‘Back to bare metal’ will almost certainly be a mechanical process

… at least some of that white fur will be Aluminium Oxide, which is chemically and physically tough stuff !

MichaelG.

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Ref. https://en.wikipedia.org/wiki/Aluminium_oxide

Something like 5% sodium hydroxide (and perhaps a wire brush) should dissolve the white corrosion products (mainly aluminum hydroxide). After that I would use phosphoric acid (citric acid should also work) to neutralize any residual lye. Clean with plenty of fresh water and dry thoroughly before painting.

I would start thinking about repairing the cover till it looks good then make a fibreglass mould, just in case the repair doesn’t last.

The aircraft industry uses chromate conversion coating to protect aircraft aluminium structures from long term corrosion. The light green coating lightly etches the metal surface to provide excellent adhesion for the paint topcoat. Google ‘chromate conversion coatings for aluminium’ for more details.

For what it’s worth … Decades ago, I had the privilege of visiting British Airways to discuss their protocol for patching damaged aircraft wings:

In brief summary

1. Mechanically abrade to provide a clean metallic surface
2. Within 20 minutes, treat the wound with Phosphoric Acid and let it dry
3. apply the epoxy pre-preg patch and cure it [hot air helps]
4. Fly away into the blue yonder

That 20 minute time-frame is considered critical.

MichaelG.

Motorcycle customisers often have bead blasting, but as the OP is ‘of Wick’ I suppose there won’t be a lot around.

Don’t worry about corrosion coming through, as long as it is thoroughly dry before you coat it.

But airplanes get wet, lathes usually don’t. If it doesn’t get wet it won’t corrode surely? I’m assuming no other corrodivre chemicals.

On 18 July 2024 at 11:57 Martin of Wick Said:…

Thanks to all for the input, I now have a plan…

 

Caustic soak for a few hours to loosen surface crust

…
 

The caustic soak is high risk.    If the part is an Aluminium alloy, the alkali won’t stop at at the crust, and the whole lot will dissolve.   And because Caustic Soda eats Aluminium faster than corrosion, it will drill new holes wherever it hits bare metal.

Supervising a soak in hope of removing the part from the chemical before deep damage is done is also risky.  The caustic will drill into any cracks and crevasses, and be extremely difficult to remove from the billions of superficial microcracks all metal parts have. It will grow the cracks and fill them with weak corrosion likely to spoil a later filler and paint job.

Always important to use the right tools for the job, and chemicals aren’t suitable for this one.   The rest of the list is fine, just start with brushes.  Won’t take long.  Most of the corrosion formed on Aluminium and Zinc Alloys is mechanically weak.  For that reason an angle-grinder might be completely over the top, so take care.   Whatever it is, the alloy will be much softer than a stainless steel brush, so don’t rip into it.   By the by sandblasting does a much better job because it gets into the cracks without gouging as a wire brush will.   All that’s needed is a sandblaster!

Just for fun, if anyone fancies the experiment,  steal rubber gloves, apron and a ceramic mug from kitchen.  Fill the mug half full of water.  Eye protection also recommended – this nasty stuff might spit.  Outside, add two or three table-spoons of Caustic Soda, stirring each in separately.  The solution will get hot.  Then add a lump of Aluminium scrap, stand back a little, and enjoy.   After a short pause, the liquid should froth, bubble and get hotter. The speed at which heat is produced is proportional to the surface area of the metal, so don’t use Aluminium swarf.    The bubbles contain Hydrogen gas that can be exploded, pop not boom, with a lit splint.  The solid matter remaining is corrosion, mostly Sodium Aluminate.    As the reaction creates a lot of heat, a chemist would take extra care to contain and control a large scale version of this reaction.  In the event the ceramic mug boils over or cracks, cool and dilute with plenty of water.  Then neutralise with Vinegar.

Dave

In another job We used a pan of the cromate converter known as Alodine over here to protect Runway lights. It was stored in a big pan on the floor in a corner of the shop. Everyone knew where it was We thought. Someone came in on the weekend and left the lights off and walked through the shop. They stepped into the pan and proceeded to leave the shop. Their foot prints were eaten into the cement floor all the way out. No one would ‘fess up. Monday morning the Boss and owner of the place seen the damage and We were told to get rid of the stuff. Some one looked it up and said it was hazardous material and would cost a small fortune to get rid of. Nothing said and it disappeared. A month or so went by and a knock on the door from the City sewage Dept happened. They said We know nothing about it. The Sewage Boss said They ran a camera down a drain from a long distance away and traced it to a loo in Our shop. It cleaned the pipes out very well. We were told not to do that again. We did not as We were out of material.

A timely thread! I’m re-building a couple of neglected accordions that I’ve acquired. The keys or buttons lift suede leather-covered valves off the light alloy ‘foundation plate’ to allow air through, to make the reeds sound. Damp storage and neglect has caused a surprising amount of corrosion under the valve faces, resembling filiform corrosion, with the white oxide/hydroxide sticking the valves down in many places.

The corrosion clearly has to go, and a smooth surface restored, so that re-faced valves will provide an airtight seal. The plate cannot be removed from the wooden body of the instrument, so I think liquid-based treatments are too hazardous. Michael G’s link says wire-brushing is forbidden, but my reading is that that applies to anodising, not bare material. Is that right? If it’s not allowed on bare metal, why not?

Would brushing with stainless steel and/or glass-fibre brushes, followed by aluminium-filled epoxy be OK? So far, it seems impossible to get the last traces of oxide/hydroxide out from the corrosion tracks. Is there any risk that corrosion may extend from these contaminated areas, which will be covered by epoxy, given that strenuous attempts will be made to protect from damp? The surface will remain bare metal, not painted.

On 19 July 2024 at 07:57 Sandgrounder Said:

And if you do you use caustic soda, google “caustic soda eye injury images” and then buy the best goggles available.

I learned as an apprentice mixing bucketfuls of caustic soda flakes into large boiler water treatment tanks, ALWAYS wash your hands after handling caustic and before going to the toilet to take a leak. Caustic residue on your hands getting on a certain sensitive part of your anatomy is something you do only once!