2018 Volume 59 Issue 2 Pages 237-243
Problems with erosion caused by water droplet impingement occur in high-temperature and high-pressure pipelines and in steam turbines in energy conservation systems and power plants. Advances in material development and the use of highly resistant materials are needed in order to insure a high-performance level of plant maintenance. The use of ceramic coatings or intermetallic diffusion coatings on pipe steels is thought to be beneficial for the improvement of corrosion and erosion resistance. Aluminizing diffusion coatings on a few types of steel tubes with different coating thicknesses were prepared for this experiment. Erosion tests on water droplet impingement were conducted on the internal surface of tubes at a droplet velocity of 148 m s−1 using a water droplet testing apparatus. Erosion resistance was evaluated based on the incubation period and an average damage-depth rate for aluminizing diffusion coatings and steel substrates. Hardness distributions of the coating materials on cross-sectional surfaces revealed a harder layer on the surface, a functional gradient hard layer, and a soft steel substrate. EDX analyses on aluminizing diffusion coatings showed irregularly large grains of alumina, a microstructural intermetallic compound of Al-Fe-C with small grains of alumina, and an Al-Fe-C gradient diffusion layer toward the substrate. Good erosion resistance was obtained with a microstructural intermetallic compound layer with small grains of alumina. However, the erosion resistance of the diffusion layer with large grains of alumina was inferior. The erosion resistance depended on the combinations of the diffusion layer, alumina grains, and steel substrate, but not necessarily on the thickness of the diffusion layers.