Evaluating The Scc Resistance Of Underwater Welds In Sodium Tetrathionate

The susceptibility of welds to stress corrosion cracking (SCC) is enhanced by the surface residual tensile stresses generated by the typical welding process. However, underwater plasma transferred arc (PTA) welding has been shown to produce compressive surface residual stresses, an encouraging result if repairs of cracked boiling water reactor (BWR) components are to be made without further endangering them to SCC. This program was designed to verify that underwater PTA welds are resistant to SCC and to determine if underwater PTA welding could mitigate SCC in potentially susceptible welds. This was achieved by exposing various welds on solution annealed (SA) and SA + thermally sensitized 304 stainless steel at 25°C in a solution of 1.5 gm/liter of sodium sulfide added to 0.05M sodium tetrathionate, titrated to a pH of 1.25 with H₂SO₄. The autogeneous welds were produced using gas tungsten arc (GTA) and plasma transferred arc (PTA) welding under atmospheric conditions, and PTA welding underwater. After 1 hour of sodium tetrathionate exposure, GTA and air PTA welds exhibited SCC while the underwater PTA weld heat affected zones were more resistant. Underwater PTA welds bisecting a GTA weld eliminated the cracking in the GTA weld heat affected zone under certain conditions. The lack of IG cracking in the region influenced by the underwater PTA weld is consistent with the measurement of compressive surface residual stresses inherent to the underwater welding process.

Underwater welding was initially investigated as a means to clad boiling water reactor (BWR) components with noble metal to impart stress corrosion cracking (SCC) resistance without the expense of draining the water from the reactor core. During the development of this underwater procedure, the plasma transferred arc (PTA) welding process was observed to produce surface compressive residual stresses adjacent to the weld.¹ Compressive stresses are advantageous since they mitigate SCC, whereas normal welding processes produce surface residual tensile stresses which promote SCC.²The main objective of this program is to demonstrate the SCC resistance of underwater welds on 304 stainless steel (SS) made by the underwater PTA process. The overall goal is to develop a welding process that does not endanger the components to SCC. A secondary objective was to develop a relatively simple method to evaluate the SCC susceptibility of 304 SS welds. Once achieved, typical GTA and air and underwater PTA welds were evaluated. Boiling magnesium chloride (ASTMG36) has often been used to provide a rapid determination of weld susceptibility, but the results can be variable and difficult to interpret. In addition, the hazards associated with this environment have lead to the consideration of alternative solutions. Sodium thiosulfate^3-6 and sodium tetrathionate^6,7 have been reported to produce SCC of 304 SS and nickel-base alloys at room temperature. Sodium tetrathionate was selected over sodium thiosulfate because it has been shown to be a more aggressive species.

EXPERIMENTAL PROCEDURE

SCC Test Solution

The solution used to evaluate the SCC resistance of the welds at 25°C was 0.05M sodium tetrathionate, titrated with sulfuric acid to a pH between l and 2.