Structural and morphological investigations of the electrodeposited Cr and Ni-Cr-P coatings and their electrochemical behaviors in chloride aqueous medium

Pedro De Lima-Neto,Adriana N Correia,Gecílio P Da Silva

doi:10.1590/s0103-50532006000700032

Abstract

Structural and morphological characteristics of the electrodeposited Cr and Ni-Cr-P coatings and their corresponding electrochemical behaviors in chloride aqueous medium were investigated here. All coatings were electrodeposited on copper. Cr coatings were obtained from an industrial plating solution and amorphous Ni-Cr-P coatings were successfully obtained at 60ºC in the range of 100 to 400 mA cm-2, using a plating solution at pH 2 containing 40 g L-1 NiCl2.6H2O; 102 g L-1 CrCl3; 14 g L-1 NaPH2O2; 30 g L-1 H3BO3; 15 g L-1 NaBr; 50 g L-1 NH4Cl, 80 g L-1 Na3C6H5O7 .2H2O and 40 mL L-1 HCOOH. The coatings were obtained using constant charges of 500 and 1600 Coulombs. The characterization of the coatings was carried out by Scanning Electron Microscopy (SEM), X-ray Diffraction (XRD), Differential Scanning Calorimetry (DSC) and Energy Dispersive X-ray Analysis (EDX) techniques. The electrochemical behavior of both coatings was evaluated by potentiodynamic polarization curves, at room temperature, in 0.1 mol L-1 NaCl aqueous solution. It was not observed cracks on the electrodeposited Ni-Cr-P coatings and the surface morphology was characterized by the presence of spherical nodules. The crystallization of these coatings occurred at 325 °C with the formation of Ni and Ni3P phases. All the annealed Cr coatings showed cracked surfaces. The presence of cracks impairs the mechanical and corrosion resistance properties of Cr coatings. The hardness of the Ni-Cr-P was increased with the increase of the annealing temperature. Spherical noodles were absent in the surface of as-annealed Ni-Cr-P coating which it was associated with the diffusion of Cr to the surface and cracks were not observed in any annealing temperature. Among the various electrodeposited Ni-Cr-P coatings studied here, the Ni66Cr12P22 coating presented the best corrosion behavior and it is a potential candidate to substitute Cr in industrial application, mainly at operational temperatures that exceed room temperature.

Highlights

Electrodeposited chromium coating is widely used as both decorative and functional coating because it presents good properties such as high hardness, good wear resistance, low coefficient of friction and excellent corrosion resistance, when used for industrial applications at room temperature
Its industrial application has been limited by public health and environmental laws because conventional industrial Cr galvanization process requires the use of carcinogenic Cr6+ ions in the chromium plating bath, and because the hardness and corrosion resistance of this coating decrease when the operating temperature exceeds the room temperature.[1]
Possible explanations for the low cathodic efficiency are: the rising in the hydrogen evolution with the increase of the current density, the formation of metal oxides due to the rising of the pH metal-solution interface during the Ni-Cr-P and Cr electrodeposition process, and the presence of organic ligands, such as citrate and formate, which act as complexing agents of both nickel and chromium ions, forming stable metal complexes that hinder the alloys deposition

Summary

Introduction

Electrodeposited chromium coating is widely used as both decorative and functional coating because it presents good properties such as high hardness, good wear resistance, low coefficient of friction and excellent corrosion resistance, when used for industrial applications at room temperature.

Results

Conclusion