modeling the corrosion of steel

[pslomski]

Dear colleagues,

I want to run a model that will be an equivalent of the laboratory experiment simulating sequestration of CO2 into shale layers. The experiment lasted 45 days and was conducted in temperature of 353K and pressure 40 MPa in brine solution (a mixture of NaCl and CaCl2). It was an analogue of in situ conditions. Comparison of XRD results for samples before and after experiments indicated dissolution of chlorites and precipitation of iron oxide-hydroxides. I use LNLL database and Peng-Robinson equation of state because the conditions for CO2 are supercritical. Initial PHREEQC simulations showed that ph during the experiment was about 4.5.
However, I think that not only the chlorites, but also the steel from pipe supplying the CO2 was a source of iron. I think this can be a treated as a simulation of corrosion of the pipe in the real life situation. So I want to model also the corrosion of steel pipe. 

In some papers and also here on the forum (posts by @snjaykdubey) I found some reactions  and  code:

Fe = Fe+2 + 2e-
log_k 12.75
delta_h -94.218 kJ
RATES
    -start
      10 MD = 7.87    # mean steel density g/cm3
      20 Atwt = 55.845    # iron atomic mass
      30 Kc = 1.585489e-14   # some kind of constant…
……..
      50 rate = -Kc
      60 rate = rate*(MD/Atwt)
      70 rate = rate*PARM(2)
      80 moles = -rate*TIME
      100 SAVE moles
      110 REM parm(2) = Reactive surface area (cm2 dm-3)
    -end

I don’t know and I couldn’t find what kind of constant is this Kc=1.585489e-14 and why the rate equation has such simple form: rate = rate*(MD/Atwt). Unfortunately I wasn't able to obtain answer from the @snjaykdubey by private message so I decided to create a new post. Does anyone here has an experience in modeling steel corrosion in PHREEQC and could suggest how to deal with such a model or where to look for the answer? I browsed few chapters in metallurgical handbooks, but didn’t found the solution.

Thank You in advance
Regards,
Peter

[dlparkhurst]

To make the units work, I think Kc would be cm/s. The RATES expression defines a constant rate of corrosion, but probably not for the conditions of supercritical CO2. You can simply adjust or fit the rate constant to try to match your experiment.