Study on stress corrosion behavior of 30CrMo steel in different concentrations of chlorine containing thiosulfate solutions

30CrMo steel is a core material for oil and gas field equipment (e.g., sucker rods, drill pipe joints), but its long-term service in sulfur-containing downhole often causes stress corrosion cracking (SCC) failure. This study systematically elucidates the SCC behavior of 30CrMo steel under H2S simulation systems constructed with chloride-containing acidic thiosulfate of different concentrations. Tests showed the corrosion rate exhibited a non-monotonic relationship with thiosulfate concentration, peaking at 0.340 mm/y in 1 × 10−2mol/L solution, with a dense FeOOH layer and loose mackinawite layer formed on the surface. Severe pitting (maximum depth 99.73 μm) occurred at 1 × 10−3 mol/L, accompanied by the highest subsurface hydrogen concentration. Moreover, the SCC mechanism evolved with concentration: low (1 × 10−4 mol/L) and high (1 × 10−1 mol/L) dominated by anodic dissolution (AD) and Hydrogen Enhanced Local Plasticity (HELP); intermediate (1 × 10−3-1 × 10−2 mol/L) exhibited significant synergy between AD and Hydrogen Enhanced Decohesion (HEDE). This study revealed thiosulfate concentration’s regulatory effect on 30CrMo steel’s HE-SCC behavior, providing a simple, safe new strategy for service safety assessment of sulfur-containing oil and gas field equipment materials.