Diffusion coatings for high temperature corrosion protection of 9-12%Cr steels

Abstract

Steels containing 9-12%Cr are of great interest for application as heat-exchanger tubes in power stations. Indeed they possess a high thermal conductivity and favourable mechanical properties at temperatures up to 650°C. However, from a mechanical point of view, even though conventional ferritic-martensitic 9-12%Cr steels are designed for service temperatures up to 650°C, their use at such high temperatures is rather limited in corrosive environments. One solution may be to protect these steels by suitable corrosion-resistant coatings. Pack cementation is one of the easiest and cheapest coating processes for high temperature applications. However, for ferritic-martensitic steels, the maximum coating temperature is limited to 650°C. Above this limit, the decomposition of martensite would be accelerated, to the detriment of the mechanical properties of the material. The objective of the present work was to use pack cementation to coat the 9%Cr steel P91 and the 12%Cr steel HCM12A without modifying their microstructure. Therefore, the coating process was either carried out at 650°C or combined with the heat treatment of the ferritic-martensitic steel. As a result of the low coating temperature, aluminide coatings were developed first. Later, a two-step Cr+Al coating was obtained. The corrosion resistance of the coatings developed was tested at 650°C for 1000 h in a simulated coal firing atmosphere composed of: 14%CO₂, 10%H₂O, 1%O₂, 0.1%SO₂, 0.01%HCl (bal. N₂). The coated samples showed better resistance to corrosion than the uncoated materials. The investigations were then extended to include aluminide coatings applied by fluidised bed chemical vapour deposition (FBCVD) on 9-12%Cr steels. Eventually, the corrosion resistance was compared with that of coated and uncoated austenitic steels as well as the nickel-based alloy IN617.