Zinc Chromate primer/Alodine preparation

[rankamateur]

Are these both Chromate treatments and if so is it neccessary to do both? Has anyone got experience with non-Chromate pre-treatments. I am having an issue with how I would handle the rinse water away from an alodine line so I don't end up poisoning something here on my farm.

 

 

[rgmwa]

I'm using Wattyl Super-Etch primer. It's available in spray cans and larger tins if you're using a spray gun. I just scotchbrite the aluminium, wash it with a mild detergent and plenty of water (dry it thoroughly), or use acetone on small parts, and then spray the primer. Job done.

 

http://www.wattyl.net.au/industrial/documents/industrial/tds/metal/Super Etch v2.pdf

 

rgmwa

 

 

[Guest Maj Millard]

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...

 

 

[aj_richo]

I used Wattyl super etch as well, but for cleaning and preparing I used Prekote applied with scotchbrite pads as per instructions.

 

Works great on 6061 (Sonex airframe) Non toxic but use gloves and eye protection.

 

I found out recently that I am allergic to Chromium compounds - via leather workboots of all things.. just as well I never used any of the more traditional metthods!

 

 

[Guest Maj Millard]

rankamateur, In answer to your initial questions, I doubt very much if Wattyl Super-etch would have any harmfull chromates in it. The original old zinc-chromate primer is still availabe within the aviation industry, but comes with many warning re: for aviation use only, by approved persons with a shitload of protective gear in place. So hardly worth the risk with Wattly super-etch easier to use and just as good.

 

As the Wattyl comes out of the industrial-sized 5 Ltr tins, we actually mix it 50/50 with a quality solvent, so it is more sprayable with a standard paint gun. This does not alter it's adhesion qualities, and it is suggested you can do so on the can. As far as Alodine solution poisioning your farm, I believe that once it has been diluted with water it would pretty much become inert. Both solutions require the use of suitable rubber gloves, and the Alodine will leave you with lovely golden hands if you don't use them !....Both the wo1 alum-etch, and Alodine, are mildly acidic and are easily diluted. This is why it is very important to remove them from your airframe with a good clean water rinse, as otherwise they could go on etching !!... However by all means refer to to saftey literature therin..................................Maj...

 

 

[aj_richo]

Super etch is Zinc Oxide based

 

Alodine is not inert, it pretty nasty stuff. mainly the Chromic acid, I understand it should not be just tipped on the ground as it can easily contaminate ground water. One of the disposal recommendations I read has it mixed in cement to lock it up

 

 

[facthunter]

If it has chromic acid in it, then it's a disposal problem> Phosphoric acid is a fertiliser and is OK diluted. I was at a SAAA builder's event yesterday. They consider the chromate pretty much a thing of the past. They reckon the "rattle cans" don't have it and epoxy etch and a coat is what's done. after the preps.. ALL as Maj said it. Nev

 

 

[Head in the clouds]

All good information there but just a word of warning from the Alodine 1201 MSDS (I'm assuming 1201 is the one you use Maj?). I'm not sure how its toxicity compares with Zinc Chromate but it's certainly a cancer hazard as well, so I wouldn't let it turn your hands yellow ...

 

Acute Toxicity:

 

A: General Product Information

 

Industrial exposure to chromium may cause dermatitis, skin ulcers, perforation of the nasal septum, as well as

 

cancers of the lungs, nasal cavity and paranasal sinuses.

 

B: Component Analysis - LD50/LC50

 

Potassium ferricyanide (13746-66-2)

 

Oral LD50 Mouse: 2970 mg/kg

 

Hydrofluoric Acid (7664-39-3)

 

Inhalation LC50 Rat: 1276 mg/kg/1H; Inhalation LC50 Mouse: 342 mg/kg/1H

 

Carcinogenicity:

 

A: General Product Information

 

No information available for the product.

 

B: Component Carcinogenicity

 

Hydrofluoric Acid (7664-39-3)

 

ACGIH: A4 - Not Classifiable as a Human Carcinogen (as F) (related to Fluorides)

 

Chromic acid (7738-94-5)

 

ACGIH: A1 - Confirmed Human Carcinogen (related to Chromium (VI) water soluble compounds)

 

Chronic Toxicity

 

Chromium III, the naturally occurring form, has low toxicity while chromium VI is highly toxic due to strong

 

oxidation characteristics and permeability through biological membranes. Excessive exposure to chromium VI

 

can produce allergic skin sensitization reactions and severe nasal irritation, scarring and damage to the lungs,

 

liver and kidney damage.

 

 

[rankamateur]

And Hydrofloric acid is the one that beads up on burnt Viton seals and belts after car fire, we train our rural firefighters about it because it can soak through the skin and enter bones where it continues migrating up the limb, this is then treated by amputation to arrest it's spread.

 

 

[rankamateur]

Super etch is Zinc Oxide based

I know Super etch says it is suitable for unspecified non-ferrous metals but I am left wondering if zinc oxide etch primer is really that well suited to aluminium for corrosion prevention or etching for that mater.

