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Chemical Process Technology

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Chemical & Process Technology

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Wednesday, August 22, 2007


Chloride stress - corrosion cracking (CSCC) is initiation and propagation of cracks in a metal or alloy under tensile stresses and a corrosive environment contains Chloride compounds. Once the crack is initiated, it will propagate rapidly and potentially lead to catastrophic failure.


Factors that influence the rate and severity of cracking include

  • chloride content
  • oxygen content
  • temperature
  • stress level
  • pH value of an aqueous solution
Higher chloride content in process fluid will increase potential of CSCC.

It has been established that oxygen is required for CSCC to occur. Detail may refer to HERE.

The severity of cracking increases with temperature. Figure below shows several Stainless Steel materials increases it susceptibility to CSCC as temperature is increased.


Source : Sandvik Material Technology

SAF 2205 (UNS 31803) = Duplex Stainless Steel
SAF 2507 (UNS 32750) = Super Duplex Stainless Steel

Material under pressure without Post weld heat treatment will experience high stress level. Higher the stress level, higher the potential of CSCC.

Acidic process(low pH) with chloride content in it tends to increase the CSCC potential.

CASE STUDIES


Hot gas (Shell) is cooled by seawater (Tube) from 220 degC to 180 degC in a Shell & Tube heat exchanger. Seawater is being heated from 30 degC to 35 degC and return to sea. The Shell and Tube material of construction are Carbon steel (CS) and Duplex Stainless Steel (DSS) respectively. After 2 months in operation, cracks occurred at the tube (DSS) and leads to major platform shutdown. Investigation found crack was caused by CSCC at tube.
Why a CSCC occurred at DSS tube although the seawater temperature only 35 degC maximum ?

Eventhough the inlet and outlet temperature are below 150 degC, thermal designer may design the heat exchanger with high heat flux in order to reduce the heat exchanger area and this result tube skin temperature exceeded 150 degC. Condition with Seawater which contains ~20,000 mg/l Chloride, high in dissolved oxygen, slightly acidic and skin temperature exceeded 150 degC is perfect combination conditions for CSCC to occur for DSS. Those heat exchanger designer shall always check skin temperature profile especially for low flow condition or specify better material i.e. Super DSS for above service.

Further Reading








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posted by Webworm, 8:48 PM

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