Impressed current anodes graphite

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.

Costanzo, F. E., Using Graphite as Impressed Current Anode for Deep Groundbeds , Mat. Prot., 9 No. 4, 26, April (1970)... 

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. 

The low cost, light weight, and exceUent electrical conductivity of graphite anodes have made this impressed current protection system valuable for cathodic protection of pipelines, storage vessels, process equipment, and also for weU casings both on- and offshore. 

The forms of corrosion which can be controlled by cathodic protection include all forms of general corrosion, pitting corrosion, graphitic corrosion, crevice corrosion, stress-corrosion cracking, corrosion fatigue, cavitation corrosion, bacterial corrosion, etc. This section deals exclusively with the practical application of cathodic protection principally using the impressed-current method. The application of cathodic protection using sacrificial anodes is dealt with in Section 10.2. 

Impressed-current CP uses an outside power supply such as a rectifier to control the voltage between the pipe and an anode (cast iron, graphite, platinum clad, mixed metal oxide) in such a manner that the pipe becomes the cathode in the circuit and corrosion is mitigated. 

As shown in Fig. 15.6, external DC current is supplied from a power source such as a rectifier. The external DC current is used to cathodicaUy polarize the pipeHne. Impressed Current System (ICS) can be used to protect bare and poorly coated pipeHnes because of high current capacity. The anodes are made of durable materials that resist wear or dissolution. Iron with 14% silicon, carbon, and graphite are some commonly used anodes for pipeline protection [17,18]. All impressed current CPs require routine maintenance because they involve a power supply and more electrical connections than sacrificial systems. 

Carbon possesses high resistance to corrosion (except against halogens and oxidizing acids) and high electrical and thermal conductivity, but it is brittle. Graphite anodes are used in cathodic protection by impressed current. 

Anodes of a corrosion-resistant material such as Pt, PbSbAg, graphite, magnetite or high-silicon iron are normally used in impressed current installations. Pt is often used as a thin layer on a substrate of another material, e.g. in the form of platinized titanium. A corroding material, such as scrap steel, can also be used, but additional anode material must be supplied regularly in this case. 

Cathodic protection by impressed current involves the use of a rectifier connected to a power line. Contrary to sacrificial anodes, which operate at a fixed potential, the use of a rectifier permits to adjust the voltage (or the current) to the particular requirements of a protection scheme. This not only allows one to optimize the electrochemical conditions for protection, but the method is also well suited to protect large surfaces. On the other hand, protection by impressed current needs more maintenance than the use of sacrificial anodes. In order to protect buried structures by impressed currents one uses consumable anodes such as scrap iron or, more often, non-consumable anodes made of iron-silicon alloy, graphite or of titanium coated with noble-metal oxides. 

Design an impressed current system to protect a coated pipeline 4 mile long, 6(5/8)" OD in a soil of 2000 ohm-cm resistivity. Graphite anodes 3" X 5 are to be used. The back voltage between the pipeline and ground-bed is 3.0 V. 

For another method of cathodic protection, the source of electrons is an impressed current from an external dc power source, as represented in Figure 17.23h for an underground tank. The negative terminal of the power source is connected to the structure to be protected. The other terminal is joined to an inert anode (often graphite), which, in this case, is buried in the soil high-conductivity backfill material provides good electrical contact between the anode and the surrounding soil. A current path exists between... 

By impressing upon the system a d-c current of a magnitude that opposes the current generated by the galvanic corrosion cell. In this case a piece of metal e.g., scrap iron or graphite) is used as the anode. 

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