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CORROSION-RESISTANT IRIDIUM-PLATINUM

ANODE MATERIAL FOR HIGH POLARIZATION c

APPLICATION IN CORROSIVE ACIDS

> By: Joseph Fanner I ' . c

43855 Olazaba Terrace Fremont, CA 94539

By: Leslie Summers 3550 Pacific Avenue #lo8 Livennore, CA 94550

2863 Briarwood Drive Livemore, CA 94550

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By: Patricia Lewis

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DISCLAIMER

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This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsi- bility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or-' represents that its use would not infringe privately owned rights. Refer- ence herein to any specific commercial product, process. or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recom- mendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.

USA

USA

USA

DISCLAIMER

Portions of this document may be illegible in electronic image products. Images are produced from the best available original document, 1

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CORROSION-RESISTANT IRIDIUM-PLATINUM ANODE MATERIAL FOR HIGH POLARIZATION APPLICATION

IN CORROSIVE ACIDS

The U.S. Government has rights to this invention pursuant to Contract No. W-7405-ENG-48 between the U.S. Department of Energy and the University of California, for the operation of the Lawrence Livermore National Laboratory.

TECHNICAL FIELD The present invention is generally directed to

highly corrosion resistant materials useful in electrochemical reactors. More specifically, the present invention relates to highly corrosion resistant materials useful as anodic materials and other components in electrochemical reactors that operate at high-polarization levels in highly corrosive acids.

BACKGROUND OF THE INVENTION Anodic materials are often subject to severely

corrosive conditions within an electrochemical cell since the anode is continuously oxidizing electron rich ions. It is generally preferred that the anodic material be resistant to corrosion since this reduces the operation costs of the electrochemical process and prevents the contamination of the electrolyte with oxidized anodic material. The level of corrosion resistivity desired for the anodic material depends largely on the electrochemical reaction conditions. Some electrochemical reactions conditions are not highly corrosive, for example when a low degree of polarization is employed and where the electrolyte is an aqueous solution with a neutral pH. Under such mild conditions, the anode may be composed of a wide variety of materials. However,

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when the electrochemical process operates under

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highly corrosive conditions, i.e. where there is a high-degree of polarization at the anodic surface and the electrolyte used is a highly corrosive acid, highly corrosion resistant anodic materials are required.

Mediated Electrochemical Oxidation (MEO) is an example of an electrochemical process that operates under highly corrosive conditions. electrochemical process used for the ambient temperature destruction of hazardous waste and for the conversion of mixed waste to low-level radioactive waste. Such a process is discussed in Farmer, et al., Trans IChemE (1992) =:158-164; Farmer, et al., J. Electrochem SOC. (1992) -:654- 662; Farmer, et al., J. Electrochem. SOC. (1992) - 139:3025-3029. Specifically, the ME0 process is designed to convert dissolved organics to CO, and H,O. Halogenated organics (Cl, Br, I, F) also produce the corresponding halide acid during their conversion to CO, and H,O, thereby forming aqua regia (i .e., HNO,/HCl). With regard to the treatment of low-level radioactive waste, the ME0 process is designed to oxidize and remove dissolved organics from the waste at ambient temperatures in order to avoid the risk of high temperature volatilization of radionuclides that might occur during incineration of the waste. Thermodynamic calculations indicate that, after removal of the dissolved organics, uranium and plutonium may be converted to volatile UO,(OH), and PuO,(OH), respectively at elevated temperatures and high partial pressures of water. Krikorian, Lawrence Livermore Laboratory Report UCRL-JC-106163 (1991); Cooper, et al., Lawrence Livermore Laboratory Report UCRL-JC-107288 (1991); Krikorian, Lawrence Livermore Laboratory Report UCRL-ID-107877 (1991).

ME0 is an

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Oxidation of the dissolved organics is achieved through the use of mediator ions including but not limited to Ag(I1) , Co(II1) , and Fe(II1) . During the ME0 process, these mediator ions oxidize the dissolved organics. continuously regenerated by their oxidation at the anode. The balanced equation for the oxidation of benzene using Ag(I1) is provided below.

The mediator ions themselves are

The choice of which mediator ion to employ depends, in part, on how strong an oxidant is needed. For example, the reversible redox potentials for Ag(II)/Ag(I), CO(III)/CO(II), Fe(III)/Fe(II) redox couples are 1.987, 1.842 and 0.700 V respectively, relative to a standard hydrogen electrode. The oxidizing power of the mediator ion is proportional to its redox potential. Hence, Ag(I1) is a significantly stronger oxidant than Fe(II1) and is preferred for the oxidation of oxidation resistant organics present in the waste being treated.

One of the problems associated with the ME0 process is the excessive corrosion of anode materials in the presence of HNO, and HC1. Farmer, et al., J. Electrochem. SOC. (1992) m:3025, 3029. As noted above, HC1 and other halide acids are generated in the process of converting halogenated organics to CO, and H20. In addition, organic chelating agents are often formed during the ME0 process which further enhance the corrosive nature of the media. The need exists for strongly corrosion resistant materials for use in Ag(I1) based ME0 processes as well as other processes that operate under highly corrosive conditions.

