<p>Stainless Steel Grade 202 (SS Grade 202) is an iron-based alloy containing a moderate percentage of chromium, leading to be resistant toward corrosion in mild environments, and thus it is used in several industrial applications. However, its corrosion behavior in aggressive media and the efficiency of sustainable and low-cost corrosion inhibitors are not adequately explored. Nanoparticles (NPs) act as corrosion inhibitors and are adsorbed on the surface of metals, acting as a physical barrier to corrosion by reducing the area exposed to the corrosive medium. NPs of CaO derived from calcination of natural materials, such as clamshells and limestone, are highly effective corrosion inhibitors due to their smaller particle size and higher surface area, and more importantly, having the ability of passivation forming an oxide film. Synthesized CaO NPs were characterized by X-ray diffraction, Fourier transform infrared spectroscopy, X-ray fluorescence spectroscopy, particle size analysis and scanning electron microscopy, while the corrosion behavior of SS Grade 202 was studied by mass loss measurements, open circuit potential measurements, electrochemical impedance spectroscopy and linear polarization. Limestone-generated CaO NPs at 0.05&#xa0;mol&#xa0;L⁻¹ strength in 0.50&#xa0;mol&#xa0;L⁻¹ HCl show a corrosion rate of 3.53 × 10⁻² mmpy, which is much lower compared to the performance exhibited by CaO NPs from clamshells (1.24 mmpy). Abundant clamshell and limestone resources ensure a green, cost-effective route to producing CaO NPs as a corrosion inhibitor. This study introduces a new inhibitor based on CaO NPs with potential for practical corrosion protection. The straightforward and scalable synthesis, along with its impressive protective capabilities, underscores its practicality for industrial use and its uniqueness relative to traditional inhibitors.</p> Graphical abstract <p></p>