Corrosion Measurements; Overview; General Corrosion Rate; Working Method Summary - Honeywell XYR 6000 User Manual

7.1

Overview

The XYR 6000 SmartCET corrosion transmitter outputs four corrosion measurements.
General Corrosion Rate - average or general corrosion rate, and is generally expressed in mils per year
•
(mpy) or millimeters per year (mmpy).
Pitting Factor - dimensionless number that indicates the presence of a pitting (localization) corrosion
•
environment.
B value - expressed in millivolts per decade, and is commonly also known as the Stern Geary constant.
•
Corrosion Mechanism Indicator – indicator representing health of the probe in regard to fouling or
•
wear.
The values are all updated every 30 seconds, which is the total measurement cycle time of the instrument.
The values for the General Corrosion Rate, the Pitting Factor and the Corrosion Mechanism Indicator are
set to output the most recent values.
7.2

General corrosion rate

The Linear Polarization Resistance (LPR) technique is used to calculate the General Corrosion Rate. This
calculation is usually the prime variable of interest since it reflects the overall rate of metallic corrosion.
Corrosion may be directly related to operational parameters such as temperatures, flow, chemical
composition, etc.
The XYR 6000 SmartCET uses three electrodes that are referred to as the working, counter and reference
electrodes. A low frequency sinusoidal voltage excitation is applied to the working electrode with respect
to the reference electrode, and the current is measured and analyzed (on the counter electrode)
synchronously with the applied signal.
Given a sinusoidal pattern, the working electrode becomes positively charged and then negatively charged
(in other words, polarized positively and negatively). It is a DC voltage applied in a sinusoidal pattern and
resembles an AC pattern. The peak-to-peak value of the sinusoidal wave is 50mV.
Strictly, this is a measurement of the real part of the low frequency impedance of the working electrode.
This method of analysis is selected due to its superior noise rejection, which is particularly useful when
studying corroding systems since they exhibit varying degrees of intrinsic noise. The result is equivalent to
measuring the linear polarization resistance of the working electrode. With this measurement, the corrosion
current (hence, the corrosion rate) is inversely proportional to the polarization resistance.
This measurement also employs the Stern-Geary approximation where the Stern-Geary constant (or B
value) is the proportionality constant. In practice, with no prior knowledge of the system, the "default"
value of B for this type of measurement is typically chosen to be in the range 25 to 30 mV; in reality, the
value of B is system-dependent.
Use of the default B value may result in the absolute corrosion rate being somewhat in error, but in some
instances, it is the general trend of the corrosion rate that could be of interest instead of the absolute value.

Working method summary

There are three electrodes in use, which are designated working electrode (WE), counter electrode (CE)
and reference electrode (RE). A sinusoidal DC voltage is applied on the WE (voltage is varied).
Revision 3 OneWireless XYR 6000 SmartCET Corrosion Transmitter User's Manual
6/24/08

7. Corrosion measurements

7. Corrosion measurements
7.1. Overview
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