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.