José Wilmar Calderón-Hernández

In this study, we explore the corrosion resistance of AA2198‐T8 alloy in friction stir‐welded zones, focusing on microstructure changes and resulting corrosion susceptibility. Macrogalvanic coupling effects are observed in weldment under corrosive conditions, particularly in the welding joint (WJ), heat‐affected zones, and base metal (BM). Severe localized corrosion is prevalent in anodic zones, revealing intricate trenching patterns around micrometric particles in cathodic domains. Investigating isolated zones exposes SLC across all welding‐affected regions and BM, underscoring the involvement of microgalvanic cells in corrosion. The novel utilization of a syringe cell as an electrochemical tool proves to be instrumental in delineating the electrochemical characteristics of welding testing zones. Notably, stir zone within WJ emerges as the region depicting the lowest corrosion resistance, a critical finding substantiated by localized electrochemical impedance spectroscopy. These insights into the interplay of electrochemical phenomena and microstructural attributes have implications for advancing corrosion mitigation strategies and alloy engineering in materials science.

In this study, an electrochemical syringe droplet cell was developed and used to evaluate the pitting corrosion resistance of Type 304 stainless steel. The cell was also adapted to perform measurements in deaerated environment, by means of continuous N2 injection. The results obtained with the syringe cells, in both conditions (aerated and deaerated), were reproducible and free of crevice corrosion because the cell configuration does not allow obtaining this condition. The proposed cell demonstrates coherency with data acquired using conventional cell arrangements, being a potential alternative to study pitting corrosion of passive metals.

The microstructure and electrochemical properties of 316L stainless steel were investigated after two conditions: aged at 640°C for 100,700 h and solution annealed at 1050°C for 2 h. While the aged samples were obtained from a pipe of a petrochemical reactor plant that was in service, the solution annealing was carried out in a conventional laboratory furnace. After aging, the precipitates present in decreasing order of quantity were sigma, Laves phase, and M₂₃C₆. After solution annealing, the microstructure was full austenitic. These results were in agreement with equilibria phase simulation with Thermo‐Calc software. Intergranular corrosion susceptibility, evaluated by means of the single loop electrochemical potentiokinetic reactivation technique and Practice A of ASTM 262, indicated a preponderant role for the sigma precipitate. The pitting potential (E_pit) was evaluated through potentiodynamic polarization curves in 0.6 M NaCl and electrochemical impedance spectroscopy was performed at the corrosion potential to complement the information about the corrosion resistance.

Low-temperature plasma nitriding (LTPN) of 410S ferritic-martensitic stainless steel was carried out in a 75 % N₂ – 25 % H₂ gas mixture, at 400°C, for 20 h. Conventional plasma nitriding (CPN) was also carried out in the same atmosphere but at 530°C. The corrosion resistance was assessed by potentiodynamic polarization (PP) testing in 3.5-wt. % NaCl. Measuring of the mass loss was carried out after immersion of the specimens in a 3 % FeCl₃ solution during 88 h. To define the sensitivity to intergranular corrosion, the degree of sensitization (DOS) was measured using the double-loop electrochemical PP technique (DL-EPR). DL-EPR results indicated that the potentiodynamic-polarization measurements showed that pitting corrosion resistance of the nitrided samples was enhanced, whereas the weight loss experiments showed that LTPN samples presented a corrosion resistance equal to the untreated samples. The DOS results displayed that the untreated samples were less resistant to intergranular corrosion correlated with the LTPN specimens. On the contrary, the CPN specimens exhibited typical behavior of general corrosion.