Corrosion Behavior of Boronized and Borochromized AISI 4140 Steel After Acid Exposure Evaluated by Electrochemical Impedance Spectroscopy

Boronizing of steel is a widely used method for superficial protection and strengthening of various components. Although it is an effective solution for most applications, it can be improved by adding metallic elements to enrich the coating with mixed borides that have exceptional properties. In the present study, we investigated the actual upgrade of a boronized AISI 4140 steel after an additional chromizing process in terms of corrosion resistance. Samples of boronized and borochromized steel were immersed in 10% vol. HCl or H 2 SO 4 solutions for 0.5-4 h and assessed with Electrochemical Impedance Spectroscopy. It is shown by the results that the borochromized specimens started with higher impedance values than the boronized ones, but they gradually degraded by immersion time and tended to approach the values of the boronized specimens, a fact attributed to the weakening of the resistance of the coating due to the corrosion. On the contrary, the boronized samples showed lower but stable values of impedance. The porosity of the boronized specimens did not contribute to their total impedance, as deduced by equivalent electrical circuit analysis. The impedance of the bulk region of both coatings did not decrease significantly during the corrosion. XRD and SEM/EDS measurements supported our findings.