Electrochemical Evaluation of 304 Stainless-Steel in Aqueous Curry-Leaf Extract: Effect of NaCl on Passivity and Food-Contact Safety
Abstract
Stainless-steel drinkware and food containers must withstand complex, plant-based beverages that may include dissolved salts. This study quantifies the corrosion behaviour of 304 stainless steel (commercially “Ever Silver”) in three media at 25 °C: de-ionised water, an aqueous extract of shade-dried curry leaves (CLE), and CLE containing 5 000 ppm NaCl. Potentiodynamic polarisation was carried out in a three-electrode cell (Ever Silver working electrode, SCE reference, Pt counter). Key parameters—including corrosion potential (E_corr), Tafel slopes, linear polarisation resistance (LPR), and corrosion current density (i_corr)—were extracted from the Tafel plots.
Relative to water (LPR ≈ 6.2 × 10⁴ Ω cm²; i_corr ≈ 0.86 µA cm⁻²), exposure to CLE alone reduced passivity markedly (LPR ≈ 4.0 × 10⁴ Ω cm²; i_corr ≈ 1.11 µA cm⁻²), indicating enhanced anodic dissolution driven by phytochemicals that adsorb on the surface. Incorporating 5 000 ppm NaCl partially restored passivity (LPR ≈ 4.4 × 10⁴ Ω cm²; i_corr ≈ 1.05 µA cm⁻²): chloride ions appear to promote a more compact, adsorbate-assisted film, offsetting some of the extract’s aggressiveness, though the alloy remains less resistant than in pure water.
The hierarchy of corrosion resistance derived from LPR and i_corr is: water > CLE + NaCl > CLE. Practically, 304 stainless vessels can safely accommodate curry-leaf infusions, and moderate chloride contents (typical of culinary use) mitigate rather than exacerbate deterioration compared with salt-free extracts. These findings support the suitability of “Ever Silver” containers for flavoured beverages while providing quantitative guidance for food-contact applications involving herbal ingredients.
Keywords:
Stainless steel 304, Ever Silver, curry-leaf extract (CLE), potentiodynamic polarisation, linear polarisation resistance, corrosion current density, food-contact safety.References
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