EOI: Custom Anodizing

In order to weaken the Aluminum you would need to take it to Annealing temperature (300-410c). The current used is a tiny 10-20 volts and the
bath stays at about 20c.

Thanks Neville… I thought that initial caustic stripping decreased the mass of metal, because wouldn’t the metal dissolve if if you leave it in the bath to long? Does it only remove the oxide layer?

If you leave it in the bath it will eventually eat away all the metal (you would have to leave it for ages) , but you only leave it long enough to take off the tiny outer layer.

I should have stuck to my guns. It seems the reduction in tensile strength due to anodizing is well documented, for example:

Hard anodising led to an appreciable reduction in the fatigue strength of 7475-T6 [aluminium] alloy of about 75% for a 60 μm thick coating.

A b

Of course, the percentage reduction depends on the initial thickness of the aluminium, but a reduction nonetheless, seems inevitable.

Ah, but we are not hard anodizing, it is a distinctively different process,
I’ll wait for Gus to chime in on the details

You have got me interested in this now…

An experiment is in order (a real world experiment)

I’m going to anodize:

My handlebars, stem, chainring and cranks on my volume cutter and thrash the shit out of it for 6 months or so, and see if I can’t snap any of those components…

Also I’m intruigued that despite the fact that these studies have been done there are still a heap of factory produced anodized bike parts…

Oooh… and that may be the source of my confusion. (Although the hard anodizing process seems not to be distinctively different, but the same, with an addtional step). Right you are though, those articles do focus on hard anodizing.

However, I still think the loss of substrate thickness (via acid bath) would result in a technical reduction in the fatigue strength, though it may not be material.

Sorry, I’m not trying to be a prick, even though it may come across like it. I just think you need to have definite/comprehensive answers when offering a service.

There’s nothing wrong with anodized parts - so long as they are sufficiently tested. Such testing may simply require that the part be made initially thicker to accomodate the process later. And again, if hard anodizing is the culprit, then there may not be a problem!

I studied materials engineering for a few years:
I’d be less worried about material loss, (especialy because material is being replaced), but I would be concerned about possible crack propagation due to any inconsistent pitting by the caustic bath.

Anyway testing is in order!

Yeah, I tend to agree: the material loss is likely to be immaterial. :-). But my point wasn’t whether or not it will be material, but that it does occur, which seems to have been confirmed:

Any anodizing always reduces the fatique strength of aluminum because some portion of the aluminum is converted to a brittle ceramic like oxide, both reducing the thickness of the aluminum and introducing a stress riser situation. But the impact of chromic acid anodizing, which is thin, is far less serious than conventional sulfuric acid anodizing.

Where did you get that last quote from? The oxide is present on EVERY piece of aluminium that comes into contact with air, it is a natural phenomenon
Anodizing is just artificially re-creating the natural oxide layer

Now, if you have a thin aluminum sheet, and sulfuric acid anodize both sides, it sounds very possible that you could reduce the thickness enough that the fatigue strength could be reduced by 50 percent. Conversely, if you had a heavy casting or extrusion which you chromic acid anodized, it’s very possible that you would reduce the thickness so little that the fatique strength would decline by 5 percent or less.

The anodizing we do is not hard anodizing and converts a much thinner layer. It is true that anodizing can reduce the strength up to 70% but only in thin material. obviously the thinner the material, the larger the proportion of material removed, the higher the percentage of strength reduction… in the case of bike parts you have solid thick material, it is safe not only to soft anodize up to 25 microns (0.025mm) as we do but also up to the 120 microns you anodize with hard anodizing which is a widely utilized process in the bike industry already. In fact by polishing or sanding a part you would remove as much or more of the material than soft anodizing converts to aluminum oxide.

Anodizing essentially thickens the harder yet brittler outer oxide by converting it from the ductile aluminum, this increases durability at the cost of strength, but with the amount of material inherrent in bike parts this strength reduction is negligible. otherwise we would see far more problems with commercially available anodized parts.

it is true that being a brittle hard layer the oxide layer can act as a stress inducer as if the aluminum is deformed to the extent that the oxide cracks it would somewhat propagate that crack… but if you’ve deformed any aluminum bike part to that extent its already failed.

Fatigue failure occurs at a fraction of the actual yield / ultimate strength of the material over a large number of cycles. The amount of material lost in the process is negligible and would barely affect the maximum load able to be sustained before plastic deformation / fracture.

In this case, fatigue life is largely controlled by surface condition, (roughness, presence of microcracking, stress concentrations etc).

Just as polishing or shot peening the surface can improve the fatigue life, having a tightly adhered, hard & brittle surface layer with micro cracks that can propagate into the material will decrease it.

Hard anodising is much thicker and will have a more significant effect on fatigue life. Cosmetic anodising will have a very minor effect.