How to choose a subsystem
The common question faced by users while generating a stability diagram is: How to choose a subsystem and how to determine the stability of metal containing species in certain subsystems?
The logical approach to take for this is the following algorithm:
Please note:
What is a redox subsystem?
- A set of species that contain a given element in all possible oxidation
states.
- Example: The iron subsystem consists of all species that contain Fe0, Fe2+ and Fe3+.
Now let us study an example:
A stability diagram calculation with Stainless Steel 304 as contact surface will look like this:
Where we have Iron, Nickel and Chromium , all three present in the same system.
When we highlight the Iron subsystem, both FeCr2O4 and NiFe2O4 will be shown to be present.
The iron subsystem will look at follows:
When we select the Nickel subsystem from the specs options, you will see that NiFe2O4 is present.
But when you highlight Chromium, only Cr2O3 will be shown as stable:
The reason being that, certain species(like FeCr2O4) will be most stable at a certain E & pH among all Fe-containing species. But this species is not the most stable among all Cr-containing species. ( In other words , it is metastable with respect to a species that does not contain Fe).