To ensure the chemical reduction of metal ions in solution, the solution needs to contain a sufficiently strong and active reducing agent; that is, it needs to have a sufficiently negative redox potential. The easier the metal ion is reduced, the greater the amount of available reducing agent. Since only the autocatalytic reduction reaction can be successfully used for the deposition of the coating, the amount of electroless plating Me-Red (metal reductant) system is not well suited to practice (see Table 28.1).
Table 28.1 Coatings obtained by electroless plating
Currently known electroless plating methods can be used to deposit 12 different metals, including metals belonging to the iron, copper, and platinum groups (various well-known catalyst reactions) as well as tin and lead (only one has been published for depositing the latter s solution). Although the deposition of chromium and cadmium coatings is described in the patent literature, no autocatalytic reduction is achieved in these cases. Coatings are deposited on some metals by dipping only electroplating.
In some widely used processes, the deposition of metals is accompanied by the precipitation of reducing agents. Reagent decomposition products - phosphorus and boron - etc., to obtain the corresponding alloy. Depositing two or more metals at once is not difficult; electroless plating methods are known to deposit over 50 alloys of different mass compositions, mainly based on nickel, cobalt, and copper.
Most reducing agents used in electroless plating are hydrogen compounds, where H is associated with phosphorus, nitrogen, and carbon. It is in the reactions of these compounds that significant catalytic effects are possible, since these reactions proceed very slowly in the absence of catalysts.
Effective autocatalysis was obtained when a stronger reducing agent - hypophosphite - was used.
In the absence of a catalyst, the reducing agent is inert and does not react even with strong oxidizing agents;
Only a few catalysts are suitable for it (for example, Ni, Co, and Pd), but they provide the highest rate of catalytic process without reducing the volume of the solution. Other reducers are more versatile, for example by using borohydride we can deposit almost all metal coatings mentioned. The reducing power of hydrogen-containing compounds increases with the pH of the solution. For this reason, most electroless plating solutions are alkaline.
Simple reducing agents such as variable valence metal ions (Fe2+, Cr2+ and Ti3+) are generally not suitable for the deposition of coatings because non-catalytic reduction occurs fairly easily. Recently, conditions for the autocatalytic deposition of tin and silver coatings using reducing agents and isometallic complexes have been established . Deposition of some metals (Ag, Au, Cu) by chemical reduction techniques has been known for a long time as in the 19th century, but it became popular after Brenner discovered (1945) a very effective electroless nickel plating using hypophosphite The electroplating process. It was then that the term "electroless plating" was coined.
