Page 53 - Reliability and Maintainability of In service Pipelines

42 Reliability and Maintainability of In-Service Pipelines

(2000) proposed a two-phase corrosion model where the first phase is a rapid
exponential pit growth and the second is a slow linear growth. There are debates
in the research community as to whether the corrosion rate can be assumed linear
or otherwise. A widely accepted model of corrosion as measured by the depth of
corrosion pit is of a power law which was first postulated for atmospheric corro-
sion by Kucera and Mattsson (1987) and can be expressed as follows:
a 5 kt n ð1:21Þ

where t is exposure time and k and n are empirical constants largely determined
from experiments and/or field data.
For underground corrosion, the constants are typically functions of localized
conditions, including soil type, the availability of oxygen and moisture, and prop-
erties of pipeline material. In many cases it may be possible to use past experi-
ence to derive estimates for the two constants in Eq. (1.21), but with somewhat
more effort than would be necessary to estimate a constant corrosion rate as used
conventionally (Ahammed and Melchers, 1997).
Rajani et al. (2000) proposed a two-phase corrosion model to accommodate
this self-inhibiting process:
A 5 αt 1 βð1 2 e 2λt Þ ð1:22Þ
where α; β; and λ are constant parameters.
In the first phase of the above equation there is a rapid exponential pit growth
and in the second phase there is a slow linear growth. This model was developed
based on a data set that lacked sufficient points in the early exposure times.
Therefore prediction of pit depth in the first 15 20 years of pipe life should be
considered highly uncertain when Eq. (1.22) is used.
An example of field data which shows the rate of internal and external corro-
sion for cast iron pipes has been illustrated in Fig. 1.7 (Marshall, 2001). As it can
be concluded from this data, external corrosion has a higher rate than internal cor-
rosion, especially during early stages. In Fig. 1.8, a sample of a cast iron pipe
taken from London water mains from the Victorian era (i.e., 1800 1900) also
shows the severity of external corrosion compared with internal corrosion.

1.6.3 DETERIORATION OF PLASTIC PIPES

Due to metals holding a higher susceptibility to corrosion, plastic piping can be
particularly useful in environments that are corrosive, including for pipes that
experience a high occurrence of temperature fluctuations.
These fluctuations create an “evaporative” action, causing vapor to form
inside pipes. This results in constant moisture inside the pipe, providing