A report from the Energy Department's Pacific Northwest National Laboratory summarizes experimental results evaluating the primary water stress corrosion cracking (PWSCC) susceptibility of high-chromium, nickel-base weld metals and interface regions in more complex dissimilar metal welds.

The research aimed to establish quantitative measurements of stress corrosion cracking (SCC) growth rates and determine relationships among cracking susceptibility, metallurgical characteristics and environmental conditions.

(Read the report)

Crack-growth rates have been determined under cyclic loading and at constant stress intensity (K) in simulated pressurized water reactor primary water environments for 21 weld metal specimens, including tests on nine alloy 152, one alloy 152M, three alloy 52, seven alloy 52M and one alloy 52MSS specimens.

In addition, SCC response was investigated from alloy 182/82 into alloy 52M for four alloy 52M/182 overlay weld specimens and two alloy 52M/82 inlay weld specimens. Finally, crack-growth behavior was evaluated at various regions in dissimilar metal (DM) welds, including nine tests at DM interfaces covering those between two alloy 152 and low alloy steel, one alloy 52M and low alloy steel, two alloy 152M and carbon steel, two alloy 52M and carbon steel, and two alloy 152 and stainless steel interfaces.

Most test specimens were machined from industry mockups to provide plant-representative materials, while a few were produced at laboratories using industry recommended procedures.

In all cases, crack extension was monitored in situ by direct current potential drop (DCPD) with an estimated length resolution of about ±1 µm, making it possible to measure extremely low growth rates approaching 5x10^-10 mm/s. Most SCC tests were performed at 360° C with a dissolved hydrogen concentration of 25 cc/kg to establish an electrochemical potential (ECP) at the nickel-nickel oxide stability line where many nickel-base alloys are most susceptible to SCC. However, environmental conditions were modified during a few experiments to evaluate the influence of temperature, water chemistry or ECP on propagation rates.

Extensive characterizations were performed on material microstructures and stress-corrosion cracks by optical and electron microscopy techniques and linked to crack-growth test results to help define material and environmental parameters controlling SCC susceptibility.

The report's main conclusion from experiments on alloy 152, 152M, 52, 52M and 52MSS weld metals with typical chromium (Cr) bulk concentrations of 28 to 30 weight percent (wt%) is that they are resistant to SCC crack growth. Some differences were observed for alloy 152 and 52M welds in regions where the local Cr concentration was found to equal to or below about 24 wt%. SCC propagation rates up to approximately 1.4x10^-8 mm/s were measured in an alloy 52M weld pass for the Ringhals overlay (CT035) and within DM weld alloy 152 dilution zones adjacent to the LAS interface.

The report said that the presence of aligned grain boundaries along the crack path were a key requirement for significant SCC growth in the dilution zone specimens and limited the extent of observed IGSCC to local areas. Overall, these results support the excellent SCC resistance of the high Cr welds in the as-welded condition.

The application of the test results to plant components is "particularly challenging," the report said, given the large number of potential variations in weld designs and configurations that are found in service.