Impressed-current

Cathodic Protection Systems. Metal anodes using either platinum [7440-06 ] metal or precious metal oxide coatings on titanium, niobium [7440-03-17, or tantalum [7440-25-7] substrates are extensively used for impressed current cathodic protection systems. A prime appHcation is the use of platinum-coated titanium anodes for protection of the hulls of marine vessels. The controUed feature of these systems has created an attractive alternative... 

Metal anodes using platinum and precious metal oxide coatings are also incorporated into a variety of designs of impressed current protection for pipeline and deep weU appHcations, as weU as for protection of condenser water boxes in power generating stations (see Pipelines Power generation). 

Oxygen-Evolving Anode. Research efforts to iacorporate the coated metal anode for oxygen-evolving appHcations such as specialty electrochemical synthesis, electro winning, impressed current, electrodialysis, and metal recovery found only limited appHcations for many years. 

Niobium is used as a substrate for platinum in impressed-current cathodic protection anodes because of its high anodic breakdown potential (100 V in seawater), good mechanical properties, good electrical conductivity, and the formation of an adherent passive oxide film when it is anodized. Other uses for niobium metal are in vacuum tubes, high pressure sodium vapor lamps, and in the manufacture of catalysts. 

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. 

It is little known that Thomas Alva Edison tried to achieve cathodic protection of ships with impressed current in 1890 however, the sources of current and anodic materials available to him were inadequate. In 1902, K. Cohen achieved practical cathodic protection using impressed direct current. The manager of urban works at... 

To protect steam boilers and their tubes from corrosion, E. Cumberland used cathodic impressed current in America in 1905. Figure 1-10 has been taken from the corresponding German patent [35]. In 1924 several locomotives of the Chicago Railroad Company were provided with cathodic protection to prevent boiler corrosion. Where previously the heating tubes of steam boilers had to be renewed every 9 months, the costs fell sharply after the introduction of the electrolytic... 

The first anode installation for the cathodic protection of gas pipelines in New Orleans consisted of a 5-m-long horizontal cast-iron tube. Later old tramway lines were used. Since in downtown New Orleans there was no suitable place to install impressed current anodes and to avoid detrimental effects on other pipelines, Kuhn recommended the use of deep anodes which were first installed in 1952 at a depth... 

In the cathodic protection of storage tanks, potentials should be measured in at least three places, i.e., at each end and at the top of the cover [16]. Widely different polarized areas arise due to the small distance which is normally the case between the impressed current anodes and the tank. Since such tanks are often buried under asphalt, it is recommended that permanent reference electrodes or fixed measuring points (plastic tubes under valve boxes) be installed. These should be located in areas not easily accessible to the cathodic protection current, for example between two tanks or between the tank wall and foundations. Since storage tanks usually have several anodes located near the tank, equalizing currents can flow between the differently loaded anodes on switching off the protection system and thus falsify the potential measurement. In such cases the anodes should be separated. 

Where stray currents are involved, several measurements have to be taken that are continually changing with time, simultaneously with each other. A double recorder is most suitable for this. Linear recorders with direct indication of the measurements cannot be used for potential measurements because the torque of the mechanism is too small to overcome the friction of the pen on the paper. Amplified recorders or potentiometer recorders are used to record potentials. In amplified recorders, as in amplified voltmeters, the measured signal is converted into a load-independent impressed current and transmitted to the measuring mechanism, which consists of a torque motor with a preamplifier. The amplifier results in an... 

Heterogeneous surface areas consist of anodic regions at corrosion cells (see Section 2.2.4.2) and objects to be protected which have damaged coating. Local concentrations of the current density develop in the area of a defect and can be determined by measurements of field strength. These occur at the anode in a corrosion cell in the case of free corrosion or at a holiday in a coated object in the case of impressed current polarization (e.g., cathodic protection). Such methods are of general interest in ascertaining the corrosion behavior of metallic construction units... 

The impressed current enters the pipe surface at a defect of radius r. The defect can be considered as a point source at a sufficiently large distance compared to the diameter of the pipe. The potential on the surface of the soil is from Table 24-1, line 5 ... 

Galvanic anodes must not be backfilled with coke as with impressed current anodes. A strong corrosion cell would arise from the potential difference between the anode and the coke, which would lead to rapid destruction of the anode. In addition, the driving voltage would immediately collapse and finally the protected object would be seriously damaged by corrosion through the formation of a cell between it and the coke. 

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. 

Data on impressed current anodes are assembled in Tables 7-1 to 7-3. 

Table 7-1 Data on impressed current anodes for use in soil...

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.

Table 7-2 Composition and properties of solid impressed current anodes (without 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). 

Impressed current anodes must be insulated from the surface that is being protected. Also, the current connections must be well insulated to prevent the free ends of the cable from being attacked and destroyed. 

The current needed for cathodic protection by impressed current is supplied from rectifier units. In Germany, the public electricity supply grid is so extensive that the CP transformer-rectifier (T-R) can be connected to it in most cases. Solar cells, thermogenerators or, for low protection currents, batteries, are only used as a source of current in exceptional cases (e.g., in sparsely populated areas) where there is no public electricity supply. Figure 8-1 shows the construction of a cathodic impressed current protection station for a pipeline. Housing, design and circuitry of the rectifier are described in this chapter. Chapter 7 gives information on impressed current anodes. 

Fig. 8-1 Construction of an impressed current cathodic protection station.

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