Scientific name for rust? 1

[CheckMate]

What is the scientific name for rust or corrosion? How do you explain the delaminating of a steel beam that has been exposed to moisture. I need to talk to a group of kids about this, and I just don't want to say it rusted. PS this is totally out of my field, but have been asked to do this as a "knowledgeable person".

 

[swall]

According to Uhlig, "hydrous ferric oxide" comprises most of ordinary rust.

 

[unclesyd]

Rust is nothing more than Iron Oxide or mixtures of the different ones.

Very simple.
What you are describing as layers peeling off are layers of Magnetite (Fe[sub]3[/sub]O[sub]4[/sub]). Magnetite isn't a true compound but a mixture of FeO and Fe[sub]2[/sub]O[sub]3[/sub]. Magnetite can be very dense, closely approaching the density of iron. As it reverts to the more stable forms of iron oxide the volume of the scale increases thus the appearance of delaminating.

What you are describing is very common in splash zones especially around salt water. That is why the Navy was always chipping and painting.

You need to look up Magnetite.

 

[EdStainless]

Metals naturally occur as compounds in the earth. That is their most stable state. We have to add energy to turn them into pure, usable metals. Nature is always trying to turn them back into dirt. Some metals (stainless steel) resist this better than others.

The usual mechanism is oxidation, iron turning into iron oxide (rust). This causes failure for a number of reasons. The loss of metal makes part thinner and they might not be able to carry the load. The corrosion products take up more volume than the metal did. This swelling effect can pry points loose, cause splitting (rusting re-bar in concrete), and even break metal parts.
The other thing that happens is that once corrosion starts it often causes more water to get trapped which accelerates the process.

There are some good corrosion overview presentations available at

http://www.nace.org

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[CoryPad]

Steel corrosion products can have many components. Ferric oxide [Fe[sub]2[/sub]O[sub]3[/sub]] can be the main component in dry environments. In wet environments, iron(III) oxide-hydroxide [FeO(OH)] can be the major component. The following Wikipedia page is useful:

http://en.wikipedia.org/wiki/Rust

In describing the condition (corroded steel), it is important to emphasize the chemical reaction(s) with the resultant free energy reduction, as well as the functional requirements (appearance, prevention of thickness loss) and the corrosion prevention treatments (barrier coatings like paint, galvanic coatings like zinc, etc.).

Regards,

Cory

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[metengr]

Great explanations above except one problem

I need to talk to a group of kids about this, and I just don't want to say it rusted.

You're not talking to a group of scientists or engineers. Keep it simple and if you can provide a demo for hands on it might make it interesting.

 

[AlanD]

>>What is the scientific name for rust or corrosion? How do you explain the delaminating of a steel beam that has been exposed to moisture. I need to talk to a group of kids about this, and I just don't want to say it rusted.

What about the term "oxidised". The terms of corroded and rusted are just increasing more specific terms.

 

[monkeydog]

I like EdStainless's description, but when you say a group of kids, what age are you talking about?

 

[CheckMate]

Thanks for the help to all that responded. I met with the kids last night and they were impressed with all your reponses. The kids varyied in age from 10-15. I think they were just as impressed in that we have this forum to get real answers to scientific questions. thanks again.

 

[HgTX]

What would have been wrong with "corroded" if you don't like "rusted"?

Hg

Eng-Tips policies: faq731-376

 

[apta]

Corrosion is the reversion of a refined product [e.g. - steel] to its natural state [iron ore] by oxidation [combination of the steel w. oxygen and water]. In the re-formation of the ore, the molecule formed is much larger than the refined steel. This molecule takes up 8 to ten times the volume of steel. The thick layers exert enormous force, pushing adjacent assemblies [bridge beams, brick walls] apart.