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

 

[CoryPad]

As far as general, performance specifications for coatings, how about MIL-STD-171 FINISHING OF METAL AND WOOD SURFACES or MIL-STD-7179 FINISHES, COATINGS, AND SEALANTS, FOR THE PROTECTION OF AEROSPACE WEAPONS SYSTEMS ?

Without knowing your requirements, it will be difficult to advise you. Why did you choose to use a conversion coating on top of the anodizing? Is there a problem using standard sealants or organic topcoats for your anodizing?

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[kenvlach]

Scott, please describe the processing that may be affecting the anodize. Can you post photos of the speckling? Also, what alloy. And, why dye black if going to paint over?

Alodine® 1200S (or other chemfilm from QPL-81706) is approved for touch-up of scratches in anodize (up to 5% of the surface unless customer disallows) per MIL-A-8625F. Normally, the scratch or bare contact point is deoxidized with dilute nitric acid, then chem filmed. The deox & chemfilm application is done via Q-tip or artist paintbrush or applicator pen, rather than immersion, as acidic solutions tend to dissolve the anodize. The only benefit to dipping in chemfilm (pH 1.7) is that it will protect bare aluminum exposed through cracks or blisters in the anodize. It's preferable to seal the anodize in near-neutral (pH 5.75) heated 5% sodium dichromate.

For pretreatment of aluminum prior to painting, I suggest either Class 2 chemfilm [(Class 1A is too thick & porous; may debond) and Class 3 is too thin (less corrosion protection)] or dichromate-sealed anodize. Also, use a primer such as MIL-PRF-23377 that will adhere to anodize (with cheaper primers, leave the anodize unsealed for more grip). Chromic acid anodize is generally best for corrosion protection, phosphoric acid anodize gives the best adhesion [see ASTM D1731 for details], but sulfuric acid anodize is most used (except for aircraft).

Class 2 chemfilm has been omitted from MIL-DTL-5541F (formerly MIL-C-5541) but remains in ASTM B449, 'Standard Specification for Chromates on Aluminum.'

http://www.astm.org/cgi-bin/SoftCart.exe/DATABASE.CART/REDLINE_PAGES/B449.htm?L+mystore+queh2642

You can specify Alodine® 1200S as pretreatment for painting per Alodine® 1200S per MIL-HDBK-509 CLEANING AND TREATMENT OF ALUMINUM PARTS PRIOR TO PAINTING [spec. apparently written by the Alodine supplier].

http://assist.daps.dla.mil/quicksearch/basic_profile.cfm?ident_number=204606

A more comprehensive specification is ASTM D1730, 'Standard Practices for Preparation of Aluminum and Aluminum-Alloy Surfaces for Painting.'

http://www.astm.org/cgi-bin/SoftCart.exe/DATABASE.CART/REDLINE_PAGES/D1730.htm?L+mystore+queh2642

 

[swertel]

I tried to keep my initial post short, so let me now elaborate.

These are warhead bodies that get an explosive material cast into them. The explosive is a hot melt, waxy substance so the bodies are preheated in order to reduce viscosity of the wax during loading.

The finish of the material is a Type II anodize, Class 2 black. That is the FINAL finish for them. But, we have a discrepancy because after loading the explosive fill, white specs appear on the body. Our customer, who is doing the loading themselves, blames a bad anodize. THE ANODIZE IS NOT THE PROBLEM AND I'M RUNNING TESTS TO PROVE IT! But, corporate politics being what they are, I still need to come up with a Plan B in case a rework is required.

For small scratches and similar defects, we alodine with the Q-tip like kenvlach states. But, repairing these warheads after they are loaded (because that's when the "problem" appears) by hand is very time consuming and costly. We want to "fix" the problem before loading and therefore need a process that can handle 15,000 parts.

Because of the loading process we can't paint prior to loading. Dealing with loaded warheads in a paint booth afterwards has its own rewards, but is still an option if nothing better can be worked out.

