6061 Al and SS or Ti threaded fitting compatability 2

[triumph406]

To all the corrosion experts on the forum:

I have a prototype of a product that is made from 6061-T6 AL. The 3 major parts are made from 6061-T6 and articulate at 2 joints. 2 of the parts are drilled to create chambers that will contain high pressure air. This device will be used in seawater by surfers or recreational divers.

I have used 3 types of anodizing on the 6061, Type II (with a color) and Type III. For the Type III I have used .0004 build-up, and a .002 build-up.

The pressure inside the chambers will be upto 3000 psi, the stress in the wall of the part will be approx 6000psi, also unfortuantly there are some stress concentrations where I have to cross-drill to connect 1 chamber to another.

The 1st question I have is;
Will I have a problem with the Type III .002 thick build-up coating cracking and initiating a crack thru to the chamber? (I haven’t done any fatigue testing yet on the parts)
Will the Type III .0004 build-up Flash Hard anodizing be better for avoiding crack initiation?
Is there any advantage in corrosion protection between Type II and Type III anodizing? (it would seem to be intuitive that Type III would be better)

The device has fittings that are threaded into the 6061-T6 (Type III .002 build-up anodized) pieces. I have tried to minimize the engagement of the threads. I have a part that has been soaked in seawater (Huntington Beaches best!) with both 304SS and Ti fittings threaded into it. The area around the 304SS fitting is getting very corroded, the area Ti fitting seems to have suffered very little corrosion.

Is there a better material to use than Ti?
Would Zinc or Cad plated Steel fittings be better for corrosion prevention purposes?
Are there thread treatments that would help to minimize corrosion?
Would Anodizing the Ti help minimize corrosion?

It seems confusing that 304SS is closer to AL in the Galvanic table, while Ti is at the opposite end to AL but appears to cause less corrosion!

I appreciate any help that the forum can provide in helping me make material choices.
Thanks Dave Cam

 

[kenvlach]

Hi Dave,
Re the anodized aluminum portion of your question:
The corrosion resistance of Type III anodize is very poor if unsealed, poor if nickel acetate sealed, decent if dichromate sealed at high temperature (205^oF), best if sealed at moderate temperature (160^oF). The moderate sealing temperature minimizes crazing, as does hard anodizing at temperatures higher than the customary 32^oF (requires anodizing solution additives). Your suspicion that thinner coating is less susceptible to cracking is true, but the cracking of hardcoat can be minimized by procedures mentioned above and described in the following threads:

Estimating wear of hard anodized aluminum
Thread342-137386
And an earlier thread:
hardcoat anodizing at it relates to hardness
thread404-66479

 

[triumph406]

Kenvlach,
Thanks for the post, and thanks for the links.

I will need to do more research into Type III anodizing thickness and sealing. The parts i have are already sealed, however I don't know what process they were sealed with, and at what temperature.

On e concern I have is, will any cracking in the hard anodized surface continue down thru into the parent material? i.e will it initiate a crack that will lead to failure, even if there is no corrosion, or corrosive atmosphere present?

What are AL scuba tanks protected with?

Thanks once again for the help.

 

[kenvlach]

Are you sure the Type III was sealed? The default for Type III per MIL-A-8625F is unsealed unless specified or dyed. Dyed is always sealed.

The brittleness of hard anodize results in lower fatigue strength. Type II does also, but not as much. The very thin Type I (chromic acid anodize) is used for aircraft for this reason (along with good corrosion resistance). Cracks in the anodize do not initially propagate through the aluminum, which is enormously more ductile, but can lead to fatigue cracking. I am sure it is possible, but I have never seen it w/o the presence of corrosion.

