B Value; Table 15 Corrosion Rate And Environment Characterization - Honeywell XYR 6000 User Manual

7. Corrosion measurements

7.3. B value

In turn, the current response is measured between the CE and WE. The ratio of voltage to current provides
the polarization resistance. The polarization resistance is not a true resistance in the traditional sense, but
can be treated as such in describing the LPR technology.
The corrosion current is inversely proportional to the polarization resistance. How does an electrical
model represent a corrosion process? What makes corrosion look like an electrical system?
Corrosion comprises an anodic process and a cathodic process, i.e. electrochemical processes that occur at
anodic and cathodic sites on the metal surface. When corrosion is occurring, there is an increase of ionic
flow between the anodic and cathodic sites (i.e. Faradaic process). A non-corrosive system would not
exhibit any ionic flow.
Table 15 shows the relationship between corrosion rate, environment characterization and the
recommendation for getting accurate General Corrosion Rate measurements.
Corrosion Rate Environment
>200 mpy Highly conductive, highly
corrosive
1-200mpy Average corrosion rate
0.01-1mpy Low conductivity or passive
system
<0.01mpy Extreme passivity or low
conductivity (for example,
organic medium)
7.3 B value
The B value represents a correction factor 'constant' that is determined by the mechanism and kinetics of
the corrosion process. In a dynamic process, research has shown that the B value is not constant. For
example, the B value for a sour system with a microbiological influence on corrosion activity could be
4mV. The average "industry-accepted" default B value is typically between 25 and 30mV. Houston tap
water gives a B value of 15mV. A severely scaled system (i.e. inorganic scale deposits on the metal
surface) would show a B value of around 100 mV.
By evaluation of the non-linearities in the current response from the LPR measurement, it is possible to
determine a B value for the system being studied. This involves the analysis of the higher order harmonic
content of the current response, and computation of a value of B for the system being studied.
With knowledge of the B value it is possible to refine the LPR-generated corrosion rate estimate, since the
uncertainty regarding the standard (default) B value is removed. The B value is directly related to the
mechanistic properties of the component anodic and cathodic corrosion processes.
The anodic process is essentially the metal oxidation and the cathodic process is, for example, the oxygen
reduction or hydrogen evolution. These are essentially non-linear processes, and the current will typically
(but not always) have a logarithmic dependence on the applied voltage.
The B value is a composite of the individual anodic and cathodic Tafel slopes.
40 OneWireless XYR 6000 SmartCET Corrosion Transmitter User's Manual

Table 15 Corrosion Rate and Environment Characterization

Comments
This could be at upper level of XYR 6000 accuracy
range. If used in this environment, electrodes with small
area should be used (for example, 1cm2).
Use correct probe type according to process application.
Electrodes with large area should be used (for example,
10cm2).
This could be at lower level of XYR 6000 accuracy
range. If used in this environment, electrodes with large
area should be used (for example, 10cm2).
Revision 3
6/24/08