On exposure to air, Aluminium will immediately form a stable oxide layer. This natural oxide film is, however, very thin, approximately 1/100th of a micron, is easily damaged and provides little corrosion resistance under normal atmospheric conditions.
Anodising is the controlled formation of an oxide coating on the surface of components made, primarily, from Aluminium and its alloys. The anodic coating is produced by passing an electric current through a dilute Sulphuric acid solution, Hydrogen is released at the cathode (the negative side of the cell) and Oxygen is formed at the anode (i.e. the work piece) forming Aluminium oxide. The thickness, density, hardness and colour of the coating can be varied depending on the solution and the processing conditions.
By modifying the processing conditions and electrolytes, a wide range of properties and coatings can be achieved, making anodising an ideal surface engineering technique for a wide range of applications.
The Main Anodising Processes
Sulphuric anodising or 'normal' anodising is used extensively for general protective and decorative applications. It produces a clear film which can be left natural or dyed to produce a range of colours, depending on the base material used.
The nature of sulphuric films enables them to be used with a range of pre-treatments to produce a variety of attractive visual finishes.
Chromic anodising is normally used for the corrosion protection of critical engineering components that require only a thin film, with minimum dimensional effect, or to ensure maximum fatigue integrity.
It is also used for crack detection and is an ideal pre-treatment for paint.
Chromic anodising is the preferred treatment for components containing deep crevices, since any residual electrolyte is non-corrosive to Aluminium.
Hard anodising uses formulated acid solutions normally operated at low temperatures, under high current density and voltage conditions. This produces an exceptionally hard coating with excellent wear, abrasion and corrosion resistance, and high electrical resistivity. "Hardas", "Hardex" and "Dioxal" are variations of the hard anodising process all in use at Acorn Surface Technology, selected for use depending on the design requirements of the application. The colour of the anodic film will depend on the alloy type used as well as the process.
Hard anodised films are usually supplied unsealed for maximum abrasion resistance, but can be further treated with dyes, solid phase lubricants or chemically sealed for high levels of corrosion protection.
By careful selection of the correct processing conditions, anodising can offer the complete answer to many Aluminium protection problems
Ion Vapour Deposition, known as IVD or Ivadising, is a physical deposition process which is used to apply pure Aluminium coatings to various substrates, improving the resistance to atmospheric and bi-metallic corrosion.
The design and manufacture of modern aircraft demands the use of high performance materials and protective systems which also need to be environmentally friendly. Historically Cadmium plating has been the mainstay for protecting both Steel and Aluminium components from corrosion but, following increased environmental concerns and recent legislation, it is increasingly becoming unpopular and expensive.
Many alternatives to Cadmium have been investigated over the years but none have fulfilled all the requirements, however IVD Aluminium has emerged as the leading single process replacement in many applications.
IVD Aluminium is a smooth, uniform coating of pure aluminium, produced by vacuum deposition and is currently used in a range of aerospace and defence product applications where critical corrosion resistance with electrical conductivity is paramount or where dissimilar metal contact can cause galvanic attack.
Typical components currently processed with an IVD Aluminium coating include – Engine and Airframe fasteners (Steel and Titanium), high tensile Steel airframe parts, Titanium bearing shells, landing gear components and assemblies, sintered magnets and electrical connectors.
Typical coating thickness is 7- 50 microns dependant on application.
We have an aerospace-standard, accredited Fluorescent Flaw Detection facility capable of inspecting Aluminium and Steel components.
Acorn Surface Technology can offer wet paint and lacquer coatings which meet UK and American Military and Aerospace Standards. Depending upon the specification, the coatings can be either air dried or force cured.