neutralising severe alloy corrosion

[Kiwi Bloke]

On Kiwi Bloke Said:

[…]  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?  […]

The logic is stated in 12.3

and yes, you are correct … the rule is not really applicable to an already-bare metal surface.

MichaelG.

[Howard LewisParticipant @howardlewis46836]

An even nastier chemical is concentrated Nitric Acid.

So you may well have great difficulty obtaining any.

It a very potent oxidising agent, so potent that when it meets the Aluminum it will produce a protective film of oxide (Akin to anodising)

But NASTY, VERY risky, hard to obtain, dangerous to use, (Goggles, plastic apron, rubber gloves) and even more difficult to dispose of.

Howard

 

[Martin of WickParticipant @martinofwick]

Semi update…

The white fluff easily brushes off, revealing a dark grey scaly layer of corroded (oxidised?) metal, distinct from the bright shiny undamaged alloy.

That dark layer is very, very hard and resists abrading/removal with wire brush (the SS one I have available is a little to soft I think). The more robust knotted angle grinder brush turns out to be steel and not SS, so not usable for fear of embedding steel particles in the alloy.

I made a poultice of caustic and applied to the affected area. Dramatic fizzing of good metal, very little impact on oxidised area. In small areas the caustic was able to penetrate beneath the scale, small circular area of clean metal would then appear within the oxidised area, resulting in shiny spots appearing within the damaged area. I interpret this as caustic solution invading some individual corrosion cells, eating the metal beneath the scale which then allows the unbonded scale layer to flake off. So, carefully applied and monitored  caustic solution would  eventually permit the scale to be brushed off. However, it would be difficult to avoid some collateral damage to the un-corroded areas.

I wish I had one of the small blasting cabinets. However, glass based media may well be softer than the scale layer and even using sharp aluminium oxide media may not be that easy – commercial AlumOx may not be much harder than the oxidised layer to be removed!

Next up… die grinder with carbide burr. Waytogo! This is relatively quick and works well, but it is surprising just how thick the scale layer is and how deep some pits go. I guess it is like a dentist drilling out a rotten tooth, you just keep going until you get to undamaged material!

So the method is simplified to a quick wash with dilute caustic, neutralise and brush off of the hydroxide dust, dry.

Remove scale down to clean metal with 6mm carbide burrs, paint with phos acid and apply primers.

 

 

[Michael GilliganParticipant @michaelgilligan61133]

Good man, Martin … you’re on the road to success !

MichaelG.

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P.S. __ I have never used one, but those “needle-descaler” tools look like they might be the weapon of choice.

[old martParticipant @oldmart]

That corrosion needs completely mechanically removing or it will restart later on. Shot blasting and then filling with epoxy or car body filler will disguise the damage. Chemicals will likely not prove a viable solution because of the intercrystalline corrosion which acids and alkalines will get into and make matters worse.

[noel shelleyParticipant @noelshelley55608]

If you have a very large commpressor then a needle gun is great. I ran mine by couplling 3 workshop sized units together – so lots of air. great for descaling. Noel.

[Alan CharlestonParticipant @alancharleston78882]

Hi,

If you or a neighbour or friend have a water blaster, it might be the answer.

Regards,

Alan C.

[noel shelleyParticipant @noelshelley55608]

All this talk of aluminium and how it is eaten by other chemicals reminds me of the current ME articles of BUTTERSIDE DOWN and the authors choice of an aluminium frame for his trailer, in the context of the salting of roads in winter. Or is he a summer operator only ? Noel.

[FulmenParticipant @fulmen]

There’s aluminum and there’s aluminum. You can make boats from aluminum that can survive a lifetime in salt water:

https://www.buster.fi/en

[Michael GilliganParticipant @michaelgilligan61133]

Of course ‘really’ there are Aluminium Alloys and Aluminium Alloys

… the Devil being in the detail.

MichaelG.

 

[Fulmen]

On Fulmen Said:

There’s aluminum and there’s aluminum. You can make boats from aluminum that can survive a lifetime in salt water:

https://www.buster.fi/en

Pedantically, pure Aluminium is rarely found in workshops because it’s too soft and weak.   What we inaccurately call Aluminium will be one of a few thousand alloys, not all suitable.    Alloys are optimised for a particular purpose, making it important to order the right one.  Casting, rolling, extrusion, corrosion resistance, strength and machinability all call for different alloys.

I recommend all Model Engineers invest in a copy of ‘Workshop Materials’ by Alex Weiss, No30 in the Workshop Practice Series.  It covers more than metals.

I made a very poor start in hobby, discovering that my mini-lathe was apparently unable to cut metal.  Obviously all that stuff on the web about Chinese rubbish must be true.   Not so, the problem was that all the scrap I’d collected to practice on was unsuitable.  None of it machined well, with the stainless steel proving to be the nastiest work-hardening variety available.   Realising I had to buy machinable alloys made all the difference: with a machinable alloy in the chuck there was nothing wrong with the lathe.

Dave

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