Interaction of Ta and Cr on Type-I hot corrosion resistance of single crystal Ni-base superalloys

Highlights

• Hot corrosion behaviour of Ni-base superalloys with different Cr and Ta contents was investigated.

• Role of Ta in hot corrosion was influenced by thermodynamic factors and kinetic factors.

• The effect of Ta on hot corrosion resistance can be described by introducing the quantitative parameter Ta/Cr (wt.%) ratio.

Abstract

Type-I hot corrosion behaviour of single crystal Ni-base superalloys with different Cr and Ta contents in sodium sulphate (Na₂SO₄) at 900 °C and 950 °C in static air were investigated using the “deposit recoat” method. It was shown that, increasing corrosion temperature shortened the incubation period. Loss of hot corrosion resistance causing by Cr reduction cannot be compensated by increasing Ta content. The effect of Ta addition on hot corrosion resistance can be described by introducing the quantitative parameter Ta/Cr (wt.%) ratio. Interaction of Cr, Ta and temperature on hot corrosion behaviour was discussed in detail in the present paper.

Introduction

Hot corrosion occurred at temperatures above 884 °C is termed as high temperature hot corrosion (HTHC) or Type-I hot corrosion [1]. As the inlet temperature of industrial gas turbine (IGT) increases, the service temperature of the turbine blades increases as well. Type-I hot corrosion is thus the dominant corrosion mode of the blade materials. Meanwhile, to accommodate the steady increase in the operating temperature of IGTs, improvement of the high temperature strength of hot corrosion resistant superalloys is always required.

As is widely accepted, Cr is the most important alloying element in resisting hot corrosion attack. Under hot corrosion environment, sufficient Cr promotes the formation of a continuous and adherent chromium oxide (Cr₂O₃) scale on the surface of the alloy [2]. Cr₂O₃ reacts preferentially with oxide ions (O²⁻) in molten Na₂SO₄ to form chromate [3], [4], [5]. The dissolution of Cr₂O₃ stabilizes the melt chemistry so that the basicity of the salt is not sufficient to cause basic fluxing but is still not low enough to cause acidic fluxing. Thus, dissolution/re-precipitation of the protective oxide scale is avoided [5]. Further, Cr₂O₃ was found to have an “inverse” Rapp/Goto effect, solid Cr₂O₃ can be re-precipitated from the melt [6]. Cr₂O₃ scale has the self-healing ability under hot corrosion circumstance. Besides, Cr can effectively capture sulphur and thereby suppress the formation of other deleterious liquid sulphides [5], [7], [8], [9]. Therefore, typical hot corrosion resistant superalloys always have more than 12 wt.% Cr [9], [10], [11], [12], [13]. However, high Cr content may induce the precipitation of TCP phases in the alloy. In order to improve the mechanical properties, one would have to make use of the design philosophy of typical aero engine materials with high refractory elements (Re, Mo, W and Ta) contents and low Cr and Ti contents, while keep the hot corrosion resistance unaffected [9], [14]. Therefore, refractory element which is beneficial to hot corrosion resistance is of high importance.

Among the refractory elements, Ta is found to be probably beneficial on hot corrosion resistance. It facilitates the formation of solid NaTaO₃ and suppresses the formation of liquid Na₂Mo(W)O₄ [15], [16], [17]. A better hot corrosion resistance would be obtained if the Ta/(Mo + W) compositional ratio of the alloy is close to unity [7], [18]. In our previous work, a series of experiments has been conducted to investigate the interaction of Ta and Cr on hot corrosion resistance. In alloys with high Cr content (12 wt.% Cr) [19], Ta promoted the formation of NaTaO₃ and (Cr, Ti)TaO₄, with the latter spinel phase acted as a diffusion barrier of ions. The corrosion kinetics was lowered by Ta addition. In alloys with low Cr content (5 wt.% Cr) [20], Ta was found to exhibit a beneficial effect by facilitating the formation of NaTaO₃ and TaS₂, and therefore inhibiting the formation of liquid phases Na₂MoO₄ and Ni-sulphides. Ta substituted Cr for sulphur catcher and more Cr was saved for Cr₂O₃ formation. The results imply that Ta may be an outstanding candidate to substitute Cr for effective hot corrosion resistant element. However, it should be noted that our previous researches about the effect of Ta on hot corrosion performance of the alloys are based on certain Cr content. It is very necessary to study the hot corrosion behaviour of low Cr high Ta alloys, and compare it with that of high Cr alloys. And then it can be determined whether Ta can replace part of Cr for hot corrosion alloy.

Up to now, most studies focus on hot corrosion at 900 °C. As the inlet temperature of the IGTs improves, the surface temperature of the blades may reach 1000 °C or higher. As the service temperature increases, corrosion behaviour of the alloy may change accordingly. On the one hand, evaporation rate of sodium sulphate increases at higher temperature. The effect of salt getting smaller and smaller as temperature increases. On the other hand, however, the protective effect of Cr against molten salt rises from Cr₂O₃, the increased oxidative vaporization rate of Cr₂O₃ may lead to a deteriorated hot corrosion resistance of Cr₂O₃-forming alloys at higher temperature [21], [22], [23], [24]. Considering that the evaporation rate of sodium sulphate, the stability of Cr₂O₃ scale and the diffusion rate of ions can all significantly affect the hot corrosion behaviour of alloys, it is of great importance to study the hot corrosion behaviour at higher temperature.

In the present paper, hot corrosion behaviour of several Ni-base single crystal superalloys with different Cr and Ta contents were investigated using the “deposit recoat” [25] method at 900 °C and 950 °C. The corrosion process and mechanism of interaction of Ta, Cr and temperature on hot corrosion resistance were discussed.