Tuning pulse frequency for improvement in the corrosion resistance of 304SS pipes inner DLC coatings deposited by hollow cathode PECVD

Metal pipes are widely used in industrial transportation system, but the inner surface of the pipes is easy to be corroded and not easy to be repaired, which leads to the great economic loss and environmental pollution. In this paper, the Diamond-like carbon (DLC) coatings was deposited on the inner surface of 304 stainless steel (304SS) pipes by the hollow cathode plasma enhanced chemical vapor deposition (HC-PECVD) equipped with the direct current power supply with the pulse frequency of 100, 500, 1000 and 1500 Hz. The aim of this paper is to improve the corrosion resistance of pipes inner DLC coatings by tuning the pulse frequency. Results indicate that the sp³-C hybridization content (16.7% ~ 26.7%), the disorder degree of carbon structure and the hydrogen content of pipes inner DLC coatings all show an increasing trend with the reduction of pulse frequency, while its stress level presents an opposite trend. Compared with 304SS pipes (0.51 V and 8.94 × 10⁻⁶ A∙cm⁻²), the pipes inner DLC coatings present more positive corrosion potential (−0.49 V ~ −0.27 V) and lower corrosion current density (3.62 × 10⁻⁶–4.46 × 10⁻⁸ A∙cm⁻²), better corrosion resistance, which is considerable as an effective protection coating. The pipes inner DLC coatings deposited at 500 Hz present the lowest corrosion current density (1.22 × 10⁻⁷–4.46 × 10⁻⁸ A∙cm⁻²). Moreover, the corrosion performances of the pipes inner DLC coatings deposited with the pulse frequency of 500 and 1000 Hz exhibit the better uniformity. Therefore, the pipes inner DLC coatings deposited at 500 Hz possesses the best corrosion resistance, which can provide important information for large-scale industrial production in the future.