The double carbide layer consisted of VC or NbC and (Cr, Fe)₇C₃ was formed on chromium-plated steels immersed in a borax bath containing powders of Fe-V or Fe-Nb was reported in a previous paper. Present paper deals with the mechanism of carbide and boride layer formation onto chromium-plated steels immersed into a fused borax bath containing powders of Fe-V, Fe-Nb, Cr, Fe-Mn or Fe-Ti, and on to the resistance spalling of the formed layers. In the bath for carbide coating, chromium plated layer near to the substrate transformed into a carbide layer of (Cr, Fe)₇C₃ by the reaction between Cr in the plated layer and carbon atoms in steel, but chromium near to the surface dissolved into the bath. When plated layer disappeared completely, a layer of VC, NbC or (Cr, Fe)₇C₃+(Cr, Fe)₂₃C₆ began to form on the (Cr, Fe)₇C₃ layer, by the reaction between V, Nb or Cr, dissolved in the borax and carbon atoms supplied from steel substrate through the layer of (Cr, Fe)₇C₃, and then the parabolic law was applied to the reaction between the thickness of those carbide layers and treating time. In the bath for bonding, chromium-plated layer near to the substrate transformed into (Cr, Fe)₇C₃, and on the surface, a boride layer of CrB was formed by the reaction between Cr in the plated layer and boron atoms in the borax. The parabolic law was applied to the relation between the thickness of layer of (Cr, Fe)₇C₃ or CrB and treating time. It was recognized that chromium plating will not accelerate the spalling of the formed VC layer.