How does pH affect rust

A. Hi, Stephen. It's actually not complicated, just a little anti-intuitive: acid attacks rust more readily than it attacks solid metal. So as long as the acid has acidity available to dissolve steel, it has acidity available to dissolve rust. Acid is widely used in industry to remove rust, and only rarely to dissolve steel.

The iron is indeed dissolved in that acid, but you won't see it come out as rust until the acid is exhausted or evaporated.

On the other hand, water does not dissolve rust, so the oxygen in the water and air combines with the steel to form iron oxides (rust). ammonia is a mild alkali, and alkalis tend to reduce the corrosion of steel, so it probably doesn't rust as quickly as in pure water. Good luck.

Regards,

Hi Melissa. Well water is not much different from "city water". Both are drinkable, and monitored for other ingredients; "city water" can come from a number of places, including wells.

"It doesn't matter how beautiful your theory is, it doesn't matter how smart you are. If it doesn't agree with experiment, it's wrong."
    -- Richard Feynman, dec., Nuclear Physicist (and considered by some to be the world's greatest teacher)

Although your son's results sound unlikely, they are his results -- and the point of a science experiment is to record the results you actually got, not what you expected to get. Most likely either 1). something was recorded wrong, or 2). your weighing procedure doesn't actually measure the mass lost to corrosion correctly, or 3). the weighing equipment isn't precise enough and leads to measurement variations that are random rather than causal. Or maybe 4). water corrodes steel nails more than lemon juice or coke.

But my guess is that it may be "2)." Rust and other corrosion products weigh more than the iron from whence they came because the corrosion products have all the original iron plus oxygen or whatever is in the corrosion compounds. If your procedure weighs the corrosion products attached to the nail, rather than just the uncorroded iron, you can get this sort of result. So how do you determine the amount of mass lost? Did you clean the nails with sandpaper or in any other way after their exposure to the corroding medium so that you were weighing just the remaining un-corroded steel? Good luck.

Regards,

A. Hi Carolina. We're not there to look over your shoulder, so you have to be careful explaining yourself or you can quickly lose us. But it seems that you are you wondering why you are not seeing rust in the steel wool in vinegar? Please see my first answer on this page: vinegar, like other acids, dissolves the iron in rust even more readily than it dissolves the iron from steel wool. You won't get rust as long as the solution is still acid, but the acid will eventually start dissolving the steel wool. Good luck.

Regards,

Hi Ritvik. Sounds like a fascinating science project: ask a stranger to guess the result of an experiment you don't feel like doing :-)

A. I'd say one minute for vinegar, 2 hours for lemon juice, 3 days for orange juice, 4 weeks for black coffee, 5 months for water. I'm pretty sure I'm right.

Regards,

A. Hi Thivina. Copper ions (Cu⁺⁺) tend to be blue. I'd say some copper from the coin dissolved into the tap water, which is probably slightly acidic. What happened to the copper coin in the vinegar?

Regards,

A. Hi Ellis. As asked earlier on this page, please tell us what grade you are in because the explanations needs to be very different for 3rd graders vs. seniors in high school. Try to read some of the answers on this page and tell us what you understand and don't understand. Are you 100% sure that you know exactly what you mean by "pH" and by "speed of rust"? It wouldn't make any sense to try to provide an answer if you don't understand the question. Good luck; we'll be here.

Regards,

 As a high school (homeschool) chemistry teacher, I really appreciate your posts, especially your recommendation to read the entire posts (i.e., do your research), present all your data and explain your results--what you observed, not what you should have seen. This is how I want my kids to learn and approach science. Thank you for such a great resource!

Thank you for the kind words, Nancy :-)

I would never have the patience to be a teacher :-)

Regards,

A. Hi Rick. I was answering a question about a student science project, and I can't claim expertise in paint formulation :-)

Students wonder why they see less rust in acidic solutions than in neutral or alkaline solutions despite the acidic solutions being more corrosive, and the answer is simply that acids dissolve rust as readily as raw metal, in fact more readily.

But, yes, I would expect a high pH paint to deter rusting more effectively than a low pH paint.

Regards,

A. Hello Edgar Z from Stockholm, Sweden. On the internet you are not as anonymous as you think, so be careful out there.

Rust

Per Wikipedia, rusts are hydrated iron oxide and hydroxide compounds like
Fe⁺⁺⁺₂O^--₃, Fe⁺⁺⁺O^--(OH)^-, and Fe⁺⁺⁺(OH)^-₃,

whereas acids are compounds like
H⁺Cl^-, H⁺₂SO₄^--, and (CH₃COO)^-H⁺

When one of the forms of rust, for example, Fe⁺⁺⁺₂O^--₃ reacts with one of the acids, for example, H⁺Cl^-, the formulas are of the form:
Fe⁺⁺⁺₂O^--₃ + 6H⁺Cl^- => 2Fe⁺⁺⁺Cl^-₃ + 3H^-₂O^--

In the given example, the iron from the rust becomes dissolved in the solution as ferric chloride. The H⁺ from the acid is what makes this dissolution possible, and pH is a measure of the available H⁺, the lower the pH the more H⁺. Good luck.

Regards,

A. Hi Anu. What do you mean by "My levels of IV..."? Sorry, that means nothing to me.

The formula which you mention predicts how many millimeters of thickness a plate will lose to corrosion in a year based on the results of a short term experiment, but I think you would find this formula difficult to implement at best because there's probably not a practical way to determine either the area or the density of your steel wool, being comprised as it is of thousands of very small diameter wires and a lot of air space. More practical would be to just use the same amount (the same weight) of steel wool for each of your liquids, and just plot weight loss to corrosion vs. pH.

