Corrosion resistant amorphous noble metal-base alloys

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to amorphous alloys which possess excellent characteristics for electrode materials in electrolysis of aqueous solutions of alkali halides.
2. Description of the Prior Arts
It has been known to use electrodes made of corrosion resistant metals such as titanium coated with noble metals. However, when such electrodes are used as an anode in the electrolysis of aqueous solutions of sodium chloride, coated noble metals are severely corroded and sometimes peeled off from the titanium substrate. It is, therefore, difficult to use these electrodes for industrial processes.
On the other hand, modern chlor-alkali industries are using composite oxide electrodes consisting of corrosion resistant metals as a substrate on which composite oxides such as ruthenium oxide and titanium oxide are coated. When these electrodes are used as an anode in the electrolysis of sodium chloride solutions, they possess the following disadvantages; the composite oxides are sometimes peeled off from the metal substrate and chlorine gas produced are contaminated by a relatively large amount of oxygen. In addition, the corrosion resistance of the electrodes is not sufficiently high, particularly at low pH.
In general, ordinary alloys are crystalline in the solid state. However, rapid quenching of some alloys with specific compositions from the liquid state gives rise to solidification in the amorphous structure. These alloys are called amorphous alloys.
The amorphous alloys have significantly high mechanical strength in comparison with the conventional industrial alloys. Some amorphous alloys with specific compositions have extremely high corrosion resistance which cannot be obtained in ordinary crystalline alloys.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide amorphous noble metal alloys which have extremely high corrosion resistance as well as high mechanical strength.
It is another object of the present invention to provide amorphous noble metal alloys which can be used as corrosion resistant electrodes for electrolysis without any trouble of peeling.
It is the other object of the present invention to provide corrosion resistant and energy saving amorphous noble metal electrode materials with a long life, by which electrolysis of aqueous alkali halide solutions at lower potentials actively generate halogen gases with a low oxygen contaminant.
The foregoing and other objects of the present invention have been attained by preparation of amorphous alloys by rapid quenching from the liquid state. The alloys consist of (1) 10-40 atomic percent P and/or Si and (2) 90-60 atomic percent of two or more Pd, Rh and Pt or (2') 90-60 atomic percent of two or more of Pd, Rh and Pt and 25 atomic percent or less Ti, Zr, Nb and/or Ta; (2") 90-60 atomic percent Pd, Rh and/or Pt and 80 atomic percent or less Ir and/or Ru; (2'") 90-60 atomic percent Pd, Rh and/or Pt, 80 atomic percent or less Ir and/or Ru and 25 atomic percent or less Ti, Zr, Nb and/or Ta.

BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a schematic view of one embodiment of an apparatus for preparing amorphous alloys of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The amorphous alloys prepared by rapid quenching of molten alloys with compositions mentioned above are single phase alloys in which the elements are uniformly distributed. On the contrary, ordinary crystalline alloys have many lattice defects which act as active surface sites with respect to corrosion. Therefore, crystalline metals, alloys or even noble metals cannot possess high corrosion resistance in very aggressive environments such as the environment to which an anode is exposed during electrolysis of sodium chloride solutions.
Electrodes which have been used for this purpose are composite oxide electrodes, that is, oxide mixture of noble metals and corrosion resistant metals such as ruthenium oxide-titanium oxide coated on corrosion resistant metals such as titanium in a thickness of several .mu.m.
On the other hand, amorphous alloys are characterized by the high reactivity unless a stable surface film is formed. The high reactivity provides the rapid formation of protective surface film. In addition, the chemically homogeneous single phase nature of amorphous alloys provides the formation of uniform surface film without weak points with respect to corrosion. Accordingly, when the amorphous alloys of the present invention are used as electrodes, the alloys are immediately covered by a uniform protective passive film of 1-5 nm thickness and show extremely high corrosion resistance.
