A comparative study of the physicochemical and electrochemical properties of Cr and Ni-W-P amorphous electrocoatings

A comparative study of the physicochemical and electrochemical properties of Cr and amorphous Ni-W-P electrocoatings is presented here. Amorphous Ni-W-P alloys were successfully produced by electrodeposition at 70 °C on copper substrate under galvanostatic control in the range of 50-400 mA cm⁻² and constant loads of 500 and 1600 C, using a solution containing 0.20 mol L⁻¹ Na₂WO₄.2H₂O; 0.02 mol L⁻¹ NiSO₄·6H₂O; 0.02 mol L⁻¹ NaPH₂O₂; 0.02 mol L⁻¹ H₃BO₃; 0.07 mol L⁻¹ (NH₄)₂SO₄; 0.20 mol L⁻¹ Na₃C₆H₅O₇·2H₂O; 0.0001 mol L⁻¹ CH₃(CH₂)₁₀·CH₂OSO₃Na. Cr electrocoatings were obtained from an industrial plating solution. The physicochemical characterization of the as-electrodeposited and as-annealed samples was carried out by scanning electron microscopy (SEM), X-ray diffraction (XRD), and energy dispersive X-ray analysis (EDX) techniques. Corrosion tests were carried out at room temperature in 10⁻¹ mol L⁻¹ NaCl aqueous solutions, using potentiodynamic linear polarization (PLP). Among the various Ni-W-P electrocoatings studied here, the Ni₆₅W₂₀P₁₅ layer presented the best corrosion behavior and a slightly superior corrosion potential than the Cr electrocoating. Heat treatments gave rise to a cracked surface morphology in the Cr layers, while the surface morphology of the Ni₆₅W₂₀P₁₅ layers remained homogeneous and devoid of cracks. Heat treatments at 400 and 600 °C led to crystallization of the Ni-W-P layer, with precipitation of the Ni₃P, Ni and Ni-W phases and increasing hardness of the Ni-W-P layer as the heat treatment temperature rose. All the annealed Cr layers showed cracked surfaces and their hardness diminished as the annealing temperature increased. The presence of cracks impairs the mechanical and corrosion resistance properties of Cr layers. Ni₆₅W₂₀P₁₅ layer is a potential candidate to replace Cr in industrial applications, mainly at operational temperatures that exceed room temperature.