Silicon-iron anodes

Anode material Iron High-silicon iron Graphite Magnetite titanium ... 

The anodes most suitable for burying in soil are cylindrical anodes of high-silicon iron of 1 to 80 kg and with diameters from 30 to 110 mm and lengths from 250 to 1500 mm. The anodes are slightly conical and have at the thicker end for the current lead an iron connector cast into the anode material, to which the cable connection is joined by brazing or wedging. This anode connection is usually sealed with cast resin and forms the anode head (see Fig. 7-2). Ninety percent of premature anode failures occur at the anode head, i.e., at the cable connection to the anode [29], Since installation and assembly costs are the main components of the total cost of an... 

In addition to anodes with a simple connecting head, there are cylindrical double anodes that have cable connectors cast on at both ends and that can be used in the construction of horizontal or vertical anode chains. Anodes of graphite or magnetite are more compact than anodes of high-silicon iron because of the danger of fracture. 

The installation costs for a single impressed current anode of high-silicon iron can be taken as Kj = DM 975 (S550). This involves about 5 m of cable trench between anodes so that the costs for horizontal or vertical anodes or for anodes in a common continuous coke bed are almost the same. To calculate the total costs, the annuity factor for a trouble-free service life of 20 years (a = 0.11, given in Fig. 22-2) should be used. For the cost of current, an industrial power tariff of 0.188 DM/kWh should be assumed for t = 8750 hours of use per year, and for the rectifier an efficiency of w = 0.5. The annual basic charge of about DM 152 for 0.5 kW gives about 0.0174 DM/kWh for the calculated hours of use, so that the total current cost comes to... 

In total, three high-silicon iron anodes of 3 kg each were installed at points a, aj and as shown in Fig. 11-3. The anodes were bedded vertically in fine-grained coke in boreholes about 2.3 m deep and J = 0.2 m so that the length of the coke backfill was about 1 m. Each anode was connected by a separate cable to the anode bus bar of the transformer-rectifier to allow the current of individual anodes to be monitored. Three cathode cables 2x4 mm were installed for the return path of the protection current and attached on the tank end to the connecting clamps of the dome support. 

As an example, a tank farm that is to be cathodically protected by this method is shown schematically in Fig. 11-4. As can be seen in the figure, injection of the protection current occurs with two current circuits of a total of about 9 A, via 16 vertically installed high-silicon iron anodes embedded in coke. These are distributed over several locations in the tank farm to achieve an approximately uniform potential drop. The details of the transformer-rectifier as well as the individual anode currents are included in Fig. 11-4. Anodes 4, 5 and 6 have been placed at areas where corrosion damage previously occurred. Since off potentials for 7/ -free potential measurements cannot be used, external measuring probes should be installed for accurate assessment (see Section 3.3.3.2 and Chapter 12). 

In the first reconstruction [27] of road slabs contaminated with CL, silicon iron anodes were embedded in a layer of coke breeze as shown in Fig. 19-4a or the current connection was achieved with noble metal wires in a conducting mineral bedding material. Slots were ground into the concrete surface for this purpose at spacings of about 0.3 m (see Fig. 19-4b). This system is not suitable for vertical structures. 

Fig. 19-4 Older anode system for roadway plates (a) Silicon iron anodes in coke breeze bed, (b) noble metal lead in conducting bedding in a trench.

The effective costs of a cathodic protection station depend very much on the local conditions, particularly on the costs of the power supply and the extent of the anode bed. Table 22-2 contains the costs of a protection station with four silicon iron anodes. 

The use of high-silicon irons as anodes for impressed-current cathodic-protection systems is described in Sections 10.3 and 10.4. 

The semi-consumable electrodes, as the name implies, suffer rather less dissolution than Faraday s law would predict and substantially more than the non-consumable electrodes. This is because the anodic reaction is shared between oxidising the anode material (causing consumption) and oxidising the environment (with no concomitant loss of metal). Electrodes made from silicon-iron, chromium-silicon-iron and graphite fall into this category. 

