Influence of target power density and substrate bias voltage on the electrochemical properties of type 304 SS films prepared by unbalanced magnetron sputtering

Abstract

Type 304 SS coatings were deposited at 200 °C onto AISI 1045 carbon steel substrate using unbalanced magnetron sputtering (UBMS) with an austenitic AISI 304 stainless steel (SS) target of 100 mm diameter. The deposition was performed at a total pressure in the active Ar gas of 2.7×10 −3 mbar and at various target power densities and bias voltages. The structure and the morphology were investigated by means of X-ray diffraction (XRD) and scanning electron microscopy (SEM). Corrosion properties of the coated specimens were examined using electrochemical polarization measurements and electrochemical impedance spectroscopy in a deaerated 3.5% NaCl solution. The porosity was obtained from a comparison of the d.c. polarization resistance of the uncoated and coated substrates. Scratch adhesion testing was used to compare the critical loads for different coatings. The sputtered films exhibit a ferritic b.c.c. α-phase. The corrosion performance of coatings could be controlled by coating porosity, penetration of the electrolyte, and coating adhesion as a function of depositional parameters. The corrosion current density decreased with the increase of the power density. The protective action of the coating should be related to the values of the capacitance and charge transfer resistance of the coating. The results of the adhesion test correlated well with the electrochemical data. Finally, an optimized deposition condition for corrosion protection was found at 40 W/cm 2 and −50 V.