Inhibitors and coatings against vanadate-containing oil ash of boilers

Corrosion in oil-fired boilers is accelerated in the presence of vanadium, sodium, and sulfur from low grade fuels. In order to avoid vanadate-induced corrosion several ceramic inhibitors and intermetallic coatings and their interaction were studied. Inhibitor ceramics such as CaO and MgO tend to react strongly with vanadium oxides forming vanadates with higher melting temperatures whereas Cr₂O₃, Al₂O₃, and SiO₂ are the most protective oxides for alloys at high temperatures even in the presence of vanadium pentoxide.

Three potential inhibitors were tested on uncoated as well as on Cr, Al, and AlSi diffusion coated 10CrMo9-10 steel by immersion in 60 mol% V₂O₅–40 mol% Na₂SO₄ salt at 650 °C. Materials performance was analyzed by calculating substrate recession rates and by metallographic characterization of the corrosion scales and of the metal subsurface zones so that the mechanism and extent of corrosion could be characterized.

In the end different materials combinations (coatings and inhibitors) could be ranked with regard to the detrimental or beneficial role of the different approaches. The present results thus provide a much deeper insight into materials resistance in vanadium-containing environments at boiler simulating conditions than previous studies.