Effect of flash-Ni plating layer on interfacial behavior of Zn-Al-Mg coating/ DH780 steel

The Zn-Al-Mg coated DH780 high-strength steel with a flash-Ni plating layer was used in this study. The occurrence state of Ni element at the coating/steel interface was investigated, and the relationship between Mn and Si elements and the internal oxidation at the interface was discussed. The results show that Ni existed between the Fe2Al5 layer and the steel substrate in the form of Fe(Ni) solid solution and Ni elemental substance. The mutual diffusion rate between Fe and Al was greatly reduced after flash-Ni plating, and the continuity of Fe2Al5 layer of the flash-Ni-plated steel was slightly lower than that of the Ni-free steel. Moreover, the diffusion coefficients of Mn and Si in the Ni layer were lower than those in the steel substrate, which indicated that the Ni layer significantly inhibited the diffusion of Mn and Si to the surface. The average depth of internal oxidation was reduced by 70.8 % after flash-Ni plating, the average Mn, Si and O contents at the interface after flash-Ni plating were reduced by 19.6 %, 6.1 % and 22.7 %, respectively, which indicated that the flash-Ni plating layer can isolate the steel substrate from the annealing atmosphere, inhibit the internal oxidation and interfacial enrichment of Mn and Si, especially for Mn. Therefore, the flash-Ni plating layer can improve the wettability of highstrength steel sheets during continuous annealing.