Platinum corrosion

Platinum corrosion and Use of a titanium reference liner with mechanical instability extended change-out frequency for a given full-scale reactor every 500 hr or longer... 

This system seems to be the only alloy to date whose hydrous oxide growth behavior under potential cycling conditions has been investigated.189 190 Burke and O Sullivan189 demonstrated that with an alloy containing 10% by weight of rhodium in platinum both components corroded on cycling between certain limits (0-1.5 V) in 1.0 mol dm-3 NaOH. However, while the platinum corrosion product was found to be soluble under these conditions the rhodium one was not—in fact the hydrous film developed on the surface in this case was apparently derived almost totally from the minor component in the alloy. 

Figure 1.6 Iron-platinum corrosion cell in an acid electrolyte dissolution of iron at the anode and reduction of protons at the platinum cathode.

The sensitivity of the analysis [19] was not sufficient to determine traces of Pt in the potential range between 0.8 V and 2.14 V in 0.05 M H2SO4 and 0.5 M H2SO4 at 25 °C. If the hump at about 1.2 V in anodic charging curves taken with a small current density on smooth platinum in 0.5 M H2SO4 is attributable to platinum corrosion, the corrosion rate can be estimated to be smaller than 0.2 pA/cm. However, this estimate [19] depends in a critical way upon the interpretation of the hump which may be due to other processes (compare section 1 in chapter VII). 

Titanium has potential use in desalination plants for converting sea water into fresh water. The metal has excellent resistance to sea water and is used for propeller shafts, rigging, and other parts of ships exposed to salt water. A titanium anode coated with platinum has been used to provide cathodic protection from corrosion by salt water. 

It is the most corrosion-resistant metal known, and was used in making the standard meter bar of Paris, which is a 90 percent platinum and 10 percent iridium alloy. This meter bar was replaced in 1960 as a fundamental unit of length (see under Krypton). 

In this pyrolysis, sub atmospheric partial pressures are achieved by employing a diluent such as steam. Because of the corrosive nature of the acids (HE and HCl) formed, the reactor design should include a platinum-lined tubular reactor made of nickel to allow atmospheric pressure reactions to be mn in the presence of a diluent. Because the pyrolysate contains numerous by-products that adversely affect polymerization, the TFE must be purified. Refinement of TFE is an extremely complex process, which contributes to the high cost of the monomer. Inhibitors are added to the purified monomer to avoid polymerization during storage terpenes such as t7-limonene and terpene B are effective (10). 

Gold and gold-based alloys ate used for corrosion-resistant equipment. Gold—platinum alloys, 75 Au-25 Pt or 84 Au-15 Pt-1 Rh, ate used as cmcible material for many molten salts (98). Spinnerets for rayon manufacture ate based on the Au—Pt system which exhibits a broad miscibility gap in the soHd state so that the alloys can be age-hardened. Spinneret alloys contain 30—40% or mote platinum modified by small additions of usually rhodium (99). Either gold or gold—platinum alloys ate used in mpture disks for service with corrosive gases (100). 

Ion implantation has also been used for the creation of novel catalyticaHy active materials. Ruthenium oxide is used as an electrode for chlorine production because of its superior corrosion resistance. Platinum was implanted in mthenium oxide and the performance of the catalyst tested with respect to the oxidation of formic acid and methanol (fuel ceU reactions) (131). The implantation of platinum produced of which a catalyticaHy active electrode, the performance of which is superior to both pure and smooth platinum. It also has good long-term stabiHty. The most interesting finding, however, is the complete inactivity of the electrode for the methanol oxidation. 

Under severe conditions and at high temperatures, noble metal films may fail by oxidation of the substrate base metal through pores in the film. Improved life may be achieved by first imposing a harder noble metal film, eg, rhodium or platinum—iridium, on the substrate metal. For maximum adhesion, the metal of the intermediate film should ahoy both with the substrate metal and the soft noble-metal lubricating film. This sometimes requires more than one intermediate layer. For example, silver does not ahoy to steel and tends to lack adhesion. A flash of hard nickel bonds weh to the steel but the nickel tends to oxidize and should be coated with rhodium before applying shver of 1—5 p.m thickness. This triplex film then provides better adhesion and gready increased corrosion protection. 

The main cause of anode wear is electrochemical oxidation or sulfur attack of anodic surfaces. As copper is not sufficiently resistant to this type of attack, thin caps of oxidation and sulfur-resistant material, such as platinum, are bra2ed to the surface, as shown in Eigure 15a. The thick platinum reinforcement at the upstream corner protects against excessive erosion where Hall effect-induced current concentrations occur, and the interelectrode cap protects the upstream edge from anodic corrosion caused by interelectrode current leakage. The tungsten undedayment protects the copper substrate in case the platinum cladding fails. 

Ruthenium and osmium have hep crystal stmetures. These metals have properties similar to the refractory metals, ie, they are hard, britde, and have relatively poor oxidation resistance (see Refractories). Platinum and palladium have fee stmetures and properties akin to gold, ie, they are soft, ductile, and have excellent resistance to oxidation and high temperature corrosion. 

Table 3. Corrosion Resistance of Platinum-Group Metals ...

Tips of platinum, platinum—nickel alloy, or iridium can be resistance-welded to spark-plug electrodes for improved reHabiHty and increased lifetime. These electrodes are exposed to extremely hostile environments involving spark erosion, high temperature corrosion, thermal shock, and thermal fatigue. 

Plating and Coatings. Thin surface coatings of platinum and platinum alloys are used as decorative finishes and in critical appHcations where it is necessary to provide finishes resistant to corrosion or high temperature, eg, coatings on jet-engine turbine components (258). Compounds used in the electro deposition of platinum are based on Pt(Il) and Pt(IV) and include H2[PtCl3] and its salts, eg, Pt—P—Salt, [Pt(NH3)2(N02)2] H2[Pt(S04)(N02)2] ... 

The formation of acids from heteroatoms creates a corrosion problem. At the working temperatures, stainless steels are easily corroded by the acids. Even platinum and gold are not immune to corrosion. One solution is to add sodium hydroxide to the reactant mixture to neutralize the acids as they form. However, because the dielectric constant of water is low at the temperatures and pressure in use, the salts formed have low solubiHty at the supercritical temperatures and tend to precipitate and plug reaction tubes. Most hydrothermal processing is oxidation, and has been called supercritical water oxidation. 

Gold [7440-57-5] Au, is the principal constituent of gold-colored alloys. It contributes gold color kicreases the specific gravity raises the melting pokit, if that of the alloy is below that of gold and kicreases ductihty, maHeabiUty, corrosion, and stain resistance. Gold produces heat-treatable compositions with copper, platinum, and zkic. It is useful ki amounts of 25—100 wt %. 

Iridium [7439-88-5] Ir, and rhodium [7440-16-6] Rh, iadividually iacrease corrosion resistance, hardness, and strength of platinum alloys. They can be used to reduce grain size (140). 

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