Some water vapor effects during the oxidation of alloys that are α-Al2O3 formers

Abstract

Oxidation studies were performed at 1100°C in dry air and air containing fixed partial pressures of water vapor on a number of alloys and coatings that form ␣-Al 2 O 3 scales under oxidizing conditions. The alloys investigated included RENÉ N5, PWA 1484, diffusion aluminide coatings (with and without Pt modification) on RENÉ N5, and a Ni-8 wt pct Cr-6 wt pct Al model alloy. The water vapor affected the oxidation of the alloys in three important ways: (1) The scales spalled more profusely during cyclic oxidation in wet air than in dry air, particularly for those alloys with alumina scales, which are only moderately adherent under dry conditions. The results were consistent with the mechanism previously proposed (Reference 1), whereby the water molecules decrease the fracture toughness of the alumina/alloy interface. (2) Thicker oxides are formed during oxidation in wet air than dry air. This effect comes primarily from accelerated transient oxidation during exposure in wet air. (3) Spinel was found to form on top of the alumina scales during long-term exposure. This phenomenon occurred in all atmospheres but was much more pronounced for exposures in wet atmospheres. Mechanisms for the preceding observations are proposed.

Moisture-Induced Delayed Alumina Scale Spallation on a Ni(Pt)Al Coating

“…A similar but less severe effect was reported for the same coating on Rene´N5 (Ref. 35), exhibiting a loss of 1 mg/cm2 in wet air compared to only 0.1 mg/cm2 in dry air. However, moisture effects for Ni(Pt)Al coatings on Rene´N5 were not very apparent.…”

The Effects of Water Vapor on the Oxidation of Nickel-Base Superalloys and Coatings at Temperatures from 700�C to 1100�C

“…These studies were performed using coupon specimens with areas about 1 cm 2 and 0.3 cm thick. The experimental procedures have been discussed previously [16]. The specimens were cyclically exposed in the hot zone of a horizontal tube furnace for 45 minutes and 15 minutes in a cool zone (200°C) outside of the furnace.…”

The Performance of Pt-Modified Alumina-Forming Coatings and Model Alloys

“…These results are consistent with prior studies of water vapor effects. [25][26][27][28] Substrates such as coated N5 that form adherent α-Al 2 O 3 scales tend to be less affected by water vapor than those that form less adherent scales, such as on 142.…”

“…Increased transient oxide formation also has been reported in previous studies of the role of water vapor. [27][28] To put these γ+γ' coating results into perspective, Figure 9 shows the mass gain of two model bond coating alloys as a function of water vapor content during cyclic oxidation at 1100°C. For Ni48Al+Hf, the addition of water vapor caused a small increase in the mass gain with increasing water vapor content.…”