Noting the rust stabilizing effect of phosphate anions (PO₄^3-) on weathering steels, we developed a rust stabilizing surface treatment that consists of phosphoric acid, iron oxide and polyvinyl butyral resin (PVB). The following results were obtained after long-term atmospheric exposure of weathering steel treated with the technique.
(1) Iron ion outflow occurred with vigor in untreated (bare) specimens, but was highly suppressed in treated specimens. The treatment causes significant reduction of rust pollution from weathering steel structures which occurs during initial atmospheric exposure.
(2) After long-term exposure in the atmosphere, the treated layer transformed into a mixed layer containing rust. Viewed through a polarizing microscope, an extinction layer (containing Cr, Cu and conspicuous P enrichment) could be clearly observed at the boundary with the steel.
(3) Through reaction with the phosphate ions in the treatment agent, the steel interface contained phosphates and the treated film contained a large amount of chromium phosphate and zinc phosphate. It is thought that these phosphates contribute to the suppression of the iron ion outflow and to the growth of the extinction layer (which is thought to be amorphous).