A recent study published in Scientific Reports examined the effects of hydrochloric acid (HCl) exposure on stainless steel (SS) 304, focusing on changes in its microstructure and mechanical properties.
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SS 304 samples were immersed in a 5 % HCl solution for 48 hours at different temperatures, and their behavior was analyzed through tensile, hardness, and bending tests, as well as microstructural analysis and X-ray diffraction (XRD).
Background
SS 304 is widely used in household and industrial applications due to its excellent corrosion resistance.
This property is attributed to its high chromium (18 %) and nickel (8 %) content, which form a passive oxide layer that protects the metal from oxidation and degradation. However, exposure to highly acidic environments, such as industrial cleaning agents and process fluids, can break down this protective layer, leading to corrosion.
This study assessed how SS 304 reacts to prolonged exposure to HCl, focusing on changes in mechanical strength and structural integrity. Understanding these effects is essential for improving material selection and protective measures in corrosive environments.
Methods
Five SS 304 plates were analyzed, with one kept as an ‘as received’ reference sample. The elemental composition of the plates was determined using energy-dispersive X-ray spectroscopy (EDS).
Four plates were submerged in separate 5 % HCl solutions at different temperatures: room temperature, 50 °C, 80 °C, and 110 °C. The heated samples were exposed to their respective temperatures for five minutes before immersion.
After 48 hours, the plates were removed from the solution and cleaned. Mechanical testing included tensile strength, bending strength, and hardness measurements. Microstructural analysis was conducted using optical microscopy, while XRD was used to detect phase transformations and assess material degradation.
The results of the corroded samples were compared to the ‘as received’ sample to evaluate the impact of HCl exposure.
Results and Discussion
Microstructural Changes:
Significant differences were observed between the ‘as received’ and HCl-treated samples. Optical microscopy images of the reference sample showed equiaxed γ-phase, fine grain boundaries, twins, and martensitic γ′ structures.
In contrast, the corroded samples exhibited carbide precipitates (M23C6), darkened and thickened grain boundaries, and pitting along γ boundaries. These changes indicated severe corrosion and material degradation.
XRD analysis further confirmed these findings. The reference sample displayed a distinct α-phase peak, which was absent in the corroded samples. The lack of oxide peaks in the HCl-treated plates suggested the removal of the protective oxide layer. The data also indicated a preferential attack on ferrite, leading to an increased concentration of the more corrosion-resistant γ-phase.
Mechanical Property Changes:
Tensile test results showed minor variations between the corroded and non-corroded samples. Yield strength and elongation remained largely unchanged, suggesting that corrosion primarily affected the surface while the core of the material remained intact.
Bending tests, however, revealed significant differences. The ‘as received’ sample withstood a higher maximum force compared to the corroded plates. Corroded samples exhibited reduced stiffness and lower resistance to bending forces, indicating structural weakening due to HCl exposure.
Hardness testing showed considerable variation across the samples, with Rockwell values ranging from 8 to 90. These inconsistencies were attributed to localized surface damage caused by corrosion.
Acid exposure led to uneven surface roughness and stress concentrations, which resulted in fluctuating hardness values across different regions of the plates. Since corrosion effects were not uniform, some areas experienced more degradation than others, leading to inconsistencies in hardness measurements.
Conclusion
This study demonstrated that prolonged exposure to 5 % HCl significantly alters the microstructure and mechanical properties of SS 304. Corrosion effects included grain boundary widening, increased carbide precipitation, and pitting along γ boundaries. XRD results confirmed the removal of the passive oxide layer and a preferential attack on ferrite, enriching the γ-phase in the material.
Although tensile properties remained mostly unchanged, bending strength decreased, and hardness values varied due to localized surface degradation. These findings highlight the importance of protective measures for SS 304 when used in high-temperature and acidic environments to prevent corrosion-related weakening.
Journal Reference
Agarwal, A., Mohite, S. A., More, P. B., & Dewangan, S. (2025). Impact of temperature changes on the microstructure and mechanical characteristics of AISI 304 submerged in 5 % HCl solution. Scientific Reports, 15(1). DOI: 10.1038/s41598-025-93164-5, https://www.nature.com/articles/s41598-025-93164-5
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