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The effect of Hot corrosion on the mechanical behavior of high temperature alloys

Hot corrosion

Introduction:

In the current context of energy growth and conventional gas field depletion (ie: North Sea), there is a need for newer resources, able to supplement intermittent renewable energies, whether these are conventional, like deeper and further offshore oil and gas, or unconventional, like shale gas or biomass. Due to economical reasons or difficulty to find alternative, an increasing number of gas turbines are running on contaminated fuel (HCl, SO3, V2O5, CdSO4, PbSO4, Na2NO3...). Alkali salts (coming from the environment) associated with sulphur (coming from the fuel) form alkali sulphate that sticks onto the blade. The focus of this work was on sodium sulphate, which is able to initiate damage to the protective scale of nickel based superalloy turbine blades, resulting in cracks growing, eventually leading to the failure of the blade by hot corrosion. The ejection of one blade results in costly downtime.

The mechanisms controlling the degradation are complex and varies with the amount of chromium. The first step of this work was to reproduce the failure environment. The second step was dedicated to the understanding of each contaminant (ie: sea salt, sodium sulphate) on two chromium content nickel single crystal superalloys. Finally, the effect of stress was investigated for different harsh environment that are present offshore. The final goal of this work (still on-going) is to build a model able to predict the lives of gas turbine blade offshore.

1) Effect of air, sea salt and sodium sulphate

 The 3 different samples strips were inserted in a furnace (not the one presented above) during one hour, weighted, and then exposed to higher temperature. This temperature change allow to:

  • quantify the salt subblimation rate (seen on figure b & c below on the green curve)
  • measure the additionnal oxide growth caused by the salt
  • determine at what temperature the mass gain started

 

Noel_result_2 Noel_result_4 Noel_result_3
a) Air only (reference) b) Sea salt sprayed sample c) Sodium sulphate sprayed sample

{More observations were made but were ommitted for clarity of this page}

2) Facility to reproduce the failure environment

A rig was built to reproduce the failure environment of the turbine section. It was composed of:

  • A vertical tube furnace (reproducing the temperature of the turbine blade)
  • A vertical actuator equipped with load cell and LVDT, able to exert stress on a sample by 4-points beam bending (with stress level comparable to stress  caused by centrifugal force on turbine blade)
  • Finally, in order to safely inject sulphur in the chamber, the furnace was sealed, with a close loop circuit equipped with SO2 sensor  (Sample were coated with salt before exposure)
  • {Other components are present but have been omitted for clarity}

Experiment lasted typically less than 100h.

carousel
a) Picture and section view of the hot corrosion type II rig b) Because the loading is made in 4-points bend, it results in a non-uniform stress distribution. The materials investigated are single-crystal (anisotropic) it requires special consideration. The finite element model was built with a orthotropic model.

 

3) Effect of stress on 4 different environments

Environment close to the ones experienced by the turbine were reproduced in the rig:

  • Air only
  • Sample sprayed with sea salt and exposed to air (with and without stress)
  • sample exposed to sulphur (with and without stress)
  • sample sprayed with salt and exposed to sulphur (with and without stress)

This incremental contaminant test allows to identify the role of each compounds and the coupling effect of stress. Characterisation of the reaction layer was done with X-ray diffraction and complemented with SEM with EDS.

Assessment of sample damage was made by specific weight gain, reaction layer thickness, tensile tests. A comparison between each damage assessment technique was also made.

Noel_result_5

Picture of a crack propagating on SCRY-4 sample.

Conclusion:

  • A facility has been built that reproduce harsh environment experienced by gas turbins offshore
  • The effect of different salts has been investigated and provided some answers to the mechanisms of failure of turbine blade
  • Various environment are affected when stress is superimposed

 

References:

[1]: https://www.cam.ac.uk/research/impact/high-flying-materials

 

Plain words abstract:

In 2013, 25% of the operating cost of the airlines were related to the engine cost (including fuel, maintenance and capital cost of the engine). Hence, new plane engine (named gas turbine) needs to be fuel efficient, easy and cheap to maintain. A specific rotating component has been developed in that exact sense, operating at very high temperature (to improve thermal efficiency) but also during long time was named Nickel "super" alloys:

  • Tip of the blades are spinning at speed ~ 1600 km/h.
  • Centrifugal force (trying to pull out the blade) exerted at the base of the blade is equivalent to the weight of a double deck bus hang to each blade
  • The power extracted by each blade (equivalent to the thrust of a formula 1) is used to compress fresh gases and "push" the plane forward
  • All this is being done at temperature 400°C above melting point (of the blade). Imagine an ice cube (that melts at 0°C) in an oven set at 400°C, would you be able to keep it at 0°C? Thanks to complex air cooling passage, nickel-superalloys resist the extreme temperature and extreme force they are operating for long time [1]

But as any super-{material}-heroes do they have their own kryptonite?

The reason why nickel superalloy has been selected is because it is stable at high temperature, and form a dense protective layer (to resist the extreme conditions). Let's take the hypothesis that a small object hit the turbine blade, it would peels or spalls off this protection. This layer will immediately grow again, very similar to skin tissue healing after a small cut. However, there are a couple of salts that disturbs this healing process. Let's take the example of table sea-salt (NaCl), at 700°C {considered low temperature for our superalloy} the chloride can migrate into the protective layer and breaks it, to continue the analogy, comparable to a cut in the skin. As mentionned above, this would typically heal.

However, if there is a small amount of sulphur available (usually there is in the fuel), the sea salt mixes with it and turns into sodium sulphate. This salt settles at the alloy interface and disturb the healing process in such a way that no protective layer can grow again. Nickel is then consumed in the environment. To continue the analogy with skin tissue, it tries to heal, but the blood does not coagulate anymore. This results in a deep crack and failure of the blade. Note that sodium sulphate is inoffensive by itself and only the 2 salts combined can cause the attack.

Superalloys have been around since Second World War. Since that time, there has been a fierce competition, driving higher efficiency and lower emissions. Have you heard about NEO plane (New engine option)? They claim up to 25% fuel saving compare to traditional engine (benefitting the airliner and supposedly the end user). This is possible thanks to higher burning temperature requiring even more resistant superalloys. Putting it simply, a painter mixes different colors to have different tone. Superalloys are in such way made of different elements. While the selection was made initially for high temperature and extreme force during long time at a decent cost and easy production, it was found later that some elements incorporated were more prone to react with salts.

Maybe an anti-kryptonite suit could protect our super? Coating is a good idea, and development is being done in that direction. However, like a blister on the back of your heel, the 2-steps attack arises in a contact zone at the lower part of the blade, and it is difficult to stick a coating that would resist. The solution at the next episode.

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