The effect of citric acid on steel

A. I recommend trying the experiment yourself. Go get some lemon juice and put it on the various metals and record the results. Your testing may take anywhere from a few minutes to a few days, and you will have the satisfaction of discovering the answer on your own.

Good luck.

Brendan,

Dan Brewer sure beat me to the punch! Yes, try things out yourself ... but to get some faster results, what about heating up the citric juice, this increases the molecular activity (so someone once said to me) and being lazy and as a teenager (well, I was one once, long, long ago) I'd want a fast answer.

Others follies teach us not
Nor much their wisdom preaches
But most of sterling worth is what
Our own experience teaches. (by ANON, not by me!)

You might remember those lines. Very true, too. And by doing something like this YOURSELF, you'll always, always remember it which you won't if you are given a simple answer.

Cheers!

A. Hello,

citric acid reacts very quickly with iron, but does not in itself rust the iron. The water in the lemon juice will help to rust the iron, however, in the presence of air. A steel nail dropped into a citric acid solution will dissolve in a matter of hours. The same thing happens if you drop it in a bottle of "Coke", which has citric acid , phosphoric acid, carbonic acid and other things in it for flavor.

If you need help with your project, let us know. We work with citric acid products every day and have a lot of data.

Thanks Lee. "Dissolve" in Coke is probably too strong a word though; let's say "begin to dissolve".

There's a lot of urban legend about Coke, and I've found that a 12-ounce bottle of Coke will not completely dissolve even the innards of a copper plated zinc penny in a month, let alone a nail in a couple of hours :-)

Part of the discrepancy probably arises from the fact that acids are consumed by the dissolution of the metals. Hollywood shows us a test tube of acid poured on a 2-foot thick block of metal eating it's way through at an inch a second, but in fact all of that acid, even the strongest acid, would be totally consumed by the first 1/32" of metal or so. Perhaps if you put a steel nail under a small waterfall or jet of fresh Coke it could dissolve in relatively short order, but probably not in a single can in a lifetime.

Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
2004

A. Pure citric acid does not corrode iron. In fact, it is a very effective rust remover and used widely for that purpose! As you have noticed, it will remove corrosion and leave clean, bare metal. Corrosion will set in due to the contact with air and humidity afterwards. I have myself cleaned rusted, chalked up cast iron cylinder heads by dumping them in citric acid solution and they came out like new.

A. Hi Ramona. The thing is, acids will attack ("begin to dissolve") metals, with variable success depending on the particular acid and the particular metal ... but they will dissolve the "tarnish" or "rust" (metal oxides) easier and faster than they dissolve raw metal. So the first thing you usually will see is tarnish removal or rust removal. That is because the metal oxide that was on the surface is now dissolved in the acid. But if you let the acid evaporate so the metal that was dissolved in it cannot stay dissolved, you'll see a rusty mess :-)

Regards,

A. Hi Jessica. In science class we have to use words more carefully than around the house. Copper doesn't "rust" because "rust" is defined as the corrosion product of iron; and copper is not iron and doesn't contain iron -- but copper can dissolve or corrode or tarnish.

As Lee says above, the citric acid itself doesn't cause rust, it's a bit complicated. The acid can dissolve copper, but it's oxygen in the water that combines with the dissolved copper to form copper oxide tarnishes. A large bowl of citric acid might completely dissolve a very thin and very small piece of copper foil in a reasonable number of days.

Regards,

A. Hi,

I believe all depends on concentrations. The nail in citric acid may be true at 100% concentration, but we actually demonstrate the OPPOSITE. At around 0.2% (2000 ppm) of citric acid in water, a nail actually coat itself gray and ABSOLUTELY NO rust appears. Pure tap water results in comparison in ugly corrosion.

I did no detail study on concentrations and their impact.

Does anyone know at what concentrations citric acid acts corrosive, and at what concentrations it acts inhibitive on corrosion? citric acid is actually termed to be an ANTI-OXIDANT. Go figure.

Looking for any clarifications.

Hans

Hi Hans, I'm not claiming that your facts are wrong -- I don't know. But most or all acids remove/deter rust while the object is immersed in them because they dissolve the rust faster than they dissolve pure metal so you will see no rust -- but that does not mean there is no corrosion. If, after a given immersion time, you evaporate all the moisture, you will see more rust from the piece that was in acid than the piece that was in tap water. But there probably is, as you imply, an ideal concentration which keeps the rust at bay during immersion, while minimizing how much raw metal is being dissolved. Certainly industrial acids are often formulated with organic 'inhibitors' whose purpose is to minimize corrosion of he base metal when the acid removes rust.

