Aqueous Anodes Data Sheet
Model AR is a thru-wall impressed current probe anode used to protect condenser waterboxes, waterfront structures, storage tanks and other wetted surfaces. It features a resilient mount between the anode rod and the fiberglass reinforced epoxy stand-off. Anode rods are typically 3/8″ or 1/2″ diameter with 2-9″ active and 1-9″ standoffs. Probes are best where current to a remote location is required. Platinum or Mixed Metal Oxide coatings can be applied that provide 20 or more years of service life. Probe anodes are intended for use at flow rates of less than 15 ft/sec (4.5 m/sec) and pressures lower than 150 psi (1 MPa). EDI also designs and builds custom anodes for special applications: see EDI Model AX Custom anodes for additional information.
Sizing and Number of Anodes – Proper cathodic protection design requires that adequate current be delivered where needed. Therefore, both the number of anodes and their location within the structure are important. Selection of rod diameter, standoff length and active length are based on many factors related to the conditions within the structure. From a cost perspective it is optimal to use the smallest diameter and shortest length rod with the necessary amount of coating to achieve the required design life while maintaining the objectives.
Substrate Selection – Titanium and Niobium are used as coating substrates for anodes because they form oxide films that prevent them from corroding in nature. Titanium is the most widely used. It is acceptable for all applications where the voltage between it and the electrolyte is below 10V. It is the only substrate to which either MMO or Pt can be applied. Niobium is used for those applications where the voltage at its surface is expected to exceed 10V. Only Pt can be applied to its surface.
Surface Coating Selection
Mixed metal oxide coatings are the best option where oxygen evolution is expected at the anode surface such as in fresh or brackish water locations. They consist of a blend of oxides of ruthenium, iridium and tantalum which have the ability to form a solid solution with the oxide film on titanium and render it conductive. The specific blend depends on the operating electrolyte and the thickness depends on the required current density and service life. The coating is applied by spraying aqueous salts of these metals onto the titanium substrate and then heating the titanium to a temperature of several hundred degrees. Further information on mixed metal oxide coatings can be found at this link.
Platinum is better for seawater applications where chlorine is generated at the anode surface. Coating thickness, operating current density, anode design life and operating environment are interrelated as shown on these Pt consumption charts. Platinum coatings are applied to a titanium or niobium substrate by electrodeposition or co-extrusion. Coating thickness will typically range from 50 in to 300 in. Thicker coatings will provide a longer service life.
Precious metal coated anodes with titanium substrates can be refurbished by removing the old coating, rebuilding or replacing the rod support and replatinizing. This process costs less than replacing the anode.
For More Information
Complete Product Information is available for downloading as pdf files. Click on the link below to download.
Probe Anode (Model AR) data sheet.
EDI products may be purchased from any corrosion control products distributor or directly from EDI.