Cathodic Protection

It is well known that a metallic vessel such as an iron vessel containing water having electrolytes dissolved therein as, for example, seawater, corrodes gradually, and in order to prevent the corrosion of such a vessel, an electric means is used, which is called "electric prevention for corrosion". The electric prevention for corrosion is also called "cathodic protection". In such electric prevention, a metallic vessel is used as a cathode and carbon, magnetite or a platinum-coated-titanium electrode is used as an anode, and a D.C. voltage is applied to the electrodes. In this case, an electrode which is used as an anode is required to have good corrosion resistance. In addition to this, in order to prevent the corrosion of "iron vessel for heating water", or "iron water heater" by the cathodic protection method, an electrode which is used as an anode is required to have good thermal shock resistance as well as good corrosion resistance because heating and cooling are repeated in the water heater.

It has been ground that the carbon electrode is characterized by poor corrosion resistance, and the magnetite electrode is characterized by poor corrosion resistance and thermal shock resistance. The platinum-coated-titanium electrode is excellent in electric conductivity and thermal shock resistance but it has low resistance for ripple current.

It is an object of the present invention to provide an electrode which is excellent in thermal shock resistance as well as the corrosion resistance and further, has a lower resistivity.

The present invention relates to an electrode for use in electric prevention for corrosion and to a process of electric prevention for corrosion by using said electrode.

The electrode of the present invention comprises a sintered body of a mixture of iron oxide of 90 to 55 mol % in terms of Fe.sub.2 O.sub.3 and a metal oxide of 10 to 45 mol % in terms of MO (M is at least one metal selected from the group consisting of Mn, Ni, Co, Cu and Zn). The sintered body is a solid solution of MFe.sub.2 O.sub.4 (wherein M is as defined above) of 15.79 to 87.1 mol % and Fe.sub.3 O.sub.4 of 84.21 to 12.9 mol %. In the sintered body, iron atoms are present in both the "ferric" and "ferrous" forms. Accordingly, the expression "in terms of" means that "all iron oxides present in the sintered body are calculated in the form of Fe.sub.2 O.sub.3 ".

Electrodes of the present invention may be manufactured for example, as follows:

Ferric oxide (Fe.sub.2 O.sub.3) and a metal oxide (MnO, NiO, CoO, CuO, or ZnO), in a ratio of 90 to 55 mol % to 10 to 45 mol % are mixed in a ball mill. The mixture is heated for about 1 to about 15 hours in air, nitrogen, or carbon dioxide, at a temperature of about 700.degree. to about 1000.degree. C. Hydrogen may be present in an amount up to about 10% in nitrogen gas. After cooling, the heated mixture is pulverized to obtain a fine powder. The fine powder is formed into a shaped body, for example, by compression molding or extrusion. The shaped body is heated at a temperature of about 1100.degree. to about 1450.degree. C. in nitrogen or carbon dioxide containing up to about 20% by volume of oxygen, for about 1 to about 4 hours. The heated body is then slowly cooled in nitrogen or carbon dioxide containing up to about 5% by volume of oxygen. The desired sintered body, i.e., an electrode of the present invention is obtained. The electrodes thus produced have relatively low resistivity, good corrosion resistance and thermal shock resistance.

Instead of Fe.sub.2 O.sub.3, metal iron or FeO may be used. Instead of the metal oxide, compounds of the metals which produce the metal oxides on heating may be used, for example, the metal carbonate or oxalate.

The following examples are given by way of illustration only:

As shown in Table 1.1, Fe.sub.2 O.sub.3 and NiO were each weighed out to provide Samples Nos. 1-6 containing various molar percentages of Fe.sub.2 O.sub.3 and NiO.

In each case, Fe.sub.2 O.sub.3 and NiO were mixed in a ball mill for 20 hours. The mixtures were then pre-heated at a temperature of 800.degree. C. for about 3 hours and then cooled. The resulting mixtures were pulverized using a ball mill to obtain powders having a particle size of less than 20 microns. The powders were compression molded at about 1 ton/cm.sup.2 to form shaped bodies 110 mm.times.15 mm.times.15 mm. The shaped bodies were heated for 5 hours at a temperature of 1440.degree. C. in nitrogen gas, and then slowly cooled in nitrogen gas over a period of 10 hours to obtain the desired sintered bodies, i.e., electrodes.

