Electrode for use in electrolytic processes and process for manufacturing it

Abstract: A metal surface is now described having enhanced adhesion of subsequently applied coatings. The substrate metal of the article, such as a valve metal as represented by titanium, is provided with a highly desirable surface characteristic for subsequent coating application. This can be initiated by selection of a metal of desirable metallurgy and heat history, including prior heat treatment to provide surface grain boundaries which may be most readily etched. In subsequent etching operation, the surface is made to exhibit well defined, three dimensional grains with deep grain boundaries. Subsequently applied coatings, by penetrating into the etched intergranular valleys, are desirably locked onto the metal substrate surface and provide enhanced lifetime even in rugged commercial environments.

Description: Background of the Invention The adhesion of coatings applied directly to the surface of a substrate metal is of special concern when the coated metal will be utilized in a rigorous industrial environment. Careful attention is usually paid to surface treatment and pre-treatment operation prior to coating. Achievement particularly of a clean surface is a priority sought in such treatment or pre-treatment operation. Representative of a coating applied directly to a base metal is an electrocatalytic coating, often containing a precious metal from the platinum metal group, and applied directly onto a metal such as a valve metal. Within this technical area of electrocatalytic coatings applied to a base metal, the metal may be simply cleaned to give a very smooth surface. U.S. Patent No, 4,797,182. Treatment with fluorine compounds may produce a smooth surface. U.S. Patent No. 3,864,163. Cleaning might include chemical degreasing, electrolytic degreasing or treatment with an oxidizing acid. U.S. Patent 3,864,163. Cleaning can be followed by mechanical roughening to prepare a surface for coating. U.S. Patent No, 3,778,307. If the mechanical treatment is sandblasting, such may be followed by etching. U.S. Patent No. 3,878,083. Or pickling with an non-oxidizing acid can produce a rough surface for coating. U.S. Patent No. 3,864.163. Such pickling can follow degreasing. U.S. patent No. Re. 28,820. The pickling may readily etch titanium to a surface roughness within the range of 150-200 or more microinches. "Titanium as a Substrate for Electrodes", Hayfield, P.C.S., IMI Research and Development Report. If there is a pre-existing coating present on the substrate metal, the metal can be treated for coating removal. For an electrocatalytic coating, such treatment may be with a melt containing a basic material used in the presence of an oxidant or oxygen. Such can be followed by pickling to reconstitute the original surface for coating. U.S. Patent No, 3,573,100. Or if a molten alkali metal hydroxide bath is used containing an alkali metal hydride, this is preferably followed by a hot mineral acid treatment. U.S. Patent No. 3,706,600. It has also been proposed to prepare the surface without stripping the old coating. U.S. Patent No. 3,684,543. More recently, this procedure has been improved by activation of the old coating, prior to application of the new. U.S. Patent No. 4,446,245. Another procedure for anchoring the fresh coating to the substrate, that has found utility in the application of an electrocatalytic coating to a valve metal, is to provide a porous oxide layer which can be formed on the base metal. It has however been found difficult to provide long-lived coated metal articles for serving in the most rugged commercial environments, e.g., oxygen evolving anodes for use in the present-day commercial applications utilized in electrogalvanizing, electrotinning, electroforming or electrowinning. Such may be continuous operation and can involve severe conditions that may lead to surface damage. It would be desirable to provide coated metal substrates to serve as electrodes in such operations, exhibiting extended stable operation while preserving excellent coating adhesion. It would also be highly desirable to provide such an electrode not only from fresh metal but also from recoated metal. The manufacture on an industrial scale of coated metal electrodes, especially coated titanium electrodes used as oxygen evolving anodes in high speed electrogalvanizing, electrotinning, electroforming or electrowinning, has involved the problem that some anodes perform satisfactorily for their anticipated lifetime, but an unacceptably high proportion of the anodes fail prematurely under the severe operating conditions and must be returned for stripping/recoating, despite strict controls in the coating procedures. The performance of coated titanium anodes is particularly erratic because the characteristics of the titanium vary substantially from one production batch to another and from one titanium sheet to another because of different heat histories and handling. This applies even to different specimens of titanium from the same batch. It was thus not possible to reliably predict anode performance based solely on careful control of the production conditions because the same etching/annealing procedure could lead to different results for different specimens. SUMMARY OF THE INVENTION There has now been found a metal surface which provides an excellent, locked on coating of outstanding coating adhesion. The coated metal substrate can have highly desirable extended lifetime even in most rigorous industrial environments. For the electrocatalytic coatings, the invention may provide for lower effective current densities and also achieve substrate metal grains desirably stabilized against passivation. In one aspect of the invention, a method of producing a coated metal electrode for use in electrolytic processes, comprises roughening the surface of a metal substrate and applying to the roughened surface an