The passive film consists mainly of hydrated noble metal oxyhydroxide whereby the alloys possess excellent catalytic activity for electrochemical reactions such as evolution of halogen gases. Consequently, the amorphous alloys of the present invention have extremely high corrosion resistance and excellent characteristics for gas evolution as energy saving electrodes with a long life.
The preparation method of amorphous alloys of the present invention is as follows:
The amorphous alloys with compositions mentioned above can be prepared by rapid quenching from the liquid state at cooling rate of higher than 10,000.degree. C./sec. If the cooling rate is slower than 10,000.degree. C./sec., it is difficult to form a completely amorphous alloys. As a principle, the amorphous alloys of the present invention can be produced by any apparatus as far as the cooling rate higher than 10,000.degree. C. is attained.
One embodiment of an apparatus for preparing the amorphous alloys of the present invention is shown in FIG. 1. In FIG. 1, a quartz tube (2) has a nozzle (3) at the lower end in the vertical direction, and raw materials (4) and an inert gas for preventing an oxidation of the raw materials are fed from the inlet (1). A heater (5) is placed around the quartz tube (2) so as to heat the raw materials (4). A high speed wheel (7) is placed below the nozzle (3) and is rotated by a motor (6).
The raw materials (4) having the specific composition are melted by the heater (5) in the quartz tube under inert gas atmosphere. The molten alloy is impinged by pressure of the inert gas onto the outer surface of the wheel (7) which is rotated at high speed of 1,000 to 10,000 rpm whereby the amorphous alloys of the present invention are formed as a long thin plate such as the plate having a thickness of 0.1 mm, a width of 10 mm and a length of several meters.
The amorphous alloys of the present invention produced by the above-mentioned procedure usually have a Vickers hardness of about 400 to 600 and a tensile strength of about 120 to 200 kg/mm.sup.2 and have excellent mechanical characteristics as the amorphous alloys such as abilities for complete bending and coil rolling at greater than 50%.
The detail of the amorphous alloys of the present invention will be illustrated.
Energy saving electrodes with a long life should have characteristics of high catalytic activity in electrolytic reactions such as high activity for gas evolution reaction along with high corrosion resistance and high mechanical strength under the electrolytic conditions.
As described above, it is important to have the amorphous structure for the alloys in order to possess extremely high corrosion resistance and excellent mechanical characteristics.
The alloys with the specific compositions defined above can form the amorphous structure and satisfy the purpose of the present invention, that is, excellent electrochemical catalytic activities and extremely high corrosion resistance.
The typical compositions are shown in Table 1.
The amorphous alloys of the present invention have excellent characteristics in comparison with composite oxides such as ruthenium oxide-titanium oxide on a corrosion resistant metal as described in Japanese Patent Publication No. 20440/1977.
For example, when the alloys are used as electrodes for electrolysis of aqueous sodium chloride solutions, the corrosion rates of the amorphous alloys of the present invention are several orders of magnitude lower than those of the conventional composite oxide electrodes. The overvoltage for chlorine evolution of the amorphous alloys of the present inverntion is substantially the same or lower than those of the conventional composite oxide electrodes. Furthermore, the oxygen content of chlorine gas produced on the amorphous alloys of the present invention is one-fifth or less in comparison with that of chlorine gas produced on the conventional composite oxide electrodes.
The amorphous alloys of the present invention also possess high corrosion resistance and high activity for gas evolution in aqueous solutions of the other metal halides such as KCl. Therefore, the amorphous alloys of the present invention have excellent characteristics for energy saving electrode materials with a long life for electrolysis. In particular, the amorphous alloys of the present invention are advantageously used for anodes for production of sodium hyroxide, potassium hydroxide, chlorine gas, bromine gas or chlorate, in a diaphragm or ion exchange membrane process.
