Influence of SnCl2 and SnC2O4 Precursors on the Corrosion Protection of SnO2 Coatings on 304 Stainless Steel in 3% NaCl Solution

This work prepared SnO2 coatings on 304 stainless steel via the sol–gel and dip-coating techniques, using tin (II) chloride (SnCl2) and tin (II) oxalate (SnC2O4) as precursors. The crystal structure analyzed by X-ray Diffraction (XRD) confirmed the cassiterite-type SnO2 in both cases. The corrosion resistance in a 3 wt.% NaCl solution was evaluated by polarization resistance (Rp) and anodic potentiodynamic polarization. Coatings derived from the SnC2O4 precursor demonstrated exceptional performance, reducing the corrosion rate by up to three orders of magnitude (from 0.0973 mpy for uncoated steel to 0.00015 mpy), corresponding to a protection efficiency of 99.8%. In contrast, coatings from the SnCl2 precursor increased the corrosion rate. X-ray Photoelectron Spectroscopy (XPS) analysis confirmed that this detrimental effect was due to the presence of chlorine (5.54 wt.%), which acted as an initiation site for pitting corrosion. Atomic force microscopy (AFM) and XRD of the effective SnC2O4-derived coatings revealed a homogeneous surface with low roughness and a textured cassiterite structure. The primary limitation of this work is that the sol–gel synthesis route using SnCl2 is unsuitable for corrosion protection in chloride environments due to the incorporation of aggressive chlorine ions, whereas the chlorine-free SnC2O4 precursor yields highly protective SnO2 coatings.