Effects of (NH4)2SO4 on the characteristics of the deposits and properties of an electroless Ni-P plating solution

The effects of ammonium sulfate (AS) in bath on the deposition rate, hypophosphite efficiency, bath stability, and characteristics of electroless nickel-phosphorous (EN) deposits were studied. The deposition rate of EN deposits was estimated by the gravimetric method. The results showed that the deposition rate of EN can be improved by the addition of AS when its concentration is lower than 12 g dm−3. Bath stability test was used to determine the stabilization effect of AS. The stability of the EN bath can be significantly decreased by a low concentration of AS, whereas a minimal influence on bath stability was observed with an increased concentration of AS. The variation in the efficiency of hypophosphite was also examined by traditional methods of chemical analysis. Characterization of the deposits was carried out by potentiodynamic polarization experiments, scanning electron microscopy, and energy dispersive X-ray, which can analyze the Ni and P content. The results indicated that a low accumulation of AS (lower than 12 g dm−3) in the plating process did not adversely affect the characteristics of the deposits. An optimum concentration of AS can decrease grain size, refine microstructure, and improve corrosion resistance. However, a very concentrated AS plating bath may result in an increase in stress of the coating, hence creating a cracked cross-section morphology.

Research highlights▶ Ammonium sulfate (AS) can be used as a rate controller on Mg AZ91 alloy in EN plating. ▶ The hypophosphite efficiency first decreases and then increases with an increase in AS concentration. ▶ A low accumulation of AS improves the corrosion resistance of the coating. ▶ A high sulfate buildup decreases the adhesion of the coating.