Effects of CVD direct growth of carbon nanotubes and nanofibers on microstructure and electrochemical corrosion behavior of 316 stainless steel

In this work, the corrosion behavior of three differently treated AISI 316 stainless steel plates is investigated, viz. (1) pristine, (2) coated with a carbon nanotube (CNT) layer and (3) coated with a carbon nanofiber (CNF) layer. CNTs and CNFs were directly grown on stainless steel via a CVD method, using ethylene as the carbon source and without the addition of an external catalyst. The corrosion behavior of these materials was investigated by a combination of microstructural and electrochemical methods. Electrochemical tests included potentiodynamic and potentiostatic techniques in hot sulfuric acid solutions. A strong deterioration in corrosion resistance was revealed by the electrochemical tests and confirmed by microstructural examination of the samples. It was found that carbon diffusion into the substrate material during the CNT/CNF growth process results in chromium depletion of the near-surface region of 316 SS and chromium carbide precipitation at grain boundaries causing accelerated intergranular corrosion. Accordingly, notwithstanding the obvious inability of CNT/CNF layers to provide protection to the substrate due to their porous nature, a real corrosion damage arises from the high temperature exposure of stainless steel to the CVD atmosphere, suggesting that a similar risk may be present even for compact carbon coating deposited by CVD process, in the event of local damage of the coating.