Abstract:
A new electrocatalytic coating to be applied onto a titanium matrix, suitable for oxygen evolution from acid electrolytes containing manganese and fluorides, comprising: 
     a) an external coating for oxygen evolution at controlled potential, immune to manganese electrochemical precipitation and capable of promoting the spontaneous removal of the same during operation, consisting of ruthenium and iridium as the major components (60-85%), tin and cobalt (2-10%) and titanium and tantalum at intermediate concentrations with respect to the previous groups of components. 
     b) an optional interlayer acting as an electroconductive system and protecting the titanium matrix against corrosion caused by fluorides, made of titanium and tantalum as the major components (&lt;95%) and iridium (&gt;5%) as the minor component. 
     At least part of the above elements are in the form of oxides.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention concerns electrocatalytic coatings for oxygen-evolving anodes. 
     2. Description of the Related Art 
     The anodic materials of the prior art for the electrometallurgy of copper, zinc and cobalt are essentially of two types: lead alloys, and cobalt-silicon alloys (cobalt only). Industrial lead anodes are made of lead alloys containing one or more elements selected in the following group: I B, IV A and V A. In particular, the lead-silver (0.2-0.8%) anode is commonly used especially in the zinc electrometallurgy, while for the cobalt electrometallurgy different alloys are used, such as lead-antimony (2-6%), lead-silver (0.2-0.8%), lead-tin (5-10%). These materials are characterized by: 
     high anodic potentials, above 1.9 V (NHE). 
     lifetimes in the range of 1 to 3 years 
     high electric resistivity and substantial electric disuniformity leading to the formation of thick solid layers of PbSO 4  (intermediate passivating layer) and PbO 2  (external electrocatalytic layer for oxygen evolution). 
     These characteristics involve the following drawbacks: 
     faradic efficiency loss (below 90% for zinc, and below 95% for cobalt) 
     uneven and dendritic aspect of the deposit 
     contamination by lead of the produced metal. 
     The cobalt alloys, used for a part of the cobalt electrometallurgy, are substantially of three types characterized by the following compositions: cobalt-silicon (5-20%), cobalt-silicon (5-20%)-manganese (1.0-5.0%), cobalt-silicon (5-20%)-copper (0.5-2.5%). 
     The cobalt-silicon alloys, with respect to the lead alloys, are characterized by a longer lifetime but are affected by a higher electrical resistivity and brittleness, while the cobalt-silicon-copper alloys have a shorter lifetime and are all the same fragile. 
     As concerns cathode poisoning, this occurs only when copper alloys are used. 
     Table 1 summarizes some examples of general operating conditions of the prior art technology. Reference is made to the process for zinc and cobalt deposition. 
     
                                           TABLE 1__________________________________________________________________________                Prior art operating materials                Anode lifetime (years)         Current        Co--SiProcess    Electrolyte         Density A/m.sup.2                Pb--Sn                    Pb--Ag                        Co--Si--Mn                              Co--Si--Cu__________________________________________________________________________Zinc    Zn.sup.2+ 300-500 g/l)                //  2-4 //    //    H.sub.2 SO.sub.4 (150-200 g/l)    Fluorides (50 ppm)    Manganese (2-8 g/l)    Zn.sup.2+ 300-500 g/l)                1-3 2-4 //    //    H.sub.2 SO.sub.4 (150-200 g/l)    Fluorides (5 ppm)    Manganese (2-8 g/l)Cobalt    Co.sup.2+ 160-250 g/l)                4-5 4-5 3-4   2-3    H.sub.2 SO.sub.4 (pH 1-3)    Manganese (10-30 g/l)    pH = 4-5,5__________________________________________________________________________ 
    
