Rusting/corrosion of nails

A. Maybe someone could e-mail me the actual textbook instructions or teacher instructions on this project, David, so I can understand the situation? We get hundreds of similar questions, and if neither the premise/purpose nor the instructions are clearly stated, we've got nothing. We can't clarify the instructions without knowing the hypothesis you are trying to prove, and we can't guess at the hypothesis from vague work instructions. All I can say about the hypothesis is that the stronger the acid the more corrosion. As for the work instructions I think the nails should be sandpapered hard so that bare steel shows through any coating that may be on them.

Acids are used in industry to remove rust, not to cause it! That is, although acid will dissolve metal, it will dissolve rust a lot faster and easier. So it is certainly possible for vinegar or soda to corrode a nail without revealing rust.

The real way to determine corrosion is by weighing the nails before and after exposure. If your school has an analytical balance, that's ideal. If not, and if you have the money and a very strong interest, highly accurate electronic pocket scales are available these days. Good luck with it!

A. Hi, Cheyenne. Water and salt water are not acidic at all -- they are neutral. So if you can determine that orange juice is acidic, your answer is that it is the most acidic of the three.

But you can play a dirty trick on your teacher by picking bleach ⇦ bleach/sodium hypochlorite in bulk on eBay or Amazon [affil links] (NaOCl, sodium hypochlorite) -- because it is very corrosive and you will find it's alkaline rather than acid :-)

Read the label and never mix bleach with anything except water or laundry detergent. Good luck.

Regards,

A. Hi, Julie. Your work instructions are fine. Be careful to not mix the vinegar with the bleach though! -- I repeat the warning for you .

If you don't have a scale, after the experiment you can at least rub the rust off of the nail and onto a coffee filter, and then pour the solution through the same filter, so you capture the rust in one place.

Everything else you asked is already here on this one page I think. Good luck.

Regards,

A. Hey Julie. This experiment is not primarily about acids or pH. Bleach is not acidic, it's quite alkaline, and yet it's very corrosive, as you have seen. You cannot judge the pH from the corrosivity. Water is not non-corrosive, it's just less corrosive than the other liquids.

By a "coffee filter", I merely mean the discardable paper element that people often use when making coffee. If you can't get one, you can use a paper towel, although it is messier.

Rust is reddish brown and accounts for the discoloration you see in the liquids.

Regards,

A. If all the nails had the same mass, your scale isn't accurate enough. The chemistry department doesn't have an analytical balance?

I can't tell you which has the most rust. If you are going to go by appearance, and you've said that the one in the bleach had the most rust, that's your answer.

Acidic liquids have an excess of hydrogen ions in them, and alkaline liquids have an excess of hydroxide ions in them. But that's not really what caused the rust in the bleach. Rust is iron oxide; i.e., iron that has come into contact with oxygen and formed iron oxide. The formula for bleach is NaOCl. It is a very unstable chemical that wants to release it's oxygen by having it contact iron and convert it to rust.

Regards,

A. Hi, Jessica. To help you find iron, let's start with what the word means. "Iron" sort of has two meanings. On the one hand it is an element, actually element no. 26, one of the 91 building blocks of nature. On the other hand "iron" is a metal of a specific composition (mostly the element iron, plus a small amount of carbon and silicon). Steel is also a metal which is made almost completely out of the element iron, and ironically it is purer iron than the metal iron. So, you can understand why I think you can probably use steel for your "iron" experiment.

Steel is the most common metal of all; it's what bridges, towers, cars, and large ships are made of. Skyscrapers may look to be made of glass or bricks or stone, but that's just a skin; a steel frame is the backbone that makes them possible. But the easiest form of steel or iron to get to corrode would be the one that has the most surface area for your liquids to attack; see if your parents have or can get you some steel wool. Good luck.

Regards,

A. Hi Lanny.

Carbonated water is water with a lot of carbon dioxide dissolved in it, which effervesces out of it over time (a few days). While the carbon dioxide is still in it, the sparkling water is mild carbonic acid, with a pH of about 3.7 to 5.7. When the carbon dioxide has all bubbled out, it becomes just plain water, with a pH of 7 (this is a slight simplification).

