corrosion analyzer 2.1 tour -- corrosion of iron

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CorrosionAnalyzer Application Demonstration problem #2 OLI Systems, Inc www. olisystems. com 973-539-4996 108 American Road Morris Plains, NJ 07950 Corrosion of Iron in Aqueous Solutions The Application Two 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 CO 2 , representative of a fluid recovered from oil production. The power of OLI/CorrosionAnalyzer becomes apparent as we study the chemistry of oxidation and reduction. OLI, Value Through Technology

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Page 1: Corrosion Analyzer 2.1 Tour -- Corrosion of Iron

CorrosionAnalyzer Application Demonstration problem #2

OLI Systems, Inc www. olisystems. com 973-539-4996 108 American Road Morris Plains, NJ 07950

Corrosion of Iron in Aqueous Solutions

The Application

Two 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.

OLI, Value Through Technology

Page 2: Corrosion Analyzer 2.1 Tour -- Corrosion of Iron

Corrosion of Iron in Aqueous Solutions • 1-2

Tour Conventions In 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 User Action

Bold and Italic The user is required to enter this information

Bold and Underlined The user is directed to look for this feature in the program windows

Click Left-mouse button

Right-Click Right-mouse button

The Tour Starts Here 1. Stability of Iron in Water

Iron 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.

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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.

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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-).

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The Tour Continues Here… 2. Corrosion Rate of Steel in Aqueous Brines

The 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 20 C Pressure 30 Atmospheres H2O 55.508 moles CO2 1.0 moles NaCl 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 titrants for this type of diagram. We also have not specified any alloys. This will be done in the rates calculations. Click on the Add Corrosion Rates Button.

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The program will automatically place you in the Definition tab. If you desire, you may enter descriptive information via the Description tab. We will forgo this at this time.

Setting up the calculation We will first perform a temperature survey. The range however, is incorrect. We wish to simulate from 0 to 200 C in 10 C increments.

Figure 19 Unfinished input

Click the drop down arrow next to the Carbon Steel entry under the Contact Surface grid.

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Figure 20 Metal Chemistry

The current list of metals are displayed. For this tour we will consider the mild steel, Carbon Steel G10100 (generic). Click on the drop-down arrow in the Flow conditions section.

Figure 21 flow conditions

There are several choices to select from:

We will simulate in static conditions. The current default is acceptable. Click on the Specs…button.

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Figure 22 Standard range specifications

This dialog is analogous to the range display in the StreamAnalyzer. Set the Starting temperature to 0C, the Ending temperature to 200 C and the increment to 10 C. Click on OK then click the Calculate button. When finished, click on the General Corr. Rate tab.

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Figure 23 The rate of corrosion v. temperature

This diagram shows that the rate of corrosion increases with increasing temperature up to about 110 oC. Then the rate decreases. Further analysis would reveal that the solid FeCO3 (siderite) has precipitated and has formed a passivating layer. Now click on the Report Tab. Scroll down to find this section.

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Figure 24 Tabulated results

This shows the tabulated results. Now click on the Polarization Curve

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Figure 25 Polarization Curve at 0 C

The curve displayed corresponds to 0oC. The customize button can be used to display the polarization curve at a different temperature. The “ ” marks the mixed potential point. It is this point that determines the rates of corrosion calculation. The appears to be on the “CO2” and “Fe” lines. This means that the carbon dioxide hydrolysis and the oxidation of iron are controlling the rates of corrosion. If the appeared on a vertical line, then we could say that the current had reached a limiting or passivating amount.

Flow survey We will now repeat our calculations but only at 20 C. This time we will perform a flow survey. We will simulate the movement of fluid through a pipe. We will need to specify the diameter of the pipe and the fluid flow. Click on the Definition tab .

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Click on the Types of Rates Calculation (The button will display Temp Survey) and select Pipe Flow. The Flow Conditions section automatically changes to pipe flow and has a default diameter of 0.1 meters.

Click the Specs… button.

Figure 26 Selecting Pipe Flow range

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Figure 27 Range setting

The range of value indicate the starting and ending values. The radio buttons in the Calculate box allow the user to perform quick conversion. The default is to set the starting and ending values along with an increment. The Number of Steps option will determine the total number of points to be calculated. The Increment option allows for the starting and ending values to be entered along with the number of steps and then the increment between point will be automatically determined. The final option; End allows the starting value, increment, and steps to be entered. The final value is automatically determined by the program. We will enter a Start value of 0 and an End value of 20. The Increment will be 1.0 Click on the OK button then click on the Calculate Button.

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Figure 28 Get set, get ready, go!!!!!

Click on the General Corr. Rate tab when the calculation finishes.

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Figure 29 The flow survey

The asymptotic behavior of the rate is due to a mass-transfer phenomena. At low flow rates, there is sufficient time for the corrosive agents to reach the pipe. At higher flows, a steady-state condition exist.

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OLI Systems, Inc.

American Enterprise Park 108 American Road

Morris Plains, NJ 07950

(973)539-4996

www.olisystems.com