electrochemically active coating, and is characterized in that the substrate surface is roughened by intergranular etching, plasma spraying or other roughening technique to produce a surface roughness having a profilometer-measured average surface roughness of about 6.35 micrometers (about 250 microinches), or more, and an average surface peaks per cm of about 15.7 (about 40 peaks per inch), or more, based on a profilometer upper threshold limit of 10.16 micrometers (400 microinches) and a profilometer lower threshold limit of 7.62 micrometers (300 microinches), and applying the electrochemically active coating to the roughened surface after checking that it meets said specifications. The invention is based on the insight that the failure mechanism of such electrodes was related to the surface morphology of the substrates. It was found that satisfactory electrodes were those whose substrate surface before coating conformed to the aforementioned specification. Electrodes made with substrates which did not meet up to this specification tended to fail prematurely. It follows that it is possible to control whether or not the surface has achieved an optimum condition for receiving a coating so that when the surface is coated the risk of premature failure is practically completely eliminated. In one way of carrying out the invention, the substrate surface is roughened by : subjecting said surface to elevated temperature annealing for a time sufficient to provide an at least sustantially continuous intergranular network of impurities, including impurities at the surface of said metal; cooling the resulting annealed surface; and etching intergranularly the surface at an elevated temperature and with a strong acid or strong caustic etchant. Thus, in one method of manufacturing a coated metal electrode a metal substrate is surface treated by strong acid or strong caustic etching and the entire etched surface intended to be coated is subjected to optical inspection to determine whether the surface has been intergranularly etched to produce three-dimensional grains with deep grain boundaries. If this inspection is positive, the intergranularly etched surface is then inspected by profilometer to determine whether the three-dimensional grains having deep grain boundaries correspond across the surface to the specifications given above, namely a profilometer-measured average surface roughness of about 6.35 micrometers (about 250 microinches), or more, and an average surface peaks per cm of about 15.7 (about 40 peaks per inch), or more, based on a profilometer upper threshold limit of 10.16 micrometers (400 microinches) and a profilometer lower threshold limit of 7.62 micrometers (300 microinches). If after the optical or profilometer inspection the surface does not meet said specifications, the surface may be subjected to further strong acid or strong caustic etching and to optical and/or profilometer inspection again after the further treatment. If said specifications cannot be attained by further etching, another substrate from the same batch may be subjected to annealing (which where the first substrate has been annealed is different to the annealing of the first substrate) followed by etching and inspection, with further etching and inspection if necessary. At the appropriate stage of the process, the electrocatalytic coating is applied to the treated metal surface, but only after optical and profilometer inspection has revealed that the surface meets said specifications. In practice, it is advantageous to deal with individual electrodes produced from a single batch of titanium as follows. A specimen is treated and inspected as set out above. If optical inspection after etching reveals that the entire surface to be coated does not have the required three-dimensional grain configuration, the same sample can be further etched and inspected as many times as required until the desired surface state is achieved. But if a specimen cannot be brought to the surface specification by further etching, it is necessary to modify the initial annealing on another specimen from the same batch, followed by etching and inspection until the desired surface state is achieved. Once optical and profilometer inspections have revealed that a specimen has reached the required specifications following a given annealing/etching treatment, all further specimens from the same batch will be treated in the same manner. But because there is still a risk that the surface may not be optimized, it is still necessary to finally inspect each individual electrode for qualification to the specifications. If necessary, adjustments such as a further etch are made and the inspection procedure is repeated until each electrode meets the specifications, or has been rejected. After final inspection, the electrocatalytic coating can be applied directly or it is possible to perform a final "clean up" etch to present a stain-free chemically clean surface for coating, for example by etching for up to 5 minutes in fresh hot hydrochloric acid, followed by rinsing and drying. The optical inspection by microscope should scan the entire surface to be coated to ensure that the three-dimensional grain boundaries extend over the entire surface. Profilometer measurements can be carried out at selected points across the surface. To reduce the number of profilometer measurements, it is convenient to measure with settings above the minimum values given. It will be appreciated that this procedure according to the invention adds to the production cost of the electrodes, but leads to a great savings during use for example as anodes in electrogalvanizing, electrotinning, electroforming or electrowinning, due to the reliability of all the anodes and the elimination of stoppages for the replacement of prematurely-failed anodes. In other embodiments of the invention, the substrate surface is roughened by plasma spraying one or more of a valve metal