Potentiodynamic polarization analysis with various corrosion inhibitors on A508/IN-182/IN-52M/308L/316L welds

Abstract

In this study, dissimilar metal welding of A508/IN-182/IN-52M/308L/316L was employed in the fabrication of pipe nozzles for pressure vessels in pressurized water reactors (PWR). This process involved the fusion of low-alloy A508 and stainless steel 316L using gas tungsten arc welding (GTAW) welding, with the use of nickel-based alloy Inconel 182 as a filler material. Additionally, Inconel 52M served as an overlay layer, and material 308 acted as a buffer layer to facilitate bonding between 316L and the overlay welding layer. The primary objective of this study was to investigate the electrochemical aspects of galvanic corrosion and analyze the surface properties of dissimilar welds in a simulated PWR environment, specifically in a simulated hot water (SHW), over a three-month period. Potentiodynamic polarization tests were conducted on the testing solution, which contained a mixture of inhibitors, including sodium molybdate (Na2MoO4) and sodium nitrite (NaNO2). The surface morphology was thoroughly examined using optical microscopy and scanning electron microscopy - energy dispersive spectroscopy (SEM-EDS), while phase analysis was performed using x-ray diffraction (XRD) testing. The results indicated that the most efficient inhibitor combination treatment for dissimilar metal welds achieved an 89.98 % reduction in corrosion on A508 metal. This was accomplished by utilizing a mixed concentration of 6000 ppm sodium molybdates combined with 4000 ppm sodium nitrites. As a result, the corrosion rate of A508 decreased significantly from 0.475 mpy to 0.047 mpy, bringing it in line with the corrosion potential of other alloys and establishing a passive zone. In summary, the corrosion rate of A508/IN-182/IN-52M/308L/316L in PWR simulations decreased as the concentration of corrosion inhibitor, specifically sodium molybdate and sodium nitrite, increased. When the inhibitor dosage concentration was optimized, a protective thin molybdenum-containing film formed on the surface, effectively guarding against random pitting caused by galvanic corrosion.