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Old 5th May 2022, 12:44 pm   #1
Uncle Bulgaria
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Default Aluminium/alloy powdery white corrosion

I've done some research online and don't quite know what to call this. The Americans talk about 'aluminum rust' or 'white rust' and I thought it was an aluminium oxide but it might be a salt.

I've taken a Cropico P.6 Thermocouple Potentiometer out of store and discovered I put it away with two D cells in. There's a lovely crop of copper (sulphate?) crystals on the terminals, and the reaction's fumes have (I guess) contributed to the eating away of the metal faceplate edge above the battery holder.

This is heavily pitted with soft white corrosion and I wondered if there was a chemical means of removal to save sanding/grinding back the painted area. Mentions from members here veer between acid or alkali and mechanical removal with little consensus.

I've sorted one of the battery terminal's alkaline deposits on the secondary mV supply section by submerging it in a citric acid solution for a few minutes - bubbles and a brilliant cleaning reaction. This didn't appear to attack the aluminium, and did clean oxide deposits from some aluminium/alloy rivets that hold the contacts. I didn't remove the contacts to clean them for this reason - each has four or so rivets so would need drilling out and replacing.

However, I'm wary of doing the acid trick for the faceplate as it could have contributed to some paint lifting on the (hidden) battery section and I don't want to remove the control markings.
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Old 5th May 2022, 1:21 pm   #2
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Default Re: Aluminium/alloy powdery white corrosion

I veer towards mechanical removal, but you still have the issue of the oxide (and i think that it will be aluminium oxide) being physically much more tough than the parent metal!
It's quite an abrasive oxide, as you know....but does have the advantage of being stable.

Dave
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Old 5th May 2022, 3:33 pm   #3
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Default Re: Aluminium/alloy powdery white corrosion

Don't know how relevant it may be, but I remember reading Les Lawry-Johns having good results on a rock star's aluminium-cased CCTV monitor with a rub-over with 3-in-1 oil.
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Old 5th May 2022, 4:10 pm   #4
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Default Re: Aluminium/alloy powdery white corrosion

Hot water can remove some corrosion salts fairly easily and cheaply, and in doing so doesn't create further problems, so it's a good first port of call. If there is any corrosion product left, then other means are needed, but you probably have less to worry about. These tasks are rarely sorted with a single action, so do the easy ones first.

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Old 5th May 2022, 6:29 pm   #5
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Default Re: Aluminium/alloy powdery white corrosion

I'm a big fan of citric acid for this type of problem; this is a very weak acid (often found in drinks and food stuffs) so it generally has little effect on metal (unless left for very long times or high temperatures) but is very good at dissolving existing corrosion salts (look up "chelating agent" on Wiki). Buy it easily on line, or on the High St, Wilko usually have it (I think homebrew beer types use it); it comes as a white powder; use Google for further info on concentrations. Most people do not feel citric acid on skin, but best wear eye protection.

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Old 5th May 2022, 10:14 pm   #6
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Default Re: Aluminium/alloy powdery white corrosion

Citric acid is also the active ingredient in "Oust" kettle de-scaler (also clears blocked shower thermostatic valves)

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Old 6th May 2022, 7:01 am   #7
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Default Re: Aluminium/alloy powdery white corrosion

Citrc acid is excellent but as noted by the op, it can damage some paint finishes. When used as an antiviral in vehicle wheel washes for animal disease control, paint damage has been a problem noted with it. I think that using a moderate concentration wouldn't do harm: test first on an inconspicuous area if possible, but mechanical removal of thick deposits first will limit immersion time. Neutralize afterwards with washing up liquid solution and then rinse.

Citric acid will remove all corrosion traces so you have to accept that there will be surface pitting.

Greg
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Old 6th May 2022, 10:47 am   #8
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Default Re: Aluminium/alloy powdery white corrosion

Thank you for all the suggestions. There were fewer connections to the battery compartment than my first inspection revealed, so I managed to unsolder the connections and immerse just the corroded battery terminal in the citric acid solution. This gave the same excellent results as the first set I tried.

The faceplate's more complicated. I was confused by the corrosion being so soft, as I am aware of the hardness of aluminium oxide and its use as an abrasive. There is a yellowish discolouration across both sections of the unit, which I surmised was nicotine/tar, grease or old varnish. It responded to no solvent I could discover, nor the citric acid solution (though perhaps it had been depleted by reacting with the battery terminals and I had no more to hand to try) and polishing has damaged the lettering.

I had that eventuality in mind, and scanned both plates before starting investigations. I think my best bet now is to sand it all back, prime and repaint and make new markings with decal paper. This has the advantage of enabling more aggressive removal of the corrosion which has become filigreed beneath the paint in a few places.

Hey ho, never say that this hobby doesn't teach you how to turn a small job into a big one!
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Old 11th May 2022, 1:46 pm   #9
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Default Re: Aluminium/alloy powdery white corrosion

The 'filigree' corrosion under paint often goes a lot deeper than you would imagine; you need to get it all off and back to bare metal or any new paint will not adhere properly.

[painters of Birmabright and Elektron alloys will be familiar with this issue.....]

Rather than spending money on citric acid, hook the slice of lemon out of this evening's G&T; there's still quite a bit of citric acid in it! I use lemon-slices for cleaning the stainless-steel splashback in my kitchen.

I've used baking soda as a mildly-abrasive cleaner on pitted aluminium surfaces; mix it to a paste and massage it into the corroded area using a "Moppet" [which is a rather-firm sponge-like thing intended for kitchen/bathroom-cleaning].
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Old 4th Jun 2022, 12:50 pm   #10
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Default Re: Aluminium/alloy powdery white corrosion

After much cleaning and delay with waterslide transfer paper delivery, I've repainted, relabelled and clear-coated the front plates. I recreated the labels using the Affinity suite's vector tools, recreating letters where they'd been damaged.

Despite much sanding and clear coats, I've not managed to remove the edges of the transfers. If I'd known they'd be so obvious, I would have cut them far more accurately. As the only thing to do would be strip them off and start again, I've decided the perfect is the enemy of the good in this instance, and I'll get the thing back in use. I have the files so can try again in the future when it irritates me enough.

Now to reassemble...

What thermocouple leads do I need for this, and does anyone have a manual?
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Old 4th Jun 2022, 10:03 pm   #11
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Default Re: Aluminium/alloy powdery white corrosion

I use ordinary vinegar for removing corrosion products of leaking alkaline batteries. The vinegar from jars of pickled onions that I used to throw away is now kept for this purpose.
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Old 23rd Nov 2022, 9:08 am   #12
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Default Re: Aluminium/alloy powdery white corrosion

Hi,Boiling water will remove corrosion from battery terminals.
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