Zinc Chromate primer/Alodine preparation

[Guest Maj Millard]

All aircraft grade sheet should be alclad as far as I know, unless otherwise maked, and specifically 6061-T6 as it is know as having good corrosion resistance ..of course when you drill or cut it, you compromise the alclad protection which is why the alodine and primer protective processes are required, as in previous posts.............................Maj...

 

 

[Head in the clouds]

I've never seen 6061T6 with an alclad coating and certainly neither Airport Metals nor Aircraft Spruce list such a thing. I don't believe it exists, and probably because 6061T6 is a Marine grade not an aircraft grade material. The aircraft grades are 2024 and 7075 in various tempers and are all available with alclad coatings.

 

No, zinc based paints, although slightly removed from aluminium on the periodic table will not provide any galvanic protection to aluminium alloys. If you wanted to achieve that you would need to use magnesium based paints, and I've not heard of their existence but magnesium anodes is what you use to protect an aly boat hull for example, if you fit zinc anodes they'll still be unmarked when your boat has dissolved. Note that the anodes on an outboard motor or outdrive leg are magnesium not zinc. Zinc anodes get fitted to steel hulls.

 

 

[Thruster87]

The Wattyl Super-etch primer is a great primer and finish base, and is now widely used in the industry in a selection of colours.It can be applied after scotchbriting as described above with some success, however the acid-etch (WO1) followed by alodining then Wattyl superetch would probabily be more longer lasting as a corrosion proofing method.

The old green or yellow zinc Chromate primer finish, commonly seen on the interiors of many aircraft, (cessna, Piper etc etc) is also very effective long-term, but the chromate side of things has been found to not be good for our long-term health. That means the aircraft would probabily last longer than the pilot !!....Most manufacturers have now replaced this with an epoxy based primer finish.

 

The best long-term corrosion-proofing method is to acid-etch with WO1, rinse with clean water, then immerse in an alodine solution which is also rinsed off with clean water. this gives the alum that nice gold appearance. the metal is then effectivly chemically corrosion-proofed, and is also paint-ready. In some cases finish coats are applied directly over this after a good clean down with prepsol or other good paint-prep solvent.

 

however the addition of any good primer coat (Wattyl super-etch or an epoxy-based primer) will add additional layers of long-term protection).

 

For your reference, as an example only, on the amphibous Cesnna Caravans and Beavers used daily in the Whitsundays we have found the follow finish to be the most durable, after much experimentation over the years. These aircraft are in the Pacific ocean as often as six times daily, and their exposure to corrision is extreme.

 

Bare alum or beed -blasted alum - wo1 acid-etch - clean water rinse - alodine dip - clean water rinse - heavy spray coat of Wattyl Super-etch - spray coat of industrial high-solids epoxy based primer - light sand with scourer to make finish coat ready - heavy coat of Poly u400 epoxy paint finish.....

 

For a lightweight aircraft where finish weight may be a factor, and where there is only minimal marine exposure, the following should do the job adequetly. Bare (new) alum roughed up with a scourer - cleaned throughly with a quality prepsol solvent - good spray coating of Wattyl Super etch - light scourer ready for final coat - good coat of epoxy based finish coat........................................................................ Maj...

Do not use an etch primer with any sort of pre-treatment [alodine or pre-kote] as the organic acids in the etch primer will remove that coating .Yes the best is the aluprep-alodine method followed by an epoxy two pack primer plus top coatsThe pre-Kote method is not as good. cheers

[XAIRVTW]

Has anybody had experience with using Pearl in their paint ? and does the pearl go in the base coat or in the top coat ?The effect I am after is to take a mauve/ lilac from a solid colour where it joins up against a striking color, then fade it as it gets further away .

 

Something like the light color purple that Toyota used in their Starlets. If you looked directly at them you would see the purple, but by looking at them from an angle they appeared to be a lighter shade of purple then disappearing into silver once you looked along them .

Hi Ron the pearl does not go in the base. You lay your base color down let it flash off then lay your pearl allow that to flash off then your clear. What you need to do first though is spray at least 3 small color cards label them 1,2,3 on the first card you put 3 coats of base 2 coates of pearl, 2nd card put 3 coats of base 3 coates of pearl, 3rd card 3 coats of base 4 coates of pearl then on them all put a coat of clear on. by doing this you will be able to work out how much pearl you need to get the effect your after it will say you time & material when it comes to putting it on the job.

 

If you need any more info Im happy to help out.

 

 

[M61A1]

I've never seen 6061T6 with an alclad coating and certainly neither Airport Metals nor Aircraft Spruce list such a thing. I don't believe it exists, and probably because 6061T6 is a Marine grade not an aircraft grade material. The aircraft grades are 2024 and 7075 in various tempers and are all available with alclad coatings..

Could it possibly be that 2000 series, aluminium/copper alloy and 7000 series, aluminium/zinc alloy are not as resistant to corrosion as the 6000 series which uses silicon?

 

 

[Head in the clouds]

Could it possibly be that 2000 series, aluminium/copper alloy and 7000 series, aluminium/zinc alloy are not as resistant to corrosion as the 6000 series which uses silicon?