or valve metal oxide, including valve metal suboxides, for example, onto a surface roughened by etching. The invention also concerns an electrode for electrolytic proposes comprising a metal substrate coated with an electrochemically active coating as claimed in claim 24, and the use of such electrodes, particularly as oxygen-evolving anode in electrogalvanizing, electrotinning, electroforming or electrowinning, or in the eletrodeposition of metal onto a substrate. When used as oxygen evolving anodes, even under the rigorous commercial operations including continuous electrogalvanizing, electrotinning, electroforming or electrowinning, such electrodes have highly desirable service life. Also, such electrodes may provide an effectively lower current density, which will aid in prolonging the life of the electrode, when used as above discussed or, for example, in water or brine electrolysis. The invention also covers electrochemical cells incorporating such anodes. DESCRIPTION OF THE PREFERRED EMBODIMENTS The metals of the substrate are broadly contemplated to be any coatable metal. For the particular application of an electrocatalytic coating, the substrate metals might be such as nickel or manganese, but will most always be valve metals, including titanium, tantalum, aluminum, zirconium and niobium. Of particular interest for its ruggedness corrosion resistance and availability is titanium. As well as the normally available elemental metals themselves, the suitable metals of the substrate can include metal alloys and intermetallic mixtures. For example, titanium may be alloyed with nickel, cobalt, iron, manganese or copper. More specifically, Grade 5 titanium may include up to 6.75 weight% aluminum and 4.5 weight% vanadium, grade 6 up to 6% aluminum and 3% tin, grade 7 up to 0.25 weight% palladium, grade 10, from 10 to 13 weight% molybdenum plus 4.5 to 7.5 weight% zirconium and so on. By use of elemental metals, alloys and intermetallic mixtures, it is most particularly meant the metals in their normally available condition, i.e., having minor amounts of impurities. Thus for the metal of particular interest, i.e., titanium, various grades of the metal are available including those in which other constituents may be alloys or alloys plus impurities. In titanium, iron may be a usual impurity. Its maximum concentration can be expected to vary from 0.2 weight percent for grades 1 and 11 up to 0.5% for grades 4 and 6. Additional impurities that may be found throughout the grades of titanium include nitrogen, carbon, hydrogen and oxygen. Since beta-titanium located at the titanium grain boundaries can be susceptible to etching, such beta-titanium is considered herein for purposes of this discussion as an impurity. Thus etching of an impurity as discussed herein may include etching of a phase of the metal itself. In addition to the beta-titanium, the titanium metal of particular interest may have beta-phase stabilizers, some of which may be present in extremely minor amounts in the manner of an impurity and include vanadium, niobium, tantalum, molybdenum, ruthenium, zirconium, tin, hafnium and mixtures thereof. Grades of titanium have been more specifically set forth in the standard specifications for titanium detailed in ASTM B 265-79. Regardless of the metal selected and how the metal surface is subsequently processed, the substrate metal advantageously is a cleaned surface. This may be obtained by any of the treatments used to achieve a clean metal surface, but with the provision that unless called for to remove an old coating, mechanical cleaning is typically minimized and preferably avoided. Thus the usual cleaning procedures of degreasing, either chemical or electrolytic, or other chemical cleaning operation may be used to advantage. Where an old coating is present on the metal surface, such needs to be addressed before recoating. It is preferred for best extended performance when the finished article will be used with an electrocatalytic coating, such as use as an oxygen evolving electrode, to remove the old coating. In the technical area of the invention which pertains to electrochemically active coatings on a valve metal, chemical means for coating removal are well known. Thus a melt of essentially basic material, followed by an initial pickling will suitably reconstitute the metal surface, as taught in U.S. Patent No. 3,573,100. Or a melt of alkali metal hydroxide containing alkali metal hydride, which may be followed by a mineral acid treatment, is useful, as described in U.S. Patent No. 3,706,600. Usual rinsing and drying steps can also form a portion of these operations. When a cleaned surface, or prepared and cleaned surface has been obtained, and particularly where applying an electrocatalytic coating to a valve metal, it is most always contemplated in the practice of the present invention that surface roughness will be achieved by means of etching. In the invention context of etching, it is important to aggressively etch the metal surface to provide deep grain boundaries providing well exposed, three-dimensional grains. It is preferred that such operation will etch impurities located at such grain boundaries. For convenience, a metal having etchable grain boundary impurities may be referred to herein as a metal having a correct "metallurgy". It is however contemplated that other roughening technique, which can be used in addition to or along with the roughness achieved by etching, such as plasma spraying of one or more of a valve metal or valve metal oxide, including valve metal suboxides, onto the metal surface can provide the surface roughness characteristics. These characteristics, as measured by profilometer, are more particularly described hereinbelow. Where etching has been selected to achieve surface roughness, an important aspect of the invention involves the enhancement of impurities of the metal at the grain boundaries. This is advantageously done