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16 MARCH 2017 — PFonline.com A higher coating flux density is therefore possible with smaller distances between anodes and cathodes. • Layers can be applied more quickly: Production output increases. • A significant improvement in separation conditions can be achieved using grids with a large effective surface area. Optimum Physical, Chemical Parameters By combining platinum and titanium, dimensional stability is possible. Both metals offer optimal parameters for hard chrome plating. Platinum has a very low specific electrical resistance of just 0.107 Ω×mm 2 /m. Lead's value is almost twice as high (0.208 Ω×mm 2 /m). Titanium's corrosion resis- tance is outstanding, however, in the presence of halides, this is reduced. The breakdown voltage for titanium in electrolytes containing chloride, for example, is between 10 and 15 V, depending on the pH value. This is significantly higher with niobium (35 to 50 V) and tantalum (70 to 100 V). Titanium does have disadvantages when it comes to corro- sion resistance in strong acids, such as sulphuric, nitric, hydrofluoric, oxalic and methane sulphonic acids. However, titanium is still a good option because of its mechanical work- ability and price. Application of the platinum layer on the titanium base substrate is best carried out electrochemically in molten salt via high-temperature electrolysis (HTE). The sophisticated HTE process offers precise coating: In a molten bath of 550°C, made with a mixture of potassium and sodium cyanide, with around 1- to 3-percent platinum, the precious metal is depos- ited electrochemically onto the titanium. The substrate is locked into a closed system with argon, where the salt bath is in a double crucible. Currents between 1 and 5 A/dm 2 allow isolation rates between 10 and 50 μm per hour at a coating tension of 0.5 to 2 V. The anodes that were platinum-plated using the HTE process are clearly superior to those coated in aqueous elec- trolytes. The purity of the molten salt platinum coating is at least 99.9 percent and therefore significantly higher than the purity of platinum layers deposited from aqueous solutions. Ductility, adhesion and corrosion resistance are significantly improved and inner tensions are minimal. When considering the optimum anode construction, opti- mizing the anode's carrier construction and power supply is TABLE 2: Some examples of areas of application for Ot/Ti anodes and recommended layer thicknesses; the thickness of the platinum layer varies depending on the area of use. Application Area Coating Density Precious metal electrolytes, strongly acidic 2.5 μm Gold electrolytes, neutral to slightly acidic 1.5 μm Gold electrolytes, alkaline 1.5 μm 1.5 μm hard chrome plating fluoride-free/containing fluoride 2.5/5 μm 2.5/5 μm nickel/nickel alloy electrolytes 1.5 μm 1.5 μm destruction of cyanides in wastewater 1.5 μm/2.5 μm TABLE 1: Standard mesh sizes for expanded metal anodes. Type Mesh length (mm) Mesh width (mm) Sheet thickness (mm) Web width (mm) Surface factor (dm 2 ) F 6 3 1 1 2.22 N 10 5 1 1 1.44 G 12 6 1 1 1.22 GS 12 6 1.5 1.5 1.75 D 12.5 7 2 2 1.92 Area with an edge length of 100 × 100 mm. of the utmost importance. The best solution is when a titanium sheeting coat is warmed and wrapped around a copper core. Copper is an ideal conductor and only has a specific electrical resistance of around 9 percent of a Pb/Sn alloy. The CuTi power supply only causes minimal power loss along the anode, so the layer thickness distribution on the cathodic component is equal. Another positive effect is the formation of less heat. The need for cooling is reduced, as well as the platinum wear on the anode. The corrosion resistant titanium coating protects the copper core. When re-platinizing the expanded metal sheet, frame constructions and/or power supplies are only cleaned and prepared. These can be reused several times. Avoiding Construction Errors CuTi power supplies offer many possibilities for error: • Too few welding spots can cause high transition resistance. • Problems occur if the bracket construction in the power supply is not ideally constructed and is not divided appropriately. • With fluoride containing Cr 6 electrolytes, it is extremely important to choose a suitable base material for the anode. • There should be no TIG/laser welding zones in the plating area: Melting the Pt/Ti coating system onto the welding join must be avoided. This could otherwise lead to alloy build-up, which reduces the corrosion resistance. In addition, heat weakens the adhesion of the Pt layer (peel- off effect). PLATING