Traitement de surfaces - Christiane Ilunga

This document addresses fundamental concepts and applications of surface treatment processes in materials engineering, with particular emphasis on steels. It first establishes a comparative framework between surface treatments and heat treatments, highlighting that surface treatments modify only the external layer to improve wear resistance, corrosion resistance, and aesthetics, whereas heat treatments alter the internal structure of the material to adjust mechanical properties such as hardness, toughness, and tensile strength. The document then examines the brunishing (brunissage) of steel, described as a chemical process forming a black magnetite (Fe₃O₄) layer either at hot or cold conditions, more commonly cold in industrial practice. The advantages and limitations of brunishing are detailed, including its aesthetic finish, dimensional stability, temporary corrosion protection, and good electrical conductivity, contrasted with its restricted corrosion resistance, maintenance requirements, and limited applicability to carbon steels. The text further discusses the choice between electrolytic zinc plating and hot-dip galvanization, recommending electrolytic zinc plating for small, precision parts where thin, uniform, and aesthetically superior coatings are required. Finally, a quantitative electrochemical calculation is presented to determine the current necessary to deposit a 5 µm chromium layer on a steel sphere of 50 mm radius in 20 minutes from a CrO₃–H₂SO₄ bath, using Faraday’s laws, leading to an applied current of approximately 70 A.