Rhodium-iron alloys

Rhodium readily alloys with platinum, stiffening it and yielding mixtures that are useful for a variety of laboratory purposes. Rhodium reduces the loss in weight of platinum by volatilisation at all temperatures above 900° C., and it has therefore been suggested that a useful alloy for best quality crucibles would be platinum containing 3 to 5 per cent, of rhodium, practically free from iron and iridium, and containing no other detectable impurities. 2... 

For general work at higher temperatures, several different types of couples are employed in this country. Up to 360°C. for extreme precision or to 500°C. for a precision of 5 to 10°C. the couple may consist of one wire of copper and the other wire of constantan. Iron-constantan or nichrome-constantan may be employed for technical processes below 900°C. For operation below 1,100°C. special patented alloys of chromium and nickel and of aluminum and nickel, chromel-alumek or nichrome-alumel are very satisfactory even for continuous service. For the temperature range 300 to 1,500°C. the Le Chatelier couple should be employed. This couple consists of one wire of platinum and the other wire an alloy containing 90 per cent platinum and 10 per cent rhodium. Other alloys and metals may be employed for special work but the above combinations are sufficient for almost all technical processes carried on in the temperature range 0 to 1,500°C. No satisfactory couple has been developed for operation much above 1,500°C. 

Alloys suitable for castings that ate to be bonded to porcelain must have expansion coefficients matching those of porcelain as well as soHdus temperatures above that at which the ceramic is fired. These ate composed of gold and palladium and small quantities of other constituents silver, calcium, iron, indium, tin, iridium, rhenium, and rhodium. The readily oxidi2able components increase the bond strength with the porcelain by chemical interaction of the oxidi2ed species with the oxide system of the enamel (see Dental materials). 

Nitric acid reacts with all metals except gold, iridium, platinum, rhodium, tantalum, titanium, and certain alloys. It reacts violentiy with sodium and potassium to produce nitrogen. Most metals are converted iato nitrates arsenic, antimony, and tin form oxides. Chrome, iron, and aluminum readily dissolve ia dilute nitric acid but with concentrated acid form a metal oxide layer that passivates the metal, ie, prevents further reaction. 

The corrosion behaviour of amorphous alloys has received particular attention since the extraordinarily high corrosion resistance of amorphous iron-chromium-metalloid alloys was reported. The majority of amorphous ferrous alloys contain large amounts of metalloids. The corrosion rate of amorphous iron-metalloid alloys decreases with the addition of most second metallic elements such as titanium, zirconium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, cobalt, nickel, copper, ruthenium, rhodium, palladium, iridium and platinum . The addition of chromium is particularly effective. For instance amorphous Fe-8Cr-13P-7C alloy passivates spontaneously even in 2 N HCl at ambient temperature ". (The number denoting the concentration of an alloy element in the amorphous alloy formulae is the atomic percent unless otherwise stated.)... 

Figure 3 shows the Mossbauer spectra for an alloy of 75% iron and 25% rhodium after two different heat treatments. Since absorption rather than transmission is plotted, these curves are right side up. The upper spectrum is taken from an alloy which was annealed in the low temperature field (cesium chloride structure), and there are two six-hne patterns... 

Figure 3. Mossbauer spectra for ordered and disordered samples of an alloy of 25% rhodium and 75% iron. The disordered sample was quenched and contains retained austenite (17)...

It is also interesting to look at alloys in this system at and near the equiatomic composition. Figure 4 shows just two lines of the six-line pattern—the two on the far right. At 50% rhodium every iron is completely surrounded by rhodium neighbors, and the lines are perfectly... 

Figure 4. Portion of the Mbssbauer spectrum for iron—rhodium alloys containing 48 and 50% rhodium (17)...

It is further important to note that all the current/voltage characteristics depicted in Fig. 6 are unchanged by the presence of liquid fuels such as methanol, formaldehyde, formic acid, or hydrazine. The phthalocyanine electrode remains completely inert toward such substances. For this reason, no mixed potential can be formed at a phthalocyanine electrode, as for example can occur at a platinum electrode, when it is used as cathode in a methanol cell containing sulfuric acid. This is shown by a comparison (see Fig. 7) of the stationary characteristics of the platinum alloy we found to be the most active in the presence of methanol, namely a Raney ruthenium—rhodium electrode, with an iron phthalocyanine electrode, both measured in 4.5 N H2SO4+2M CH3OH. 

To measure temperatures not exceeding 800 °C, one should use thermocouples made from copper and constantan (the latter is an alloy of 45-60% copper, 40-55% nickel, and 0-1.4% manganese it usually also contains about 0.1% carbon), Alumel (an alloy of 95% nickel, 2 % aluminium, 2% manganese, and 1% silicon), and Chromel (90% nickel and 10% chromium), or iron and constantan. Platinum-platinum/rhodium thermocouples are generally used for measuring high temperatures (up to 1600 °C). 

For anodic processes the choice of materials for the electrode is much more limited than for cathodic ones, as the anode could bo easily attacked by the products of the electrolysis (chlorine, oxygon etc.), or electrochemioally dissolved. In alkaline solutions the selection will be restricted to the application of platinum (or alloys of platinum with irridium or rhodium), palladium, carbon (or rather graphite) iron and nickel, while for acid solutions only metals of the platinum group and graphite will be suitable in a special case of the electrolysis in sulphuric acid solutions lead has found wide use, it getting coated with a conductive film of lead dioxide. 

Deoille and Delray s Method.4—This method consists in melting the crude platinum, from which the gold has been removed, with excess of lead. On cooling, the mass is treated first with nitric add, and then with aqua regia, whereby lead, platinum, and some rhodium pass into solution iron, ruthenium, and rhodium remaining behind as a crystalline alloy. From the solution ammonium chlor-platinate is precipitated, and from it metallic platinum is prepared as described above. 

When platinum is heated in a luminous coal-gas flame a black layer is formed. If the carbon is now burnt off in air the metal is left in a rough and brittle condition, but without having undergone any loss in weight. The action is considerably enhanced by the presence of alloyed rhodium or iron, whilst iridium has a less marked effect.8... 

At present, the main industrial catalyst of ammonia oxidation is platinum and its alloys with aluminium and rhodium. Taking into account the deficit and high cost of platinum metals, the dcCTcasing of the consumption and losses of platinum metals is an urgent problem. Therefore, several compositions of complex oxide catalysts have been developed with iron (111), cobalt and chromium oxides as an active component. Complex oxides with perovskite structure are used as new catalysts they provide selective oxidation of ammonia with an yield not less than 90 %. The authors of [33] proposed to use perovskite powders LaMeOj, where Me=Fe, Co, Ni, Cr, Mn, and La,.,Sr,Me03, where Me=Co, Mn and x=0.25-0.75. To prepare these compounds, they used the precipitation by tetraethyl ammonia from diluted nitrate solutions taken at necessary ratios. The powders as prepared are poorly molded as in the form of honeycomb stractures as well as in the form of simple granules. 

An interesting application of adsorption microcalorimetry was used by these researchers to examine changes in adsorption behavior of graphite-supported iron/rhodium bimetallic catalysts as a function of oxidation and reduction treatments. The differential heat of oxygen adsorption on the bimetallic catalysts after various treatments was compared to the values obtained for the monometallic materials to determine the relative contributions to the total adsorption. Reduction at 673 K produced an alloy whose... 

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