Electrochemical impedance spectroscopy and indentation studies of pure and composite electroless Ni–P coatings

• Published in: Surface & coatings technology Vol. 236; pp. 262 - 268
• Main Authors: Islam, Mohammad, Azhar, Muhammad Rizwan, Fredj, Narjes, Burleigh, T. David
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier B.V 15.12.2013; Elsevier
• Subjects: Aluminum oxide; Applied sciences; Composite coatings; Corrosion resistance; Cross-disciplinary physics: materials science; rheology; EIS; Electrochemical impedance spectroscopy; Electroless coatings; Electroless nickel; Exact sciences and technology; Materials science; Metallic coatings; Metals. Metallurgy; Microhardness; Nanostructures; Nickel; Physics; Production techniques; Protective coatings; Silicon carbide; Surface treatment; Surface treatments; Electroless nickel; Nanostructures; Composite coatings; Microhardness; EIS; Nickel base alloys; Surface treatments; Indentation; Metal coating; Composite materials; Composite coating; Chemical deposition; Nickel; Impedance
• ISSN: 0257-8972; 1879-3347
• DOI: 10.1016/j.surfcoat.2013.09.057

Electroless Ni–P coatings offer excellent corrosion and wear resistance and ability to withstand acidic and salt solutions. Medium and high phosphorus Ni–P coatings were produced using plating baths with 10g/L or 25g/L of sodium hypophosphite as reducing agent (RA) with composition of the resulting deposits to be 91.5 Ni: 8.5 P and 87.6 Ni: 12.4 P, respectively. From field-emission scanning electron microscope (FE-SEM) examination, the deposit morphology was found to change from nodular with surface porosity and cracks to dense, smooth upon increasing the RA content. Addition of nanostructures such as nanoparticles of alumina (Al2O3) or silicon carbide (SiC) or multi-walled carbon nanotubes (CNT) into Ni–P matrix, at low loading levels, was investigated for their effect on corrosion resistance and hardness of Ni–P–Al2O3, Ni–P–SiC and Ni–P–CNT composite coatings. Electrochemical impedance spectroscopy (EIS) studies in 4wt.% NaCl solution revealed 91.5 Ni: 8.5 P coating to offer much superior corrosion resistance than 87.6 Ni: 12.4 P coating even after immersion for 42days. Among all composite coatings, however, Ni–P–Al2O3 produced from 1.0g/L Al2O3 in plating solution exhibits higher impedance values at low and intermediate frequencies. Nyquist plots for different frequencies were analyzed for comparison between different composite coatings. Microhardness tests indicate higher hardness value of 8.46GPa for Ni–P–SiC coating as compared to 7.42GPa for pure Ni–P coating. •91.5 Ni:8.5 P coatings offer impedance and hardness values of 1.8×105Ω·cm2 and 7.42GPa.•Ni–P based composite coatings were investigated with low loading of Al2O3, SiC or CNT.•EIS studies show better corrosion resistance in 4wt.% NaCl for pure Ni–P coatings.•Composite coatings with Al2O3 nanoparticles or CNT offer better corrosion properties.