Magnesium, anodized

Electroreductive coupling of ben2otrifluorides with sacrificial aluminum or magnesium anodes in the presence of acetone, carbon dioxide, or /V, /V-dimethylformamide provides a novel route to ArCF2-derivatives (310). 

In order to pilot this process, SNPE has developed an annular ceU having a cone-shaped electrode section. The magnesium anode is placed into the conic section and fed like a pencil into a sharpener as it is consumed during electrolysis. The carbon dioxide is introduced into the electrolyte externally to the cell. 

The factor a varies considerably with individual alloys and is also dependent on the application conditions. It varies between wide limits from about 0.98 for zinc to below 0.5 for magnesium anodes. The a values given in Tables 6-1 to 6-4 apply to cold seawater. Deviations in application (i.e., cooling water) temperature and loading can lead to considerable changes. 

Magnesium anodes usually consist of alloys with additions of Al, Zn and Mn. The content of Ni, Fe and Cu must be kept very low because they favor selfcorrosion. Ni contents of >0.001% impair properties and should not be exceeded. The influence of Cu is not clear. Cu certainly increases self-corrosion but amounts up to 0.05% are not detrimental if the Mn content is over 0.3%. Amounts of Fe up to about 0.01% do not influence self-corrosion if the Mn content is above 0.3%. With additions of Mn, Fe is precipitated from the melt which on solidification is rendered harmless by the formation of Fe crystals with a coating of manganese. The addition of zinc renders the corrosive attack uniform. In addition, the sensitivity to other impurities is depressed. The most important magnesium alloy for galvanic anodes is AZ63, which corresponds to the claims in Ref. 22. Alloys AZ31 and M2 are still used. The most important properties of these alloys are... 

Figure 6-7 shows the effect of water conductivity on the rest potential of type AZ63, and M2 as well as of zinc and aluminum [23]. In cold waters with chlorides or sulfates, the polarization of magnesium anodes is low — even in the case of high current densities. This is demonstrated for 70°C with current-density vs. 

In the application of magnesium anodes for enamelled boilers, the consumption rate of the anodes is determined less by current supply than by self-corrosion. The calculation of life from data on protection current requirement, /, and anode mass, m, is difficult because the a value is so low. 

On the basis of available practical experience, the consumption rate of magnesium anodes in enamelled boilers is lower than 3 mm a". For a rod anode with a diameter of 33 mm, this corresponds to a life of over 5 years. As a guideline for the required anode mass, 200 to 250 g per m of internal surface is recommended [27]. 

Hydrogen is involved in cathodic protection with magnesium anodes on account of the high contribution of self-corrosion. This must be considered in its use in closed containers, e.g., boilers. In enamelled boilers there is no danger from deflagration of the oxy-hydrogen gas under normal service conditions [2] however safety requirements must be observed [28,29], particularly with routine maintenance work. 

In enamelled boilers, magnesium anodes for the most part are insulated and connected with the container by a ground lead (see Fig. 6-16). This method of... 

Anodes for boilers can be tested by such methods. Good-quality magnesium anodes have a mass loss rate per unit area < 30 g m d", corresponding to a current yield of >18% under galvanostatic anode loading of 50 /xA cm" in 10 M NaCl at 60°C. In 10 M NaCl at 60°C, the potential should not be more positive than t/jj = -0.9 V for the same polarization conditions [27],... 

Cathodic protection with magnesium anodes can be just as economical as impressed current anode assemblies for pipelines only a few kilometers in length and with protection current densities below 10 xA m" e.g., in isolated stretches of new pipeline in old networks and steel distribution or service pipes. In this case, several anodes would be connected to the pipeline in a group at test points. The distance from the pipeline is about 1 to 3 m. The measurement of the off potential... 

If the distribution network consists of pipes with screwed rubber sockets and only the service pipes are of steel, these can be protected with zinc or magnesium anodes. This requires an insulating service clip near the insulating connection. It is usually sufficient just to insulate the stirrup of the service clip since the rubber seal of the service pipe acts as an insulator. In the United Kingdom, the protection of numerous service pipes has been proved with magnesium anodes [31]. 

The grounding resistance of different types of anodes can be calculated from the equations in Section 24.1 (see Table 24-1). The use of magnesium anodes is convenient and economical for relatively small protection currents. In the case of an increase in the protection current demand, because the voltage is fixed at about 0.6 V, the current can only be raised by lowering the grounding resistance of the anodes, i.e., by installing more anodes. Alternatively, the voltage can also be increased by an impressed current system. 

