FIELD OF INVENTION

The present invention relates to new catalytic alloys based on Fe, Al, Ta and catalytic species such as Ru.

The present invention also relates to the use of such catalytic alloys as electrode material for the synthesis of sodium chlorate.

TECHNOLOGICAL BACKGROUND

Nanocrystalline alloys of the formula

Fe _3-x Ali _+!C M _y T _z

wherein

M represents at least one catalytic specie selected from the group consisting of Ru, Ir, Pd, Pt, Rh, Os, Re and Ag;

T represents at least one element selected from the group consisting of Mo, Co, Cr, V, Cu, Zn, Nb, W, Zr, Y, Mn, Cd, Si, B, C, O, N, P, F, S, Ci and Na;

x is a number higher than -1 and smaller than or equal to +1

y is a number higher than 0 and smaller than or equal to +1

z is a number ranging between 0 and +1 have been disclosed recently as efficient cathodic materials for the synthesis of sodium chlorate (see CA 2,687,129 and the corresponding international application WO 2008/138148). These catalytic materials, when use as cathode for the electrosynthesis of sodium chlorate show very low cathodic overpotentials and they do not absorb hydrogen when the H ₂ evolution reaction takes place on their surfaces. These materials also show good corrosion resistance in the sodium chlorate electrolyte under typical industrial operating conditions (NaC10 ₃ : 550g/l; NaCl: 1 10 g/1; NaCr ₂ 0 ₇ : 3g/l; NaCIO: 1 g/1; pH= 6.5 and temperatures around 70°C).

Although corrosion resistance of these Fe3 _-x Ali _+x M _y T _z alloys is quite good at pH 6.5, it is not the case in acidic conditions. Standard industrial practises use acid wash from time to time to clean electrochemical cells and electrodes. To do so, HC1 solutions at concentrations varying between 3% (~ 1M) and 9% (~ 3M) are often used. When the above mentioned alloys are put in contact with such concentrated acidic solutions, they can be severely damaged. Indeed, the corrosion resistance of these new catalytic alloys in HC1 solutions at low pH is not so good.

SUMMARY OF THE INVENTION

To solve this problem, the inventors of record have search new formulations and have discover surprisingly that the addition of a small amount of Ta to these materials could make these new alloys not only highly resistant to corrosion in chlorate electrolyte but also in acidic (HCl) solutions without loosing any performance regarding the electrochemical synthesis of sodium chlorate.

Fe ₃ Al(Ru) alloys are often single phase solid solutions usually prepared in a nanocrystalline form by mecanosynthesis. A powder mixture of ruthenium and iron aluminide is milled intensively for several hours until the Ru catalytic element enters and gets highly dispersed into the cubic crystalline structure of iron aluminide (Fe ₃ Al). The nanocrystalline Fe ₃ Al(Ru) alloy thus formed is highly active thanks to its high surface area and highly dispersed electrocatalytic element.

The present inventors have actually found by investigating various ternary phase diagrams that Ta (tantalum) which is known to be a good corrosion resistant element, is quite soluble in Fe-Al alloys. Therefore, Fe3Al(Ru)Ta _t with various Ta concentration "t" can be prepared as a single phase material by mechanosynthesis very easily and these new alloys show not only good electrocatalytic activity towards the electrosynthesis of sodium chlorate but also good corrosion resistance in the sodium chlorate electrolyte as well as in concentrated HC1 solutions.

Therefore, the first object of the present invention is an alloy characterized by the following formula:

Fe3. _x Ali _+x M _y T _z Ta _t

wherein:

M represents at least one catalytic specie selected from the group consisting of Ru, Ir, Pd, Pt, Rh, Os, Re and Ag;

T represents at least one element selected from the group consisting of Mo, Co, Cr, V, Cu, Zn, Nb, W, Zr, Y, Mn, Cd, Si, B, C, O, N, P, F, S, CI, Na and Ti;

x is a number higher than -1 and smaller than or equal to +1

y is a number higher than 0 and smaller than or equal to +1

z is a number ranging between 0 and +1

and t is a number higher than 0 and smaller than or equal to +1, preferably lower than 0.4 and more preferably lower than or equal to 0.2

The alloy of the invention is preferably in a nanocrystalline state. If nanocrystalline, the crystallites are smaller than 1 OOnm. The alloy is also preferably a single phase material with a cubic crystallographic structure but can also be multiphase depending on the x, y, z and t composition. Most of the time, these alloys are metastable. In other words, they decompose or transform into a different state when heated at high temperatures. But again, they can also be thermodynamically stable depending on the x, y, z and t composition.

