Sodium Hydroxide + Iron Oxide: Green Tint Explained?

DrStupid said: But there are stable mixtures of both oxidation states (e.g. Fe3O4).

Black.

There is even a green rust mineral: https://en.wikipedia.org/wiki/Fougèrite.

Somehow I don't expect it on the rusted grill

It is not like there are no green Fe(II) compounds, sure they exist. But - apart from some specific cases, like Mohr's salt - they are not stable in the presence of oxygen, and chances of finding them on a random, rusted iron surface are next to zero. How green are we talking here?

BWV said: FeO + 4NaOH = Na₄FeO₃+ 2H₂O

Rust isn't FeO. It's a mix of almost exclusively iron(III) compounds.

Keith_McClary said:

NaOH is not really a reducing agent, and I seriously doubt this reaction is occurring in any appreciable amount. Also, the author of that site references no sources and looks like he might be 15 years old.

I think @Borek is on the right track if you have a vivid green material. However, if it's more of a dull greenish beige, it could easily be some sort of hydrated rust compound (still probably iron(III), though):
Picture of hydrated iron(III) oxide

TeethWhitener said: How green are we talking here?Rust isn't FeO. It's a mix of almost exclusively iron(III) compounds.NaOH is not really a reducing agent, and I seriously doubt this reaction is occurring in any appreciable amount. Also, the author of that site references no sources and looks like he might be 15 years old.

I think @Borek is on the right track if you have a vivid green material. However, if it's more of a dull greenish beige, it could easily be some sort of hydrated rust compound (still probably iron(III), though):
Picture of hydrated iron(III) oxide

The shade was a little darker than a copper patina, looked almost like mold - dull greenish beige would be a good description

Vanadium 50 said: Immediately after gook has been sprayed on them? It's not like they have had years of exposure to oxygen. More like minutes.

Yes, immediately. In my experience iron that corrodes on air gets directly to red, I have never seen it getting through a green phase (nor heard about such a situation).

DrStupid said: But they usually don't instantly turn into Fe(III) compounds when exposed to air.

Yes, dry salts (especially if stored in controlled conditions) are a bit more stable, but we are taking about a highly alkaline solution, not a dry solid. Pourbaix diagram for such systems says that around 0.4 V (which is more or less what I would expect here) and at high pH the only stable form is Fe2O3/Fe(OH)3.

In general Fe(II) salts are unstable and can't be used as Fe(II) source in analytical chemistry. Only Mohr's salt is stable enough to be considered a primary substance, in all other salts enough Fe(II) is oxidized to Fe(III) to make them unreliable.

Borek said: Yes, immediately. In my experience iron that corrodes on air

I don't think that's what it is. (Or rather, that's not what the son who is taking AP chemistry thinks it is, nor what the title says it is) The title chemical has Fe(2+) in it. That is, it's not corroded iron, but it's an iron reaction product.

Borek said: In my experience iron that corrodes on air gets directly to red, I have never seen it getting through a green phase (nor heard about such a situation).

Yes rust is red, but no it is not only Fe(III).

Borek said: Pourbaix diagram for such systems says that around 0.4 V (which is more or less what I would expect here) and at high pH the only stable form is Fe2O3/Fe(OH)3.

That doesn't mean Fe(II) turns into Fe(III) immediately. It just means that the conversion starts spontaneously.