Wustite formation on machined iron products

[AndrewLewis]

Rip,

In a video a while ago, you talk about barbell selection and purchase. You mentioned that a black patina forms on barbells if they are oiled and taken care of over time.

I was reading a corrosion engineering book and reviewing phase diagrams of FeO (wustite), Fe2O3 (hematite/common rust), and Fe3O4 (magnetite), and it got me thinking, so I spoke with some coworkers.

Our conclusion from these discussions was that, because of the thermodynamic considerations of FeO (see Ellingham diagram for steelmaking), FeO formation doesn't occur at room temperature. The same is true of Fe3O4.

As-cast surfaces oxides form in the following layering: Fe (substrate) -> FeO -> Fe3O4 -> Fe2O3 -> open air
These are a result of the steelmaking process, so seeing FeO post processing makes sense on an as-cast surface.
However, a machined surface should not have FeO formation (based solely on the metallurgy and chemistry).

Therefore, my question is the following:
How do you explain the observation you've had that barbells (a machined product) turn black over time on the surface?

[Mark Rippetoe]

Remembering my minerology, common rust is limenite, hematite is the higher oxidation state. I had assumed the patina was hematite. But I know a guy. Hang on...

Okay, here's my guess: Wustite is FeO, and does not occur under normal subareal conditions -- it is iron meteorites, one deposit in Greenland, and a transient product of the steel/ironmaking process. The common oxidation states of iron oxide are Fe2O3, limonite-- common orange/red rust with an orange streak test -- and hematite, a much darker compound with a dark red streak. Magnetite is Fe3O4. We believe that the bar patina is magnetite, essentially the same thing a a gun bluing, and that the higher oxidation state is cause by skin contact and the oils and acids found there. This may explain the fact that racks and rack pins not exposed to as much handling oxidize at a slower rate. Basically, steel left outside in the rain rusts, and steel in the gym gets a bluing job over time.

[AndrewLewis]

I didn't see you edit this on the 10th, but I haven't stopped thinking about this over the last week.

After doing more research, your hypothesis that it's magnetite is consistent with the scientific studies. The overwhelming metallography testing indicates that Fe2O3*H2O forms and then partially electrochemically converts to FeOOH and Fe3O4 as a deeper layer.

So based on that, it seems like our congruent conclusion that it's magnetite is correct, but we differ on how it gets there. My hypothesis based on my research is that Fe2O3*H2O is constantly forming, but with the oxygen shielding effect from chalk and skin oil, it slows down and doesn't have time to flake off so it forms a thicker magnetite layer.

I don't see how the oil from skin would assist in this process other than to actually prevent fast oxidation (or is that what you meant?). The sebum released by the skin appears to be relatively inert with the exception of the carboxylic acids which comprises about 16% of the sebum.

What do you think of that analysis?

[Mark Rippetoe]

I think we are very impressive.

[AndrewLewis]

We're alright I guess. I didn't post this to seem impressive. It's been bothering me for like two weeks. I just needed to have an explanation.

[Dave_G]

I think we are very impressive.

I don't know about anyone else, but I was impressed as I read this thread.

[Greg Ruhl]

Have you experimented with phosphate conversion coating on old bars? Specifically, phosphoric acid (Jasco prep and prime, etc.) to remove rust and convert to iron phosphate or something like Brownells zinc phosphate (parkerizing)?

[Mark Rippetoe]

I haven't experimented at all. I am not an experimenter.

[Troy May]

One of the best things about WFAC is the patina on the equipment...

[AndrewLewis]

Have you experimented with phosphate conversion coating on old bars? Specifically, phosphoric acid (Jasco prep and prime, etc.) to remove rust and convert to iron phosphate or something like Brownells zinc phosphate (parkerizing)?

If you're going to go through that work, you might as well just wire brush all of the rust off of the equipment and then take care of it properly.