The reason of the definitions of the components in the amorphous alloys of the present invention will be illustrated as follows:
Addition of P and/or Si is necessary for forming the amorphous structure and also effective for rapid formation of protective passive film. However, when the total content of P and Si is less than 10 atomic percent or higher than 40 atomic percent, it is difficult to form the amorphous structure. Therefore, the total content of P and Si must be in a range of 10 to 40 atomic percent. In particular, the amorphous structure can be easily obtained when the total content of P and Si is in a range of 16 to 30 atomic percent.
It has been known that addition of B or C is also effective in forming the amorphous structure for iron-, cobalt- or nickel-base alloys. The amorphous noble metal alloys of the present invention, however, become brittle to some extent by the addition of B or C, and hence all of P and/or Si cannot be substituted by B and/or C but substitution of P and/or Si in 7 atomic percent or less by B and/or C is possible since the ductility of the alloys is maintained.
The elements Pd, Rh and/or Pt are main metallic components of the amorphous alloys of the present invention and are effective in forming the amorphous structure and evolving halogen gases. The element Pd or Rh is especially effective in evolving the gases whereas the element Rh or Pt is effective in improving the corrosion resistance of the electrodes. Thus, unless Ir and/or Ru are added, the alloys must contain at least two of Pd, Rh and Pt. When one of Pd, Rh or Pt is the main metallic component of alloys which do not contain Ir and/or Ru, it is preferable that the alloys contain 10 atomic percent or more of the other one or two of Pd, Rh and Pt in order to provide high activity for gas evolution and high corrosion resistance.
The elements Ir and Ru are both effective in increasing the activity for gas evolution and the corrosion resistance. Accordingly, when Ir and/or Ru are added to the alloys, it is not necessary that the alloys contain two or more of Pd, Rh and Pt. It is, however, preferable for the high activity for gas evolution and high corrosion resistance that, when the amorphous alloys contain only one of Pd, Rh or Pt and do not contain Ti, Zr, Nb and/or Ta, the total content of Ir and Ru is more than 20 atomic percent.
On the other hand, Ir or Ru alloys containing P and/or Si hardly form the amorphous structure by rapid quenching from the liquid state, unless Pd, Rh and/or Pt are added to the alloys. It is, therefore, necessary for the formation of amorphous structure that the total content of Ir and Ru is 80 atomic percent or less and the total content of Pd, Rh and Pt is 10 atomic percent or more.
The elements Ti, Zr, Nb and Ta are significantly effective in increasing the corrosion resistance and facilitating the formation of the amorphous structure. However, the addition of Ti, Zr, Nb and Ta in a large amount lowers the activity for gas evolution. Therefore, when Ti, Zr, Nb and/or Ta are added, the total content of these elements in the amorphous alloys muut be 25 atomic percent or less.
In addition, when the amorphous alloys contain only Pd or Rh among Pd, Rh and Pt and do not contain Ir and/or Ru, it is preferable for the high corrosion resistance that the total content of one or more of Ti, Zr, Nb and Ta is 1 atomic percent or more. On the other hand, when alloys contain only Pt among Pd, Rh and Pt, it is preferable for the high activity for gas evolution that the total content of Ir and Ru is 2 atomic percent or more.
As described above, the alloys of the present invention are the amorphous alloys having the specific compositions consisting of elements selected from the elements for improving the activity for gas evolution such as Pd, Rh, Ir or Ru and the elements for improving the corrosion resistance such as Rh, Pt, Ir, Ru, Ti, Zr, Nb or Ta.
Consequently, these alloys possess both the high activity for gas evolution and high corrosion resistance and hence can be used as energy saving electrode materials with a long life for electrolysis of aqueous solutions of alkali halides.
The purpose of the present investigation can be also attained by addition of a small amount (about 2 atomic percent) of other elements such as V, Cr, Mo, W, Fe, Co, Ni, Cu, Ag, and Au.
The amorphous alloys of the present invention will be further illustrated by certain examples which are provided only for purpose of illustration and are not intended to be limiting the present invention.
