Corrosion Mechanism Indicator; Understanding Cmi Values; Table 17 Pitting Factor Values - Honeywell XYR 6000 User Manual

7. Corrosion measurements

7.5. Corrosion mechanism indicator

General corrosion processes typically have low levels of intrinsic noise, with the ratio of noise to the
general corrosion current typically being ≤1% (Pitting Factor ≤0.01). With the onset of instability (pit
initiation), localized corrosion occurrence leads to increasingly higher levels of current noise with respect
to the general corrosion current such that the Pitting Factor may reach a value of 1. The Pitting Factor can
be viewed as the probability that the corrosion mechanism is localized.
Spontaneous changes in the environment may also cause the instantaneous value of the Pitting Factor to
approach a value of 1 in the short term; however, for localized corrosion, the Pitting Factor will remain
unstable and secondary evidence may be observed in terms of the overall stabilities of both the general
corrosion rate estimate and the B value.
XYR 6000 uses electrochemical noise (ECN) to calculate the Pitting Factor.
A useful analogy to explain the difference between general corrosion and localized corrosion (Pitting
Factor) is that of a flashlight with its beam constantly ON (general corrosion) and one that is flickering
(localized corrosion).
PF Value Comments
0.1 or higher
0.01 to 0.1
0.01 or lower
7.5 Corrosion mechanism indicator
The metallic corroding interface is complex and dynamic. The general corrosion rate, the B value, and the
Pitting Factor all help to characterize the Faradaic corrosion processes (current flow that is the result of
electrochemical process) quite thoroughly. However, in order to be more complete in the analysis of the
electrochemical response there is at least one more factor which needs to be taken into account.
During the measurement of the low frequency impedance, a reactive, phase shifted component of the
current response may be detected. This is a consequence of the physical nature of the metal/environment
(electrolyte) interface, and may reflect mechanistic properties such as the presence of films, film formation
and surface adsorption processes.
The values obtained are likely to be characteristic of a particular system being studied. For example sulfide
filming may cause the reactance to become more positive, whereas adsorption processes may cause the
values to go negative. The absolute values obtained may provide the corrosion expert with extra
knowledge regarding the corrosion behavior of any particular system.

Understanding CMI values

The CMI is a qualitative indicator of whether a surface film is present or not. If there is no film and only
corrosion is present, the CMI will have an intermediate value. Inorganic scale, or thick passive oxide films
with little or no conductivity, will show a low CMI value.
Analysis of the Corrosion Mechanism Indicator is shown in Table 18.
42 OneWireless XYR 6000 SmartCET Corrosion Transmitter User's Manual

Table 17 Pitting Factor Values

Pitting/localized corrosion – initiation (Note: check corrosion rate value; if very low, PF
could be misleading).
Intermediate level; general corrosion but check PF does not increase above 0.1.
General corrosion.
Revision 3
6/24/08