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    Effect of heat treatment on corrosion behavior of S275 mild steel using accelerated DC voltage, LPR, and EIS
    (2024) Jacob, Liyamol; Parapurath, Shahid; Vahdati, Nader; Günister, Ebru
    This study used an external DC voltage of 1.5 V to accelerate corrosion in heat-treated S275 mild steel samples at different time intervals. LPR and EIS were used to study the corrosion behavior of original and quenched steel samples. There was only a negligible difference in the corrosion rate (CR) for the original and the quenched samples up to 30 min of voltage application in a 3.5% NaCl electrolyte media. When the exposure time increased to 60 min, the original sample showed seven times higher CR than the quenched samples. The pits on the surface of the original samples acted as cathodes, enhancing the reaction rate on the surface and increasing its CR dramatically. This led to bimodal corrosion, where the first part is led by concentration and diffusion; while, the second part is led by localized corrosion. The smaller pits on the original surface samples served as cathodic reaction centers, exacerbating corrosion. The corrosion rate of the original samples ranged from 0.8 to 7.8 mmpy; whereas, the corrosion rate of the quenched samples remained consistently around 0.8 mmpy. This trend can be observed in long-term corrosion in different metals. The uniformly oriented martensitic microstructure and the quenched samples’ small grain size prevented the enhanced ion penetration due to applied voltage. This study analyses the long-term stability of structural steel samples in marine environments by accelerating the corrosion rate by an applied external DC voltage.
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    Effect of microstructure on electrochemical properties of the EN S275 mild steel under chlorine-rich and chlorine-free media at different pHs
    (MDPI, 2022) Parapurath, Shahid; Jacob, Liyamol; Günister, Ebru; Vahdati, Nader
    This study examines microstructural modification as an effective strategy for reducing corrosion and its impact on the mechanical properties of mild steel. The effect of heat treatment on morphology, strength, toughness, and ductility was studied using optical microscopy, SEM, Scherrer equation, Vickers's hardness test, and tensile-strength measurement. The heat treatment changed the microstructures, grain sizes, and particle sizes of the samples. It also increased the material strength by 56% and 25% for the quenched and tempered samples, respectively. The hardness was increased to 95% by quenching. The effect of the microstructural changes on the corrosion rate in chlorine-rich and chlorine-free media at different pH was studied using linear-polarization-resistance and dynamic-polarization-resistance methods. In both media, the quenched samples showed a lower corrosion rate compared to the original and tempered samples. The heat treatment resulted in the formation of homogenous martensite with coarse grains and small particle sizes that seemed to reduce the corrosion rate significantly. It also had an impact on the corrosion mechanism of these materials. The original and tempered samples showed pitting-corrosion behavior with high corrosion rates, while the quenched samples were more susceptible to intergranular corrosion. The rate of corrosion was investigated further at different pH, and it was shown to decrease when the pH was raised. This study confirms the impact of microstructural changes on the corrosion behavior of S275 structural steel.
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    Influence of microstructure changes on the stress corrosion properties of u-bend S275 mild steel
    (Springer Nature Link, 2025) Parapurath, Shahid; Jacob, Liya; Vahdati, Nader; Günister, Ebru
    Current research investigates the material degradation behavior of thermally processed S275 mild steel under continuous stress in chloride and sulfate media at different pH values (4, 7, and 10). The mild steel samples were quenched at 1000 °C, followed by tempering at 600 °C, followed by U-bending the samples and attaching a bolt and nut to develop continuous stress according to ASTM standards. A U-bend sample without heat treatment was used as a reference. The corrosion behavior of the samples was evaluated using linear polarization resistance and potentiodynamic polarization scans. Optical microscopy, scanning electron microscope, roughness measurements, and Raman spectroscopy have been used to examine the wear behavior of S275 samples. Quenching followed by tempering caused the quasi-polygonal ferrite and granular bainite to convert to martensite. This microstructural change impacted on the material’s mechanical properties and corrosion behavior. Tempering caused a 38% increase in ultimate tensile strength and a 103% increase in yield strength. It also caused a 7–27% increase in corrosion resistance in sulfate media. The pH of the electrolyte solution also influenced corrosion. The corrosion rate of the original and tempered U-bend samples in acidic sulfate media showed approximately a 700% increase in corrosion rate compared to media alkaline and neutral sulfate media samples. The prime reason for the exponential increase in the corrosion rate is the effect of the chemical reaction of highly oxidative species on stress-induced surface defects. Raman analysis confirmed a variety of ferrous oxides and hydroxide formations on the surface of S275 steel.