EXAMPLE 1
Amorphous alloys whose compositions are shown in Table 1 were prepared by rapid quenching from the liquid state by using the apparatus shown in FIG. 1. The amorphous alloy sheets prepared were 0.02-0.05 mm thick, 1-3 mm wide and 10 m long. Specimens cut from the amorphous alloy sheets were used as anodes in electrolysis of stagnant aqueous 4 M NaCl solution at 80.degree. C. and pH 4.
Corrosion rates of amorphous alloys were obtained from the weight loss of specimens after electrolysis for 10 days at a constant current density of 50 A/dm.sup.2. The solution was renewed every 12 hours during electrolysis.
Table 2 shows corrosion rates and potentials of specimens measured during chlorine evolution at a current density of 50 A/dm.sup.2. Potentials shown in Table 1 are relative to the saturated calomel electrode.
The corrosion resistance of almost all the amorphous alloys of the present invention is several orders of magnitude higher than those of the composite oxide electrodes used in modern chlor-alkali industries. In particular, all the amorphous alloys which show the corrosion rate lower than 1 .mu.m/year in Table 2 passivate spontaneously in the hot concentrated sodium chloride solution and can be used as anodes for several tens of years for electrolysis of the sodium chloride solutions.
On the other hand, the oxide electrode consisting of ruthenium oxide on titanium has higher activity for chlorine gas evolution than the composite oxide electrodes which are used in modern chlor-alkali industries, although ruthenium oxide on titanium has lower corrosion resistance than that of the composite oxide electrodes. The overvoltage of the ruthenium oxide electrode on titanium for chlorine evolution measured galvanostatically at 50 A/dm.sup.2 was about 1.095 V (SCE), and the current used for the evolution of oxygen which is contaminant of chlorine gas is 18% of total current passed on the ruthenium oxide electrode on titanium under the present experimental conditions.
In contrast, the current used for oxygen evolution on the amorphous alloys of the present invention is less than 0.4% of the total current passed under the present experimental conditions.
Furthermore, when the amount of chlorine gas produced potentiostatically at 1.10 V(SCE) on the amorphous alloys of the present invention is compared with the amount of chlorine gas produced on the ruthenium oxide electrode on titanium under the same conditions, the amount of chlorine is 1.5 times on the specimen No. 61, 1.3 times on the specimens No. 46, 60, 62, 66, 67 and 71, and 1.2 times on the specimens No. 26, 36, 40, 48, 50, 53 and 62. The oxygen content of chlorine gas produced on these amorphous alloys is less than 0.05%.
Consequently, the amorphous alloys of the present invention can be used as energy saving electrodes with a long life for electrolysis of alkali halide solutions to produce high purity halogen gases.
EXAMPLE 2
Electrolysis was carried out by using the amorphous alloys as anodes in 4 M NaCl solution at pH 2 and 80.degree. C. (this is further severe corrosive environment comparing to Example 1).
The results of the overvoltages for chlorine evolution and the corrosion rates are shown in Table 3.
The corrosion rates are higher than those measured in 4 M NaCl solution at pH 4 shown in Table 2. However, they are remarkably lower than the corrosion rates of the composite oxide electrodes. The high corrosion resistance and the low overvoltages for chlorine evolution clearly reveal that the amorphous alloys of the present invention have excellent characteristics as the anode for electrolysis of alkali halide solutions.
EXAMPLE 3
Electrolysis was carried out by using the amorphous alloys as anodes in the saturated KCl solution at 80.degree. C.
For example, the corrosion rates of the specimens No. 35, 37, 46 and 61 are 2.50, 2.14, 3.45 and 2.90 .mu.m/year, and hence they possess high corrosion resistance.
