neutralising severe alloy corrosion

[Martin of WickParticipant @martinofwick]

The lathe gear cover and other alloy parts have suffered severe and perforating chemical corrosion of some sort as per photo.

An internet search proved particularly unhelpful as all manner of chemicals from caustic soda to nitric acid were proposed and other esoterics such as vinegar or ammonium sulphate (even lemon juice!). I always thought that caustic attacked alloy  quite severely in any case? and strong acids likewise. Maybe it is possible to use any sort of chemical on alloy, but I somehow doubt it.

I was wondering if anyone could provide any experience led guidance as how to best to completely kill the corrosion back to bare metal, so the cover can be primed, repaired with GRP etc. and re-sprayed without risk of the corrosion bubbling up again in future.

 

 

 

 

 

[Michael GilliganParticipant @michaelgilligan61133]

‘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

[HopperParticipant @hopper]

Was the lathe left lying in a puddle on the beach for the past 50 years? Looks like electrolytic / galvanic corrosion caused by electrical currents generated between the different elements of aluminium alloy or Mazak diecasting material.

In a wet, salt environment, there is galvanic current flow between the aluminium, zinc, copper, magnesium and whatever else happens to be in the alloy.It eats the metal away as it goes in a sort of reverse electroplating process.

We see a lot of it where I live in a harsh wet (8 feet of rain a year) tropical coastal environment. Old motorbike components start to sort of delaminate or simply fret away as in your picture if left lying around in the wet rainforest environment or beach. Military aircraft that crashed on remote beaches in World War 2 and sat there since turn into piles of white powder through the process. (The hard chromed landing gear struts still look like new in the middle of it all!)

I don’t know of any magic chemical that will fix it. Wire brushing wheel and even grinder to get rid of all the crystalised and corroded parts and then seal it up with some very good two-pack epoxy primer type paint to keep it away from further wet, salty air. Maybe wash it well first to get rid of any acidic pH residue. Then maybe bake it to get any moisture out of the pores before painting.

Some of our resident aircraft experts may have more idea as it is a recognised problem in aluminium alloy airframes and skins etc. Aluminium boat guys deal with it too. Alumabrite is one product they use after sanding down.

But if you google around at electrolytic or galvanic corrosion you may find more info.

[FulmenParticipant @fulmen]

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.

[Michael Gilligan]

On Michael Gilligan Said:

‘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.

.

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

I agree.  Elbow-grease is the only safe potion for this.

The unknown alloy could be either Aluminium or Zinc based.   What’s needed is a chemical that removes the corrosion without attacking  the underlying metal.  May not exist!  Whilst Aluminium Oxide is removed by Sodium Hydroxide (Caustic Soda), the alkali also proceeds to scoff the Aluminium underneath.  Ditto acids used on Zinc alloys, making it quite risky to splash chemicals about in hope one of them might work.   Light metal alloys are much less tolerant of chemical cleaners than steel, copper, brass and silver etc.

If sand-blasting or a wire-wheel were off the menu, I might try a toothbrush and 50% washing soda solution.  Don’t leave it to soak unattended!  Although Washing Soda is less aggressive to skin and eyes than caustic, still a rubber gloves and eye protection job.  Chemical burns aren’t recommended!

If the alloy is Zinc based, could be much worse than corrosion.   See Zinc Plague, an unfixable problem afflicting many but not all Zinc alloy castings made before about 1960.

Dave

[Dave HalfordParticipant @davehalford22513]

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.

[Martin of WickParticipant @martinofwick]

Hmm, thank s for the advice. To answer Hoppers question, I suspect the covers have been lying in a pile of bits at the bottom of someone’s leaky shed for the past 50 years.

Covers are from  a 50’s Bormilathe on which the alloy parts still attached to the lathe were in good condition, so I expect the covers were regarded as superfluous and just dumped in heap somewhere out of the way and forgotten about. The lathe was indescribably filthy and encased in a thick cake of denatured oil, dust and rust. Surprisingly, underneath the 50 year deposit of crap, the unit appears little used and in good mechanical condition. The scraping marks on the ways still visible and the spindle and taper bores perfect. Spindle turns a smooth as silk, but for a  12″ or so between centres machine, it is the most colossal weight!

