Rate of penny corrosion

A. Hi Dr. Kevin. I can't decide if you're posing your question in this style to trick people into doing your homework for you, or if you are enthusiastically playing the role described for your hypothetical science research project. But you sound pretty cool :-)

But why 1989 pennies rather than 1988 or 1990? What do you think them different?

Why Coca-cola, which is a hodgepodge of secret ingredients, and whose acidity is supported by volatile carbonic acid, rather than a repeatable and documentable reagent like .01 N HCl or something?

After you do your trial, let us know what corrosion rate you got; it would be fun to discuss. But the thing is, trying to look up the answer first when you're still in school seems like carefully training students in the art of "junk science": the kids look up what the answer is "supposed to be", and learn to fudge their experiments to interpret them that way -- chalking up their real observations to experimental error, or otherwise discrediting them, giving far too much weight to the scantest result that supports their preconceived notions, etc. I suggest you somewhat alter the scientific method by doing the experiment first and recording your results, and then do the research after you know what happens :-)

A. Will you see some long term attack on the penny, Yes. Far too much depends on how clean the penny was when you started (oxides, grease, etc.), the temperature that it is kept at, the amount of agitation, can the Coke effectively get to both sides, how often are you cleaning the resulting smut off of the penny, the effect of the increasing sugar content as it slowly evaporates. Too many variables and no one cares enough to document (publish) the results if they had done any work. Coke certainly is not going to talk about it. If you want a real life shock, put a 1" cube of lean steak in the same 300 mL of Coke and put cover on the flask or beaker [beakers on eBay or Amazon].

A. How long does it take for the hour hand on a clock to move, Katherine. How big a change? How long have your coins been immersed without showing any change? Coins wouldn't be very useful if they quickly corroded away. Good luck.

A. You're doing fine, Christina. Cover the cups if you expect the test to last longer than about 2 days.

I think every day for 3 days is much more reasonable than every 5 minutes for 15 minutes, Mercedes. I'd suggest using post-1982 pennies, and cutting them into pieces.

A. Hi Andrew. Sodium hydroxide is very strong and dangerous; I don't think even a tenth-grader should use it, but if you do, be careful to wear goggles [on eBay or Amazon]. Sodium hydroxide probably has less effect on solid copper pennies (before 1982), but I'm sure that you are right that it is murder on zinc core pennies (after 1982). I suggest you chop them into pie slices before putting them into your liquids.

A. Hi, Melinda. I think Coke will have little effect on solid copper pennies (before 1982). It would seriously corrode (make pits in) the zinc core of post-1982 pennies in a few days depending upon finding any holes in the copper plating.

Iron spikes, unfortunately, I'm not very familiar with firsthand. I'm sure it will dissolve a good portion of the rust on them though. I would not expect it to attack the base metal while there is still significant rust on it because rust dissolves into acid much faster and easier than metal does. Good luck.

Regards,

A. Hi Halona. My advice on this subject is always the same. Please share my answer with your teacher and I think s/he will agree --

1. If you don't know the answer to a question, that's what research is for and about -- get busy.

2. But if you don't understand the question, tell your teacher and get clarification; because nobody else can clarify the question for you ... all they can do is take wild guesses that will certainly confuse you and possibly mislead you.

My *guess* is that you misunderstood the teacher's question and are trying to bull on through anyway. It probably won't turn out well unless you go back to him/her and ask what s/he means by "crystallization on pennies" :-)

Regards,

A. Hi Violet. Does it? Please don't ask why it happens until you've put pennies in lemon juice and found for yourself whether it does in fact happen or not. As the renowned physicist and wonderful teacher Richard Feynman said: "It doesn't matter how beautiful your theory is, or how smart you are, if it disagrees with experiment, it's wrong.".

Regards,

A. Hi Daniel. Go through your pockets and change drawer and find the date of the oldest penny you can find. Maybe ask your family members if they have an older one.
Let X = 2019 - that date ...
... and your answer will be "It takes at least X years for pennies to corrode."

Regards,

A. Hi Chris. I think it's unlikely. We find gold as a metal in nature, but do not find aluminum as a metal in nature ... and the reason is that gold and some other noble and precious elements are in a low energy state as metals whereas aluminum (and iron, zinc, etc.) give off energy when converting to oxides. Thus, time and tide corrode non noble metals into oxides and sulfides because this is a lower energy state, and energy always flows from high states to lower states (if you put a bottle of beer in a chest with ice, the beer doesn't get warmer and the ice colder; rather the two eventually reach the same temperature).

There is another factor, however. Sometimes the corrosion products form a tough, glassy, relatively impermeable layer which help protect the metal from corrosion (as with aluminum), in other cases (as with iron) the corrosion products (rust) are loose and fluffy and do not form such a barrier. Some alloys like stainless steel and bronze, even though not noble, exhibit good corrosion resistance because of this relatively impermeable layer of corrosion products.

Regards,