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SUMMARY OF THE INVENTION The present invention relates to highly

corrosion resistant components for use in an electrochemical cell. Specifically, these components

conditions such as exposure to aqua regia in the presence of a constant current density of 100mA/m2. The components are comprised of an iridium-platinum alloy that comprises less than 30% iridium.

iridium-platinum alloy comprises 15-20% iridium. another preferred embodiment of the present invention, the iridium-platinum alloy is deposited on the surface of an electrochemical cell component by

The present invention also relates to a method

5 are resistant to corrosion under very extreme

In a

10 preferred embodiment of the present invention, the In

15 magnetron sputtering.

for conducting an electrochemical reaction in the presence of highly corrosive acids under a high degree of polarization wherein the electrochemical

containing an iridium-platinum alloy that comprises less than 30% iridium.

20 cell comprises a component, preferably the anode,

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 depicts the relative stability of oxide

Figure 2 depicts the amount of iridium measured

Figure 3 depicts the amount of platinum measured

25 powders under the Powder Test.

in the anolyte during the Galvanic Test.

in the anolyte during the Galvanic Test.

30 DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT The present invention relates to highly

corrosion resistant compositions useful in electrochemical processes that operate under high current densities and in the presence of highly

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corrosive electrolytes such as nitric acid, sulfuric acid, hydrochloric acid and especially nitric acid containing dissolved halide anions (Cl- or F-1.

Previously, it was known that certain noble metals and noble metal oxides including ruthenium, iridium and platinum and alloys thereof possess a high degree of corrosion resistivity. Hayfield, U.S. Patent No. 4,422,917; WO 89/10981. In the present invention, it has been determined that iridium- platinum alloys comprised of less than 30% iridium possess unexpectedly superior corrosion resistivity as compared to other relatively corrosion resistant compositions. withstand the corrosive conditions present in the ME0 process, namely resistance to corrosion under high current densities (100 mA/m2) in the presence of HNO,/HCl. In addition to being highly corrosion resistant, these iridium-platinum alloys are preferred because they employ at least 70% platinum, thereby substantially reducing the cost of the anode material when compared to pure iridium.

that the superior corrosion resistance exhibited by these Pt-Ir alloys is due to the formation of a thin protective layer of Ir0,on the surface of the alloy.

Iridium alloys useful in the present invention may be produced by any of the various methods known in the art including but not limited to melting, arc melting, melting in an induction furnace, sputtering, cosputtering, chemical vapor deposition technology and grinding and pressing the metal into a composite. Electrodes Of Conductive Metallic Oxides (S. Trasatti, ea.) Elsevier Scientific Pub. Co., New York (1980-1981); Thin Film Processes (J. L. Vossen & W. Kern, ed.), Academic Press, New York (1978); Makowiecki, et al., Journal of Vacuum Science and

These compositions have been found to

Without being bound by theory, it is believed

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Technolow (1990) 8:6 3910-3913; Atlas Of Electrochemical Equilibria In Aqueous Solutions (Marcel Pourbaix) National Association of Corrosion

Engineers, Houston, TX (1974). Shaped Ir-Pt alloys may be produced by melting and then cooling the alloy to form a desired shape.

In order to reduce the cost associated with producing corrosion resistant anodes and other electrochemical cell components, it is preferred that an iridium alloy film be deposited on the surface of the anode or component. the iridium alloy film be generated by magnetron sputtering which produces non-porous iridium alloy films. With regard to magnetron sputtering, the alloy may be produced from a single target source comprised of the desired final composition or may be produced from dual target sources wherein metal from each target source is sputtered at rates such that the desired final alloy composition is achieved.

20 In the case of single magnetron sputtering, it is necessary to first generate the bulk iridium alloy (ingot) used as the target source. Bulk alloys may be generated by arc melting the alloy metals in the desired ratio on a water-cooled hearth inside an

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25 argon-filled glove box. The resulting ingot is then ground into a fine powder. poured into a graphite die and heated to approximately 80% of the alloy melting point while being compressed at a pressure of several thousand

30 pounds per square inch. Ingots useful as target sources may also be produced by melting the alloy components under vacuum in an induction furnace.

sputtering has the particular advantage that the resulting film has a low degree of porosity which

The pulverized ingot is

. Formation of an iridium-platinum alloy film by

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further enhances the corrosion resistivity of the alloy film.

The following examples set forth the test methods used to determine and compare the corrosion resistivity of various materials. The examples also set forth the method by which the preferred embodiments were determined. Further objectives and advantages of the instant invention, other than those set forth above, will become apparent from the examples, which are not intended to limit the present invention.

EXAMPLES

1.

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Determination Of Corrosion Resistant Anodic Materials

In order to identify materials possessing

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significant corrosion resistance, a variety of materials were subjected to the Powder Test and the Galvanic Test. As a preliminary matter, the materials to be tested were first selected based on the criteria that they be 1) stable at the operating potential and pH of the anode based on relevant Pourbaix diagrams (potential-pH diagrams) , 2) insoluble in aqua regia (HNO,/HCl) and 3 ) possess reasonable electrical conductivity. A list of materials satisfying these criteria are set forth in Table 1 below.

