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Oxidation was detected by EDS on the cross-section of a surface crack in cast iron. I am trying to determine if the crack occurred during casting (oxide formed at high temperature), or after casting (the resulting oxidation in the crack is from aqueous corrosion). No additional thermal exposure occurred following the casting process. The oxide appears dark. There is no evidence of red rust within the crack.

Hello Brian,

First of all you will have to determine if the crack you are seeing is a result of high stresses or casting problems such as misruns or merging fronts. If the "crack" occurred as a result of casting related issues, you should see a difference in microstructure near the crack as compared to the region away from the crack due to the fact that area near the crack froze first. On the other hand if the crack formed during the cooling process as a result of high stresses, you should see uniform microstructure in the cracked region.

That said, if the oxidation product formed when the material was molten, it is typically wustite, FeO. If the product formed during the cooling process after the casting was solid, it is likely to be wustite or magnetite, Fe3O4, lower temperatures favoring magnetite. X-ray radiography of the cracked region may give you an indication if the crack formed before or after solidification. The product formed during the corrosion process is haematite, Fe2O3.xH2O. Both wustite and magnetite are black while haematite is rust colored. You may distinguish between these products using x-ray diffraction if you are able to scrap the oxidation product and subject it to the x-ray diffraction. If that is not possible, you may have to use techniques that are more complicated and expensive.

Oxidation was detected by EDS on the cross-section of a surface crack in cast iron. I am trying to determine if the crack occurred during casting (oxide formed at high temperature), or after casting (the resulting oxidation in the crack is from aqueous corrosion). No additional thermal exposure occurred following the casting process. The oxide appears dark. There is no evidence of red rust within the crack.

Oxidation was detected by EDS on the cross-section of a surface crack in cast iron. I am trying to determine if the crack occurred during casting (oxide formed at high temperature), or after casting (the resulting oxidation in the crack is from aqueous corrosion). No additional thermal exposure occurred following the casting process. The oxide appears dark. There is no evidence of red rust within the crack.

Oxidation was detected by EDS on the cross-section of a surface crack in cast iron. I am trying to determine if the crack occurred during casting (oxide formed at high temperature), or after casting (the resulting oxidation in the crack is from aqueous corrosion). No additional thermal exposure occurred following the casting process. The oxide appears dark. There is no evidence of red rust within the crack.

Original Message:
Sent: 10/23/2023 11:20:00 AM
From: Brian DeForce
Subject: Oxide source – high temperature or aqueous corrosion?

Oxidation was detected by EDS on the cross-section of a surface crack in cast iron. I am trying to determine if the crack occurred during casting (oxide formed at high temperature), or after casting (the resulting oxidation in the crack is from aqueous corrosion). No additional thermal exposure occurred following the casting process. The oxide appears dark. There is no evidence of red rust within the crack.

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Brian DeForce
Materials Laboratory Manager
Wabtec
Wilmerding PA
4128251458
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Oxidation was detected by EDS on the cross-section of a surface crack in cast iron. I am trying to determine if the crack occurred during casting (oxide formed at high temperature), or after casting (the resulting oxidation in the crack is from aqueous corrosion). No additional thermal exposure occurred following the casting process. The oxide appears dark. There is no evidence of red rust within the crack.

Can you explain a little more why the high temperature oxide would be more enriched in active elements than a low temperature oxide?

Oxidation was detected by EDS on the cross-section of a surface crack in cast iron. I am trying to determine if the crack occurred during casting (oxide formed at high temperature), or after casting (the resulting oxidation in the crack is from aqueous corrosion). No additional thermal exposure occurred following the casting process. The oxide appears dark. There is no evidence of red rust within the crack.