Acid etch and alodine

[Eriselle]

Hello,

I'm in the early stages of planning an upcoming RV-9A build, which will be my first build. A lot of information on the internet about etching and alodining is confusing. I've spent hours searching the forums here and elsewhere. I'll probably ask a bunch of questions over time, but here are a few specific questions I have right now:

1.) https://www.pilotsofamerica.com/community/threads/alodine-or-no-alodine.118147/post-2712823

This post states, "Aluma-prep is an etch, so is alidine" (spelling as-is in the post).

Is this actually a correct statement that alodine is an etch? It would make more sense to me if this is not correct, because otherwise why would you need to acid etch before alodining if alodine is already an etch?

2.) I've read that alodining weakens the metal because the chemically altered surface of the aluminum is weaker than the unaltered aluminum. This would be, as far as I understand it, similar to how anodizing weakens the metal, for the same reason. I've read that the wing spars from Vans compensate for the anodizing weakening intentionally, as it is a known quantity of weakening.

Is this understanding of the weakening caused by alodining correct, and is it a significant factor? How much of a factor is it? Is this planned on by Vans' design?

3.) It is my understanding that acid etching creates a less smooth surface by eating away at smooth surfaces, which helps the primer to bond better. However, an amount of the prep work for parts is removing sharp edges, corners, and deep scratches. It is my understanding that any of those kinds of non-smooth or curved edges become failure points. Maybe I'm overthinking this, but wouldn't taking a smooth surface and etching it to be non-smooth weaken the metal, in addition to the fact that simply removing material would assumedly weaken it? Is this not an issue because the amount of material we are talking about, and the depth of the etching is very light in comparison to what would be considered a "deep scratch"?

4.) https://www.rvplane.com/priming/ This link discusses using an electric engraver to mark your parts since the priming prep process will remove your sharpie markings. Is this a concern for weakening the parts?

Thank you for your time and knowledge.

 

[terrye]

1. Alumiprep is a mild acid etch and detergent cleaner. It is a necessary first step before Alodine. Alumiprep is normally only in contact with the surface for about 5 minutes, after which it is rinsed off with clean water (be careful if your water contains contaminants)
2. Alodine is a conversion process that transforms the aluminum oxide (microscopically thin) layer on the surface of aluminum parts to aluminum chromate which is a more corrosion resistant layer. It does not weaken the part as it does not penetrate the surface. Anodizing is a different process, it artificially thickens the aluminum oxide surface layer. It results in a slightly lower fatigue life than the un-anodized surface, but makes up for it in increased corrosion resistance.
3. "Eating away" is far too strong a term. See comments 1.
Aluminum is notch sensitive (most metals are) under fatigue loading, so deburring holes and polishing edges reduces or eliminates crack initiation sites, as does removing scratches. Yes, the Alumiprep/Alodine process is recommended for additional processes such as primer/paint. However, for the interior surfaces the alodine process is probably between the "left bare" and "primer painted" in terms of corrosion resistance.
4. Although I am aware of electric engraver marked parts on some (certified) aircraft, I didn't do it on my airplane.

 

[DonMcKee]

Electric pencil OK, but...

4.) https://www.rvplane.com/priming/ This link discusses using an electric engraver to mark your parts since the priming prep process will remove your sharpie markings. Is this a concern for weakening the parts?

I asked Van's about using an electric (vibratory) pencil on parts. Sterling's reply:

This is not something we have studied extensively. If you have an engraving tool with a rounded blunt end that would likely be fine, I would not recommend a sharp point. We typically would not use this on item thinner than .063.

 

[rocketman1988]

Comment about anodizing

A previous post stated that “It results in a slightly lower fatigue life than the un-anodized surface, but makes up for it in increased corrosion resistance.”

This would lead one to believe that anodizing is ok. I would be very cautious about this line of thinking; a bit of research and you will find that there have been instances of failures due to anodizing parts…unintended consequences.

 

[terrye]

A previous post stated that “It results in a slightly lower fatigue life than the un-anodized surface, but makes up for it in increased corrosion resistance.”

This would lead one to believe that anodizing is ok. I would be very cautious about this line of thinking; a bit of research and you will find that there have been instances of failures due to anodizing parts…unintended consequences.

Point taken, thanks Bob. Anodizing of aircraft primary structure is not something that is normally done by the home builder, whereas alodining is. I know that Vans anodizes their main spar and spar carrythrough (as in my RV-9A) and understand that they have (up)sized the structural components to compensate accordingly.

I did a quick internet search and found this abstract, although it is for 7075 rather than the 2024 used in Vans aircraft.

The Effects of Anodizing Process on the Corrosion rate and Fatigue Life of Aluminum Alloy 7075-T73

This research aims to study the effect of using the anodizing process on the corrosion rate, mechanical properties as well as the fatigue life for aluminum alloy (7075-T73), which is one of the most commonly used aluminum alloy in production of aircrafts, vehicles and ships structures. The anodizing process was employed through using sulfuric acid for time (20) min in a salty atmosphere. The mechanical properties and fatigue life of the AA7075-T73 were obtained before and after the anodizing process. All the results were listed in detailed tables and figures for comparison purpose. Generally, these results showed a decrease in corrosion rate by (155.06%) in comparison with untreated, an increase in hardness by (21.54%) and a slight decrease in fatigue life by (7.7%) due to anodizing for a time of 20 min at the stress level of (σa = 491.10 MPa). It was concluded that this technique could be applied on other aluminum alloys in order to know the magnitude of change in the mechanical characteristics and their fatigue life.

