Electrochemical Frequency Modulation - Setup Parameters

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Electrochemical Frequency Modulation - Setup Parameters

Description

 

1 -  Electrochemical Frequency Modulation

 

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.

 

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.

 

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.

 

Base Freq.

The repeat time of the EFM waveform applied to the cell. The two frequencies simultaneously applied to the cell are:

 

 

hertz (Hz)

Multiplier A

Multiplier A and Multiplier B define the EFM waveform applied to the cell. The two frequencies simultaneously applied to the cell are:

 

 

 

Multiplier B

 

Amplitude

The amplitude of each of the two sine waves. Keep in mind that the total EFM waveform amplitude typically ranges between 1 and 2 times of the selected value due to the principle of superposition.

 

Choose an amplitude that is small compared to the Tafel constants but large enough to give a reasonable signal. For many systems, 10 to 40 mV is a good starting point.

mV

DC Voltage

The constant potential applied to the cell throughout the EFM scan. For most EFM experiments, the DC Voltage is 0 V versus the open-circuit potential

volts (V)

Cycles

The number of times the scan is repeated during the experiment.

 

The Number of Cycles and the Base Frequency determine the duration of an EFM experiment. The greater the cycle number, the longer the experiment takes, but the resolution between the peaks of the intermodulation spectrum increases. Although you may enter any number of cycles, you get better results if Number of Cycles has a power of two: 2, 4, 8, ... 128, with a maximum number of 255. The first repetition of the Base Frequency (the first cycle) is not used for data analysis. The duration of an EFM experiment can be calculated as follows:

 

 

I/E Range Mode

Select the current range mode and choose between Auto mode or Fixed mode.

 

Because the EFM waveform is a complicated AC waveform, better results are generally obtained in the Fixed mode. In Fixed mode, the current scale is selected based on Max Current. In Auto mode, Max Current is used as an initial guess for the proper scale, but subsequent decisions are based on the value of the most recently measured current.

 

Max Current

Max Current controls the current measurement range if I/E Range is set to Fixed mode. If I/E Range is set to Auto mode, Max Current specifies the maximum expected starting current.

 

The software adjusts the current range based on the Max Current input. In order to use the most sensitive range that will not overload, the software chooses the current range based on a value that is 89% of the full-scale current range.

 

Gamry tip bulb

The Max Current parameter is a current, not a current density.

 

If your current data look very choppy and stepped, the problem could be a poorly selected current range. If you enter a Max Current value of 10 mA and the maximum current in your sweep is only 100 mA, you are only using 1/100th of the potentiostat's A/D converter range. The result is a significant quantization error. Rerun the test entering a smaller Max Current value.

 

If your current data show perfectly flat and horizontal regions, the current has most likely overloaded the potentiostat's current-measurement circuits. Check that the value that you entered for the Max Current parameter is larger than the actual measured cell current. Try rerunning the test with a larger value for the Max Current.

mA

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)

Init. Delay

The Initial Delay phase of the experiment is the first step to occur in the experimental sequence. This phase of the experiment stabilizes the open-circuit potential of the sample prior to any applied signal and measures that open-circuit potential.

 

The Initial Delay is turned On or Off via the check box in the Setup dialog. The Initial Delay Time parameter is the time that the sample is held at the open-circuit potential prior to the scan. The delay may stop before the Initial Delay Time if the Stability criterion for Eoc is met. The minimum Delay Time 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.

 

In many cases, you really do not want to set a delay for a fixed time, but you may want to a delay until Eoc stops drifting. The Stability parameter allows you to set a drift-rate that you feel represents a stable Eoc. If the absolute value of the drift-rate falls below the Stability parameter, the Initial Delay phase of the experiment ends immediately, disregarding the programmed Initial Delay Time. Enter a Stability setting of zero to ensure that the delay will last for the full Initial Delay Time. A typical value is 0.05 mV/s. The upper limit of this parameter is 8 V/s, well above the range of practical stabilities with real cells, while the lower limit is set by your patience. A stability of 0.01 mV/s indicates a drift of less than 1 mV within 100 seconds. The software will always take data long enough to resolve a 1 mV change in the potential at the requested drift rate.

 

No open-circuit voltage measurement occurs if the initial delay is turned off. In this case, the open-circuit voltage defaults to 0.0 V.

seconds (s), mV/s

Corrosion Type

The calculation of the corrosion current from EFM data depends on the mechanism of the corrosion process. The three choices are Active, Diffusion control, and Passive film control. The choice here only determines the equation for the run-time display. You may change your choice later in the Echem Analyst 2™ and recalculate the corrosion rate without having to rerun the experiment.