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Protection and Restoration
maintenance at elevated temperatures, etc. Shourgeshty et al. (2016) divide hightemperature damage into three general groups in line with temperature ranges:
1. High-temperature corrosion type II (600–850°C), where sulfates are
formed from the substrate at a certain partial pressure for sublimation of
sulfur trioxide. The sulfate reaction with alkali metal forms low-meltingpoint particles that prevent formation of a protective layer.
2. High-temperature corrosion type I (750–950°C), includes transportation
of sulfur from a deposit (sulfate base like Na 2 SO 4 ) through an oxide layer
into a metal substrate resulting in formation of stable oxides. After a reaction between a stable sulfide and sulfur moving through a scale, the base
metal sulfides form a disastrous sequence in the molten phase due to high
temperatures. Thus, formation of NiS 2 (molten at 645°C) and Co x S y (lowest liquids at 840°C) can lead to serious component degradation. The most
suitable materials which can resist type I hot corrosion are PtAl 2 -(Ni-Pt-Al)
coatings (aluminide coatings modified with platinum) and MCrAlY coatings containing up to 25 wt% Cr and 6 wt% Al.
3. Oxidation (950°C and higher), which depends on transportation of cations
or anions through the structure of an oxide layer and grain boundary. In
order to form a continuous oxide layer for cobalt base superalloys, Cr content should be at least 25%. To increase oxidation resistance of chromia,
addition of aluminum is preferred, especially for severe and critical conditions experienced by gas turbine blades. However, when thermal cycling
conditions prevail, oxide scales can spall from a substrate surface as a result
of thermally induced stresses. In the event, the oxidation resistance can be
improved by addition of reactive elements to alloys and coatings, such as Y,
Hf, and Ce.
Thus, the main purpose of coatings is to palliate for a poor oxidation resistance of a
base alloy (aluminide, Pt–aluminide, and MCrAlY) at a high temperature. By analogy, the authors distinguish three types of high-temperature coating:
1. Diffusional coatings. Chromium-rich coatings are resistant toward type
II high-temperature corrosion and provide some major benefits in protecting Ni-based alloys from corrosion by sulfatic deposits in chemical plants.
Aluminide coatings usually consist of an outer layer with an aluminumrich β-NiAl phase and an internal area rich in Ni. Aluminide coatings are
fabricated through aluminizing by two different processes which differ in
terms of aluminum activity in the gas phase and temperature of the process,
namely the low activity–high temperature (LAHT) process aimed at development of a β-NiAl coating and the high activity–low temperature (HALT)
method providing a δ-Ni 2 Al 3 coating which requires subsequent heat operations to convert it to β-NiAl phase. However, aluminide coatings lose their
flexibility at temperatures less than 750°C due to the internal diffusion and
oxidation and subsequent loss of aluminum protection from the surface. To
Protection and Restoration
maintenance at elevated temperatures, etc. Shourgeshty et al. (2016) divide hightemperature damage into three general groups in line with temperature ranges:
1. High-temperature corrosion type II (600–850°C), where sulfates are
formed from the substrate at a certain partial pressure for sublimation of
sulfur trioxide. The sulfate reaction with alkali metal forms low-meltingpoint particles that prevent formation of a protective layer.
2. High-temperature corrosion type I (750–950°C), includes transportation
of sulfur from a deposit (sulfate base like Na 2 SO 4 ) through an oxide layer
into a metal substrate resulting in formation of stable oxides. After a reaction between a stable sulfide and sulfur moving through a scale, the base
metal sulfides form a disastrous sequence in the molten phase due to high
temperatures. Thus, formation of NiS 2 (molten at 645°C) and Co x S y (lowest liquids at 840°C) can lead to serious component degradation. The most
suitable materials which can resist type I hot corrosion are PtAl 2 -(Ni-Pt-Al)
coatings (aluminide coatings modified with platinum) and MCrAlY coatings containing up to 25 wt% Cr and 6 wt% Al.
3. Oxidation (950°C and higher), which depends on transportation of cations
or anions through the structure of an oxide layer and grain boundary. In
order to form a continuous oxide layer for cobalt base superalloys, Cr content should be at least 25%. To increase oxidation resistance of chromia,
addition of aluminum is preferred, especially for severe and critical conditions experienced by gas turbine blades. However, when thermal cycling
conditions prevail, oxide scales can spall from a substrate surface as a result
of thermally induced stresses. In the event, the oxidation resistance can be
improved by addition of reactive elements to alloys and coatings, such as Y,
Hf, and Ce.
Thus, the main purpose of coatings is to palliate for a poor oxidation resistance of a
base alloy (aluminide, Pt–aluminide, and MCrAlY) at a high temperature. By analogy, the authors distinguish three types of high-temperature coating:
1. Diffusional coatings. Chromium-rich coatings are resistant toward type
II high-temperature corrosion and provide some major benefits in protecting Ni-based alloys from corrosion by sulfatic deposits in chemical plants.
Aluminide coatings usually consist of an outer layer with an aluminumrich β-NiAl phase and an internal area rich in Ni. Aluminide coatings are
fabricated through aluminizing by two different processes which differ in
terms of aluminum activity in the gas phase and temperature of the process,
namely the low activity–high temperature (LAHT) process aimed at development of a β-NiAl coating and the high activity–low temperature (HALT)
method providing a δ-Ni 2 Al 3 coating which requires subsequent heat operations to convert it to β-NiAl phase. However, aluminide coatings lose their
flexibility at temperatures less than 750°C due to the internal diffusion and
oxidation and subsequent loss of aluminum protection from the surface. To