SDStaff Ken is a retard: Water and Rust

Cecil's Columns/Staff Reports
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  1. He was right about the galvanic cell stuff. Sort of, but not really. The rest was COMPLETELY wrong

  2. Water does NOT dissociate into O2 and H2. Never has, never will. Water dissociates into H+ and OH- (hydrogen and hydroxyl). OH- will react with the iron on the surface of the metal, creating some Fe(OH)3 and Fe(OH)2. The iron hydroxides then further oxidize to form iron oxides Fe2O3 and FeO. These are rust. Iron does not rust from oxygen, it rusts from water. O2 may help with the second oxidation, but has no effect on the iron without the formation of the hydroxides, which requires water. The reason it takes longer in the air is because water vapor, being some 1000 times less concentrated than liquid water, can’t do the job as quickly.

Cecil should be ashamed of this answer, and you should as well, Ken

Jason R Remy

“Open mindedness is not the same thing as empty mindedness.”
– John Dewey Democracy and Education (1916)

2

The Mailbag item that jayron refers to:
http://www.straightdope.com/mailbag/mrust.html

It’s helpful if the first person commenting on a topic would include the url or link in the first post.
[Note: This message has been edited by CKDextHavn]

3

If the rain involved is acid rain, does this increase / decrease the rate at which it rusts? I know that some acids are used to get rid of rust.

4

Alright alright, one more time. I’ll agree that Ken was wrong, but I believe that Jay is just as wrong.

The two biggest problems with Jay’s hypothesis are almost immediately obvious. He says that OH- reacts with Fe to oxidize it and that oxygen is not involved. If this were true, then iron would rust faster in a high pH environment, and it would rust just as easily without oxygen. In fact, iron rusts less in a high pH environment and will not rust in the absence of oxygen (or a similar oxydizing agent). In fact, oxygen is reduced in both steps on the way to rust.

In addition, an OH- reaction with iron to form rust would not explain the fact that iron rusts more readily in the presence of salts. The reduction of oxygen explains it quite nicely.

Fe(OH)3 is commonly formed using NH3 as a reagent to establish a high pH (high OH concentration). Fe+++ ions will not spontaneously react to form Fe(OH)3 at a neutral pH in any noticable amount. Also note that iron does not rust in a solution with a pH above 9 under normal conditions.

You’ll also want to consider that OH- reacts with Fe++ or Fe+++. If it reacts with atomic Fe (and frankly I can’t imagine why it would), then it doesn’t do it very well. This is, of course, witnessed by the previously mentioned nature (or rather effective nonexistence) of iron corrosion at high pH. In short, it would appear to me that iron must be oxidized prior to forming a precipitate with OH-. Fe++ will reduce oxygen well enough on its own to achieve the second step of oxidation, so any formation of Fe(OH)2 or Fe(OH)3 would seem to be largely irrelivant.

-Bob

5

So, if I were to expose my hatchet to water in an oxygen-free environment (say, ancient Earth’s oceans or lakes before all those pesky cyanobacteria started messing up the place), it wouldn’t rust?

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I don’t have anything to add to the discussion on rust, but I would like to ask Jayron: Wouldn’t it have been possible to correct Ken without calling him a “retard”? Or are you perfect?

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Ken happens to be one of the nicest, smartest people I know. You can disagree with his column, but that title was unecessary!

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And it was unNecessary, too.

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Jayron32: Whatever small meaningful bit of information you had to share is completely overshadowed by this subject line. You only made yourself look like a “retard”.

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While parts of these posts have been correct so far, no one has gotten it completely correct. In order for iron to be turned into rust it needs to be oxidized by something. An oxidant is an electron acceptor. Given that OH- is negatively charged (i.e. has plenty of electrons) it does not make a very good oxidant. Oxygen from the air is a quite good oxidant, but its reaction with metallic iron is quite slow because it is a gas. However, H+ ions (from the dissociation of water) are quite an admirable oxidant and are quite capable of oxidizing iron to iron oxide (rust), and the byproduct of this oxidation is H2. This is why low pH solutions oxidize iron faster; there are more H+ ions.

The oxygen from the air plays no part in this oxidation by H+ ions and rusting will occur in the absence of oxygen (like the ocean floor). The reason you don’t see H2 gas evolution is that the rate of reaction is so slow that it is unnoticeable.

