A new enabling technology for coating the internal surfaces of pipes with a hard, corrosion and wear resistant diamond-like-carbon (DLC) coating is described. The improvement in corrosion and wear resistance is shown based on changes in film chemistry, structure and thickness. Corrosion resistance is measured based on exposure to HCl, NaCl and H2S environments. Mechanical properties include high hardness, high adhesion, and good wear resistance in dry and wet sand slurry environments. It is suggested that this new technology enables wide spread use of DLC based coating to increase component life in applications where internal surface of pipes are exposed to corrosive and abrasive environment especially in the oil and gas industry.
Diamond-like carbon (DLC) coatings have excellent properties such as high wear resistance, very low friction coefficient and high corrosion resistance. [1-3] Because of these excellent properties, DLC coatings have attracted great attention for use in various applications in industries such as oil and gas, semiconductor, medical and automotive. In the oil and gas industry, DLC coatings are especially expected to improve tribological and corrosion performance of components that experience extreme environments provided the coating can be applied to internal surfaces of pipes, pipe joints, drilling fixtures, and drilling bores, etc. For piping or tubing that delivers corrosive material, obviously the interior surface that is in contact with the corrosive material is the surface that must be coated. There are several methods available to deposit DLC or other coatings at the outer surface of components; such as chemical vapor deposition (CVD), physical vapor deposition (PVD), electroplating, flame spray and sol-gel. However, coating internal surfaces remains a challenge especially for large aspect ratio (length to diameter ratio) components and very limited information is available in the literature.4 In the case of very low-pressure techniques such as PVD, where the pressure is below or near the molecular flow region, coating internal surfaces has been limited to tubing with large diameters and short lengths, due to line of sight deposition. CVD techniques are limited in this application as well, due to the need to supply heat for the chemical reaction, which damages heat sensitive substrates. PECVD (plasma enhanced chemical vapor deposition) can be used to lower the temperature required for reaction, but then there is difficulty in maintaining a uniform plasma inside the pipe and preventing depletion of the source gas as it flows through a pipe placed inside a vacuum chamber. This article reports result of a study demonstrating the potential of a new PECVD technology to deposit DLC based films on internal surfaces of pipes with excellent corrosion and wear resistance characteristics. The results are obtained on rough ( Ra ~ 100 ? 200µin ) carbon steel substrates through the use of a multi-layer coating that provides strong adhesion to the substrate and reduced stress, as demonstrated for the former by the lack of corrosive undercutting between the substrate and the coating and for the later by the ability to deposit thick coatings. The surface roughness is controlled through blasting of the pipe interior, to bring the roughness into the desired range.
