K. Ravichandran

Deposition of zinc-zinc phosphate composite coatings on steel by cathodic electrochemical treatment

The present work aims at the development of an energy-efficient and eco-friendly approach for the deposition of zinc phosphate coatings on steel. The study describes the possibility of preparing zinc–zinc phosphate composite coatings by cathodic electrochemical treatment using dilute phosphoric acid as an electrolyte and zinc as an anode. The methodology enables the preparation of coatings with different proportions of zinc and zinc phosphate by suitably varying the applied current density, pH, and treatment time. Adhesion of the coating on mild steel and adhesion of paint film on the phosphate coating were found to be good. The surface morphology of the coatings exhibited platelet-type features and small white crystals (agglomerated at some places) which represented zinc and zinc phosphate, respectively. An increase in current density (from 20 to 50xa0mA/cm2) increased the size of the zinc crystals, and coatings prepared at 40 and 50xa0mA/cm2 resembled that of electrodeposited zinc. Since the proportions of zinc and zinc phosphate could be varied with applied current density, pH, and treatment time, it would be possible to use this methodology to prepare coatings that would offer different degrees of corrosion protection.

A facile electrochemical approach for the deposition of iron–manganese phosphate composite coatings on aluminium

A novel electrochemical approach to deposit an iron–manganese phosphate composite coating on aluminium, which would otherwise be difficult to achieve by a conventional chemical conversion coating method, is addressed. An electrolyte solution containing manganese and phosphate ions and, a steel anode, were used to deposit the iron–manganese phosphate composite coating on the aluminium cathode. During the electrochemical treatment, besides the aluminium cathode, coating deposition was also observed on the steel anode. The resultant coatings were characterized by Fourier-transform infrared spectroscopy, Raman spectroscopy, scanning electron microscopy, energy dispersive X-ray analysis, X-ray photoelectron spectroscopy and X-ray diffraction measurements to ascertain the type of functional groups, morphological features, chemical composition and phase constituents, respectively. The surface coverage of the coatings was estimated by immersion plating of copper. The corrosion resistance of the iron–manganese phosphate composite coated aluminium, in 3.5% NaCl, was evaluated by potentiodynamic polarization, current–time transient and electrochemical impedance spectroscopy studies. Optical micrographs were also acquired after corrosion testing to understand the corrosion mechanism. The novel electrochemical approach addressed in this study provides a new avenue to deposit an iron–manganese phosphate composite coating on aluminium that offers good corrosion protection for aluminium in 3.5% NaCl.