Protection of cylinders by ion-beam sputter deposition: corrosion of carbon-coated aluminium tubes

In application, cylinders and tubes may chemically fail when they are exposed to aggressive media. As for other objects, coating with a protective film may improve the situation. However, coating the inner walls of hollow objects, such as tubes, by means of physical vapour deposition techniques is not feasible, because the material to be deposited has to enter the tube under very flat angles to the surface normal of the walls, depending on the ratio of the inner diameter to the tube length. This problem can be overcome when the source of the material to be deposited is located inside the tube. This is possible when sputter deposition is performed with an ion beam. A sputter target is located inside the tube; energetic ions are accelerated into this tube and impinge onto the target. Thus, material is sputtered from the target onto the inner walls of the tube. With this technique, aluminium tubes were coated inside with thin, amorphous carbon films. The corrosion performance in an aqueous chloride-containing environment, where aluminium suffers from pitting corrosion, was evaluated by means of electrochemical polarisation measurements. These showed that the films exhibited low microporosity with a good corrosion protection effect when appropriate deposition process parameters were used.