The following flow chart provides an overview of interaction with POLEXPERT.
This tutorial uses the following example to give step-by-step instructions. Alloy 316ss was
one of several alloys being considered as a material of construction for a fluid containing
inorganic anions having a pH of about 4, and being at a temperature of about 40°C. In the
laboratory, an electrode of 316ss was immersed in process fluid for about 18 hours to establish
the corrosion potential that was fairly representative of that expected in the process after
longer exposure. The polarization scan was generated at a scan rate of about 0.5 mV/s to attempt
to balance the need for timely information and the need to avoid artifacts caused by too high of
a scan rate. The current at the point of scan reversal was about 100 microamp/cm2 to minimize
possible artifacts caused by large alteration of the surface. Background information on the
corrosion potential, scan rate, and point of scan reversal are presented later in this tutorial.
This figure shows the resulting polarization scan.
Following is the detailed procedure for using POLEXPERT to make its prediction of the propensity
for general corrosion, crevice corrosion, and pitting for this example. This table shows
the values of the features that are to be used by POLEXPERT.
They are explained in more detail in the section entitled "Features Useful in Interpreting Scan"
as well as when you get to item 7 below. When in POLEXPERT, each of these items has a Help screen
associated with it to help you to determine the appropriate input.
Input Parameter
Value
Repassivation Potential - Corrosion Potential
0.0 volts
Pitting Potential - Corrosion Potential
0.28 volts
Hysteresis
Negative
Passivation Potential
No
Passive current density
0.1 microamp/cm2
Anodic-to-Cathodic Transition
0.0 volts
Detailed Instructions:
POLEXPERT is located at the web site www.argentumsolutions.com.
Enter POLEXPERT. On the left hand side of each page including this one is a
column with the heading "Intelligent Tools". POLEXPERT is the first icon listed. Double
left click on that icon. You are taken to an opening page which overlays the page you are on.
This page contains general information. When you have finished reading this information,
left click on the button labeled "Enter POLEXPERT" at the top or bottom of the page.
This action takes you into the POLEXPERT environment.
If you wish, read the "Overview" and "Instructions" to get a brief introduction
to POLEXPERT and what you are asked to do. If you wish to go immediately to the "Instructions",
click on the word "Instructions". Note that you can change an entry at any time before
running the tool.
Enter your name, company affiliation, and email address. An example using
Argentum Solutions, Inc. is shown below. That information is only for internal identification
and will not be shared with anyone. It will aid in data retrieval in case you make an inquiry
about the results.
In POLEXPERT:
Step 1
Please enter the following information:
First Name:
Middle Initial:
Last Name:
Affiliation or Company Name:
Email Address:
Fill in the alloy and any additional information about it you desire. For this
example 316ss is entered. Below this entry you are asked to choose which of four polarization
scans most closely resembles the generated scan. This information is used as part of a
consistency check among the scan appearance, electrode appearance, and prediction from POLEXPERT.
In this case, you would click on the figure showing "Polarization Scan for Localized Corrosion".
You can expand the picture by clicking on the thumbnail. The pictures also can be used to help
identify the location on the scan of some of the inputs for POLEXPERT. You can expand the figure
by clicking on the thumbnail. If the appearance of your scan does not resemble any of the ones
shown, select the option "Not Shown". The only effect that choice will have is to prevent a
consistency check between the choice of the scan appearance and the prediction by POLEXPERT.
It will not affect the prediction by POLEXPERT.
In POLEXPERT:
Step 2
What alloy are you using? Please input appropriate designation
information. Format can be generic (examples: alloy
825, 316, Al 2024, Ti Gr2), the UNS number (examples: UNS N08825, UNS
S31600, UNS A92024, UNS R50400), or the manufacturer's identification
(See Woldman's "Engineering Alloys", 9th edition).
Please identify which of the following four polarization scans most closely resembles your scan.
If your scan does not resemble those shown, please select "Not Shown"
Click on the image for a full size image.
Not Shown
Next you need to examine the electrode used in the test and select the
condition that most resembles the appearance after the experiment. This selection provides
another consistency check, this time between the prediction and the appearance of the
electrode. In this case, the electrode showed localized corrosion where it contacted the
holder so the choice of "localized corrosion" was selected. If you select "Help on this
topic", you are provided with some guidelines. If the appearance is other than the
descriptions is listed, type in the appearance. Selecting other only prevents the
consistency check from being made but still allows the prediction to be made.
