NEPTUNE Circuit

Allows the anodes to form a thin film of silicon dioxide which reduces consumption Consumption ratings from 0.5 to 1.0 lb/amp-yr. in sea water Recommended operating current density is at or below 1 amp/ft 2 High Silicon Cast Iron Anodes

Duriron Electrodes 1 lb x 10 A x 30 yr = 300 lb consumption A-yr Need to restrict consumption to 33% loss of diameter 300 lb x 2.5 = 750 lb starting weight 1 ft2 x 10 A = 10 ft2 (will be less due to large surface area A and volumefor consumption) 20 electrodes (?) 1 A Tyco systems use 4-16, depending on earth resistance

Groundbed anodes are usually surrounded by a carbonaceous backfill. Reduces the groundbed resistance by increasing the effective size of the anode Provides surface on which oxidation reactions can occur which prolongs anode life. Carbonaceous Backfill

Platinized Niobium/Titanium Probe Anodes Titanium or niobium rod substrates covered with a platinum coating Oxidizing film coats the surface of titanium and niobium, assuring that the substrates will remain stable and not be consumed The film also makes the substrate surface relatively non-conductive, and electrical discharge therefore occurs through the platinum coating Then coating is electroplated to the substrates in thicknesses ranging from 100 to 300 micro- inches 8-16 mg/amp-yr consumption amps/ft 2 in salt water electrolytes

Ti/Pt Anode Sizing 15 mg x 1 A x 30 yr = 450 mg A-yr 450 mg x cm3 x in3 x 1 = 4.28 in x 103 mg (2.54 cm) x 10-6 in 1 A x 144 in2 = 33.6 A/ ft in21 ft2 Shaped for uniform wear

Mixed Metal Oxide Probe Anodes Very low consumption rates (under 1.0 mg/amp- yr High current densities - over 50 amps/ft 2 in salt water Catalytic coating “recipe” can be varied to select for

The reactions that take place at the anode in sea water and brackish water are: 2 H 2 O → 4H+ + O 2 (g) + 4e- E0 (vs. H 2 /Pt)= 1.23 V 2 Cl- → Cl 2 (g) + 2 e-E0 (vs. H 2 /Pt)= 1.36 V Oxygen evolution is favored since it requires less energy MMO coatings allow controlling the reaction to select oxygen evolution

If the catalyst has enough surface area, the residual chlorine evolved will not appear as gas but will form a hypochlorite (OCl-): Cl 2 (g) + H 2 O ↔ HCL + HOCl HOCL ↔ H+ + OCl- The hypochlorite on a large anode is less harmful because the buffering capacity of the carbonate in the sea water will not be exceeded and pH will not decrease. Hypochlorite can attack cable insulation.

The reaction that takes place at the cathode is: 2 H 2 O + 2 e- → 2 OH- + H 2 (g) OH- ions will achieve equilibrium with the carbonate backfill and the effect of the pH will be reduced.