Resistance of Plasma-deposited a -C: H/fluorocarbon Films to Anodic Breakdown in Aqueous Electrolytes

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Resistance of plasma-deposited a-C : Hyfluorocarbon films to anodic breakdown in aqueous electrolytes C. Srividya Center For Advanced Materials Processing and Department of Chemical Engineering, Clarkson University, Potsdam, New York 13699

M. Sunkara Chemical Engineering Department, University of Louisville, Louisville, Kentucky 40292

S. V. Babu Center For Advanced Materials Processing and Department of Chemical Engineering, Clarkson University, Potsdam, New York 13699 (Received 5 September 1996; accepted 26 March 1997)

The corrosion characteristics of a-C : Hyfluorocarbon composite films deposited on type 301 stainless steel substrates were investigated using potentiometry and electrochemical impedance spectroscopy. The films were deposited by radio frequency (13.56 MHz) plasma deposition from different mixtures of hexafluoroethane, acetylene, and argon. A 10 nm thick polysilicon film was plasma-deposited prior to a-C : H film deposition to improve adhesion. The anodic current densities recorded with all the coated samples, in an electrolyte consisting of 0.1 M NaCl and 0.1 M Na2 SO4 at 1.5 V (Standard Calomel Electrode), were at least three orders of magnitude less than that for a bare steel sample. They were also at least 20 times less than that obtained with diamond-like carbon film-coated substrates. EIS spectra obtained for these samples, while exposed to 0.6 M NaCl solution over an extended period of time (30–40 days), confirm their barrier properties. All the coatings showed near capacitive behavior in the frequency range 10 mHz–4 KHz.

I. INTRODUCTION

Several studies have been carried out on the protective coatings used for enhancing the corrosion resistance of stainless steel surfaces.1 These coatings include noble metal films, sacrificial coatings of metals such as zinc, organic coatings of epoxies, and ceramic coatings. Recently, diamond-like carbon (DLC) films were proposed as promising anticorrosion coatings on aluminum2 and stainless steel.3,4 In our previous work,2 it was observed that the anodic current density, measured under potentiostatic control,5 was reduced by three orders of magnitude when Al surfaces were covered with DLC films. This work suggested that the DLC films offered much better resistance to anodic breakdown in aqueous electrolytes containing 0.1 M NaCl and 0.1 M Na2 SO4 when compared to polyimide films. In this communication, we show that in the same electrolyte, some radiofrequency (rf) plasma-deposited fluorocarbon films4 with surface energies of about 20 mNym offer superior resistance to anodic breakdown when compared to the DLC films. The characteristics of these fluorocarbon films have been investigated here using potentiostatic measurements and electrochemical impedance spectroscopy (EIS).6,7 Various parameters in the impedance spectrum can often be used for characterizing and ranking the coatings in J. Mater. Res., Vol. 12, No. 8, Aug 1997

terms of their performance for corrosion protection.8–10 Electrochemical impedance spe

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