So here's the problem in the nutshell.
We have a final finished product with all certs and passed all tests including tape test and salt fog.
All the anodize bath data, temperatures, materials, etc. are to spec and within acceptable tolerances.
The anodize looks great on the bodies prior to loading.
SOME bodies get white spots after loading.
Customer blames a bad anodize and will charge us for the rework if I don't prove our innocense.
Customer is not informing us on the details of their process or what detergents are used during cleaning of the spilled explosive material. Why? Probably because if they did we could pinpoint the problem on them and they couldn't back-charge us for the rework.

The good news is, by the time I finish this investigation, eng-tips will have another anodize and conversion coating expert on the forums. I got a pile of MIL and ASTM specs on my desk that would turn a forest into a grassland.

--Scott

http://wertel.eng.pro

 

[swall]

Very interesting product and problem. My guess is that the temperature control of the explosive filling operation leaves a lot to be desired and some of the anodize is being exposed to temperatures high enough to degrade it.

 

[CoryPad]

Because of the loading process we can't paint prior to loading

But can you seal? Immersion sealing (boiling water, chromate solution, etc.) may meet your requirements (improvement at imperfections, immersion process).

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[swertel]

Yes, we could seal. I believe MIL-8625 leaves sealing open. I know we don't specify it on our drawing. I'll have to review my certs to see if it is something the anodizer may be doing anyway.

And since I'm not really worried about coating hardness or wear resistance, nor am I worried about the exact coloring, I can deal with any side effects of sealing if the cost is reasonable. If I remember what I have been reading, corrosion resistance is not degraded by sealing. I just really don't want to issue a change order for something that's not broken.

--Scott

http://wertel.eng.pro

 

[CoryPad]

Sealing IMPROVES corrosion resistance. Al₂O₃ has pores as it forms, which allows corrosive media to reach the substrate. The sealing material fills the pores, thus reducing the media:substrate contact, and thus improving corrosion resistance.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[israelkk]

As far as I know the unsealed coating will not stand in the long period (336 hours) of the salt fog test. Therefore, if you do not specifically ask for unsealed coating I believe it was sealed. Unsealed coating is mainly used as a paint base coating but the painting should be done as soon as possible after the anodize.

We had similar problem in the past with black died coating. What we have found is that the sealing was done in a lower bath temperature than the spec asks for (90 to 100 Celsius) degrees . When the finished part was put in a higher temperature the anodized coat break and the die fell off the surface exposing white spots on the surface.

 

[israelkk]

One more question, why are you using black dyed coating if you paint over it? Black dye is mainly used for optical reasons.

 

[swertel]

We don't paint over it. The black anodize is the final finish. But, our customer believes that the white specs are areas where the anodize has been removed and therefore as a workaround, since we can't re-anodize in the condition the parts are in, are going to paint.

How can I tell if the anodize is good but the coloring is somehow eskew?

--Scott

http://wertel.eng.pro

 

[israelkk]

First you need to check the routing charts of the specific process that was made on the part to make sure that the bath temperature was correct during sealing. I recall that there is some chemical spot test that can check that the sealed surface was sealed in the correct temperature.

 

[swertel]

You can do the stain resitance check per ASTM B136 to determine sealing. A properly sealed anodic coating should not stain.

--Scott

http://wertel.eng.pro

 

[kenvlach]

What is the alloy?

Some Al alloys, most notably 7075 (also 2024) have Cu-rich intermetallic inclusions ('constituent phases') remaining from the initial solidification that can survive both the wrought processing & solutionizing heat treatment. These intermetallics cause anodizing defects. More obvious when doing hard anodize; can cause pimples & blisters. On Type II, less obvious since the lesser anodize growth into the metal doesn't create enough strain to fracture on its own. Roll a smooth metal cylinder over the anodize surface & see if any pimples pop.

http://www.finishing.com/429/61.shtml

 

[swertel]

As a matter of fact, it is 7075-T6 in accordance with ASTM B221.

