Investigation of diamond-like carbon coated steel corrosion: Enhancing the optical detection of defects by a controlled electrochemical activation

It is often reported that DLC thin films have good corrosion properties due to chemical inertness and low electrical conductivity. Nevertheless, the performance of these coating is highly sensitive to the presence of growth defects. An in situ optical microscopy coupled to an electrochemical test was then developed to evaluate corrosion protection performance of DLC coated steel. A square wave voltammetry was applied to increase the sensitivity of electrochemical techniques based on the detection of the dissolution of the bare metal surface triggered by the presence of uncoated spots due to inherent defects. With this in situ electrochemical test, a clear improvement in optical detection ``natural'' defects is obtained. SEM coupled with FIB milling technique was used to generate artificial defects with controlled dimensions down to 2.5 mu m diameter to define the sensitivity of the proposed counting procedure. Results showed that it was possible to detect cylindrical defects with diameter down to 2.5 mu m. Moreover, the electrochemical approach allowed to characterize the propagation of the damage under the DLC coatings.