Page 52 - Reliability and Maintainability of In service Pipelines

Introduction 41

The passive layer in stainless steel is susceptible to attacks from chlorides,
caustic acid, and polythionic acid, which generally occur at localized points.
Stainless steels consisting of a higher level of alloy material, such as the combina-
tion of nickel, have a higher susceptibility to corrosion in chloride-rich environ-
ments. According to a study conducted by Khalifeh et al. (2017), a high incidence
of failures was observed in equipment derived from these materials.
However, for SCC to occur, the presence of tensile stresses is required, which
can be either applied, thermal, or residual and it is usually the combined effect
that accelerates SCC.
For SCC to take place solely as a result of applied stresses, a very high magni-
tude of these stresses is needed. On the contrary the effect of residual stresses
was observed to cause a higher susceptibility to SCC.
Residual stresses are defined by the tensile or compressive force of a material
in the absence of any other external loading. The process of welding and mechan-
ical forces in the production of these materials can cause residual stress. The
expansion and contraction action caused by temperature variations that occur in
welding are what cause residual stress. The mechanical forces which occur during
the fabrication process of these materials cause residual stress by the bending
action of steel during pipe formation. This process can also cause localized defor-
mations, which can lead to an early onset of SCC.
In sensitized stainless steels, the formation of chromium carbides also causes
overheating, and high temperatures are known to feed the growth rate of crack
formation. As a result, these cracks further allow chlorides and other caustic
agents to leak straight through and allow SCC.

1.6.2.1 Corrosion Rate in Ferrous Pipes

Corrosion pits have a variety of shapes with characteristic depths, diameters (or
widths), and lengths. They can develop randomly along any segment of pipe and
tend to grow with time at a rate that depends on environmental conditions in the
immediate vicinity of the pipeline (Rajani and Makar, 2000).
The corrosion rate of in-service ferrous pipes is believed to be higher in the
beginning and then decreases over time as corrosion appears to be a self-
inhibiting process (Shreir et al., 1994). Furthermore, due to the variation of ser-
vice environment it is rare that the corrosion occurs uniformly along the pipe but
is more likely locally in the form of a corrosion pit.
A number of models for corrosion of ferrous pipes have been proposed to esti-
mate the depth of corrosion pit (e.g., Sheikh et al., 1990; Ahammed and
Melchers, 1997; Kucera and Mattsson, 1987; Rajani et al., 2000; Sadiq et al.,
2004). For example, Sheikh et al. (1990) suggested a linear model for corrosion
growth in predicting the strength of cast iron pipes. A decade later, Rajani et al.