What is Chloride Stress Corrosion Cracking (SCC)?


Chloride Stress Corrosion Cracking (CL-SCC)

Austenitic Stainless steel or commonly known as 18Cr-8Ni alloys offers very good corrosion resistance. Usually, the chromium is kept above 19% and this chromium helps to forms a passive layer that will gives the corrosion resistance as shown in below figure.

As stated above that stainless steel offer high corrosion resistance although when they are exposed to chloride containing environment along with tensile stresses on the material, they exhibit cracking which is known Chloride Stress corrosion cracking (CL-SCC). CL-SSC once started in the material will leads to sudden and unexpected catastrophic failure of materials (as shown in below picture) which are normally ductile such as Stainless steel. Carbon steels, low alloy steels and 400 Series SS are not susceptible to Chloride Stress Corrosion Cracking.

Nickel content of the alloy has a major affect on resistance. The greatest susceptibility is at a nickel content of 8% to 12%. Alloys with nickel contents above 35% are highly resistant and alloys above 45% are nearly immune.

Affected materials in CLSCC

  1. 300 Series stainless steels
  2. Nickel base alloys although less susceptible than 300 series.

Chloride Stress Corrosion Cracking (CL-SCC) phenomena

Chloride Stress Corrosion Cracking (CL-SCC) is the result of combined effect of tensile stresses, presence of aqueous chloride environment and temperature. The dissolved oxygen in the chloride atmosphere work as catalyst and increases the propensity of the chloride stress corrosion cracking. The presence of tensile load (Either internal or residual stresses) is must for this type of cracking to occur. Residual stresses can be induced due to deformation or cold work on any other external factors. The residual stresses can combine with the operating stresses and add to the internal stresses.

The cracking initiation include a race among local corrosion (That is mainly reliant on Cl concentration although has a weak dependence on the temperature) along with crack growth (that has a solid dependence on temperature but is fairly unaffected by Cl concentration & pH value). The chloride stress corrosion cracking should be evaluated from the peak temperature touched during any time of operation or maintenance procedures, regardless of the period of the high-temperature service., chloride cracking has been found in specimens exposed to temperatures as low as 25ᵒC during lab tests. CLSCC appearance is typically transgranular, branched and detected as a grid of multiple cracks as shown in below picture. The actual material will not shows any visible sign of corrosion and fracture surface often appear as brittle in nature.

Prevention of CL SCC

a) Use resistant materials of construction.
b) When hydrotesting, use low chloride content water and dry out thoroughly and quickly.
c) Properly applied coatings under insulation.
d) Avoid designs that allow stagnant regions where chlorides can concentrate or deposit.
e) A high temperature stress relief of 300 Series SS after fabrication may reduce residual stresses.
However, consideration should be given to the possible effects of sensitization that may occur,
increasing susceptibility to polythionic SCC, possible distortion problems and potential reheat
cracking.

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