Alodine (Iridite) over an Anodize finish 2

[swertel]

I'm still doing all the research and brushing up my knowledge on chemical conversion coatings, but I want to pass a whacky idea passed the experienced pros to shoot down before I waste too much time on it.

I have some Type II, Class 2, Black per MIL-A-8625 parts that pass the tape test and salt fog test. Upon further processing during product assembly, speckling occurs around portions of the part.

I'm currently conducting experiments to see if processes used during assembly are attacking the anodize, but in the meantime I need to have a Plan B - also known as a rework plan.

Two options:
1) Find a reasonable MIL-Spec paint process. MIL-F-18264 is inactive for new design and also so long-in-the-tooth that no one can certify to it. Does anyone know of other paint processes/specifications?

2) We currently iridite any small scratches in the anodize as we notice them during the assembly process. But, for this particular problem we don't want to have to do it on the production line nor do we want to have to do a 100% screening process. As one of my wild brainstorms, I'm considering batching the entire lot of materials with alodine directly over the anodize in order to save the time and expense of removing the anodize. (If I'm going to take that risk, I'd just re-anodize it.) Any obvious pitfalls for doing a "dip" process (for lack of better terms) of alodine over anodize. I'll have a processor spray or brush on the alodine per the spec, but it would sure be easier to have a tank of chemical and just dunk each part.

--Scott

http://wertel.eng.pro

 

[swertel]

By the way, I duplicated the speckling of the warhead loader by baking the aluminum bodies at 95-100C for 2.5 hours. That is what they do in their process.

The anodize passess all tests prior to this heating process. After heating, I can use a paper towel, even a Kimwipe, and rub it across the surface to reveal speckling. No speckling without physical contact so the anodize doesn't just "fall off."

I don't have any metalurgy books. I also can't seem to find a good phase diagram for 7075 aluminum. I can't see 100C having any effect on the aluminum but it appears we are dealing with inclusions within the materials structure that requires a bit more specialization beyond my learnings in college.

--Scott

http://wertel.eng.pro

 

[kenvlach]

This part of the behavior isn't complicated. The anodize bumps atop the intermetallic inclusions are in tension. Heat the material up, the aluminum expands a lot more and fractures the brittle anodize (low CTE, low strain to fracture). It's frequently observed with hard anodize (even w/o defects) formed at ~0 C, that crazing (network of fine cracks) occurs when dipped in a hot dye or sealing tank.

Anyhow, the only good a phase diagram might do is convince you to switch to a modern 7000-series alloy with low Fe, Si, Cr. Note that the solubility of Fe in Al at 500 C is only 0.006 wt% whereas 7075 is allowed 0.5%. Impossible to solutionize w/o melting the aluminum matrix. So, need rapid casting procedures to reduce segregation, plus a lot of hot deformation to create the wrought microstructure. The anodizing defects I've only seen in billet material, not stuff that's been extruded.

FYI, some papers: first 2 are both free (along with a lot of other good papers on aluminum alloys), at

http://www.materialsaustralia.com.au/Services/Publications/Materials_Forum/Vol28/Papers/

A Microstructural Engineering-Based Approach to 7xxx Series Alloy Optimisation, M.R. Clinch et al.

'Figure 1: The quaternary system Aluminium-Copper-Magnesium-Zinc at 460°C and 6wt.% Zn. The compositions of the two alloys investigated are indicated. [16]'
Reference 16. D.J. Strawbridge, W. Hume-Rothery, A.T. Little, J. Inst. Met., 74, 191, 1948.

Microstructure Property Relationships in a High Strength Al-Zn-Mg-Cu-Zr Alloy, A.K.Mukhopadhyay et al.
“The reason for the selection of the particular Cu and Mg contents of the base alloy is not to allow the undesirable S (Al₂CuMg) phase to survive commercial homogenization treatments [4], thereby improving workability as well as avoiding degradation of mechanical properties and SCC resistance of the wrought alloy [3,5].”

Note that Fe in the alloys investigated was 0.06 wt%, unlike 7075 which has 0.5% Fe (max). Likewise, Si was 0.02-0.04% vs. 0.4% (max) in 7075. Also, Cr was left out of all but one alloy investigated.

