Diffusion aluminizing – diffusion enrichment of surfaces with aluminum, process, properties and applications

Diffusion aluminizing is a process thermochemical treatment consisting in the diffusion enrichment of the surface layer of the material in aluminum AlThe process is primarily used to increase resistance to oxidation and corrosion at high temperatures, as well as resistance to certain aggressive industrial atmospheres.
During aluminization, aluminum does not simply form a coating on the surface. Al diffuse into the material and react with the substrate components, creating diffusion layer containing solid solutions and intermetallic phases.
In the case of steel and other alloys on the matrix iron the phases of the Fe–Al system are of particular importance, while in nickel-based alloys layers containing phases of the Ni–Al system may be formed.
The most important effect of the technology is the ability of the aluminum-enriched surface to produce a compact, stable aluminum oxide layer. Al₂O₃, which limits further oxidation of the material.

What is diffusion aluminizing?

During the process, the material surface is in an environment that provides active aluminum material.
At elevated temperatures the following occur:

1. production of active forms of aluminum in the process environment,
2. transport of aluminum to the material surface,
3. adsorption of aluminum on the surface,
4. diffusion of aluminum into the substrate,
5. mutual diffusion of material and layer components,
6. formation of solid solutions and intermetallic phases,
7. creation of a surface layer capable of producing a protective scale rich in Al during operation₂O₃.

The process is largely controlled by diffusion, therefore its course depends on temperature, time, aluminum activity and the chemical composition of the substrate material.

Why does aluminum protect the surface at high temperatures?

Aluminium has a very high affinity for oxygen.
If the surface of a material contains a sufficient amount of aluminum, a layer may form upon contact with an oxidizing atmosphere:

Correctly formed scale rich in Al₂O₃ it is compact, thermodynamically stable and is characterized by a relatively low rate of transport of components through the layer.
Thanks to this, it limits:

• access of oxygen to the material,
• transport of metal towards the surface,
• further growth of scale,
• the rate of substrate degradation at high temperature.

This mechanism makes aluminum one of the most important elements used in materials and protective layers intended for high-temperature operation.

Layer structure after diffusion aluminization

The layer formed during aluminization is not a homogeneous layer of pure aluminum.
Its structure depends on:

• substrate material,
• process temperature,
• aluminization time,
• aluminum activity,
• content of alloying elements,
• layer growth mechanism.

In the case of steel, the layer may contain different phases of the Fe–Al system , while in nickel alloys the phases of the Ni–Al system may be important.

Fe–Al phases in aluminized layers

In the case of a substrate based on iron diffusion aluminum material may lead to the formation of intermetallic phases of the Fe–Al system.
Depending on the local chemical composition and process conditions, there may be, among others, aluminum-rich phases such as FeAl and Fe₂Al₅ and other phases characteristic of the Fe–Al system.
Their share depends on technology.
Intermetallic phases can provide high chemical and high-temperature resistance, but some of them have limited ductility.
Therefore, in practice it is important to control not only the total layer thickness, but also its phase composition.

Aluminizing nickel alloys

Diffusion aluminization is of particular importance in the case of nickel-based superalloys used at high temperatures.
During the process, aluminum reacts with nickel, and one of the important phases occurring in aluminide layers may be β-NiAl.
This layer is a reservoir of aluminum, enabling the formation of a protective Al scale.₂O₃.
Solutions of this type are used, among others, in gas turbine components and other parts operating at very high temperatures.

Diffusion aluminizing temperature

Diffusion aluminizing requires a temperature that allows for sufficiently rapid diffusion. aluminum material.
Depending on the material and technology, processes can be carried out in a wide temperature range, often around 700-1100 ° C.
However, this range should not be treated as a universal technological parameter.
The temperature is selected depending on:

• substrate material,
• aluminization methods,
• required layer thickness,
• desired phase composition,
• process time,
• further heat treatment.

The effect of time on layer growth

Since the layer growth is diffusion-based, increasing the process time increases the depth of the material's enrichment in aluminum.
As in other diffusion processes, layer growth may, under certain conditions, exhibit a relationship close to parabolic:

where:

• x – characteristic layer thickness,
• k – coefficient depending on, among other things, temperature and material,
• t - time.

Increasing the temperature increases the diffusion rate, but at the same time it may accelerate the formation and growth of intermetallic phases and affect the microstructure of the substrate.

Diffusion aluminizing methods

Aluminization using the pack cementation method

One of the classic methods of diffusion aluminization is pack cementation.
The element is placed in a mixture containing:

• source aluminum material,
• activator,
• inert filling material.