at an early stage of the overall process of metal preparation. One manner of this enhancement that is contemplated is the inducement at, or introduction to, the grain-boundaries of one or more impurities for the metal. For example, with the particularly representative metal titanium, the impurities of the metal might include iron, nitrogen, carbon, hydrogen, oxygen, and beta-titanium. Although impurities introduction procedures that might he used can include surface deposition, e.g., vapor deposition, which might be followed by a heat treatment for surface impurity diffusion, one particular manner contemplated for impurity enhancement is to subject the titanium metal to a hydrogen-containing treatment. This can be accomplished by exposing the metal to a hydrogen atmosphere at elevated temperature. Or the metal might be subjected to an electrochemical hydrogen treatment, with the metal as a cathode in a suitable electrolyte evolving hydrogen at the cathode. Another consideration for the aspect of the invention involving etching, which aspect can lead to impurity enhancement at the grain boundaries, involves the heat treatment history of the metal. For example, to prepare a metal such as titanium for etching, it can be most useful to condition the metal, as by annealing, to diffuse impurities to the grain boundaries. Thus, by way of example, proper annealing of grade 1 titanium will enhance the concentration of the iron impurity at grain boundaries. Where the suitable preparation includes annealing, and the metal is grade 1 titanium, the titanium can be annealed at a temperature of at least about 500°C. for a time of at least about 15 minutes. For efficiency of operation, a more elevated annealing temperature, e.g., 600°-800°C. is advantageous. Annealing times at such more elevated temperatures will typically be on the order of 15 minutes to 4 hours. Alternatively, a short, high temperature anneal, e.g., on the order of 800°C. for a few minutes such as 5-10 minutes, may be continued, after rapid or slow cooling, at a quite low temperature, with 200°-400°C. being representative, for several hours, with 10-20 hours being typical. Suitable conditions can include annealing in air, or under vacuum, or with an inert gas such as argon. Subsequent cooling of the annealed metal can appropriately stabilize the grain boundaries for etching. Stabilization may be achieved by controlled or rapid cooling of the metal or by other usual metal cooling techniques including quenching. For convenience, a metal having such stabilization may be referred to herein as a metal having a desirable "heat history". For enhancing coating adhesion when the surface is roughened by etching, it can be desirable to combine a metal surface having a correct grain boundary metallurgy as above-discussed, with an advantageous grain size. Again, referring to titanium as exemplary, at least a substantial amount of the grains having grain size within the range of from about 3 to about 7 is advantageous. Grain size as referred to herein is in accordance with the designation provided in ASTM E 112-84. Titanium with a grain size below about 3 (above about 125 micrometers) produces a high percentage of broad grains which is detrimental to coating adhesion. Grain sizes above about 7 (below about 32 micrometers) are not desired for best three-dimensional grain structure development. Preferably for titanium, the grains will have size within the range from about 4 to about 6 (about 90 micrometers to about 45 micrometers). After the foregoing operations, e.g., cleaning, or coating removal and cleaning, and including any desired rinsing and drying steps, followed by any impurity enhancement for grain boundary etching, the metal surface is then ready for continuing operation. Where such is etching, it will be with a sufficiently active etch solution to develop aggressive grain boundary attack. Typical etch solutions are acid solutions. These can be hydrochloric, sulfuric, perchloric, nitric, oxalic, tartaric, and phosphoric acids as well as mixtures thereof, e.g., aqua regia. Other etchants that may be utilized include caustic etchants such as a solution of potassium hydroxide/hydrogen peroxide in combination, or a melt of potassium hydroxide with potassium nitrate. The etch solution is advantageously a strong, or concentrated, solution, such as an 18-22 weight% solution of hydrochloric acid. Moreover, the solution is advantageously maintained during etching at elevated temperature such as at 80°C or more for aqueous solutions, and often at or near boiling condition or greater, e.g., under refluxing condition. Following etching, the etched metal surface an then be subjected to rinsing and drying steps to prepare the surface for coating. Regardless of the technique employed to reach the desired roughness, e.g., plasma spray or intergranular etch, it is necessary that the metal surface have an average roughness (Ra) of about 6.35 micrometers (about 250 microinches), or more, and an average number of surface peaks per cm (Nr) of at least about 15.7 (about 40 peaks per inch). The surface peaks per cm can be typically measured at a lower threshold limit of 7.62 micrometers (300 microinches) and an upper threshold limit of 10.16 micrometers (400 microinches). A surface having an average roughness of below about 6.35 micrometers (about 250 microinches) will be undesirably smooth, as will a surface having an average number of surface peaks per cm of below about 15.7 (about 40 peaks per inch), for providing the needed, substantially enhanced coating adhesion. Advantageously, the surface will have an average roughness of on the order of about 6.35 micrometers (about 250 microinches), or more, e.g., ranging up to about 19.05-38.1 micrometers (about 750-1500 microinches), with no low spots of less than 5.08 micrometers (about 200 microinches). Advantageously, for best avoidance of surface smoothness, the surface will be free from