A second object of the present invention is the use of such alloys as electrode material for the synthesis of sodium chlorate. In order to prepare an electrode of these alloys, several methods can be used. A preferred one is thermal spray such as the high velocity oxyfuel (HVOF) technique using the alloy in powder form as feedstock for the spray gun. If the method of preparation involves a rapid quenching process, the alloy can be prepared in a nano crystal line state.

Even though, the preferred application of these new materials is sodium chlorate, several other electrochemical processes can take advantage of these alloys such as industrial and swimming pool water treatment.

Moreover, since the corrosion resistance of these new alloys is very good in various conditions, a third object of the present invention is the use of these alloys as coating for the protection against corrosion. If the targeted application is a coating for protection against corrosion, there may be no advantage of adding a large amount of expensive catalytic element to the alloy. In these cases, the molar content "y" can be chosen small to reduce costs. Moreover, it may be advantageous to add some Ti (titanium) to the alloy since Ti is also known for its good corrosion resistance and the inventors of the present invention found that Ti like Ta is quite soluble in iron-aluminium alloys.

The invention and its associated advantages will be better understood upon reading the following more detailed but not limitative description of preferred modes of achievement of it, made with reference to the enclosed drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 shows an equilibrium ternary phase diagram of the Fe, Al and Ta at 1000°C.

Fig. 2 shows pictures of a corrosion test in 5% HC1 solution for a sample containing Ta according to the invention (right end side) and a similar sample not containing Ta (left end side). Fig, 3 represents the hydrogen released as a function of time during corrosion tests in a 5% HCl solution for samples according to the invention containing Ta with composition t of 0.1 , 0.2, 0.3 and 0.4 and a similar sample not containing Ta.

Fig. 4 represents a figure similar to Fig. 3 where in addition to the previously presented results, the hydrogen released as a function of time is shown for a sample of the invention Fe _3-x Al _1+x M _y T _z Ta _t containing both Nb (element T) and Ta with a respective molar content of z = 0.1 and t = 0.2.

Fig. 5 shows X-ray diffraction spectra of a mixture of a powder of the prior art Fe ₃ . xAli _+x M _y T _z and a powder of Ta at an equivalent molar content of t = 0.2 as a function of milling time during a mecanosynthesis process.

Fig. 6 shows a picture of a ball milled powder of the invention containing Nb and Ta at a respective molar content of z = 0.1 and t = 0.2.

Fig. 7 a) and b) show pictures of a coating according to the invention made from the powder of Fig. 6 using a thermal spray technique at two different magnifications 120 and 5000x.

Fig. 8 show samples of coated electrodes after lhour immersion in 5% HCl solution. The left end side is an electrode of the prior art while the right end side is an electrode according to the present invention containing Ta.

Fig. 9 shows cyclic voltametric curves (current - voltage curves) in a chlorate solution taken at a rate of 5mV/sec at 20°C and pH-6.5 for a sample of the invention and a standard stainless steel 316 sample.

Fig. 10 shows an equilibrium ternary phase diagram of the Fe, Al and Ti at 1200°C. Fig. 11 shows an electrochemical test in a standard chlorate solution using an electrode according to the invention containing Ta at a molar content of t = 0.1.

DETAILED DECRIPTION OF THE INVENTION

As it can be seen, Fig. 1 shows a ternary phase diagram of Fe, Al and Ta at 1000°C. Ta is quite soluble in Fe _x Ali _-x alloys especially near the equiatomic composition (x = 0,5). By inserting Ta into the cubic FeAl alloy at high temperature, a single phase FeAlTa _t material can be prepared at room temperature using a rapid quenching process. If the Ta content is high, the single phase obtain at room temperature will most likely be metastable.

Fig. 2 represents a corrosion test of an alloy containing Ta according to the invention in comparison with a similar alloy not containing Ta. The samples are immerged in a 5% HC1 solution. On the left end part of the picture, we see for the alloy not containing Ta, a lot of hydrogen bubbles indicating severe corrosion in the acidic solution. On the contrary, for the alloy containing Ta in the picture on the right end side, very little bubble formation is observed indicating a much better corrosion resistance in the HC1 solution.