During heating, the activator participates in the formation of volatile compounds that transport aluminum to the surface of the element.
Chemical reactions occur on the surface, releasing aluminum, which then diffuses into the material.
The process therefore includes both chemical transport in the gas phase and diffusion in the solid state.

Low- and high-activity aluminization

In aluminization technologies, especially superalloys, a distinction is made between processes that differ in the activity of aluminum.
In simple terms:

• highly active process promotes intensive transport of aluminum towards the substrate,
• low-activity process may lead to a different layer growth mechanism, with a greater proportion of diffusion of substrate components towards the surface.

The growth mechanism influences the location and composition of diffusion zones and the distribution of alloying elements.
The issue is of particular importance when designing aluminide layers on nickel superalloys.

Diffusion aluminizing and oxidation resistance

The most important function of the aluminized layer is to increase resistance to oxidation at high temperatures.
When the surface is exposed to oxygen, aluminum reacts to form Al₂O₃.
The protective oxide layer can:

• slow down further oxidation,
• limit material loss,
• reduce the rate of scale growth,
• protect the substrate material from direct contact with the atmosphere.

The ability to renew the protective layer depends on the availability of aluminum in the diffusion layer.
During long-term operation, aluminum is gradually consumed to form oxide and may also diffuse towards the substrate.

Aluminizing and resistance to high-temperature corrosion

Aluminized layers can increase resistance not only to oxidation in air, but also to some types of high-temperature corrosion.
They are used in environments containing, among others:

• exhaust gases,
• sulfur compounds,
• fuel combustion products,
• hot industrial atmospheres.

However, the effectiveness of protection depends on the specific environment. The Al ₂ O ₃ layer does not provide identical resistance in all chemical conditions.

Diffusion aluminization and diffusion chromization

Aluminization and chromium diffusion plating are processes that increase the resistance of a surface primarily by enriching it with an element capable of creating a protective oxide layer.

Both processes can be used to protect materials operating at high temperatures, but the protection mechanism, layer stability and suitability in a specific environment depend on the material and operating conditions.

Diffusion aluminizing and boriding

Drilling It is primarily used to obtain very hard boride layers with high abrasion resistance.
Diffusion aluminizing has another primary purpose – protection of the material against chemical degradation and oxidation at high temperatures.

Diffusion aluminizing and nitriding

Nitriding consists in enriching the surface in nitrogen and serves primarily to increase hardness, wear resistance and fatigue resistance.
Aluminizing leads to enrichment of the surface in aluminum material and is primarily focused on chemical resistance at high temperatures.
The processes also differ in temperature and the type of phases formed.

Diffusion aluminizing and carburizing

During carburization the steel surface is enriched with carbonand then usually subjected to hardening in order to obtain a hard martensitic layer.
In diffusion aluminizing, the primary goal is not to obtain martensite or maximum hardness, but a change in the chemical composition of the surface enabling the formation of a protective layer rich in Al₂O₃.

Diffusion aluminizing of superalloys

One of the most important areas of application of technology is nickel superalloys designed to work at high temperatures.
Aluminide layers are used, among others, on:

• turbine blades,
• turbine guides,
• combustion chamber elements,
• aircraft engine parts,
• gas turbine components.

The aluminium-rich layer enables the formation of a protective Al ₂ O ₃ scale , while the high-temperature mechanical properties are ensured by a suitably designed substrate alloy.

Aluminization as a bonding layer in TBC systems

Aluminide layers can also be part of more complex protective systems used at high temperatures.
In systems TBC – Thermal Barrier Coatings the metallic layer underneath the ceramic thermal insulation coating should enable the formation of a stable oxide layer.
Aluminum contained in the bonding layer can form Al₂O₃, which plays an important role at the interface between the metallic protective layer and the ceramic outer coating.
Aluminides can be used alone or as a substrate for more complex protective systems.

The influence of alloying elements on the aluminized layer

Degradation of the layer during operation

The aluminized layer is not an inexhaustible source of protection.
During long-term operation at high temperatures, aluminum is gradually worn away by:

• creation and renewal of Al₂O₃,
• diffusion towards the substrate,
• mutual diffusion processes of the components of the layer and the material,
• loss of oxidation products due to cracking or chipping of scale.

If the aluminum content at the surface falls below a level that allows effective regeneration of the protective scale, the protective properties of the layer may deteriorate.

The importance of thermal expansion

Components operating at high temperatures are often heated and cooled repeatedly.
The protective layer, the substrate material and the resulting scale may have different thermal expansion coefficients.
This causes thermal stresses to occur during temperature cycling.
A good layer should therefore not only create a chemically stable Al₂O₃, but also maintain adequate adhesion during repeated temperature changes.

