Chloride Stress Corrosion Cracking (Cl-SCC) is a type of Stress Corrosion Cracking (SCC) and is one of the most well known forms of SCC in the refining and chemical processing industries. It can be detrimental to austenitic stainless steels, one of the main reasons these steels are not considered a cure-all for corrosion problems. Damage due to Cl-SCC is easily identifiable by the telltale spiderwebbed and lightening-array type network of highly branched cracks.

Despite the facts that we know much about this mechanism and there have been many failures due to it in the past, it continues to plague the industry. This is typically due to inadvertent contamination of equipment with chlorides that was not anticipated by design engineers who are unaware of the potential consequences of using austenitic stainless steels where chlorides may be present.

Fortunately, catastrophic failures from Cl-SCC are rare because of the very high toughness of stainless steel - although they can occur. The consequences from most leaks tend to be economic in nature, although this can still be devastating to some plants due to the high costs associated with replacing equipment.

Chloride cracking of 300 series stainless steels continues to occur in a number of places, including:

1. Cracking from corrosion under insulation (CUI) which contains small amounts of chloride or where chlorides are present in the atmosphere;

2. When a process is inadvertently contaminated with chlorides by unsuspecting people;

3. Equipment that is is hydrotested with chloride contaminated water and left to dry out (concentrating the chlorides into small pools of highly aggressive salt solutions), which causes cracking on startup;

4. Stainless steel deadlegs which collect chloride contaminated water;

5. Instrument tubing that is normally not welded but contains high residual stresses comes in contact with chloride contaminated atmospheres; and

6. Stainless steel bellows which typically have high stress levels come in contact with chloride contaminated environments especially during down time.   

This topic is covered in more detail in API RP 571 - Damage Mechanisms Affecting Fixed Equipment in the Refining Industry.

 

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