Degradation mechanisms of Co-based alloy and WC metal-matrix composites for drilling tools offshore

The efficient performance of drilling tools in aqueous environments requires materials that can effectively resist excessive material loss due to the combined effects of erosion and corrosion. In this paper, the interactions between electrochemical corrosion and mechanical erosion have been assessed in liquid-solid slurry. The following materials were used: (a) a cobalt-based alloy which is currently used in the manufacture of oil tools, (b) a nickel-based composite coating with tungsten carbide and (c) a tungsten carbide copper-based solid cermet. The material degradation is assessed as a function of aqueous environment, sand loading and angle of impingement. Their degradation shows a distinctive electrochemical behaviour (corrosion) which has been shown to represent an important factor in the total degradation of these materials. The relative importance of corrosion and its synergistic effect when combined with aggressive erosion are quantified for different angles of impingement. The cobalt-based bit body material has a superior degradation resistance in the erosion-corrosion environments and although there is potential to use other materials to protect the areas of major damage near the nozzles in drilling tools. It is shown that simply replacing the materials with a harder cermet is not sufficient. The implications in terms of material performance and the potential protection from degradation is discussed in the paper. © 2003 Elsevier Science B.V. All rights reserved.