TABLE 1______________________________________Compositions of Amorphous Alloys of the Invention(atomic percent)Speci-men No. Pd Rh Pt Ru Ir Ti Zr Nb Ta P Si______________________________________1 71 10 192 61 20 193 55 25 204 56 25 195 51 30 196 10 70 207 20 60 208 20 60 209 30 50 11 910 61 20 10 911 56 25 10 912 42 25 10 2313 53 25 2 2014 51 25 5 1915 46 25 10 1916 36 25 20 1917 30 41 10 1918 54 25 2 1919 51 25 5 1920 41 30 10 1921 54 20 2 2422 56 20 5 1923 51 20 10 1924 49 20 16 1525 55 25 1 1926 54 25 2 1927 51 25 5 1928 46 25 10 1929 41 25 15 1930 46 30 5 1931 30 46 5 1932 46 30 5 1933 51 25 5 1934 25 51 5 1935 46 25 5 5 1936 46 25 5 5 1937 46 25 5 5 1938 46 25 5 5 1939 45 25 5 5 10 1040 46 25 5 5 1941 56 10 5 5 5 1942 51 5 15 10 1943 51 10 10 5 5 1944 31 10 40 1945 25 5 50 2046 41 40 1947 31 50 1948 46 5 30 1949 46 5 30 1950 41 10 30 1951 30 20 30 2052 41 10 30 1953 36 10 10 25 1954 20 20 20 20 2055 15 30 35 2056 39 10 30 2157 21 10 50 1958 46 34 2059 10 10 60 2060 41 35 5 1961 47 30 5 1862 41 30 10 1963 41 25 15 1964 36 40 5 1965 41 30 10 1966 44 5 28 5 1867 45 10 25 2 1868 39 10 20 15 1669 10 10 20 35 5 2070 15 30 30 5 2071 41 35 5 10 972 41 35 5 10 973 41 35 5 10 974 40 30 10 10 1075 30 10 25 5 15 1576 25 10 25 10 12 18______________________________________
TABLE 2______________________________________Corrosion Rates and Overvoltages for Chlorine Evolu-tion of Amorphous Alloys of the Present InventionMeasured by Galvanostatic Polarization at 50 A/dm.sup.2in 4 M NaCl Solution at pH 4 and 80.degree. C. Overvoltage forSpecimen Corrosion rates chlorine evolutionNo. (.mu.m/year) V(SCE)______________________________________ 4 18.50 1.11 5 4.87 1.1119 15.31 1.1026 11.36 1.0927 5.19 1.1028 4.22 1.1429 2.01 1.1730 1.23 1.1035 0.00 1.1236 2.17 1.0937 0.00 1.1038 1.91 1.1439 2.21 1.1240 1.91 1.1241 1.01 1.1142 2.03 1.1143 1.07 1.1044 7.01 1.0945 10.24 1.1246 1.45 1.0847 0.81 1.1148 5.27 1.0949 3.02 1.1150 0.25 1.0951 0.34 1.1152 0.57 1.1353 0.12 1.0954 0.57 1.1354 0.03 1.1455 11.45 1.1556 5.68 1.1257 2.45 1.1658 0.00 1.1959 0.04 1.1760 0.06 1.0961 0.29 1.0862 0.02 1.0963 0.00 1.1264 5.46 1.1465 1.75 1.1266 0.03 1.0967 0.01 1.0868 6.00 1.1269 0.00 1.1470 1.27 1.1571 1.18 1.0972 1.03 1.1073 2.11 1.1374 15.29 1.1175 0.04 1.1376 0.00 1.15______________________________________
TABLE 3______________________________________Corrosion Rates and Overvoltages for ChlorineEvolution of Amorphous Alloys for the Present InventionMeasured by Galvanostatic Polarization at 50 A/dm.sup.2in 4 M NaCl Solution at pH 2 and 80.degree. C. Overvoltage forSpecimen Corrosion rates chlorine evolutionNo. (.mu.m/year) V(SCE)______________________________________30 16.23 1.1035 11.68 1.1136 39.02 1.0937 71.39 1.1046 7.85 1.0848 32.49 1.0960 17.65 1.0961 45.27 1.0862 3.21 1.0967 8.45 1.08______________________________________

Corrosion resistant amorphous noble metal-base alloys

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