Two more papers (free abstract, $30 for entire article):
A three-dimensional study of the microstructure of an aluminium alloy as revealed within its thick anodically formed oxide, P. J. E. Forsyth, Materials Letters, Volume 13, Issues 4-5, Pages 184-193 (April 1992).
Abstract
"Most of the microscopically visible constituents in the aluminium alloy Hiduminium 48 have been observed to persist, with unchanged morphology, in the relatively transparent coatings formed by anodic oxidation in a sulphuric acid electrolyte. This has made it possible to study the features of the original microstructure, in three dimensions, using the optical microscope to view detail directly through the top surface of the coating. This internal examination of the distribution of the micro-constituents has been found to be particularly helpful in the assessment of residual “ingotism”. It can also aid the study of machining-induced disturbance of “near surface” microstructure by avoiding the problems presented by section preparation, with the special requirement to preserve the specimen edge. The influence of the disturbed surface layer, as induced by machining or other forms of surface deformation, on both the fatigue and the stress corrosion behaviour of aluminium alloys, is well known. The approach described in this paper should provide a better understanding of the way this disturbance affects these properties."
[Hiduminium 48 is similar to 7010 but with lower Cu so a bit lower strength.]

http://www.sciencedirect.com/scienc...serid=10&md5=59b235303cc801314f2f9e6fa65db366

Some further observations on the various features of aluminium alloy microstructures that are transferred, bodily, into the oxide coating formed by anodising, P. J. E. Forsyth, Materials Letters, Volume 16, Issues 2-3, Pages 113-122 (March 1993).
Abstract
"When anodised in a sulphuric acid electrolyte, many of the microstructural features of aluminium alloys are transferred into the oxide coating so formed. Even with thick coatings these features can be seen, in depth, with the optical microscope, although certain procedures may be required for optimum clarity. Depending on their oxidation characteristics, some micro-constituents appeared in the oxide unchanged, others were convened to a transparent form, and some disappeared entirely to leave cavities. In most cases, particularly where the features within the oxide had characteristic shapes, they could be related back to the original constituent type as seen in the alloy itself. This identification was aided by the three-dimensional viewing capability that gave a direct appreciation of particle morphology and distribution. Magnesium silicide (Mg₂Si), when anodised, was converted to a transparent substance that retained its original rounded form. It could be specifically stained, notably with the aniline dye Malachite Green. This provided a useful marker for that constituent's three-dimensional distribution within any microstructure, as none of the other particle species were stained by this reagent. Contrast enhancement by the use of video microscopy proved invaluable, particularly for viewing fine precipitates in suspension within the depths of the oxide."

http://www.sciencedirect.com/scienc...serid=10&md5=a18f213702d35cf0cfa6272d3526f26e

 

[swertel]

Thanks!

I can't give you enough stars for all the superb information you have provided. This info, mixed with the research I've done and the testing performed by me and my vendors should be enough to prove to our customer that there is nothing wrong with the processing and our corrective action will have to be
1) Change their pre-heat process or
2) Change raw material.

Thanks again. When I see you around Arizona, USA look me up. I owe you a drink.

--Scott

http://wertel.eng.pro

 

[Colordude]

I am sorry that I came to this forum so late in the game.
We had an almost identical problem with a customer in the midwest, I am in Indiana. We were getting the same complaints from a customer of ours who is producing warheads for an end customer in Scandanavia.

What we finally found out, and it does go along with the other information provided by the other respondants, is that our clean-up phase of the anodizing process was not as agressive as it should have been due to the tolerancing of the parts.

We processed these parts with due care due to high the tolerance of some of the diameters and threaded nose of the warhead. This means that the cleanup and etch of the parts was held to a minimum so as to minimize metal removal in these areas.

This is, what we believe, was the cause of the white flecks which later showed up during the explosive loading and preheat of the parts. We did not remove the copper inclusions or, the cleanup was not sufficient enough to remove small spots of machining oil that might have been "baked" dry onto the surface of the parts. Either way, when we discussed this with the machine shop producing the parts and decided that a more agressive cleanup was possible on these parts, the problem no longer occurred even after several test runs.

I know this is late but I hope that it helps.

 

[swertel]

That's funny. Colors, Inc., based on the midwest and closely matching your username, does our anodize of the warhead parts working with a machine shop out of Louisville.

Strange the connections one can make in the online community.

--Scott

http://wertel.eng.pro

 

[Colordude]

That's exactly what I thought when I read this thread.

Mike