     In the electrolysis of solutions containing, besides the salt of the metal to be deposited, also significant quantities of manganese (5-20 g/l and more), two reactions take place at the anode, and precisely: 
     oxygen evolution: 2H 2  O=4H+O 2  +4e 
     manganese dioxide deposition (parasitic reaction): 2Mn 2+  +4H 2  O=2MnO 2  +8H +  +4e. 
     This anodic by-product is an electrically resistive oxide (resistivity equal or higher than that of the PbO 2  -PbSO 4  mixture formed on lead anodes); as a consequence, its precipitation on the surface of the electrode, if compact and continuous with time, involves a progressive increase of the electrode potential, which negatively affects prior art electrodes. In industrial practice, to avoid or at least control this phenomenon, the anodes (lead alloys or cobalt alloys) are periodically cleaned by mechanical brushing carried out outside the electrolysis cell. 
     It is known that titanium electrodes, activated by conventional coatings based on tantalum and iridium oxides, when used in electrolytes containing manganese, are negatively affected by the same drawbacks as lead anodes, with the only difference that the mechanical cleaning is not applicable due to the insufficient mechanical stability of the coating. Therefore, possible alternatives to the mechanical removal of the MnO 2  have been considered, such as periodical washing outside the cell with reducing solutions such as H 2  O 2  ; H 2  O 2  +nitrates or nitric acid; ferrous salts and nitrates, ferrous salts and sulphates, etc. or actions carried out in the cell, such periodical current reversal, periodical current interruption, scheduled shut-downs etc. As the results were either negative or unsuitable for industrial scale application, efforts have been focused on the spontaneous removal of manganese dioxide electrodeposited onto the anode directly in the electrolysis cell. 
     SUMMARY OF THE INVENTION 
     It is the main object of the present invention to provide for an anode capable of promoting the spontaneous and continuous removal of the manganese dioxide, MnO 2 , formed by the aforementioned parasitic reaction, so that the growth in thick layers is prevented. 
     DETAILED DESCRIPTION 
     The anode of the invention comprises an electrocatalytic surface coating for oxygen evolution applied on a titanium matrix, suitable for operation at controlled potential. Optionally an inter-layer may be provided, which acts as an electroconductive system for protecting the titanium matrix (stabilizing action towards fluorides and acidity). The following complementary criteria are used for selecting the surface coating: 
     a) addition of highly catalytic metals for oxygen evolution, for example ruthenium and cobalt, to the main components consisting of tantalum and iridium, to fix the voltage at low and controlled values. 
     b) further addition of metals capable of stabilizing ruthenium and cobalt, such as titanium and tin. 
    
    
     The invention will be better illustrated making reference to some examples, which are not intended to limit the same. 
     For all of the Examples, the samples, consisting of a matrix made of titanium grade 1, having the dimensions of 40 mm×40 mm×2 mm, were prepared according to the following steps and control procedures: 
     I. surface treatment with corindone sand+pickling in 20% HCl for 30 minutes; 
     II. application of optional protective layers; 
     III. application of the surface electrocatalytic layer for oxygen evolution; 
     IV. electrochemical characterization tests (electrode potential) in electrolytic media simulating industrial process working conditions; 
     V. comparison with reference samples prepared according to prior art technologies. 
     EXAMPLE 1 
     27 reference samples have been prepared according to the prior art teachings. The titanium matrix was pre-treated as described above (step I). Then, 9 samples, identified as A, were activated with a surface coating based on Ta-Ir (65% by weight) (step III only); 9 samples, identified as B, were activated with an interlayer based on Ti-Ta (49% by weight) (step II) and, subsequently, with a surface coating of Ta-Ir (65% by weight) (step III) and 9 samples, identified as C were activated with an interlayer based on Ti-Ta (44% by weight)-Ir (12% by weight) (step II) and, subsequently, with a surface coating of Ta-Ir (65% by weight) (step III). 
     The compositions of the paints, interlayers and surface coatings are reported herebelow: 
     
         __________________________________________________________________________Paints for the interlayers                   Paints for the surface coatings  Components       mg/ml as metal                   Components                             mg/ml as metal__________________________________________________________________________A =         =           TaCl.sub.5 IrCl.sub.3.3H.sub.2 O                             50(Ta) 90(Ir)B TiCl.sub.3 TaCl.sub.5 HCl       5,33(Ti) 5,03(Ta)                   TaCl.sub.5 IrCl.sub.3.3H.sub.2 O                             50(Ta) 90(Ir)C TiCl.sub.3 TaCl.sub.5 IrCl.sub.3 HCl       5,00(Ti) 5,00(Ta) 1,36(Ir)                   TaCl.sub.5 IrCl.sub.3.3H.sub.2 O                             50(Ta) 90(Ir)__________________________________________________________________________Interlayers            Surface coatings      % by weight             g/m.sup.2 as                        % by weight                              g/m.sup.2 as noble  Components      as metal             total metal                  Components                        as metal                              metal__________________________________________________________________________A =        =      =    Ta.sub.2 O.sub.5 --IrO.sub.2                        35(Ta) 65(Ir)                              10B Ta.sub.2 O.sub.5 --TiO.sub.2      50(Ta) 50(Ti)             1    Ta.sub.2 O.sub.5 --IrO.sub.2                        35(Ta) 65(Ir)                              10C Ta.sub.2 O.sub.5 --TiO.sub.2 --IrO.sub.2      44(Ta) 44(Ti)             2    Ta.sub.2 O.sub.5 --IrO.sub.2                        35(Ta) 65(Ir)                              10      12(Ir)__________________________________________________________________________ 
    
