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Inspectioneering Journal

99 Diseases of Pressure Equipment: Chloride Stress Corrosion Cracking

By John Reynolds at Intertek. This article appears in the November/December 2003 issue of Inspectioneering Journal

Chloride stress corrosion cracking (SCC) is about as well known as any SCC mechanism can be, so I won't dwell much on it here, but want to mention it for the sake of completeness and hopefully mention something that is not as commonly known about it. Chloride SCC is clearly the bane of austenitic stainless steels and one of the main reasons they are not the "miracle cure" for many corrosion problems. Most people have seen the spiderwebbed and lightening array type network of highly branched cracks that is common with chloride cracking of SS's. One would think that with all that is known about this mechanism and with all the failures, that it would be a thing of the past, but not so - it continues to plague the industry. Why? Typically because of inadvertent contamination with chlorides that was not anticipated by design engineers that are unaware of the potential consequences of using austenitic SS's where chlorides may be present. Fortunately because of the very high toughness of stainless steel, catastrophic failures from chloride SCC are rare (but can happen). Most consequences are economic from leaks, but these days, few plants can afford the economic consequences of chloride SCC leaks because it nearly always means equipment replacement is necessary. Repair welding of chloride cracked equipment is a fools play ground. Places where chloride cracking of 300 series stainless steels continues to occur include: 1) cracking from corrosion under insulation 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 is hydrotested with chloride contaminated water and left to dry out (concentrating the chlorides into small pools of highly aggressive salt solutions) that cause cracking on start up; 4) stainless steel deadlegs 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.

Do all the right people (everybody but the cleaning lady) in your facility know about the chloride SCC susceptibility of 300 series SS's and what their role is in preventing such failures? I'll bet not.


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