corrosion analyzer 2.1 tour -- corrosion of iron
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CorrosionAnalyzer ApplicationDemonstration problem #2
Corrosion of Iron in Aqueous SolutionsThe ApplicationTwo illustrations of corrosion analysis: Iron in water develops a passivation layer at specific conditions of pH and Eh. An aqueous, carbon dioxide bearing brine in contact with generic steel corrodes at a calculated rate of uniform corrosion.
In the first analysis, a real-solution stability diagram will be generated for water in contact with an iron surface metal. Sulfuric acid and sodium hydroxide will be used to vary the pH of the water. The effect of the sulfur and sodium in solution will be taken into account. In the second part of the application, the uniform rate of corrosion will be calculated for an aqueous brine in contact with a generic steel surface metal. The brine is composed of water, NaCl and CO2, representative of a fluid recovered from oil production. The power of OLI/CorrosionAnalyzer becomes apparent as we study the chemistry of oxidation and reduction.
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Tour ConventionsIn this tour, and all subsequent tours, when action is required by the user, the instruction will be in Bold and Italic type. When you are referred to a feature on a screen, the information will be Bold and underlined. Any mouse clicks are left-mouse button clicks unless otherwise noted. This is summarized below:Type Face Bold and Italic Bold and Underlined Click Right-Click User Action The user is required to enter this information The user is directed to look for this feature in the program windows Left-mouse button Right-mouse button
The Tour Starts Here 1. Stability of Iron in WaterIron in water develops a passivation layer at specific conditions of pH and Eh.
Double-Click the CorrosionAnalyzer icon on the desktop or select it from the Start button. After the splash screen displays, click the Close button to remove the Tip-of-the-Day if it displayed. This screen is similar to other OLI Analyzer Software. Click the Add New Stream icon to begin.
Corrosion of Iron in Aqueous Solutions 1-2
Figure 1 Corrosion Analyzer Main Window
After clicking the Add New Stream icon, you will display the Definition Tab. As in the OLI/StreamAnalyzer, you may add Single Point Calculations and Surveys. You may also study stability diagrams and per form rate calculations.
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For this tour, we will first define the stream.
Figure 2 Corrosion Analyzer Definition Tab
Click on the Description Tab.
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It is advisable to enter a brief description of the stream so we can later identify what our thoughts were when we created this file.
Figure 3 Enter a description
Enter a Description and then click the Definition Tab.
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The units for this stream may not be in the set required for the tour. Click on the Tools menu item.
Figure 4 Default Stream Definition Grid
The Tools menu will be displayed. Select Units Manager from the list.
Figure 5 Tools Menu
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We wish to use metric units. Click the down-arrow in the drop-down list box under the Standard radio button
Figure 6 Default Units Manager
Scroll down to find the word Metric.
Figure 7 Select metric
Click the OK button.
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We now have the units correct for the tour. Enter into the inflows grid the species: Fe NaOH H2SO4 Note: The display name may change these to a Spelled out display. You can use the Names Manager in the Tools menu to alter the display as you desire. For these examples, select Formula display.
Figure 8 Stream Definition in correct units
Water is the default species and is always defaulted to a value of 55.508 moles. Leave the remaining fields blank. Thus, we will simulate the behavior of iron in water at ambient conditions. Note that it is not necessary to include any elemental iron in the stream composition. Although it is permissible to include a corroding metal in the stream, it would not correspond to reality (e.g., a steel pipe is not a component of a stream) and would actually increase the computation time. We now need to verify that oxidation and reduction have been turned on in the chemistry model. Click on the Chemistry menu item and then select Model Options Warning! This step may take a few seconds, depending on the speed of your computer.
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Figure 9 The Model Options dialog.
There are many options to select from. Currently, we are only concerned with Redox. Click on the Redox tab.
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Figure 10 Selecting redox subsystems.
