Aluminising (sometimes referred to as AlonizingTM and Calorizing) is a high temperature chemical process whereby aluminium diffuses into the surface of the base metal to form a metallurgical aluminide surface layer. Diffusion Alloys is a global leader in the provision of aluminide diffusion coatings, offering the widest range of applications for aluminising of any company in the market.
Properties and Uses of Aluminising
Aluminide diffusion coatings offer protection in medium and high temperature operating environments against:
- Hot Corrosion
- Metal Dusting
This protection is conferred by the aluminium oxide that forms at the aluminide coating surface in service. This oxide constitutes an extremely effective barrier. The aluminide coating essentially acts as an aluminium reservoir to maintain and support the aluminium oxide. Materials suitable for aluminising include mild steel, low alloy steels, cast/wrought stainless steels, nickel-base alloys and nickel-iron alloys.
Areas of application for aluminising are still evolving. As turbine inlet gas temperatures increased to enhance output and efficiency, aluminide diffusion coatings became widely applied to protect the external and internal cooling channel surfaces of gas turbine blades and vanes.
Diffusion Alloys is also a global market leader in the application of aluminising into process industries such chemical/ petrochemical, iron and steel, energy and biomass and, most recently, fuel cells.
Diffusion Alloys' Aluminising Facilities and Processes
For the application of aluminide diffusion coatings for small components, we operate at our facility in Hatfield five air furnaces (glass sealed box method) and a further six furnaces that are deployed across fifteen retorts operating under protective atmospheres.
We have been aluminising long tubes/pipes and large parts in our facility on Teesside since 1985 and have recently acquired the largest aluminising furnace (at 21m length) of any supplier in the world, which allows us to coat tubes of up to 18.5m in length.
Aluminising of components is carried out using one of four processes.
Our most widely used process is performed in either a glass seal box in an air furnace or in a reducing/inert gas atmosphere retort.
The chosen components are embedded in a proprietary powder mixture referred to as the 'pack' or 'compound' which consists of:
- A source metal: Al (may be moderated)
- An activator (e.g. Halide)
- An inert diluent (e.g. Al₂O₃; prevents pack powder particles sintering together)
At the required coating temperature, the activator reacts with the source metal to form a gaseous halide compound, which is transferred to the substrate surface. As this gas decomposes, the halogen activator is released and the coating element is deposited at the substrate surface, leaving the activator to return to the pack and react with the source metal again.
Coating thickness is controlled by temperature and time at temperature. For example, at our coating facility on Teesside for long tubes/large parts, the parts to be aluminide diffusion coated are packed in aluminising compound within steel retorts, heated slowly to a temperature within the range 900 - 1050°C, held at temperature for 24 - 48 hours then slow cooled to ambient temperature.
Parts may be simultaneously aluminised both internally (by filling them with compound) and externally, or external or internal only aluminide coatings may be applied.
Very similar to the pack cementation method; however, components to be coated are placed above and hence out of contact with the pack.
Con: increased path length for halide transport generally involves reduced deposition rates.
This is a process uniquely developed by DAL to apply diffusion coatings to serpentine internal cooling channels in gas turbine blades and vanes. The method is an extension of our Above- Pack process but differs in two crucial aspects:
- Each component is situated above its own dedicated reactive powder mixture.
- The volatile metal halides generated in this powder mixture are picked up by carrier gas and passed via a manifold into the serpentine internal cooling channels of the turbine blades/vanes.
This method allows coating of specific areas and local diffusion coating repair. Slurries consist of an organic binder mixed in equal parts with water, to which variants of reactive pack mixture are added.
Depending on their composition, slurries can be applied either by painting or pasting. Areas not to be coated are masked to ensure no slurry is applied to these areas.
In concert with an extensive menu of diffusion coating compounds we are able to offer tailor made solutions to any customer's diffusion coating requirements.
To minimise costs, all but a very few of these compounds can be regenerated for continual re-use.
Stringent quality control procedures are in place to guarantee compound properties, with each batch individually checked prior to release.
Similarly, representative and strategically located test pieces accompany every diffusion coating run to facilitate in-process control and certification.
Aluminising can also be referred to as Aluminizing, AlonizingTM and Calorizing.