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De-Rusting - Experiments with Citric Acid.

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Pete W.:
Hi there, all,

Several Modders have recommended the use of Citric Acid for de-rusting.  I thought I'd try it out so I bought a 1 kgm bag on eBay.

Another eBay purchase of mine had been a job lot of workshop odds and ends, some of which were very rusty so I picked out three items as guinea pigs.

The first one is a derelict caliper, here are its pictures, before:



It was particularly rusty on the adjusting screw:



The other two items were someone's workshop test pieces (not mine, I hasten to add):



and



If they could speak, these last two could probably tell a tale or two, I bet the person who made them kept them in his toolbox for the whole of his career.  I bet they weren't allowed to rust while he was fit and active.

I selected a food container left over from a Chinese take away and almost filled it with hot water from a recently boiled kettle.  Then I immersed the test objects and added three level dessert spoons of the anhydrous citric acid.



The two test pieces started to fizz immediately and there was no visible rust remaining after about 45 minutes.  So I removed them from the bath, rinsed and dried them and took 'after' photos:



and



They still look rusty in those pictures but I think that's a camera problem - to the Mk 1 eyeball in daylight, they look silvery and rust free.  Maybe I'll try another couple of photos in daylight.

The calipers didn't seem to be very active so I added another dessert spoon of citric acid and parked the bath in a safe place.  That was 48 hours ago and I think I can see some reduction in the rust on the caliper arms.  The encrustation on the adjustment screw has collapsed but not all dissolved.  I didn't de-grease the calipers before putting them in the bath so maybe they were oily and that's isolated them from the citric acid.  I'll see how they look in daylight and maybe try some IPA on one leg and some washing-up liquid on the other and put them back in the citric acid.

So, that's all for now, more photos to follow. 

awemawson:
Pete,

I've been known to add a squeeze  of washing up liquid to the citric acid bath to lower the surface tension where bits are a bit grubby - look at my thread on rebuilding the Edwards box pan folder where I de-rust the folding teeth

krv3000:
hi dam good post i my self use old car battery acid i wunder haw long thows test peces took to mack and did he cheet we will never no i don't think they do out like that thes days when the yung ones are serving ther time

Pete W.:
Hi there, all,

Andrew and Bob, thank you for your comments.

As promised (threatened?), I took a couple more photos of the test pieces, in daylight this time.  They've come out a more realistic colour than the previous shots that were taken by artificial light.



and



I also acted on Andrew's suggestion and added a few squirts of washing-up liquid yesterday evening to the bath containing the calipers.  I didn't stir the bath then but, by this morning, the washing-up liquid had mixed in.  It had formed a sort of flaky deposit floating on the surface of the liquid.  I removed as much of that as I could to show the contents of the bath below.  The first picture seemed to show the beginnings of a steely grey colour.



To try to make it clearer, I used MicroSoft Photo Editor to tweak the gamma, with this result:



Later today, I'll remove the calipers from the bath, rinse and dry and take another photo or two.  I'll be particularly interested in the screw thread and the parts that have to run in contact with each other. 

awemawson:
To encourage you regarding screw threads:

Some years ago I acquired a Roundhead Colchester Student that was in bits - previous owner had dismantled it for transport and never put it together with the result that the lead screw, which was left in a damp garage was very rusty and, I thought, a right off. I rebuilt the lathe having the ways re-ground and started hunting for a replacement lead screw. While this was going on I popped the old one in a vertical length of underground drain pipe with a stopper at the lower end, full of citric acid. Pulled it out a few days later not expecting much, and it shone ! Careful examination showed that it was barely touched and perfectly usable, and indeed I used it for best part of ten years ! I suspect that different steels come out better than others. I expect that leadscrew was EN8 or similar

I understand the the volume of rust is something like 14 times that of the previous steel that made it, so quite horrible looking things probably haven't lost much original material.

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