Evaluation of corrosion resistance of diamond-like carbon films deposited onto AISI 4340 steel

The corrosion resistance of amorphous diamond-like carbon (DLC) coatings deposited by radio frequency plasma enhanced chemical vapor deposition (rf-PECVD) technique on AISI 4340 steel substrates was evaluated under saline (5% NaCl) and acid (1700 ppm H2SO4) atmospheres. The corrosion process was investigated by surface characterization and electrochemical methods, such as potentiostatic polarization and electrochemical impedance spectroscopy (EIS). DLC coatings effectively protected the substrate after 48 h in a salt fog chamber and after the first Kesternich cycle. For comparison, under the same conditions, titanium nitride (TiN) coatings did not protect the substrate even for 2 h of saline exposure and even for the first Kesternich cycle. Although the DLC coatings resisted well to the corrosive action of the aggressive media, nucleation and growth of homogenous and micro-sized pinholes uniformly distributed on DLC coatings were observed as a result of the corrosion processes. The observed results suggest that the development of techniques which would reduce the porosity of the DLC films could promote further improvement on their corrosion protection ability.