Regards,

A. Hi Ruth.

Acids like citric acid dissolve metals but they dissolve rust (metal oxide) faster. Your chemistry book may tell you that an acid plus metal will yield hydrogen plus a salt of that metal, so the rust dissolves as a metal salt. Good luck.

Regards,

A. Hi Susan. Surely, yes, the acid in lime juice or vinegar will dissolve some amount of iron, but iron is an essential nutrient, not a poison. But does it dissolve so much that it could make for a harmful overdose, or is it such a minimal amount that it has no effect on health? Sounds like a great topic for a research paper: you'd have to experiment to figure out how much iron it dissolves, then compare that to what a normal daily intake of iron should be. Sounds very interesting! Good luck.

Regards,

A. Hi Jonah. Let us know what happens when you put samples of those metals in lemon juice, and we'll try to help you explain your project results. Best of luck.

Regards,

A. Hi Anna. Get four non-metal bowls, put those four liquids in the bowls, put a nail in each (fully submerged) and start recording what you did and what you saw in your lab book (a notebook where you've pre-numbered the pages, so you are not tempted to rip a page out). If you think you recorded something incorrectly, strike it through once but leave it legible.

Then come back in a week with your observations and we'll be happy to comment on them. But please don't make the mistake of trying to figure out which of the four liquids should cause the most rust until you've actually done the experiment. Because if you do, you will be tempted to discount contrary observations and give too much weight to observations which agree with your expectations. You would then be learning a lesson in "junk science" instead of real science; junk science should be avoided until you are a hungry climatologist looking for a grant :-)
Best of luck.

Regards,

Hi cousin Dharani. Yes, I do think it can be helpful.

Please detail your situation and why this question has come up and I'm confident we can very quickly offer a more helpful answer. Thanks!

Regards,

A. Hi Continue. You already recognize that the cleaning is better with some salt, than with "no" salt, but you probably also recognize that there is no such thing as "no" when it comes to chemical concentrations. Surely there are one or more molecules of salt in your quintillions of molecules of lemon juice even if it is ultrapure. So you already know the answer, that at least within some possible limits, increasing the salt concentration improves the cleaning. Exactly how much salt you must add to reach maximum efficiency without wasting any salt probably depends on the size of the lemon and it's ripeness.

Regards,

A. Horseshoes are soft iron with a low carbon content.
Tilling discs will be high strength steel with a relatively high carbon content. Acid removes the rust and dissolves some iron exposing the black carbon. Another pass through the sand blaster will remove the carbon.

A. Hi Rebecca. The assertion is that the Kool Aid will dissolves some of the metal into it -- enough to taste. It is probably a combination of chelation and metal dissolution by acid as previously explained on this page.

But what are you testing for and how?

Regards,

A. It depends on the metal!

I expect most metal containers meant for holding beverages will be stainless steel, which ought to be fine. I would see nothing wrong with using titanium, either. Aluminum would be okay in the short term, though there is a reason that aluminum beverage cans are coated on the inside.

But a whole discussion on what acids at what strengths will dissolve which metals is probably more than the class assignment is looking for. ;)

A. Using hydrochloric (muriatic acid) to de-rust steel is asking for trouble down the road. Sure, it's very effective at dissolving surface rust, but no matter how you neutralize it with lye or sodium bicarbonate (baking soda), or wash with water, it leaves behind chloride salts trapped in the surface. Unless you immediately follow up with another form of surface treatment, the highly electronegative chloride will attract oxygen from the air, causing rust to develop. Once started, that form of rust is like a cancer, and slowly eats away the base metal.

3% citric acid in hot water is my preferred solution to dissolve rust off of steel. This dilute weak acid WILL NOT attack the base metal, unlike muriatic acid. I've used it many times. It's cheap, it's relatively green, and it's safe.

Alternatively, phosphoric acid, at about 3% in hot water is much safer to work with than is muriatic acid, and will remove rust very quickly, but you must keep it warm. If left in too long, black iron phosphate will deposit on the surface, which might not be what you want. Nevertheless, that coating is very rust resistant, and forms an excellent bond with metal primers if the piece is to be painted. I keep an old ceramic electric crock pot in my shop for derusting small parts. I mix either 3% citric acid , or 3% phosphoric acid in the pot, drop in the parts and turn the pot on low. A few hours later the parts are clean. A few times, I've left the pot running overnight for very heavily rusted parts.

And that's all I have to say about that!

A. Hi Rody. When we use citric acid for industrial purposes there may be additives or specific concentrations for specific applications. But for the student science projects on this page, yes, food grade citric acid should be fine. Good luck with it.

Regards,

A. Normally hydrochloric acid is used, same as muriatic acid.
The concern is what is called white nubbing caused by the chlorides, a good rinsing before use will prevent this. The reputable suppliers also add fluorides in small amounts to the make up and replenisher to prevent this.