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(11) | EP 1 026 288 A1 |
| (12) | EUROPEAN PATENT APPLICATION |
| published in accordance with Art. 158(3) EPC |
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| (54) | FREELY DETACHABLE INSOLUBLE ANODE |
| (57) The invention provides an insoluble anode for use in an electrolytic plating apparatus
for electrolytically plating a steel strip or like sheet continuously, the anode being
rendered free of the likelihood that no nonconductive portions will develop locally
from a plurality of recessed portions formed in the surface thereof for fitting bolt
heads therein to thereby prevent uneven plating of the resulting product. In the insoluble
anode (7) of the invention comprises an anode plate (1) removably fixed to an electrode
substrate (2) with a plurality of bolts (3) screwed in from the anode plate side,
a plurality of recessed portions (9) formed in an anode surface (8) for fitting heads
of the bolts therein are so arranged that the sum of the areas occupied by the recessed
portions (9) positioned within a rectangular hypothetical region having the length
of the anode plate and an arbitrary width in the overall width thereof is not more
than 10%, preferably not more than 8%, most preferably not more than 5%, of the area
of the arbitrary hypothetical region. |
FIELD OF THE INVENTION
[0001] The present invention relates to insoluble anodes for use in electrolytic plating as the main component of electrolytic plating apparatus for electrolytically plating sheets with zinc, tin or the like continuously, and more particularly to an insoluble anode comprising an anode plate removably fixed to an electrode substrate.
BACKGROUND ART
[0002] Conventionally used in electrolytically plating a steel strip or like sheet with zinc, tin or the like continuously are insoluble anodes which comprise an electrode substrate overlaid with lead or a lead alloy for the electrode reaction and conduction of current over the surface thereof to be opposed to the steel strip. However, the anode has the problem that the lead dissolving out from the overlay contaminates the plating solution and degrades the plating. As substitutes for such anodes, various insoluble anodes have been proposed which comprise an anode material coated with an electrode active substance layer containing an iridium oxide as an electrode active substance.
[0003] Since the electrode active layer coating the anode material of this type of anode has a greatly reduced thickness as compared with the anode having an overlay of lead or lead alloy, the steel strip is likely to come into contact with the anode, locally causing damage to the electrode active layer on the anode surface to inactivate the anode due to the fluttering of the steel strip traveling in the vicinity of the anode at a high speed as in the zinc electroplating line. Such local inactivation produces irregularities in the plating on the product to degrade the product, giving rise to a need for repair or replacement of the anode.
[0004] For use in electrolytic plating, Japanese Unexamined Utility Model Publication No. 136059/1990 proposes a segmented insoluble anode comprising an anode plate divided into a plurality of segments which are removably fixed to an electrode substrate with electrically conductive bolts screwed in from the electrolysis side in order to facilitate the repair of the anode. Further for use in electrolytic plating, Japanese Unexamined Utility Model Publication No. 131764/1988 proposes a composite electrode comprising a first electrode substrate and a second electrode substrate coated with an electrode active substance and removably fixed to the first electrode substrate with titanium bolts screwed in from the active layer side. Thus, these publications disclose that an anode plate is removably fixed to an electrode substrate with bolts screwed in from the anode plate side, whereas nothing whatever is disclosed, for example, about the fixing positions of the bolts.
[0005] With the electrode wherein an anode plate is fixed to an electrode substrate with bolts from the electrolysis side, i.e., from the electrode active layer side as disclosed in the above publications, the anode plate can be replaced by a greatly facilitated procedure, whereas the portions defining the bolt holes thereof are generally not coated with the electrode active layer and electrically nonconductive. Even if the bolt hole defining portions are coated with the active layer, the fixing tool will damage the active layer, or the coating of these portions with the active layer requires much labor because of the special shape of the portions, hence an economical disadvantage. Moreover, the electrode active layer coating the bolt hole defining portions will be separated off within a short period of time owing to a flaw or the like, failing to pass current therethrough. The bolt heads may be coated with the active layer, but the active layer become incapable of passing current therethrough because of damage or a shortened life like the coating over the bolt hole defining portions. Such nonconductive portions, if present locally, pose a problem in the appearance of the product such as irregularities in the plating.
[0006] An object of the present invention is to provide an insoluble anode for use in an electrolytic plating apparatus for electrolytically plating a steel strip or like sheet continuously, the anode being rendered free of the likelihood that no nonconductive portions will develop locally from a plurality of recessed portions formed in the surface thereof for fitting bolt heads therein to thereby prevent uneven plating of the resulting product.
