International Journal of Science and Technology Volume 3 No. 11, November, 2014 IJST © 2014– IJST Publications UK. All rights reserved. 728 The electrochemical characteristics of an ultra–fine grain Ȗ / -Ni3Al coating in 3.5% NaCl solution Onyeachu B.I, Oguzie E.E, Njoku D.I, Ukaga I Electrochemistry and Materials Science Research Laboratory Department of Chemistry, Federal University of Technology Owerri, PMB 1526, Owerri, Nigeria ABSTRACT The study of the electrochemical characteristics of an ultrafine–grain γ / –Ni3Al coating was undertaken, after exposure for 72 h in 3.5% NaCl solution, using open circuit potential, electrochemical impedance spectroscopy and potentiodynamic polarization techniques. The ultrafine–grain γ / –Ni3Al was fabricated by annealing an electrodeposition of a Ni –Al composite coating at 600 o C for 1 h. Compared with an arc–melted γ / –Ni3Al alloy; the ultrafine–grain γ / –Ni3Al exhibited more positive corrosion potential with lower corrosion current density. This was attributed to the highly refined microstructure of the ultrafine –grain γ / –Ni3Al which promoted a lower transient time for the formation an Al2O3– enriched corrosion product layer. Keywords: Grain refinement, γ / –Ni3Al, EIS, Al2O3, corrosion resistance 1. INTRODUCTION One of the most effective means of improving the general material characteristics of many metals and alloys is by refining their surface microstructure; such as reducing the grain sizes down to the micron or submicron sizes to form ultrafine– grain (UFG) metals and alloys. The UFG metals and alloys usually possess better strength because of the abundance of grain boundary atoms and triple junctions [1]. Understanding how such surface modification can influence the chemical stability of these metals and alloys would go a long way to determine their technological application. The corrosion resistance of many metals and alloys has been greatly increased in wet corrosion environments through grain size reduction [2– 5] because the UFG materials provide abundance of nucleation sites and high particle–particle proximity for the effective formation of a highly continuous, adherent and protective corrosion product layer, compared with their polycrystalline coarse grain counterparts. The conventional γ / –Ni3Al intermetallic is well–known for its high melting point, low density and is an excellent candidate for high temperature application due to its ability to form a mature Al2O3 scale [6–10]. Compared with the coarse grain counterpart, UFG γ / –Ni3Al possesses higher resistance to oxidation and better scaling mechanism in dry corrosion environments at high temperature because of a lower transient time for the formation of the protective and highly adherent Al2O3 [10, 11]. Electrodeposition of Ni–Al composites followed by subsequent heat treatment in vacuum leads to the formation of the UFG γ / –Ni3Al as coatings on desired metal substrates [10]. The content and distribution of the Al particles in the Ni–Al composite and the annealing treatments like temperature and time are important factors which control the phase transformation of the pure Ni grains and Al particles into the γ / –Ni3Al phase [11]. UFG γ / –Ni3Al coating was obtained after annealing a Ni–Al composite with 20 vol. % of Al particles at 825 o C for 3 h [10], and a Ni–28wt. %Al composite at 600 o C for 2 h [11]. In a wet corrosion environment, it is envisaged that the electrochemical corrosion of the γ / –Ni3Al could be improved through such grain size refinement, provided a lower transient time for the enrichment of the corrosion product layer with Al2O3 can be guaranteed in aqueous solutions. Unfortunately, such studies have not been given adequate attention. In the present work, we electrodeposited a Ni–37wt.%Al composite on Ni substrate, and subjected it to vacuum annealing treatment at 600 o C for 1 h. Thereafter we employed electrochemical methods to characterize the corrosion behaviour of the resultant UFG γ / –Ni3Al after 72 h exposure in 3.5% NaCl solution, compared with a pure arc–melted γ / –Ni3Al alloy fabricated by arc melting. We hope that the study will encourage the application of UFG γ / –Ni3Al as coating for low temperature applications. 2. EXPERIMENTAL 2.1. Materials Preparation The γ / –Ni3Al alloy coupons had dimensions 12 X 10 X 2 mm 3 . The coupons were grinded to a final 800 grit size with SiC paper, washed with distilled water and ultrasonically cleaned in acetone. Electrodeposition of the Ni–Al composite was performed at 2 A/dm 2 from a Ni–sulphate bath containing 150 g/L NiSO4.6H2O, 0.1 g/L Sodium Dodecyl Sulphate, 15 g/L NH4Cl, 15 g/L H3BO3 and loaded with 300 g/L of 1μm size Al particles. Electrodeposition was performed at 30 o C for 2 h, using a reciprocating perforated plastic as agitator at 200 rpm speed. During the electrodeposition, the cathodes were pure Ni coupons with dimension 12 X 10 X 2 mm 3 . The average Al content in the Ni–Al composite was 37wt.%, according to EDAX surface analysis. The annealing treatment of the Ni–Al composite was performed under a pressure of 10 –5 Pa at 600 o C for 1 h. The phase characterization of the arc–melted γ / –Ni3Al alloy and Ni–37wt.%Al composite before and after annealing treatment was performed using XRD.