low spots that are less than about 5.33 to 5.59 micrometers (about 210 to 220 microinches). It is preferable that the surface have an average roughness of from about 7.62 to about 12.7 micrometers (about 300 to about 500 microinches). Advantageously, the surface has an average number of peaks per cm of at least about 23.6 (about 60/inch) but which might be on the order of as great as about 51 (130/inch) or more, preferably with an average from about 31.5 (80/inch) to about 47.2/cm (120/inch). It is further advantageous for the surface to have an average distance between the maximum peak and the maximum valley (Rz) and average peaks height (Rm) of about 25.4 micrometers (about 1000 microinches), or more. All of such foregoing surface characteristics are as measured by a profilometer. More desirably, the surface for coating will have an Rm value of about 38.1 to about 88.9 micrometers (about 1500 microinches to about 3500 microinches), or more, and have a Rz characteristic of about 38.1 to about 88.9 micrometers (about 1500 microinches to about 3500 microinches), or more. As representative of the electrochemically active coatings that may then be applied to the roughened surface of the metal, are those provided from platinum or other platinum group metals or they can be active oxide coatings such as platinum group metal oxides, magnetite, ferrite, cobalt spinel or mixed metal oxide coatings. Such coatings have typically been developed for used as anode coatings in the industrial electrochemical industry. They may be water based or solvent based, e.g., using alcohol solvent. Suitable coatings of this type have been generally described in one or more of US Patents Nos. 3,265,526, 3,632,498, 3,711,385 and 4,528,084. The mixed metal oxide coatings can often include at least one oxide of a valve metal with an oxide of a platinum group metal including platinum, palladium, rhodium, iridium and ruthenium or mixtures between themselves and with other metals, Further coatings in addition to those enumerated above include manganese dioxide, lead dioxide, platinate coatings such as M x Pt₃O₄ where M is an alkali metal and x is typically targeted at approximately 0.5, nickel-nickel oxide and nickel plus lanthanide oxides. It is contemplated that coatings will be applied to the metal by any of those means which are useful for applying a liquid coating composition to a metal substrate. Such methods include dip spin and dip drain techniques, brush application, roller coating and spray application such as electrostatic spray. Moreover spray application and combination techniques, e.g., dip drain with spray application can be utilized. With the above-mentioned coating compositions for providing an electrochemically active coating, a modified dip drain operation can be most serviceable. Following any of the foregoing coating procedures, upon removal from the liquid coating composition, the coated metal surface may simply dip drain or be subjected to other post coating techniques such as forced air drying. Typical curing conditions for electrocatalytic coatings can include cure temperatures of from about 300°C up to about 600°C. Curing times may vary from only a few minutes for each coating layer up to an hour or more, e.g., a longer cure time after several coating layers have been applied. However, cure procedures duplicating annealing conditions of elevated temperature plus prolonged exposure to such elevated temperature, are generally avoided for economy. In general, the curing technique employed can be any of those that may be used for curing a coating on a metal substrate. Thus, oven curing, including conveyor ovens may be utilized. Moreover, infrared cure techniques can be useful. Preferably for most economical curing, oven curing is used and the cure temperature used for electrocatalytic coatings will be within the range of from about 450°C to about 550°C. At such temperatures, curing times of only a few minutes, e.g., from about 3 to 10 minutes, will most always be used for each applied coating layer. The following examples show ways in which the invention has been practised, as well as showing comparative examples. However, the examples showing ways in which the invention has been practiced should not be construed as limiting the invention. EXAMPLE 1 There is used a titanium plate measuring 2 inches by 6 inches by 3/8 inch (about 5.1 x 15.3 x 0.95 cm) and being an alloyed grade 1 titanium, as determined in accordance with the specifications of ASTM B 265-79. This titanium sheet thus contained 0.20 percent, maximum, iron impurity. This plate, which was a fresh grade 1 titanium plate, was degreased in perchloroethylene vapors, rinsed with deionized water and air dried. It was then etched for approximately 1 hour by immersion in 20 weight percent hydrochloric acid aqueous solution heated to 95°C. After removal from the hot hydrochloric acid, the plate was again rinsed with deionized water and air dried. By this etching, the plate achieves a weight loss of 500-600 grams per square meter of plate surface area. This weight loss is determined by pre and post etching weighing of the plate sample and then calculating the loss per square meter by straight forward calculation on the basis of the surface area of both large flat faces of the plate. The surface structure of the sample plate, on both broad surfaces, is then examined under a stereo microscope under magnification varying during the study from 40X to 60X. Such plate surface can be seen to have a well defined, three dimensional, grain boundary etch. The etched surface was then subjected to surface profilometer measurement using a Hommel model T1000 C instrument manufactured by Hommelwerk GmbH. The plate surface profilometer measurements are average values computed from eight separate measurements conducted by running the instrument in random orientation across one large flat face of the plate. This gave average values for surface