DISCLOSURE OF THE INVENTION
[0007] The present invention provides an insoluble anode 7 comprising an anode plate 1 removably fixed to an electrode substrate 2 with a plurality of bolts 3 screwed in from the anode plate side, the removable insoluble anode being characterized in that a plurality of recessed portions 9 formed in an anode surface 8 for fitting heads of the bolts therein are so arranged that the sum of the areas occupied by the recessed portions 9 positioned within a rectangular hypothetical region (hereinafter referred to as "arbitrary hypothetical region") having the length of the anode plate and an arbitrary width in the overall width thereof is not more than 10%, preferably not more than 8%, most preferably not mare than 5%, of the area of the arbitrary hypothetical region.
[0008] Stated more specifically, relative to the area of one rectangular arbitrary region Z1, Z2 having the length L of the anode plate and an arbitrary width W1, W2 in the overall width W thereof, the ratio of the sum of the areas of the recessed portions in the region is 10%, preferably 8%, most preferably 5%, if maximum.
[0009] The expression that the areas occupied by the recessed portions within an arbitrary hypothetical region means the entire area of the recessed portion when the recessed portion is entirely positioned within this region, and the area of the part of the recessed portion within the arbitrary hypothetical region when the recessed portion is positioned within this region only partly.
[0010] Throughout the specification, the term "longitudinal direction" refers to a direction in coincidence with the direction, indicated by an arrow D, of travel of a steel strip or like sheet to be plated, and the term "widthwise direction" to a direction orthogonal to the direction of the arrow D. Further the term the "area of a recessed portion 9" refers to the area of the recessed portion 9 in a plane flush with the anode surface 8.
[0012] Preferably, at least two recessed portions 9 adjacent to each other in the longitudinal direction are so arranged that both are not positioned on an arbitrary line in the longitudinal direction. More specifically, when one of the two recessed portions 9 adjacent to each other in the longitudinal direction is positioned on an arbitrary line in the longitudinal direction, it is desired that the other recessed portion 9 be invariably out of alignment with the straight line.
[0013] Preferably, the anode plate 1 comprises an anode material coated with an electrode active layer containing a platinum group metal or an oxide thereof.
[0014] The insoluble anode of the present invention is suitable for use as the main component of a continuous electrolytic plating apparatus for electrolytically plating a steel strip or like sheet continuously with zinc, tin or the like.
[0015] The insoluble anode according to the invention will be described in greater detail with reference to the drawings.
[0016] FIG. 1 is a plan view showing an example of insoluble anode of the invention, and FIG. 2 is a view in vertical cross section showing an exemplary arrangement wherein the insoluble anode of the invention is installed in a plating cell. With reference to FIGS. 1 and 2, the insoluble anode 7 comprises an anode plate 1 removably fixed to an electrode substrate 2 with a plurality of bolts 3 screwed into the electrode substrate 2 from the anode plate side. The insoluble anode 7 is disposed with the anode plate 1 up in a plating solution 6 contained in a plating cell 5. A steel strip 4 serving as the sheet to be plated is caused to travel through the solution 6 in a direction D along the anode 7 at a predetermined distance therefrom. The steel strip 4 serves as a cathode. Although another insoluble anode is usually disposed above the steel strip to plate opposite surfaces of the steel strip with the upper and lower anodes, the upper insoluble anode is not shown for simplified illustration. The plating apparatus shown in FIG. 2 is of the horizontal type wherein the insoluble anode 7 is disposed horizontally, whereas the present invention is applicable also to vertical plating apparatus having an insoluble anode as positioned vertically.
[0017] With reference to FIG. 1, the anode surface 8 of the insoluble anode 7, i.e., the surface thereof facing the steel strip, is formed with a plurality of recessed portions 9 for fitting therein the heads of the bolts 3 for fixing the anode plate 1 to the electrode substrate 2. No recessed portion 9 exists in a left rectangular arbitrary hypothetical region Z1 having the length L of the anode plate 1 and an arbitrary width W1 at the left side of the anode surface 8. Based on the area of the arbitrary hypothetical region Z1, the ratio of the sum of the areas occupied by the recessed portions therein is 0%. A plurality of recessed portions 9 exist entirely or partially in a right rectangular arbitrary hypothetical region Z2 having the length L of the anode plate 1 and an arbitrary width W2 at the right side of the anode surface 8 (see hatched areas in FIG. 1). The sum of the areas occupied by these recessed portions 9 is not more than about 7% of the area of the arbitrary hypothetical region Z2.
[0018] With the insoluble anode 7 shown in FIG. 1, the anode plate 1 is made of a metal plate in match with the electrode substrate 2 in size, whereas the anode plate 1 may comprise a plurality of, e.g., four, divided segments 10 as shown in FIGS. 3 and 4. The segments 10 are closely arranged on the electrode substrate 2 and removably fixed to the substrate 2 with a plurality of bolts screwed in from the anode plate side.
