Functionally Graded Coatings for Enhanced Corrosion Resistant Steel Tubing for Ultrasupercritical Boilers

DOE has a need for cost effective materials that can meet the high temperature creep and oxidation/corrosion requirements for the operation of ultrasupercritical boilers at high operating temperatures. To meet these requirements, this project will develop a new process to produce novel composite coatings on metal shapes. For the case of steel tubes for an ultrasupercritical boiler, a plasma transferred arc (PTA) torch will be used to melt a small pool in the surface of the steel tube, and chrome powder will be added to the melt pool. A localized N2 atmosphere results in the instantaneous formation of a Cr-Fe-N composite in a solid solution of Cr-Fe. The concentration of the nitride phase will be functionally graded, from very high at the surface to zero in the original substrate, which results in exceptionally strong bonding between the coating and the substrate. The concentration of the nitride phase can be controlled by the process conditions. For example, as nitride content increases, hardness increases, and corrosion resistance is enhanced. In Phase I, the process conditions, coating composition, microstructure, and thickness will be optimized for scale-up in Phase II, where components will be tested in an operating boiler. Commercial Applications and other Benefits as described by the awardee: In addition to the tubing in ultrasupercritical boilers, the new coating technology should find application in other heat exchangers where corrosion resistance is an issue. The high hardness and erosion resistance of the coatings also should find use in applications requiring erosion resistance, such as turbine engines.