Effect of Phenyl Thiourea as a Stabilizer on Electrochemical Properties of Electroless Ni-P Coatings on 6063 Aluminum Alloy

Abstract

Polarization curves and AC impedance spectroscopy were used to investigate the effects of different concentrations of stabilizer phenyl thiourea(PHTU) on the electrochemical properties of electroless Ni-P coatings on a 6063 aluminum alloy.The polarization curves show that the coatings formed in phenyl thioureacontaining electroless nickel solution have a more positive corrosion potential(Ecorr) and pitting corrosion potential(Epit),and a lower corrosion current density(icorr) than the bare aluminum alloy.To explain the electrochemical properties of the coatings,a simple equivalent circuit is designed.Electrochemical parameters of EIS,such as the surface resistance(Rcoat) and capacitance(Qcoat),charge transfer resistance(Rct) and double layer capacitance(Qdl),have been obtained by fitting the EIS plots.The EIS results show that the coatings formed in 6 ～ 10 mg/L PHTU electroless nickel solution present a higher resistance(Rcoat and Rct) and lower capacitance(Qcoat and Qdl) values.The EIS and polarization curve results indicate that the prepared coatings have a better corrosion resistance and the EIS and polarization curve results are in agreement with each other.The morphology and composition of the Ni-P coatings have been examined by using a SEM and EDS,respectively.The results show the coating surface is in a uniform state and the mass fraction of P element on the coating surface is more than 10%.