Noble metal coatings electrodeposited

Zn-Al LDHs can be prepared by electrodeposition from aqueous solutions of the nitrates of such metals (Yarger et al., 2008). Cathodic depositions, conducted at room temperature without stirring over noble metal-coated electrodes, were achieved by reducing nitrate ions to generate hydroxide ions on the working electrode, the optimal potential being -1.65 V vs. AgCl/Ag in 4 M KCl. The overall reactions involved are... 

Cladding may be less expensive than sdective electrodeposition when coatings greater than 1 Jim of a noble metal are required, but may be more expensive than electro deposition for thinner coatings. Selective techniques are most easily used for sheet metal substrates that are to be machine stamped and formed into contacts. Clad noble metals are considerably more ductile (and less hard) than comparable electrodeposits and, therefore, are better suited to forming operations. Contacts that are made into separate parts from rod by screw machining are usually coated on all exposed surfaces by barrel electroplating. 

High preconcentration factors in the analysis of environmental samples were obtained using electrolysis of the examined solutions and electrodeposition of noble metals (e.g., Pd and Pt in airborne particulates [89] and ft, Pd, Rh, and Ru in road dust [90]) on the iimer surface of p3Tolytically coated graphite tube. 

For the electrodeposition of laminar metal coatings, two conditions must be fulfilled (1) The reversible potentials for metals A and B must be sufficiently different so that at a given current density, the less noble one (B) virtually does not electrodeposit during the electrodeposition of the more noble one (A) until complete concentration polarization with respect to ions of metal A takes place (2) within the duration of the current density pulse, Send s equation [15] for diffusional polarization is obeyed with respect to concentration change, resulting in transition from electrodeposition of metal A to electrodeposition of metal B after well-defined transition time. 

At the start the cathode is invariably a metal different from that to be deposited. Frequently, the aim is to coat a base metal with a more noble one, but it may not be possible to do this in one step. When a metal is immersed in a plating bath it will corrode unless its potential is sufficiently low to suppress its ionisation. Fortunately, a low rate of corrosion is tolerable for a brief initial period. There are cases where even when a cathode is being plated at a high cathodic (nett) current density, the substrate continues to corrode rapidly because the potential (determined by the metal deposited) is too high. No satisfactory coating forms if the substrate dissolves at a high rate concurrently with electrodeposition. This problem can be overcome by one or more of the following procedures ... 

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