Intergranular Stress Corrosion Cracking
SIB-96-139

The Problem
Austenitic stainless steels are utilized in applications where corrosion resistance is an important characteristic. However under specific conditions, localized attack in the form of pitting, intergranular, or transgranular cracking may occur. One prominent corrosion mechanism affecting austenitic stainless steels, which has been of significant economic concern to the electrical utility industry, is intergranular stress corrosion cracking (IGSCC).

IGSCC results from the localized attack of the grain boundaries of a susceptible material in an aggressive environment under load. The grain boundary regions undergo attack and physical cracks are formed. The cracks propagate with little or no evidence of plastic deformation, and failure of the component is possible.

Three simultaneous conditions must be present for IGSCC to occur: susceptible material, tensile stress, and environment. All of these critical factors are necessary for the formation and propagation of a stress corrosion crack, as shown schematically below. IGSCC-related research has produced mitigation methods that deal with each of these contributors.

The Solutions
Some materials are more susceptible to IGSCC than others. The high carbon content for some types of stainless steel (i.e., Type 304) makes them more subject to sensitization of grain boundaries than stainless steels with a lower carbon content (maximum of 0.035%), like Type 316L stainless steel. Although the reduced carbon content of the IGSCC-resistant materials lower the yield and tensile strength, nuclear grades (Type 304NG and 316NG) with IGSCC resistance have been developed which possess the strength levels of standard grade stainless steels.

Another IGSCC mitigation measure is the weld overlay technique. Weld overlays provide an excellent deterrent to IGSCC in the boiling water reactor (BWR) environment by producing a new corrosion-resistant pressure boundary on the outside surface of the piping component, as shown below. The duplex microstructure created by Type 308L carbon steel weld metal overlay makes it far more resistant to IGSCC than the underlying sensitized base metal.

The Nuclear Regulatory Commission (NRC) has endorsed the use of weld overlays as a means of mitigating IGSCC in NUREG-0313, Revision 2. ASME has also issued Code Case N-504 approving this process and defining the requirements for use in weld overlays.

The second condition necessary for IGSCC to occur is a tensile stress, typically produced during butt welding of piping structures. Induction heat stress improvement (IHSI) employs the simultaneous application of heat to the pipe outer diameter surface while cooling the pipe inside surface with water. This produces beneficial compressive yielding on the inside surface of the pipe. More recently, the mechanical stress improvement process has also demonstrated ability as a stress mitigation measure.

The third and final condition necessary for IGSCC is environment. Modifications of water chemistry to decrease its oxidizing power, dearation during startup and full power operation, improved BWR water purity with very strict contaminant control (chlorides, sulfates, etc.) and hydrogen water chemistry, have been shown to be extremely beneficial to IGSCC resistance of austenitic stainless steels. Structural Integrity Associates (SI) has been involved in all aspects of IGSCC-related research, both in its mitigation and elimination. SI has assisted utilities in demonstrating that the Code requirements of the nuclear grade IGSCC-resistant materials are satisfied. SI's technical experts have been involved with the structural weld overlay process since early development and implementation. SI has designed many of the weld overlays in service, provided third party review of designs by others, and worked closely with the Electric Power Research Institute (EPRI) to document the historical background of the process and proper application. Additionally, SI has worked closely with EPRI, the BWR Owners Group, BWR Vessel and Internals Program, individual utilities, and the NRC on the implementation of IGSCC mitigation processes.

If you would like more information regarding IGSCC or SI's services in this area, please contact SI.