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Table 1

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Metals Ir Pt Ta Nb Ti Ru

Conductive Oxides PbO

SnO, IrO, RuO,

Pb304

Carbides

Cr&3 Sic B4C wc Hf C TiC TaC

Cr3C2

Miscellaneous Glassy Carbon MoSi,

Powder Test The Powder Test measures the amount of a

particular compound that becomes dissolved in aqua regia (HCl:HN03 = 3:l) over time. In the Powder Test, a 100 mg pulverized sample was placed in 20 mL of aqua regia. Aliquots of the acid were then removed every 2-3 days and analyzed for dissolved metals by inductively coupled plasma mass spectrometry (ICP- MS). After 17 days, the solid residue was recovered and weighed. In order to pass the Powder Test, the material being tested must not become significantly dissolved in the aqua regia. Significant dissolution corresponds to greater than 1 mg dissolution. Of the materials listed in Table 1, only SnO,, IrO,, RuO,,

Ru(,), Pt-20%-Ir alloy, Pt-10%-Rh, glassy carbon and MoSi, passed the Powder Test. Figure 1 depicts the relative stability of oxide powders under the Powder Test wherein stability is based on the dissolution of the oxide into the aqua regia. As can be clearly seen, the amount of IrO, that becomes dissolved in the

Cr3C2, Cr7C3, sic, B4C# Ir(m), Pt(m), Ta(m), a(,), Ti(m),

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aqua regia is over an order of magnitude smaller than any of the other oxides tested.

Galvanic Test The Galvanic Test measures the corrosion

resistivity of a 0.625" diameter x 0.25" piece of material used as an anode wherein aqua regia is employed as the anolyte. During the Galvanic Test, a constant current density of approximately 100mA/cm2 was applied. material were measured. also removed and analyzed. As was the case with the Powder Test, in order to pass the Galvanic Test, the material being tested must not become significantly dissolved in the electrolyte.

Changes in the weight of the anode Aliquots of the anolyte were

Of the materials that passed the Powder Test, only Ir02, Galvanic Test. As previously mentioned, it was already known that certain noble metals and noble metal oxides including ruthenium, iridium and platinum and alloys thereof possessed a high degree of corrosion resistivity in other electrolytes. Hayfield, U.S. Patent No. 4,422,917; WO 89/10981. However, it was surprising that iridium based compositions, as a class, exhibited superior extreme corrosion resistivity as compared to other noble metals in aqua regia. and in accordance with the present invention, it was

determined that Ir02, Ir,,, and iridium-platinum alloys are significantly preferred for use in electrochemical cells under highly corrosive conditions.

and Pt-20%-Ir alloy also passed the

Based on these test results,

2. Determination Of Preferred Pt-Ir Alloy ComDosition

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After determining the desirability of Pt-Ir alloys relative to other corrosion resistant materials for use in the fabrication of corrosion resistant components, the preferred platinum to iridium alloy ratios were determined. Iridium-

platinum alloys comprising 5%, IO%, 151, 20% and 30% iridium were subjected to the Galvanic Test for three days and tested with regard to their degree of iridium and platinum dissolution. composition with minimal dissolution of both alloy constituents is preferred. However, given the significant cost of iridium metal relative to platinum, alloys having minimal iridium dissolution are particularly preferred.

in the anolyte during the Galvanic Test. indicated by Figure 2, a platinum-iridium alloy

comprising between 15% and 20% iridium exhibited the least iridium disolution (greatest stability) during the Galvanic Test. platinum measured in the anolyte during the Galvanic Test. The 30% iridium alloy exhibited the lowest degree of platinum dissolution, followed by the 15%

iridium alloy. Based on these test results, it was determined that an iridium-platinum alloy comprising less than 30% iridium is the preferred corrosion resistant composition, wherein an iridium-platinum alloy comprised of between 15% and 20% iridium is the most preferred composition (a composition giving the

least overall dissolution).

An alloy

Figure 2 depicts the amount of iridium measured As

Figure 3 depicts the amount of

While the invention of this patent application is disclosed by reference. to the aforementioned examples, it is to be understood that these examples are intended in an illustrative rather than limiting sense, as it is contemplated that modifications will readily occur to those skilled in the art, within the

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s p i r i t of the invention and the scope of the appended claims.

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ABSTRACT

The present invention relates to highly corrosion resistant components for use in an electrochemical cell. Specifically, these components

conditions such as exposure to aqua regia in the presence of a constant current density of 100mA/m2. The components are comprised of an iridium-platinum alloy that comprises less than 30% iridium. In a

iridium-platinum alloy comprises 15-20% iridium. In another preferred embodiment of the present invention, the iridium-platinum alloy is deposited on the surface of an electrochemical cell component by

The present invention also relates to a method

5 are resistant to corrosion under very extreme

10 preferred embodiment of the present invention, the

15 magnetron sputtering.

for conducting an electrochemical reaction in the presence of highly corrosive acids under a high degree of polarization wherein the electrochemical

containing an iridium-platinum alloy that comprises less than 30% iridium.

20 cell comprises a component, preferably the anode,

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