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Electrochemical machining is based on the anodic dissolution of most metals and generates high quality polished surfaces. However, ferrite-perlite 42CrMo4 steel reveals local optical changes at the surface after electrochemical finishing, such as a widely variable surface finish from shiny (reflective) to rough (dark) surfaces even after one processing step. The optical different surface areas of ferrite-perlite 42CrMo4 steel (AISI 4140) are studied by different electron microscopy techniques, x-ray diffraction and x-ray photoelectron spectroscopy to gain information about the local chemistry of the reaction layers and residual stresses of the rim zone. The results show that the rim zone for the different surface areas are about 50 nm-100 nm thick and contain oxygen. Selected area diffraction reveals the formation of iron(II/III) oxide (Fe3O4) and x-ray photoelectron spectroscopy confirms the formation of a mixed iron oxide (Fe3-xO4) with a variation of the oxidation state for both near-surface rim zones. Furthermore, the reflective surfaces reveal a homogeneous dissolution of ferrite and cementite lamellae whereas the rough surfaces show a preferred dissolution of cementite and an inhomogeneous dissolution of ferrite within the rim zone. X-ray diffraction measurements do not show any introduced residual stresses in the rim zone. [ABSTRACT FROM AUTHOR]