Letter 34032

Stumped by my experiment results on corrosion of iron  

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Hi,

I'm a 7th grade student conducting a science experiment similar to others on this site. I am studying the role that salt plays on the rate of iron corrosion. The problem is that I did a lot of research prior to beginning my experiment which led me to expect that the higher the salinity of the water, the higher the rate of iron corrosion. I used sterilized covered glass jars filled with distilled bottled water for all of my 5 samples. I then added varying amounts of kosher salt to each sample to create different salinity environments. I placed an iron nail in each of the jars and covered them for 24 days. By the end of the first week, the nail in zero salinity was corroding at a quick pace, while the one submerged in the highest salinity environment (175 ppt) wasn't corroding much at all. At the end of the 24 day period, the data showed that the lower the salinity, the higher the amount of corrosion and the higher the salinity, the lower the amount of corrosion. In addition, I had read that an iron nail placed in a dish of dry salt would corrode because the salt would attract moisture from the air, thus starting the rusting process. After 24 days, the iron nail that I had placed in a dish of salt hadn't corroded at all.

All of the online research that I have found does not support my results and I'm wondering if you can offer any advice.

Thank you.

J. S.
Student - Worcester, Massachusetts

What is Kosher salt?

Trevor Crichton R&D practical scientist - UK

Rust-takes water and oxygen. Salt just increases the conductivity of the solution which will normally accelerate the process. I will guess that you put the cover back on the jar. This reduces the amount of oxygen that can get to the nail. I will also guess that you covered the nail completely with solution. I will further guess that you did not sand all of the coating off of the nail before you started. Try it again in wide mouth jars that are only half as high as the nail, so half of the nail is exposed to air (oxygen). Make sure that they are all the same temperature. You could also take a very small fish tank aerator and put a air rock in the solution with a few bubbles per second coming out of the rock and compare the rusting of this solution to the others. Also, ALL of the coating on the nail has to be removed. Guess what, the surface roughness will also have a small effect on rusting as will fingerprints. As far as the salt dish, you are in a heated room,so the humidity is rather low. Salt is hygroscopic, but it takes a very high humidity for it to take enough moisture out of the air to actually make a solution. It normally just turns to a rock and quits taking moisture out of the air.

Overall, your hypotheses are correct, but your execution did not go along with what was required, so, your experiment proved that under your conditions that the hypotheses were wrong. This is one of the most important lessons of science that you will ever learn. Test conditions mean everything in science.

James Watts
- Navarre, Florida

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