Passivities of Steels Weathered for Nine Years

A carbon steel (CS) and a weathering steel (WS) were weathered in a temperate urban atmosphere for nine years. Phases of their rust layers were examined. Their corrosion behaviors were probed in aerated, neutral 0.5 M sodium sulfate (Na2SO4) bulk solution at room temperature using the potentiodynamic anodic polarization technique. Passivities by the layers with a corrosion rate of 0.05 mm/y or less at the corrosion potential of –150 mV vs saturated calomel electrode (SCE) or higher were observed, but they survived temporarily in the solution for 20 h and 5 h on the WS and CS covered with magnetite plus hematite phases, respectively, and 4 h and 1 h on the WS and CS with lepidocrocite plus goethite phases, respectively. They degraded with continued immersion in the solution accompanied by decreasing corrosion potential, finally near the active corrosion of bare steels. The passivities are under diffusion control of the iron ion dissolved from the base steels through the water in open pores of the layers but degrade by thinning and pore-opening of the layers under hydration and reductive reactions of the layers. The passivities are strengthened by integration and pore-sealing of the layers with corrosion products of alloying elements for the WS and by more compact and less-reductive magnetite-containing phases than lepidocrocite-containing ones in the layers. These findings may be applied in the evaluation and improvement of corrosion resistance for WS.