Black Layers on Aluminum Substrate – Evaluation of Corrosion Behavior

Abstract

The paper presents some preliminary experimental results dealing with electrochemical synthesis and corrosion behavior of various black layers types onto Al substrates, suitable to be used as selective absorber coatings in the case of thermal energy production from solar energy, involving several different procedures, respectively: (i) electrodeposition of black Ni-Mo alloy after an initial anodizing in nitric acid electrolyte that offers a very good adherence of the further applied coatings; (ii) cathodic deposition of black Mo based layers after an initial double zincating procedure; (iii) black coatings through a.c. electrochemical coloring of aluminum anodic oxides, using AgNO3/H2SO4, NiSO4 based solutions; (iv) black composite polypyrrole films. The obtained black layers are characterized by a good throwing power, homogeneity, smoothness and adherence, as shown by SEM and AFM investigations. Also, they present a good solar absorption and low values of infrared emittance. To evaluate the resistance of black coating layers against corrosion, several accelerated corrosion tests have been performed, respectively: (i) continuous immersion in 0.5M NaCl for 408 hours with intermediary visual examinations and recording of corrosion potential; (ii) potentiodynamic polarization curves in 0.5M NaCl; (iii) impedance spectra in 0.5M NaCl at open circuit potential using a Zahner IM6e potentiostatic equipment. The corrosion performances are discussed taking into account the applied preparation procedures.