Now, refer to my first response in this thread, where I mention that acid is used in industry to remove rust rather than to make rust. While the sparking water is still acid, it dissolves any rust on the nail and a little iron from the nail, so the nail remains rust-free. But as the carbon dioxide effervesces out, and the pH rises, it can no longer keep that iron dissolved, and the dissolved iron settles out as rust. Good luck.

Regards,

A. Hi Kendra.

Acids will dissolve iron, but it takes quite a lot of very powerful acid (like strong hydrochloric acid), plus time, to dissolve a substantial amount of iron.

I don't understand what you mean by "break" in this context. If you're in jail, with time on your hands, you'll probably need a gallon or two to eat through the tops and bottoms of two bars :-)

Regards,

A. Hi Nasuha. I'm not understanding what you mean by "mass increment". Do you mean they all increased in weight? Or do you mean they decreased in weight? Or they changed in varying directions? Are you including the weight of any loosely attached rust?

I can say that ...

- Copper sulphate should have plated out onto the steel nails, and that you should be able to research that reaction and decide whether the copper displacement of iron should increase or decrease the weight of the nail.
- Hydrochloric acid should dissolve some of the iron of the nail, causing a loss of weight.
- Distilled water and salt water should cause the nail to rust, i.e., some of the steel will combine with oxygen; if you remove the rust, the nail will have lost weight. If you don't remove the rust, some will loosely adhere, which would increase the weight, but some will fall off -- so it's hard to say whether the nail will be heavier or lighter.

Regards,

A. Hi James. As you probably know, water is H2O (sometimes written as HOH) ... 2 parts hydrogen and one part oxygen. If you apply electricity to your experiment, the pole connected to the "-" end of the battery will become negative and the pole connected to the "+" end of the battery will become positive. What that actually means is that the battery pumps electrons from the positive pole to the negative pole, where they accumulate.

Some of the water in the experiment dish may be pulled apart by this into positively charged H ions that are drawn to the negative pole and negatively charged O ions which are attracted to the positive pole.

The hydrogen ions that accumulate on the negative pole become hydrogen gas bubbles. The oxygen ions that accumulate on the positive pole become oxygen gas. Because there are two parts of hydrogen to each part of oxygen, you'll always see more vigorous bubbling on the negative pole, due to twice as much hydrogen as oxygen.

Even without electricity, you may see some bubbling if you have used an acid like vinegar or lemon juice because acids have a lot of excess hydrogen they would like to trade for metal. If you used diluted hydrochloric acid, HCl, for example, the iron (Fe) in your nails will replace the H in the HCl and become FeCl. When this happens the freed up H ions will bubble off as hydrogen.

Fe + 2HCl => FeCl2 + H2 (gaseous)

I don't fully understand what you used for a sacrificial anode, and the conditions of use, so I don't know about the yellowish buildup on the sacrificial anode. Still, generally the sacrificial anode dissolves into the solution, similar to the equation above. So the yellow may be tramp ingredients in the anode which did not dissolve.

Regards,

Q. Thanks for your reply Ted, neat to know where the bubbles came from. As our corrosive solution was white vinegar, salt and hydrogen peroxide, which was fairly acidic, I understand why there were so many bubbles.

With the sacrificial anode, we were protecting an iron nail with a smaller nail as our anode. I'm curious if you think the solution may have contributed to the build up. There was no orange or red coloured rust, so was curious if this was a different type of corrosion or oxidation. It is possible that whatever was in the container before may have left residue which I suppose could have resulted in build up. Appreciate any insight you can provide.

Cheers,

A. Hi again James. That looks like a nylon string connecting the two nails -- is it actually a thin insulated wire? Is/was there a metal wire connecting them? You can't do sacrificial protection unless the two metals have a metallic connection between them. I don't think you've told us what the two nails were made from so that we can conjecture deeply about the reactions; but yes, a galvanized nail connected to a plain steel/iron nail by a wire will sacrificially protect the iron/steel nail until the zinc coating is gone. But offering sacrificial protection is not quite the same thing as guaranteeing pristine appearance.

Regards,

Thanks again Ted. It is a thin wire, found it worked better than standard household wire. It seems like the sacrificial anode worked pretty well and the build up on the cathode could have come from several different sources.No worries as it doesn't appear to be rust!
Thanks again for your insights.