Failure Analysis
Oxygen Enhanced Corrosion
 Oxygen enhanced corrosion will be most prevalent on couplings,
with a few instances found on rod upsets. Oxygen enhanced corrosion
is rarely seen on the rod body. In fact, aggressive oxygen
enhanced corrosion can erode couplings without harming the
sucker rods on either side. The rate of oxygen enhanced corrosion
is directly proportional to the dissolved oxygen concentration,
chloride content of the produced water and/or presence of other
acid gases. Dissolved oxygen can cause severe corrosion at
extremely low concentrations and evaporate large amounts of
metal. Pitting is usually shallow, flat-bottomed, and broad-based
with the tendency of one pit to combine with another. Pit shape
characteristics may include sharp edges and steep sides if
 accompanied by CO2 or broad, smooth craters with beveled edges
if accompanied by H2S. Corrosion rates increase with increased
concentrations of dissolved oxygen.
Figures 24 and 25
are examples of oxygen
enhanced corrosion.
The coupling sample
on the left in Figure
24 is an example
of the effects of
oxygen enhanced CO2
corrosion (left),
H2S corrosion (middle),
and chloride corrosion
(right) while the
rod samples in Figure
25 show the effects
of oxygen enhanced
CO2 corrosion near
the upset (left)
and CO2 corrosion
on the rod body (right).
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