     As regards the formation of the interlayer and surface coating, the paint was applied by brushing or equivalent technique. This procedure was repeated as many times as necessary to obtain the desired quantity of deposited metal. Between an interlayer and the other layer of applied paint, drying was carried out at 150° C., followed by thermal decomposition in oven under forced air ventilation at 500° C. for 10-15 minutes and subsequent natural cooling at ambient temperature. 
     EXAMPLE 2 
     18 samples made of titanium were prepared according to the invention following the procedures described above. The compositions of the interlayer and surface coatings are illustrated in Table 2.1. 
     
                                           TABLE 2.1__________________________________________________________________________Interlayer                   Surface coatingsSample        % by weight                   g/m.sup.2 as      % by weight   g/m.sup.2 as                                                   noblecode Components         as metal  total metal                        Components   as metal      metal__________________________________________________________________________2.1 a,b,cTa.sub.2 O.sub.5 --TiO.sub.2 --IrO.sub.2         44(Ta) 44((Ti) 12(Ir)                   2    Ta.sub.2 O.sub.5 --IrO.sub.2 --RuO.sub.2                                     30(Ta) 65(Ir)                                                   10Ru2.2 a,b,c&#34;        &#34;         &#34;    Ta.sub.2 O.sub.5 --IrO.sub.2 --RuO.sub.2                                     35(Ta) 50(Ir)                                                   &#34;5(Ru)2.3 a,b,c&#34;        &#34;         &#34;    Ta.sub.2 O.sub.5 --IrO.sub.2 --RuO.sub.2                                     35(Ta) 32.5(Ir)                                                   &#34;2.5(Ru)2.4 a,b,c&#34;        &#34;         &#34;    Ta.sub.2 O.sub.5 --IrO.sub.2 --RuO.sub.2                                     35(Ta) 15(Ir)                                                   &#34;0(Ru)2.5 a,b,c&#34;        &#34;         &#34;    Ta.sub.2 O.sub.5 --TiO.sub.2 --IrO.sub.2                        --RuO.sub.2  17.5(Ta) 12.5(Ti) 35(Ir)                                                   &#34;5(Ru)2.6 a,b,c&#34;        &#34;         &#34;    Ta.sub.2 O.sub.5 --TiO.sub.2 --IrO.sub.2                        --RuO.sub.2  20(Ta) 10(Ti) 60(Ir)                                                   &#34;0(Ru)__________________________________________________________________________ 
    
     The interlayers and surface coatings of Table 2.1 were obtained by thermal treatment starting from paints containing precursors as described in Table 2.2. 
     
                       TABLE 2.2______________________________________Composition of the paints used for obtaining theinterlayers and surface coatingsSample Interlayer       Surface coatingcode   Components            mg/ml as metal                       Components                               mg/ml as metal______________________________________2.1 a,b,c  TiCl.sub.3            5.00       TaCl.sub.5                               39  TaCl.sub.5            5.00       IrCl.sub.3                               85  IrCl.sub.3            1.36       RuCl.sub.3                               6.5  HCl       110        HCl     1102.2 a,b,c  &#34;                    TaCl.sub.5                               45.5                       IrCl.sub.3                               65                       RuCl.sub.3                               19.5                       HCl     1102,3 a,b,c  &#34;                    TaCl.sub.5                               45.5                       IrCl.sub.3                               42.3                       RuCl.sub.3                               42.3                       HCl     1102,4 a,b,c  &#34;                    TaCl.sub.5                               45.5                       IrCl.sub.3                               19.5                       RuCl.sub.3                               65                       HCl     1102.5 a,b,c  &#34;                    TaCl.sub.5                               20                       TiCl.sub.3                               14.3                       IrCl.sub.3                               40                       RuCl.sub.3                               40                       HCl     1102.6 a,b,c  &#34;                    TaCl.sub.5                               22.9                       TiCl.sub.3                               11.4                       IrCl.sub.3                               69                       RuCl.sub.3                               11.4                       HCl     110______________________________________ 
    
