Corrosion behaviour of magnetron sputtered α- and β-Ta coatings on AISI 4340 steel as a function of coating thickness

The corrosion behaviour of magnetron sputtered alpha- and beta-Ta coated AISI 4340 steels was studied with potentiodynamic polarization and electrochemical impedance spectroscopy. The coating porosity was observed to decrease with increasing coating thickness. For coatings less than 10 mu m thick (alpha- or beta-Ta), porosity was significant and open pores resulted in severe localized corrosion of the steel substrate, coating delamination, and overall coating failure. Additionally, the beta-Ta coatings were more susceptible than the alpha-phase to delamination. As for the 50 and 100 Pm thick alpha-Ta coatings, the electrochemical impedance behaviour was comparable to that of Ta foil, demonstrating the coating viability and corrosion resistance. (c) 2005 Elsevier Ltd. All rights reserved.