Unmasking the Hidden Threat: Conductive Under-Deposits and Their Role in Preferential Weldment Corrosion of Carbon Steel under Sour Conditions

This work incorporates precorrosion conditioning and conductive deposits (FeS or FeS₂) to explore corrosion mechanisms across base metal (BM), weld metal (WM), and heat-affected zone (HAZ) regions under simulated sour conditions (NaCl (3.5 wt %), Na₂S₂O₃ (1000 ppm), CH₃COOH (100 ppm), CO₂, 60 °C, pH ∼ 4, and 1000 rpm). Electrochemical studies demonstrated various corrosion rates (CR) hierarchies (CS-BM (0.34-0.42 mmpy) < CS-WM (0.38-0.73 mmpy) < CS-HAZ (0.48-1.75 mmpy)), which were exacerbated by FeS or FeS₂ deposition. This was ascribed to a variety of potential variations and localized acidification. Also, there were notable changes in the welded CS areas' compositions and microstructures both before and after corrosion testing. By creating protective layers that reduce microgalvanic interactions, the commercial amine-based inhibitor CRW11 (200 ppm) showed remarkable potency by lowering the CRs (<0.1 mmpy) threshold with inhibition efficiency (IE = >80%). Corrosion products (α-Fe^₂O₃, γ-FeOOH, and Fe₃O₄) were most significant, identified by Raman spectroscopy. 1D artificial pit tests showed varied pit propagation dynamics, especially under FeS-induced heterogeneity, with CS-BM-FeS having the highest pit depth (71.4 ± 11.8 μm), but increased pit density (568.0 mm²) was recorded for CS-HAZ-FeS₂. This proves the impact of conductive deposits on the pitting of welded CS regions. These findings corroborated deposit-induced electrochemical heterogeneity with localized attack, which was most significant with FeS₂ deposit. Machine learning (ML) models, like random forest (RF), decision tree (DT), and extreme gradient boost (XGBoost), demonstrated excellent IE predictive ability (<i>R</i>² = 0.99) for welded CS without/with conductive deposits. However, RF and XGBoost are ideal with the least RMSE/MAE (0.2/0.2 and 0.1/0.1) for welded CS without/with FeS and FeS₂, respectively. These results provide a practical foundation for pitting and PWC in preconditioned welded CS with conductive deposits, allowing for the best material selection and corrosion control techniques in sour service applications.