The effect of alloy composition on the rate-control process of electrolytic boriding of Fe-Cr-Ni alloys has been examined in a molten alkali salt with the low melting point using electrochemical and metallographic techniques. Electrochemical measurements showed no significant difference between the cathodic polarization curves for Fe-Cr and Fe-Ni alloys containing Cr or Ni up to 30 mass %. Meanwhile, metallographic measurements showed that an increase in the thickness of boride layer with decreasing potential occurs in the case of boriding of Fe, Ni, and Fe-20Ni alloy but does not occur in that of Cr, Fe-20Cr, and Fe-18Cr-8Ni alloys. This means the diffusion rate of B is high in the boride layers of the former metals and alloy but low in those of the latter metal and alloys. Therefore, it is presumed that the electrodeposition rate of B should control the bording rate on the former metals and alloy but the diffusion rate of B in the boride layers on the latter metal and alloys. Cr in Fe-Cr-Ni alloys inhibits the diffusion of B in the boride layer of the alloys.
