Kinetics of electrochemical boriding of low carbon steel

In this study, the growth kinetics of the boride layers forming on low carbon steel substrates was investigated during electrochemical boriding which was performed at a constant current density of 200 mA/cm2 in a borax based electrolyte at temperatures ranging from 1123 K to 1273 K for periods of 5-120 min. After boriding, the presence of both FeB and Fe2B phases were confirmed by the X-ray diffraction method. Cross-sectional microscopy revealed a very dense and thick morphology for both boride phases. Micro hardness testing of the borided steel samples showed a significant increase in the hardness of the borided surfaces (i.e., up to (1700 ± 200) HV), while the hardness of un-borided steel samples was approximately (200 ± 20) HV. Systematic studies over a wide range of boriding time and temperature confirmed that the rate of the boride layer formation is strongly dependent on boriding duration and has a parabolic character. The activation energy of boride layer growth for electrochemical boriding was determined as (172.75 ± 8.6) kJ/mol.

► The growth kinetics of the boride layers forming on low carbon steel was investigated during electrochemical boriding. ► Very dense and thick boride layers could be formed in a short period of electrochemical boriding (i.e., (19 ± 1) μm thick boride layer at 850 °C). ► The systematic studies over a wide range of boriding time and temperature confirmed that the rate of the boride layer formation is strongly dependent on boriding duration and has a parabolic character. ► The activation energy of the borided steel were determined to be (172.75 ± 8.6) kJ/mol which is about 24% lower than that found in the conventional paste boriding process. ► An empirical equation was derived to estimate the thickness of boride layers at different time and temperatures. Additionally the study resulted in a diagram that can be used to predict the thickness of boride layer for a given process time and temperature.