3.15 Caustic Stress Corrosion Cracking

Question 1

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 _________________

Answer 1

non-PWHT’d welds ; concentration of the caustic solution.

Question 2

Affected Materials are: _________, _________, __________, ___________. The alloys more resistant are ________

Answer 2

CS, LAS and 300SS, Duplex ; Nickel based alloys

Question 3

Susceptibility to caustic SCC in caustic soda (NaOH) and caustic potash (KOH) solutions is a function that includes:

Answer 3

1. caustic strength
2. metal temperature
3. stress level

Question 4

Increasing caustic concentration and increasing temperature ____________ the likelihood and rate of cracking.

Answer 4

Increase

Question 5

Cracking can occur at low caustic levels if a concentrating mechanism is present. In such cases, concentrations of _______________ are sufficient to cause cracking.

Answer 5

50 ppm to 100 ppm

Question 6

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 ___________

Answer 6

cracking ; caustic SCC

Question 7

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.

Answer 7

uncommon

Question 8

Thermal stress relief (PWHT) is _____________ in preventing caustic SCC

Answer 8

effective

Question 9

Special care must be taken with__________________________ design as well as steam out of non-PWHT’d piping and equipment

Answer 9

steam tracing or heat tracing

Question 10

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

Answer 10

sulfides ; increase

Question 11

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________________________________________________________________________________________.

Answer 11

temperature; alternating wet and dry conditions, localized hot spots,
or high-temperature steam out

Question 12

Caustic SCC can occur in non-stress-relieved piping and equipment that handles caustic, including:

Answer 12

1. H2S and mercaptan removal units.
2. quipment that uses caustic for neutralization in sulfuric acid and HF alkylation units.
3. feed to the crude tower for chloride
   control.

Question 13

Caustic SCC can occur in equipment as the result of _______________ after being in caustic service.

Answer 13

steam cleaning

Question 14

Cracking can occur in boilers at rolled tube joints due to ________________ between the tube and tubesheet

Answer 14

caustic concentrating

Question 15

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.

Answer 15

carryover

Question 16

Caustic SCC typically propagates _________ to the weld in adjacent base metal, i.e:

Answer 16

parallel; in the zone of highest
welding residual stress, weld deposit or HAZ and can be transverse to the weld.

Question 17

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.

Answer 17

metallographic ; intergranular

Question 18

Cracking in 300 series SS is most often transgranular but can be intergranular even in _____________ material. In ________________ has been observed as either intergranular

Answer 18

non-sensitized ; nickel-based alloys

Question 19

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

Answer 19

black magnetite

Question 20

Cracking can be effectively prevented by means of a

Answer 20

stress-relieving heat treatment (e.g. PWHT)

Question 21

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.

Answer 21

1150 °F (620 °C)

Question 22

______________ offer little advantage in resistance over carbon steel, and ______________ are more resistant to cracking

Answer 22

300 series SS ; nickel-based alloy

Question 23

In ________________________, stress relief may be needed to prevent cracking of carbon steel even at low, otherwise “safe” temperatures

Answer 23

contaminated caustic solutions

Question 24

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:

Answer 24

molten caustic cracking

Question 25

Steam out of non-PWHT’d carbon steel piping and equipment should be avoided. Where necessary, equipment should be:

Answer 25

ater washed before steaming out
only low-pressure steam

Question 26

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 __________

Answer 26

300 series SS; Alloy 800 and Alloy 825

Question 27

Proper _____________ of the injection system is required to ensure that caustic is properly dispersed before entering the high- temperature crude preheat system

Answer 27

design and operation

Question 28

MT, WFMT, ACFM, and ECT, AET can be ________techniques to detect these surface-breaking cracks and SWUT and PAUT can be effective to __________ cracks

Answer 28

effective ; detect and size

Question 29

PT and RT may not be effective in detecting _____________

Answer 29

fine, tight cracks