# What's the weight of rust? **URL:** \ **Category:** Factual Questions\ **Created:** [September 2, 2001, 2:20pm UTC](https://boards.straightdope.com/t/whats-the-weight-of-rust/79404 "2001-09-02T14:20:14Z")\ **Posts on this page:** 8\ **Page:** 1
**Author:** ![FairyChatMom](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/fairychatmom/32/21906_2.png) [@FairyChatMom](https://boards.straightdope.com/u/FairyChatMom)\ **Post date:** [September 2, 2001, 2:20pm UTC](https://boards.straightdope.com/t/whats-the-weight-of-rust/79404/1 "2001-09-02T14:20:14Z")
It’s been too many years since Chem I - but intuitively, I would expect that if you weighed a hunk of steel, then let it corrode and weigh it again, it would weigh more. After all, oxygen combines with the iron to make iron oxide, right? How much more would it weigh, tho? 1%? .001%? 10%? I can’t remember how to balance chem equations - could a balanced equation tell you how much the rust would weigh? I suppose I could just leave a hunk of steel soaking in my bath tub and weigh it before and after, but I don’t want to risk a nasty tub ring if the weight change wouldn’t be detectable on the bathroom scale. My Google search has just given me a headache - I must not be searching the correct words. Dare I hope that among the Teeming Millions someone can ease my pain and satisfy my curiosity? Thanking you in advance… ---
**Author:** ![Squink](https://avatars.discourse-cdn.com/v4/letter/s/b5e925/32.png) [@Squink](https://boards.straightdope.com/u/Squink)\ **Post date:** [September 2, 2001, 3:07pm UTC](https://boards.straightdope.com/t/whats-the-weight-of-rust/79404/2 "2001-09-02T15:07:38Z")
Iron has an atomic mass of 55.84. The most common oxide, hematite, Fe[sub]2[/sub]O[sub]3[/sub] has a mass of 159.69. The reddish brown stuff you find on old railroad spikes etc. is the hydrate of hematite: Fe[sub]2[/sub]O[sub]3[/sub]-XH[sub]2[/sub]O. The X in that formula means that the amount of water is variable. In real life, the amount of water varies with temperature and humidity. That makes it impossible to calculate exactly how much weight an object will gain when it rusts, but the minimum percentage gain would be 159.69/55.84 or roughly 286%. ---
**Author:** ![Squink](https://avatars.discourse-cdn.com/v4/letter/s/b5e925/32.png) [@Squink](https://boards.straightdope.com/u/Squink)\ **Post date:** [September 2, 2001, 3:12pm UTC](https://boards.straightdope.com/t/whats-the-weight-of-rust/79404/3 "2001-09-02T15:12:18Z")
I should NOT post before achieving consciousness :o with 2 iron per rust molecule the weight gain works out to be 159.69/111.68 or ~143%. ---
**Author:** ![FairyChatMom](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/fairychatmom/32/21906_2.png) [@FairyChatMom](https://boards.straightdope.com/u/FairyChatMom)\ **Post date:** [September 2, 2001, 3:16pm UTC](https://boards.straightdope.com/t/whats-the-weight-of-rust/79404/4 "2001-09-02T15:16:40Z")
Wow - way more than I expected… Of course, only the surface would corrode, right? Doesn’t the surface corrosion protect the innards from corroding, to a certain extent? So in order for something to rust away, it’d have to flake off and continue to corrode inside, right?? Anyway, thanks for the info!! ---
**Author:** ![tomndebb](https://avatars.discourse-cdn.com/v4/letter/t/b9e5f3/32.png) [@tomndebb](https://boards.straightdope.com/u/tomndebb)\ **Post date:** [September 2, 2001, 3:21pm UTC](https://boards.straightdope.com/t/whats-the-weight-of-rust/79404/5 "2001-09-02T15:21:05Z")
That would be in a closed environment, right? I was under the impression that the act of oxidizing burned off energy (using the iron as fuel) in such a way that the effective mass of the rusting object was reduced. (I realize that comparing car fenders does not work because there is constant flaking going on, but I had thought that there was a direct reduction in mass, as well.) ---
**Author:** ![bibliophage](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bibliophage/32/7615_2.png) [@bibliophage](https://boards.straightdope.com/u/bibliophage)\ **Post date:** [September 2, 2001, 3:23pm UTC](https://boards.straightdope.com/t/whats-the-weight-of-rust/79404/6 "2001-09-02T15:23:36Z")
The fraction is 160/112 = 1.43, implies an increase in mass of 43%, not 143%. Also, I should note that it takes a long time for a hunk of iron to turn completely to rust. The surface layer of rust tends to protect the underlying metal from corroding. ---
**Author:** ![Carpal\_Tunnel](https://avatars.discourse-cdn.com/v4/letter/c/9d8465/32.png) [@Carpal\_Tunnel](https://boards.straightdope.com/u/Carpal_Tunnel)\ **Post date:** [September 2, 2001, 3:39pm UTC](https://boards.straightdope.com/t/whats-the-weight-of-rust/79404/7 "2001-09-02T15:39:45Z")
As for your bathtub, here is a more practical experiment you could try: [http://www2.ncsu.edu/ncsu/pams/science\_house/learn/CountertopChem/exp2.html](http://www2.ncsu.edu/ncsu/pams/science_house/learn/CountertopChem/exp2.html) ---
**Author:** ![FairyChatMom](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/fairychatmom/32/21906_2.png) [@FairyChatMom](https://boards.straightdope.com/u/FairyChatMom)\ **Post date:** [September 2, 2001, 3:55pm UTC](https://boards.straightdope.com/t/whats-the-weight-of-rust/79404/8 "2001-09-02T15:55:38Z")
> [@](#): > > \*Originally posted by Carpal Tunnel \* > \*\*As for your bathtub, here is a more practical experiment you could try: > > [http://www2.ncsu.edu/ncsu/pams/science\_house/learn/CountertopChem/exp2.html](http://www2.ncsu.edu/ncsu/pams/science_house/learn/CountertopChem/exp2.html) \*\* Thanks for the site. Incidentally, I have a shower, so the bathtub part of the question was rhetorical. 😃