Abstract Scope |
Keywords: Pitting corrosion, Syringe cell, No-backing-gas welding.
Introduction: Stainless steel pipe welds for service applications in corrosive environments typically require the use of an inert backing gas in order to minimize or prevent root bead contamination and oxidation. This adds significant cost and complexity to the welding of stainless steel pipe due to access restrictions, personnel safety, and/or economic factors. Gas metal arc welding (GMAW) offers productivity and economic benefits as compared to conventional (GTAW) welding processes for stainless steel pipe. In this study, waveform controlled GMAW was used for welding of stainless steel with no backing gas (NBG). In this work we compared metallurgical characteristics and corrosion resistance of austenitic stainless steel 304L/ER308LSi GMAW welds without the presence of backing gas to matched control welds (GMAW and GTAW) with the use of backing gas. A syringe cell setup was used for electrochemical corrosion testing locally on the backside heat-affected zone and the root weld metal.
Experimental: Welds on 304L plates were made with ER308LSi wire using modified short arc GMAW for the root pass, followed by a GMAW-P hot pass for half of the total weld length. Plates were welded with different backing gases, 100% Ar and 95%Ar/5% O2, and no backing gas. GTAW welds with 100% Ar were also made as a reference. Microstructural analysis and quantification of weld metal ferrite (using digital image analysis) were done on weld cross sections. The backside heat-affected zone region was ground to a 600-grit finish prior to corrosion testing. The root weld metal was ground flat and also finished with 600-grit because a flat surface is needed for corrosion testing. Pitting corrosion tests were performed using 0.1M NaCl solution and an electrochemical syringe cell arrangement that adapts a typical three electrode immersion test into a localized droplet test. The electrodes used in this study were an Ag/AgCl reference electrode, platinum wire for the counter electrode, and the working electrode being the weld sample. A Gamry potentiostat was used to control the applied potential and measure the induced current. The area exposed to the droplet was typically in the range of .15-.2cm2. Four to six replicate experiments were completed in the heat-affected zone and root weld metal of each weld. As a reference the 304L base metal was analyzed for pitting corrosion resistance. Corrosion resistance was determined by obtaining cyclic potentiodynamic polarization (CPP) scans until a pitting and repassivation potential were developed through a positive hysteresis. Optical microscopy of each test was performed to ensure pitting occurred on the surface.
Results/Discussion: The pitting potential (Epit) and repassivation potential (ERP) for the base metal and all tested welds (including the NBG welds) obtained with the syringe droplet test were significantly above the ±2σ band of what is reported in ASTM G-61 for 304 stainless steel. This is due to the fact that crevice corrosion is avoided using the syringe cell arrangement. In addition, the scan rate used in this work is higher than in ASTM G-61, which should further shift the results to a higher potential. All welded samples showed a decrease in pitting potential (Epit) and repassivation potential (ERP) as compared to the 304L base metal. However, there was less of a difference between the welds made with 100% Ar and 95%Ar/5% O2, and no backing gas. The NBG welds showed a slight decrease in pitting potential (Epit), but were comparable to all other welds in terms of repassivation potential (ERP). Overlapping scatting bands indicate that all tested welds perform similarly in terms of resistance to pitting corrosion. In this study, the repassivation potential (ERP) results are particularly significant because the repassivation potential is approximately the minimum potential that metastable pitting occurs at. Excessive metastable pitting can be damaging over time. In this work, welds made with and without backing gas were shown to have nearly the same pitting corrosion resistance. The additional hot pass did not have a significant effect on the pitting potential (Epit) and repassivation potential (ERP). As a next step, intergranular corrosion behavior will be analyzed using the syringe droplet test to fully understand corrosion resistance and degree of sensitization as a function of backing gas. In addition, results from microstructural analysis and weld metal ferrite content will be correlated to the corrosion results.
Conclusion: The syringe droplet test enabled measurements of corrosion properties locally on the backside heat-affected zone and root weld metal without the need for sample masking. Thus, the syringe cell approach holds an advantage over conventional methods for assessment of corrosion susceptibility in different weld regions. Pitting corrosion testing showed no significant difference between NBG welds and welds made with 100% Ar and 95%Ar/5% O2 backing gas. Our results thus far suggest that NBG welds utilizing modified short arc GMAW for the root pass will have acceptable corrosion resistance properties.
This work was performed within the Manufacturing and Materials Joining Innovation Center (Ma2JIC), made possible through an award (1822144) from the National Science Foundation (NSF) Industry University Cooperative Research Center program (IUCRC). |