Growth Modes of Electrodeposited Cobalt

Structure, morphology and electrokinetics in cobalt electrocrystallisation from sulphamate electrolytes, either without or with addition of boric acid, are studied and an interpretation of the structure characteristics and growth modes of electrodeposited cobalt is proposed. The correlation between crystallographic structure and electrokinetic behaviour, analysed by short galvanostatic pulses over-imposed during steady-state growth at fixed current density, is investigated, pointing out the main factors which affect structure and texture of electrodeposits and emphasising the electrolyte pH influence. Three well-defined growth modes are recognised and univocally related to the transient electrokinetic parameters. Growth structures characterised by low crystallinity and/or texture modifications occur in relation with the disturbance of the hydrogen discharge reaction or as a result of ageing of not-buffered electrolyte. The basic growth modes are classified as follows: outgrowth or perpendicular growth, in condition of surface stabilisation of hydrolysed species resulting in low nucleation activity and growth of relatively large-grained columnar deposits; lateral growth, when hydrolysed species are stable in the bulk solution and cellular crystallisation occurs as a consequence of full surface coverage by discharge intermediates and precipitation of insoluble hydrolysis products at grain boundaries; cluster growth, in the presence of boric acid, in conditions of nucleation control, possibly related to the formation of adsorbed complexes stabilised at the surface by boric acid.