Cathodic protection anode resistance

It is somewhat less corrosion resistant than tantalum, and like tantalum suffers from hydrogen embrittlement if it is made cathodic by a galvanic couple or an external e.m.f., or is exposed to hot hydrogen gas. The metal anodises in acid electrolytes to form an anodic oxide film which has a high dielectric constant, and a high anodic breakdown potential. This latter property coupled with good electrical conductivity has led to the use of niobium as a substrate for platinum-group metals in impressed-current cathodic-protection anodes. 

Cathodic Protection Circuit Resistance The resistance of a vertical anode to ground can be calculated from the following equation based on the dimensions of the anode package [48]... 

The basic design of sacrificial CP system includes calculation of cathodic protection circuit resistance, potential difference between the anode and structure, anode output, number of anodes, and the anode life expectancy. A schematic of the cathodic protection test is given in Fig. 15.11. To estimate current requirements, a test is needed to determine the current i ) necessary to provide adequate protection for the pipeline. This can be done by applying current using a temporary test setup and adjusting the current from the rectifier until the cathodic protection criteria is reached. 

The cathodic protection circuit resistance includes the anode to soil resistance, resistance of the wire/cable, and the structure to soil resistance. 

The system was straightforward. One of the popular impressed current pipeline cathodic protection anodes of that time was made of a corrosion resistant silicon iron, surrounded by a carbon cokebreeze backfill. A well was dug near the pipeline, the anode put in surrounded by the backfill and the system connected to a DC power supply, with the negative terminal connected to the pipeline to make a cathodic protection system. Richard Stratfull look pancake silicon iron anodes, fixed them on a bridge deck and applied a carbon cokebreeze asphalt overlay (Stratfull, 1974). The systems installed in 1973 and 1974 were reviewed in 1989 and were still working (Broomfield and Tinnea, 1992),... 

I n addition to the seal applications mentioned above, molded graphite has many applications in areas where chemical resistance is the major factor. Such applications are found in chemical reactors, heat exchangers, steam jets, chemical-vapor deposition equipment, and cathodic-protection anodes for pipelines, oil rigs, DC-power lines, and highway and building construction. 

A unique condition encountered on land that has been built up from coral deposits is the presence of blowholes, fissures and caves, which augments the penetration of seawater to areas remote from the actual seashore. Knowing that seawater makes for a very extensive, uniform, low resistivity "ground bed" for cathodic protection anodes, the above condition facilitates the design of unique cathodic protection systems. 

Titanium has potential use in desalination plants for converting sea water into fresh water. The metal has excellent resistance to sea water and is used for propeller shafts, rigging, and other parts of ships exposed to salt water. A titanium anode coated with platinum has been used to provide cathodic protection from corrosion by salt water. 

Silver reduces the oxygen evolution potential at the anode, which reduces the rate of corrosion and decreases lead contamination of the cathode. Lead—antimony—silver alloy anodes are used for the production of thin copper foil for use in electronics. Lead—silver (2 wt %), lead—silver (1 wt %)—tin (1 wt %), and lead—antimony (6 wt %)—silver (1—2 wt %) alloys ate used as anodes in cathodic protection of steel pipes and stmctures in fresh, brackish, or seawater. The lead dioxide layer is not only conductive, but also resists decomposition in chloride environments. Silver-free alloys rapidly become passivated and scale badly in seawater. Silver is also added to the positive grids of lead—acid batteries in small amounts (0.005—0.05 wt %) to reduce the rate of corrosion. 

This value can be considerably smaller. It corresponds in Fig. 6-1 to the ordinate of the intersection of the resistance graph of slope cCq with a 7(f/-r) curve that deviates markedly to the left of that plotted. The maximum current density is an important quantity for the setting up of cathodic protection with galvanic anodes and is dependent on the anode geometry and conductivity of the medium. 

Current control can be more advantageous where rail/soil potentials are predominantly positive. Current control is also preferred in the cathodic protection of steel-water construction if the anode resistance fluctuates due to changes in electrical conductivity. 

Deep anodes are installed where the resistivity is high in the upper layers of soil and decreases with increasing depth. This type of installation is recommended for densely populated areas and for local cathodic protection (see Chapter 12) on account of the small space needed and the smaller voltage cone, which avoids interference with foreign structures. 

At the relatively low protection current density of 200 llA m and with the anode positioned on one side, it is to be expected that with this storage tank sufficient reduction in potential would be achieved on the other side of the tank from the anode. The off potential was measured using a measurement point at a depth of about 2 m as f/cu-cuso4 = -0.88 V at the tank. At the other side of the tank as well as above it, off potentials of-0.90 to -0.94 V were found. These potentials were measured with a protection current of 10 mA (anode 1 6 mA, anode 2 4 mA) with an additional resistance of 8 Q in the protection current circuit (see Fig. 11-2). With a direct connection between the tank and the group of magnesium anodes, the initial current was about 16 mA, which after 1 h of polarization decreased to about 14 mA. The reserve current, based on a long-term current of 10 mA, amounted to ca. 40% in the operation of the cathodic protection installation. 

A proposal for a draft standard on the requirements for the internal cathodic protection of fuel tanks has been put forward by a working party entitled Internal cathodic protection of fuel tanks [18]. This contains the following information an electrolyte is produced by dissolving sodium bicarbonate in drinking water with a resistivity not greater than 2000 Q cm. The solution should completely cover the anodes in the tank. 

Fig. 12-6 Local cathodic protection of a tank farm in high-resistance soil using the anodic voltage cones of distributed anodes the lines indicate soil potential values for an increase of 0.5 V relative to a remote ground numerical pairs volts.

Cathodic protection, complete or partial (stem and bow), is arranged by the distribution of the anodes so that the desired current distribution is maintained correctly in the relevant areas. Galvanic anodes, depending on their dimensions and current output, deliver a certain maximum current which depends on the conductivity. The calculated maximum current from Eq. (6-12) based on the driving voltage and grounding resistance is reduced in practice on working anodes due to film for-... 

Ships with nonmetallic hulls frequently have metallic attachments which can be cathodically protected. Here the anodes are screwed onto the timber or plastic hull and electrically connected with low resistance via the interior of the ship to the objects to be protected. The metallic foundation serves for flotation and copper bands. 

Since docks are usually situated in brackish water, the anodes must have large surface areas to keep the grounding resistance and the driving voltage low. Cathodic protection of the interior is not necessary because the dock is only flooded for a short time and can be otherwise maintained. 

Cathodic protection of enamelled tanks with Mg anodes has long been the state of the art, with potential-controlled equipment being used with increasing frequency in recent years. A high-resistance coating with limited defects according to Ref. 4 enables uniform current distribution to be maintained over the whole tank. 

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