Abstract:

Propionic acid as an additive in electroless nickel plating will accelerate chemical deposition and raise P contents in the deposits.  In this work, the action mechanism of propionic acid in the electroless plating were studied by cyclic voltammetry and infrared reflection spectroscopy.  The voltammetric curves at various concentrations of propionic acid indicate that propionic acid will promote both cathodic Ni^2＋ reduction and anodic NaH₂PO₂ oxidation.  According to the differences between the infrared reflection spectra of adsorbed propionic acid on Ni substrates and that of the adsorbed species when propionic acid co-exists with NaH₂PO₂ or NiSO₄, it was proposed that the acceleration of chemical deposition was attributed to the formation of surface complexes of  propionic acid with NaH₂PO₂ and NiSO₄,respectively.  Propionic acid could form intermolecular hydrogen bonds with NaH₂PO₂, which promotes splitting of P-H bond in the reductant and producing  PHO^-₂ intermediates, hence facilitates the oxidation of NaH₂PO₂.  At the same time, propionic acid is coordinated with Ni^2＋ via   its  —OCO— group to form bridge complexes so as to promote NiSO4 reduction.  The accelerating oxidation of NaH₂PO₂ in turn helps the deposition of P, as a result, P contents in the deposits increase with accelerating NaH₂PO₂ oxidation. 

Key words: Propionic acid; Organic additive; Electroless nickel plating;Deposition rate