     The samples thus prepared were subjected to electrochemical anodic characterization in three types of electrolytes, each one simulating industrial operating conditions as shown in Table 2.3. 
     
                                           TABLE 2.3__________________________________________________________________________Electrochemical characterization: description of the tests.Test   Samples   Operating Conditions                            Simulated industrialcode   Sample code        Electrolyte                  Operating parameters                            process__________________________________________________________________________M  present invention:        H.sub.2 SO.sub.4            150 g/l                  500 A/m.sup.2                            zinc   from 2.1a → 2.6a        F.sup.-            50 ppm                  40° C.                            (above 90% of the   references: A1,B1,C1        Mn.sup.2+            5 g/l           worldwide electrolytic                            production)N  present invention:        H.sub.2 SO.sub.4            150 g/l                  500 A/m.sup.2                            zinc   from 2.1b → 2.6b        F.sup.-            5 ppm 40° C.                            (the remaining 10% of   references: A2,B2,C2        Mn.sup.2+            5 g/l           the worldwide electro-                            lytic production)O  present invention:        Na.sub.2 SO.sub.4            100 g/l                  500 A/m.sup.2                            cobalt   from 2.1c → 2.6c        H.sub.2 SO.sub.4            (pH = 2-3)                  40° C.   references: A3,B3,C3        Mn.sup.2+            20 g/l__________________________________________________________________________ 
    
     The electrochemical characterization comprised the determination of the electrode potential as a function of the working time (expressed in the normal hydrogen reference electrode scale as Volt (NHE)) and visual inspection of the sample at the end of the test. 
     The results obtained are summarized in table 
     
                       TABLE 2.4______________________________________Electrochemical characterization: Experimental results.                          MorphologicalTest Sample  Potential (V(NHE))                          observationscode code    initial               100 h                    1000 h                          3000 h                                at the end of the test______________________________________2.1a    1.70   1.72 1.90  ≧3.0                                MnO.sub.2 compact deposit2.2a    1.68   1.70 1.95  ≧2.5                                &#34;2.3a    1.65   1.68 1.90  ≧2.2                                &#34;2.4a    1.62   1.75 ≧2.5 &#34;2.5a    1.64   1.65 1.67  1.65  MnO.sub.2 partial coverage:                                spontaneous removal2.6a    1.68   1.72 1.74  1.75  MnO.sub.2 partial coverage:                                spontaneous removalA1      1.69   1.85 2.10  ≧3.0                                MnO.sub.2 compact depositB1      1.72   1.82 2.10  ≧3.0                                &#34;C1      1.72   1.70 1.95  ≧3.0                                &#34;N    2.1b    1.65   1.70 1.90  ≧2.5                                MnO.sub.2 compact deposit2.2b    1.63   1.66 1.85  ≧2.2                                &#34;2.3b    1.60   1.62 1.80  ≧2.0                                &#34;2.4b    1.58   1.70 ≧2.0 &#34;2.5b    1.62   1.64 1.65  1.65  MnO.sub.2 partial coverage:                                spontaneous removal2.6b    1.64   1.65 1169  1.69  MnO.sub.2 partial coverage:                                spontaneous removalA2      1.65   1.72 2.00  ≧2.8                                MnO.sub.2 compact depositB2      1.69   1.80 2.11  ≧3.0                                &#34;C2      1.68   1.70 1.90  ≧2.5                                &#34;O    2.1c    1.80   1.85 2.10  ≧3.0                                MnO.sub.2 compact deposit2.2c    1.76   1.78 2.00  ≧2.5                                &#34;2.3c    1.75   1.74 1.90  ≧2.2                                &#34;2.4c    1.70   1.72 ≧4.00                                &#34;2.5c    1.72   1.74 1.70  1.75  MnO.sub.2 partial coverage:                                spontaneous removal2.6c    1.74   1.75 1.77  1.80  MnO.sub.2 partial coverage:                                spontaneous removalA3      1.80   1.95 ≧2.2 MnO.sub.2 compact depositB3      1.84   1.95 ≧203 &#34;C3      1.78   1.90 ≧2.3 &#34;______________________________________ 
    
