Electrochemical Evaluation for Defects in TiN Films Coated on SUS 304 Stainless Steel.

The amount of defects in TiN films coated on SUS304 stainless steel has been evaluated by three electrochemical measuring techniques in a sulfuric acid solution containing potassium thiocyanate. Three different TiN films were prepared by a hollow cathode ion plating technique. A critical passivation current density (icrit) can readily provide the apparent amount of the defects. However, this method was found to overestimate the amount of the defects because active dissolution of the substrate at the measurement caused enlargement of the defects. An activation time, which is a period until free corrosion potential (Ecorr) descends to that of the bare substrate steel, was not useful to evaluate the amount of defects rapidly because about a half the specimens did not activate within a test time of 90 ks. More precise and less destructive evaluation could be achieved by using a reciprocal of polarization resistance (RP-1) which was obtained by an electrochemical impedance spectroscopy immediately after activation. It is concluded that icrit is a suitable parameter for an approximate estimation of the defect area, and RP-1 amount of the original defects.