Ion Selectivity and Mechanism of Stable Rust Formation of Phosphoric Acid-Iron Oxide-PVB Rust Stabilization Surface Treatment Film

Tsuneyasu WATANABE, Yoichi ITO, Noboru KAWABATA, Atsumu KANDA
1996 Journal of The Surface Finishing Society of Japan  
Despite thier thinness, films applied to weathering steel using a phosphoric acid-iron oxide-PVB rust stabilization surface treatment act to inhibit rust formation and to suppress "rust outflow". The following conclusions were reached in a study of the reasons for the treatment's effectiveness. (1) The treatment film exhibits cation selectivity in a NaCl solution and KCl solution and anion selectivity in a FeCl2 solution and FeCl3 solution. Accordingly, by acting to suppress the diffusion of
more » ... the diffusion of Cl-, Fe2+ and Fe3+, the film inhibits corrosion of the iron matrix and the occurrence of "rust out-flow". (2) Examination of artificial rust (amorphous rust) revealed that a compound layer comprised of the treatment film and rust acts as a bipolar film with both cation selectivity and anion selectivity. The compound layer also suppresses the diffusion of Cl-, Fe2+ and Fe3+. (3) Because the treatment film has a relatively high permeability for moisture and oxygen, PO43-and other phosphate ions easily flow out of the film during prolonged atmospheric exposure. It is thought that PO43-, in addition to imparting ion selectivity to the rust, contributes, together with the alloying elements, to rust stabilization in weathering steel.
doi:10.4139/sfj.47.366 fatcat:va4iax23dvflpgk3ksab4win5y