Rusting in tap water vs. saltwater

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, New York

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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, Florida

Please describe the results you got and the experiment that you conducted, Kayla. Asking what the results of an experiment are supposed to be before you do the experiment creates pressure to get that answer in your own trials, leading to "junk science" which might earn you an "F" :-)

Ted Mooney, P.E.
finishing.com
Brick, New Jersey

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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, Texas

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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.
- Philadelphia., Pennsylvania

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Hello Joshua.

I claim that the nails rust because invisible fairies dance in the water and chip away at the nails with their tiny invisible tap shoes, and the fairies prefer dancing in salt water because it soothes their bunions. If you can get to the point where you no longer are getting conflicting results, then you try to fine tune the experiment to see if you can demonstrate anything about causality. Like, what happens if you use half as much salt or twice as much salt?

You must first do an experiment and tabulate the results, then try to formulate a testable hypothesis that you think might 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. If you can't test the explanation, it's only a guess, and anybody's wild guess is as good as anyone else's, and is not science :-)

Parents and teachers:

It might surprise you, but acids are used in industry primarily to remove rust from steel, not to make steel rust. That's because corrosive solutions dissolve rust faster than they dissolve iron. So, the more corrosive your 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 can give erroneous results. This may be why some kids are seeing contradictory results.

The real way to measure corrosion is to weigh the nail before the test and after the test, (after rubbing off any rust and drying the nail. Whichever nail lost the most weight is the one that corroded the most. This requires a good "analytical balance" (scale), which sometimes isn't available in most grade schools. If the school doesn't have one, and you have the money and a strong interest, you can buy a very accurate pocket scale pretty inexpensively, and see if it will do the job -->
Disclosure:finishing.com may get a commission if you buy this scale.

If you don't have an accurate scale, 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 too much about why one type of water creates more visible rust. Good luck!

Ted Mooney
finishing.com
Brick, New Jersey

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, North Carolina

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

January 19, 2009

Hi, Poppy Anne. That sounds believable, thanks. Sounds like a student should put their nails in the water very early on to see this effect.

Hint: for a long term experiment you must cover your experiment dish with saran wrap / plastic wrap. Otherwise the water will evaporate and build up in salt content over time as you keep adding replacement water.

Regards,

Ted Mooney
finishing.com
Brick, New Jersey

January 19, 2009

I an doing a experiment to what rust first and the salt water rust first it started rusting on the 2 day and the tap water started rusting on the 3 day.

emmalee e
student - ironton, Ohio

March 1, 2009

I am doing science experiment on iron nail rusting faster in tap or salt water. I notice the results posted say that the tap water will rust first but the salt will be more effective over long period of time. Can you send me some information on how long I will have to run this experiment to show the results of the salt water corrosion. Also how do I set up a log book and practical experiment for the accurate recording of this experiment. I don't know where to start.

Darren A
- Sydney NSW Australia

March 1, 2009

Hi,Darren. If you don't know where to start, you simply have to get more complete instruction from your teacher. It's bad enough for you to guess; to ask someone 10,000 miles away guess what your teacher might have said or meant in your class one day is ridiculous :-)

But the lab book part is easy. Get a composition pad, number the pages, get an ink pen that you can't erase, and mark the date and time of the things you do. Then write down what you see or otherwise observe. Start by putting identical nails in a little dessert dish of fresh water and salt water. Ideally you should do three or four of each rather than just one. Good luck!

Regards,

Ted Mooney
finishing.com
Brick, New Jersey

April 19, 2009

hey everyone =D...
I'm currently having my school holidays... and i have one week left. I'm supposed to finish this (assignment) science experiment about whether salt water rusts steel faster or tap water. i have read many other questions similar to my one on this site but none of them match what I'm looking for.

i have done a few tests which proves my hypothesis wrong (my hypothesis is: salt water rusts steel faster). in my test, i had 3 identical cups. i made the strongest salt solution (by adding heaps of salt until no more could be mixed in at room temperature) and poured 100 ml of salt water into the 1st cup, 50 mls of salt water and 50 ml of tap water in the 2nd cup and 100 ml of tap in the 3rd. i put a steel nail (around 3 cm long) into each cup. the cup with the 100 ml's of salt water didn't rust until after 5 day's while the cup with the 100 ml's of tap water rusted within hours.. (tap was faster).. the one with 50-50 was rusted on the side facing the top. the other side turned black..

my hypothesis was proven wrong, and all my background information is now useless. i have to now gather more information (background info) on why salt water would slow down the rusting of steel as opposite to what i have done earlier. i had many support, through websites saying that salt water will rust steel faster. so going with my experiment and against my support earlier, i can't find any site's or books proving that tap water will rust steel faster then salt.

PLEASE if anyone knows any sites.... or can explain to me why salt water slows down the rusting of steel. (for example; the salt displacing the oxygen? maybe?) but I'm not sure... and most importantly i don't have proof of this.
my teacher said that i needed at least 2 articles and a book to support my findings PLEASE HELP.... if you know anything about this topic =D

Alice G
student - Sydney, NSW, Australia

September 24, 2009

OH MY GOD Alice, i did the exactly same experiment for my science assignment and the same thing happened to me!

apparently salt does make the nail rust faster but it takes longer...everything on the internet goes against what my experiment proves which is that 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 this helps a bit.

alanna n
- Byron Bay, NSW, Australia

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