Of course "unintended consequences" seem to be everywhere in aircraft, I'd be interested in case studies you have found on the fatigue life of anodized aluminum.

 

[Larry DeCamp]

Kelsie, some food for thought.

Etch primer has been used for decades on antiques, home builts and campers and truck trailers. You will find it very hard to strip off and finish paints bond very well. A good scuffing with scotchbrite and degreasing solution will condition the aluminum oxide / alclad or mill finish 6061 ready to prime.

That said, epoxy has a lot of merit because it sprays thin and smooth like finish coats. Minimal scuffing or tac-ing for finish coats (2K primer, base,clear). But, some epoxy folks say grind the **** out of it to assure good bond. No problem for car resto but not smart for airplanes. Also, most components have obscure corners and crevices you can't grind, sand or scuff uniformly. This might take you back to considering the ancient option of "etch primer ".

 

[wirejock]

Priming

No comment on #1 & #2. Other comment cover the topic very well

#3. Some sort of etch must be done before Alodine or priming to remove aluminum oxide. It can be a self etching primer, Alumiprep scrub or my preference, Bon Ami and scotchbrite. The goal is a water break free surface. Hydrophillic. Dry and treat. Aluminum oxide starts reforming immediately.

Please wear PPE and capture all run off products from the Alodine process. I made a make shift sluice. Save to a bin with a mesh screen. When it evaporates to a manageable amount, transfer to a stable container and deliver to the local Haz Mat facility.

#4. Depending on primer, seeing markings is not the problem. My preference P60G2, but I can see my markings through a light coat of Epoxy 2K pimer too. The problem is most surface treatments wipe the markings off. Temporary tags made from scrap work well. Great use for the engraving pen too. A small piece of tape over a label works for scrubbing. Usually I could see enough to remark them after scrubbing.
Lots of ways to skin the beast without engraving. By the way, sharpie markings disappear pretty quickly with Alumiprep.

 

[Eriselle]

These two quotes from the previous replies seem to contradict each other.

#3. Some sort of etch must be done before Alodine or priming to remove aluminum oxide.

2. Alodine is a conversion process that transforms the aluminum oxide (microscopically thin) layer on the surface of aluminum parts to aluminum chromate which is a more corrosion resistant layer.

If the aluminum oxide layer is removed by the etching before the Alodine, what aluminum oxide layer is the Alodine supposed to be converting into aluminum chromate?

Which one is correct? Or is it both somehow?

 

[Brantel]

#4 - Vans has been vibratory engraving spars for as long as I can remember. The correct engraver will just make a series of dots and not a contiguous line. Unlike cracked dimples in LCP’s you can rest easy that this practice should not be an issue if done within reason with the proper tool.

 

[wirejock]

Alodine

These two quotes from the previous replies seem to contradict each other.

If the aluminum oxide layer is removed by the etching before the Alodine, what aluminum oxide layer is the Alodine supposed to be converting into aluminum chromate?

Which one is correct? Or is it both somehow?

I'm no chemist but my understanding is the Aluminum Oxide layer must be removed so the Alodine can convert the pure aluminum to Aluminum chromate which doesn't oxidize. Stable surface. Also prepped for paints.
I could be way off base.

 

[Eriselle]

I'm no chemist but my understanding is the Aluminum Oxide layer must be removed so the Alodine can convert the pure aluminum to Aluminum chromate which doesn't oxidize. Stable surface. Also prepped for paints.
I could be way off base.

If the alodine conversion is aluminum oxide to aluminum chromate, then we are taking a layer of what presumably is already weakened material and converting it to presumably equally weakened material. Whereas if the alodine conversion is pure aluminum to aluminum chromate, then we have weakened the part in the alodining process. Although perhaps you could look at it as the etching is what really did the weakening, because if we didn't alodine after etching, that same layer the alodine would be converting would instead oxidize.

Since Vans obviously supports priming components, and as far as I understand it, etching is a prerequisite to priming regardless of whether you use alodine, it seems fair to assume that whatever weakening is occurring due to etching, however negligible or significant & measured that it is, is within the design intent of Vans.

Thus I would conclude that there is not a concern here at all to my #2 question.

I'm still curious what the actual chemical process is doing and which of these two seemingly conflicting explanations is actually correct.

 

[Avanza]

Link to Alodine Chromate conversion coating.

https://en.wikipedia.org/wiki/Chromate_conversion_coating

Good luck

 

[Eriselle]

Link to Alodine Chromate conversion coating.

https://en.wikipedia.org/wiki/Chromate_conversion_coating

Good luck

I'm not particularly educated on the topic of chemistry, but the chemistry explanation in this link seems to be pretty clear that it is pure aluminum that the reaction is occurring with, not with aluminum oxide.

 

[Kyle Boatright]

If the alodine conversion is aluminum oxide to aluminum chromate, then we are taking a layer of what presumably is already weakened material and converting it to presumably equally weakened material.

The thickness of the layers we're talking about here is microscopically thin. It doesn't matter from a strength standpoint.