, LARISSA, PETROVNA
Application year: 1997
Main classification: C25D3
IPC codes: C25D3, C25D5