     The analysis of the experimental data leads to the following observations: 
     the prior art coatings are irreversibly passivated by the manganese present in the electrolyte, after about 1000 hours of operation in simulated industrial conditions; 
     the presence of ruthenium in the electrocatalytic surface coating together with iridium and tantalum improves the behaviour of the electrode with respect to manganese without however eliminating the inconveniences. In fact, only a delay with time of the passivation phenomena is experienced, delay which depends on the ruthenium content in the active layer. In particular, an optimum concentration (=35%) is observed, which corresponds to longer lifetimes; 
     the concurrent presence of ruthenium and titanium in the surface coating together with iridium and tantalum permits to obtain an electrochemical system durable with time and not passivated by manganese. 
     EXAMPLE 3 
     Following the same procedures described above, 18 samples made of titanium were prepared with a second type of surface coating of the invention containing ruthenium, iridium, titanium and tantalum as major components, (for a total of 90-95%), cobalt and tin as minor components (for a total of 5-10% max.). The compositions of the interlayers and surface coatings are reported in Table 
     
                                           TABLE 3.1__________________________________________________________________________Interlayers                  g/m.sup.2                      CoatingsSample        % by weight                  as total              % by weight  g/m as no-code Components         as metal metal                      Components        as metal     ble__________________________________________________________________________                                                     metal3.1 a,b,cTa.sub.2 O.sub.5 --TiO.sub.2 IrO.sub.2         44(Ta) 44(Ti) 12(Ir)                  2   Ta.sub.2 O.sub.5 --TiO.sub.2 IrO.sub.2                      --RuO.sub.2 CoO.sub.x                                        17.5(Ta) 17.5(Ti)                                                     10                                        32(Ir) 32(Ru) I(Co)3.2 a,b,c&#34;        &#34;        `   &#34;                 17.5(Ta) 17.5(Ti)                                                     &#34;                                        31.25(Ir) 31.25(Ru) 2.5(Co)3.3 a,b,c&#34;        &#34;        &#34;   &#34;                 17.5(Ta) 17.5(Ti)                                                     &#34;                                        30(Ir) 30(Ru) 5(Co)3.4 a,b,c&#34;        &#34;        &#34;   -                 17.5(Ta) 17.5(Ti)                                                     &#34;                                        27.5(Ir) 27.5(Ru) 10(Co)3.5 a,b,c&#34;        &#34;        &#34;   Ta.sub.2 O.sub.5 --TiO.sub.2 IrO.sub.2                      --RuO.sub.2 CoO.sub.x SnO.sub.x                                        15(Ta) 10(Ti)                                                     &#34;5(Ir)                                        35(Ru) 2.5(Co)                                        2.5(Sn)3.6 a,b,c&#34;        &#34;        &#34;   &#34;                 15(Ta) 10(Ti)                                                     &#34;                                        33.75(Ir) 33.75(Ru)                                        2.5(Co) 5(Sn)__________________________________________________________________________ 
    
     The interlayers and surface coatings of Table 3.1 have been obtained by thermal treatment starting from paints of precursor salts as illustrated in Table 3.2. 
     