roughness (Ra) of 9.98 micrometers (393 microinches), peaks per cm (Nr) of 38.8 (86/inch) and an average distance between the maximum peak and the maximum valley (Rz) of 53.44 micrometers (2104 microinches). The peaks per cm were measured within the threshold limits of 7.62 micrometers (300 microinches) (lower) and 10.16 micrometers (400 microinches), (upper). COMPARATIVE EXAMPLE 2 A titanium plate sample (5mm thick) of unalloyed grade 1 titanium, but from a different batch than the plate sample of Example 1, was etched in 18% HCl at boiling temperature for 1 hour. Visually, the resulting etched surfaces of the titanium plate sample, as viewed in the manner of Example 1, were found not to have a well defined grain boundary etch. Subsequent profilometer measurements, conducted in the manner of Example 1, provided average values of 2.54 micrometers (100) (Ra), 0 (Nr) and 16.92 micrometers(666) (Rz). The sample was etched an additional 1 hour. Profilometer values were only slightly improved, 2.97 micrometers (117) (Ra), 3.15/cm (8) (Nr) and 19.56 micrometers (770) (Rz), but were still well below those of Example 1. EXAMPLE 2 A second sample plate from the same batch of unalloyed titanium as was used for the plate sample of Comparative Example 2, was subjected to annealing operation. In this operation, the sample was placed in an oven and the oven was heated until the air temperature reached 700°C. This air temperature was then held for 15 minutes, cooled to 450°C, and held for 30 minutes. Thereafter, while the sample was maintained in the oven, the oven air temperature was permitted to cool to about 200°C in a period of 1.5 hours. The sample was then removed for cooling to room temperature. The resulting test sample was then etched in boiling 18 weight percent HCl for one hour, then rinsed and dried as described in Example 1. Subsequently, under visual examination in the manner of Example 1, the etched sample plate was seen to have a highly desirable, three dimensional grain boundary etch. This was confirmed by profilometer measurements which provided average values of 10.11 (398) (Ra), 29 (76) (Nr) and 51.82 (2040) (Rz). EXAMPLE 3 A grade 1 titanium plate sample prepared in the manner of Example 1, and having highly desirable three dimensional and well defined grain boundary etching as described in Example 1, was provided with an electrochemically active coating of tantalum oxide and iridium oxide using an aqueous, acidic solution of chloride salts, the coating being applied and baked in the manner as described in Example 1 of US Patent 4.797.182. The resulting sample was tested as an anode in an electrolyte that was a mixture of 285 grams per liter (g/l) of sodium sulfate and 60 g/l of magnesium sulfate. The test cell was maintained at 65°C and operated at a current density of 15 kiloamps per square meter (kA/m²). About once per week the electrolysis was briefly interrupted. The coated titanium plate anode was removed from the electrolyte, rinsed in deionized water, air dried and then cooled to ambient temperature. There was then applied to the coated plate surface, by firmly manually pressing onto the coating, a strip of self-adhesive, pressure sensitive tape. This tape was then removed from the surface by quickly pulling the tape away from the plate. After 3000 hours of operation, including approximately 18 tape tests, the coated anode continued to exhibit excellent coating adhesion to the underlying titanium substrate. COMPARATIVE EXAMPLE 3 A sample of 2 mm thick, grade 1 titanium sheet was etched in 20% HCl at 90-95°C. After etching, the profilimeter measurements were found to be 3.15 (124) (Ra), 4.7 (12) (Nr) and 19.43 (765) (Rz). A coating was applied to the etched sheet as described in Example 3. A sample of the sheet was tested in the electrolyte and under the conditions as described in Example 3. After 424 hours of operation, a tape test was performed on the sample resulting in the nearly complete removal of the coating from the tested area, exposing the underlying substrate and effectively terminating the testing. EXAMPLE 4 At the completion of the testing of the sample of Example 3, the oxide coating was stripped from the surface by means of a molten salt bath. The sample was etched about 30 minutes in 20% HCl at 95-100°C. Profilometer measurements were 8.55 (337) (Ra), 3.38 (86) (Nr) and 46.12 (1816) (Rz), which agreed well with the values obtained before the coating was removed 8.97 (346) (Ra), 30.7 (78) (Nr) and 52.25 (2057) (Rz). The sample was then recoated using the same coating and procedure as in Example 3. Operation of the recoated anode under the conditions of Example 3 resulted in a lifetime of 2956 hours with tape tests comparable to those of the original sample. COMPARATIVE EXAMPLE 4 A sample of titanium which had been previously coated with an electrochemically active coating, was blasted with alumina powder to remove the previous coating. By this abrasive method, it was determined by X-ray fluoroescence that the previous coating had been removed. After removal of any residue of the abrasive treatment, the resulting sample plate was etched in the composition of Example 1 in the manner of Example 1. Under visual inspection as described in Example 1, it was seen that there was no evidence of desirable grain boundary etching. Furthermore, under profilometer measurement, the resulting average values were found to be 3.48 (137) (Ra), 4.7 (12) (Nr) and 21.36 (841) (Rz). The sample was nevertheless coated with the electrocatalytic coating of Example 3 in the manner as described in Example 3 and utilized as an anode also in the manner as described in Example 3. After 91 hours of operation, the sample was removed and the coating adhesion tested utilizing the tape test of Example 3. In this test, and after only the 91 hours of testing, the tape removed the majority of the coating exposing the underlying substrate and thus terminating further testing.