Canister anodes consist of a spirally wound galvanised steel outer casing containing a carbonaceous based extender which surrounds the primary anode element which may be graphite, silicon iron, magnetite, platinised titanium, mixed metal oxide-coated titanium or platinised niobium, etc. 

Groundbeds consist of a carbonaceous extender generally coke breeze and graphite, silicon-iron scrap steel, platinised titanium or niobium anodes. 

The high cost of platinised materials for use in borehole groundbeds as opposed to conventional silicon-iron anodes may also be offset by the reduction in required borehole diameter, hence lower installation cost, with the relative economics between the different systems dependent upon a combination of both material and installation costs. 

Magnetite anodes exhibit a relatively low consumption rate when compared with other anode materials, namely graphite, silicon iron and lead and can be used in seawater, fresh water and soils. This low consumption rate enables a light-weight anode construction to be utilised. For example, the anode described by Linder is 800 mm in length 60 mm in diameter, 10 mm wall thickness and 6 kg in weight. 

Graphite anodes when used in soils are invariably placed in a carbonaceous backfill. This helps to compensate for the lower electrical resistivity of graphite when compared with silicon iron. In such an environment, no build-up of a film of high resistance between the anode and backfill occurs, unlike silicon-iron anodes where the resistance can increase with... 

Graphite, silicon-iron and scrap-steel anodes used for buried structures and landward faces of jetties, wharves, etc. 

Voltage drop caused by groundbed resistance, as previously explained. Back voltage polarisation between groundbed and pipeline. In the case of both graphite and silicon-iron anodes, an allowance of 2 V is normally used. This back voltage is that which exists between the anodes and the structure in opposition to the applied voltage. 

Impressed current systems are normally based upon anodes of silicon iron, platinised titanium or platinised niobium. The method of anode installation is usually by suspension. The anode configuration and number must be such as to ensure uniform current distribution. Considerable use is made of wire-type platinised-titanium, and niobium anodes which offer minimal weight and relative ease of mounting/suspension. 

Silicon-iron and Graphite Anodes of these materials are now frequently manufactured with the anode cable tail connected at the centre of the rod... 

DcMilIc Campbell, E., High Silicon Iron Anodes for Cathodic Protection , Corrosion, 27 No. 4, 141 (1971)... 

After many attempts with several anode materials we found a stable anode. Silicon carbide and iron silicide, etc. in a conducting form are stable towards chlorine. The chlorine formed on the anode then reacts with the solvent (THF) forming chlorinated organic compounds. 

In Alternative 3 (Fig. 3), the electrolysis may be operated on a semicontinuous basis with the cadmium eventually being stripped completely from the electrolyte, which is then discarded after suitable treatment. Instead of the usual silver—lead anodes, high silicon—iron anodes, such as Duriion, are commonly used. 

Aluminum spraying is used to coat less corrosion-resistant alloys. In the case of some composites, corrosion is due to the galvanic action between the aluminum matrix and the reinforcing material. Aluminum thermal spraying has been successfully used for the protection of the discontinous silicon carbide/aluminum composites, and continuous graphite/aluminum. Other protection procedures include sulfuric acid anodizing and iron vapor deposition on aluminum.44... 

The anodes are made of various materials and the choice is determined by the physical conditions, the electric field pattern, current densities, cost and anode corrosion. Anode current densities vary between 10 amperes per metre squared for silicon iron to more than 1000 amperes per metre squared for platinised and lead alloys. 

Coke-asphalt anode system used high silicon iron anode and required wear surface. The estimated cost for this anode system is 92/m with service life of 20 years (29). 

The high-alloy silicon irons are used in draining pipelines, pumps, valves, other process equipment, and anodes for cathodic protection with 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. 

The first anodes, developed by Stratfull, were silicon iron primary anodes in contact with a conductive coke breeze asphalt overlay (the secondary anode). This design was based upon CP designs for pipelines where a silicon iron anode is embedded in a carbon coke breeze backfill to give a large contact area and low resistance. The anode is then linked, via the transformer rectifier, to the pipeline to be protected. A modified form of the conductive... 

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