The improvement of weathering resistance by increasing P contents in cast strips of low carbon steels

Cast strips of low carbon steels with different P contents (0.08, 0.15, and 0.26 wt%) were fabricated by using a pilot twin roll strip caster, whose mechanical properties were measured to be as good as those of the conventional counterparts. The weathering resistance of cast strips was assessed by the dry and wet cyclic accelerated corrosion test with the corrosive reagent in a 0.005 mol/l NaHSO3 solution (pH 4.5) at 30 °C with the humidity of 60%. Surface and cross-section morphologies of rust layers formed on the cast strips were observed and analyzed by SEM and EDX. The rust surfaces became to be more homogeneous and compact with increasing P content in cast strips. After 120 cycles of corrosion exposures, the average thickness of rust layers formed on the strips containing 0.15 and 0.26% P was less than 60% of that on the strips with 0.08% P. The P enrichment was observed at the interface between rust layer and substrate, which was believed to be responsible for the improvement of weathering resistance by retarding corrosive agents from further penetrating the substrate.