Hot Corrosion

Question 1

How can hot corrosion occur?

Answer 1

Many Ni based super alloys components are used in environments where they are exposed to hot gases. Typically the most severe conditions arise between ~760-1000C (i.e. turbine blades).

In these operating conditions, other additions of oxygen (sulphur) and conditions where condensed phases are present on the surface of the alloy. The condensed phases, have an effect on the protective oxide layers, designed to provide resistance to attack.

Question 2

Condensed _______ are thought to be the primary responsible reason for hot corrosion? From what sources?

Answer 2

Alkali metal salts (e.g. Na2SO4)

1. Ingestion of sea salt in marine environment.
2. Combustion of fuels.

Question 3

What happens during combustion in the gas turbine?

Answer 3

Sulphur from the fuel reacts with sodium chloride from ingested air at elevated temperatures to form sodium sulphate.

The sodium sulphate then deposits on the hot-section components, such as nozzle guide vanes and rotor blades, resulting in accelerated oxidation (or sulphidation) attack. This is commonly referred to as “hot corrosion”

Question 4

What is the temperature range for hot corrosion attack?

Answer 4

~800-950C

Question 5

What are the two types of hot corrosion?

Answer 5

Type 1 - has two stages. an incubation period exhibiting a low corrosion rate, followed by accelerated corrosion attack

Type 2 (~670-750C) - characterised by:
Pitting attack with little or no internal attack underneath the pit
Generally observed in marine and industrial gas turbines, which operate at lower temperatures

Question 6

Describe the incubation period.

Answer 6

Initial stage:
Quite long time period.

Initiation of accelerated corrosion attack is believed to to be related to the breakdown of the protective oxide scale.

Protective oxide layer may dissolve into the salt deposit and precipitate as non protective acidic layer. (especially when deposit is a liquid)

Known as “fluxing reaction”.

Propagation stage:
Reaction accelerated

Less protective reactive layer (Cr2O3, Al2O3)

Part should be removed from service

Question 7

What is the effect of chromium in hot corrosion?

Answer 7

The effect of chromium is dominant and a minimum 15wt% is required.

Cr has been shown to inhibit “fluxing”

Cr decreases the rate of low temperature hot corrosion