Alodine

[SkyPunk]

Have heard several arguements/discussions on the effectiveness of Alodine (Chromate conversion coating per, MIL-C-5541 Class 1A) as a final coating. Most indications are that Alodine is an excellent base coat, but needs an organic topcoat. Anyone know of any study data, or higher level specifications that provide the skinny, and do not just require organic finish?

Thanks,

Skypunk

 

[kenvlach]

It's fine as a final coating for the interiors of aircraft & electronics cases. It's not OK for surfaces exposed to weather, handling, any abrasion, most liquids, high temperatures, etc. So depends on the application. A general answer is that topcoating with paint is required for exterior surfaces.

If you haven't already done so, read MIL-DTL-5541F CHEMICAL CONVERSION COATINGS ON ALUMINUM AND ALUMINUM ALLOYS

http://assist.daps.dla.mil/quicksearch/basic_profile.cfm?ident_number=5499

Now, for the serious study: Non-Chromate Aluminum Pretreatments (NCAP) Report (August 2003) gives salt spray test results for Alodine 1200S® and selected non-hexavalent chromium alternatives on several Al alloys in section 5. Compare the number of hours of testing for these samples (24, 48, 168, 336 hours) with the testing periods (e.g., 500, 1000, 3000 hours, 4, 8 & 12 month) for the painted samples in the bulk of the report. Note also that most painted samples were scribed (X-scratched per ASTM D1654) prior to testing.

http://www.jgpp.com/projects/pretreatments/documents/phase_i_.pdf

Generally, for best resistance to multiple types of corrosion, Alodine 1200S® & the trivalent TCP pretreatments, coated with Mil-PRF-23377 Primer and Mil-C-85285 Topcoat give best results.

 

[israelkk]

Here is a quote from MIL-DTL-5541F regarding temperature effect on the conversion coating. Therefore, exterior (or even interior) surfaces subjected to direct sun light (such as aircraft body) may well exeed those temperatures and adversly affect the coating.

"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."

 

[40818]

What i remember about the whole process was that the alocrom/alodine protects the aluminium. The primer then protects the alodine/aluminium, and the final paint finish stops the underlying layers from the outside world and to look pretty.

 

[israelkk]

40818

I have seen painted parts of a flight system where the paint was completely separated from the base metal because the conversion coating was exposed to higher than 60 ºC.

The correction was to coat the parts by anodic coating before painting instead of the conversion coating.

 

[SkyPunk]

Thanks for the assistance. I have gotten my hands on a table from a study performed on Trivalent Chromium Coating, and Chromate Conversion Coatings in both Salt Spray, and 1 year Marine Environment Exposure. Both the Trivalent, and Chromate coatings perform quite well under 336 hours of Salt Spray testing (Presumably 5%) only traces of corrosion were noted. However both jump to moderate corrosion, and present as a corrosion failure when exposed to Marine Environments for 1 year. Is this be related to all of the metals that are in sea water?

Thanks,

Skypunk