                       TABLE 3.2______________________________________Composition of the paints used for obtaining theinterlayers and surface coatingsSample Interlayer       Surface Coatingcode   Components            mg/ml as metal                       Components                               mg/ml as metal______________________________________3.1.a,b,c  TiCl.sub.3            5.00       TaCl.sub.5                               24.2  TaCl.sub.5            5.00       TiCl.sub.3                               24.2  IrCl.sub.3            1.36       IrCl.sub.3                               45  HCl       110        RuCl.sub.3                               45                       CoCl.sub.2                               1.4                       HCl     1103.2.a,b,c  &#34;         &#34;          TaCl.sub.5                               25.2                       TiCl.sub.3                               25.2                       IrCl.sub.3                               45                       RuCl.sub.3                               45                       CoCl.sub.2                               3.6                       HCl     1103.3.a,b,c  &#34;         &#34;          TaCl.sub.5                               26.3                       TiCl.sub.3                               26.3                       IrCl.sub.3                               45                       RuCl.sub.3                               45                       CoCl.sub.2                               7.5                       HCl     1103.4.a,b,c  &#34;         &#34;          TaCl.sub.5                               25.5                       TiCl.sub.3                               25.5                       IrCl.sub.3                               40                       RuCl.sub.3                               40                       CoCl.sub.2                               14.5                       HCl     1103.5.a,b,c  &#34;         &#34;          TaCl.sub.5                               17.1                       TiCl.sub.3                               11.4                       IrCl.sub.3                               40                       RuCl.sub.3                               40                       CoCl.sub.2                               2.8                       SnCl.sub.4                               2.8                       HCl     1103.6.a,b,c  &#34;         &#34;          TaCl.sub.5                               17.3                       TiCl.sub.3                               11.5                       IrCl.sub.3                               38.9                       RuCl.sub.3                               38.9                       CoCl.sub.2                               2.9                       SnCl.sub.4                               5.7                       HCl     110______________________________________ 
    
     The samples thus prepared have been subjected to anodic electrochemical characterization in 3 types of electrolyte, each one simulating industrial operating conditions as shown in Table 3.3. 
     
                                           TABLE 3.3__________________________________________________________________________Electrochemical characterization: description of the tests.Test   Sampling  Operating Conditions                            Simulated Industrialcode   Sample code        Electrolyte                  Operating parameters                            Process__________________________________________________________________________M  present invention:        H.sub.2 SO.sub.4            150 g/l                  500 A/m.sup.2                            zinc   from 3.1a → 3.6a        F.sup.-            50 ppm                  40° C.                            (above 90% of the   references: A4,B4,C4        Mn.sup.2+            5 g/l           worldwide electrolytic                            production)N  present invention:        H.sub.2 SO.sub.4            150 g/l                  500 A/m.sup.2                            zinc   from 3.1b → 3.6b        F.sup.-            5 ppm 40° C.                            (the remaining 10% of   references: A5,B5,C5        Mn.sup.2+            5 g/l           the worldwide electro-                            lytic production)O  present invention:        Na.sub.2 SO.sub.4            100 g/l                  500 A/m.sup.2                            cobalt   from 3.1c → 3.6c        H.sub.2 SO.sub.4            (pH = 2-3)                  40° C.   references: A6,B6,C6        Mn.sup.2+            20 g/l__________________________________________________________________________ 
    
     The characterization comprised the determination of the electrode potential as a function of the working time and visual inspection of the sample at the end of the test. 
     The results obtained are summarized in table 3.4. 
     
