Metal Ion and Exopolymer Interaction: A Surface Analytical Study

Various concentrations of molybdate (MoO₄²⁻) were added to the protein-containing and deproteinated exopolymers of the marine bacterium Deleya marina. Interaction was investigated using x-ray photoelectron spectroscopy (XPS) and electron spin resonance (ESR). MoO₄²⁻ reduction was observed exclusively in the deaerated protein-containing exopolymer, resulting in the formation of a Mo⁵⁺ species. This species appeared susceptible to reoxidation in the presence of soluble oxygen. Only hexavalent molybdenum (Mo⁶⁺) was seen in the aerated suspension. The reducing agents may have been the residual proteins that remained in the exopolymer without subsequent deproteination. The influence of this reduction on corrosion resistance of Mo-bearing stainless steels (SS) was simulated with an austenitic type 304 SS (UNS S30400), whose surface was treated with MoO₄²⁻ before exposure to the exopolymer. In addition to formation of the Mo⁵⁺ species, a small amount of molybdenum dioxide (MoO₂) was detected, and hydration of the passive film increased. No evidence was found to indicate the exopolymer attachment compromised corrosion resistance of the SS in deaerated 0.1 M hydrochloric acid (HCl).