Re: Need recipe for 'Do coins corrode more in salt or fresh water'?

Dear Susan:

I hope this answer is not coming in too late. The chemical processes 
behind how iron (or other metals) corrodes in the presence of salt water 
is covered in this answer: 
 http:
//www.madsci.org/posts/archives/oct98/908687315.Ch.r.html 

and to avoid duplication I shall not go into it here.

There are several problems I think could be affecting your experiment:

1. The salt water is insufficiently oxygenated
    As the oyxgen and not the salt is the substance doing the 
oxidation, you will need more oxygen to speed up the corrosion process. A 
possible way to do this would be to constantly bubble air through the 
water. Alternatively, you could only moisten the coin with the water and 
leave it in a closed jar (to prevent evaporation). This is the same reason 
why steel pilings rust faster when exposed to sea spray than when 
submerged permanently underwater -- in the former case there is more 
oxygen to oxidize the metal.

2. The coin is not clean
    Coins are handled very very often, as such they may have a 
layer of oil and grime built up, which impedes the corrosion. Hence, you 
might want to wash it with detergent and dip in acid before carrying out 
the experiment.

3. The time-frame is too short
    The process of rusting on exposure to oxygen at room 
temperature, especially in the low concentrations dissolved in water, will 
take a long time. I'm quite sure that 48 hours will be insufficient, 
however I do not know the exact details.

4. The coin is made of materials resistant to corrosion
    I'm not sure about the United States, but where I live, 
the common materials used to make coins is a copper-nickel (cupro-nickel, 
CuNi) alloy, which is much more resistant to corrosion than, say, iron, 
since both copper and nickel are further down the reactivity series than 
iron. This is logical, because coins are made to last for a long time, and 
the metals selected to produce them are those which would not corrode 
easily under normal conditions of use. Other materials commonly used to 
make coins are (pure) copper, (pure) nickel, aluiminium, and even silver. 
The common characteristic of these metals is that they do not corrode 
easily (aluminium is the exception, but it has a protective oxide layer). 
Hence, using coins may be a bad idea, if you wish to actually see 
corrosion for yourself.

However, the point of an experiment is to test a hypothesis. If it turns 
out to be untrue, the experiment is still a success, because you have 
invalidated it. Hence, if your original hypothesis is 'coins will rust 
within 2 days when submerged in salt water as opposed to plain water', 
then you will have found that the hypothesis is untrue if your experiment 
does not work. Don't feel too bad about 'failed' experiments, because the 
data you have is still useful. Your class could analyse why the corrosion 
did not take place, and so on. But I understand that for a third grade 
class, having 'no results' can be quite disapppointing. Hence I suggest 
using iron nails (preferably not stainless steel...), zinc, or other more 
reactive materials rather than coins for your experiment. Perhaps you 
could even try different household objects made of metal, rather than 
restrict yourself to coins. 

But those are only my suggestions. I hope what I have written is useful to 
you. If you have any other questions, feel free to post them to the Mad 
Science Network in the future!

Regards,
Tan "50 cents" Thiam Hock