From Seaweed to Corrosion Inhibitor: A Multi-Method Validation of λ‑Carrageenan as a Non-Toxic Inhibitor under Acidic Hydrodynamic Conditions.

Corrosive processes are responsible for direct and indirect costs in industrial systems, which can be mitigated by corrosion inhibitors. The awareness of environmental risks has motivated sustainable approaches, such as naturally derived biopolymers, due to low toxicity and inhibition potential. λ-carrageenan, a sulfated biopolymer obtained from red algae, is explored in this work as an environmentally benign corrosion inhibitor. Gravimetric analyses indicate that the addition of 1500 ppm of λ-carrageenan markedly decreases the corrosion rate of mild steel, yielding inhibition efficiencies of 87.1% at 303 K and 72.0% at 333 K. Electrochemical techniques were employed under static conditions (0 rpm) and under hydrodynamic conditions at 500 and 1500 rpm to mimic flowing conditions expected in transport pipelines, achieving 89.8% at 500 rpm. Density functional theory calculations demonstrated that both the neutral and protonated forms of λ-carrageenan exhibit favorable electronic properties, with electron density distributed across hydroxyl and sulfate functionalities that promote interaction with the steel surface. Molecular dynamics simulations further confirmed the spontaneous adsorption onto the iron substrate at 1500 ppm, with interaction energies ranging from -158 to -179 kcal/mol across the investigated temperature interval. The protective layer, enabled by multiple adsorption sites involving SO