Difference between revisions of "Corrosion FAQ's"

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== When standard database predicts precipitation of Fe(III), Zn(II), Cu(II) as Fe(OH)3, Zn(OH)2 and Cu(OH)2 respectively, Inclusion of the Corrosion database produces the more stable hematite Fe2O3, goethite FeO(OH), ZnO and CuO solids.==
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Frequently Asked Questions about OLI and Corrosion
  
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When you include a corrosion data bank, surely it predicts more stable oxides. That is because when a corrosion databank is included, the solver automatically assumes a surface rather than a simple solid precipitation. And then the speciation changes. The hydroxides are still considered a part of the system, but the possibility of oxides forming increases because there will be corroding surface involved in that system. Thus the stable oxide predictions.
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== Databanks ==
  
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[[When standard database predicts precipitation of Fe(III), Zn(II), Cu(II) as Fe(OH)3, Zn(OH)2 and Cu(OH)2 respectively, Inclusion of the Corrosion database produces the more stable hematite Fe2O3, goethite FeO(OH), ZnO and CuO solids]]
  
  
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== When to use Corrosion  and Geochemical databank ==
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== Phenomena ==
  
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[[Elemental Sulfur Corrosion]]
  
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When you use OLI software, there are certain calculations that need additional databanks to be included for specific predictions. The main special databanks are geochemical (i.e. GEOCHEM in the AQ model and GEMSE in the MSE model) and corrosion-related (i.e., CORROSIO in the AQ model and CRMSE in the MSE model).
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== ISO Corrosion standards ==
  
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Comparison with ISO15156.
  
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[[File:2014-12-11 12-31-49.png|500px]]
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ISO15156 is a standard for corrosion-resistant alloy selection for Oil and Gas production.
  
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There is a fine difference between scenarios which govern whether the databanks should be included or not. One of the key factors is the '''time of equilibration'''. Would a particular species be formed under process conditions? Or can it only be formed over a significant period of time, for example a geological timeframe?
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The environmental limits for the alloys in ISO15156-3 are based on experimental data with chloride concentration up to 180000 mg/l using NaCl.
  
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For the geochemical databanks (GEOCHEM and GEMSE), the controlling parameter is only the equilibration time. If a component forms only over a geological time scale, then it is necessary to include one of the geochemical databanks. Please remember that under chemical process circumstances this solid would not realistically form and would not be predicted as a part of the system.
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A plot for Ecorr and Erp at 150 C and 5.7 m NaCl (180000 mg Cl/l) with varying partial pressure of H2S:
  
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A good example for this is the SiO2 system. OLI extensively worked on this system.
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[[File:ISO.png]]
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Now SiO2 and its two forms
 
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                          -> '''SiO2 ( Amorphous)'''
 
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                          -> '''SiO2 ( Quartz)'''
 
  
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are included in '''MSE''' and '''GEMSE''' databanks respectively. Now under chemical process conditions, one would expect the amorphous silicon dioxide to be present in a system. But when a geological timescale (or a special crystal growth process) is involved, then it becomes necessary to include the quartz structure, and then the geochemical databank comes into picture.
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Plot Analysis: The transition to localized corrosion is predicted at about 50 psi H2S in OLI's predictive model, whereas ISO 15156 predicts that the alloy is OK up to 200 psi H2S. The electrochemical model is more conservative than the standard. But the transition would move to a substantially higher P(H2S) if  the chloride concentration  is lowered(because Erp strongly depends on Cl).
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The driving force for localized corrosion, i.e., the difference between Ecorr and Erp in the plot above is small, i.e., it is only about 50 mV at 200 psi H2S, which is within the experimental uncertainty.
  
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Lets consider corrosion databank now.
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Note: The above mentioned results are still under development at OLI.
  
