Electroless cobalt coating of BNi-2 powders: process development and characterization

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Göynük T., KARAKAYA İ.

Journal of Electrochemical Science and Engineering, vol.16, 2026 (ESCI, Scopus)

Abstract

Electroless cobalt deposition and examination of how cobalt enrichment on the particle surface modifies the chemical composition and thermal behaviour of the commercial BNi-2 brazing alloy powder was investigated in this study. A multi-step surface preparation sequence involving oxide removal, Sn-based sensitization, and Pd activation was employed prior to cobalt deposition in an alkaline CoSO₄-NaH₂PO₂ bath. A design of experiments approach was used to evaluate the influence of plating temperature, cobalt sulphate concentration, hypophosphite concentration, and coating time on cobalt uptake. ICP-OES analyses showed that cobalt incorporation increased from 1.1 to 7.0 wt.% across the selected parameter range. Differential scanning calorimetry revealed that coating levels up to 7 wt.% Co elevate the solidus temperature, while decreasing the liquidus temperature, resulting in a narrower melting interval. Thermodynamic simulations confirmed these trends, showing suppression of borides and enhanced γ/γ′ stability where γ denotes the Ni-rich face-cantered cubic matrix phase and γ′ represents the ordered Ni₃(Al,Ti)-type strengthening phase with increasing cobalt content. EDS mapping of coated powder showed continuous cobalt distribution at the particle perimeter. Overall, the results provided a quantitative basis for linking coating parameters to cobalt incorporation and corresponding modifications in melting behaviour.