It is always much less embarrasing when slinging insults when one has one’s facts straight.

TheDude

11

Dude, I think you are mostly, but perhaps not entirely, correct.

I pulled out my Introduction to Materials Science for Engineers textbook and it has a diagram of the rusting process. It describes it as an “example of oxygen reduction as a cathodic reaction.” According to it, you do need O2 from the air (dissolved in the water), along with H20, reacting with the Fe (taking 3 electrons) to create 3 OH- groups and an Fe3+ group. These combine to form Fe(OH)3, which is rust, which then precipitates back onto the iron.

The textbook Materials Science and Engineering: An Introduction goes a bit further in explaining that it is actually a two-step proces. First, Fe is oxidized to Fe2+ as Fe(OH)2 by reacting with 1/2 an O2 molecule plus an H2O molecule (as 2 OH- ions). Then, in the second stage, 2 Fe(OH)2 molecules react again with 1/2 an O2 molecule plus an H2O molecule to form 2 Fe(OH)3 molecules – rust.

Now, if that doesn’t answer this question, nothing will.

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One at a time, folks, I only have two arms.

TheDude:

I’m not sure what you find incomplete here. You mention H+, but perhaps you fail to realize that reduction in an aqueous solution is assumed to involve H+ when looking at the final product as H2O. If you look at OH- as the final product, then you assume H2O is involved. Now, H+ can be reduced to H2, but not by iron in anything but an acidic solution.

You guys must truly hate me, because you have forced me to pull out my old Chem manual, which I promised I would never look at again for fear that it might turn me to stone.

To summarize, the reduction potential for Iron is -0.44, the reduction potential for H+ is 0, and the potential for O2 is 0.40. This means that Iron, in theory, will always reduce oxygen over hydrogen ions when both are present (you’ll note that H+ is only going to be in abundance in an acidic solution). In reality, though, iron will not oxidize to rust by either oxygen or H+ unless both are present. Under these condition, O2 is reduced. I quote:

“The rusting of iron is known to require oxygen; iron does not rust in water unless O2 is present. Rusting also requires water; iron does not rust in oil, even if it contains O2, unless H2O is also present.”

The reaction proceeds as:

"Cathode: O2(g) + 4H+(aq) + 4e- --> 2H2O(l)
[Energy of reduction] =1.23V
Anode: Fe(s) --> Fe++(aq) + 2e-
[Energy of reduction] =-0.44V

The Fe++ formed at the anode is eventually oxidized further to Fe+++, which forms the hydrated iron (III) oxide known as rust:
4Fe++(aq) + O2(g) + 4H2O(l) + 2xH2O(l) —>
2Fe2O3xH2O(s) + 8H+(aq)"
-Brown, et al. Chemistry: The Central Science

You’ll note that the anode step must be doubled to balance the sum of the half reactions, and that gaseous oxygen is involved in both steps.

Issue resolved.

Additionally, I don’t know where you got the idea that there is no oxygen at the ocean floor. You’ll note that many aerobic organisms do just fine down there.

David:

It seems that there is some contention as to what rust is. Fe(OH)2 and Fe(OH)3 are red-brown, kinda icky looking, and both precipitate when formed. Fe2O3 is red, kinda icky looking, and precipitates when formed.
The first oxidation of iron would also tend to alkalize the solution, possibly facilitating Fe(OH)2 formation (see my previous mention of NH3). You’ll note that the second oxidation step in the electrochemical model would seem to work with only a small adjustment, if Fe(OH)2 is substituted (and let’s ignore the hydration for simplicity’s sake).

4Fe(OH)2(s) + O2(g) --> 2Fe2O3(s) + 4H2O

This, of course, assumes that Fe(OH)2 precipitate (a solid, not aqueous) will react.

If the iron is in contact with another oxidizing agent, then it may form Fe+++ seperate from iron oxide. This could then form Fe(OH)3, but it would not technically be rust. However, it would be corrosion which is synonymous enough with rusting for most purposes.

In short, iron will corrode in an acidic environment, or when it is in contact with a suitable oxidizing agent (say nickle or tin or lead). Iron will rust in the presence of oxygen and an aqueous solution with a pH at or below 9. Iron will not rust due simply to exposure to an acid, to water of any pH, to oxygen, or to any other oxidizing agent. Iron will not corrode at all without oxygen unless in an acidic environment or in contact with a suitable oxidizer. Oxygen and water must both be present for real rust to form. Anything else is just an imitator, apparently.

