© 2006 by Taylor & Francis Group, LLC

Corrosion Testing — Practice 147
it. It can be a powerful research tool and has been used, for example, to characterize
the anodic oxide films on platinum that resulted from different anodizing methods
[34]. It is also extremely useful for confirming theories of mechanisms in cases
where the presence or absence of one or more elements is significant.
8.2.6.5 Electrochemical Noise Measurement
Electrochemical noise measurement (ENM) has attracted attention since it was first
applied to anticorrosion coatings in the late 1980s [35, 36]. The noise consists of
fluctuations in the current or potential that occur during the course of corrosion. The
underlying idea is that these fluctuations in current or potential are not entirely
random. An unavoidable minimum noise associated with current flow will always
be random. However, if this minimum can be predicted for an electrochemical
reaction, then analysis of the remainder of the noise may yield information about
other processes, such as pitting corrosion, mass transport fluctuations, and the formation
of bubbles (i.e., hydrogen formed at the cathode).
The theoretical treatment of electrochemical noise is not complete. There does
not yet seem to be consensus on which signal analysis techniques are most useful.
It is fairly clear, however, that understanding of ENM requires a good working
knowledge of statistics; anyone setting out to master the technique must steel themselves
to hear of kurtosis, skewness, and block averages rather frequently.
In the future, this technique may become a standard research tool for localized
corrosion processes that give strong electrochemical noise signals, such as microbial
corrosion and pitting corrosion.
8.3 CALCULATING AMOUNT OF ACCELERATION
AND CORRELATIONS
Accelerated tests are most commonly used in one of two ways:
1. To compare or rank a series of samples in order to screen out unsuitable
coatings or substrates (or conversely, in order to find the most applicable
ones)
2. To predict whether a coating/substrate combination will give satisfactory
performance in the field — and for how long
This requires that it be possible to calculate both the amount of acceleration the test
causes and how uniform this amount of acceleration is over a range of substrates
and coatings.
In order to be useful in comparing different coating systems or substrates, an
accelerated test must cause even acceleration of the corrosion process among all the
samples being tested. Different paint types have different corrosion-protection mechanisms;
therefore, accentuating one or more stresses — such as heat or wet time —
can be expected to produce different amounts of acceleration of corrosion among a
group of coatings. The same holds true for substrates. As the stress or stresses are
further accentuated — higher temperatures, more wet time, more salt, more UV light
© 2006 by Taylor & Francis Group, LLC