Claims: A method of producing a coated metal electrode for use in electrolytic processes, comprising roughening the surface of a metal substrate and applying to the roughened surface an electrochemically active coating, characterized in that the substrate surface is roughened by intergranular etching, plasma spraying or other roughening technique to produce a surface roughness having a profilometer-measured average surface roughness of about 6.35 micrometers (about 250 microinches), or more, and an average surface peaks per cm of about 15.7 (about 40 peaks per inch), or more, based on a profilometer upper threshold limit of 10.16 micrometers (400 microinches) and a profilometer lower threshold limit of 7.62 micrometers (300 microinches), and applying the electrochemically active coating to the roughened surface after checking that it meets said specifications. The method of claim 1, wherein the substrate metal is selected from metals, alloys and intermetallic mixtures of titanium, tantalum, niobium, aluminium, zirconium, manganese and nickel. The method of claim 1 or 2, wherein the substrate surface is roughened by : subjecting said surface to elevated temperature annealing for a time sufficient to provide an at least substantially continuous intergranular network of impurities, including impurities at the surface of said metal; cooling the resulting annealed surface; and etching intergranularly the surface at an elevated temperature and with a strong acid or strong caustic etchant. The method of claim 3, wherein said metal is titanium and said annealing is conducted in one or more of air, vacuum or inert gas at a temperature reached during said annealing of at least about 500°C. The method of claim 3 or 4, wherein said cooling includes quenching. The method of any one of claims 3 to 5, wherein said metal is unalloyed titanium whose surface prior to etching has intergranular impurities from the group consisting of iron, nitrogen, carbon, hydrogen, beta-titanium, beta-phase stabilizers and mixtures thereof. The method of any one of claims 3 to 6, wherein said elevated temperature etching is conducted with an aqueous etch solution maintained at an elevated temperature of at least about 80°C. The method of any one of claims 3 to 7, wherein said etching employs a strong acid selected from hydrochloric, sulfuric, perchloric, oxalic and phosphoric acids and their mixtures. The method of any one of claims 3 to 7, wherein said etching employs a strong caustic selected from potassium hydroxide/hydrogen peroxide and potassium hydroxide/potassium nitrate mixtures. The method of any one of claims 3 to 9, wherein the substrate is titanium and the intergranular etching results in a substrate weight loss of at least 500 grams per square meter. The method of any one of claims 1 to 10, wherein : (a) the metal substrate is surface treated by strong acid or strong caustic etching, optionally after annealing; (b) the etched surface is subjected to optical inspection to determine whether the surface has been intergranularly etched to produce three-dimensional grains with deep grain boundaries; (c) the intergranularly etched surface is subjected to profilometer inspection to determine whether the three-dimensional grains having deep grain boundaries correspond to said profilometer-measured average surface roughness specifications; and (d) the electrochemically active coating is applied to the treated metal surface only after optical and profilometer inspection has revealed that it meets said specifications. The method of claim 11, wherein if after step (b) or (c) the surface does not meet said specifications, the surface is subjected to further treatment by strong acid or strong caustic etching treatment followed by another optical inspection and profilometer inspection, or followed by another profilometer inspection. The method of claim 12, wherein if after said further treatment the surface does not meet said specifications, another metal substrate from the same batch is subjected to annealing, which where the first substrate had been annealed is different to the annealing of the first substrate, followed by etching and inspection and possible further treatment as in steps (a), (b) and (c). The method of claim 11, 12 or 13, wherein when a substrate surface is qualified for coating, all further substrates from the same batch of metal are subjected to the same annealing and etching procedures, followed by optical and profilometer inspection. The method of claim 1 or 2, wherein the substrate surface is roughened by plasma spraying one or more of a valve metal or valve metal oxide, including valve metal suboxides. The method of claim 15, wherein the valve metal and/or valve metal oxide is plasma sprayed onto a surface roughened by etching. The method of any preceding claim, wherein said surface with a profilometer-measured average roughness of about 6.35 micrometers (250 microinches), or more, has no low spots of about 5.08 micrometers (about 200 microinches), or less, based on the aforesaid profilometer threshold limits. The method of any preceding claim, wherein said surface has a profilometer-measured average surface peaks per cm of about 23.6 (about 60 peaks per inch), or more, based on the aforesaid profilometer threshold limits. The method of any preceding claim, wherein said surface has profilometer-measured average distance between the maximum peak and the maximum valley of at least about 25.4 micrometers (about 1,000 microinches), or more, based on the aforesaid profilometer threshold limits. The method of claim 19, wherein said surface has profilometer-measured