posted in Processes in the Spotlight
Anodising is used to increase surface hardness, wear resistance and dielectric strength of metals such as Aluminium, Titanium and Magnesium. Anodising can also provide a good base for painting or colouring, creating an extremely stable but porous oxide layer on the surface of the metal. And there you were thinking it was a decorative finish!
What is Anodising?
Anodising is very simply an oxidation process, as is the corrosion of steel into iron oxide (rust). However, the oxides of aluminium, titanium and magnesium are highly stable ceramic compounds which are integral to the metal surface, they will not flake or peel.
Anodising is the process of submerging the metal in an acidic solution, and applying a voltage to promote the absorption of oxygen into the surface. In the case of anodising aluminium, this causes the formation of the hard ceramic compound, Aluminium Oxide.
Types of Anodising
Anodising can be categorised in a number of ways, down to material application, chemical composition of the acid used, end use application as well as the basic measure of the oxide layer thickness formed. There are three types of anodising capability that are commonly found in the UK. So let’s get into it…
Chromic Acid Anodising
The least commonly used of the three, chromic anodising is a preferred choice in certain applications such as the anodising of castings and in situations where a reduction in fatigue strength is not acceptable. Film thicknesses are the thinnest of the three common processes, being anywhere between 1-10Microns. For that reason, they aren’t ideal for applications which require colouring, as the oxide layer can be too thin to provide a consistent all over colouring.
Sulphuric Acid Anodising
This is by far the most widespread process and varies from Chromic Anodising in the chemical composition of the solution and process variables such as the level and method of application of voltage. A range of oxide layer thicknesses can be achieved as can effective colouring, most commonly achieved by the application of a secondary colouring treatment. Film thicknesses in aluminium are in the range of 5-25 Microns, with 10-15 required for effective colouring using dyes. Coating thicknesses in titanium are significantly less, being in the range of 0.1-0.3Microns.
Hard Anodising is ultimately is the thickest form of anodising, creating an oxide layer which is between 20-100 Microns thick, depending on the base alloy being treated. It offers the highest increases in hardness, wear resistance and electrical resistance and is primarily a functional coating. It is also performed in a sulfuric acid solution, but with higher acid concentration, low temperature and a higher voltage applied.
What processes typically accompany Anodising
Anodising is largely understood as a finishing process as it provides mechanical and decorative property enhancement of a component largely in the finished state. However, anodising can be used very effectively as a pre-treatment for painting processes - providing micro-pores for the paint to adhere to. In the case of Magnesium anodising, it is almost exclusively used to provide a solid base for further painting. The porous and low density surface is not anywhere near as protective as the oxide layers produced on aluminium and titanium.
Strictly speaking the anodising process itself does not provide all the property enhancements described. Particularly in the case of Aluminium, it’s the process of forming the oxide layer, followed by a sealing of the surface that provides the full benefits of anodising. Sealing can be done in a variety of ways, but the ultimate mechanism is the plugging of the surface pores, and the conversion of the metal oxides into even more stable oxide form.
Effective sealing of Hard Anodised components can provide protection in deep sea, or harsh saltwater environments.
It’s not typically possible or necessary to seal the surface of anodised titanium, mainly because the oxide layer thickness is so thin that sealing would not provide any further enhancement.
Coating & Colouring
Although anodising in itself is a conversion coating - coatings such as PTFE fluoropolymer and colouring coatings using dyes can be applied after anodising and before sealing. With the addition of a PTFE coating, not only does the surface have increased hardness, wear resistance and corrosion resistance, but it’s lubricity can also be greatly improved. The application of colour dyes is typically performed in this way, as a post process and able to create a range of colours. Colour intensity and tone is impacted greatly by the exact chemical composition of the material, therefore the same treatment can produce different results between material batches.
In the anodising of titanium, the colouring is actually provided by the anodised surface itself, therefore an additional dyeing process is not required. An array of colours can be achieved with titanium anodising by varying the surface chemistry.
Colouring can be effective not only for decorative purposes, but is highly applicable in situations where easy and assured component recognition is required.
Laser marking can remove the oxidised layer of material locally, when combined with colour anodising this can create a high contrast between the base material and the coloured anodised layer. This can be used for decoration, such as the marking of a logo, or more functional purposes such as part numbering or marking.
When colouring an anodised layer, it can be noticed that sharp edges are either devoid of, or have a lower intensity of colour. This is because the oxide layer created during anodising grows perpendicularly from the surface of the material, sharp corners are therefore protected, but dye’s will not be as prominent. A simple deburring operation by vibratory deburring or tumbling prior to anodising can be highly effective in overcoming this problem.
Chemically Brightening & Etching
Enhancement of the material surface can be achieved using various processing involving submerging the component in chemical solutions. Solutions can be used to:
Depending on the alloy, the surface finish required after anodising and how the alloy has been processed a mixture of these chemical pre-treatments will be required.
For decorative finishes where high reflectivity is required a Chemical Brightening is ideal, capable of reaching intricate geometries unlike mechanical polishing. It's worth noting that some aluminium alloys can only produce a matt finish regardless of their surface condition prior to anodising.
Etching can be used to remove a very thin layer of material, impurities and naturally formed oxides from the surface of a component prior to anodising. This pretreatment can be carried out to different degrees, more intense etching reducing surface imperfections, providing a more uniform appearance.
