Evaluation of Defects in Plasma CVD Coating Film by Electrochemical Measurements under Coupling Condition with Pt Counter Electrode

Abstract

Corrosion resistance of TiN-coated stainless steel by drying coating process is extremely influenced by the existence of various defects in the thin films, such as pinhole defects. Recently, the critical passivation current density (CPCD) method have been frequently used for evaluating the defect ratio (pinhole-ratio) from among various tests employing electrochemical methods. However, CPCD method showed much difficulties when applying to plasma CVD film, because relatively large scale spalling-off of the coated film was recognized after the electrochemical test. In this study, a new method, in which potential and current density changes were measured under coupling condition between specimen and counter electrode of Pt, was proposed for conducting less destructive electrochemical testing. Testing method under coupling condition was less destructive than CPCD method, for causing little peeling off in coated film. Therefore, good correspondence between the defect region in coated film and the penetrated area of it to cause substrate corrosion by electrochemical test was recognized. Also in this coupling test method, the pinhole-ratio Ra of coating can be determined quantitatively from the quantity of electricity Qa through evaluating corrosion pit morphology.