API RP 571 3.15 Caustic Stress
Corrosion Cracking Exam Questions
and Answers (Latest Update 2026)
3.15.1 Description of Damage -
correct answer ✅Caustic SCC is characterized by surface-initiated
cracks that occur in piping and equipment exposed to caustic
(alkaline hydroxide solutions) at elevated temperature, primarily
adjacent to non-PWHT'd welds. It is a form of ASCC. The
temperature above which caustic SCC occurs depends on the
concentration of the caustic solution.
3.15.2 Affected Materials -
correct answer ✅Carbon steel, low-alloy steels, and 300 series SS
are susceptible. Duplex stainless steels are also susceptible but
have shown improved resistance compared to the 300 series SS.
Nickel-based alloys are more resistant.
3.15.3 Critical Factors -
correct answer ✅a) Susceptibility to caustic SCC in caustic soda
(NaOH) and caustic potash (KOH) solutions is a function of caustic
strength, metal temperature, and stress level.
b) Increasing caustic concentration and increasing temperature
increase the likelihood and rate of cracking. Conditions likely to
result in cracking have been established through plant experience
and are presented in (Figure 3-15-1)
, API RP 571 3.15 Caustic Stress
Corrosion Cracking Exam Questions
and Answers (Latest Update 2026)
c) Cracking can occur at low caustic levels if a concentrating
mechanism is present. In such cases, caustic concentrations of
50ppm to 100ppm are sufficient to cause cracking.
d) Residual stresses from welding (in non-stress-relieved welds) or
from cold working (such as bending and forming) will cause
cracking, and these high residual stresses are what typically lead to
caustic SCC. (Figure 3-15-2 to Figure 3-15-8). Applied stresses, e.g.
from pressure or mechanical loading, can also cause caustic SCC,
but this is uncommon since applied stresses are normally low
relative to the yield point and are lower than residual stresses from
welding or forming.
e) Thermal stress relief (PWHT) is effective in preventing caustic
SCC. (Figure 3-15-1)
f) Crack propagation rates increase dramatically with temperature,
and cracks can grow through wall in a matter of hours during
temperature excursions, especially in concentrated caustic or if
conditions promote caustic concentration. Concentration can occur
as the result of alternating wet and dry conditions, localized hot
spots, or high-temperature steam out.
g) Special care must be taken with steam tracing or heat tracing
design as well as steam out of non-PWHT'd piping and equipment.
h) Contaminants in the caustic solution, especially sulfides,
regardless of concentration, will increase the likelihood of SCC in
Corrosion Cracking Exam Questions
and Answers (Latest Update 2026)
3.15.1 Description of Damage -
correct answer ✅Caustic SCC is characterized by surface-initiated
cracks that occur in piping and equipment exposed to caustic
(alkaline hydroxide solutions) at elevated temperature, primarily
adjacent to non-PWHT'd welds. It is a form of ASCC. The
temperature above which caustic SCC occurs depends on the
concentration of the caustic solution.
3.15.2 Affected Materials -
correct answer ✅Carbon steel, low-alloy steels, and 300 series SS
are susceptible. Duplex stainless steels are also susceptible but
have shown improved resistance compared to the 300 series SS.
Nickel-based alloys are more resistant.
3.15.3 Critical Factors -
correct answer ✅a) Susceptibility to caustic SCC in caustic soda
(NaOH) and caustic potash (KOH) solutions is a function of caustic
strength, metal temperature, and stress level.
b) Increasing caustic concentration and increasing temperature
increase the likelihood and rate of cracking. Conditions likely to
result in cracking have been established through plant experience
and are presented in (Figure 3-15-1)
, API RP 571 3.15 Caustic Stress
Corrosion Cracking Exam Questions
and Answers (Latest Update 2026)
c) Cracking can occur at low caustic levels if a concentrating
mechanism is present. In such cases, caustic concentrations of
50ppm to 100ppm are sufficient to cause cracking.
d) Residual stresses from welding (in non-stress-relieved welds) or
from cold working (such as bending and forming) will cause
cracking, and these high residual stresses are what typically lead to
caustic SCC. (Figure 3-15-2 to Figure 3-15-8). Applied stresses, e.g.
from pressure or mechanical loading, can also cause caustic SCC,
but this is uncommon since applied stresses are normally low
relative to the yield point and are lower than residual stresses from
welding or forming.
e) Thermal stress relief (PWHT) is effective in preventing caustic
SCC. (Figure 3-15-1)
f) Crack propagation rates increase dramatically with temperature,
and cracks can grow through wall in a matter of hours during
temperature excursions, especially in concentrated caustic or if
conditions promote caustic concentration. Concentration can occur
as the result of alternating wet and dry conditions, localized hot
spots, or high-temperature steam out.
g) Special care must be taken with steam tracing or heat tracing
design as well as steam out of non-PWHT'd piping and equipment.
h) Contaminants in the caustic solution, especially sulfides,
regardless of concentration, will increase the likelihood of SCC in
