This work reveals a stage-dependent reversal of corrosion susceptibility between CGHAZ and FGHAZ of welded 650 MPa high-strength low-alloy steel under wet–dry cyclic in 0.01 mol/L NaHSO3. In the initial bare steel stage, FGHAZ exhibits the most negative corrosion potential (-718.8 mV) and the highest corrosion current density (48.955 µA cm− 2) relative to both BM and CGHAZ. This is attributed to its high density of active grain boundary dissolution sites. With prolonged exposure, however, a corrosion reversal occurs. FGHAZ develops a compact, α-FeOOH rich rust layer with α/γ* of 33.5% and a polarization resistance of 3076 Ω cm2 at 384 h. In contrast, the porous CGHAZ rust permits sustained bisulfite penetration, which suppresses α-FeOOH formation to an α/γ* of only 12.7%. This leads to a 51.1% decline in polarization resistance of CGHAZ from 192 h to 384 h. A random forest regression model (R² = 0.9736) for predicting corrosion current identifies polarization resistance and the α/γ* ratio as the dominant predictors. These findings demonstrate that the regulation of corrosion resistance in welded high-strength steels involves a multi-dimensional correlation between microstructure and rust layer. Differences in HAZ microstructure directly govern the formation and phase evolution of the surface rust layer, which in turn determines the long-term corrosion resistance under sulfide-containing atmospheres.