Impressed current anode materials

Numerous materials fall into the category of electronic conductors and hence may be utilised as impressed-current anode material. That only a small number of these materials have a practical application is a function of their cost per unit of energy emitted and their electrochemical inertness and mechanical durability. These major factors are interrelated and —as with any held of practical engineering—the choice of a particular material can only be related to total cost. Within this cost must be considered the initial cost of the cathodic protection system and maintenance, operation and refurbishment costs during the required life of both the structure to be protected and the cathodic protection system. 

These are iron alloys that contain 14-18% Si and are reported as first being developed in 1912 , although it was not until 1954 that they were first evaluated for use as impressed-current anode material in cathodic protection. Its major disadvantage is that it is a hard brittle material unable to sustain thermal or mechanical shock. 

Table 1.31 Impressed current anode materials and their properties75(Copyright Pearson Education Ltd)...

J.A. Jakobs, F.W. Hewes, Evaluation of impressed current anode materials for use in wet soils and sediments containing chlorides and sulfates, Proc. Int. Conre. Met. Corros., Edmonton, AB, Canada, (1984), pp. 78—85. 

PRINCIPLES OF CATHODIC PROTECTION Table 10.1 Impressed current anode materials... 

A wide range of materials can be considered for impressed current anodes, ranging from inexpensive scrap steel to high-cost platinized material. The following properties would all be ideally desirable in an impressed current anode material [13] ... 

There are two types of impressed current anodes either they consist of anodically stable noble metals (e.g., platinum) or anodically passivatable materials that form conducting oxide films on their surfaces. In both cases, the anodic redox reaction occurs at much lower potentials than those of theoretically possible anodic corrosion. 

This general statement does not of course mean that materials with stoichiometric weight are completely unsuited as impressed current anodes. 

Fig. 7-1 Material consumption from impressed current anodes. graphite anode without coke backfill, O graphite anode with coke backfill, FeSi anode without coke backfill, A FeSi anode with coke backfill.

Impressed current anodes of the previously described substrate materials always have a much higher consumption rate, even at moderately low anode current densities. If long life at high anode current densities is to be achieved, one must resort to anodes whose surfaces consist of anodically stable noble metals, mostly platinum, more seldom iridium or metal oxide films (see Table 7-3). 

Galvanic or impressed current anodes are used to protect these components. The anode material is determined by the electrolyte zinc and aluminum for seawater, magnesium for freshwater circuits. Platinized titanium is used for the anode material in impressed current protection. Potential-regulating systems working independently of each other should be used for the inlet and outlet feeds of heat exchangers on account of the different temperature behavior. The protection current densities depend on the material and the medium. 

The manufacture, processing and application of a particular material as an impressed-current anode requires knowledge of several physical characteristics. Knowledge and attention to these characteristics is necessary to design for anode longevity with maximum freedom from electrical and mechanical defects. 

Swedish iron is sometimes used as galvanic wastage plates in heat exchangers, particularly for marine applications. This is possibly based on tradition, since it cannot be the most economical method in the light of current cathodic-protection practice. The material is not currently used as an impressed-current anode. 

A variety of materials is used for impressed current anodes. Among the oldest... 

Impressed current anodes must be corrosion resistant and otherwise durable in the environment in which they are used. They must have low consumption rates when coimected to a CP source. All materials used for impressed current anodes are cathodic (more noble) than steel. High silicon cast iron (Fe-0.95C-0.75Mn-14.5Si-4.5Cr) is used for onshore CP applications and in other locations where abrasion resistance and other mechanical damage considerations are important [1]. Graphite anodes are extensively used for onshore pipeline CP applications in which they can be buried in multiple-anode ground beds. However, graphite must be stored and handled carefully because of its brittle nature. 

Precious metals are used for impressed current anodes because they are highly efficient electrodes and can handle much higher currents than anodes fabricated from other materials. Precious metal anodes are actually platinized titanium or niobium anodes the platinum is either clad or electroplated on the substrate. 

Rectifiers are used more than any other source of impressed-current power. Areas discussed include rectifier types, rectifier selection, specification requirements, and typical installation details. Various types of impressed-current anodes and components that make up an impressed-current system are also presented. Impressed-current-type cathodic protection systems provide cathodic current from an external power source. A direct current (DC) power source forces current to discharge from expendable anodes through the electrolyte and onto the structure to be protected. Although the current is not generated by the corrosion of a sacrificial metal/alloy, the energized materials used for the auxiliary anodes do corrode. 

Embrittlement by internal hydride formation (e.g., Ti, Zr, Nb, Ta) [These metals are important as valve metals for impressed current anodes (Section 7.1) and as materials in chemical plants (Section 21.4.3).]... 

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