ELECTROCHEMICAL EVALUATION FOR DEFECTS IN TIN FILMS COATED ON SUS-304STAINLESS-STEEL

The amount of defects in TiN films coated on SUS 304 stainless steel h as been evaluated by three electrochemical measuring techniques in a s ulfuric acid solution containing potassium thiocyanate. Three differen t TiN films were prepared by a hollow cathode ion plating technique. A critical passivation current density (i(crit)) can readily provide th e apparent amount of the defects. However, this method was found to ov erestimate the amount of the defects because active dissolution of the substrate at the measurement caused enlargement of the defects. An ac tivation time, which is a period until free corrosion potential (E(cor r)) descends to that of the bare substrate steel, was not useful to ev aluate the amount of defects rapid ly because about a half the specime ns did not activate within a test time of 90 ks. More precise and less destructive evaluation could be achieved by using a reciprocal of pol arization resistance (R(p)(-1)) which was obtained by an electrochemic al impedance spectroscopy immediately after activation. It is conclude d that i(crit) is a suitable parameter for an approximate estimation o f the defect area, and R(p)(-1) provides the more precise amount of th e original defects.