The materials chosen in the design of Anomet's anodes are based primarily on the technical merits of the metals utilized. While the list of possible material combinations is virtually unlimited, the current standard line of Anomet material limits itself to those materials which possess specific properties necessary in the design of an effective impressed current anode material.
Platinum is used on the surface as a primary anode material due to its excellent corrosion resistance under anodic conditions coupled with its ability to pass current in all electrolytes without forming an insulating film. The corrosion resistance of platinum manifests itself in its low consumption rate. Its major disadvantage is its high cost, thus making it clear that it is most desirable to use as little platinum as is necessary. In order to restrict the amount of platinum used and to maintain an anode of some structural integrity, it is necessary to use some type of substrate material.
Included among the properties of the substrate should be the ability to form an insulating film under anodic conditions, such that a dimensionally stable anode is obtained, good conductivity, and relatively low cost. As there are no individual materials which fully meet all of these criteria, it is necessary in most cases to use a combination of materials.
Both niobium and titanium have the ability to form insulating oxide films under anodic conditions, and both possess unique advantages and disadvantages as a substrate for platinum. The major advantage of titanium is its low cost, particularly when considering its lower density. Unfortunately, there are many applications where the disadvantages of titanium far outweigh its cost advantage. First, in environments with high chloride contents, the insulating oxide film formed by titanium tends to break down, thus undermining dimensional stability needed for complete platinum consumption. A second disadvantage of titanium is its poor conductivity; approximately 5 times less than niobium and 50 times less than copper. Low conductivity makes titanium poorly suited for long wire lengths and restricts its use to larger diameter, shorter anodes.
The use of niobium as a substrate to platinum eliminates many of the problems with titanium. The breakdown voltage of niobium oxide film is approximately 120 volts in most commonly found cathodic protection environments. Although its conductivity is higher than that of titanium, it is often still too low for use in small diameters and long lengths. The principal disadvantage of niobium, however, is its relatively high cost.
Copper is an ideal conductor at virtually any length and diameter suitable for cathodic protection. The combination of high conductivity and low cost allows copper to be an ideal candidate for use in the design of an anode material.