Precious metals, coatings

There are obviously situations which demand considerable over-design of a cathodic protection system, in particular where regular and efficient maintenance of anodes is not practical, or where temporary failure of the system could cause costly damage to plant or product. Furthermore, contamination of potable waters by chromium-containing or lead-based alloy anodes must lead to the choice of the more expensive, but more inert, precious metal-coated anodes. The choice of material is then not unusual in being one of economics coupled with practicability. 

Laister and Benham have shown that under more arduous conditions (immersion for 6 months in sea-water) a minimum thickness of 0-025 mm of silver is required to protect steel, even when the silver is itself further protected by a thin rhodium coating. In similar circumstances brass was completely protected by 0 012 5 mm of silver. The use of an undercoating deposit of intermediate electrode potential is generally desirable when precious metal coatings are applied to more reactive base metals, e.g. steel, zinc alloys and aluminium, since otherwise corrosion at discontinuities in the coating will be accelerated by the high e.m.f. of the couple formed between the coating and the basis metal. The thickness of undercoat may have to be increased substantially above the values indicated if the basis metal is affected by special defects such as porosity. 

INEOS chlor offers both precious metal and nickel alloy coatings [192]. The precious metal coatings are deposited on a nickel substrate, and the electrocatalytically active layers are applied to the smface by thermal deposition or electroplating. Thermal spraying or vacuum deposition techniques form the nickel-alloy coating. These materials have been used in the FM 21 electrolyzers for over four membrane cycles. 

Mercury also acts as a poison for precious metal-coated cathodes. If present in a brine circuit as a result of conversion from mercury to membrane technology, it would pass through the membranes and contaminate the cathodes. For this reason, a brine circuit... 

In general, precious metal electrodes or precious metal coated electrodes are used in batteries, fuel cells. 

Catalysts are materials that speed up chemical reactions or even enable them to proceed without themselves being consumed. An example of a catalyst is the one located in the catalytic converter of an automobile exhaust system that consists of small quantities of precious metals coated onto the solid honeycomb-like surface that acts as a catalyst support. Such a catalyst speeds the reaction with oxygen of toxic carbon monoxide from the engine exhaust to produce nontoxic carbon dioxide ... 

While this is another figure of merit which, away from totally specific circumstances is difficult to quantify, it is clear that the cost, performance and lifetime of all cell components wilt affect the design of the electrolysis cell, e.g. the benefit of catalytic electrodes will depend on the initial cost and their lifetime, which in turn wiU depend on current density certainly the saving in electricity consumption during the lifetime of the electrodes must exceed the diflerence in cost between the catalytic and poorer electrode materials Cells are usually taken off-stream OR a routine cycle because of the inconvenience and cost of failure of components such as electrodes and separators (although tn successful processes this may be once in several years) and the components replaced or, at least, inspected closely, e.g. the thickness of a precious metal coating may be deter-... 

Dusing, W. Seal Metals with Precious Metal Coatings for Microwave Tubes, Glastech. Ber, vol. 31, pp. 137-142, April, 1958. 

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