Erosion corrosion
Erosion corrosion also known as flow-enhanced corrosion or
impingement attack takes place in flowing systems where turbulence occurs and
solid particles i.e. sand is associated, typically in pipe bends (elbows), flow
constrictions, and other internals, pipe fittings that alter flow direction or
velocity. The mechanism for this type of corrosion
is the continual flow of water, which removes any protective film, such as
corrosion inhibitor or metal oxide from the metal surface. The exposed surface
quickly corroded and the resulting oxide is in turn eroded away.
Erosion is a complex issue;
dictated by fluid phase, flow regime, density, solids content, solids hardness,
solids geometry and flow-enhanced corrosivity (erosion-corrosion). Erosion
corrosion involves a synergistic relationship between erosion and corrosion,
where the combined rate of metal loss can be higher than the sum of the rates
expected for erosion and corrosion separately. A related corrosion mechanism is
flow-induced corrosion in which the fluid is the erosive component. The
corrosion rate of carbon steels increases with increasing mass transfer rate
and wall shear stress.
Erosion corrosion can lead to rapid failure. The primary method of avoiding
erosion and erosion-corrosion is to design facilities with velocities below the
limit given by API Recommended Practice 14E. Although this limit is known to be
conservative in most cases, it provides a reliable design that is tolerant of
design changes and unexpectedly erosive flows (e.g. from slugs of solids).
However, it is only applicable to straight pipe and does not consider geometry
or entrapped solids.
Sand production has been
considered in design basis. Presence of solid or sand in the system is
considered as a major threat if not properly mitigated. It may cause erosion
corrosion and plugging the facilities that will reduce the mechanical integrity
of the facilities.
Material selection is affected
by sand and harder material for valve sheets is provided to minimize erosion.
The design of piping should also taking into consideration to evaluate the
possibility of using thicker pipe or additional corrosion allowance, using long
radius bends, etc. Erosion resistant
materials, such as cermet’s (tungsten carbides with metallic binder) or
ceramics can be applied as coatings on local parts of the system being most
exposed to erosion.
The threat also can be reduced
by installing sand jetting system in production separator and sand handling
facilities downstream of production separator so that the sand can be prevented
from entering to all systems. Unlike
copper alloys, stainless steels generally offer very good resistance to impingement
attack.
Higher surface hardness of 22Cr
or 25Cr DSS compared to austenitic stainless steels (i.e. SS 316L) can in some
cases be an advantage.
Sand monitoring should be performed in order to control the
sand production effectively. Inspection of the most exposed components to
erosion is recommended to be performed regularly to control the material
degradation.