Effect of Cr(III)+Ni(II) solution chemistry on electrodeposition of CrNi alloys from aqueous oxalate and glycine baths

Abstract

CrNi alloy coatings with various concentrations of Cr and Ni were deposited from trivalent chromium plating baths containing bivalent nickel and glycine or oxalate as a complexing agent. The deposition rate, composition and morphology of CrNi alloy coatings and chemistry of the solutions employed have been studied. It has been found that the substrate surface initially was coated with a layer of Ni, which facilitates the underpotential nucleation of Cr on Ni giving an alloy. The formation of a Ni rich alloy starts at a less negative potential in the oxalate bath as compared to that in the glycine bath. The CrNi alloy coatings are characterised by nodular structures with microcracks and the coatings deposited from the glycine bath have pores and cavities, whereas no pores were detected in the deposits from the oxalate bath. The solution chemistry substantially affects the formation of CrNi alloy. The formation of the Ni(II)–Cr(III) ring structure capable of oligomerisation most probably impedes the electrodeposition making it impossible to obtain a thick CrNi alloy coating in the glycine bath. However, in the oxalate bath, no noticeable effect of Ni(II) ions on the structure of oxalic complexes was revealed. Moreover, active [(H₂O)_nCr(C₂O₄)]⁺ and [(H₂O)_nCr₂(C₂O₄)]⁴⁺ complexes may be restored which allow the prolonging of the lifetime of the oxalate bath.

Keywords:

• Trivalent chromium
• Oxalate
• Glycine
• Cr–Ni alloy
• Electrodeposition