                       TABLE 3.4______________________________________Electrochemical characterization: Experimental results.                          MorphologicalTest Sample  Potential (V(NHE))                          observationscode code    initial               100 h                    1000 h                          3000 h                                at the end of the test______________________________________M    3.1a    1.65   1.65 1.68  1.72  MnO.sub.2 partial coverage:                                spontaneous removal3.2a    1.64   1.65 1.67  1.68  MnO.sub.2 partial coverage:                                spontaneous removal3.3a    1.60   1.63 1.65  1.69  MnO.sub.2 partial coverage:                                spontaneous removal3.4a    1.58   1.62 1.65  1.65  MnO.sub.2 partial coverage:                                spontaneous removal3.5a    1.62   1.60 1.55  1.58  MnO.sub.2 partial coverage:                                spontaneous removal3.6a    1.64   1.62 1.64  1.68  MnO.sub.2 partial coverage:                                spontaneous removalA4      1.69   1.85 2.20  ≧3.0                                MnO.sub.2 compact depositB4      1.72   1.80 1.95  ≧3.0                                &#34;C4      1.68   1.75 1.90  ≧3.0                                &#34;N    3.1b    1.60   1.62 1.60  1.64  MnO.sub.2 partial coverage:                                spontaneous removal3.2b    1.62   1.60 1.62  1.70  MnO.sub.2 partial coverage:                                spontaneous removal3.3b    1.58   1.60 1.62  1.65  MnO.sub.2 partial coverage:                                spontaneous removal3.4b    1.55   1.58 1.65  1.75  MnO.sub.2 partial coverage:                                spontaneous removal3.5b    1.60   1.62 1.58  1.63  MnO.sub.2 partial coverage:                                spontaneous removal3.6b    1.62   1.64 1.70  1.74  MnO.sub.2 partial coverage:                                spontaneous removalA5      1.65   1.80 2.20  ≧2.8                                MnO.sub.2 compact depositB5      1.70   1.75 1.90  ≧3.0                                &#34;C5      1.65   1.70 1.90  ≧2.5                                &#34;O    3.1c    1.75   1.77 1.77  1.80  MnO.sub.2 partial coverage:                                spontaneous removal3.2c    1.72   1.72 1.74  1.75  MnO.sub.2 partial coverage:                                spontaneous removal3.3c    1.68   1.64 1.68  1.70  MnO.sub.2 partial coverage:                                spontaneous removal3.4c    1.64   1.65 1.67  1.65  MnO.sub.2 partial coverage:                                spontaneous removal3.5c    1.70   1.68 1.70  1.72  MnO.sub.2 partial coverage:                                spontaneous removal3.6c    1.65   1.67 1.68  1.70  MnO.sub.2 partial coverage:                                spontaneous removalA6      1.80   2.0  ≧2.3 MnO.sub.2 compact depositB6      1.85   2.1  ≧2.4 &#34;C6      1.75   1.90 ≧2.3 &#34;______________________________________ 
    
     The analysis of the data of table 3.4 leads to the following observations: 
     the prior art coatings are irreversibly passivated by the manganese present in the electrolyte after about 1000 hours of operation at simulated industrial conditions; 
     the presence of cobalt, in the system comprising ruthenium, iridium, tantalum (already examined in previous Example 2) further decreases the electrode potential, mainly at the beginning of the operation; 
     the concurrent presence of cobalt and tin in the above system not only decreases the initial electrode potential but furthermore causes its stabilization with time. 
     EXAMPLE 4 
     6 samples made of titanium have been prepared following the aforementioned procedure, without any interlayer, with the 4- or 6-component surface coatings selected among the best from the tests of the previous examples. The compositions of the surface coatings are given in Table 4.1. 
     
                                           TABLE 4.1__________________________________________________________________________Interlayers           CoatingsSample     % by weight            g/m.sup.2 as                        % by weight                                g/m as no-code Components      as metal            total metal                 Components                        as metal                                ble metal__________________________________________________________________________4.1 a,b,c/     /     /    Ta.sub.2 O.sub.5 --TiO.sub.2                        17.5(Ta) 12.5(Ti)                                10                 IrO.sub.2 --RuO.sub.2                        35(Ir) 35(Ru)4.2 a,b,c/     /     /    Ta.sub.2 O.sub.5 --TiO.sub.2                        12.5(Ta) 12.5(Ti)                 IrO.sub.2 --RuO.sub.2                        35(Ir) 35(Ru)                 CoO.sub.x --SnO.sub.x                        2.5(Co) 2.5(Sn)__________________________________________________________________________ 
    
     The surface coatings of Table 4.1 were obtained by thermal treatment from paints of precursor salts as shown in Table 4.2. 
     
                       TABLE 4.2______________________________________Compositions of the paints used for preparing the surfacecoatings of Table 4.1Sample code    Components                    mg/ml______________________________________4.1.a,b,c      TaCl.sub.5                    17.1          TiCl.sub.3                    17.1          IrCl.sub.3                    40          RuCl.sub.3                    40          HCl       1104.2.a,b,c      TaCl.sub.5                    14.3          TiCl.sub.3                    14.3          IrCl.sub.3                    40          RuCl.sub.3                    40          CoCl.sub.2                    2.9          SnCl.sub.4                    2.9          HCl       110______________________________________ 
    
     The samples thus prepared were subjected to anodic electrochemical characterization in 3 types of electrolytes, each one simulating the industrial operating as shown in table 4.3. 
     
