Cyclic Polarization - Setup Parameters

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Cyclic Polarization - Setup Parameters

Description

 

4 - Cyclic Polarization

 

Parameter

Description

Units

Pstat

Select the potentiostat/galvanostat to perform the experiment. Each radio button corresponds to an installed potentiostat. You can select only one potentiostat at a time. Potentiostats that are already in use are marked with an asterisk. They can be selected but cannot be used.

 

Test Identifier

A string that is used as a name. It is written to the data file, so it can be used to identify the data in database or data manipulation programs. The Identifier string defaults to a name derived from the technique's name. While this makes an acceptable curve label, it does not generate a unique descriptive label for a data set. The Identifier string is limited to 80 characters. It can include almost any normally printable character. Numbers, upper- and lower-case letters, and the most common punctuation characters including spaces are valid.

 

Output File

The name of the file in which the output data are written. By default, it is saved in the default file directory.

 

The input can be a simple file name with no path information. In this case the output file is located in the default file directory. The default file directory is specified in the Gamry.INI file under the [Framework] section with a Key named DataDir. The default path can be changed using the Path command in the Options menu. It can also include path information, such as C:\MY GAMRY DATA\YOURDATA.DTA. In this example, the data are written to the YOURDATA.DTA file in the MY GAMRY DATA directory on drive C. The default value of the Output File parameter is an abbreviation of the technique name with a *.DTA filename extension. We recommend that you use a *.DTA file name extension for your data file names. The data analysis package assumes that all data files have *.DTA extensions.

 

If the script is unable to open the file, an error message box, Unable to Open File, is generated. Common causes for this type of problem include:

•Invalid file name.

•The file is already open under a different Windows® application.

•The disk is full.

 

After you click the OK button in the error box, the script returns to the Setup where you can re-enter the file name.

 

Notes...

Enter several lines of text that describe the experiment. A typical use of Notes is to record the experimental conditions for a data set.

 

Notes defaults to an empty string and is limited to 400 characters. It can include all printable characters including numbers, upper- and lower-case letters, and the most common punctuation including spaces. Tab characters are not allowed in the Notes string. Press the Notes button on the right-hand side to open a separate Notes dialog box.

 

Initial E

The starting potential of the initial potential sweep during data acquisition. The allowed range is ±10 V with a resolution of 0.125 mV. Its accuracy is determined by the settings of Initial E, Apex E, and Final E.

volts (V)

Forward Scan

The Scan Rate for the potential sweep during the initial scan (Initial E to Apex E). A practical bound on the Scan Rate is 1 mV/s. Higher Scan Rates may run but yield unreliable data because the potentiostat's compensation and filtering are set for long time constants. A lower bound on the Scan Rate is given by the minimum Step Size divided by the longest Sample Period. For scans shorter than 2 V, the slowest Scan Rate is 52.1 nV/s or 0.1875 mV/hour. Multiply these limits by four for scans longer than 2 V.

mV/s

Apex E

One of the two conditions to reverse the potential sweep. The sweep reverses when it reaches Apex E, if Apex I is not reached first. The allowed range for Apex E is ±10 V with a resolution of 0.125 mV. Its accuracy and resolution are determined by the settings for Initial E, Apex E, and Final E.

volts (V)

Reverse Scan

The Scan Rate of the potential sweep following a scan reversal (Apex E to Final E).

 

A practical upper bound on the Scan Rate is 1 mV/s. Higher Scan Rates may run but yield unreliable data because the potentiostat's compensation and filtering are set for long time constants.

 

A lower bound on the Scan Rate is given by the minimum Step Size divided by the longest Sample Period. For scans shorter than 2 V, the slowest Scan Rate is 52.1 nV/s or 0.1875 mV/hour. Multiply these limits by four for scans longer than 2 V.

mV/s

Final E

The final potential of the potential sweep following the scan reversal. The allowed range is ±10 V with a resolution of 0.125 mV. Its accuracy and resolution are determined by the settings for Initial E, Apex E, and Final E. The Scan Range, defined as the most anodic voltage in the scan minus the most cathodic voltage, must be less than 8 V.

volts (V)

Apex I

One of the conditions to cause a reversal of the initial ramp. The scan reverses as soon as the absolute value of the current exceeds Apex I. If you always want to reverse at a potential rather than at a current, enter a value larger than your potentiostat's maximum current for Apex I. The allowed range for Apex I ranges between zero and the potentiostat's maximum rated current.

