Stress Rupture Behavior of Disk Superalloys Exposed to Low-Temperature Hot Corrosion Environment

Siemens Energy has a large fleet of aero derivative gas turbines. There are currently more than 1100 installed power turbine variants coupled with aeroderivative gas generators worldwide being used in mechanical drive and power generation applications.

Many aeroderivative gas turbines operate in offshore and marine environments wherein the inlet air and fuel often contain corrosive contaminants that can cause serious high temperature surface attacks and degradation of mechanical properties of critical power turbine components. Additionally, there is an increased market demand on OEMs for designing gas turbines that offer much higher fuel flexibility leading to potentially higher risk of hot corrosion induced damage.

Hot corrosion attack occurs due to exposure of turbine components to sulfur-bearing, corrosive compounds during turbine operation.

Two distinct hot corrosion mechanisms have been recognized: Type I or High Temperature Hot Corrosion (HTHC) occurs between 800 and 925 °C (1470 and 1700 °F), while Critical components such as gas turbine disks and shafts are specifically susceptible to a type of Hot corrosion attack known as Low temperature hot corrosion (LTHC) or type II hot corrosion. LTHC occurs at temperatures ranging from 545 °C (1013°F)-melting temperature of sodium-sulfate – cobalt-sulfate eutectic compound) to about 760°C (1400°F) where partial pressure of SO₃ is relatively high..

The application of superalloys in gas turbines is often limited by their susceptibility to environment assisted intergranular crack growth at high temperatures in fatigue with extended hold times. Presence of hot corrosion further accelerates the rate of time dependent crack growth.

In layered type high temperature corrosion (LTHC) surface attacks, i.e., diffusion of corrosive elements into the base alloy results in a favored consumption of hardening, matrix stabilizer, and oxidation resistance elements . This along with localized increase in state of stress due to pitting leads to a significant reduction in crack initiation lives either due to low cycle fatigue or due to creep

This Paper compares the detriment to creep damage resistance and corresponding microstructural degradation of commonly used gas turbine disk alloys: Inconel 718, Incoloy 901 and A-286 when exposed to type II hot corrosion attack.

The Creep Rupture testing for the study was performed on smooth specimens in air and on specimens coated with sea salt in corrosive environments under uniaxial, constant load and at temperatures of 510 °C (950°F) and 593 °C (1100°F) per ASTME139-06. Testing was performed in customized/hermetically sealed constant load tensile test frames designed to contain SO2 / air mixture with H2O vapor.

Testing confirmed that Inconel 718 and Incoloy 901 maintain creep strength advantage over A286 in a hot corrosion environment, within the temperature range of 950-1100°F. All three materials exhibited an equivalent life reduction in the corrosive environments at 1100°F.