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Impressed Current Cathodic Protection

Cathodic protection can be applied if the metal to be protected is coupled to the negative pole of a direct current source (schematic), while the positive pole is coupled to an auxiliary anode. Since the driving voltage is provided by the DC source there is no need for the anode to be more active than the structure to be protected.

There are basically three types of anode materials:
  • Inert or non consumable anodes
  • Semi-consumable anodes
  • Consumable anodes

All items to be protected shall be electrically connected and should have a welded or brazed connection to an anode. For bolted or clamped assemblies without an all welded brazed electrical grounding, the electric resistance should be less than 0.10 ohm. Coating on contact surfaces shall be removed prior to assembly.

Non Consumable Anodes

This type of anode supports other anodic reactions on their surfaces. In environments where water and chloride ions are present, chlorine evolution and oxidation of water are possible.

A. Platinized substrates: Platinum is the ideal permanent impressed current anode material. It is one of the most noble metals and in practically all environments forms a thin invisible film which is electrically very conductive. In addition, the exchange current densities of most anodic reactions on the Pt surface are greater than on other anode materials. Due to its high cost, platinum is applied as a thin coating (1-5 mm) on metallic substrates such as titanium, niobium and tantalum.

Platinized titanium is often used in marine environments. 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 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.

B. Magnetite: Magnetite is a cheap and naturally occurring material. It is a non-stoichiometric oxide and has an electrical conductivity of 1.25 W-1 m-1. Due to its brittleness, the anode is cast as a hollow cylinder and closed at one end. The inner surface is then copper plated and the cylinder is filled with polystyrene. Epoxy resin is used to fill any remaining space. The anode cable is soldered to the copper plate. Magnetite anodes have been successfully used in the cathodic protection of buried structures and those immersed in seawater. The maximum operating current density is 0.115 kA m-2 and the anode consumption rate is approximately from 1 to 4 g A-1y-1.

C. Lida: This is a recently developed anode. It is claimed that it has superior mechanical, consumption and electrochemical properties compared with conventional anodes. The anode is composed of an inert metal oxide , ruthenium oxide coated titanium. The operating current density is 0.8 kA m-2 and the consumption rate is in the range of 0.8 mg A-1 y-1.

Semi-consumable anodes

Semi-consumable anodes such as graphite and high silicon iron have been in service since the first industrial electrochemical systems were built.

A. Graphite: Graphite anodes are widely used. Carbon has been used as an anode in chlorine production since the end of the nineteenth century. Graphite, which is less porous and more electrically conductive, is now preferred for use in impressed anode materials. However, graphite can still be highly porous, with the porosity being exacerbated by gas evolution. For this reason, graphite is often impregnated with resins to reduce solution ingress and improve mechanical strength. Graphite anodes are inert when chlorine evolution is occurring, chlorine being produced efficiently at low polarizations. But if oxygen formation predominates, as in low chlorine media, graphite is oxidized to carbon dioxide. Graphite deterioration also increases with decreasing pH and increasing sulfate ions concentration.

To eliminate the possibility of galvanic corrosion caused by detached pieces, graphite is not recommended for use in closed systems. In addition, graphite suffers high consumption rates in water at temperatures above 50oC. Consumption rates measured for graphite depend on the environment and thus range from 0.045 in seawater to 0.45 kg A-1 y-1 in freshwater. Similarly the corresponding operating current densities vary from 2.5 to 10 A m-2. The maximum operating voltage for graphite anodes is only limited by excessive consumption rate and brittleness of the material. The main disadvantages of graphite compared to other impressed anodes are low operating current densities and inferior mechanical strength. Graphite is generally used in conjunction with carbonaceous back-fills in soil based impressed anode systems.

B. High silicon iron (HSI) alloys: These anodes are widely used. HSI anodes contain about 14.5% silicon and certain alloys have 4.5% chromium. Chromium has now replaced molybdenum as an alloying element in this type of anode. The high silicon content ensures that the alloy forms a protective film containing silicon dioxide, SiO2. A prerequisite for the formation of this film is that the anode must initially corrode during the first few hours of operation. The mechanism of this passivating film is not well understood. The high electrical conductivity of the film is believed to be due to the presence of iron oxides. Silicon dioxide is highly resistant to acids but it is readily dissolved in alkaline conditions. High silicon iron anodes are extremely hard and cannot be machined easily. They are generally cast and then stress relieved by annealing. Although brittle these anodes have superior abrasion and erosion characteristics compared to graphite.

High silicon iron anodes are widely used usually in conjunction with carbonaceous backfills in soils. They have also found limited use in marine and freshwater environments. The maximum operating current density is determined by the type of alloy and the environment. For instance, in groundbeds with backfills the current density is limited to between 10 and 20 A m-2 because of problems caused by gas entrapment. In marine environments, a high iron chromium anode can be operated up to 50 A m-2. As for graphite, the maximum operating voltage is limited by excessive consumption and brittleness of the material. The consumption rate of these anodes is influenced by the operating current density and the nature of the environment. Generally, a lower current density reduces the consumption rate. Wastage rates range from 0.10 to 0.50 kg A-1 y-1. Sulfate ions in particular have been noted to enhance the dissolution rate of these materials.

C. Lead alloys: The function of lead as an impressed current anode depends on the formation of a protective and electrically conductivity film (101-102 Sm-1) of lead dioxide, PbO2. This film is non-stoichiometric oxide and exists in two forms:

  • alpha - PbO2 (orthorhombic)
  • beta - PbO2 (tetragonal).

Lead dioxide is surprisingly stable in the presence of chloride ions. The insoluble lead chloride, PbCl2, is believed to be responsible for healing the defects in the film. This ensures that Pb/PbO2 behaves as an inert electrode and hence allowing at high polarization the evolution of chlorine and oxygen. To form an adherent and stable film of PbO2, lead is generally alloyed with Ag and Sb. A typical alloy composition is Pb 6 Sb 1 Ag.

Owing to the low overvoltage of chlorine evolution on the surface of these anodes, lead alloys are mostly used in seawater applications. Maximum operating voltage and current density of these anodes are 24 V and 1 kA m-2 respectively. The consumption rate is in the range 1-10 g A-1 y-1. It should be added that lead alloy anodes are sometimes used with platinum pins. It has been found that a platinum microelectrode inserted into the surface of the lead enhanced the formation of PbO2. It is also worthwhile noting that the performance of lead alloy anodes (with and without Pt pins) is adversely affected at operation depths greater than 30 m in seawater.

Consumable Anodes

Examples of this type of anode include scrap iron or steel and cast iron. The anode is deliberately dissolved to provide the electrons required to polarize the structure. Consumable anodes can be used in buried or under immersed conditions. They have consumption rates of approximately 9 kg A-1 y-1. and maximum current densities are in the order of 5 A m-2. Due mainly to their high consumption rate the use of such anodes is rather rare unless a redundant source of iron or steel is readily available such as an old ship beached below low tide, a disused pipeline, well string etc. However, since such structures are frequently massive, they represent a very low resistance to earth and therefore can make the cathodic protection engineer's life much simpler.