So you measure the pH of your 5 liquids, weigh the chunks of steel wool before immersion, and weigh them again after you take them back out of the corrosive solutions. There are, of course, other practical difficulties with this experiment: other things besides pH may affect the corrosion rate; and I don't know how you're actually going to remove the adhering corrosion products from the steel wool after immersion so that you can weigh how much raw steel you have left. Good luck.

Regards,

A. Hi Albert. Good luck with it.

pH is a measure of acidity, the lower the pH the more acidic. And what that really means is the lower the pH or the more acidic the acid is, the more hydrogen ions available to dissolve metal. Hydrochloric acid is a very strong acid which we abbreviate as HCl; using HCl as the example because it's easy to write, it spontaneously separates or 'ionizes' to H⁺ and Cl^-: positively charged hydrogen ions and negatively charged chlorine ions.

Steel wool is primarily made of the element iron, which we abbreviate as Fe. If you put steel wool into hydrochloric acid, the positively charged hydrogen ions are searching strongly for electrons to balance them and they have a stronger pull on the electrons than iron does. So they will 'steal' an electron from the iron they touch, converting it from Fe⁰ to Fe⁺, or two hydrogen ions may between them steal 2 electrons converting the iron to Fe⁺⁺, or 3 hydrogen ions might steal 3 electrons and convert it to Fe⁺⁺⁺. Exactly which form the iron goes to depends on some other stuff and is beyond 8th grade science, but in any case this converts the Fe atoms into positively charged Fe ions which dissolve into the solution and combine with the chlorine atoms to form somewhat soluble salts like FeCl3. That's how acid dissolves metal, but it's also what it does to rust (Fe2O3). So you don't immediately see more rust with stronger acids, you see less because the iron in the rust as well as the iron in the object gets ionized into soluble salts like FeCl3.

So a point I have been trying to make is that acid attacks and corrodes steel, but generally this is not seen as rust because the acid dissolves rust even more readily than it dissolves steel.

What you could do is try to dissolve the steel wool in pyrex bowls of acids for a fixed amount of time, say 2 days, then dry the bowls of liquid and steel wool in a warm oven so the ferric salts cannot remain dissolved; the lower the pH, the more rust you might now see.

Regards,

 Aloha Albert. I'm not sure that 'affected' and 'influenced' are exactly the best words, but this is science class not English class, and I think your hypothesis is perfect! Thanks for the kind words.

Regards,

A. Hi Katie. Lots of things will not corrode steel wool, including most liquids which don't contain water, like oils & alcohols, and most liquids which do contain water but are alkaline (high pH) like bicarbonate of soda, sodium hydroxide, calcium hydroxide, and, yes, ammonia (with its pH of about 11). The question is not quite "why doesn't ammonia corrode steel wool?", because who says it should really? The better question is why did those other substances corrode steel wool.

Vinegar corroded the steel wool because it is acetic acid, and acids (low pH) are materials which will dissolve metal because they contain hydrogen in an active form which is hungry to steal electrons from metals when it has the opportunity. When hydrogen succeeds in stealing those electrons (with their negative charges) from the metal, the atom of metal stops being a neutral atom of metal, and becomes a positively charged ion which dissolves into the watery solution as a metal 'salt'.

The best known salt is sodium chloride (NaCl), which is table salt and the most prevalent of the salts in the ocean. One theoretical, although impractical, way of making table salt would be to put sodium metal (Na) into hydrochloric acid (HCl); the hydrogen in the HCl would steal an electron from the Na metal, which would produce free hydrogen gas plus NaCl metallic salt. Good luck.

Regards,

A. Hi Giovanni -- interesting idea. I am not a chef nor a machinist nor a chemist, so I can't comment on the practicality of this mix, but I would agree that baking soda reduces the pH and thereby discourages rusting. Good luck.

Regards,

A. Hello Sun,

There are more variables affecting corrosion speed in steel than pH. Though pH is a strong one, redox activity is stronger (and bleach has a strong oxidizing power). Coffee has many tannins, and they oxidize readily in presence of oxygen, rather than steel. Coffee is preventing steel to rust while oxidizing itself first.

So, if you want to conduct pH vs corrosion rate, you should make artificial samples of different pH with the same redox behaviour (distilled water with sulfuric/caustic soda [affil links] to certain pH is an idea). Don't use hydrochloric because chlorides are a corrosion initiator and will interfere positively for corrosion speed.

So, best of luck! Let us know your results when you have them :)

A. Hi Jude. Not knowing your age or what grade you're in, it's difficult to know how to properly build on what you already know, and neither talk down to you nor use terms that might be meaningless to you. But as you corrode this steel wool into acid, how are you going to determine "the mass of the steel wool after corrosion"?

Before corrosion you can weigh it, but after corrosion unless you do things quite rightly it may be covered with rust, and rust is far from weightless -- how can you know how much of the weight is steel wool vs. rust? So I think the first thing in your setup is to make sure you have plenty of acid and a short enough immersion time, free of excessive evaporation, so no rust accumulates on the steel wool. Good luck.

Regards,

A. Hi John. It sounds like your teacher has provided enough detail for you to do an interesting experiment and learn from it. Please don't ask what the result is "supposed to be" because that is a recipe for doing science exactly the wrong way, stultifying boredom, learning nothing, and misleading your teacher, your parents, and yourself.

Make yourself a "lab book" -- a little note pad where you number the pages so you can't rip anything out; use a pen so nothing can be erased; write the time and date for everything you do; start observing & writing. Bingo, you are a real scientist doing actual science ... and you'll probably get an A+ on your report.

Regards,