Application of diffusion aluminization

Aluminizing is primarily used for components operating at high temperatures and in aggressive atmospheres.
Typical applications include:

• gas turbine components,
• turbine blades,
• aircraft engine components,
• parts of combustion chambers,
• elements of industrial furnaces,
• pipes and components of high-temperature installations,
• elements of the petrochemical industry,
• parts of energy equipment,
• elements operating in hot gases and exhaust gases,
• parts exposed to high-temperature corrosion.

Advantages of diffusion aluminizing

• high resistance to oxidation at elevated temperatures,
• possibility of creating protective Al scale₂O₃,
• metallurgical bonding of the layer to the substrate,
• lack of the classic sharp boundary characteristic of many applied coatings,
• the possibility of increasing resistance to certain types of high-temperature corrosion,
• can be used on steels and superalloys,
• the possibility of combining the mechanical properties of the core with high chemical resistance of the surface.

Limitations of diffusion aluminizing

• high process temperature,
• possibility of changing the microstructure of the substrate,
• risk of growth of brittle intermetallic phases,
• possibility of changing the dimensions of the element,
• gradual consumption of the aluminum reserve during long-term use,
• strong dependence of the layer properties on the substrate material and process parameters,
• the need to take into account mutual diffusion during long-term operation at high temperature.

Quality control of the aluminized layer

Assessment of the layer after aluminization may include:

• layer thickness measurement,
• metallographic tests,
• content profile analysis aluminum material,
• identification of intermetallic phases,
• diffusion zone analysis,
• oxidation resistance tests,
• cyclic oxidation studies,
• high-temperature corrosion resistance tests,
• assessment of adhesion and continuity of the layer,
• assessment of changes in the structure of the substrate material.

In high-temperature applications, not only the initial structure of the layer is particularly important, but also its stability during long-term exposure and cyclic temperature changes.

Diffusion aluminizing – comparison with other thermochemical treatment processes

Diffusion aluminizing is a thermochemical treatment process consisting in the diffusion enrichment of the surface layer of the material in aluminum material.
The most important goal of the process is to obtain a surface capable of creating a compact and stable layer Al₂O₃, limiting the rate of oxidation and degradation of the material at high temperature.
In the case of steel, Fe–Al phases may form in the layer, while on nickel superalloys Nickel aluminides, including the β-NiAl phase, are of particular importance.
Diffusion aluminizing differs fundamentally from processes aimed primarily at increasing hardnessUnlike carburizing, nitriding or boriding, its most important function is chemical protection of surfaces when working at high temperatures.
The process is therefore used primarily in the energy, aviation, petrochemical industries, gas turbines, industrial furnaces and other devices exposed to long-term exposure to high temperatures and aggressive atmospheres.

Methods of heat and thermochemical treatment of steel

Diffusion aluminizing is just one of the processes used to modify the microstructure and properties of steel. Depending on the desired effect, normalizing, stress relieving, hardening, tempering, and thermochemical processes such as carburizing, nitriding, and others are also used.

Steel heat treatment methods

• Normalizing steel – grain refinement and microstructure unification.
• Stress relief annealing – reduction of residual stresses.
• Softening of steel – reducing hardness and improving susceptibility to further processing.
• Full annealing – austenitization and slow cooling.
• Isothermal annealing – transformation carried out while holding at a properly selected temperature.
• Ball annealing – spheroidizing – change in the morphology of cementite or other carbides.
• Recrystallization annealing – removal of the effects of crushing and restoration of plasticity.
• Homogenizing annealing – reducing segregation and homogenizing the material composition.
• Hardening and heat treatment – shaping the microstructure and properties by hardening and tempering.
• Tempering steel – controlled modification of steel properties after hardening.
• Induction hardening – surface induction heating and rapid cooling.
• Flame hardening – surface hardening after local heating with a flame.
• Oversaturation – obtaining a supersaturated solid solution by an appropriate thermal cycle.
• Aging – controlled phase separation leading to material strengthening.

Methods of thermochemical treatment of steel

• Nitriding of steel – diffusional enrichment of the surface layer with nitrogen.
• Carburizing steel – diffusional enrichment of the surface with carbon.
• Carbonitriding – simultaneous enrichment of the surface layer with carbon and nitrogen.
• Nitrocarburization / nitrocarburization – enrichment of the layer primarily with nitrogen and carbon.
• Drilling – diffusion saturation of the surface with boron.
• Diffusion chrome plating – diffusion enrichment of the layer with chromium.
• Diffusion aluminizing – diffusion enrichment of the layer in aluminum.