To my knowledge - waiting to confirm with the manufacturing on the process details - the rough body shape gets punched and then post machined.

--Scott

http://wertel.eng.pro

 

[israelkk]

To my best knowledge and experience the correct and recommended anodize coating for 7075-T6 should be Type I group and not type II. Look in table V page 17 of MIL-A-8625F and see that for 7075-T6 there is no data for typical minimum thickness at all. There is no need for Type II for 7075 to withstand the 336 hours salt test.

 

[kenvlach]

A few points:

All anodizing that is dyed must be sealed.

Al 7075 can be anodized by all anodizing processes. Every commercial anodizer that I know does monthly qualification testing of anodized 7075 test panels: Both Type II (sealed) for salt spray & Type III (unsealed) for Taber abrasion testing. 7075 is popular with many anodizers for Type III (Hard anodize), as one can readily grow a coating of 0.004” (can sometime save overmachined material).

http://www.anodizing.org/reference_guide.html

However, there is a material problem of ~micron-sized intermetallic inclusions in this particular piece or batch of Al 7075 which is causing defects in the anodic coating. I studied this extensively in 1997 (SEM, EDS, etc.) & discussed it in 1998 at SUR/FIN with a Bell Textron engineer who had published an article on hard anodizing of 7075 for aerospace. Either you find a clean batch of material (where all the initial solidification phases have been broken up & solutionized by mechanical & thermal processing) or you do this workaround: Shot peen or burnish the 7075 such that the surface metal layer is homogenized. If you are growing Type II anodize of 0.0005”, then the material must be worked to 0.0003”. For hard anodizing 0.002”, the surface must be worked to at least 0.001.”
Ken

 

[swertel]

New Title: Verifying ASTM B136 dye.

Since this is dealing with a continuation of the same issue, I was to resurrect my thread.

I finally got some time after the regeants arrived to be able to perform the spot test per ASTM B136. We didn't get the blue dye from Sandoz Colors and Chemicals, as it states in the spec, but we did get the same 2LW dye and adjusted it to the proper pH.

In order to verify that I can read a spec, we tried the procedure on same non-anodized aluminum samples. The result, no blue stain. The spec states that to verify the stain I need to get a freshly anodized part that hasn't been sealed. I don't have that available.

1) Shouldn't the procedure stain a non-anodized aluminum? Or is there something with the surface treatment of anodize that makes the dye work? I know I can't get a Sharpy to stain aluminum, but I don't know about the dye especially after I used the nitric acid solution.
2) How obvious is the stain? I have a black anodize with speckling. I'm expecting the speckles to turn blue. Will I also be able to see a stain against the black background?
3) I'm not seeing any staining. That either means that the seal is good, or the operators are bad. I can't seem to prove/disprove either, so any methods would be helpful.

Thanks again.

--Scott

http://wertel.eng.pro

 

[kenvlach]

Some major dye suppliers for anodized aluminum in the US are Clariant (formerly Sandoz)

http://pigments.clariant.com/pa/int...3551911e139e6028c1256c85004bf8cc?OpenDocument

Keystone Aniline Corp

http://www.dyes.com/Default.html

and
U.S. Specialty Color Corporation

http://www.usspecialty.com/color.php

The dye stain seal test won't work on bare aluminum (lots of people would skip anodizing if aluminum could be easily colored!).
Unsealed anodize has open pores (like honeycomb, only smaller) which can accept dye molecules. Sealing fills & caps the pores.

Sealing is irrelevant to the origin of the pimple defects in anodized 7075, which are due to non-anodizing intermetallic particles within the alloy. White spots are corrosion due to the acidic electrolyte getting beneath the anodize layer through cracks at the intermetallic particle. These intermetallics aren't the tiny heat treat precipitates which give strength but are residuals from the initial casting and shouldn't be present in wrought material.

 

[swertel]

Thanks. You verified my suspicions.

--Scott

http://wertel.eng.pro