99 Diseases of Pressure Equipment: External Chloride Stress Corrosion Cracking (ECSCC)

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By John Reynolds at Intertek. This article appears in the May/June 2004 issue of Inspectioneering Journal

Chloride cracking of austenitic stainless steels (300 series SS) is an off-shoot of CUI, and there’s nothing really magical about it. If you have insulated solid stainless steel equipment operating in the CUI temperature range you are likely to eventually experience ECSCC. Good coatings, properly selected for the purpose and properly applied will give you some protection for a period of time, but the vast majority of coatings break down after about 10 (5-15) years and allow chloride laden moisture to be exposed to the surface of the stainless steel. Low chloride insulation and well-applied weather barriers will also help avoid the on-set of ECSCC. The older version of calcium silicate insulation which contained chlorides is especially prone to causing ECSCC. Although the temperature range of 140 F (60C) to 300F (150C) is likely to be the most active region for ECSCC, there are numerous data points reported outside of that temperature range (above and below), including severe ECSCC of hydroprocess stainless steel piping operating well above 600 F (315C). The later was exposed to periodic drenching from testing of fireprotection deluge systems.

The good news about ECSCC on austenitic SS is that 300 series SS’s are normally very tough steels and chances are therefore very likely that you will experience a leak before break scenario, and that leakage is likely to be small. So the potential for a large safety event are reduced over other forms of SCC and even less than CUI on carbon or low alloy steel equipment. That of course does not mean that the chances for catastrophic rupture are zero because if the equipment is so extensively cracked with ECSCC, then there may be the potential for what the fitness for service analysts like to call “plastic collapse”. The bad news is, of course, that even small leaks can be hazardous and/or produce an undesirable reliability impact.

Inspection methods for ECSCC are normally relegated to surface techniques such as liquid penetrant (LPT) or specialized eddy current probes.

 

Does your CUI program/project include inspection planning for ECSCC of susceptible stainless steel equipment?

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