Effect of Inhomogeneous Composition on the Passive Film of AISI 431 Coating Fabricated by Extreme-High-speed Laser Cladding

The effect of inhomogeneous composition of Cr and Fe on morphology and formation mechanism of passive film was investigated. A bi-layer structure passive film with a Cr2O3 inner layer and a Fe2O3 outer layer obtained on EHLA coating. The passive film is heterogeneous (thickness, compactness) between inter-dendrite and dendrite core due to the element segregation and dislocation. Aspects of the passive film formation mechanism are discussed according to the thermodynamics. It indicates the prior oxidation of chromium resulting in the formation of inner Cr-rich layer, while higher diffusion rate of Fe3+ is the main reason for the formation of outer Fe-rich layer. This provides a new sight on the formation of multilayer oxide films.