Electrodeposition substrates

At potentials positive to the bulk metal deposition, a metal monolayer-or in some cases a bilayer-of one metal can be electrodeposited on another metal surface this phenomenon is referred to as underiDotential deposition (upd) in the literature. Many investigations of several different metal adsorbate/substrate systems have been published to date. In general, two different classes of surface stmetures can be classified (a) simple superstmetures with small packing densities and (b) close-packed (bulklike) or even compressed stmetures, which are observed for deposition of the heavy metal ions Tl, Hg and Pb on Ag, Au, Cu or Pt (see, e.g., [63, 64, 65, 66, 62, 68, 69 and 70]). In case (a), the metal adsorbate is very often stabilized by coadsorbed anions typical representatives of this type are Cu/Au (111) (e.g. [44, 45, 21, 22 and 25]) or Cu/Pt(l 11) (e.g. [46, 74, 75, and 26 ]) It has to be mentioned that the two dimensional ordering of the Cu adatoms is significantly affected by the presence of coadsorbed anions, for example, for the upd of Cu on Au(l 11), the onset of underiDotential deposition shifts to more positive potentials from 80"to Br and CE [72]. 

J. W. Dini, Electrodeposition The Materials Science of Coatings and Substrates, Noyes Data Corp., Park Ridge, N.J., 1993. 

A.STM B650, Std. Spec, for Electrodeposited Engineering Chromium Coatings of Ferrous Substrates, American Society for Testing and Materials, Philadelphia,... 

Adhesion of copper films to PMDA/ODA polyimide was determined by peel tests conducted on samples that were prepared by vapor-depositing a thin layer of copper onto the polyimide and then building the thickness of the metal layer to about 18 p,m by electrodeposition of copper. Results of the adhesion measurements correlated well with substrate pretreatment. When the substrate... 

Electroplating—the process of electrodeposition onto a metallic substrate of a thin adherent layer of a metal or alloy having desirable chemical, physical and/or mechanical properties. 

Germanium In situ STM studies on Ge electrodeposition on gold from an ionic liquid have quite recently been started at our institute [59, 60]. In these studies we used dry [BMIM][PF<3] as a solvent and dissolved Gel4 at estimated concentrations of 0.1-1 mmol 1 the substrate being Au(lll). This ionic liquid has, in its dry state, an electrochemical window of a little more than 4 V on gold, and the bulk deposition of Ge started several hundreds of mV positive from the solvent decomposition. Furthermore, distinct underpotential phenomena were observed. Some insight into the nanoscale processes at the electrode surface is given in Section 6.2.2.3. 

Pt electrodeposits may also be produced from molten salt electrolytes. Such a high-temperature process has the advantage that the deposits are diffusion bonded to the titanium substrate and thus have good adhesion, and, if necessary, thick deposits can be produced. However, they have the disadvantage that because of the complexity of the process there is a limitation on the size and shape of the object to be plated, and the resultant deposits are softer and less wear resistant than those from aqueous solutions... 

Magnetite may also be used in combination with lead or electrodeposited onto a titanium substrate". The latter anode system has been shown to exhibit good operating characteristics in seawater but at present it is only of academic interest. 

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 ... 

Pseudomorphism received methodical study from about 1905. A micro-section taken across the interface between a substrate and an electrodeposit shows the grain boundaries of the former continue across the interface into the deposit (Fig. 12.5). As grain boundaries are internal faces of metal crystals, when they continue into the deposit the latter is displaying the form of the substrate. Hothersall s 1935 paper contains numerous excellent illustrations with substrates and deposits chosen from six different metals, crystallising in different lattice systems and with different equilibrium spacing. Grain boundary continuation and hence pseudomorphism is evident despite the differences. 

Fig. 12.5 Pseudomorphism grain boundaries in the substrate (5) are continued in the electrodeposit (D)...

Fig. 12.6 (a) Co-ordination across a substrate S-electrodeposit D interface on the atomic scale produces epitaxy, (b) a non-epitaxial deposit has no co-ordination and (c) epitaxy would be expected to produce grain boundary continuation at the interface, though in fact grain boundaries often continue to thicknesses far greater than those at which epitaxy disappears... 