average distance between the maximum peak and the maximum valley of about 38.1 micrometers (about 1,500 microinches) to about 88.9 micrometers (about 3,500 microinches, based on the aforesaid profilometer threshold limits. The method of any preceding claim, wherein said surface has a profilometer-measured average peaks height of about 25.4 micrometers (about 1,000 microinches), or more, based on the aforesaid profilometer threshold limits. The method of claim 21, wherein said surface has a profilometer-measured average peaks height of about 38.1 micrometers (about 1,500 microinches) to about 88.9 micrometers (about 3,500 microinches), based on the aforesaid profilometer threshold limits. The method of any preceding claim, wherein the metal substrate is a coated metal electrode substrate to be recoated and wherein, prior to the roughening step, the coated metal substrate is treated with a melt containing basic material to remove the old coating. An electrode for use in electrolytic processes, comprising a metal substrate having a roughened surface to which an electrochemically active coating is applied, which surface is roughened by intergranular etching to produce three dimensional grains with deep grain boundaries, or by plasma spraying valve metal and/or valve metal oxide, or by another toughening technique, to provide said roughened surface which has a profilometer-measured average surface roughness of about 6.35 micrometers (about 250 microinches), or more, and an average surface peaks per cm of about 15.7 peaks per cm (about 40 peaks per inch), or more, based on a profilometer upper threshold limit of 10.16 micrometers (400 microinches) and a profilometer lower threshold limit of 7.62 micrometers (300 microinches). The electrode of claim 24, wherein the substrate metal is selected from metals, alloys and intermetallic mixtures of titanium, tantalum, niobium, aluminium, zirconium, manganese and nickel. The electrode of claim 24, wherein said metal is unalloyed titanium. The electrode of claim 24, 25 or 26, wherein said metal surface is intergranularly etched and has at least substantially all grains of size within the range of about 125 to about 32 micrometers (grain size about 3 to about 7, according to ASTM E 112-84). The electrode of claim 24, 25 or 26, wherein the substrate surface is roughened by a plasma sprayed valve metal and/or plasma sprayed valve metal oxide, including valve metal suboxides. The electrode of claim 28, wherein the plasma sprayed valve metal and/or valve metal oxide is on an etched, roughened surface. The electrode of any one of claims 24 to 29, wherein said surface has a profilometer-measured average roughness of about 6.35 micrometers (about 250 microinches), or more, with no low spots of about 5.08 micrometers (about 200 microinches), or less, based on the aforesaid profilometer threshold limits. The electrode or any one of claims 24 to 30, wherein said surface has a profilometer-measured average surface peaks per cm of about 23.6 (about 60 peaks per inch), or more, based on the aforesaid profilometer threshold limits. The electrode of any one of claims 24 to 31, wherein said surface has profilometer-measured average distance between the maximum peak and the maximum valley of about 25.4 micrometers (about 1,000 microinches), or more, based on the aforesaid profilometer threshold limits. The electrode of claim 32, wherein said surface has profilometer-measured average distance between the maximum peak and the maximum valley of about 38.1 micrometers (about 1,500 microinches) to about 88.9 micrometers (about 3,500 microinches), based on the aforesaid profilometer threshold limits. The electrode of any one of claims 24 to 33, wherein said surface has a profilometer-measured average peaks height of about 25.4 micrometers (about 1,000 microinches), or more, based on the aforesaid profilometer threshold limits. The electrode of claim 34, wherein said surface has a profilometer-measured average peaks height of about 38.1 micrometers (about 1,500 microinches) to about 88.9 micrometers (about 3,500 microinches), based on the aforesaid profilometer threshold limits. The electrode of any one of claims 24 to 35, wherein the electrochemically active coating contains a platinum group metal or metal oxide or their mixtures. The electrode of claim 35, wherein the electrochemically active coating contains at least one oxide selected from platinum group metal oxides, magnetite, ferrite and cobalt oxide spinel. The electrode of claim 36, wherein the electrochemically active coating contains a mixed crystal material of at least one oxide of a valve metal and at least one oxide of a platinum group metal. The electrode of claim 24, wherein the electrochemically active coating contains one or more of manganese dioxide, lead dioxide, platinate substituent, nickel plus nickel oxide and nickel plus lanthanide oxides. Use of the electrode of any one of claims 24 to 39, as oxygen-evolving anode in electrogalvanizing, electrotinning, electroforming or electrowinning, or in the eletrodeposition of metal onto a substrate. A cell for the electrolysis of a dissolved species contained in a bath of said cell and having an anode immersed in said bath, wherein the anode is an electrode as claimed in any one of claims 24 to 39.

Inventors: CARLSON RICHARD C, ERNES LYNNE M, HARDEE KENNETH L, CARLSON, RICHARD C., ERNES, LYNNE M., HARDEE, KENNETH L.
Applicants: ELTECH SYSTEMS CORP, ELTECH SYSTEMS CORPORATION
Application year: 1990
Main classification: C23F1
IPC codes: C25D17, C25C7, C23F1, C23C4, C23C8, C25B11, C22C14, C23C28, C23C18, C21D1, C23C26