If I had a bead blasting cabinet I would use that to blow away the rotten material. I will try to source some SS wire brushes to mount on an angle grinder to remove the dross.

Problem is, even when you are back to what appears to be bare metal, are you really? those micro pits and fissures likely go vey deep, which is why I think there needs to be some form of additional chemical treatment (possibly quite aggressive).

As suggested, if there is no readily available chemical treatment, probably best to go a far as possible with mechanical debriding, seal it up with something very sticky and moisture proof and hope for the best. If it erupts in pustules after a couple of years, smooth off and make a mould for the GRP version! ( or just throw them away, as likely, they are a bloody nuisance when in place!).

 

[mike TParticipant @miket56243]

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.

[mike T]

On mike T Said:

Google ‘chromate conversion coatings for aluminium’ for more details.

Do a search for ‘hexavalent chromium risk’ while you’re at it. It’s not the most healthy chemical to toss around. Phosphoric acid should also passivate aluminum (phosphate conversion coating). Probably less durable than chromate but at least you won’t grow a third head.

As others have pointed out, sodium hydroxide will attack the base metal as well as the corrosion products. However this reaction is fairly slow, I’ve stripped bad anodizing on parts many times and a few minutes doesn’t do any damage.

 

[Michael GilliganParticipant @michaelgilligan61133]

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.

[Robert Atkinson 2Participant @robertatkinson2]

The cromate conversion products are now banned for general use and aviation ban is nigh.
Mechanical removal of most of it. A quick brush with dilute phosphoric acid if you must but rinse well and dry immediately. Build up with aluminium filled epoxy ideally or polyester filler if nothing else.

Robert.

[duncan webster 1Participant @duncanwebster1]

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.

[Bantam BillParticipant @bantambill]

In the aircraft industry mechanical abrasion is the only approved method of corrosion removal on aluminium alloys, once all visible corrosion is removed a further abrasion of the surface is required to remove any remaining microscopic pits. If it ain’t done right it’s back within a year. We had one aircraft company who reported the same corrosion nearly every year just a bit deeper than last year, not sure what happened to that one.

 

[duncan webster 1Participant @duncanwebster1]

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.

[Michael GilliganParticipant @michaelgilligan61133]

A little light reading … obviously more relevant to aircraft than the covers of a Bormilathe:

https://www.caa.co.uk/publication/download/16188

… but we can often benefit from the experience of others.

MichaelG.

[Martin of WickParticipant @martinofwick]

Phew! Some scary pictures of corroded aircraft parts, hope they are not all like that!

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

 

Caustic soak for a few hours to loosen surface crust

Angle grinder with SS brush and Dremel with carbide burr to chase out any obvious pits to clean metal

Phos acid soak for affected area, rinse and dry

polyester resin + filler with Al mesh to bridge perforations followed by surface abrasion

Polyester car body filler to finished surface

etch primer then filler primer then paint finish

 

 

[HopperParticipant @hopper]

Sounds good. The Bormilathe is an interesting machine. Please post some pics when it is done.

[Martin of Wick]

On 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

 

 

[duncan webster 1Participant @duncanwebster1]

Ally also reacts with cement (because it is strongly alkaline) so if you want to concrete in an ally post you’re well advised to wind sticky tape round to keep them apart.

 

[mark costello 1Participant @markcostello1]

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.

[Bantam BillParticipant @bantambill]

Noooo don’t use caustic soda, there will be nothing left. Just do it mechanically

[Kiwi BlokeParticipant @kiwibloke62605]

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.

[SandgrounderParticipant @sandgrounder]

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

[Sandgrounder]

On 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!

[Martin of WickParticipant @martinofwick]

Perhaps ‘soak’ was too strong a descriptive. Had in mind to flush the surface with a weak solution, scrub in leave for 10 mins and wash off. Caustic – some say yay some say nay, so perhaps will use washing soda or weak ammonia solution for the first flush prior to debriding.

I may initially try using a electric drill with SS brush instead of angle grinder, but they always seem a bit feeble. Really do wish I had a grit blasting cabinet though!

BTW, wholly endorse comments regarding caustic – nasty stuff, I have a half inch crater in my heel to prove it!

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