The base metal is required to have electrochemical corrosion resistance, a certain mechanical strength, easy to manufacture, easy to form passivation film on the surface and good electrical conductivity.
The electrode matrix can be made of any conductive material, such as iron, nickel, lead, copper and other metals and their alloys, as well as high silicon iron containing chromium and magnesium. But it is best to use valve metal or called film metal.
Valve metal, which is conductive when used as a cathode in brine electrolysis, but not conductive when used as an anode, has a one-way current-carrying property. Valve type metals include
titanium,
niobium,
tantalum,
zirconium,
tungsten, aluminum, or an alloy of two or more of the above-mentioned metals. Titanium is usually used in the electrode matrix, and tantalum and niobium are used in special cases.
When titanium, tantalum and niobium are polarized at the anode, a solid passivation film is produced. In the presence of chloride ions, titanium locally corrodes.
When the applied voltage is below 10V, the reaction of the three metals is almost the same, but above 10V, the current density of titanium can be seen to increase, which is the result of local corrosion of titanium. However, tantalum and niobium show strong corrosion resistance even in applied voltage above 10V polarization. Compared with tantalum and niobium, titanium has the advantages of cheap and small density, so the relative price per unit volume is about 1/20 of tantalum and 1/10 of niobium. However, the resistivity of titanium is 3-4 times that of tantalum and ni, and 25 times that of copper.
The passivation characteristics of the anodic base metal are expressed by the destruction potential (Ec) and protection potential (Ep).
