Yes...and no ... The etch primer will not have a top coat. The Alodine will just be left as it is after the process. Due to large number rivets and holes. "Thick" paint is a problem during assembly. So just Alodine would be ideal ...
Alodine is almost certainly superior, it gives long-term corrosion protection to internal aircraft parts.
An etch primer (presumably a phosphoric acid type, please inform if otherwise) roughens the surface more & the phosphate absorbs moisture & isn't as corrosion resistant as the oxide-chromate of Alodine. Needs a topcoat (altho the polyvinyl, butyral with zinc chromate can be pretty good). But, it is advantageous for riveted surfaces that the Alodine is very thin. Plus, Alodine gold color shows that it is 'on the job' although of course there are clear Alodines (e.g., 1500).
If you mean something like an FPL etch (hot sulfuric-dichromate), this is only good for adhesive bonding pretreatment.
Thanks for that answer kenvlach. The spar is made from two webs and a 5 top and 5 bottom cap strips. The Alodine will be ideal due to its low thickness…and no primer filling up the holes. It was our intention of using the 1200, golden yellow. In the UK it is sold by Trimite (the product is made by Henkel).
One worry is possible moisture ingression (very humid in England) in-between the riveted laminations...consider using a very thin application of zinc chromate joining compound spread on very thinly…..
Be careful using Alodine on already assembled parts.
The Alodine must be washed off (Flushed - water break)prior to assembly.
Suggest assemble with thin sealant fay surface coat after Alodine treatment.
One issue to take into account is that Alodine (conversion coating MIL-C-5541) will not hold for long exposure to temperatures above 60 C degrees. Therefore, sealed chromic anodize or sealed sulfuric anodize are much superior. I have seen painted parts over conversion coating where the paint chipped and pealed off after exposure to temperatures (the parts where outdoor) due to the failure of the conversion coating.
Appreciate all you comments. The parts will potentially be Alodined first. Then riveted together. Using a chromatic non hardening joining compound. You are looking at the main spar and a spar web with 6 spar flange strips, top and bottom.
I did not know about the heat issue. It was not our intention to paint or do any further surface treatment of the spar after assembly.
The 60oC limit is correct for unpainted Alodine, but doesn't apply if painted. Chromate coatings can be powder coated, baked at 400oF, then X-scratched and still pass a salt spray test for filiform corrosion. Adhesion problems as mentioned by israelkk can be due to overly thick coatings. A thinner coating than Class 1A is better for adhesion; e.g., the Class 2 coating in ASTM B449 & the original MIL-C-5541.
MIL-DTL-5541F:
"6.14 Temperature effects on corrosion protection. Unpainted conversion coatings will commence losing corrosion resistance properties if exposed to temperatures of 140 ºF (60 ºC) or higher, during drying, subsequent fabrication, or service. As temperatures and exposure times increase, the corrosion protection of unpainted conversion coated parts decreases. The reduction is believed to result from the coating dehydrating and the resulting insolubility of the chromates within the coating."
There are also no-rinse conversion coatings, both chromated and non, but Alodine/Alocrom 1200 isn't one of them.
Sealed chromic anodize is a great substrate. Definitely better than chemfilm in uncoated or abrasion situation, and high temp., too. Bare chemfilm is basically for interior surfaces, while one of the first uses for chromic acid anodize was for seaplane hulls & pontoons.
The problems with chromic anodize aren't due to lack of properties, but with EPA, OSHA & equipment required (power supply, heated electrolyte, high-efficiency, EPA-approved scrubber). Alodine can be easily field applied, and very little respiratory hazard (if not spray applied!).
