Failure Analysis
H2S Corrosion
H2S pitting is round based, deep with steep walls and beveled
edges. It is usually small, random, and scattered over the
 entire surface of the rod. A second corrodent generated by
H2S is iron sulfide scale. The surfaces of both the sucker
rod and the pit will be covered with the tightly adhering black
scale. Iron sulfide scale is highly insoluble and cathodic
to steel which tends to accelerate corrosion penetration rates.
A third corroding mechanism is hydrogen embrittlement, which
causes the fracture surface to have a brittle or granular appearance.
A crack initiation point may or may not be visible and a fatigue
portion may not be present on the fracture surface. The shear
tear of a hydrogen embrittlement failure is immediate during
fracture due to the absorption of hydrogen and the loss of
ductility in the steel. Although a relatively weak acid, any
 measurable trace amount of H2S is considered justification
for chemical inhibition programs when any measurable trace
amount of water is also present.
Figure 21 and 22
are examples of H2S
corrosion. The three
rod body samples
on the left in Figure
21 are examples of
localized corrosion
(pitting) and the
two rod body samples
on the right in Figure
21 are examples of
general thinning
corrosion from under-
scale deposit corrosion.
The sample in Figure
22 is an example
of a pin failure
due to hydrogen embrittlement.
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