High temperature corrosion of coatings and boiler steels in reducing chlorine-containing atmosphere

Unacceptably high corrosion rates are often experienced, when chlorine-containing fuels are combusted. Reducing conditions that may occur in various boilers accelerate corrosion even further. Protective oxide scales are not formed on low-alloy steels if partial pressure of oxygen is too low. Materials rich in oxide formers, such as chromium and aluminum, are needed to resist corrosion in reducing combustion atmospheres, but processibility of such bulk alloys is very limited. Various coating technologies are considered as potential solution for corrosion problems in high temperature combustion environments with low partial pressure of oxygen. High temperature corrosion tests were performed on one ferritic boiler steel, one austenitic boiler steel, five high velocity oxy-fuel (HVOF) coatings, one laser-melted HVOF coating, and one diffusion chromized steel. Synthetic atmosphere simulating reducing conditions in combustion of chlorine-containing fuels was created for the tests. The test atmosphere contained 500 ppm HCl, 600 ppm H2S, 20% H2O, 5% CO, and Ar as a balance. The test temperature was 550 °C and the test duration was 1000 h. Corrosion resistance of steels and homogeneous coatings was mainly determined by chromium content. Homogeneous and dense coatings with high chromium content performed well and were able to protect the substrate. Some of the HVOF coatings were attacked by corrosive species through interconnected network of voids and oxides at splat boundaries.