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The phase evolution of AISI 321 stainless steel was studied by directional solidification and quenching techniques. Two interfaces, solid/liquid and the peritectic reaction interface, were found to exist in the directional solidification structure. With increasing growth velocity the solid/ liquid interface changed in the sequence of planar, cellular, dendritic and the primary phase changed from austenite to ferrite. The phase and morphology selection was verified by the interface response functions (IRFs) and the maximum growth temperature criterion. The ferritic island banding structure was observed, not only in the austenite cellular primary growth condition (3 mm s 21 ), but also in the dendritic ferrite primary growth one at relatively low growth velocity (5 mm s 21 ). It is deemed that the former resulted from the nucleation of ferrite in the continuous matrix of austenite phase, yet the latter is the residual primary ferrite attributed to the growth of austenite. Both of them do not come from the nucleation near the solid/liquid interface.

The corrosion mechanism of the low carbon low alloy steel with Al2O3 particles and rare earth (RE) oxide particles was compared in a simulated marine environment. It is shown that when the Al2O3-containing particles are introduced, the number density of nonmetallic particles of the steel increases twice, and the average particle size decreases from approximately 2.4 μm to 1.4 μm. With the introduction of Al2O3-containing particles, the amount of pitting corrosion increases. Furthermore, pitting corrosion occurs more uniformly owing to the fineness of the Al2O3 particles, thereby leading to smaller, shallower pits after the Al2O3 particles are shed. Hence, the corrosion performance of the steel with Al2O3 particles is significantly improved than that of the steel without Al2O3 particles. By adding RE oxide particles into steel, the nonmetallic particles in steel are refined but not as effectively as that achieved by adding the Al2O3-containing particles. Different from Al2O3 particles, Cu is obviously enriched in the location of RE oxide particles at the initial corrosion stage, which makes the steel exhibit the best corrosion resistance. Cu enrichment is attributed to the mobile Cu present in the rust layer and to the micro acid region formed around the RE oxide particles.