The effect of salt chemistry on the zero and cyclic load performance of a high temperature nickel-based superalloy

Unpeened fine grain RR1000 flat plate specimens were exposed to type II hot corrosion conditions (600°C, 300 ppm SO_x in air) for 200 hours cumulative exposure. Four different salt compositions; 60% Na₂SO₄ – 40% K₂SO₄, 55% K₂SO₄ – 45% KCl, 98% Na₂SO₄ – 2% NaCl, 100% Na₂SO₄, were selected for corrosion testing due to the range in melting temperatures of the salts and their associated eutectic products. Results obtained through the use of both microscopy and surface profilometry methods provided a good comparison for each of the applied salts.

The 55% K₂SO₄ – 45% KCl salt has been identified as producing the highest level of attack followed by the 98% Na₂SO₄ – 2% NaCl salt commonly used in type II hot corrosion studies. Cumulative probability distributions have been utilised and indicate the 55% K₂SO₄ – 45% KCl produces a gross metal loss type attack whereas the remaining salts created damage through a pitting type mechanism. The role of environmental degradation is an important factor for turbine rotor materials, which operate under significant loads. In order to quantify the impact of hot corrosion at this lower temperature, low cycle fatigue specimens have been subjected to hot corrosion conditions and the fatigue lives and failure initiation points compared to those of specimens exposed at higher temperatures.

This paper aims to address the following key question: what salt chemistry is most likely to produce type II hot corrosion damage at turbine rotor operating temperatures and to what extent? Our work on investigating the effects of introducing stress through fatigue testing of peened and unpeened samples will also be presented.