IGSCC

Cracks, Corrosion, Pitting, Wall thickness, IGSCC... 

In-service Inspection by X-Ray Tomosynthesis, A New Method for Sizing IGSCC. 

Many instances of intergranular stress corrosion cracking (IGSCC) of stainless steel and nickel-based alloys have occurred in the reactor water systems of BWRs. IGSCC, first observed in the recirculation piping systems (21) and later in reactor vessel internal components, has been observed primarily in the weld heat-affected zone of Type 304 stainless steel. 

Laboratory experiments have shown that IGSCC can be mitigated if the electrochemical potential (ECP) could be decreased to —0.230 V on the standard hydrogen electrode (SHE) scale in water with a conductivity of 0.3 ]lS/cm (22). This has also been demonstrated in operating plants. Equipment has been developed to monitor ECP in the recirculation line and in strategic places such as the core top and core bottom, in the reactor vessel during power operation. 

Intergranular stress-corrosion cracking (IG-SCC) can occur in some sensitized materials when placed under tensile stress. Thus DL-EPR has been used to study the effects of aging time on the susceptibility of Alloy 600 to IGSCC, as shown in Fig. 41 (39). This work also shows the need to modify the experimental parameters of the test to achieve optimal correlation for alloys other than Type 304SS, in this case lowering the KCNS concentration and the temperature while raising the peak potential and the scan rate. 

Fig. 8. Potential-temperature diagram showing domains of IGSCC and ductile failure in sensitized Type 304SS in 0.01 m Na2S04 as determined using CERTs at an extension rate of 10 s [29]. Reproduced from Proc. 9th Int. Congr. Met. Corros., Vol. 2, pp. 185-201 (1984) by permission of the National Research Council of Canada.

Practical use of this phenomenon is now being made in the control of IGSCC in the heat-affected zones adjacent to welds in Type 304SS recirculation piping in commercial nuclear boiling-water reactors (BWRs). The criterion for protection is that the potential should be displaced to a value more negative than -0.23 Vshe indeed, this value has been accepted by the Nuclear Regulatory Commission based upon short-term in-reactor tests [30]. 

Fig. 18. Polarization curves measured for Alloy 600 in lithiated water at 350 °C and percentages of IGSCC fracture in slow strain rate test specimens with humps [40], Reproduced with permission.

M. P. Manahan, K. E. Newman, D. D. Macdonald, A. J. Peterson, Experimental Validation of the Basis for the Coupled Environment Fracture Model, in Rw. EPRI Workshop on Secondary-Side Initiated IGA/IGSCC, Minneapolis, MN, October 14-15, 1993. EPRI, Published while at the Center for Advanced Materials, The Pennsylvania State University, University Park, PA 16802 in cooperation with MPM Research and Consulting, 915 Pike St. PO Box 840, Lemont, PA in cooperation with Global Technical Consultants, Inc., Centre Hall, PA 16828 and in cooperation with Niagara Mohawk Power Corp. Research and Development, 300 Erie Blvd. W. Syracuse, NY 13202, 1993. 

Fig. 13 Dependence of calculated crack growth rate on the electrochemical crack length for IGSCC in Type 304 SS in NaCl solution.

An important finding of this previous work [54] is that the critical potential for IGSCC is predicted to depend upon the ECL. Thus, with reference to Fig. 15, it is evident that L igscc increases (becomes more positive) as the ECL increases, changing from — 0.40Vshe to —0.15 Vshe as the ECL increases from... 

ECL the critical potential becomes more positive than the corrosion potential [54]. The reader will note that the predicted critical potential for IGSCC for the shortest ECL considered (0.1 cm), —0.40 V he> is significantly more negative than the often-quoted value of —0.23 Vshe for BWR coolant conditions [3]. Notwithstanding that the ECL corresponding to the BWR... 

Fig. 16 IGSCC crack growth rate versus corrosion potential (ECP) for sensitized Type 304 SS in high-temperature water as a function of solution conductivity (rc25) for given values for the degree of sensitization (15 C cm 2) and stress intensity (27.5 MPa.m1/ 2). The experimental data were measured for ambient temperature conductivities, K2S, ranging from 0.1 to 0.3 pS cm-1. The citations given in the figure are those presented in Ref. 43.

Key components of the BWR. the parts names in red indicates areas of IGSCC. 

Fig. 17 Cut-away schematic of a BWR vessel and associated piping system for a BWR equipped with external coolant pumps. The underlined labels are indicating regions of IGSCC. The control rod drive mechanisms and penetrations that are located at the bottom of the vessel are not shown.

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