Hard Anodising

Hard anodising is anodising produced under very specific operating conditions that produces very functional coatings.  These hard anodised coatings are typically 50 microns in thickness and have excellent wear and corrosion resistance characteristics.  Hard anodising has many uses and is typically found where either a high degree of wear resistance or corrosion resistance is required i.e. sprockets, pulleys, boat parts, pistons etc....

Functional characteristics of hard anodising

While the typical hard anodised coating is 50 microns thick, coatings ranging from 30 - 75 microns can be readily achieved and if required a 100 microns may be possible.  One important point point about the coating thickness is that a greater coating thickness may not necessarily improve the functional performance of the hard anodised coating.  Coatings over 75 microns can have worse wear performance than the standard 50 micron.  Also a coating of 100 microns may not perform any better with regards to corrosion resistance due to the high likely hood of the coating cracking.

Interestingly enough there is no set standard for how hard, hard anodising must be.  The hardness can vary quite a lot and depends on having operating condition that favours smaller pores, choice of alloy and the actual coating thickness.  While the hardness of hard anodising in comparison to hard chrome and heat treated steel is certainly lower, hard anodised coatings are normally equal or better when it comes to comparing wear resistance.

What are the best alloys for hard anodising

Alloys from the 5000, 6000 and 7000 series are regarded as best for hard anodising.  Those alloys that are high in Copper (>%) and/or Silicon (>%) are generally not suitable for hard anodising.  Due to the high copper content found in 2000 series alloys, there is a high risk of 'burning', and operating parameters have to be very carefully controlled to try to avoid this.

Hard anodised colours

As hard anodised coatings are both very thick (50+ microns) and dense, hard anodising in its natural state varies in colour from a dark grey to brown shades.  Due to this hard anodising is not suitable for most colour dyes except black.  In most cases this is not an issue as the purpose of the coating is functional and not decorative.

Considerations for hard anodising

• Size of the parts - With hard anodising you can expect to see a dimensional change of 1/2 the coating thickness that is applied. E.G If anodised to 50 microns then a dimensional chage can be expected to be 25 microns on every surface.  This will need to be factored in prior to machining.  Just as the outward size of part increases, similarly bores will shrink by the same ratio and overall by the total coating thickness, so masking may be required.
• Surface finish - If wear resistance is critical for your components, then the surface finish prior to anodising must as smooth as possible.  If not there is a likely hood that any raised spots of the coating may fracture off and become trapped between the 2 sliding surfaces, resulting in a greater rate of wear than would otherwise be the case.
• Joins - Any areas that are riveted or spot welded should be avoided
• Earth point - Should an earth be required, this must be masked prior to anodising due to the high electrical resistance of the anodised coating and the difficulty in removing the coating after anodising.
• Non-Aluminium Parts - As with standard anodising, attachments such as those made from steel will be severely damaged
• Mixed Alloys - Avoid this where possible, where as with standard anodising where certain alloys can be anodised together this may not be the case for hard anodising.  Slight differences in the makeup of the alloy can result in one anodising at a much greater rate than the other.  In other words, slight variations that do not rate in standard anodising are magnified in hard anodising.
• Forming - As with all anodising any bending/forming of the product needs to be done prior to the anodising process.  Aluminium Oxide will crack and fracture if bent
• Sharp Edges - Anodising grows perpendicular to the surface.  This means that on sharp outer edges there will be a 'void' where there is no anodising, while on sharp inside corners there will be what is termed a 'crash' where the coating grows into itself.  Neither is desirable as it will lead to quality defects.  There are some guidelines that can be followed to avoid these issues and these are laid out in the specification MIL-A-8625F.  The following table identifies an optimum amount of curvature relative to coating thickness that will help avoid 'voids' and 'chrashes'.

• Threads - There can be quite a dramatic change in thread dimensions during the hard anodising process.  It is commonly quoted that the pitch diameter will change by a ratio of 4 to 1 to hard anodising build up.  To deal with this either the threads need to be made under size or alternatively masked, if in doubt it could be worthwhile getting some samples done.
• Sealed or unsealed - This will depend on whether your primary purpose for hard anodising is wear or corrosion resistance.  If wear resistance is the more important then the parts would be left unsealed as this maintains the maximum wear resistance.  Unsealed anodising has the added functionality of 'holding' lubricants such as oils in the case of hard anodised pistons.  In the event that corrosion resistance is the critical factor then closing the pore structure by sealing ensures maximum corrosion resistance or alternatively using a secondary coating to close over the pore structure  E.G. Teflon or similar.  One down side to sealing is that the wear resistance of the coating is slightly reduced.  Sealing may also be required where the hard anodising has been dyed.