Evalution of protective ability of rust layers on weathering steels by potential measurement

摘要

The protective ability of rust layers formed onweathering steels exposed in an industrial region for variousperiods has been studied by the potential measurement, and the rustcomposition and structure were also studied. Potential ofweathering steel in 0.1 mol/L Na_2SO_4 solution tends tobecome noble with exposure time. Potential of weathering steelswith rust layers, which were exposed for a long time and hadsuperior atmospheric corrosion resistance, became over -0.3 V (vs.SCE). The rust layer possesses the double layer structurecomposed of inner dark layer of #alpha#-FeOOH and outer brightlayer under reflected polarized light, which is observed typically inthe protective rust layer. On the other hand, potential of mild steelexposed for a long time is less noble at about -0.45 V (vs. SCE).These results mean that potential reflects the difference ofcharacteristic of rust layer. Potential became noble with increasingamount of #alpha#-FeOOH in rust, which suggests that potentialdepends on the composition of the rust layer. Furthermore,potential correlates with the ratio of rust composition#alpha#/#gamma#*: #alpha#-FeOOH/(#gamma#-FeOOH#+#beta#-FeOOH+Fe_3O_4), and potential of -0.3 V (vs.SCE) corresponds to alpha#/#gamma#*=2. Therefore potential aswell as the ratio alpha#/#gamma#* is able to use for theevaluation of the protective ability of the rust layers formed onweathering steels.