Pitting corrosion behavior of Cu–P–RE weathering steels

Abstract

The weathering steels are prone to pitting corrosion in an environment containing chloride ions. The pitting behavior of Cu–P–RE weathering steels and its effect on the corrosion resistance of steels were investigated by multifarious analytical techniques, such as field emission-scanning electron microscopy (FE-SEM), electron probe microanalysis (EPMA), scanning Kelvin probe force microscopy (SKPFM), electrochemical workstation and a series of immersion tests. The results show that the original strip-shaped MnS inclusions and Al2O3 inclusions with sharp angles are modified into the fine spherical rare earth (RE) inclusions with small average size, which are mainly RE oxysulfides after adding appropriate amount of mischmetal (48.9 wt%Ce–42 wt%La–5 wt%Nd–Fe) into the Cu–P weathering steel. In the environment containing Cl–, the pitting corrosion in RE weathering steel is induced by the preferential dissolution of RE inclusions in that the RE inclusions have a more negative potential than steel matrix at the initial corrosion stage. With the increase of corrosion time, the driving force of pitting expansion is weakened as a result of the continuous dissolution of RE inclusions, which makes the pitting tend to propagate horizontally around the RE inclusions. Once the RE inclusions completely dissolve, the open corrosion pits with shallow depth are formed in steel. The dispersed pitting pits with small size and shallow depth induced by RE inclusions are conducive to the formation of uniform and dense corrosion products layer on the steel surface, which reveals that the addition of RE can improve the corrosion resistance of weathering steels.