6061-T6 anodized fatigue issues

[triumph406]

Hi All.

I have an application that needs to be made from 6061-T6 and will need to be anodized for intermitant service in seawater.

I have been told that Type III Hard Anodizing (.002 build-up) may cause fatigue cracks in the Anodized layer, that could lead to corrosion at the crack.

Does this cracking in the hard anodized layer also propogate down into the material leading to cracking of the part.

The part will experience tensile and compressive stresses in the 15k range, for maybe 1000 cycles before replacement.

Will type I anodizing or Type III anodizing with a .0002-.0004 buildup be more appropriate.

Thanks for any help or leads you can give me.

DC

 

[swall]

Sulfuric acid anodizing in the .0002-.0004" thickness will reduce fatigue life. This has been well known in the aluminum aircraft wheel industry for 30 years or so. But, you may be better off than with the .002" thick hardcoat currently in use.

 

[sreid]

General Magnaplate offers excellent anti corrosion coatings. I'd call and talk to them about your application.

http://www.magnaplate.com/

 

[kenvlach]

For so few cycles, the effect of anodizing on fatigue is nearly irrelevant.
For high cycle fatigue (10⁶), hard anodizing greatly reduces the fatigue strength of 6061-T6: from 15,000 down to 6,000 psi. -- Materials and Methods, vol. 32, p. 62-64 (1950).

Hard anodizing is usually only used where high abrasion resistance is required. When corrosion resistance is also required, the hard anodizing is dichromate sealed. This also lessens the reduction in fatigue strength, although with some decrease in abrasion resistance.

In this application, there is a danger that the applied tensile stresses will open cracks in the hard anodize leading to corrosion. Hard anodizing is typically conducted at ~0 °C, so it already has high residual tensile stresses in the anodic oxide (as the aluminum substrate expands more) upon warming to room temperature.

For corrosion protection, consider chemfilm (MIL-DTL-5541, Class 1A) + either powdercoating or wet painting (epoxy primer and polyurethane topcoat).
If some abrasion resistance is also required, then either Type I or Type II anodize (dichromate sealed) + painting.
If high abrasion resistance is also required, then hard anodize (dichromate sealed) + painting.

 

[triumph406]

Kenvlach/sreid/swall,

Thanks for the information.

It would appear that hard anodizing is out, Kenvlach, the devices may be used for greater than 1000 cycles, until the devices are put into service we don't know what the service life will be.

I think the chemfilm or Type I or II anodizing, plus painting will be the way to go. Unfortuantly my customer
is reluctant to pay for any testing, plus the devices are being made in Thailand which complcates quality control.

Thanks again for the help!

DC

 

[NickE]

If yoru customer doesnt want to pay for testing, then dont garentee any performance. Thats pretty easy.

(IE: We'll make exactly what you tell us to, however we dont care if it works or not, just that it meets your specifications.)

If you have already agreed to meeting some sort of specs, life, properties, etc.... Then I think out of good moral practice you should eat the testing cost. (And then tell your sales department that you really dont like to loose money on projects and should be consulted before/as quotes are being produced so that you dont get caught again.)

Nick
I love materials science!

 

[triumph406]

NickE
I agree we should eat the testing cost, we’ve done a simple one time pressure test, but we need to do a 10’000 cycle pressure test. Unfortuantly I’m a contractor working for my customer, and seem to have very little say in what happens. Same old story .

DC