                                           TABLE 4.3__________________________________________________________________________Electrochemical characterization: description of the tests.Test   Sampling  Operating Conditions                            Simulated industrialcode   Sample code        Electrolyte                  Operating parameters                            process__________________________________________________________________________M  present invention:        H.sub.2 SO.sub.4            150 g/l                  500 A/m.sup.2                            zinc   from 4.1a → 4.2a        F.sup.-            50 ppm                  40° C.                            (above 90% of the   2.5a (Example 2),        Mn.sup.2+            5 g/l           worldwide electrolytic   3.5a (Example 3),             production)   references:   A7,B7,C7.N  present invention:        H.sub.2 SO.sub.4            150 g/l                  500 A/m.sup.2                            zinc   from 4.1b → 4.2b        F.sup.-            5 ppm 40° C.                            (the remaining 10% of   2.5b (Example 2),        Mn.sup.2+            5 g/l           the worldwide electro-   3.5b (Example 3),             lytic production)   references:   A8,B8,C8.O  present invention:        Na.sub.2 SO.sub.4            100 g/l                  500 A/m.sup.2                            cobalt   from 4.1c → 4.2c        H.sub.2 SO.sub.4            (pH = 2-3)                  40° C.   2.5c (Example 2),        Mn.sup.2+            20 g/l   3.5c (Example 3),   references:   A9,B9,C9.__________________________________________________________________________ 
    
     The characterization comprising the determination of the electrode potential as a function of the working time and visual inspection of the sample at the end of the test, gave the experimental results summarized in 4.4. 
     
                       TABLE 4.4______________________________________Electrochemical characterization: Experimental results.                          MorphologicalTest Sample  Potential (V(NHE))                          observationscode code    initial               100 h                    1000 h                          3000 h                                at the end of the test______________________________________M    4.1a    1.67   1.68 1.70  1.74  MnO.sub.2 partial coverage:                                spontaneous removal4.2a    1.66   1.68 1.67  1.70  MnO.sub.2 partial coverage:                                spontaneous removalA7      1.69   1.85 2.20  ≧3.0                                MnO.sub.2 compact depositB7      1.72   1.80 2.20  ≧3.0                                &#34;C7      1.68   1.75 1.90  ≧3.0                                &#34;2.5a    1.64   1.65 1.67  1.65  MnO.sub.2 partial coverage:(Exam-                          spontaneous removalple 2)3.5a    1.62   1.60 1.55  1.58  MnO.sub.2 partial coverage:(Exam-                          spontaneous removalple 3)N    4.1b    1.67   1.70 1.70  1.74  MnO.sub.2 partial coverage:                                spontaneous removal4.2b    1.65   1.68 1.72  1.70  MnO.sub.2 partial coverage:                                spontaneous removalA8      1.65   1.80 2.20  ≧2.8                                MnO.sub.2 partial coverage:                                spontaneous removalB8      1.70   1.75 1.90  ≧3.0                                MnO.sub.2 partial coverage:                                spontaneous removalC8      1.65   1.70 1.90  ≧2.5                                MnO.sub.2 partial coverage:                                spontaneous removal2.5b    1.62   1.64 1.65  1.65  MnO.sub.2 partial coverage:(Exam-                          spontaneous removalple 2)3.5b    1.60   1.62 1.58  1.63  MnO.sub.2 partial coverage:(Exam-                          spontaneous removalple 3)O    4.1c    1.78   1.75 1.80  1.80  MnO.sub.2 partial coverage:                                spontaneous removal4.2c    1.74   1.70 1.75  1.78  MnO.sub.2 partial coverage:                                spontaneous removalA9      1.80   2.00 ≧2.20                                MnO.sub.2 compact depositB9      1.85   2.10 ≧2.30                                &#34;C9      1.75   1.90 ≧2.30                                &#34;2.5c    1.72   1.74 1.70  1.75  MnO.sub.2 partial coverage:(Exam-                          spontaneous removalple 2)3.5c    1.70   1.68 1.70  1.72  MnO.sub.2 partial coverage:(Exam-                          spontaneous removalple 3)______________________________________ 
    
     From the analysis of the experimental results it is possible to make the following observations: 
     the prior art coatings are irreversibly passivated by manganese present in the electrolyte after about 1000 hour of simulated industrial conditions. 
     the coatings of the present invention, without any interlayer, although operating at slightly higher potentials with respect to those typical of anodes provided with the interlayer are equally stable to fluorides and are not passivated by manganese.