Fig. 3 represents corrosion tests similar to the ones of Fig. 2 showing the amount of hydrogen released during the test as a function of time. The sample of the prior art not containing Ta, Fe3 _-x Ali _+x M _y T _z releases 1 1ml of hydrogen in about 7,5min while the sample of the present invention Fe3 _-x Ali _+x- vi _y T _z Tao _,2 containing Ta at a molar content of y = 0.2 releases only 3.9 ml in 340min. Fig. 3 also shows that, even with a small concentration of Ta of t = 0.1, significant improvement in the corrosion resistance of the alloy can be achieved. This very large improvement in the corrosion resistance of the alloy of the prior art with very small additions of Ta was unexpected.

Fig. 4 represents corrosion tests similar to the ones of Fig. 3 where, in addition to the curves presented in Fig. 3, the results of a sample containing both Nb and Ta are presented. When Ta is added at a composition t = 0.2 to a sample of the prior art already containing Nb (Fe ₃ . _x Ali _+x M _y T _z where T is Nb at a molar content of z = 0.1) a synergetic effect takes place and a huge improvement in the corrosion resistance is achieved. The sample Fe3. _x Ali _+x MyNbojTao _,2 released only 1,5ml of hydrogen in 500min. This synergetic effect when both Nb and Ta are present in the alloy and which gives incredible improvement in the corrosion resistance of the alloy in HC1 solution was also unexpected.

Fig. 5 shows X-ray diffraction spectra of a mixture of an alloy powder of the prior art with a powder of Ta at a molar content of t = 0.2 as a function of the milling time during a mecanosynthesis process. We see the characteristic X-ray peak of the iron aluminide powder (Fe ₃ Al) cubic structure around 44° and a peak corresponding to Ta at about 38.4°. By increasing the milling time from lh to 12h, we observe that the intensity of the Ta peak decreases and vanishes after about 12h of milling. This indicates that all of the Ta has penetrated into the crystalline structure of iron aluminide to form a metastable solid solution.

Fig. 6 shows a scanning electron micrograph taken at a magnification of 2000x of a ball milled powder of the invention Fe _3-x Ali ₊ xMyT _z T _t which comprises both Nb and Ta at a molar content of Nb ₀₌₁ Tao _,2 (T is Nb, z = 0.1 and t = 0.2). The average particle size of this nanocrystalline powder is around 10 microns.

Fig. 7a) and b) represent scanning electron micrographs at 120x and 5000x magnification respectively of the surface of a coating according to the invention made by HVOF thermal spray using the powder shown in Fig. 6. Thus, the material of the coating contains both Nb and Ta elements. This electrocatalytic coating is not only corrosion resistant in the chlorate electrolytic but also in hydrochloric acid solutions.

Fig. 8 represents images of electrodes after immersion in a 5% HC1 solution for one hour. The left side is a picture of an electrode of the prior art (Fe ₃ . _x Ali _+x M _y T _z ) while the right side shows a picture of an electrode of the present invention (Fe3 _-x Ali _+x M _y T-Ta _t ). The electrode of the prior art has been destroyed by the acid wash treatment. The catalytic coating has peeled off from the substrate. On the contrary, the electrode of the present invention shown on the right end side is intact and shows no damage.

Fig. 9 shows current versus voltage curves taken at a scan rate of SmWsec in a highly corrosive environment (chlorate solution at 20°C and pH=6.5) for a coating of the invention containing both Ta and Nb and a standard stainless steel 316 sample. The breakdown potentials on the anodic side (positive voltages) are almost the same for the two samples indicating that under these conditions, the coating material of the invention is a corrosion resistant material as good as stainless steel 316.

Fig. 10 represents a ternary phase diagram of Fe, Al and Ti at 1200°C. One can see that, on the Fe rich side of the Fe-Al system, Ti is quite soluble in the alloys. Therefore, for applications as corrosion resistant coatings, it may be advantageous of adding not only Ta but also Ti to the alloys since Ti is known to be a good corrosion resistant element especially in chlorine environment. However, the addition of Ti is not recommended in applications as electrode for the hydrogen evolution reaction since Ti is known to form stable hydrides as discussed in CA 2,687,129 mentioned hereinabove. Fig. 1 1 shows an electrochemical test conducted in a standard chlorate solution using a DSA as anode and an electrode material of the invention as cathode. The material according to the invention contains Ta at a molar content of t = 0.1. The anodic and cathodic voltages are measured with respect to a Ag/AgCl reference electrode. 1.3 volt has been substracted from the Anode-Cathode voltage difference in order to show the three traces on the same figure. Open circuit (OC) events for durations of 30sec. lmin and 2min have been conducted during the test. Has it can be seen, the voltage of the cell remains stable in spite of these events.