[0019] The anode plate 1 may be removably fixed to the electrode substrate 2 alternatively by forming a plurality of bosses on each of the opposed surfaces of the plate and the substrate, forming a clearance between the plate 1 and the substrate 2 with the opposed bosses in each pair butting on each other and screwing a bolt into the butting bosses from the anode plate side. Preferably, the butting faces of the bosses are coated as with platinum to ensure electrical contact between the anode plate 1 and the electrode substrate 2.
[0021] In the case of the examples shown in FIGS. 3 and 4 wherein an anode plate 1 comprises four segments 10, the sum of the areas occupied by recessed portions 9 in an arbitrary hypothetical region is not more than 10% of the area of the hypothetical region.
[0022] The anode plate 1 comprising segments 10 and shown in FIG. 4 is also an exemplary arrangement wherein both recessed portions 9 adjacent to each other in the longitudinal direction are not positioned on an arbitrary line in the longitudinal direction. This arrangement is effective for precluding plating spots or stains from occurring on the steel plate successively in the direction of travel of the strip.
[0023] FIG. 5 shows an example of insoluble anode which is not included in the present invention. With reference to FIG. 5, an anode plate 1 similarly comprises four divided segments 10 which are closely arranged on an electrode substrate and attached to the substrate with bolts, whereas in the case of the anode plate 1 comprising the four segments 10, the sum of the areas occupied by recessed portions 9 in an arbitrary hypothetical region is over 10% of the area of the hypothetical region.
[0024] The electrode substrate 2 of the invention is generally made from metallic titanium, while the substrate may alternatively comprise a core of steel or the like covered with a corrosion resistant material, such as titanium, over the outer surface of the core. Suitable anode materials for making the anode plate 1 are metallic titanium and titanium-tantalum, titanium-tantalum-niobium, titanium-palladium and like titanium-base alloys. The anode material is generally made into a plate. Alternatively, a metal plate may be cladded with an anode material in the form of a net or porous sheet. The anode material is, for example, a titanium plate treated by blasting with alumina grit.
[0025] The electrode active layer for coating the anode material consists primarily of a component selected from among platinum group metals and oxides thereof. Especially desirable are iridium oxides, and mixed oxides of iridium and a valve metal such as titanium, tantalum, niobium, tungsten and zirconium. Typical of such examples are iridium-tantalum mixed oxide, iridium-titanium mixed oxide, etc. Highly durable are mixed oxides comprising 10 to 97 wt. % of iridium calculated as metal, and 90 to 3 wt. % of a valve metal.
[0026] The insoluble anode of the invention has the following advantages because of the foregoing structure.
a) The sum of the areas occupied by the recessed portions 9 within an arbitrary hypothetical region is not more than 10% of the area of the hypothetical region. This prevents the recessed portions from developing nonconductive portions locally, thereby obviating uneven plating of the product.
b) The two recessed portions 9 adjacent to each other in the longitudinal direction are so arranged that both are not positioned on an arbitrary line in the longitudinal direction, whereby uneven plating of the product can be precluded more effectively.
c) The anode plate 1 is removably fixed to the electrode substrate 2 with a plurality of bolts 3 from the anode plate side and is therefore replaceable easily.
d) The anode plate to be attached to the electrode substrate comprises a plurality of segments 10 and can therefore be repaired or replaced with a further improved work efficiency.
BRIEF DESCRIPTION OF THE DRAWINGS
[0027]
FIG. 1 is a plan view showing an example of insoluble anode according to the invention.
FIG. 2 is a view in vertical cross section showing an exemplary arrangement wherein the insoluble anode of the invention is disposed in a plating cell.
FIG. 3 is a plan view showing an example of anode plate comprising a plurality of divided segments.
FIG. 4 is a plan view showing another example of anode plate comprising a plurality of divided segments.
FIG. 5 is a plan view showing an example of insoluble anode not included in the invention.
BEST MODE OF CARRYING OUT THE INVENTION
[0028] Examples of insoluble anodes according to the invention will be described below, whereas the invention is not limited to these examples.
Example 1
[0029] The anode plate 1 of this example comprises four segments 10 as shown in FIG. 3. The segments were made from an anode material which was prepared by blasting a commercial titanium plate (15 mm in thickness) with alumina grit at an air pressure of 4 kgf/cm2. The anode material was coated with a solution of the following composition.
| TaCl5 | 400 mg |
| H2IrCl6 6H2O | 1300 mg |
| 35% HCl | 1 ml |
| n-C4H9OH | 20 ml |
[0030] The coated material was dried at 120 C for 10 minutes and then heated in an electric oven maintained at 500 C for 20 minutes. This procedure was repeated 5 times to form on the surface of the anode material an electrode active layer containing an electrode active substance in an amount of 10 g/m2 calculated as metallic iridium. In this way, a plurality of anode segments 10 were prepared which comprised an anode material coated with an electrode active layer containing an iridium oxide.
