Nứt ăn mòn ứng suất clorua (Cl SCC)

This type of SCC occurs in austenitic alloys & duplex stainless steels exposed to solutions containing chlorides. This cracking is often transgranular & highly branched, although it can be intergranular if the alloy microstructure has been significantly sensitised. The cracking can also be mixed mode in some cases. Cl SCC results in brittle fractures of ductile materials.

Cl SCC seldom occurs at metal temperatures below 50ºC or above 200ºC. However, isolated cases have been reported to occur at lower temperatures. The likelihood of Cl SCC increases with stresses, but it can also occur at relatively low stresses. Presence of Cl ions at ppm levels is sufficient to cause SCC of austenitic SS if other conditions are met. Concentration mechanisms (e.g. evaporation) can cause high local concentrations of chlorides. Evaporative concentrations in dead space, crevice or surface film on heat-rejecting surfaces (e.g., heat exchanger tube) are common mechanisms encountered in practice. Chlorides can also concentrate in areas such as threaded or socket-welded connections, tube-to-tube sheet joints, under deposits & locations where dew point or wet/dry conditions are present.

Generally, Cl SCC does not occur in oxygen free environments that do not contain other oxidising agents. Parameters affecting likelihood of Cl SCC include acidity, chloride/oxygen concentrations & temperature. Cracking is likely to occur under acidic or neutral pH conditions. There are no well-established thresholds for temperature, pH, Cl/oxygen concentrations because of the complex interrelationships. Some guidelines established for SS grade 304 & similar austenitic SS are:

1) At 2 < pH < 7 in presence of oxygen, likelihood of Cl SCC is low at temperatures below 50 °C with Cl concentrations up to 100 ppm.
2) At 7 < pH < 12 in presence of oxygen, likelihood of the cracking is low at temperatures below 80 °C with Cl concentrations up to 1000 ppm.
3) At pH > 12 in presence of oxygen, likelihood of Cl SCC is low at temperatures below 100 °C with Cl concentrations up to 1000 ppm.

Normal operating & transient conditions (e.g., start-ups, shutdowns or process upsets) are also important considerations. External Cl SCC can occur under insulation that contains Cl from either manufacture or sources such as rainwater, firewater, seawater or cooling water tower. It could also occur on bare metals (i.e., not insulated) in hot, humid & coastal areas.

Increasing Ni content in SS imparts greater resistance to SCC. Carbon/low-alloy steels & 12Cr steels are not susceptible to Cl SCC but are not used as alternative because of their susceptibility to general/pitting corrosion in Cl-containing environments.

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