1. Field of the Invention
This invention relates to an anode coated with .beta.-lead dioxide which has dimensional stability and excellent adhesion between the .beta.-lead dioxide coating layer and a titanium substrate. More particularly, this invention relates to an anode coated with .beta.-lead dioxide which comprises a titanium substrate on at least one side of which both titanium expanded metal and titanium reinforcing bars are welded, at least one medium layer (i.e., intermediate layer) being applied on said substrate, said intermediate layer consisting either of an alloy or an oxide of platinum group metals, and a .beta.-lead dioxide coating layer being applied on the said intermediate layer.
2. Description of the Prior Art
An anode for use in electrolytic industries should have not only excellent anodic characteristics and corrosion resistance, but also excellent workability, mechanical strength and dimensional stability. Recently, the use of coated metallic anodes in the chlor-alkali industry has been proposed. Such coated metallic anodes have excellent anodic characteristics, corrosion resistance, and dimensional stability and have brought about a number of remarkable improvements in the chlor-alkali industry. However, these anodes have disadvantages in that they are expensive and have relatively short life when they are used under oxygen generation.
On the other hand, the .beta.-lead dioxide anode has been known from old times, and has been recognized to have high oxygen overvoltage characteristics, high corrosion resistance, peculiar catalytic properties and economic advantages resulting from its low manufacturing cost. However, the .beta.-lead dioxide anode has not been adapted very widely in the electrolytic industy in spite of its excellent characteristics. This is due mainly to its lack of mechanical strength, dimensional stability, and workability. Planar .beta.-lead dioxide anodes have conventionally been manufactured by anodically electrodepositing .beta.-lead dioxide on a concave surface of a substrate from a lead nitrate bath, and then by detaching .beta.-lead dioxide layer from the substrate. Such anodes are not free from strong internal stresses and hence are very brittle, thus limiting the size and shape that can be manufactured and substantially reducing their practical value in industry. The attempt has been made to electrodeposit .beta.-lead dioxide on a graphite or on a flash coated platinum which is coated on titanium substrate (See U.S. Pat. No. 3,207,679). This was not very successful, however, because .beta.-lead dioxide is apt to detach from the substrate because of low adhesion and the growth of cracks, and such anodes were too heavy for easy handling. More recently, the manufacture of a .beta.-lead dioxide anode by electrodepositing .beta.-lead dioxide on a screen was tried (See U.S. Pat. No. 2,872,405). The adhesion of the coating layer to the anode was better, but it was difficult to obtain even plates. Thus, the surface was uneven, the dimensional stability was insufficient, and it was impossible to obtain even anodes of the substantial dimensions required in industry. In addition, difficulty was encountered in applying a current connection to the anode.
Accordingly, an anode coated with .beta.-lead dioxide is not satisfactory for extended use in the electrolytic industries, mainly because of the lack of mechanical strength, and dimensional stability and the difficulty of manufacturing large anodes.