Étude de l’influence des traitements thermochimiques sur le comportement a la corrosion des aciers

Abstract

Steel remains the only material capable of simultaneously supporting considerable forces and loads, but sometimes its use is limited by some of their surface-related properties, the main one being a very bad corrosion behavior, associated with a low hardness, it therefore becomes imperative to use different surface treatments. This work aims to improve the physico-chemical behavior of carbon steels, 20MnCr5 and C15 by different thermochemical treatments. The first treatment consists of a gaseous carburization carried out at 950 °C, the second treatment is a nitriding in salt baths carried out at 580 °C. Microscopic observations allowed us to delimit the carburizing and nitriding layers formed during each treatment. X-ray diffraction analysis and Raman spectroscopy of the treated samples allowed us to identify the different carbides (Fe2C, Fe3C) formed during the cementation treatment and the iron nitrides (Fe2-3N, Fe4N) formed during the nitriding. The Vickers microhardness tests carried out after each treatment showed the increase of the hardness of the surface relative to that of the matrix following the formation of carbides and nitrides of iron. We also studied the influence of these two treatments on the corrosion behavior of the two grades in an alkaline solution at 3 % NaCl assimilated to seawater. The cyclic polarization curves obtained after carburizing and nitriding treatment clearly shows the decrease in corrosion current density of the treated samples, therefore the treatments have led to a considerable increase in polarization resistance. The impedance diagrams of the two steels plotted before and after treatment clearly showed that the increase of the charge transfer resistance following the formation of carbides and nitrides of iron on the surface of the treated samples.