High-silicon iron anodes

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. 

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). 

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

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

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). 

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 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. 

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... 

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. 

Anode Selection High silicon, chromium bearing cast iron and graphite have similar characteristics. Dissipation rate varies with the environment, but... 

Design of cathodic protection for marine structures in both fresh and salt water require special techniques. Galvanic systems usually employ zinc or aluminum alloy anodes. Impressed current systems frequently use high silicon, chromium bearing iron, platinized niobium, or mixed-metal oxide/titanium anodes. The structure being protected affects the design. Stationary facihties such as bulkheads and support piles require different techniques from ship hulls [55]. 

In this case of a cathodic protection system, the weight and number of anodes to be used is calculated from the cycle life of the system. Prolonged life of the anode is essential to decrease maintenance cost of the impressed current cathodic protection system. Dissipation rate of the anode is a measure of the life of the anode. For high silicon, chromium bearing cast iron, the dissipation rate is 0.5 kg/A year. The weight of the anode is given by... 

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

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