Environmental Factors Affecting the CaSO₄-Induced Corrosion of Ni-Based Superalloys N5 and N500

Abstract

In previous work, it was shown that CaSO₄-induced corrosion can cause significant internal oxidation in single-crystal Ni-based superalloys similar to that observed in field-exposed components when subjected to a bi-thermal exposure (8 h at 1150 °C → 60 h at 871 °C) in air + 30% steam. In this work, the influence of (1) 1150 °C initiation stage temperature, (2) propagation stage temperature, and (3) CaSO₄ deposit mass on the mechanism of CaSO₄-induced corrosion were investigated further. The results of these experiments showed that subsurface depletion of Al and Cr caused by rapid Ca₄Al₆O₁₆S, CaAl₂O₄, and CrS formation due to CaSO₄ corrosion is required for internal oxidation to occur in N5 and N500, but that these alloys can “heal” when held for long durations at 1150 °C due to the enhanced diffusion of Al within the alloy at elevated temperatures.

Appendix

The following outlines the steps used to quantify the Al-consumption rate of CaSO₄-induced degradation relative to that due to Al₂O₃-scale formation at 1150 °C. The solution used is based on work from [14]. Using the boundary conditions given in Fig. 12 and the measured denuded zone depth of 40.3 μm after 86400 s (24 h), the growth of the γ’ denuded zone can be modeled by Eq. 1 as presented by Carter et al. [14], i.e.,

$$\frac{{N}_{Al}^{\gamma }-{N}_{Al}^{i}}{{N}_{Al}^{o}-{N}_{Al}^{\gamma }}=\sqrt{\pi } \alpha \mathit{exp}\left({\alpha }^{2}\right)\left[\mathit{erf}\left(\alpha \right)-\mathit{erf}\left(u\right)\right]$$

(1)

where ${N}_{Al}^{\gamma }$ is the Al concentration at the γ − γ + γ’ interface (9.9 at%), ${N}_{Al}^{o}$ is the Al concentration in the bulk alloy (13.9 at%), and ${N}_{Al}^{i}$ is the Al concentration at the scale-alloy interface (7.0 at%).

The quantity α from the standard solution is given in Eq. 2, with α = 0.354 when solved using X_d = 4.03 × 10^–3 cm for the γ’ depletion zone depth, ${D}_{Al}^{Ni}$ = 3.746 × 10^–10 $\frac{{cm}^{2}}{s}$ at 1150 °C (based on data from [13]), and t = 86400 s.

$$\alpha =\frac{{X}_{d}}{2\sqrt{{D}_{Al}t}}$$

(2)

Substituting α = 0.354 into Eq. 1 and simplifying yields erf(u) = − 0.636, which corresponds to u = − 0.642. The u in Eq. 1 is given by Eq. 3, where k_c $\left(\frac{{cm}^{2}}{s}\right)$ is the corrosion constant that represents the Al consumption rate in terms of equivalent Al metal recession and ${D}_{Al}^{Ni}$ = 3.5 × 10^–10 $\frac{{cm}^{2}}{s}$ at 1150 °C. Solving yields k_c = 6.1 × 10^–10 $\frac{{cm}^{2}}{s}$

$$u=\sqrt{\frac{{k}_{c}}{{4D}_{Al}}}$$

(3)

This k_c can be converted to the equivalent growth-rate constant of Al₂O₃ that would form a γ’ denuded zone that is 4.03 × 10^–3 cm thick in 86400 s using Eq. 4. This rate constant is ${k}_{p}^{DZ}$ = 3.5 × 10^–3 $\frac{{mg}^{2}}{{cm}^{4}s}$.

$${k}_{p}^{DZ}={k}_{c}{\left(\frac{{3M}_{O}}{2{V}_{Al}}\right)}^{2}$$

(4)

Thus, the equivalent Al₂O₃ growth-rate constant deduced from the γ’-denuded-zone depth in CaSO₄-deposited alloys is roughly 1000–4000 times greater than that for typical Al₂O₃-scale growth.