The resistivities of the sintered bodies (Electrodes Nos. 1-6) obtained were measured by the four contact method. The values obtained are shown in Table 1.2. The corrosion resistance of each sintered body (i.e., the anodes) was evaluated by measuring the loss in weight of each anode in the electrolysis of the standard seawater under the following conditions. The values obtained are shown as "corrosion loss" in Table 1.2.

The thermal shock resistance of each sintered body (i.e., the anodes) was evaluated by counting the number of repetitions of "heating" and "cooling" as follows:

The sintered body is dipped in hot water at a temperature of 98.degree. C. for 3 minutes and then dipped in cold water at a temperature of 5.degree. C. for 3 minutes. This "heating and cooling" (referred to as one cycle of heating and cooling) is repeated until the sintered body breaks down. The number of repetitions obtained are shown in Table 1.2.

As seen from the results shown above, Sample 1 has a low resistivity, but it is inferior in both corrosion resistance and thermal shock resistance.

In FIG. 1 of the accompanying drawings, Curves A and B respectively show the variation in resistivity and corrosion loss with variations in the relative molar percentages of Fe.sub.2 O.sub.3 and NiO.

As shown in Table 2.1, Fe.sub.2 O.sub.3, MnO, CoO, CuO, ZnO and NiO were each weighed out to provide Samples Nos. 7-14 containing various molar percentages of Fe.sub.2 O.sub.3, MnO, CoO, CuO, ZnO and NiO.

The sintered bodies (Electrodes Nos. 7-14) were obtained by the same procedure as that of Example 1 except that "pre-heating" and "heating" were carried out under the conditions as shown in Table 2.2.

The resistivities, the value of corrosion resistances and the number of repetitions of "heating and cooling" of the sintered bodies (Electrodes Nos. 7-14) are shown in Table 2.3.

From the results as shown above, it is understood that electrodes for use in cathodic protection having a low resistivity, a high corrosion resistance and a high thermal shock resistance can be obtained from a sintered body of a mixture of iron oxide of 90 to 55 mol % in terms of Fe.sub.2 O.sub.3 and metal oxide of 10 to 45 mol % in terms of MO, wherein M is at least one metal selected from the group consisting of Mn, Ni, Co, Cu and Zn.

Additional tests were carried out as follows:

Electrodes Nos. 2-6 having dimension of 120 mm.times.10 mm were prepared by repeating the procedures as shown in Samples 2-6 of Example 1, and thermal shock tests of Electrodes Nos. 2-6 and corrosion tests of Electrodes No. 2 were carried out as follows:

1. Thermal shock tests:

Electrodes Nos. 2-6 were dipped in an iron vessel provided with an electric heater. Cold water at a temperature of 5.degree. C. was poured into the vessel over a period of 6 minutes and then the cold water was heated by energizing the electric heater for 30 minutes. The temperature of the water reached 98.degree. C. After the energizing was stopped, the hot water was drained off from the vessel over 4 minutes and cold water at a temperature of 5.degree. C. was poured again into the vessel over a period of 6 minutes. The second energizing was begun. The heating and cooling of the electrodes as above was repeated 1050 times by an automatic controller. The breakage of Electrodes Nos. 2-6 could not be detected.

2. Corrosion tests:

Electrode No. 2 was set in a vessel (enamelled ironware) containing hot water. The tests were carried out under the following conditions:

The energizing was continued for 8 hours and 7 days; the concentration of iron ion in the water was 0.007 ppm (for 8 hours) and 0.15 ppm (for 7 days), respectively. These values are less than that of the Standard of the Food Sanitation Law (Fe ion 0.3 ppm). The electrode of this invention can be used as an anode for a water heater (kettle) for drinking purposes.