Development of novel protective high temperature coatings on heat exchanger steels and their corrosion resistance in simulated coal firing environment

摘要

Improving the efficiencies of thermal power plants requires an increase of the operating temperatures and thus of the corrosion resistance of heat exchanger materials. Therefore, the present study aimed at developing protective coatings using the pack cementation process. Two types of heat exchanger steels were investigated: a 17%Cr/13%Ni austenitic steel and three ferritic-martensitic steels with 9 (P91 and P92) and 12 %Cr (HCM12A). The austenitic steel was successfully aluminised at 950 °C. For the ferritic-martensitic steels, the pack cementation temperature was decreased down to 650 °C, in order to maintain their initial microstructure. Two types of aluminides, made of Fe2Al5 and FeAl, were developed. A mechanism of the coating formation at low temperature is proposed. Furthermore, combining the pack cementation with the conventional heat treatment of P91 allowed to take benefit of higher temperatures for the deposition of a two-step Cr+Al coating. The corrosion resistance of coated and uncoated steels is compared in simulated coal firing environment for durations up to 2000 h between 650 and 700 °C. It is shown that the coatings offer a significant corrosion protection and, thus, an increase of the component lifetime. Finally, the performance of coated 9-12 % Cr steels is no longer limited by corrosion but by interdiffusion between the coating and the substrate.