Consultant Laurie Brooking considers the hot topic of hot corrosion.
Hot corrosion can cause severe damage, and shorten the life of key assets. It is an issue that has come to light over the last 40 years, as a degradation mechanism in gas turbines (GTs), operating in both power generation and propulsion applications. Hot corrosion has developed as a challenge, partly due to the pursuit of higher efficiencies: often achieved through increasing temperatures, and requiring the growth in use of more creep resistant single crystal (SX) alloys and superalloys. These more creep resistant alloys often have lower concentrations of refractory elements, and are therefore more susceptible to hot corrosion.
Chemically, hot corrosion is a relatively complex degradation mechanism, and is more likely a group of mechanisms which vary with alloy systems, and for temperatures ranging from around 450°C to 750°C. Generally, the mechanism is thought to occur in the presence of Sulphur trioxide (SO3) gas and deposited salt species, which results in an acidic fluxing mechanism or sulphidation.
It is increasingly understood, however, that lower temperature mechanisms may exist which are less reliant on a partial pressure of SO3, and require only the deposition of electrochemically active contaminants, with several mechanisms proposed. It is also now understood that hot corrosion can act in combination with stress and fatigue, leading to cracking and fracture – and resulting in catastrophic damage to your assets.
The complex chemical nature of hot corrosion makes developing accurate life prediction methods difficult, as during operation gas turbines are subjected to a range of contaminants, from both the surrounding environment and fuel, which are difficult to measure and quantify. However, models for hot corrosion and hot corrosion cracking are being developed across the industry, utilising a range of probabilistic, deterministic and data science methods, and helping to pave the way for a new generation of more sustainable gas turbines which can operate more flexibly and more robustly.