Corrosion Behaviour of SS316 in TiO₂ Nanofluids and Phase Change Materials for Dairy Heat-Exchange Applications — Gravimetric, XRD, and Spectrophotometric Evaluation

Aims: To evaluate the corrosion behaviour of SS316 exposed to TiO₂-based nanofluids under dynamic flow and repeated freezing–thawing conditions, with particular emphasis on surface changes and dissolved iron release. Study Design: An experimental study investigating the effects of two TiO₂ concentrations (0.5% and 1.0%, w/w) under dynamic flow and phase change material (PCM)-associated freezing–thawing conditions. Place and Duration of Study: The study was conducted in the Dairy Engineering Section of the Southern Regional Station, ICAR-National Dairy Research Institute, Bengaluru, over an exposure period of 80 days. Methodology: TiO₂ nanofluids were prepared by magnetic stirring followed by ultrasonication at 40 kHz and 350 W for 2 h. SS316 coupons were exposed under dynamic flow at 80 °C and 100 L h⁻¹ for 8 h daily over 80 days. For PCM-associated exposure, coupons underwent freezing at −15 °C and thawing at ambient temperature (28–35 °C). Corrosion behaviour was evaluated using gravimetric analysis, X-ray diffraction (XRD), and spectrophotometric determination of dissolved iron by the 1,10-phenanthroline method. Results: PCM-exposed coupons exhibited initial weight gain followed by progressive weight loss, whereas dynamically exposed coupons showed gradual weight gain that tended to stabilise with time. XRD analysis indicated changes in surface crystallinity and possible formation of Fe₂O₃ and FeCr₂O₄. After 80 days, dissolved iron concentrations in the 0.5% and 1.0% PCM suspensions reached 2.365 ppm and 4.552 ppm, respectively. Conclusion: SS316 exhibited measurable surface and corrosion-related changes under both exposure conditions. The higher dissolved iron concentration in the 1.0% TiO₂ PCM suspension highlights the importance of evaluating material compatibility when employing TiO₂-based heat-transfer systems.