What to do about this corrosion when it comes to re-spray

[justincarter]

Wise Mooney folks,

Its time for a respray on my 1967F. Its looking a little tired and we look like were destined to be together for the foreseeable future!

One of the reasons is that along the underside of each wing, where the (many!) inspection panels are, stainless screws were used and these have caused a little metal on metal corrosion to start. It starts around the screw holes in the skin. The engineers have inspected and its only superficial.

I attached a picture to illustrate the point.

When it comes to the re-spray, at some point we'll have a bare metal aircraft. What should be done with this corrosion then?

I guess mechanically remove (wire brushing, sanding etc - being careful not to remove too much of the outer metal layer) but then what? - once corrosion free - direct to etch primer, primer and repaint - OR should we be looking at some metal treatment or other to reduce the risk of return?

And after the respray- any thoughts on stainless or CAD plated screws or any other tips on reducing risk of recurrence? The holes are countersunk so no chance of a plastic washer really.

In the second picture, you can see the original screws around the windows (They were painted over at last respray). Around some of them you can see the slight yellow / orange stain of rust - I guess from the screws themselves - Im presuming these will all need to come out, holes treated and screw replaced before respray otherwise well have corrosion underneath the new paint from the outset?

(Once all is done Im planning an ACF50/corrosion X strategy)

[N201MKTurbo]

I would prime the cleaned corrosion with yellow zinc chromate before you prime the whole plane.

Were you going to strip the plane? You should. 

The zinc chromate is anti corrosion paint. It will react with the corrosive chemicals before the aluminum will. The yellow zinc chromate has no carbon black filler so you won't see it under the paint.

[justincarter]

yes - planning to strip the whole aircraft. Thanks for the zinc chromate tip.

[markejackson02]

If you're going to have the entire plane painted, the paint shop should remove the corrosion, acid etch or alodine, prime, and then paint.  I would talk to whoever is going to paint before you do anything.  If the paint job is a year away or you are just going to touch up, I would remove the corrosion (use the maroon 3M Scotchbrite pads), brush on some alodine, then prime with Zinc Chromate or an expoy primer and then touch it up.

Edited October 24, 2015 by markejackson02
grammar

[jetdriven]

That under wing photo is filiform corrosion. After you strip it it looks darker than the surround aluminum.  You must remove all this to a shiny later before acid etch, alodine, primer and paint. Red scotchbrite by hand is preferred to remove the corrosion  for local small areas use a red scotchbrite on a twist loc in a drill  

Variprime etching primer is ok for spot jobs, but it's soft since it's a lacquer.  It actually must be primed over. For a whole plane the best you can buy is a MIL-P23377 strontium chromate epoxy primer.  One coat then topcoat it.  

Note do not let stripper, acid or alodine into any lap seams or under any screws or inspection panels.   Tape these with aluminum tape. Further, alodine and chromate primers contain hexavalent chromium  which kills you slowly, or at least takes your golden years from you.  Wear a bunny suit with a N95 organic solvent cartridge respirator when working with this stuff.

I just stripped the front half of my wing to profile it with micro balloons and epoxy filler primer.  It had a few areas like you have. 

 

[carl]

I have be working on this too. And as Jetdriven states this stuff is a poison. The industry is coming out with new improved systems that are non chromatic. 

I am using PreKote®   

http://www.pantheonchemical.com/prekote-surface-pretreatment/      this is a non chromate prep.

This is fairly easy to use, after this treatment you prime, then paint. 

So it is  1)Stripping  2) PreKote 3)Epoxy Priming 4) Painting

I use an organic cartridge respirator (1,2,3)and I would use a full air replacement mask for urethane (4). Also these chemicals can be absorbed through skin.

(Yes, I even use a mask for stripping )

 

carl

 

[justincarter]

thanks guys, Carl the Prekote looks interesting.

even if the prekote  or alodine just gets used on the 'high erosion areas' and where there was corrosion before the primer might be a start. it seems to reduce corrosion be getting great adhesion of the primer to the metal surface.

Any thoughts about the fixings for the inspection panels once all is done? stainless or otherwise or other strategies ?

[cliffy]

AC 43.13-1B has an entire chapter devoted to Corrosion- Chap 6. The Mooney Maintenance manual should be consulted first and then if found lacking in directions go to 43.13. I've included a clip from Chap 6 on filiform corrosion. 

Everyone should have a copy of the MM and 43.13 if only for reference and general information

 

6-16. FILIFORM CORROSION. Filiform
corrosion is a special form of oxygen concentration
cell which occurs on metal surfaces
having an organic coating system. It is recognized
by its characteristic worm-like trace of
corrosion products beneath the paint film.
(See figure 6-8.) Polyurethane finishes are especially
susceptible to filiform corrosion. Filiform
occurs when the relative humidity of the
air is between 78 and 90 percent and the surface
is slightly acidic. This corrosion usually
attacks steel and aluminum surfaces. The
traces never cross on steel, but they will cross
under one another on aluminum which makes
the damage deeper and more severe for aluminum.
If the corrosion is not removed, the area
treated, and a protective finish applied, the corrosion
can lead to inter-granular corrosion, especially
around fasteners and at seams. Filiform
corrosion can be removed using glass
bead blasting material with portable abrasive
blasting equipment or sanding. Filiform corrosion
can be prevented by storing aircraft in an
environment with a relative humidity below
70 percent, using coating systems having a low
rate of diffusion for oxygen and water vapors,
and by washing the aircraft to remove acidic
contaminants from the surface.
 

[justincarter]

Thanks Cliffy,

I guess living in Arizona means you're 90% of the way there. The dank NE Coast of England is another story

Justin

[N601RX]

Do a google search for soda blasting. It basically uses baking soda and will remove the corrosion without distorting or damaging the aluminum like other more harsh abrasives will. 

[cliffy]

Except we had 8 straight days of rain last week (first time ever for us) with flash floods drowning my garages.

At least I don't live in a dank, dark, wet cave in the NE :-)