In the pretreatment of Cast Aluminum alloys where magnesium content is a chemical etch is essentials to remove the buildup of magnesium oxides on the material surface to avoid patchy or inconsistent creation of the main base material oxide as intended.
Chromate Conversion Coating
Anodising produces an electrically insulative oxide layer. In instances where electrical conductivity is preferable, or a particular requirement of a component, Chromate Conversion Coating can be used in combination with Anodising. This is often referred to as “Dual Finishing”. The process involves masking, stripping and sequential application of the two different finishes. The processes are usually combined in situations where the physically hard wearing nature of anodising is required, however electrical conductivity is required, for example at earthing points.
What processes are alternatives?
Anodising is a dominant form of finishing in the world of aluminium, and this is largely due to it’s cost effectiveness, availability and suitability across a whole host of applications. Whether the purpose is purely decorative, or more technical in nature, anodising can satisfy your needs. Anodising of Magnesium and Titanium is less widespread, and they each have many alternatives for finishing. Here we’ll primarily list alternatives for anodising of Aluminium.
Anodising vs Chromate Conversion Coating
Chromate Conversion Coatings are often referred to by their commercial names, such as “Alocrom 1000”, “Iridite NCP” or “Surtec 650”
Anodising vs Electroless Nickel Plating
Electroless Nickel Plating can also be used as an intermediary process for further electroplating of aluminium alloys. The process involves several steps, including deoxidation, coating with zinc and then removal and substitution of that zinc for Nickel in the electroless nickel plating process. Beyond this a number of electroplated metals can be applied, such as Tin, Silver, Copper and even Gold.
What to Consider When Choosing and Specifying Anodising
Final Finish or Pre-treatment
The first thing to establish is whether the Anodised finish is the final finish you require. As anodising may be used as a porous pre-treatment for processes such as painting, a sealing process must also be specified if Anodising is the final required finish.
Importance of Electrical Conductivity
If electrical conductivity, even if only for earthing purposes is required, removal of the anodised surface locally is necessary. Ideally, to maintain sufficient levels of conductivity, a conductive coating needs to be applied to the bare aluminium substrate.
With increasing thickness comes increasing cost and surface roughness, therefore choosing an optimum thickness is necessary. Consulting with your anodising supplier will allow you to establish this for your given application
Film Thickness Variation
Deposit uniformity is not a strong point of the Anodising process, or any electrolytically deposited process. Geometry such as sharp corners, stepped areas and holes can all result in high surface thickness variation, down to zero coverage. Specifying a toleranced fit with an anodised aluminium component is therefore difficult, however impregnations of PTFE into the porous surface can significantly help.
Jigging allows Only 99.9% Coverage
As Anodising is an electrolytic process there needs to be at least one electrically conductive point on the component at all times. This results in the area of conduction achieving no coverage. For this reason it’s important to talk to your anodising supplier about jigging requirements during design. In many cases the conductive points can be placed on non-critical or decorative surfaces. These are usually in the form of threaded holes for good electrical contact.
Fatigue Life Reduction
If fatigue life is an important factor in your design application, Anodising may play a factor in mechanical design. The anodising process reduces the fatigue life of aluminium, more so with higher thicknesses. For this reason, thinner films of Chromic Anodising are often used in Military and Defence applications.
Non-Aluminium not compatible
During the Anodising process high voltages can cause non-aluminum inserts and foreign components to melt. Therefore it’s important that any helicoils or inserts made from anything other than aluminium are inserted or attached only after anodising.
Colour Variation - Same Alloy
Variations in colour are significant dependent on the process, thickness of the oxide layer and the composition of the base aluminium alloy to mention a few. If colour matching is of vital importance, as with decorative applications, it’s important to ensure your components are all made from the same batch of material. Most material stockists will be able to satisfy this requirement. Obviously this is not viable in a high volume environment, however colour matching with the batch will be effective.
Anodising is a versatile process, being applicable as a pre-treatment as well as a final finish. The process is however multi-stage and requires careful consideration, and accurate specification in a technical environment. Even for decorative only purposes, the variation in exact anodising process, the sealing process, when and if dyes are applied can all make a significant difference to the output.
Anodising is used across a whole range of industries, for many different reasons. It has the ability to transform the soft underlying base material Aluminium, into a hard, wear and corrosion resistant engineering material.
Applications do go beyond that of aluminium anodising, and Titanium in particular can have it’s own set of high performing characteristics further improved by the process. Magnesium on the other hand, although still readily anodised benefits from anodising as a pre-treatment for further coating.
Sourcing Anodising services, whether as a final finish or as a pre-treatment can benefit component design in a number of areas. With the oxide film being an integral part of the substrate it offers a significant advantage over other plating and corrosion protection surface treatments. The oxide film will never flake or chip, whilst thicknesses can be designed to provide the level of ruggedness required.
Where can I find Anodising Services?
In the UK we have suppliers offering each of the Anodising processes covered, across a variety of materials. With the extensive use of aluminium in both technical and decorative environments, at high and niche volume it is easy to find a supplier that fits with your requirements.
You can start your search today - need Sulphuric Acid Anodising and Chromate Conversion Coating from one supplier? No problem - just head on over to the Manufacturing Network Supplier Search Engine and you’ll find suppliers offering the exact combination of processes you require.
You’ll be amazed at how you can start improving and simplifying your supply chain through using our Supplier Search Engine to find suppliers with capability that matches your exact needs.
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