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Nail is anodic in center as well as at ends

I'm a grade twelve student doing a lab on the corrosion of iron. When I placed a nail in agar solution with phenolphthalein ⇦ this on eBay & Amazon [affil links] and potassium_ferricyanide [affil links], I observed anodic points at the tip, head, and in the center of the nail. For the life of me, or anyone in the class, I cannot figure out why the center of the nail would act as an anodic point. I think it might be relevant only to our brand of nails, because I've read from many sources that anodic points form frequently on the head and tip. Can you help us?

Kim C.
student - New York City, New York, USA
2006

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The truth is it probably doesn't matter why there is an anodic hotspot in the center of the nail, there just is. It could be that the nail was straightened there during manufacture or after a carpenter bent it; or there could be a slightly different material composition there. But for something to note in your report, you might say that one of the reasons things are phosphatized is to counteract this sometimes intractable problem of anodic hotspots.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
2006