Its been ages since I've SCUBA dived, but from the silvery color, I would say clear Type II anodize. Mostly hoop-wrapped with fiberglass, anyway. I think the earliest tanks were a deep drawing quality marine grade alloy such as 5052, but desire for higher pressures lead to the use of 6351 beginning in 1971. Ruptures due to Sustained Load Cracking peculiar to 6351 caused a switch to 6061 ca. 1988-89:

http://www.psicylinders.com/library/cracking.htm

 

[triumph406]

kenvlach,
The TypeIII anodizing was dyed, and therefore I assume sealed.
One reason for going to TypeIII anodizing was to gain some abrasion resistance, which I guess would be superior to Type I and II. However if there is a problem with cracking, it might be worth looking at Type I anodizing, with another coating of some sort to provide some abrasion resistance. I assume an epoxy paint would probably be OK.
Thanks again

 

[kenvlach]

I don't know your loading & cycles, but unless very extreme, I don't mean to discourage the use of Type III anodize. Dichromate-sealed Type III (taking a little care to avoid thermally-induced crazing, as described in Thread342-137386), with epoxy primer & polyurethane topcoat, is the superior protection for aluminum against abrasion and corrosion. Even 7075 after 5 years on ocean bottom showed no damage (except from rolling around on deck on the return trip).

One more comment on fatigue resistance of Type III anodized aluminum. An effective method to increase the fatigue life is to shot peen the aluminum, caustic etch to remove shot peening debris, desmut & then anodize. The hard anodize on this roughened surface is then a fine-grained mosaic. Looks ugly, but resists cracking due to difference of CTE vs. aluminum. Kind of like many little expansion loops a la pipelines. Used on mission-critical Al 7075 & 7511 helicopter parts.

 

[triumph406]

Kenvlach,
we will be tsteting the finished product to conform to DOT guidlines. Although we don't need to meet a DOT spec (3AL)(the air volume is too small) we will be testing the assembly to if I remember 5000psi for 10'000 cycles.

I didn't realise that people shot peened AL.

I need to talk to the local anodizers and see what it would take to specify the process I want. I've had very little luck, with 1 anodizer especially to follow even the most basic of instructions.

For the fatigue testing I will make 3 samples of each type of anaodizing and 3 shot peened parts, and test them all at once. It would probably make sense to expose them to salt water first before fatigue testing. Although DOT 3AL does not callout for fatigue/corrosion testing.

One thing I want to avoid, is having a finish that looks OK, but maybe corroding and not visible to the naked eye. And as such I'd like to avoid paint or plastic coatings. If there is corrosion I'd like the consumer to be able to detect it before it becomes critical
Thanks Dave

 

[kenvlach]

Shot-peening of high strength aluminum alloys to improve fatigue life has been done for a long time, before any benefits to subsequent hard anodize were known. (It also breaks up subsurface intermetallic particles which can cause anodizing defects.) Pretty much only used for aircraft applications of high strength alloys. Go to

http://www.shotpeener.com,

click 'Library,' then 'Search,' and type 'Aluminum.' There are 190 results.

Shot peening also reduces exfoliation corrosion in aluminum:

http://www.metalimprovement.com/exfoliation_corrosion.php

However, glass bead blasting is much more commonly used for aluminum-- paint & scale removal, matte finishing, etc. Shot peening for surface cleaning is prohibited in MIL-A-8625F. A 'Peentex®' process is used for texturing effects on architectural aluminum (no details or indication which photos show Al & which are SS):

http://www.metalimprovement.com/peentex.php

 

[triumph406]

Kenvlach,
I will go and talk to a local shotpeener and show him some parts and see what proceesing cost would be for production size lots of parts.

Maybe, shotpeening typeIII anodizing and then epoxy priming and polyurethane topcoat would be the ultimate protection. As these are intended to be used by sporstman, I dought they will looked after, they will be kept in constantly damp gearbags, and banged around when used, we will probably need all the protetcion we can get!

 

[kenvlach]

Also consider bead blasting as alternative to shot peening. You probably don't have a fatigue application; the idea is to slightly roughen the surface such that the hard anodize grows as a mosaic of small platelets (rather than a large flat plane), which alleviates large CTE stresses.

A little roughness aids the epoxy primer adhesion. You may have to try several to find one with good adhesion to sealed anodize. Don't get talked into using non-sealed anodize; water vapor permeates through all paints. The combination of hard anodized Al 7075-T6, mid-temperature dichromate sealing, epoxy primer & PU topcoat competed successfully against titanium for a Navy contractor.
Ken