Groundwater in Jeju Island is extensively utilized for living, agriculture, and industry, with over 20 million tons consumed monthly. This heavy usage raises concerns about groundwater pollution due to aging and corrosion of pipes, with over 90% being outdated. Effective management and corrosion prevention measures are essential to address these issues. This study aims to compare the corrosion characteristics of Shielded Metal Arc Welding (SMAW) and Gas Tungsten Arc Welding (GTAW) to analyze weld joint corrosion in the groundwater environment of Jeju Island and under ordinary temperatures. The corrosion behaviors of mild steel and stainless steel were examined by measuring weight changes over time in both conditions. Results indicate that, with the SMAW process, corrosion progressed slowly at the start and end of the weld under ordinary temperatures, while in the groundwater environment, it occurred gradually across the entire weld area. In contrast, GTAW exhibited corrosion at the weld start, end, and heat-affected zone in ordinary conditions, with gradual corrosion also observed under ground.