Platinum is one of the six metals in
the "Platinum" group of noble or precious metals, all of which are
excellent catalyst materials. Platinum is conductive, chemically stable
and highly resistant to oxidation and corrosion. It has a silvery-gray
color and can be polished to a variety of finishes.
Because of its
high conductivity and low consumption rate, Platinum is an excellent anode
material. Due to its high cost, Platinum is made practical for use by
electroplating a thin layer over a high corrosion resistance substrate.
Since Titanium, Niobium and Tantalum substrates are having the ability to
form an insulating oxide film under anodic conditions, they are all most
commonly used as anode in ICCP. Among them, Titanium is less expensive;
however, it has a much lower breakdown potential than Niobium or Tantalum.
The titanium oxide breaks down at anodic potentials in the 12 V range.
In the case of Marine application, to avoid the dissolution of
titanium at unplatinized locations on the surface, the operating voltage
of the anode is limited by the anodic breakdown potential of titanium,
which is in the range of 9 to 9.5 V in the presence of chlorides. Hence
the maximum recommended operating voltage of platinized titanium anodes is
8 V. The corresponding maximum current density output is approximately 1
kA m-2. For cathodic protection systems where operating
voltages are relatively high, niobium and tantalum based anodes are
generally selected. This is because these two substrates have anodic
breakdown potentials greater than 100 V in chloride containing
electrolytes. The wastage rate of platinized anodes is approximately 8 mg
A-1 y-1.
The rate of platinum consumption
has been found to accelerate in the presence of AC current ripple. Most
wastage was observed to occur with AC frequencies of less than 50 Hz. The
repeated oxidation/reduction processes result in the formation of a
brownish layer of platinum oxide. To avoid the occurrence of this
phenomenon, a single or a three phase full-wave rectification is
recommended. The consumption rate of platinized anodes is also adversely
affected by the presence of organic impurities such as sugar and diesel
fuel.
Pt/Ti-Nb-Ta rod anodes are widely used for internal protection of
condensers, process equipment etc.
Pt/Ti-Nb-Ta wire anodes are used for internal protection of water
tanks and pipelines.
Pt/Ti-Nb-Ta mesh anodes are used for protection of reinforced
concrete structures.