Non-epitaxial electrodeposition occurs when the substrate is a semiconductor. The metallic deposit cannot form strong bonds with the substrate lattice, and the stability conferred by co-ordination across the interface would be much less than that lost by straining the lattices. The case is the converse of the metal-metal interface the stable arrangement is that in which each lattice maintains its equilibrium spacing, and there is consequently no epitaxy. The bonding between the met lic lattice of the electrodeposit and the ionic or covalent lattice of the substrate arises only from secondary or van der Waals forces. The force of adhesion is not more than a tenth of that to a metal substrate, and may be much less. 

Pseudomorphism has less desirable consequences, and usually means are sought to suppress it. If the substrate has been scratched, ground or abrasively polished, or if it has been cold rolled or cold formed, the surface is left in a peculiar state. Cold working reduces the surface grain size, and produces deformed, shattered and partly reoriented metal. It may produce microcrevices between the deformed grains, and, with some processes, non-metallic impurities and oxides are embedded in the surface. The disturbed state of the substrate is copied by a pseudomorphic electrodeposit with several consequences (Fig. 12.7). One is aesthetic it has often been noted that almost invisible abrasion of the substrate develops as more prominent... 

Continuous plating of wire and strip is, unlike the preceding techniques, a prefabrication process. The production of tinplate is the largest scale continuous operation, but any electrodeposit may be applied this way. Subsequent fabrication processes arc likely to damage the coating, so that pre-coating is best reserved for ductile coatings which are anodic to the substrate in service, as is the case for tin. 

Electrodeposits are usually in a state of internal stress. Two types of stress are recognised. First order, or macro-stress, is manifest when the deposit as a whole would, when released from the substrate, either contract (tensile stress) or expand (compressive stress) (Fig. 12.12). Second order or microstress, occurs when individual grains or localities in the metal are stressed, but the signs and directions of the micro-stresses cancel on the larger scale. The effects of first order stress are easily observed by a variety of techniques. 

Fig. 12.14 Spread of silver sulphide from discontinuities in gold electrodeposits on silver substrates. The gold was deliberately scratched and the specimen exposed for 24 h to an atmosphere containing 10< o SO2. Immediately after this the sulphide stain extended 0-2 mm. Five years later, the stain extends to about 13 mm, after storage in a normal indoors atmosphere...

Compared with other methods, vacuum evaporation produces coatings that have a most satisfactory corrosion resistance, e.g. 0.005 mm of evaporated Cd gives a degree of protection to steel similar to that afforded by 0.01 mm of electrodeposited Cd. Cadmium coatings on ferrous and other substrates can meet authoritative specifications concerning corrosion... 

The principal use of gold is as a very thin coating about 0-05 /xm thick for electrical and electronic applications. Because of the thinness of gold electrodeposits, porosity must be very carefully controlled since seepage of corrosion products from substrate or undercoat exposed at these pores can have serious adverse effects on both appearance and electrical properties of the composite. The porosity can vary with the thickness of the deposit (Fig. 13.1), and with the type of plating bath and with its method of operation (Fig. 13.2), and the phenomenon has been extensively studied by Clarke and many other workers. 

With tin coatings on brass, the interdiffusion of coating and substrate brings zinc to the surface of the tin the action can be rapid even with electrodeposited coatings. The effect of zinc in the surface layers is to reduce the resistance of the coating to dulling in humid atmospheres, and the layer of zinc corrosion product formed makes soldering more difficult. An intermediate layer of copper or nickel between brass and tin restrains this interdiffusion . 

Bellows Nickel bellows can be made by electrodeposition onto a grooved cylinder. In this case, the nickel coating cannot be slid off, and so the substrate must be removed destructively. The grooved cylinders or mandrels are frequently of aluminium alloy which is dissolved away in caustic alkali when the nickel deposition is completed. Uses include pressure switches, flexible couplings, and pressure transducers . 

Electrodeposited chromium coatings. Electrolytic corrosion testing Coatings cathodic to the substrate-rating of electroplated test specimens subjected to corrosion tests... 

---