Hot corrosion and erosion resistance of new coating alloys for biomass and waste boiler heat transfer surfaces

In the present work new coating materials have been tested in laboratory conditions simulating high temperature corrosion in biomass boilers and waste incinerators with molten salt deposits. The work also included some thermodynamic modelling and high temperature erosion testing. In the deposit corrosion tests the dense Ni- and Cr-based materials performed somewhat better than iron-based materials in means of corrosion. In the porous coatings corrosion had propagated into the coating through the pores. Inconel 625, as well as some of the other high Ni-materials, had good corrosion resistance, but the coating detached from the low alloy steel substrate material in many cases. Welded materials (MIG, laser) formed a very dense coating and therefore reacted like bulk materials and HVOF coatings were denser than arc sprayed coatings. It can be concluded that tough and dense coatings succeed best in the erosion conditions. However, due to their lamellar structure coatings are very sensitive to the size of erosive particles and the impact angle. The thermodynamic modelling supported quite well the corrosion test results, in spite of some weaknesses in the databases, e.g. content of boron. The new salt database used in some modelling cases gave good results, but should be applied with some caution.