-Bob

13

Well done, Bob. I stand corrected. While it is still theoretically possible (look at the reduction potentials) for the H+ in water to oxidize iron, it apparently does not do so in the absence of oxygen. I’m afraid I’m not so clear on what the distinction between rusting and corrosion is in this case; it seems somewhat artificial. If you have more insight on this distinction, I would love to hear it.

All of this still fails to explain why water is required for and is a catalyst for rust formation. Is it bringing a higher relative concentration of O2 to the surface? Is it providing additional stabilization of the Fe++ and Fe+++ oxidation states? Is it providing a (very weak) electrolyte solution for the reaction to take place in? This is what I want to know.

TheDude

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Actually, you people are answering the second half of the topic.
IS SDStaff Ken a retard?

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It would seem that rust is a very specific form of iron corrosion, the formation of Fe2O3. If you’ve ever seen Fe(OH)2 and Fe(OH)3 precipitate out of a Fe++/Fe+++ solution (and, honestly, who would pass up the chance to see something as exciting as that?) you know that they look somewhat, but not entirely unlike the rust you see on iron tools. Now, I don’t know if I would be able to take the Pepsi challenge with Fe(OH)2 and Fe2O3, but I can see that there’s a difference.

H+ will still oxidize iron, but the pH has to be quite low and it still won’t give you Fe2O3.

Water provides the electrolyte, which bridges the gap between the anode and cathode portion of the metal and the cathode portion. The electrons travel through the metal, and the ions travel through the solution, just like a battery.

-Bob

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Come to think of it, since when has water ever disassociated into H[sup]+[/sup] and OH[sup]-[/sup]? The last I heard 2 H[sub]2[/sub]O disassociated into H[sub]3[/sub]O[sup]+[/sup] and OH[sup]-[/sup].

John W. Kennedy
“Compact is becoming contract; man only earns and pays.”
– Charles Williams

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Re: JWK

Man, and I thought I was a geek. No longer.

How you represent the dissociation of water is almost invariably irrelevent to the reaction that you’re studying. You say tomato and I say tomato and yet it all balances in the end.

-Bob

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This is a very interesting thread, but there are other reasons why you need water to cause iron to rust, and there are other corrosion mechanisms which are not being discussed here. Now, as a disclaimer, I haven’t been working in this area for about 5 years, but I was a mechanical engineer specializing in materials at one time. If only I had my materials book here at the office… But onward!

The first reason you need water is because you need a galvanic cell to cause any kind of corrosion. That is, you need an anode, a cathode and a medium to transport the ions/electrons from the anode to the cathode and vice-versa. The medium for transporting ions in this case is water. No water, no transport, no galvanic cell. The presence of oxygen is necessary because rust is an oxide, and, well, no oxygen, no oxide. The oxygen usually comes from the dissolved oxygen in the water, but not always!

Interestingly, at least to me, is the fact that a great deal of corrosion in cars is caused by a corrosion mechanism called “oxygen deprivation corrosion”, which occurs when water gets trapped in a crack or seam in a metal - e.g. in the edge of your car door where the metal is folded over and welded.

The water in the crack or seam quickly loses its dissolved oxygen and turns into an anodic cell (due to the fact that the water turns basic when the oxygen is exhausted). Since the water cannot “replenish” its dissolved oxygen because it’s not in contact with the air, this anodic cell continues until the water evaporates or gets its oxygen replenished (i.e. by rusting through the car door). This type of corrison can be the nastiest type because it’s usually hidden, and the cell can last a long time.

Water droplets can also cause this type of corrosion in some cases, due to their slow absorbtion of oxygen. That sometimes explains the “spotted” rust pattern you get on hatchets and the like left out overnight, when the dew forms droplets on the hatchet. The droplet itself becomes the anodic cell.

If anyone is curious, I can dig up my old materials book and give more specific details on this.

Ken Clark
Toronto, Canada

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Comment from the Forum Moderator: OK, I let the title go because it had some black humour appeal, although I personally found it pretty offensive.

That seems to have been taken as permission for others to follow suit.

Policy: this is not the BBQ Pit. This is a forum about Cecil’s Mailbag. Titles to topics that are personally insulting are inappropriate and will be edited in the future.

Any complaints, I will be happy to discuss with you personally in e-mail.