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Corrosion Rates and PH Levels

April 6, 2010

I am in 9th grade and doing a chemistry project in a biology class (just like the subject more) but the problem is the fact that I haven't been studying chemistry for a year. My project is testing the effects of steel nails (an iron alloy), a penny (copper alloy), and aluminum foil (AL alloy) and corrosion. My project is slowly turning into a series of experiments with all the research I have been doing, but I still have some questions. Temperature is sort of going to be my control. I am putting one of each sample in a hot water bath, one in room temperature and one in my refrigerator. Then I will do the same thing with salt added to the water. Lastly, I am going to suspend all the metals above the solution and calculate that as well. I was told pH is a good way to measure corrosion. Is this true? I also need to know what corrosion does in relation to pH levels. (Does pH go up when more corrosion occurs or down?)

Hannah M
student - MARYVILLE, Tennessee

April 7, 2010

Hi, Hannah. Sounds like an ambitious project with a lot of variables. Good luck!

I don't think that I would agree that pH is a good way to "measure" corrosion, although it is generally true that the lower the pH, the more aggressively corrosive the acid will be. Better would be to weigh the metal on an analytical balance before immersion, then lightly sand off any corrosion products and reweigh afterwards, judging corrosion by the weight loss.

But it is true that the pH will rise as the corrosion proceeds because pH is a measure of free hydrogen ions in solution. Before immersing the metal you will have:
HCl <==> H+ + Cl-, i.e., a lot of free hydrogen ions.

After immersion, you will have:
2HCl + Cu0 ==> CuCl2 + H20^(gas) <==> Cu++ + 2Cl- + H20^(gas)


Ted Mooney, finishing.com
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

April 8, 2010

pH is a poor measurement as it is logarithmic. As soon as you get any distance from 7.0, it takes a significantly greater reaction to change the pH even a tiny amount.
Sanding the part to remove the corrosion is terribly sensitive to who is doing it as it is highly probable that you will remove an undetermined amount of the base metal which will vary by how hard it is.
Weighing the part, and then corroding it, drying it and weighing for the new weight does not take into account the amount of metal that went into solution or is compounded with the corrosion product. It will give a relative value and not a finite value.

James Watts
- Navarre, Florida

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