Magnesium anodes were chosen as the source of the protection current in this old example, because on one hand a sufficient current, including current reserve, could be achieved due to the relatively low soil resistivity, and on the other hand, use of an impressed current protection system would have required much greater expenditure. 

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. 

The life of the magnesium anodes with a current content of about 1.2 A a for 10 mA according to Table 6-4 was calculated from Eq. (6-9) as 120 years. This assumes that the protection current is equally distributed over both anodes. The calculated life would certainly not be reached because uniform anode current distribution cannot be achieved over a long period of time. It would, however, be substantially longer than the minimum required life of 25 years. For this length... 

Magnesium anodes must not be used in tanks. Aluminum anodes may be installed in all tanks according to the agreements of the International Association of Classification Societies, which are included in the individual regulations [5,6], but in tanks (b) in the event of the anode falling off, the kinetic energy must not exceed 275 J, i.e., a 10 kg anode must not be fixed more than 2.8 m above the bottom of the tank. There are no restrictions on the use of zinc anodes. The restrictions on the use of aluminum anodes are due to the possible danger of sparks if the anode falls off. 

The range of applications for magnesium anodes includes the internal protection of boilers, feedwater tanks, filter tanks, coolers, pipe heat exchangers and condensers. They are mainly used in conjunction with coatings and where impressed current equipment is too expensive or cannot be installed. 

Magnesium anodes are frequently used as an additional protection measure at a later stage for stainless steel tanks. In this case the anodes are connected through a 5- to 10-Q resistor to the tank to avoid an unnecessarily high current for the cathodic protection of the tank and simultaneous high consumption of the anodes. 

Magnesium anodes are widely used in conjunction with enamel coatings. This type of corrosion protection is particularly economical and convenient in small-and medium-sized boilers. The anode only has to ensure protection of small de-... 

Magnesium anodes are usually built into the object to be protected through isolating sockets or holes (see Fig. 20-6) and joined to them with cables. They must be readily accessible and easily exchangeable for convenient control [4]. The directions for use have to indicate the necessity for an inspection after 2 years of operation. During operation, control can be exercised by electrical measurements (current, resistance). In addition, acoustic and optical methods exist to determine the amount of anode consumption [4]. The life of the anodes is usually more than 5 years (see Section 6.6). 

Cathodic protection with impressed current, aluminum or magnesium anodes does not lead to any promotion of germs in the water. There is also no multiplication of bacteria and fungi in the anode slime [32,33]. Unhygienic contamination of the water only arises if anaerobic conditions develop in the slurry deposits, giving rise to bacterial reduction of sulfate. If this is the case, HjS can be detected by smell in amounts which cannot be detected analytically or by taste. Remedial measures are dealt with in Section 20.4.2. 

Table 22-1 Installation costs of a cathodic protection station with three magnesium anodes for = 100 mA, r = 30 W m and service life of 36 years.

Fig. 22-1 Economic application range for cathodic protection with magnesium anodes or with impressed current.

The required number of anodes n is given by the protection current requirement of the object and the current output of a single magnesium anode... 

The decision on whether cathodic protection with impressed current or with magnesium anodes is more economical depends on the protection current requirement and the soil resistivity. This estimate only indicates the basic influence of the different variables. In the individual case, installation costs can vary widely so that a specific cost calculation is necessary for every project. 

Since the K radiation emitted from a magnesium anode consists of the intense Kbinding energies by 8.0 and 10.2 eV [20]. The satellites excited by the Kas and K<,6 lines are usually too weak to be observed. Similar features can be observed in XPS spectra excited using Ko, X-rays from aluminum. 

Cathodic protection (CP) is an electrochemical technique of corrosion control in which the potential of a metal surface is moved in a cathodic direction to reduce the thermodynamic tendency for corrosion. CP requires that the item to be protected be in contact with an electrolyte. Only those parts of the item that are electrically coupled to the anode and to which the CP current can flow are protected. Thus, the inside of a buried pipe is not capable of cathodic protection unless a suitable anode is placed inside the pipe. The electrolyte through which the CP current flows is usually seawater or soil. Fresh waters generally have inadequate conductivity (but the interiors of galvanized hot water tanks are sometimes protected by a sacrificial magnesium anode) and the conductivity... 

Fisher, A. O., Magnesium Anodes Control Pitting in an Inhibited Circulating Cooling Water System , Mater. Protect., 3, 64 (1964)... 

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