Alodine
Alodine
(OP)
We are discussing the pro/con of Alodining v Etch priming on a riveted spar assembly. Anyone with practical long term experince with Alodining ?
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RE: Alodine
RE: Alodine
RE: Alodine
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.
RE: Alodine
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…..
RE: Alodine
The Alodine must be washed off (Flushed - water break)prior to assembly.
Suggest assemble with thin sealant fay surface coat after Alodine treatment.
RE: Alodine
RE: Alodine
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.
RE: Alodine
RE: Alodine
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.
RE: Alodine
But why not use sealed chromic anodize instead which should be better for corrosion?
RE: Alodine
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!).