Corrosion Protection

 
  I spent a lot of time reading up on the various aspects of corrosion protection. Many sources listed the inherent corrosion resistance of 6061T6 aluminum. The general consensus is that this alloy offers superior resistance compared to other aluminum alloys. Some builders report virtually no corrosion on 6061, others have reported signs of surface corrosion on the inside of skins and other large areas in as little as 6-10 years (that plane was tied down about 10 miles from the ocean).

My own research has lead me to the following conclusions:

  • All aluminum will corrode under the right conditions
  • 6061 does offer superior corrosion resistance compared to other alloys, BUT...
  • 6061 will corrode under less than ideal conditions
  • The type of primer used is not as important as simply using one
Having decided to use a primer, I considered several approaches to priming:
  • Prime everything, including rivets
  • Prime only one mating surface to prevent metal-to-metal contact
  • Prime any aluminum that mates with anything else to protect from trapped moisture
  • Prime skins and other large, open sheets
Based on my own thoughts and considerations, and balancing that against the added cost and time of priming, I developed the following approach to applying primer on my plane:
  • Prime all webs in the main wing spar and horizontal stab spars (these assemblies are impossible to detect and correct corrosion once riveted)
  • Prime all angle stock mating surfaces (this is easy to do with spray cans)
  • Prime mating surfaces of small reinforcements, tabs, brackets, ect. that are attached to larger sheets (easy and cheap)
  • Spray a light mist coat of primer on wing skins, aft fuse box, and turtledeck skins (easy to do with spray cans, and keeps weight gain to a minimum)
  • Do not prime the forward fuselage skins (corrosion not likely, cosmetically unappealing with primer)
  • Do not prime rivets prior to setting (I donít think this is really a concern)
There are a few things that I have not decided firmly, namely which primer will I use? I considered the following primers:

Primer
Pro's
Con's
Zinc Chromate: Proven track record, offers scratch protection, comes in spray cans, easy to apply, minimal cleanup Toxic (requires protective gear), may not adhere well (need to test this), hazmat shipping charge
Zinc Oxide: Similar to Zinc Chromate, effective, offers scratch protection, comes in spray cans, easy to apply, minimal cleanup Not as tough as Zinc Chromate, toxic (but less so than zinc chromate), may not adhere well (need to test this), hazmat shipping charge
Napa Self-Etching
(spray cans):
Cheap, readily available, self etching promotes adherence, relatively non-toxic, easy to apply, minimal cleanup Not as high-tech or effective as newer primers (or zinc chromate)
AFS Water-bourn
Epoxy Primer:
Non-toxic (uses water as the reducing agent), high volume when used with spray gun, effective Complex (requires spray gun, compressor, and paint booth/open area to use), more expensive, extensive cleanup of spray gun required after each use
Alodyne Provides good tooth for primer to adhere to, adds protection Messy, not well suited to long build periods, toxic
Wash Primer: Adds some protection, easy to apply, provides good tooth for paint Requires spray gun, expensive, setup and cleanup time

My plan for prepping the aluminum will go something like this: Use lacquer thinner to remove all ink marks and printing from the parts. Using a spray bottle, spray parts with acid based etch solution (AFS Cleaner/Etch, PPG DX533 Cleaner/Etch, Alumiprep, ect.) and let sit for a couple of minutes. Scrub the parts with ScotchBrite pad (use gloves!) to abrade the surface and remove surface corrosion. Rinse with clean water and dry. Prime parts as soon as practical (within a couple of days).

Based on my priming strategy, my initial plan for priming interior structures is to purchase 12 spray cans of Tempo brand Zinc Oxide primer (Aircraft Spruce PN 7-9100). This should provide me with 300 sq. ft. of application (approximately 25 sq. ft. per can), and should cost less than $75. I feel this offers the best compromise between cost, simplicity, and performance.

For the exterior paint, I plan on using the AFS 1-part water-bourn Primer/Sealer, followed by AFS 2-part polyurethane color Top Coat. At the current time, I have no spray equipment and only experience spraying PolyFiber paint and products. I will continue to research and talk to other builders, and my choice of exterior paint may change.


Update: After completing the fuselage priming, the Zinc Oxide primer is really easy to use. It took me about 3-4 hours to prep all my fuselage parts with Alumiprep (acid etch), and then about 2-3 hours to prime the parts. I used 4 cans on the fuselage. At 12 oz per can, that is 48 oz of liquid primer. Once you subtract overspray and the weight of the solvent carrier, and there is probably only 1/3 of the weight left on the parts. That means I added a pound or two of weight, $30, and 1 days work to fully prime my fuselage. Sounds good to me!

You may notice in some pictures that ink marks have bled through the primer. In many instances the cause of the bleed through is failing to remove the old ink marks (part numbers, alignment marks, construction lines, etc) with lacquer thinner prior to scuffing the aluminum surface with a scotchbrite pad. The scotchbrite removes the top layer of ink as you scuff but leaves lots down in the pores of the metal. When the solvent from the primer hits the ink, it raises it from its' hiding places and bleeds it through the primer. The best way to prevent this is to 1) clean the ink from the part with lacquer thinner, 2) scuff with a scotchbrite pad, alumiprep etch, 3) rinse with water and dry completely, and finally 4) prime.


Ultimately, you will have to make up your own mind on priming, but I know how frustrating it was to get the same non-commital, "It's a personal choice" answer. The links below are some of the oppinions and information I found during my search on the internet. I place them here for information purposes only, and make no claim as to their accuracy.

TAB Group - Primer Tests
EAA Chapter 1000 Corrosion Control Articles
Gary Liming's Zenith 801 Web Page - Corrosion
Andy Karmy: Priming his RV-9
Primer wars by Andy Karmy
Aircraft Painting - Solving the Mystery by Ron Alexander
The Priming Issue from Robert's RV-7 Website
DETCO Aircraft Coatings
Preparation and Priming - Just one view
Priming with PolyFiber Epoxy Primer
That Priming Thing... by Sam Buchanan

 

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Updated: 22 Jan 07