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Introduction
The Pyxis CR-301 corrosion sensor measures the metal corrosion rate in an aqueous environment based on
the principle of linear polarization resistance (LPR). A small polarization DC voltage is applied to two test
metal electrodes and the resulting current is measured by the sensor. The polarization resistance value is
calculated from the measured current and the applied polarization voltage. The metal corrosion rate in the
unit of thousandths of inch per year (or mils per year, MPY) is then determined as:
M P Y
=
B
/
Rp
(1)
where Rp is the polarization resistance (LPR) and B the proportional constant.
To calculate Rp originated from the electrochemical reaction at the metal water interface, the CR-301 sensor
subtracts the solution resistance Rs due to the test solution conductivity from the total resistance measured
between the two test electrodes. The CR-301 sensor accurately measures the test solution conductivity us-
ing the two test electrodes and the bipolar pulse technique, which has been successfully used in other Pyxis
conductivity sensors and handheld meters.
A challenge of using the LPR method to measure corrosion rate below 0.01 MPY is to measure electric
current in the range of pico and nano-amperes. The CR-301 sensor adapts a range of techniques that are
practiced in our fluorometers where low pico ampere current is measured. These techniques include elec-
tromagnetic interference shielding, special analog circuit designs and digital signal processing. The propor-
tional constant B has a theoretical value for a given metal type and size. The CR-301 assumes B equal to
1.24 MPY∙Ω for a 5 cm
2
mild steel electrode, which is a typical value that has been used in many LPR cor-
rosion researches. This proportional constant may be also referred to as the alloy factor of the test metal
and normalized to 1.0 for the steel electrode for convenience. It can be adjusted by the user to account for
variations in a real application environment.
To determine the corrosion situation of real process equipment is not a simple matter. The corrosion rates
for various metal surfaces contacting aqueous fluids in a process depends on many parameters, including
the corrosivity pertaining to the chemistry of the aqueous fluid, physical parameters such as temperature
and the velocity of the fluid, and the metallurgical composition of the process equipment itself. Because
of this, the corrosion rate measured by the CR-301 should not be used alone to predict or assess the real
corrosion rate of the process equipment. Nevertheless, the science of using the LPR technique to quantify
the corrosion rate has been well established. The corrosion rate measured by the CR-301 sensor can be
used to understand the corrosivity trend of the aqueous fluid over a period and its correlation to changes
in the process parameters.
Measuring corrosion rate using a weight loss corrosion coupon is still widely practiced in many industry
applications. LPR is an instantaneous method compared with the corrosion coupon method. It can indi-
cate a change in corrosion rate in a time scale of a few minutes. In theory, a time averaged corrosion rate
measured by the Pyxis LPR sensor should agree with that from the weight loss coupon method if both are
practiced according to the common practices known in the industry. For monitoring mild steel corrosion
rate in an industrial cooling water system, one could also treat the LPR metal electrode itself as the weight
loss corrosion coupon and compare the corrosion rate calculated from the weight loss of the electrode to
the averaged LPR corrosion rate in order to calibrate the LPR corrosion rate by adjusting the alloy factor.
The electrochemical noise measurement has long been used to quantify the localized corrosion or pitting
corrosion rate. The CR-301 sensor measures the short circuit electric current flowing through the electrodes
CR-301 User Manual
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