In POLEXPERT:
Step 3
Please examine your electrode for corrosion and identify the description most constent with your electrode
appearance. If two types of attack are present, pick the most sever of the two. This answer will be compared
to the type of polizartion scan selected in Step 2 to provide an initial consistency check. The answers
will also be compared with the prediction made by POLEXPERT.
Examine the electrode carefully. Enter the best description shown.
Help on this topic
No Attack
Localized Corrosion
Discoloration
Other, please specify:
At this point you are ready to enter the parameters that will be used as inputs
to the artificial neural network in POLEXPERT. The values used for this example have been
entered in the spaces below. Note that all voltages have the units of "volts" and the current
density has the units of microamp/cm2. If you need additional information, a help window can
be opened when you select "Help on this topic" to provide more details. You can also scroll up
to Step 2 and expand the thumbnails to get more information on selection of the variables for
different types of scans. All of the information must be filled in. Since you are trying to
make predictions about long term behavior, the corrosion potential should arise from the
surface conditions that might exist after longer exposure. This requirement imples that
the electrode be immersed at a minimum for about 12 hours prior to generating the polarization
scan especially for self-passivating type of alloys or alloys that are being passivated by
the environment.
In POLEXPERT:
Step 4
Enter the Repassivation Potential minus the corrosion potential.
( Erepass - Ecorr ) VoltsHelp on this topic
Enter the Pitting Potential minus the corrosion potential.
( Epit - Ecorr )
If there is no pitting potential, enter 10 for the voltage. VoltsHelp on this topic
Is there hysteresis between the forward and reverse portions of the polarization scan?
Help on this topic
Negative
None
Positive
What is the Anodic to Cathodic transition potential minus the corrosion potential?
( Ea-to-c - Ecorr ) VoltsHelp on this topic
At this point, you are ready to run POLEXPERT. If your inputs are as you want
them, click on the button labeled "Run POLEXPERT" and the prediction will emerge on the next
screen. If you want to change one or several items, you can retype the entry or click on a
different selection depending on the step above. Only if you want to reenter ALL of your inputs
should you click on the button labeled "Start Over".
In POLEXPERT:
Step 5
After you have clicked on "Run POLEXPERT" a new screen appears which contains
the prediction and its consistency with the choices of the scan and electrode appearance.
In this case, crevice corrosion is predicted. The wording also provides some additional
information in terms of watch-outs and possible additional testing the may be suggested from
experience. Farther down, the consistency check among the prediction, the scan appearance,
and the electrode appearance is provided. In this case, crevice corrosion is predicted
in agreement with the corrosion observed on coupons placed in the process. Following the predictions,
you are asked to submit your comments. Your name, affiliation, and email
address information will already appear based on your original input in Step 1. You are asked
to fill in the space with any comments and click on the “Submit” button. This information is
optional but feedback would be appreciated especially if the prediction is contrary to your
experience. You can return to any of the pages on this web site by clicking the appropriate
tab.
In POLEXPERT:
This interactive program is being provided for test purposes only. Under NO circumstances should any materials decisions be made based on the output you receive.
CREVICE CORROSION PREDICTED
Risk of localized corrosion in the form of crevice corrosion is present. Such areas as close proximity of surfaces, areas under deposits, and metal-gasket interfaces may be prone to such attack. Longer term immersion tests with, for example, artificial crevice formers, are implied to confirm crevice corrosion prediction.
Though general corrosion is not predicted, the general corrosion rate may be at most a contamination rate. If metal ion contamination is important, corrosion rate should be checked by alternative experimental methods.
Your initial choice of the type of pattern of the polarization scan, the appearance of the electrode, and the corrosion that is predicted from the polarization scan by POLEXPERT are consistent with each other.
David C. Silverman, Ph.D. - Primary Consultant E-Mail:dcsilverman@argentumsolutions.com Phone: 314-576-3586 Fax: 314-754-9825 Address: The Argentum House
14314 Strawbridge Ct.
Chesterfield, MO 63017
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