Certainly, and don't forget the manganese ... and is why 6061 and 5083 are the marine grades suitable for highly corrosive environment. There's nothing wrong with designing to use them in aircraft of course but you need to be careful about different alloys in contact with each other.

 

When you build a (good) boat for example you make sure that all of the aly plate comes from the same smelt, the very same batch, not just the same alloy in general. Not only that, if you really want to do the job properly you then send several billets of the same batch over to Italy and get the welding wire made from it. That way you hopefully won't have any electrolytically induced corrosion problems at the weld-lines.

 

 

[facthunter]

What HITC is saying relating to slightly different metals in contact with each other in an electrolyte, Like salt water or acid or alkali solution is the same for impure metals. some alloys may behave like this ??? A physical barrier that is impervious to the solutions will stop corrosion, until it is damaged.( Stone chips etc) Someone put a stainless steel prop on an Aluminium boat that apparently went to the bottom quickly. I can't imagine how hard it must be to keep seaplanes from rotting away. There were some Grumman Mallards in the states where the wing spars went bad. Nev

 

 

[rankamateur]

Certainly, and don't forget the manganese ... and is why 6061 and 5083 are the marine grades suitable for highly corrosive environment. There's nothing wrong with designing to use them in aircraft of course but you need to be careful about different alloys in contact with each other.When you build a (good) boat for example you make sure that all of the aly plate comes from the same smelt, the very same batch, not just the same alloy in general. Not only that, if you really want to do the job properly you then send several billets of the same batch over to Italy and get the welding wire made from it. That way you hopefully won't have any electrolytically induced corrosion problems at the weld-lines.

So there could be an issued where the 6061-T6 sheet is supported by a softer dull grade used to press the nose ribs for the wings, for instance. The best solution is to not allow it to come in contact with the electrolyte, don't know how you do that if you fly from a coastal aerodrome.

 

 

[facthunter]

When you are assembling the contact areas are painted and some additional wet paint applied at time of assembly. I reckon the idea of using a glue at the same time would be a good idea (instead of the paint). Corrosion in lap joints is common. The glue makes the whole thing much stronger especially with thin sheet. Nev .

 

 

[rankamateur]

ICP supply a black water based mastic which is supposed to be applied to all contact areas, dried and then assembled. This acts like contact cement in the joint to stop the pieces ring-barking the rivets as the joint works under stress. I am now planning to give the alodine a miss and etch prime all parts before applying the mastic like the factory now also does. It must add a fair bit of time to the build because it adds $3000 to the factory built price.

 

 

[Kyle Communications]

They use some epoxy etch primer now I think...and it will add a lot to the build time....just file all the edges of all sheets and parts and use lots of the black crap....I have no corrosion at all on my Sav. I am sure it is because the sharp edges of the sheets rub each other and when they paint the aircraft they don't pay enough attention to all the edges where the sheets meet or overlap with any etch or paint

 

 

[Kyle Communications]

Oh and I have a anticorrosion unit fitted too...who knows it may even work

 

 

[rankamateur]

Oh and I have a anticorrosion unit fitted too...who knows it may even work

Gundy has one fitted too and last report he had no corrosion either, I might get one of those Peter Brock endorsed black boxes that you put in the engine bay but don't connect to anything, it might work too.

 

I have already ordered a second tin of black goop from the local distributor, I will pick it up when I pick up his cleco collection.

 

 

[Head in the clouds]

So there could be an issued where the 6061-T6 sheet is supported by a softer dull grade used to press the nose ribs for the wings, for instance. The best solution is to not allow it to come in contact with the electrolyte, don't know how you do that if you fly from a coastal aerodrome.

Yes, there would be the potential for that if it's bare metal-to-metal contact and the alloys are different. But are you sure the alloys are different? I was looking at kit parts some while ago and was surprised to see that most of the ribs were pressed from 6061T6 which was the same as some of the skins, other kits had 2024T3 Alclad skins.

 

Nonetheless I beat my ribs out of a soft grade and skin them with either 6061 or 2024 Alclad and haven't had corrosion issues at the rib/skin interface because water generally doesn't get there.

 

The worst case would be if you use stainless rivets. Where high strength pull rivets are required I use monel and if it's an area that will get moisture then I dip each rivet in Duralac immediately prior to fitting it.

 

I can't imagine how hard it must be to keep seaplanes from rotting away. There were some Grumman Mallards in the states where the wing spars went bad. Nev

I have a friend who used to be an engineer on PBY Catalinas. Apparently they had a fabric (cotton?, linen?, flax?) inserted between the lap joints of the sheeting on the hulls, it was intended as some form of caulking. The sheets were all the same alloy of course but the rivets weren't ... You can imagine the electrolysis. I'm told that the Cats that survived the war and went into private collections had to be completely de-rivetted and then re-rivetted, to remove the fabric strips.

 

 

[M61A1]

The polysulphide aircraft sealants do a very good job for faying surface seals (pr-1440, ps-870, pr-1771). I've never actually purchased one ( all consumables are supplied by defence in my job), so I don't know how much they cost, but those that I mentioned are specifically designed for the task. The only thing is, if you need to disassemble once it's cured, it can be very difficult not to damage the parts, especially if they're thin.