Sciencemadness Discussion Board

Molten salt or acid baths for cleaning metals

RogueRose - 16-4-2016 at 12:20

I'm trying to figure out what the best compound to use for cleaning some metals that can't be cleaned mechanically or other ways.

I have heard of NaOH being used for this. It is heated to melting point and the metals are given a quick dip and then thoroughly rinsed.

The goal is to get stains, corrosion, oxidation, paints, varnish, etc off the metal and return the metal to a new/like-new condition.

I have been told that NaCl should be added to the NaOH and then melt the mixture. The amount of salt isn't mentioned, only "a little" to the 2oz of lye mentioned. Can anyone suggest how much and why the salt is added?


I've also been told that KOH can be used as well as sulfuric acid (~95%) although the latter seems potentially more dangerous.

I wanted to come up with a better base or acid to use for this that didn't require such a high heat to melt. I was thinking of Oxalic acid as it has a much lower MP and I know it does a very good job in cleaning some metals with cold water and scrubbing, so I have a feeling it might do a good job when heated - though IDK if it will fume or if there are dangers I should know about.







Metacelsus - 16-4-2016 at 14:09

What kind of metal are you trying to clean? (Make sure it isn't aluminum!)

RogueRose - 16-4-2016 at 19:39

Quote: Originally posted by Metacelsus  
What kind of metal are you trying to clean? (Make sure it isn't aluminum!)


Copper, brass, bronze, stainless. Copper to start.

Any idea if the salt is needed and how much to add and what is it for? Flux maybe. If that is so, would something work better like borax? I thought it might also be for the chlorine in it as it might release with the heat and help clean somehow.. IDK, thoughts?

bobm4360 - 16-4-2016 at 19:52

They will all clean well with an aqueous solution of citric acid. No muss, no fuss, no burns! I've used it many times on brass, copper and bronze, and "green" industry uses it for passivating stainless steel. For paint, etc., try MEK or DCM.
Regards,
Bob

[Edited on 17-4-2016 by bobm4360]

RogueRose - 16-4-2016 at 21:27

Quote: Originally posted by bobm4360  
They will all clean well with an aqueous solution of citric acid. No muss, no fuss, no burns! I've used it many times on brass, copper and bronze, and "green" industry uses it for passivating stainless steel. For paint, etc., try MEK or DCM.
Regards,
Bob

[Edited on 17-4-2016 by bobm4360]


Thanks. I will try that for my second cleaning as the lye has left a slight residue (it had quite a bit of varnish or laquer on it) which is a black something or other that floats on top of the lye.

I was amazed at how well it cleaned the copper - I've never seen copper as shiny.

I forgot to add the salt so I added some after it was melted and it looked like it vaporized as it hit the surface which is odd considering the MP is 1475F. When it hit the surface it sizzled, turned white and floated until it "evaporated". Ideas what happened?

MrHomeScientist - 20-4-2016 at 10:01

That was just absorbed water boiling off. NaCl and NaOH have similar densities, so I imagine the salt sat on top due to no mixing, and 'evaporation' was just the salt sinking and dissolving in the melt.