Origin mechanism of pitting corrosion induced by cerium inclusions

In this paper,the origin mechanism of pitting corrosion induced by Ce_(2)O_(3),Ce_(2)O_(2)S,and CeAlO_(3) inclusions in a microalloyed steel was investigated by field emission scanning electron microscopy with energy dispersive spectroscopy and electron backscattered diffraction,conductivity atomic force microscopy,immersion test,and first-principles calculation.The results show that the Ce_(2)O_(3),Ce_(2)O_(2)S,and CeAlO_(3) inclusions are non-conductive,which are impossible to form corrosion couples with the steel matrix.There are no obvious lattice distortion zones in the steel matrix around the Ce_(2)O_(3),Ce_(2)O_(2)S,and CeAlO_(3) inclusions,so it is difficult to form micro-galvanic corrosion near the Ce inclusion.The order of work functions of the Ce inclusions and the steel matrix from small to large is Ce_(2)O_(2)S<Ce_(2)O_(3)<CeAlO_(3)<steel matrix,which is consistent with their dissolution sequence in the immersion test in _(3).5 wt%NaCl solution.Consequently,it is effective and reliable to use work function to predict or judge the dissolution behaviors of the Ce inclusions or steel matrix in corrosive solution.The Ce_(2)O_(3),Ce_(2)O_(2)S,and CeAlO_(3) inclusions have the tendency of self-dissolution and dissolve preferentially to the steel matrix in the solution by the salt effect,which lead to pitting corrosion of Ce-containing microalloyed steel.

Experimental and DFT study on cerium inclusions in clean steels

To provide insights into deforming Ce-O-S-Al inclusions in steels and improving the mechanical properties,the evolution process of such harmful inclusions in clean steels was investigated by thermodynamic calculation,metallographic examination and first-principles calculation in this paper.For the tested IF steel,the thermodynamic analysis results are consistent with the calculated formation enthalpy.After Ce addition,the inclusions are transformed from Al_(2)O_(3)and TiN-Al_(2)TiO_(5)-Al_(2)O_(3)to Ce2O_(3),Ce_(2)O_(2)S,CeAlO_(3),TiN-Al_(2)TiO_(5)-Ce_(2)O_(3)and TiN-Al_(2)TiO_(5)-Ce_(2)O_(2)S composite inclusions,which can be confirmed by metallographic examination.The elastic constants were calculated,and the bulk modulus,Young's modulus,shear modulus and Poisson's ratio were evaluated by the Voigt-Reuss-Hill(VRH)approximation.All inclusions except Ce_(2)O_(3)show apparent brittleness.TiN,Al_(2)O_(3),Al_(2)TiO_(5)and CeAlO_(3)present much higher hardness than iron matrix,while the hardness of Ce2O_(3)or Ce_(2)O_(2)S is close to that of iron matrix.The thermal expansion coefficients of Ce_(2)O_(3)and CeAlO_(3)are close to that of iron matrix,whereas,Ce_(2)O_(2)S inclusion has largely different thermal expansion coefficient from iron matrix and may deteriorate the steel performance at higher temperatures.The relatively small differences between Ce inclusions and iron matrix in terms of hardness,toughness,brittleness,and thermal expansion coefficient can explain the improvement of the mechanical properties of the tested steel.