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De-Rusting - Experiments with Citric Acid.

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Arbalist:
I tried some citric acid derusting on some nuts and bolts as it would have been a fag setting up the battery charger. It worked well enough but the surface finish was terrible compared to washing soda and a battery charger. Never again unless I want to blacken the surface afterwards.

Pete W.:
Hi there, all,

Well, I decided to leave the parts in the citric acid solution and they did eventually start to bubble but only slowly.

This morning, I decided to check what progress had been achieved.  Here's a photo of the parts straight out of the 'bath':



I then rinsed the parts in fresh water and then in methylated spirit and allowed to air dry.  That got me to here:



The parts were covered with a fine dark-coloured sediment.  I went over them with a brass bristled suede brush and they came out quite well:



I finished off by coating the parts with 3-in-1 oil.  I think that these parts are now in a good enough condition to be returned to service.

It seems that my stock of citric acid crystals hadn't 'gone off', rather that there was some coating on the parts under the rust that slowed up the reaction or made progress less apparent. 

Eugene:

--- Quote ---I have tried electrolysis on drills , someone mentioned that this causes  "hydrogen embrittlement " of the tools? They still cut OK so don't know how true , or critical for use this is.
--- End quote ---
There's certainly the possibility it could happen, especially given the extended times the parts are subjected to.

Hydrogen embrittlement is a potential problem with hardened and spring steels, electro-plating and it's associated cleaning processes being a prime cause; even common or garden rusting can cause it on susceptible steels. Mild steel is unaffected. The process of embrittlement can be reversed by heat treatment so within industry it's SOP to remove the hydrogen immediately after plating. The usual temp is around 200 C for two hours, so the home brew de-rusting of taps, dies and suchlike can be usefully followed by a stay in the domestic oven; it certainly won't do any harm and if it avoids a broken tap (such fun :() then why not do it?

There was a time in my life when I designed a test rig for plated parts that were safety critical and used it to check what felt like several million spring washers to establish some sort of statistical regime. It was the second most boring job I ever did. Weeks and weeks of sitting on your duff waiting for something to go "ping" .... ooh the excitement.  H & S rules forbid the publishing of my most boring job; simply reading about it can permanently shut down a complete cerebral hemisphere.

The effect of cider on rust is probably due to the tannins as much as anything else. Some commercial de-rusters are made from manipulating the lees left over from the fermentation of grapes in wine making and utilising the tannins; we imported hundreds of barrels of the stuff from Italy and sold the resulting product as "FerTan". Or was it FerroPro? Or both? I think it was under licence anyway.

Eug

 

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