Quantitative Analyses of Crystalline Compositions of Iron Rust on Automobiles under Service Conditions.

Abstract

Quantitative analyses of the crystalline composition of rust that formed on automobiles under service conditions was carried out to investigate the corrosion mechanism of zinc-coated steel panels. The composition of the rust was quantitatively analyzed by X-ray diffraction using the internal-standard technique. The rust compositions in the crevice and on the surface of the outer panels were located within each characteristic region in the a-FeOOH/(g-FeOOHϩFe 3 O 4 )/remainder ternary diagram that we proposed in the present study. The rust in crevices contained more (g-FeOOHϩFe 3 O 4 ) than that on the surface. The largest amount of amorphous iron rust formed in the crevices of galvanized steel panels under zinc corrosion products during the initial stage of steel corrosion. Crystalline b-FeOOH then appeared until the amount reached a maximum content of 13 %. The composition of iron rust on zinc-coated steel sheets progresses towards that of the ungalvanized regions. It was found that the production of crystalline iron rust was retarded when zinc corrosion product formed in the crevice of lapped areas.