You are free to choose all redox systems, but this will usually result in long computation times. It is advisable to choose the redox systems that are relevant to the studied corrosion processes. In our example, we will choose the iron and sulfur systems. This means that the program will consider all redox states of iron (i.e., 0, +2 and +3) and those for sulfur (-2 to +6). For the moment, leave the sulfur subsystem unchecked. We will eventually select sulfur when we add the hydrogen sulfide. Click on the OK button to return to the definition. We have now defined the stream. Click on the Explorer Tab to continue. Now click on the Add Stability Diagram Icon.
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The default display is to show the Definition Tab. If you want to enter a description about this diagram, use the Description tab. We will not be entering a description at this time.
Figure 11The definition Tab.
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We now have some work to do to set up the calculation. The Calculate button is red indicating that we are not ready to calculate. We need to specify our surface metal. The program will attempt to determine the surface metal automatically from the list of inflows. Frequently this will be iron, as it is in this case, but we may use other metals. We also need to specify the titrants that will adjust the pH of the solution. The summary box displays the current information about the calculation. Click in the box under the Contact Surface header and enter the species Iron
Figure 12 Entering the surface metal
Since the stability diagram uses real titrants to adjust pH, we must specify them.Corrosion of Iron in Aqueous Solutions 1-12
Click on the Specs button to begin to fill out the remaining missing information. The Diagram Type is a Potential vs. pH (Pourbaix) type diagram. This plots potential on the Y-Axis and pH on the X-Axis. The program will default to using hydrochloric acid (HCl) and sodium hydroxide (NaOH) as the acid and base titrants. We do not wish to do so (adding chlorides can complicate the iron plots).
Figure 13 Select Titrants
Click the Select radio button Click the pH Titrants button.
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We now need to select an acid and a base. Select H2SO4 as the acid (Sulfuric Acid) and NaOH as the base (sodium hydroxide).
Figure 14 Choose an acid and base
Click on the OK button. Click on the Subsystems Tab.
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Figure 15 Make sure the sulfur subsystem is off.
This tab will only display the selected subsystems. In this case only iron and water will be displayed. The subsystems are still calculated if they are not checked, merely not displayed. Accept the default entries. Click on the Diagram Choices tab.
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Figure 16 Shade the iron solids
For this tour, we will accept the defaults. Click on the Close button. We are now ready to calculate. Click the Calculate button and wait for the calculation to finish. When the calculation has finished , click on the Diagram tab.
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Figure 17 A Pourbaix diagram for iron
The obtained diagram is useful for assessing the corrosion behavior of iron. First, the equilibrium lines between elemental iron (i.e., Fe(s)) and other species can be found. As shown in the diagram, elemental iron can be oxidized to the Fe2+ ions (i.e., FE+2) in acidic, neutral and weakly alkaline solutions (for pH below ca. 9.5) and to the Fe(OH)3-1 ions (i.e., FEIIOH3-1) in alkaline environments (for pH above ca. 11.5). The oxidation of iron can be coupled with the reduction of the H+ ions because the H+/H2o equilibrium line (denoted by a) lies always above the lines that represent the oxidation of iron. Therefore, corrosion of iron can occur with the evolution of hydrogen and formation of soluble iron-containing ions (either Fe2+ or Fe(OH)3-).Corrosion of Iron in Aqueous Solutions 1-17
The Tour Continues Here 2. Corrosion Rate of Steel in Aqueous BrinesThe uniform corrosion rate of generic mild steel in a carbon dioxide containing brine will be calculated. This is representative of fluid recovered from oil production. As you did in the previous tour, you will create a stream and then perform some calculations. Click on the Streams line in the left-hand tree view. Add a stream with the following composition. Temperature Pressure H2O CO2 NaCl 20 C 30 Atmospheres 55.508 moles 1.0 moles 1.0 moles
The following figure shows the input. Verify that only water is selected in the redox subsystems in the Chemistry menu item in the Model Options/Redox item.
Figure 18 The stream definition
Notice that there are no ti