Chemical Engineering June 2010 - 23

Corrosion Monitoring teChniques
here are several methods of monitoring corrosion, which can be broken down into
two main categories: traditional and automated corrosion monitoring techniques.
Within each of these categories, there are a variety of methods. The most popular
are defined here by Pepperl + Fuchs.
T
Traditional
Traditional corrosion monitoring techniques include the following:
* Mass loss: Often referred to as coupons, mass loss methods incorporate inserting
pieces of metal directly into the process. Coupons are weighed before insertion and
after extraction. Typically, after 90 days they are again weighed and studied to determine
the corrosion rate and type
* Resistance measurement: Similar to coupon measurement, resistance measurements
use wires that are exposed to the process. Resistance is measured and, as the wires
corrode, resistance increases, providing an indication that corrosion is occurring
* Polarization resistance: This technique measures the inhibition of the corrosion process.
This measurement is inversely proportional to the corrosion current
* Acoustic emission: Different types of corrosion emit different sounds. These sounds are
recorded and provide information about the process relative to corrosion
* Ultrasonic examinations: As corrosion occurs, the wall thickness of pipe deteriorates.
Ultrasonic examination of the pipe can determine the remaining wall thickness. This
calculation is based on the time it takes for non-audible acoustic waves to travel back
and forth
Automated methods
Automated corrosion monitoring techniques include:
* Linear polarization (LPR): This technique involves the measurement of the polarization
resistance of a corroding electrode to determine the corrosion current. Since the
voltage-current response of a corroding element tends to be linear over a small range,
determination of the polarization resistance allows the corrosion current to be determined.
The slope of the response, the polarization resistance, is inversely proportional
to the corrosion current, allowing a corrosion rate to be calculated
* Harmonic distortion analysis (HDA): HDA measures the resistance of the corrosive
solution by applying a low frequency sine wave to the measurement current. Using harmonic
analysis, the solution resistance is determined and combined with the polarization
resistance of the LPR method to calculate a more-accurate general-corrosion rate
* Electrochemical noise (ECN): This method evaluates the fluctuation in current and voltage
noise generated at the corroding metal-solution interface. This technique is generally
used to detect non-uniform or localized corrosion.
While some experts suggest that in this limited economy spending on corrosion monitoring
is currently down because it is not an absolute necessity when it comes to getting
process to the barrel or to the customer, ignoring the practice is not advisable.
" Deciding not to monitor corrosion is like deciding not to maintain a car, " says James
Gray, divisional president of Metal Samples Corrosion Monitoring Systems division
of Alabama Specialty Products. " If money is tight, you have to buy gas to make the
car go, but you don't have to change the oil, but you know that sooner or later this is
going to become an expensive problem. That's what happens with corrosion monitoring
if you chose not to do it. Due to a lack of corrosion monitoring, a pipe will become
too thin and an accident will occur and someone will get hurt or something will be
released to the environment. "
To avoid this worst-case scenario, Gray advises selecting the best corrosion monitoring
technique within your budget. " Using coupons is better than using nothing at all if
you can't afford a more proactive system, because by using nothing at all, you're just
prolonging the inevitable, " warns Gray.
❏
ultrasonic sensing technology that attaches
to the outside of the asset and
measures wall thickness.
Rightrax
sensors can be polled periodically, allowing
users to assess not only the
useful life by comparing current values
to minimum thickness limits, but
to establish historical trends.
This ability to trend data provides
several benefits. First users can understand
the rate at which corrosion/
erosion is progressing, allowing better
maintenance planning. Second, users
can correlate data with process data,
providing insight into cause and effect
relationships that can help operators
understand and avoid conditions that
accelerate corrosion/erosion rates and
asset degradation.
Integration provides users with the
ability to display, trend, analyze, plot
and correlate data. Software also permits
import/export functionality for
integration with reporting tools and
programs, as well as automation capability
that can automatically analyze
Distillation and
Rectification
Recovering recyclables
Treating solvents
Producing alcohol
Cleaning waste water
This is just a small choice of examples
for which we can offer you a plant
conception. We will find a way to
design your process with our distillation
plants so that it will be even
more economic.
GEA Process Engineering
GEA Wiegand GmbH
Einsteinstrasse 9-15
76275 Ettlingen, Germany
Telefon: +49 7243 705-0
Telefax: +49 7243 705-330
E-Mail: info.gewi.de@geagroup.com
Internet: www.gea-wiegand.com
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Chemical Engineering June 2010

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