Abstract
Electrodeposition, although clearly an old technology in its basic operating principles enunciated by Michael Faraday, remains a major process in worldwide coating technology taking a leading place in the spectrum of process choice. Despite being an old technology, however, it is still susceptible to improvement opportunities and several areas can be considered as active at the present time. While the intrinsic character of electrodeposition is its relative cheapness based as it is on water as a solvent and air as an agitation medium, other solvents such as fused salts and organics have been explored but are not the most desirable for fast efficient processing but offer some special features for noble metals which are difficult to dissolve and active metals which are difficult to cathodically discharge. The term Ionic Liquids, although a misnomer to linguistic purists, has become commonly used to describe the novel organic solvents being developed with some considerable promise for specialised applications.1 Three areas of focused advancement can be identified for discussion.
D. Gabe
Among the main process parameters of metal concentration C, current I, time t and temperature T, the optima have long been established although the search for novel salts based obscure complexants continues. At the present time, the main opportunity for advancement seems to lie with agitation as a means of maximising forced convection near the cathode surface. For over 100 years, agitation has depended upon the use of air as a bubbling agent, air still having the advantage of being cost free but disadvantageously oxidative especially towards transition metal ions and organic additives. On this basis, solution jetting was developed as a means of focusing the convection at the jet target thus enabling, for example, spot coating to be achieved especially valuable for precious metal electrical contacts. However, less well focused jets can enhance convection over larger surfaces, cavities, etc.; such jets have the generic term Eductors and are based on the old idea of Giovanni Venturi in about 1780. 2 2,3 Two bonuses are immediately apparent: first that plating additive usage was reduced by virtue of the absence of chemical oxidation although anodic electrochemical oxidation at anodes, especially catalytic type anodes, can still occur. Second, corrosive fume containing acid was substantially eliminated thereby reducing environmental corrosion and pollution. This last characteristic may turn out to be the most critical with the additional bonus of eliminating air bubbles from the solution which as an insulative second fluid phase can occupy 20–30% volume space representing 20–30% increase in electrical power consumption for the solution.
Pulsed current deposition has long been known and generally understood but its exploitation still depends on optimisation for particular solutions and types of deposit. It is likely to be of value for a few specialist metals and coatings.
Improving the product has several aspects. The oldest is probably corrosion and wear resistance for which alloys have been sought for some years, both solid solution and two phased for particular applications. Physical property improvement of strength and hardness has been approached by producing metals and alloys having very small grain size or even amorphous usually by the use of pulsed current. An alternative approach is to use the time honoured concepts of dislocation theory to develop structures having dislocation barriers of two dimensions rather like wave guides. Thus, multilayering was conceived which at the nanoscale allows dramatically enhanced physical properties to be developed through ‘designer coatings’. 4 4,5 At the microscale, they have long been known to improve corrosion resistance but at the nanoscale, mechanical, electrical and magnetic properties are much increased the last named to feed the insatiable demands of electronic memories and information storage for computers, mobile phones and music. The competition here is largely between physical vapour deposition, electroless and electrodeposition technologies.
Ten years ago, the author discussed certain challenges for the first decade of the millennium and highlighted the possible elimination of toxic metals such as cadmium and reconsideration of certain toxic solutions such as cyanide and chromate.6 Cadmium has substantially disappeared from use except for a few critical applications while cyanide is still in use but a much better understanding of its treatment is common and chromate is likely to suffer a political not scientific fate through its poor management.
Environmental considerations have long been important besides toxicity but are now dominant thinking. Impact with the environment comes in several process stages each with its own characteristic features. In the pretreatment stages of cleaning and pickling, a number of issues are important. Cleaning is usually an alkaline process and a pickling acid process; in principle their waste products can neutralise each other but this is rarely possible except in general terms because the presence of metal complexants in solution is widespread practice as a means of lengthening the effective life of the solutions. Unfortunately, they also make conventional effluent treatment more difficult preventing, for example, effective precipitation of metal hydroxides. Choice of complexant may ease the situation and alternative precipitants may be possible but the process is still with some risk. Organic solvent cleaning has long been practiced but CFC cleaners remain a problem even though good choice should eliminate ozone layer depletion. Newer solvents such as n-propyl bromide have received considerable publicity but are not yet fully proven through long term use as risk free chemicals with ‘ozone friendliness’, the very laudable aim.
Effluent treatments have traditionally involved precipitation and recycling as process feedstock. But recovery is not always feasible without resort to processes which are deemed ‘difficult’ to operate; e.g. ion exchange, solvent extraction, etc. It is probable that specialist effluent treatment should be offered more widely as a service to manufacturers with suitable incentives introduced. The importance of not mixing different effluent streams, in this context, is a lesson that has not been fully learnt.
The pollution mode of atmospheric fume emission has a number of aspects but for electrodeposition and its associated stages is generally attributable to evaporation of hot solutions and entrainment of solution chemicals by air used for agitation. As already mentioned, the elimination of air agitation in favour of eductor jetting was driven originally by the need to speed up processes. However, its rapid adoption by industry in the last 5 years can reasonably be attributed to the bonus of fume minimisation.
Protection of the environment is also about careful use of resources, two of which have been of special concern – energy and water. Energy in electroplating involves a number of aspects among which heating has long been a problem. In general, hotter process solutions operate faster but above ∼65°C evaporation can be a serious issue. Consequently, the development of solutions capable of being efficient at lower temperatures has been an important aim although the greatest successes have been for associated processes such as phosphating and anodising sealing where chemical energy is in effect replacing thermal energy. Water recycling has been a great success where it is often a byproduct of effective effluent treatment – up to 80% recovery is now quite common. This has, of course, the double advantage of reducing costs of both water supply and effluent discharge.
The rivalry between various coating techniques is a long standing tradition but is one that is rapidly disappearing. The need to train staff in chemical, physical and mechanical manufacturing processes is now fully recognised such that the techniques are now complementary not competitive. The combining of conversion coating to passivate electrodeposits has long been standard practice; combining physical vapour deposition with electrodeposition or electrodeposition with thermal diffusion treatments must now be considered as a standard means of achieving superior engineered surfaces which are designed for specific mixes of properties and service life. Such approaches are widely adopted and the integration of the technologies is essential for future engineering design of technological surfaces. The use of these techniques for product forming, especially microelectronic components and integrated circuits has already become a part of standard manufacturing technology.
In coating technology, the drive towards cheaper or thinner coatings to obtain superior performance has long been a driving force; this means improved coating integrity and continuity, the use of current modulation being commonplace. But the use of lasers and plasma is making a new spectrum of coating materials feasible, both for localised coating and localised high temperatures enabling high melting point coatings to be deposited on substrates of much lower melting point themselves. The sky is now the limit!