Electrolysis: processes – compositions used therein – and methods – Electrolytic coating – Depositing predominantly single metal coating
Reexamination Certificate
1999-09-28
2001-06-26
Bell, Bruce F. (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic coating
Depositing predominantly single metal coating
C205S284000, C205S288000, C205S289000, C204S290010, C204S290030, C204S290060, C204S290080, C204S290090, C204S290120, C204S290130, C204S290140
Reexamination Certificate
active
06251254
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an electrode for use in chromium plating. More particularly, this invention relates to an anode which is suitable for use in chromium plating from trivalent chromium baths and is effective in diminishing the oxidation of trivalent chromium to hexavalent chromium.
BACKGROUND OF THE INVENTION
Chromium plating is widely used for the corrosion protection of iron-based metals, decoration, etc. Although plating baths containing chromic acid, which is a compound of hexavalent chromium, as a chromium material have been used for chromium plating, the discharge of hexavalent chromium into the environment is strictly restricted because of the problem of environmental pollution. Attention is hence directed to a plating method in which trivalent chromium, which is less toxic, is used as a feed material in place of hexavalent chromium.
Theoretically, the plating method in which trivalent chromium is used as a feed material is capable of depositing the metal at a rate two times that in plating from a hexavalent chromium bath at the same plating current. This plating method is characterized in that it is excellent in covering power, throwing power, etc., and that wastewater treatment is easy. However, it has problems, for example, in that electrode reactions including the anodic oxidation of trivalent chromium into hexavalent chromium shorten the life of the plating bath and reduce the deposit quality. In the case where a metal electrode made of lead, a lead alloy, or the like is used as an anode for plating from a trivalent chromium bath, a sludge generates which is the same as the lead compound sludge resulting from dissolution of the lead electrode used in plating from hexavalent chromium baths. In addition, the lead oxide yielded on the anode surface accelerates the oxidation of trivalent chromium to enhance the generation of hexavalent chromium. Thus, the problems inherent in hexavalent chromium have remained unsolved.
JP-B-56-43119 (the term “JP-B” as used herein means an “examined Japanese patent publication”) proposes to prevent the anodic generation of hexavalent chromium by using, for plating from a trivalent chromium bath, an anode comprising at least one of iron, iron alloys, nickel, nickel alloys and nickel oxide. JP-B-61-22037 proposes the use of a ferrite electrode. However, use of these electrodes as an anode has a problem in that an electrode component contained in the anode dissolves away to generate a sludge or adhere to the surface of the work, resulting in a decrease in deposit quality.
JP-A-54-134038 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”), JP-A-61-23783, and JP-A-61-26797 disclose a plating technique in which an ion-exchange membrane is used to partition an electrolytic cell into an anode chamber and a cathode chamber. In this technique, an aqueous solution of a salt of trivalent chromium is fed to the cathode chamber, while a solution not containing trivalent chromium, e.g., a solution of an acid containing the same anion as the salt of trivalent chromium, is fed to the anode chamber.
When the solution to be fed to the anode chamber is a sulfuric acid solution, the anode is, for example, an electrode comprising a lead or titanium base coated with either a noble metal or an oxide thereof. When the solution to be fed to the anode chamber is a chloride solution, the anode is, for example, an electrode comprising a graphite or titanium base coated with either a noble metal or an oxide thereof. However, this technique has a problem in that the plating vessel has a complicated structure due to the use of an ion-exchange membrane.
Furthermore, JP-A-8-13199 discloses the use of an electrode comprising an electrode base coated with an electrode catalyst comprising iridium oxide as an anode in a trivalent chromium bath. Use of iridium oxide as an electrode catalyst is effective in attaining improvements including a prolonged electrode life. However, it has been found that the bath becomes unstable through long-term use due to the hexavalent chromium ions which generate in a slight amount and due to the decomposition products resulting from the electrolytic oxidation of an organic additive contained in the bath. There is hence a need for an electrode with which the bath components are stable over a long period of operation and the generation of hexavalent chromium is further diminished.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a method of chromium plating from trivalent chromium baths which minimizes the generation of hexavalent chromium in anodic reactions, keeps the plating bath components stable over a long period of operation, and exerts a limited influence on the environment.
The above object of the present invention has been achieved by providing an electrode for chromium plating from a trivalent chromium bath which comprises a conductive base, an electrode material layer comprising iridium oxide formed on the base, and a porous layer formed on the surface of the electrode material layer.
In a preferred embodiment, the porous layer is made of an oxide containing at least one element selected from the group consisting of silicon, molybdenum, titanium, tantalum, zirconium and tungsten.
DETAILED DESCRIPTION OF THE INVENTION
The present invention has been completed based on the finding that an electrode constituted by forming a porous layer on an electrode material layer formed on a conductive base functions as an anode for chromium plating from trivalent chromium baths, and is effective in preventing the oxidation reaction in which trivalent chromium present in the plating baths is oxidized to hexavalent chromium.
The characteristic feature of the electrode for chromium plating of the present invention resides in that it has a porous layer on the electrode catalyst layer.
The porous layer can be made of an oxide containing at least one element selected from silicon, molybdenum, titanium, tantalum, zirconium and tungsten. Examples of the oxide include SiO
2
, TiO
2
, Ta
2
O
5
, ZrO
2
and WO
3
.
Of these, SiO
2
, TiO
2
, and ZrO
2
are preferred.
The porous layer preferably covers the surface of the electrode material layer in a thickness of from 2 to 50 &mgr;m. The thickness of the porous layer is more preferably from 5 to 20 &mgr;m. It is, however, necessary that the electrode material layer be completely covered with the porous layer even when examined with an electron microscope.
For forming the porous layer, the following methods can be used. First, a sol is prepared by the sol-gel method from, e.g., an alkyl compound containing a material for porous-layer formation such as an organosilicon compound. At least one of phosphorus pentoxide, phosphoric acid and boric acid is added to the sol, and the resultant fluid is applied to the surface of an electrode. The coating is burned to form a layer. Thereafter, the phosphorus pentoxide, phosphoric acid and boric acid are dissolved away with warm water or the like to form the target porous layer. Other usable methods include: a method which comprises applying an aqueous solution of a compound for porous-layer formation such as sodium silicate on the surface of an electrode, burning the coating, and then dissolving away the resultant soluble ingredient with warm water or the like; and a pyrolytic method in which a solution of a salt for porous-layer formation, e.g., titanium chloride, zirconium chloride, molybdenum chloride or tantalum chloride is applied, and the resultant coating is pyrolyzed to form a porous oxide film.
Still another method usable for obtaining the desired porous layer comprises adding a sodium salt, phosphoric acid, or boric acid to a material for porous-layer formation, forming a layer through burning, and then dissolving away the added substance with warm water or the like.
The conductive base for use as an electrode base in the present invention is preferably made of a highly corrosion-resistant metal capable of forming a thin film, such as, eg., titanium
Katoh Masaaki
Matsumoto Yukiei
Nara Miwako
Ogata Setsuro
Bell Bruce F.
Permelec Electrode Ltd.
Sughrue Mion Zinn Macpeak & Seas, PLLC
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