Improving the structural properties and corrosion behaviour of electroless deposited Ni-P-Zn coatings on mild steel for advanced processes

Hydrated crystal of ZnS[O.sub.4] * 7[H.sub.2]O was co-deposited with NiS[O.sub.4] * 6[H.sub.2]O, and Na[H.sub.2]P[O.sub.2] * [H.sub.2]O processed electrolyte to develop a ternary phase electroless coating on mild steel for advanced application. The coating was produced in an aqueous nickel electrolyte solution with zinc at a temperature of 90[degrees]C and varying time conditions between 20 and 50 min. The effects of the developed coatings on microstructure and physical properties were investigated using scanning electron microscopy (SEM), Energy dispersive spectroscopy (EDS), and optical microscopy. The corrosion characteristics of the coatings examined in 0.5 M [H.sub.2]S[O.sub.4] and 3.5% NaCl were analyzed using linear potentiodynamic polarization technique. The microstructure revealed that the constituent deposited on the steel. A more uniformly distributed crystallite with minimal pores was observed at 20 min of Ni-P-Zn coating, while the tiniest film was observed at 50 min of the coating without crack. There was also a fully compact grain within the intermetallic matrix of the coating due to the synergetic effect of the electrolyte constituent with the strengthening phase of [Zn.sub.7][Ni.sub.4][P.sub.2], [Zn.sub.2][N.sub.i5]P, and [Zn.sub.3]Ni. Corrosion study established that Ni-P-Zn exhibits significant corrosion resistance at the optimum time within the examined environments with little S[O.sub.4.sup.2-] and [Cl.sup.-] ions. Keywords: microstructure; electroless; coating; corrosion; application