Background
The oil and gas and petrochemical industries have a need for thick diamond-like carbon (DLC) coatings on the interior surface of long pipes to increase corrosion and erosion resistance. Using a plasma immersion ion deposition (PIID) process, SwRI has demonstrated that thick (>10 µm) DLC coatings can be deposited on the outer surfaces of components. The coatings typically have shown excellent wear and erosion resistance and also provide corrosion resistance in salt water and CO2 environments, which are commonly seen in oil and gas fields. SwRI also has developed a PIID-based coating process by which a thin functional (hydrophobic) coating can be deposited on the interior surface of long pipes up to 80 feet long to prevent hydrate from clogging the pipes. To date, however, SwRI has been unable to deposit thick DLC coatings (5-10 µm) for erosion/corrosion protection in long pipes. Therefore, the objective of the project is to develop a coating technology by which thick wear-, erosion-, and corrosion-resistant DLC coatings can be deposited on the interior surface of long pipes.
Approach
Figure 1: Deposition of thick DLC coatings at the interior surfaces of a long pipe at SwRI. Initial coating development is carried out on 7-foot-long pipes (4-inch diameter) to optimize key deposition parameters and power supply configurations. Stainless steel witness coupon samples are placed inside the pipe for characterization and process optimization purposes. The efforts are focused on developing a stable PIID process for obtaining high-quality thick DLC coatings in long pipes. The corrosion resistance of the DLC coatings on coupon samples will be evaluated using electrochemical impedance spectroscopy (EIS) tests, salt spray test, and scaling resistance test. After successfully depositing high-quality thick DLC coating (5-10 µm) on coupon samples in 7-foot-long pipes, the coating process will be transferred and demonstrated on large aspect ratio pipes (30-feet long, 4-inch diameter).
Accomplishments
A new HC-PIID coating technology using a combination of AC and pulsed DC power technologies has been successfully developed to coat the internal surface of long pipes with thick DLC coatings (up to 20 mm) and high deposition rates (2-3 mm/hr). The coating consists of an Al metal bond layer, a SiCx transition layer, and a thick DLC layer. Overall, the thick DLC coatings on the internal surface of long pipes exhibit dense structure and good adhesion. The developed coating technology provides enhanced wear and erosion resistance of the internal surface of long pipes and improves their durability. The corrosion test of coupon samples sectioned off from the coated pipe in salt spray testing and electrochemical impedance testing show that the thick DLC coatings developed on the internal surface of long pipes provides certain levels of corrosion resistance. The developed coating technology can be jointly applied with the previously commercialized hydrophobic thin films for long pipes to significantly improve the durability and performance in industrial working environments.
Figure 2: At left is a cross-sectional SEM micrograph of a thick DLC coating (14 mm) deposited on the interior surface of a 7-foot long stainless steel pipe (4-inch diameter). At right is an HRC indentation performed on the surface of the coating showing no coating delamination and cracking.