3.15 Caustic Stress Corrosion Cracking Flashcards
Characterized by surface-initiated cracks that occur from expose to caustic (alkaline hydroxide solutions) at elevated temperature, primarily adjacent to ______________. It is a form of ASCC. The temperature above which occurs depends on the _________________
non-PWHT’d welds ; concentration of the caustic solution.
Affected Materials are: _________, _________, __________, ___________. The alloys more resistant are ________
CS, LAS and 300SS, Duplex ; Nickel based alloys
Susceptibility to caustic SCC in caustic soda (NaOH) and caustic potash (KOH) solutions is a function that includes:
- caustic strength
- metal temperature
- stress level
Increasing caustic concentration and increasing temperature ____________ the likelihood and rate of cracking.
Increase
Cracking can occur at low caustic levels if a concentrating mechanism is present. In such cases, concentrations of _______________ are sufficient to cause cracking.
50 ppm to 100 ppm
Residual stresses from welding (in non-stress-relieved welds) or from cold working (such as bending and forming) will cause _________, and these high residual stresses are what typically lead to ___________
cracking ; caustic SCC
Applied stresses, e.g. from pressure or mechanical loading, can also cause caustic SCC, but this is ______________ since applied stresses are normally low relative to the yield point and are lower than residual stresses from welding or forming.
uncommon
Thermal stress relief (PWHT) is _____________ in preventing caustic SCC
effective
Special care must be taken with__________________________ design as well as steam out of non-PWHT’d piping and equipment
steam tracing or heat tracing
Contaminants in the caustic solution, especially ________, regardless of concentration, will ____________the likelihood of SCC in non-PWHT’d carbon steel, including in the lower temperature area
sulfides ; increase
Crack propagation rates increase dramatically with ___________, and cracks can grow through wall in a matter of hours- especially in concentrated caustic or if conditions promote caustic concentration as the result of________________________________________________________________________________________.
temperature; alternating wet and dry conditions, localized hot spots,
or high-temperature steam out
Caustic SCC can occur in non-stress-relieved piping and equipment that handles caustic, including:
- H2S and mercaptan removal units.
- quipment that uses caustic for neutralization in sulfuric acid and HF alkylation units.
- feed to the crude tower for chloride
control.
Caustic SCC can occur in equipment as the result of _______________ after being in caustic service.
steam cleaning
Cracking can occur in boilers at rolled tube joints due to ________________ between the tube and tubesheet
caustic concentrating
Caustic SCC can also occur as the result of ______________ unintended of caustic into equipment not designed to handle hot caustic (e.g. not stress relieved), such as steam condensate piping or process equipment downstream of a caustic treating section of a process unit.
carryover
Caustic SCC typically propagates _________ to the weld in adjacent base metal, i.e:
parallel; in the zone of highest
welding residual stress, weld deposit or HAZ and can be transverse to the weld.
Cracks can be confirmed through ____________ examination. Cracks are typically branched and predominantly ______________. Cracking that occurs in as-welded carbon steel typically appears as a network
of very fine, oxide-filled cracks.
metallographic ; intergranular
Cracking in 300 series SS is most often transgranular but can be intergranular even in _____________ material. In ________________ has been observed as either intergranular
non-sensitized ; nickel-based alloys
When the cracks are transgranular, can be difficult to distinguish from Cl− SCC. However Caustic SCC should exhibit a _______________layer on the crack surface, whereas Cl− SCC should not produce that type of oxide scale
black magnetite
Cracking can be effectively prevented by means of a
stress-relieving heat treatment (e.g. PWHT)
A heat treatment at ___________ minimum with a minimum holding time of 1 hr is considered an effective stress relieving heat treatment for carbon steel and repair welds.
1150 °F (620 °C)
______________ offer little advantage in resistance over carbon steel, and ______________ are more resistant to cracking
300 series SS ; nickel-based alloy
In ________________________, stress relief may be needed to prevent cracking of carbon steel even at low, otherwise “safe” temperatures
contaminated caustic solutions
When caustic SCC occour at high temperatures that promote the formation of molten caustic in the absence of free water [604 °F (318 °C), at atmospheric pressure It is commonly called:
molten caustic cracking
Steam out of non-PWHT’d carbon steel piping and equipment should be avoided. Where necessary, equipment should be:
ater washed before steaming out
only low-pressure steam
Ensure all soda ash (sodium carbonate) solution that may have been used in 300 series SS equipment is drained prior to heating up as this soda ash can result in caustic SCC of __________
300 series SS; Alloy 800 and Alloy 825
Proper _____________ of the injection system is required to ensure that caustic is properly dispersed before entering the high- temperature crude preheat system
design and operation
MT, WFMT, ACFM, and ECT, AET can be ________techniques to detect these surface-breaking cracks and SWUT and PAUT can be effective to __________ cracks
effective ; detect and size
PT and RT may not be effective in detecting _____________
fine, tight cracks
