19024. Rusting in tap water vs. saltwater

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Rusting in tap water vs. saltwater

Letter 19024

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I am doing a science project on rusting. I placed one iron nail is plain tap water and one in saltwater (I added salt to regular tap water). The one in the tap water rusted within just a few hours while the one in the saltwater is rusting much slower. All of the info I find says that the one in the saltwater should rust faster. Why isn't it?

Thanks, Nick age 10

Nick
- Staten Island, NY USA

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I am doing a science project on whether salt water, rain water, or tap water rusts a metal nail the fastest but I can't find any info for the research paper. Where do you find it?

Kayla
- Jacksonville, FL, USA

Please describe the results you got and the experiment that you conducted, Kayla. Never ask what the results of an experiment are supposed to be before you do the experiment. That is called "junk science" and is supposed to earn you an "F", and it just might :-)

Ted Mooney, P.E.
finishing.com
Brick, NJ

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I think that the nail rusts much earlier in the salt water.

Kayla M.
- Chicago, Illinois, America

Thanks, Kayla M. But is that a guess, or is it that what your experiment demonstrated? :-)

Ted Mooney, P.E.
finishing.com
Brick, NJ

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My son is 8 years old and he is doing a science project to see in what solution the nail is going to rust faster. He used bottle water, tap water, tap water + salt and tap water + sugar. We were expecting that the nail in salt + water would rust faster but the one in the bottle water was the first one to rust followed by the one in the tap water. Why did it happen? could you help us?

Denise Rossi
- Weslaco, TX, USA

January 3, 2006

Please answer letter #19024 asap...this would greatly be appreciated.we are very confused about a iron nail rusting quicker in saltwater or tap water quicker and why?

Joshua D.
- Phila., PA, USA

January 4, 2006

Hello Joshua.

What is confusing you? Exactly what results did you get? How long did you leave the nails in the water? All the way submerged, or halfway in? What did you observe?
If you are trying to do science backwards, no wonder you are very confused! You must first do the experiment and tabulate the results, then you formulate a testable theory that you think may explain the results. Then you either retest to verify your theory or you leave your theory to be tested next semester or by the next class.

To the previous inquirers,

Please recognize that it would do no good for me to offer an explanation or advance a theory unless you can test it. If you can't test the theory, we may just as well say the nails rust faster because invisible fairies dance in the water and chip away at the nails with their tiny invisible tapshoes, and the fairies prefer dancing in salt water because it heals their bunions. One of the greatest teacher-lecturers, Richard Feynman, often made the point that we can explain HOW things work but we can never explain WHY.

Parents and teachers:

Acids are used in industry primarily to remove rust from steel, not to make steel rust! Corrosive solutions dissolve rust easier than they dissolve iron. The more corrosive the solution, the more it will corrode the iron -- but the less rust there may be. So trying to judge the corrosive effect by the amount of rust you see gives variable and erroneous results.

The real way to measure corrosion is to weigh the nail before and after, but this requires a good "analytical balance" (scale), which isn't available in most grade schools. So use this opportunity to teach the scientific method and allow students to learn laboratory method and how to keep a lab book, but don't worry about why one type of water made more rust. Good luck!

Ted Mooney, P.E.
finishing.com
Brick, NJ

February 5, 2008

Why does the saltwater rust slower than tap water. I need research for a science fair project. everything says that salt should rust faster than tap water.

7th grader thx

Thomas B.
- Tarboro, NC, United States

August 13, 2008

G'day,

In response to many of your questions, Iron rusts faster in fresh water. Although many people say that it is quicker to corrode in salt water this is not the case. The freshwater used in all of your experiment rusts faster, however after a short time creates a thin film around the rust block preventing it from rusting any further. The SALTY water however rusts at a slower rate, but because it uses electrochemical corrosion (because the salt in it creates a more conductive environment) it does not create the thin film that fresh water does. It is because of this that the salty water can rust until there is no more iron left.

People tend to think that salty water rusts faster - it doesn't. Fresh water will rust faster until it has created the protective film, whereas salty water can rust until no more iron is left.

Hope that helps :D

Poppy Anne
- Perth, Western Australia

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