FAQ for MICORP Version 1.x Software

1. How do I find the biofilm aggressiveness value?
   This value is like the rate constant in a chemical reaction. It reflects the ability of the biofilm to catalyze sulfate reduction. You can use a pit depth data point to calibrate it. Fortunately, biofilm aggressiveness is a sensitive parameter for deep (several mm) pit growth because mass transfer is in control. This means cathodic sulfate reduction is limited by the ability of sulfate to diffuse through mass transfer barriers to get to the metal surface, regardless how aggressive an SRB biofilm is. In such a case, you can use a typical value (say -10). Remember, you can always use different biofilm aggressiveness value to test its sensitivity! 
   
   We have seen published pit data in NACE papers for SRB indicate pit depth of up to 10 microns in the first 24 hours at 37^oC. Some reported only 20 microns in 7 days. These lab pit data give biofilm aggressiveness between -9.5 to -12.3  When using MICORP to calibrate such shallow pits, mass transfer parameters are not sensitive at all because charge transfer is limiting. Our research group is performing state of the art tests to probe whether SRB can be super-corrosive under some conditions that can happen in the field. We believe, a much larger biofilm aggressiveness is possible!
   
   
2. Is it true that almost all short-term lab tests are in the charge transfer control region?
   Most pit depth data from lab tests lasting a few weeks are shallow (say 5 - 20 microns). Sulfate concentrations are high (above 20 mM in most SRB media). In such cases, MICORP simulation will show that mass transfer is not limiting. This means sulfate concentration variations have no impact on pit depth. There are exceptions. For example, some researchers grow SRB biofilms in a rich culture medium and then switch to wastewater that contains less than 1 mM sulfate. In such a case, it is likely that charge transfer and mass transfer are both important. 
   
   
3. How do I paste MICORP generated figures to Microsoft WORD?
   You can take a screen shot of the figure and paste it to WORD. This carries the background grey color with it. To have a neat result, you can use the "Copy Figure To Clipboard" menu under the MICORP Edit menu. You can then use paste it to WORD (without the grey background). This figure will look similar to an Excel figure with only lines and words. MATALB figure files can be opened using MATLAB software. You can edit the figure in many ways including changing labels, line styles, adding/deleting curves, and adding drawings, etc.)
   
   
4. How do I extract x-y data from figures generated by MICORP to an Excel file?
   MICORP Version 1.3 and above has a built-in utility under the Tools menu to do the job. 
   
   
5. MICORP assumes a constant bulk phase sulfate concentration. What do I do for lab tests in bottles with decreasing sulfate concentration with time?
   Lab tests typically lasts for only a week or a month during which charge transfer is likely dominating because pit depth is relatively shallow. This means variations in sulfate concentration do not matter much for pit growth. You may use an estimated average sulfate concentration to do simulation. Remember, you can always plug in different sulfate concentration values to test its sensitivity. This can tell you how accurate your approximation is. For long term tests under stagnant condition, please contact Prof. Gu to custom build the software for you. (In some lab tests by civil engineers, a sulfate concentration of less than 1 mM is sometimes used to simulate wastewater. In such tests, mass transfer is important even for shallow pits.)
   
   
6. When I change corrosion temperature, biofilm aggressiveness does not automatically change. Why?
   When you enter a different temperature, MICORP will adjust sulfate diffusivities based on the Stokes-Einstein relationship. It will also adjust electrochemical parameters internally, except biofilm aggressiveness. This is because there is no fixed temperature relationship for it. It may increase or decrease with temperature depending on the deviation from its optimal operating temperature. It is up to the user to adjust manually based on the actual situation or simply assume that its change is not significant. 
   
   
7. What about 2-D simulation?
   In most cases, 1-D simulation should be sufficient. 2-D simulation is usually not that meaningful because you don't know the pit population and depth distribution.. Prof. Gu will custom make MIC software for specific 2-D simulation for you under a research agreement for pipe flow or stagnant water tank, etc. Be aware that such software may be computationally demanding unless a large time step is used.
   
   
8. Can incomplete biofilm treatment be bad?
   Yes. When a biofilm is not removed completely by pigging and/or biocide dosing, sessile cells on a metal surface may survive. Usually, a pig's brush is long and flexible enough to remove biofilms in shallow pits. However, this may not be true for deep pits. If they were previously starved of nutrients due to a thick layer of top biofilm or debris, they now have better access to nutrients (including sulfate) because of reduced mass transfer resistance. If we assume that these sessile cells are not injured by chemicals, they may actually increase pitting. MICORP simulation shows increased pit growth when the mass transfer resistance layer is reduced. By the way, incomplete treatment may enhance biocide resistance.
   
   
9. Is there such a thing as a protective biofilm?
   Yes, but be careful. A docile biofilm can become aggressive. There are several reason to turn aggressive. Some biofilm cells can never be aggressive themselves, but they help the aggressive cells. Aggressive cells will become dominant on the iron surface when the need arises. See our future publications. 
   
   
10. Does a thicker SRB biofilm mean faster MIC pitting?    
    If a mathematical model directly relates the flux of sulfate entering the biofilm to MIC pitting rate, a thick SRB biofilm will mean more digestion of sulfate by SRB and thus faster pitting rate. Unfortunately, this is inconsistent with biofilm catalyzed electrochemical kinetics. A thicker SRB biofilm may help protect the biofilm community, but it does not necessarily promotes sulfate reduction at the iron surface. A thicker biofilm actually presents a thicker mass transfer barrier for sulfate diffusion. It also consumes more sulfate for biofilm maintenance. Both will lead to a smaller pitting rate.  Remember, electrons from the iron dissolution reaction can only reach the SRB cells very close to the iron surface. Cells are very poor electron conductors! When SRB cells cannot receive the electrons from iron dissolution, they will use lactate as electron donor. This redox reaction is not electrodic, thus does not lead to electrochemical corrosion!
    
    
11. How does SRB biofilm benefit from MIC of iron?   
    It is all about bioenergetics! Iron dissolution with concomitant sulfate reduction is exergonic. The released energy helps support metabolic activities of sessile SRB cells. Although the redox reaction is spontaneous, sulfate reduction does not occur at an appreciable rate. SRB enzymes (namely hydrogenase enzymes) promote sulfate reduction.