Porosity electrodeposits

Electrodeposits of Pt can only be applied as relatively thin coatings that are porous. Although the porosity decreases with increase in deposit thickness, so does the internal stress and if the platinum adhesion is poor the coating may exfoliate. As a consequence, thicknesses of 2-5 to 1-5 fim Pt... 

It will be seen that the design of articles to be electroplated can have a considerable effect on the corrosion resistance of the electrodeposited coating. The chief effects are the result of variations in deposit thickness, but also important are features which can influence the adhesion, porosity and physical properties of the deposit. Good design will also avoid features of the plated article capable of trapping liquids or solid contaminants which might cause more rapid corrosion. 

Fig. 12.9 Corrosion resistance of tin-nickel electrodeposit impaired by pseudomorphic porosity originating on cold-rolled steel surface (left). Panel on right has had the shattered grain surface removed by chemical polishing (0-125 iim removed). Coating thickness 15 iim-, panels exposed 6 months to marine atmospheric corrosion (Hayling Island)...

Fig. 12.16 Increase in porosity of an electrodeposit caused by mechanical polishing. Left, 7-5/im unpolished coating right, polished with lime finishing compound. The average thickness removed by abrasian was 0-1 /im...

Tin is applied by hot-dipping or electrodeposition and has a similar corrosion behaviour to that of zinc. Coating thicknesses are usually in the range 12-50 tm, and in the lower portion of this range coating porosity can be a factor to be taken into account (see discussions by Kochergin and Gonser and Strader ). 

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. 

In view of the high cost, when tarnish resistance of the surface is the only requirement it is customary to use the thinnest possible coatings of rhodium (0-000 25-0-000 5 mm). Since rhodium deposits in this thickness range, like thin electrodeposits of other metals, show significant porosity, readily corrodible metals, e.g. steel, zinc-base alloys, etc. must be provided with an undercoating deposit, usually of silver or nickel, which is sufficiently thick to provide a fairly high level of protection to the basis metal even before the final precious metal deposit is applied, and, in this way, to prevent accelerated electrochemical corrosion at pores in the rhodium deposit. 

Smooth Pt and Rh electrodes have been compared [105, 269] with electrodepos-ited layers to investigate the effect of roughness (which may be of the order of 103). While electrodeposited Pt absorbs hydrogen and bright Pt does not, no surface area effect has been observed as for hydrogen evolution. This indicates that the internal surface (pores) of rough electrodes does not work because of exclusion due to gas formation. Thus, the porosity of the active layers also needs to be characterized. It has been shown that in the case of Raney Ni [270] this can be conveniently done by means of impedance measurements [271]. 

The pulsed electrodeposition technique (PED) is a versatile method for the preparation of nanostructured metals and alloys [47]. In the last two decades PED has received much attention worldwide because it allows the preparation of large bulk samples with high purity, low porosity and enhanced thermal stability. 

The necessary porosity for thicker layers was introduced by appropriate current densities [321-323], by co-deposition of composites with carbon black [28, 324] (cf. Fig. 27), by electrodeposition into carbon felt [28], and by fabrication of pellets from chemically synthesized PPy powders with added carbon black [325]. Practical capacities of 90-100 Ah/kg could be achieved in this way even for thicker layers. Self-discharge of PPy was low, as mentioned. However, in lithium cells with solid polymer electrolytes (PEO), high values were reported also [326]. This was attributed to reduction products at the negative electrode to yield a shuttle transport to the positive electrode. The kinetics of the doping/undoping process based on Eq. (59) is normally fast, but complications due to the combined insertion/release of both ions [327-330] or the presence of a large and a small anion [331] may arise. Techniques such as QMB/CV(Quartz Micro Balance/Cyclic Voltammetry) [331] or resistometry [332] have been employed to elucidate the various mechanisms. 

Continuous Pt films at the surface of porous silicon cannot be applicable as catalytic coats for fuel cells electrodes. Quite the contrary, Pt coats should save its porosity to allow an easy penetration of gaseous iuel and to have the effective surface area as high as possible. So, the layers of electrodeposited Pt of about 100 nm in thickness, as illustrated in Fig. Ic, are optimal catalytic Pt films for micro fuel cell electrodes. 

An important feature in electrodeposition is the efficiency of the cathodic process, i.e. the percentage of the cathodic current which is used to deposit the metal rather than produce hydrogen. A higher value is important in giving a better coating because hydrogen evolution causes porosity within the metal. 

Ultrasound not only influences the hardness of the electrodeposited material but can also greatly affect the porosity of the coating. This has been shown by work carried out by Chistyakov et al. [86] who studied the influence of ultrasound upon... 

The development of a well-ordered metal surface during electrodeposition is of considerable theoretical and practical importance since it affects the electrocatalytic processes that occur later on it. In this sense the porosity, the surface roughness and the compactness of the deposit define the formation of the oxide layers, especially, in the case of the non-noble metals. 

---