[0031] Four of the segments 10 were laid over a single electrode substrate in a close arrangement and fixed to the substrate with bolts to prepare an insoluble electrode. With the anode plate 1 of this example comprising four segments 10, the sum of the areas occupied by the recessed portions 9 within an arbitrary hypothetical region for fitting in the bolt heads was not more than 5% when greatest based on the area of the hypothetical region.
Example 2
[0032] An insoluble anode was fabricated by using four segments 10 (amount of electrode active substance: 10 g/m2 calculated as metallic iridium) prepared in the same manner as in Example 1 for an anode plate 1, laying these segments 10 over a single electrode substrate in a close arrangement as shown in FIG. 4 and fixing the segments to the substrate with bolts. With the anode plate 1 of this example comprising four segments 10, the two recessed portions 9 adjacent to each other in the longitudinal direction were so arranged that both were not positioned on an arbitrary longitudinal line.
[0033] With the anode plate 1 of this example comprising four segments 10, the sum of the areas occupied by the bolt head fitting recessed portions 9 within an arbitrary hypothetical region was not more than 8% when greatest based on the area of the hypothetical region.
Comparative Example 1
[0034] An insoluble anode was fabricated by using four segments 10 (amount of electrode active substance: 10 g/m2 calculated as metallic iridium) prepared in the same manner as in Example 1, laying these segments 10 over a single electrode substrate in a close arrangement as shown in FIG. 5 and fixing the segments to the substrate with bolts. With the anode plate 1 of this example comprising four segments 10, the sum of the areas occupied by the bolt head fitting recessed portions 9 within an arbitrary hypothetical region was not more than 15% when greatest based on the area of the hypothetical region.
Evaluation Test
[0035] Each of the insoluble anodes fabricated in Examples and Comparative Example was installed in a plating apparatus as shown in FIG. 2, and the plating apparatus was operated for continuously plating a steel strip with zinc in the following manner. The sheet used for plating was a usual A1 killed steel strip. ZnSO4 7H2O and H2SO4 were dissolved in ion exchange water to a concentration of 300 g/l and to a concentration of 50 g/l, respectively, to prepare a Zn plating solution. The plating solution was placed into the plating cell and heated at 60 C, and the sheet to be plated was immersed, as degreased and treated by acid pickling, in the plating solution. The speed of travel of the sheet to be plated was thereafter so adjusted that the flow rate of the plating solution was 2 m/sec relative to the sheet, and the sheet was electrically plated at a current density of 100 A/dm2 and a zinc deposition rate of 20 g/m2 to obtain a zinc-plated steel sheet.
[0036] The zinc-plated steel sheets obtained were checked for appearance. The appearance was evaluated with the unaided eye according to three criteria. Table 1 shows the result.
Table 1
| Maximum ratio of sum of recessed portion areas in arbitrary hypothetical region | Uneven plating of product | |
| Example 1 | 5% | A |
| Example 2 | 8% | B |
| Comp. Ex. 1 | 15% | C |
| A: No plating irregularities | ||
| B: Almost no noticeable plating irregularities | ||
| C: Marked plating irregularities |
INDUSTRIAL APPLICABILITY
1. An insoluble anode (7) comprising an anode plate (1) removably fixed to an electrode
substrate (2) with a plurality of bolts (3) screwed in from the anode plate side,
the removable insoluble anode being characterized in that a plurality of recessed
portions (9) formed in an anode surface (8) for fitting heads of the bolts therein
are so arranged that the sum of the areas occupied by the recessed portions (9) positioned
within a rectangular hypothetical region having the length of the anode plate and
an arbitrary width in the overall width thereof is not more than 10% of the area of
the arbitrary hypothetical region.
2. An insoluble anode according to claim 1 which is characterized in that the plurality
of recessed portions (9) are so arranged that the sum of the areas occupied by the
recessed portions (9) positioned within the rectangular hypothetical region is not
more than 8% of the area of the arbitrary hypothetical region.
3. An insoluble anode according to claim 1 which is characterized in that the plurality
of recessed portions (9) are so arranged that the sum of the areas occupied by the
recessed portions (9) positioned within the rectangular hypothetical region is not
more than 5% of the area of the arbitrary hypothetical region.
4. An insoluble anode according to any one of claims 1 to 3 which is characterized in
that the anode plate (1) comprises a plurality of divided segments (10).
5. An insoluble anode according to any one of claims 1 to 4 which is characterized in
that at least two recessed portions (9) adjacent to each other longitudinally of the
anode plate are so arranged that both are not positioned on an arbitrary line in the
longitudinal direction.
6. An insoluble anode according to any one of claims 1 to 5 which is characterized in
that the anode plate (1) comprises an anode material coated with an electrode active
substance layer containing a platinum group metal or an oxide thereof.
7. An insoluble anode according to any one of claims 1 to 6 which is characterized in
that the insoluble anode (7) is an anode of a continuous electrolytic plating apparatus.