3.17 Chloride Stress Corrosion Cracking

Question 1

Also referred to as chloride cracking. Surface initiated cracking of 300 series SS and some nickel-based alloys under the combined action of:

Answer 1

• tensile stress.
• temperature,
• aqueous chloride environmen

Question 2

Affected Materials:

Answer 2

• All 300 series SS are highly susceptible.
• Duplex stainless steels are more resistant but still susceptible.
• Nickel-based alloys are highly resistant but not immune.

Question 3

Welds in 300 series SS normally contain some ferrite, producing a duplex structure that is usually more _________ to chloride stress corrosion cracking (Cl– SCC) than the base metal.

Answer 3

resistant

Question 4

Critical Factors

Answer 4

• Chloride content
• Temperature,
• pH,
• tensile stress,
• presence of oxygen,
• Alloy composition

Question 5

Increasing levels of chloride ___________ the likelihood of cracking.

Answer 5

increase

Question 6

Increasing temperatures _____________ the potential for cracking, as long as the other required elements (stress and aqueous chloride solution) are present concurrently

Answer 6

increase. Although there are exceptions at lower temperatures and even ambient temperature, particularly with highly cold worked or sensitized materials, cracking usually occurs at metal temperatures above about
140 °F (60 °C)

Question 7

The potential for cracking ____________at lower pH; however, SCC usually does not occur at pH values below 2.

Answer 7

increases

Question 8

The most common area of concern are:

Answer 8

• non-stress-relieved
  welds.
• highly stressed or cold worked components, such as expansion bellows

Question 9

Nickel content of the alloy has a major effect on resistance. The greatest susceptibility occurs in stainless steels with a nickel content of

Answer 9

8 % to 12 %.

Question 10

Alloys with nickel contents above _______ are highly resistant, and alloys above ______ are nearly immune in refining applications, but cracking can still occur in severe conditions.

Answer 10

35 % ; 45 %

Question 11

Affected Units or Equipment

Answer 11

a) Most non-stress-relieved 300 series SS piping and equipment in any process unit.
b) water-cooled condensers and on the process side of crude tower overhead
condensers.
c) Stainless steel drains in hydroprocessing units.
d) Bellows and instrument tubing.
e) insulated 300 series SS surfaces when insulation gets wet.
f) boiler drain lines.
g) exchanger tube bundles.
h) Units processing or co-processing bio-based or renewable feedstocks

Question 12

Units processing or co-processing bio-based or renewable feedstocks (biomass, natural fats and oils, etc.) are particularly susceptible to Cl− SCC due to ____________ of organic chlorides converting to inorganic chlorides in the reactor effluent.

Answer 12

high levels

Question 13

Stainless steel drains in hydroprocessing units are susceptible to cracking, particularly during start-up, if not __________

Answer 13

properly purged

Question 14

Surface-breaking ___________ can occur from the process side or externally under insulation

Answer 14

cracks

Question 15

The component usually shows ____________ signs of corrosion

Answer 15

no visible

Question 16

cracks have many branches and may be visually detectable as _______________ on the surface.

Answer 16

spider web or craze cracking

Question 17

Metallography of cracked samples typically shows branched, ______________ cracks. However Cracking of sensitized 300 series SS may be __________________

Answer 17

transgranular ; intergranular

Question 18

Cl− SCC in nickel-based alloys, which can occur under ______________ , appears similar to Cl− SCC in stainless steel.

Answer 18

severe conditions

Question 19

____________ typically have a brittle appearance.

Answer 19

Fracture surfaces

Question 20

Materials not susceptible to Cl− SCC.

Answer 20

Carbon steels, low-alloy steels, and 400 series SS

Question 21

For hydrotesting, low-chloride-_____________ should be used, followed quickly by thorough dryout.

Answer 21

low-chloride-content water

Question 22

A _________________ should be applied to stainless steel piping and equipment prior to insulating. Also,
PVC labels or adhesives with _____________________ should be avoided.

Answer 22

suitable coating ;

high levels of chlorides or other halogen
ions

Question 23

Avoid exchanger designs with ______________________ and a high delta T between the shell and tube side
where localized condensation can occur on or in the tubes.

Answer 23

300 series SS steel tubes

Question 24

a suitable high-temperature ____________of 300 series SS after fabrication will reduce residual stresses. However, consideration should be given to the possible effects of sensitization that may occur, increasing susceptibility to _____________________________

Answer 24

stress relief ; PTA SCC, distortion problems, and the potential for stress relaxation cracking

Question 25

Cl− SCC may be detected using the following methods:

Answer 25

• VT (only to advanced stages of cracking)
• PT however extremely fine cracks may be difficult to detect.
• ECT can be used.
• UT (SWUT and PAUT) however, detection and characterization of the cracking will be difficult due to the craze-cracked, multi-branched
  appearance.
• RT is often not sensitive enough to detect cracks except in advanced stages where a sufficient network of
  cracks has developed.