Page 402 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

380 CONSEQUENCES OF CORROSION

An additional 50% volume of inhibitor is added to compensate for the loss of
inhibitor because of adsorption on solids.
In continuous treatment, Nowcorr 800 is added to the drilling fluid or to the acid
blends. The concentration of the inhibitor depends on downhole conditions, temper-
ature, total pressure, and partial pressure of H S.
2
In the case of underbalanced acid wash treatments in sour environments, Nowcorr
800 is used in addition to the acid corrosion inhibitor such as Nowsco’s AI-275 and
CI-30.

5.3.7.4.17 Environmental Cracking Environmental cracking refers to corrosion
cracking caused by a combination of conditions that can result in SCC, corrosion
fatigue, and HE. Stresses that cause environmental cracking arise from cold work,
welding, grinding, thermal treatment or may be externally applied stresses during
service and must be tensile to be damaging.
Stress variables: mean stress, maximum stress, minimum stress, constant
load/constant strain, strain rate, plane stress/plane strain, biaxial, cyclic frequency
wave shape.
The origin of stress can be: (i) intentional or applied stress consisting of quenching,
thermal cycling, thermal expansion, vibration rotation, bolting, dead load, pressure,
residual stress consisting of shearing, punching and cutting, bending, crimping and
riveting, welding, machining, grinding, and products of corrosion reaction.
Stress cells can be present in a single piece of metal where a portion of the metal’s
microstructure possesses more stored strain energy than the rest of the metal. Metal
atoms are at their lowest strain energy state when situated in a regular crystal array.
Applied stresses include cyclic stressing consisting of cyclic frequencies and wave
shapes. Although less well defined, the most important contribution to the modes of
corrosion, such as SCC, are residual stresses. Applied stresses are usually less than
half the yield stress or lower. The residual stresses are usually in the range of yield
stress. Quantifying such residual stresses is often omitted in design with the erroneous
conclusion that such stresses are irrelevant to design and performance.
Accumulation of corrosion products can result in stresses at restricted geometries
where the specific volume of the corrosion product is greater than the corroding metal.
These stresses can cause cracks to initiate and grow. Stresses from expanding corro-
sion products can cause adjacent metals to flow plastically, as occurs in nuclear steam
generators in a process called “denting.” Denting results from the corrosion of the car-
bon steel support plates and the buildup of corrosion product in the crevices between
the tubes and the tube support plates. This process is known as “denting” as, when
seen from the inside of tubes, these deformations seem to produce dents at the tube
sheet locations. Similar stresses from the buildup of steel corrosion products cause the
degradation of reinforced concrete. An equivalent expansion ratio of 3.0–3.2 has been
measured because of the formation of corrosion products on steel bars embedded in
concrete (33).
“Pack rust” is an example of the tremendous forces created by expanding steel cor-
rosion products. The effect of pack rust that developed on an important steel bridge
under repair has been documented (Roberge (16), Fig. 1.10, p. 14). The force of