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[[File:2014-12-11 15-00-37.png|500px]]
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[[user:RNIMKAR | Editor: Rasika Nimkar, Author: Andre Anderko]]
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[[Category: Corrosion]]
  
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As you can see in a typical corrosion example, where rust i.e iron oxide forms on iron surface. The layers shown in above image are representation of the scenario. Under regular circumstances when you add a corrosion rate calculation, OLI's solver will consider speciation in a such a way that the fe surface is taken into consideration. But when you have to create a stability diagram, you need to include the CEMSE databank. Because then the time factor comes into picture. When CEMSE databank is turned on, it means the the process will not have any iron oxides by itself. But given a number of years under right circumstances the well developed rust could form along with Fe2O3 ( hematite).
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== Pitting Current Density ==
  
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A pitting can be represented by the following diagram:
  
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For a list of components in the Corrosion databank , please follow the following link:
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[[File:2017-05-26 15-28-09.jpg|500px]]
  
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[[File:Corrosion list.pdf]]
 
  
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Source:https://en.wikipedia.org/wiki/Pitting_corrosion
  
  
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OLI's corrosion model predicts the current density as a function of potential which is given by the following equation:
  
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[[File:2017-05-26 15-15-39.jpg]]
  
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The basic idea behind the pitting current density is as follows:
  
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*  The repassivation potential model gives an i vs. E relationship within a localized corrosion environment. Since it is a limiting model (i.e., it is rigorous only in the limit of repassivation), it does not    include mass transfer effects and does not handle the spatial distribution of current. Therefore, it can be viewed as representing a one-dimensional, maximum propagation rate.
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*  At the corrosion potential, it will give the maximum propagation rate for one-dimensional propagation of localized corrosion. The corrosion potential is calculated from a general corrosion model (on a passive surface).
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[[user:RNIMKAR | Editor: Rasika Nimkar]]
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[[Author: Andre Anderko]]
 
[[Category: Corrosion]]
 
[[Category: Corrosion]]

Latest revision as of 13:39, 11 January 2018

Frequently Asked Questions about OLI and Corrosion

Databanks

When standard database predicts precipitation of Fe(III), Zn(II), Cu(II) as Fe(OH)3, Zn(OH)2 and Cu(OH)2 respectively, Inclusion of the Corrosion database produces the more stable hematite Fe2O3, goethite FeO(OH), ZnO and CuO solids


Phenomena

Elemental Sulfur Corrosion

ISO Corrosion standards

Comparison with ISO15156.

ISO15156 is a standard for corrosion-resistant alloy selection for Oil and Gas production.

The environmental limits for the alloys in ISO15156-3 are based on experimental data with chloride concentration up to 180000 mg/l using NaCl.

A plot for Ecorr and Erp at 150 C and 5.7 m NaCl (180000 mg Cl/l) with varying partial pressure of H2S:

ISO.png

Plot Analysis: The transition to localized corrosion is predicted at about 50 psi H2S in OLI's predictive model, whereas ISO 15156 predicts that the alloy is OK up to 200 psi H2S. The electrochemical model is more conservative than the standard. But the transition would move to a substantially higher P(H2S) if the chloride concentration is lowered(because Erp strongly depends on Cl). The driving force for localized corrosion, i.e., the difference between Ecorr and Erp in the plot above is small, i.e., it is only about 50 mV at 200 psi H2S, which is within the experimental uncertainty.

Note: The above mentioned results are still under development at OLI.

Editor: Rasika Nimkar, Author: Andre Anderko

Pitting Current Density

A pitting can be represented by the following diagram:

2017-05-26 15-28-09.jpg


Source:https://en.wikipedia.org/wiki/Pitting_corrosion


OLI's corrosion model predicts the current density as a function of potential which is given by the following equation:

2017-05-26 15-15-39.jpg

The basic idea behind the pitting current density is as follows:

  • The repassivation potential model gives an i vs. E relationship within a localized corrosion environment. Since it is a limiting model (i.e., it is rigorous only in the limit of repassivation), it does not include mass transfer effects and does not handle the spatial distribution of current. Therefore, it can be viewed as representing a one-dimensional, maximum propagation rate.
  • At the corrosion potential, it will give the maximum propagation rate for one-dimensional propagation of localized corrosion. The corrosion potential is calculated from a general corrosion model (on a passive surface).


Editor: Rasika Nimkar Author: Andre Anderko

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