Unusual reaction: citric acid

[The Z'eer]

Last week, rather than make up more citric acid gel i thought I'd try the a bath of citric acid. Submerging the small rusty fittings/nuts and bolts, I left them for 24 hours.

Upon removing them the water had become black. The parts were rinsed, mechanical abrasion took place. They had a few stubborn areas of corrosion so I repeated the above process except I mistakenly left the parts submerged for nearly three days.

Oh dear, upon removal the parts look as if they've been at the bottom of the sea for a while! A nasty thick green/black crustaceous layer of what looks lime scale has enveloped them.

I'm now having to grind off this scale. Any idea what's occurred here? Perhaps something to do with a heavy lime scale content in the local tap water? Or the citric acid itself?

Thanks

 

[rcx132]

I don't know exactly but the acid doesn't remove the rust, it converts it to something water soluble so you end up with by products. When I take the parts out of my stripping tank they never look shiney, they are covered in smut and all sorts. My acid tank is also black which I'm told is the output of the chemical reaction, but I'm using phosphoric, not citric.

Unfortunately nothing beats blasting!

 

[rikrobson]

You should try electrolysis. This works very well and does remove rust from parts as opposse to converting it. There are may threads on thiis and you tubes as well.

 

[Screwdriver]

Last week, rather than make up more citric acid gel i thought I'd try the a bath of citric acid. Submerging the small rusty fittings/nuts and bolts, I left them for 24 hours.

Upon removing them the water had become black. The parts were rinsed, mechanical abrasion took place. They had a few stubborn areas of corrosion so I repeated the above process except I mistakenly left the parts submerged for nearly three days.

Oh dear, upon removal the parts look as if they've been at the bottom of the sea for a while! A nasty thick green/black crustaceous layer of what looks lime scale has enveloped them.

I'm now having to grind off this scale. Any idea what's occurred here? Perhaps something to do with a heavy lime scale content in the local tap water? Or the citric acid itself?

Thanks

The black stuff is a different type of rust (iron oxide) called magnetite.

I'm guessing anything green is probably from a zinc/galvanised coating.

Neither should be that hard but I guess if you leave it long enough it might go a bit crusty.

 

[The Z'eer]

I don't know exactly but the acid doesn't remove the rust, it converts it to something water soluble so you end up with by products. When I take the parts out of my stripping tank they never look shiney, they are covered in smut and all sorts. My acid tank is also black which I'm told is the output of the chemical reaction, but I'm using phosphoric, not citric.

Unfortunately nothing beats blasting!

Do you use neat phosphoric or diluted? Any reason for opting for phosphoric over citric?

 

[The Z'eer]

You should try electrolysis. This works very well and does remove rust from parts as opposse to converting it. There are may threads on thiis and you tubes as well.

I might give that a go, so far the citric gel I made up work very well. Didn't have any nasty black gunk or really dull steel when I used the gel.

 

[The Z'eer]

The black stuff is a different type of rust (iron oxide) called magnetite.

I'm guessing anything green is probably from a zinc/galvanised coating.

Neither should be that hard but I guess if you leave it long enough it might go a bit crusty.

It went very crusty and requires a great deal of mechanical abrasion to remove it. I'm unsure if the bolts were galvanised, they had rust on em' so I guess they weren't. I've left citric acid gel on parts for days and never had a problem like this, only that it dries out a bit and needs a thorough soaking in soapy water to wash it off.

 

[123hotchef]

I had the same thing when I left the bits in the citric acid tank too long, try vinegar next time

 

[rikrobson]

you should watch these You Tubes by MrPete222

 

[rcx132]

I used phosphoric because it's more popular but I don't know the advantage over citric.

I compared electrolytic cleaning to acid using similarly rusted parts and found that acid is quicker on shallow rust and electrolytic much quicker on deep rust. With acid you also get flash rusting. I use both depending on what I'm doing.

From memory the phosphoric acid is used at 30%. It comes as 85% from chemical suppliers.

 

[The Z'eer]

you should watch these You Tubes by MrPete222

Thanks for the links there, great information and very helpful

 

[The Z'eer]

I used phosphoric because it's more popular but I don't know the advantage over citric.

I compared electrolytic cleaning to acid using similarly rusted parts and found that acid is quicker on shallow rust and electrolytic much quicker on deep rust. With acid you also get flash rusting. I use both depending on what I'm doing.

From memory the phosphoric acid is used at 30%. It comes as 85% from chemical suppliers.

Thanks for the percentages there, I have a few litres of the 85% phosphoric acid. I've encountered the flash rusting before, annoying! What do use wash off the phosphoric acid, a wash of bicarbonate soda?

 

[The Z'eer]

I had the same thing when I left the bits in the citric acid tank too long, try vinegar next time

Thank god I'm not the only one who's made this mistake! How do you find vinegar? I've heard it is a little too passive?

 

[123hotchef]

Thank god I'm not the only one who's made this mistake! How do you find vinegar? I've heard it is a little too passive?

The vinegar worked ok I smelled of vinegar for ages afterwards though. I also find with the citric acid solution it works better and faster if it is how/warm not tried warming the vinegar yet though

 

[Seadog]

All acids will work better heated. Normal white vinegar is between 5% and 9% and is more than adequate. I did buy 5 litres of Ethanoic acid (which is what vinegar is) but have yet to dilute and use any. It's 99% if I remember correctly.

 

[123hotchef]

All acids will work better heated. Normal white vinegar is between 5% and 9% and is more than adequate. I did buy 5 litres of Ethanoic acid (which is what vinegar is) but have yet to dilute and use any. It's 99% if I remember correctly.

vinegar is acetic acid or the stuff I make is

 

[grim_d]

vinegar is acetic acid or the stuff I make is

Same stuff, ethanoic acid is the posh name

 

[rcx132]

Thanks for the percentages there, I have a few litres of the 85% phosphoric acid. I've encountered the flash rusting before, annoying! What do use wash off the phosphoric acid, a wash of bicarbonate soda?

I rinse with water! Ideally a pressure washer. Thats the trouble with acid and caustic stripping, acid causes rusting, caustic damages paint. And the stuff always gets inside welds and lap joints and leaks out a week or so later. That's another reason for switching to electrolytic cleaning. Both methods still leave smutting behind so you still need a polish.

 

[MG David]

Having tried a number of ways, I use citric acid to remove the rust. You are left with some black deposits that are soft and wash off easily with a washing up brush. I then use dilute phosphoric acid 5% as an anti flash dip. Then into fresh water to rinse of the surplus phosphoric acid and dry. Owing to the anti flash dip, the part will keep for weeks in a dry garage before being epoxy primed.

Often there is oily muck and old paint on things I want to de-rust. So they are treated to a bath in a warm caustic soda tub first.

 

[The Z'eer]

Having tried a number of ways, I use citric acid to remove the rust. You are left with some black deposits that are soft and wash off easily with a washing up brush. I then use dilute phosphoric acid 5% as an anti flash dip. Then into fresh water to rinse of the surplus phosphoric acid and dry. Owing to the anti flash dip, the part will keep for weeks in a dry garage before being epoxy primed.

Often there is oily muck and old paint on things I want to de-rust. So they are treated to a bath in a warm caustic soda tub first.

Thanks for sharing the info there several other methods of corrosion removal I wasn't aware of.