Background: Nickel–titanium (Ni–Ti) rotary files improve root canal therapy due to their flexibility and cutting efficiency but remain susceptible to fracture from fatigue, torsional stress, and corrosion. Sodium hypochlorite (NaOCl), a common irrigant, can weaken files by causing surface defects. Stainless steel (SS) files generally show uniform corrosion, while Ni–Ti files develop localized pitting. Previous studies, including in Bangladesh, indicate higher overall corrosion in SS and more surface irregularities in Ni–Ti. This study aimed to compare the corrosion behavior, rate, and surface changes of SS and Ni–Ti files in NaOCl under controlled conditions. Methods: In this in vitro study, 30 endodontic files (15 stainless steel hand K-files and 15 NiTi rotary files) of similar lengths and diameters (15, 20, 25, 30mm) were assessed for corrosion after 24-hour immersion in 5.25% sodium hypochlorite at room temperature. Corrosion susceptibility was evaluated via Initial Corrosion (IC), Corrosion Rate (CR), and Corrosion Potential (CP), along with overall corrosion frequency. Visual inspection and SEM analysis were used to assess surface degradation. Data were analyzed using independent t-tests and chi-square tests, with p<0.05 considered significant. Results: A total of 30 endodontic files (15 stainless steel [SS] K-files and 15 nickel-titanium [NiTi] rotary files) were evaluated after 24-hour immersion in 5.25% sodium hypochlorite (NaOCl). Corrosion frequency was higher in SS files (80%) than NiTi files (40%). SEM analysis revealed more pronounced pitting and surface roughening in SS files, while NiTi files showed localized pitting and minor micro-cracks (mean severity score: 2.4 ± 0.6 vs. 1.3 ± 0.5). Quantitatively, SS files exhibited significantly higher corrosion rates at all diameters (p < 0.05), with notable differences in initial corrosion and corrosion potential at select points. Overall, stainless steel files were more susceptible to NaOCl-induced corrosion than NiTi instruments. Conclusion: Stainless steel (SS) hand K-files are more susceptible to NaOCl-induced corrosion than nickel–titanium (NiTi) rotary files, exhibiting higher corrosion frequency, greater initial corrosion, and increased corrosion rates. NiTi files demonstrate superior structural integrity and resistance, highlighting their advantage for safer and more durable root canal instrumentation.