What thickness should I use for …?
The thickness of anodising that you should use depends on the service environment of the component. We have produced some recommendations based on what we find is suitable for most of our customers. The corrosion resistance and long term performance of the anodising depends on two things:
- anodising thickness
By far the most important factor in the corrosion resistance of anodising is sealing, not thickness. Even very thin coatings have good corrosion resistance when sealed. The quality of sealing is difficult to evaluate non-destructively, so it is important to use a reputable anodising company that has good process controls, which include periodic salt spray testing. Good sealing is also essential for dye colour retention.
Thickness is important for wear and scratch resistance, as well as a host of other features which are discussed below.
Wear and Scratch Resistance
In general, thicker coatings will be more wear resistant and more scratch resistant than thinner coatings. The explanation for this is simple; it is because there is more material to wear/scratch through. For this reason and others, chromic anodising is relatively poor for wear/scratch resistance. Sulphuric anodising or hard anodising is a better choice for improving wear qualities. Hard anodising provides the best resistance to both wear and scratching but has other trade-offs, one of which is cost.
Beyond approximately 50µm the wear resistance of hard anodising starts to decrease in relative terms, so it shouldn’t be assumed that more thickness is automatically better. 40 to 60µm is a typical thickness range for unsealed hard anodising in a wear environment. Note that unsealed coatings have better wear resistance than sealed coatings (in particular hydrothermally sealed coatings), but is not well suited to corrosive atmospheres. In relatively non-corrosive environments, the application of PTFE or an oil may be a suitable compromise.
Corrosion resistance is not highly influenced by coating thickness as stated above, sealing is the key concern. However a higher thickness is likely to provide some improvement in corrosion resistance. In this way, for best corrosion resistance, we suggest sulphuric anodising (in excess of 8µm) or hard anodising (up to 60µm). While hard anodising above 60µm is possible and a service we offer, at higher thicknesses there is a higher risk of crazing, which may impact corrosion resistance.
The benefit of increased thickness is relatively limited though. Good corrosion resistance (in excess of 336 hours NSS exposure) is readily obtainable with our chromic anodising process and our thin film sulphuric anodising process (both yielding a thickness under 5µm).
With increased thickness, as a rule of thumb, there is a reduction in fatigue strength. Some of which may be restored by the sealing process. Loss of fatigue strength isn’t entirely determined by thickness, but also partly by the method of coating formation. Thinner coatings, such as chromic anodising and, to a lesser extent, thin film sulphuric anodising are generally preferred when fatigue is a concern.
Sulphuric anodising is preferred for parts that are to be coloured by dying because the anodising itself doesn’t interfere with the colouring. Typically a thickness range of 8 to 25µm should be allowed for parts that are to be dyed. We suggest that you don’t specify a tighter range that this for dyed work because coating thickness can affect the type of dye colour which can be achieved. Please contact us for advice if you have a particular coating thickness requirement for your dyed anodised coating.
Hard anodised parts can be dyed, but the natural grey colouration of the coating makes the range of colours less appealing. Lower thicknesses (below 40µm) are recommended for dyed parts because beyond this the grey dominates. The exception to this is black dyed parts, which are unaffected by the background grey colouration Chromic anodising has poor suitability for dying at any thickness, although it can be tinted black at higher thicknesses.
Anodising naturally dulls the surface of the aluminium as coating thickness increases. You are recommended to request flash anodising on parts that you wish to be as bright as possible because this minimises dulling. Flash anodising is typically slightly thicker than thin film sulphuric (which may have an iridescence that is undesirable), but a lower thickness than standard sulphuric anodising. Chromic anodising and hard anodising should be avoided when brightness is desired.
Common thickness ranges
To benefit from economies of scale, it is advisable to choose “defacto” standard thickness ranges.
|Finish||MIL-PRF-8625||Our recommended thickness|
|Sulphuric anodising, undyed||Type II||8 to 13µm or 5 to 15µm|
|Sulphuric anodising, dyed||Type II||8 to 25µm or 5 to 25µm|
|Hard anodising, preferred for cosmetic dyed work||N/A||25 to 40µm|
|Hard anodising, general/dyed||Type III||40 to 60µm|
|Hard anodising, for wear resistance||Type III||40 to 60µm|
|Chromic anodising||Type I||Thickness not specified because inherent to process, subject to other controls|
|Thin film sulphuric anodising||Type IC or Type IIB||Thickness not specified, subject to other controls|
Tighter ranges and alternative ranges are available, for optimum quality, price and turnaround, you are recommended to use the above.
ISO 7599 System ("AA" System)
An alternative method of specifying thickness is by specifying minima instead of ranges. This method is promoted by ISO 7599 (and its predecessor EN 12373-1). An example would be AA10, which means a minimum average thickness of 10µm). For many applications we believe this is not sensible because it can lead to a coating thickness that the designer wasn't expecting but that is perfectly compliant. We suggest using a range in all cases.
Your Purchase Order
Please tell us what thickness you require on your purchase order. We suggest that you use a range rather than the ISO 7599 system.
More Info and Specialist Advice
Please email us if you would like advice on anodising thickness. This page just gives general suggestions and our specialists will be able to provide more in-depth information.
This page is provided for information only, it should not be considered advice and we cannot accept any responsibility or liability for your use of the information on this page. The information on this page is used and relied on at your own risk and you bear the sole responsibility for any outcomes. E&OE.