Alloy 617, a candidate material for the intermediate heat exchanger in the helium-cooled very high temperature reactor (VHTR), is susceptible to carburization, decarburization, and/or oxidation by impurities (H 2/H2O, CO/CO2, CH4, etc) in the He coolant, with deleterious effects on its structural integrity. The chromia scale formed naturally by this alloy does not provide adequate protection at the target operating temperatures, 950+/-50°C. Alpha alumina, which has been proven as an effective diffusion barrier to protect nickel alloys in extreme environments such as gas turbine engines, is attractive as an environmental barrier for 617. To assure that this phase forms (rather than less protective metastable structures) in the low pO2 environment at 800°C≤T≤1000°C, surface modification is required. Lifetime requirements demand stability of the modified surface layers against interdiffusion with the substrate, and the ability of the oxide to re-form in the event of cracking or spallation due to thermo-mechanical stresses in the prospective applications.

To provide an aluminium reservoir and enable the formation of alumina, alloy 617 was aluminized by pack cementation. NiAl formed on further heating as well as carbide and sigma interlayers that were found to control the diffusion behavior between the coating and substrate. Because NiAl coatings are known to form persistent transient aluminas at T≤1000°C, FeCrAlY, which readily form alpha alumina under the same conditions, was also explored as an outer layer on 617.

Although cladding 617 directly with FeCrAlY resulted in extensive interdiffusion, cladding 617 that had previously been aluminized produced a more stable microstructure. While the interlayers formed by aluminizing limited diffusion in the multilayer system, there was still diffusion of nickel into FeCrAlY that resulted in the formation of ordered B2 (beta') precipitates in the disordered, solid solution bcc (beta) FeCrAlY layer. The beta/beta' microstructure resembles the gamma/gamma' microstructure of some nickel superalloys and could enhance the mechanical properties of the coating. However, continued nickel diffusion caused the formation of a solid solution fcc (gamma) layer at the expense of the beta/beta' microstructure, indicating further improvements to the stability are necessary.

Oxidation in low pO2 environments promoted the formation of alpha alumina over transient aluminas for both coatings. However, the clad + aluminized coatings more readily developed an alpha alumina scale, a difference that became more pronounced at lower oxidation temperatures. Samples for evaluation in impure He environments were pre-oxidized in a low pO2 environment prior to exposure to form a thin alpha alumina scale. Exposures were carried out in flowing He environments with carefully controlled CO:CO2 ratios of ∼9 (decarburizing) and ∼1272 (carburizing). The alpha alumina scale formed on pre-oxidation was an effective barrier to carburization/decarburization in both aluminized and aluminized + clad samples.