 

Gamry tip bulb

Negative values for Apex I are not allowed because of the absolute value in the comparison.

mA/cm2

Sample Period

The spacing between data points. The shortest Sample Period we recommend is 0.1 s. The longest Sample Period allowed is 715 s for Reference potentiostats and 750 s for Interface potentiostats.

 

Use Sample Period to determine the Number of Points in the data curve as follows:

 

 

The Number of Points must be less than 262 143 (218 - 1). If you have more points, the experiment aborts just before the data acquisition phase of the sequence. The resolution in the Total Time is controlled by the Sample Period. Times that are not integer multiples of the Sample Period are rounded down. For example, if the Sample Period is set for 8 seconds, and a Total Time of 900 seconds is requested, the actual Total Time will be 896 seconds, which is 112 Sample Periods.

seconds (s)

Sample Area

The surface area of the sample that is exposed to the solution. The software uses the sample area to calculate the current density and corrosion rate (if applicable). If you do not want to enter an area, we recommend that you leave it at the default value of 1.00 cm².

 

Gamry tip bulb

Do not enter a value of zero!

cm2

Density

The density of the metal sample, used in calculating the corrosion rate. You may disregard this parameter if absolute corrosion rates are not required for analysis.

g/cm3

Equiv. Wt

Theoretical mass of metal lost from the sample after one Faraday of anodic charge is passed. One Faraday of charge is equivalent to an Avogadro's number of electrons. The Equivalent Weight can be used to calculate the corrosion rate. You may disregard this parameter if absolute corrosion rates are not required.

 

To calculate the equivalent weight for an alloy, you need to know:

•The composition of the metal sample, expressed in mole fractions.

•The atomic weight AW of each alloy constituent.

•The number of electrons n, lost by each component of the sample as it oxidizes.

 

Suppose we have a 40:60 mole-percent Cu:Ni alloy dissolving in an acidic Cl–-solution. Ni forms Ni2+ as it dissolves and Cu forms a Cu+ complex. The equivalent weight is

 

 

 

The above discussion assumes that the sample oxidizes without changing the composition. This is not always the case. The de-zincification of brass is a well-known problem, where under special conditions brass becomes enriched in Cu as it corrodes. If selective dissolution occurs in your system, the best way to find the equivalent weight may be to measure the concentration of your corrosion products in the solution.

g/equivalent

Conditioning

You may condition the electrode as the first step of the experiment, e.g., to remove an oxide film from the electrode or to grow one. Conditioning ensures that the metal sample has a known surface state at the start of the experiment. This step is done potentiostatically for a set amount of time.

 

Turn the conditioning phase On or Off with the Conditioning switch check box. Conditioning E is the potential applied during the conditioning phase of the experimental sequence. The conditioning potential has an allowed range of ±8 V. The resolution is 0.25 mV. If you have iR-compensation enabled for the data acquisition phase of your experiment, it is also turned on during conditioning. Conditioning E is always specified vs. Eref, because the open-circuit potential is not measured until after Conditioning is completed.

 

The Conditioning Time is the length of time that the sample is potentiostatted at Conditioning E. The minimum time is 1 s and the maximum time is 400000 s (more than 4 days). Below 1000 seconds, the time resolution is 1 s. Between 1000 and 10000 seconds, the resolution is 10 s, and 100 s above 10000 seconds.

seconds (s), volts (V)

Initial Delay

Use the Initial Delay parameter to tell the system your definition of a stable potential and when to begin the actual measurement. If the absolute value of the Eoc drift-rate falls below the Stability parameter, the Initial Delay phase ends immediately and the experiment begins, disregarding the Time parameter. The drift rate can never fall below zero, so entering a Stability value of zero ensures that the Initial Delay will not end prematurely. A typical value is 0.05 mV/s. The lower limit of the Stability parameter is set by your patience. For example, a stability of 0.01 mV/s indicates a drift of less than 1 mV within 100 seconds.

seconds (s), mV/s

IR Comp

Choose to turn iR-compensation either On or Off. Turning on IR Comp causes the applied potential to be adjusted for the estimated iR-drop.

 

Gamry potentiostats are able to estimate uncompensated voltage-drop caused by cell resistance. They do so by performing a current-interrupt experiment after every data point.

 

Equil. Time

The duration during which the cell remains at the initial potential, with the cell turned on before data acquisition begins. It allows the system to equilibrate before the actual experiment starts. No data are recorded during this step.

 

Gamry tip bulb

The input parameter must be an integer. If equilibration is not required, set this value to 0.

seconds (s)

 

Related Topics

Multiplexed Cyclic Polarization