 

 

[aj_richo]

It is for aluminium, it has an acidic component to etch the material. That said I have left it in my aluminium pot spraygun for weeks, no real probelms with eating the pot out (looks a bit grainy though)

 

I believe the zinc oxide forms part of a sacrificial barrier in the presence of moisture.

 

Time will tell if it will do the job, certainly better than bare metal on bare metal with dust/crud in between.

 

Spare bare 6061 channel I have in the shed is suffering from filiform corrosion just from dust accumulation, whereas the dusty primed scrap parts have none.

 

My main reason for using it was that alodine and other hexavalent compounds are too hazardous to the body and environment, especially trying to dispose of it safely.

 

 

[rankamateur]

My main reason for using it was that alodine and other hexavalent compounds are too hazardous to the body and environment, especially trying to dispose of it safely.

I share the same concerns about toxicity, it will kill you if you don't take it seriously. I retired as a chemical lab-technician after only seven years because amongst other things, my senior colleges were all retiring unfit with various cancers from working lives spent before OH&S took on. But I have a kit on the way and it is no good deciding what pre-treatment to use when you are half finished building.

 

 

[dazza 38]

Great, we used Zinc Chromate in rattle cans and Alodine all the time in the RAAF as Airframe fitters. Loved the smell of Zinc Chromate, No PPE at all for the Zinc Chromate. Wore those disposable gloves when I used Alodine on the Pave Tack Pods. Well most of the time (yes I did get Yellow hand syndrome with alodine). Anyway it couldn't be as bad as spending 12 years as a plumber with my hands covered in PVC Pipe cleaner (MEK). I'm surprised that I'm still here:crazy:

 

PS- Although I did get a pay out for my time spent in F111 fuel tanks .

 

 

[aj_richo]

Ha ha .. yep as an ex RAAFie I hear you Dazza... are you sure you're still breathing?

 

Loved that MEK as a cleaner, started using it when they stopped us from using trichloroethane to wash carparts.. er aircraft components

 

 

[Ron5335]

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 .

 

 

[Guest Maj Millard]

There is a Lancair in Shute harbor with a full-on pearl paint job (purple). I seem to recall he said he put it in the base paint with clear over the top......................................Maj...

 

 

 

 

[Guest Maj Millard]

I do have Cols Email if you want to talk with him about it directly, looks pretty stunning as it goes past the hangar approaching 160 kts !

 

...............................Maj...

 

 

[facthunter]

Paint helps protect a surface in 2 basic ways. Firstly it provides a protective barrier by covering the surface.

 

secondly, SOME paints on SOME metals have a galvanic capability. This feature allows for the particles in the paint to sacrificially be removed chemically in preference to the parent metal. ( Like an anode in your water heater or outboard motor cooling system). Steel is easier to provide this protection to than the more "active" metals like aluminium and magnesium. Nev

 

 

[Mick]

looks pretty stunning as it goes past the hangar approaching 160 kts !...........Maj...

Does that mean if I paint the Skylark like that it will do 160kts too?

 

 

[Guest Maj Millard]

Only if u put an IO-360 in it with a custom 3-blade Hoffman prop !!!...actually that Lancair will do about 199 at full tilt.....................................................................Maj...

 

 

[rankamateur]

SOME paints on SOME metals have a galvanic capability. This feature allows for the particles in the paint to sacrificially be removed chemically in preference to the parent metal. ( Like an anode in your water heater or outboard motor cooling system).

So since Zinc is a little higher than Aluminium in the galvanic table, does that mean that Zinc Oxide paint safrificially protects aluminium under the top coat of two-pack or what ever?

 

 

[facthunter]

If it (the material in the paint) is more active, it does. This will even occur when there is a reducing coverage due to wear of the paint surface. Steel will rust through chromium and nickel but not zinc. ( Zinc is the original "Galvanizing" as in corrugated iron(steel) sheets which are hot dipped. Zinc can also be plated or incorporated in a paint. the protection is lost when significant areas of the surface become devoid of the protecting medium. Nev

 

 

[Guest Maj Millard]

Good question there, not sure about the answer though. Alclad, as the name suggests is an alloy (6061-2024 or whatever) with a very thin coating of pure aluminun on both sides. if you put a block of alum alloy in salt water, and a block of pure alum besides it and came back in a week only the block of pure aluminum would be there.........................................................Maj...

 

 

[facthunter]

Very pure metals do not corrode as much as alloys with impurities in do. eg. Steel containing sulphur is more corrosive whereas wrought iron ( the proper stuff) lasts a long time in sea water. early anchors and chains were made from it

 

Alclad has a PURE Al skin on it which resists corrosion as Aluminium quickly forms a protective oxide on its surface that protects it from further corrosion. This also makes it hard to weld without a gas shield or using a flux to exclude oxygen. Nev

 

 

[rankamateur]

Alclad has a PURE Al skin on it which resists corrosion as Aluminium quickly forms a protective oxide on its surface that protects it from further corrosion.

But is 6061-T6 aircraft sheet Alclad or not?