Ways to differentiate type of rust

[Naruwan]

Hi guys,
is there any way to determine the difference between Fe2+ and Fe3+? I don't think telling the difference by the colour of the rust will be reliable enough.

In addition, had anyone heard of black rust from steel? They should be iron oxide which had been oxidized in a high temperature environment. Will a EDX evaluation tell the difference as compare to the normal rust e.g. Fe3+ ?

Thanks guys!

 

[gr2vessels]

I would start with inet, there is a miriad of info on the iron oxide.

 

[CoryPad]

X-ray diffraction will tell the difference between the different iron oxides (magnetite [black] and hematite [orange]), but energy dispersive x-ray spectrometry will not.

 

[TVP]

CoryPad is correct-- x-ray diffraction can differentiate the two. I have done this before, and the cost is usually not too high even with independent/outside labs.

 

[blacksmith37]

Nothing is ever easy .
Assuming by "rust" you mean iron oxides- FeO (black) ,forms at highest temperature (> 1300F +/-). Fe3 O4(black) forms at intermediate temps. Fe2 O3 (red) forms at 700F +/-, and at room temp (lookup fretting corrosion). Fe3 O4 -H2O (brown) ambient temp (it can be written with different formula like FeO2(OH)2). Finks Index will give you more pages of compounds than you want and you will find using the mineral names is not straightforward .When using XRD ,keep in mind some have strong crystallinity and give misleading patterns if you are trying to quantify them.

 

[emxhps06]

I personally had run a permanent magnetic across both magnetite (Fe3O4) and hematite (Fe2O3). From my whimsical experiment, I was able to move the magnetite and couldn't with hematite.

I agree with blacksmith,the XRD spectrum can give misleading patterns. In my own experience, spinel oxide (FeCrO type oxide) overlaps with the Fe3O4 and Fe2O3.

Just a thought.

 

[TomDOT]

The easy qualitative answer is using potassium ferricyanide papers to detect Fe2+. You can purchase them, or easily make your own by saturating filter paper in potassium ferricyanide solution (5% in water.) Allow the papers to dry, and store in a clean Ziplock bag. When you want to test a surface, place the paper on the surface and wet it. If you have Fe2+, the yellow filter paper will show blue spots.

Similarly, potassium ferrocyanide will react with Fe3+.

Large-scale crevice corrosion or "pack rust" is often quite dark in color, similar to millscale. It is also more tenaciously adhered, like millscale. I see it all the time on steel bridges after the "rust" colored surface layers are blasted off.

 

[Guest102023]

I order XRD testing fairly routinely in failure investigations involving corrosion, but have also learned to identify certain compounds by appearance. For example malachite and azurite are copper hydroxides that come up in brass pitting corrosion. Iron hydroxide is orange and superficial; its the stuff that develops when you put a clean piece of steel outside for a week. The lab I use usually also does the elemental analysis (EDS), and it can be a useful supplement to the XRD.

I was not aware of the overlap between certain compounds though, so thanks for that - I will quiz the testing people.

 

[Guest102023]

gr2vessels,
Please give some directions to 'inet'; googling did not turn up very much. Thanks.

 

[cloa]

gr2vessel means the internet when he say inet. Search for iron oxide.

 

[Guest102023]

Sorry, I never learned text-speak. I have searched heavily on several occasions, with mixed success.

 

[deco0404]

Its been a while since I have been involved in power plant / hi temperature work, but if I remember correctly, Magnetite is a black, hi-temp corrosion which is Fe3O4. It is generally found in dry steam piping such as superheaters and the like. Haematite is red and is Fe2O3 and is generally found in wet steam such as reheaters

Declan